Concrete technology : theory and practice [Fifth edition.] 9781259062551, 1259062554

Table of contents :
Title
Contents
1. Concrete as Construction Material
1.1 Introduction
1.2 Classifi cation of Concrete
1.3 Properties of Concrete
1.4 Grades of Concrete
1.5 Advantages of Concrete
1.6 Disadvantages of Concrete
1.7 Concept of Quality Control
1.8 Concrete Industry
1.9 Challenges Faced by the Concrete Industry
Review Questions
Multiple-Choice Questions
Answers to MCQ
2. Concrete Making Materials—I: Cement
2.1 Introduction
2.2 Portland Cement
2.3 Types of Cements
2.4 International Cement Classifi cations
2.5 Storage of Cement
2.6 Cement Certifi cation and Rejection
Review Questions
Multiple-Choice Questions
Answers to MCQ
3. Concrete Making Materials—II: Aggregate
3.1 Introduction
3.2 Classifi cation of Aggregates
3.3 Characteristics of Aggregates
3.4 Deleterious Substances in Aggregates
3.5 Soundness of Aggregate
3.6 Alkali-Aggregate Reaction (AAR)
3.7 Thermal Properties of Aggregates
3.8 Fineness Modulus
3.9 Maximum Size of Aggregate
3.10 Grading and Surface Area of Aggregate
3.11 Testing of Aggregates
3.12 Aggregate Processing, Handling and Storing
3.13 Marine-Dredged Aggregate
3.14 Recycled Concrete
Review Questions
Multiple-Choice Questions
Answers to MCQ
4. Concrete Making Materials—III: Water
4.1 Introduction
4.2 Quality of Mixing Water
4.3 Curing Water
Review Questions
Multiple-Choice Questions
Answers to MCQ
5. Chemical Admixtures and Mineral Additives
5.1 Introduction
5.2 Functions of Admixtures
5.3 Classifi cation of Admixtures
5.4 Physical Requirements of Admixtures
5.5 Indian Standard Specifi cations
5.6 Mineral or Supplementary Additives
Review Questions
Multiple-Choice Questions
Answers to MCQ
6. Properties of Fresh Concrete
6.1 Introduction
6.2 Workability
6.3 Measurement of Workability
6.4 Factors Affecting Workability
6.5 Requirements of Workability
6.6 Estimation of Errors
6.7 Air Content Test
Review Questions
Multiple-Choice Questions
Answers to MCQ
7. Rheology of Concrete
7.1 Introduction
7.2 Representation of Rheological Behaviour
7.3 Measurement of Rheology by the Modifi ed Slump Test
7.4 Factors Affecting Rheological Properties
7.5 Mixture Adjustments
Review Questions
Multiple-Choice Questions
Answers to MCQ
8. Properties of Hardened Concrete
8.1 Introduction
8.2 Strengths of Concrete
8.3 Stress and Strain Characteristics of Concrete
8.4 Dimensional Stability— Shrinkage and Creep
8.5 Creep of Concrete
8.6 Permeability of Concrete
8.7 Durability of Concrete
8.8 Concrete in Marine Environment
8.9 Acid Attack
8.10 Effl orescence
8.11 Fire Resistance
8.12 Thermal Properties of Concrete
8.13 Micro-Cracking of Concrete
Review Questions
Multiple-Choice Questions
Answers to MCQ
9. Quality Control of Concrete
9.1 Introduction
9.2 Factors Causing Variations in the Quality of Concrete
9.3 Field Control
9.4 Advantages of Quality Control
9.5 Statistical Quality Control
9.6 Measure of Variability
9.7 Application
9.8 Quality Management in Concrete Construction
Review Questions
Multiple-Choice Questions
Answers to MCQ
10. Proportioning of Concrete Mixes
10.1 Introduction
10.2 Basic Considerations for Concrete Mix Design
10.3 Factors Infl uencing the Choice of Mix Proportions
10.4 Methods of Concrete Mix Design for Medium Strength Concretes
10.5 Trial and Adjustment Method of Mix Design
10.6 New European Standards on Concrete
10.7 British Doe Method of Concrete Mix Design
10.8 The ACI Method For Mix Proportioning
10.9 Concrete Mix Proportioning – Is Guidelines
10.10 Concrete Mix Proportioning using FlY Ash - Is Guidlines
10.11 Rapid Method for Mix Design
10.12 Concrete Mix Design Illustration
10.13 Comparison of Mix Proportioning Methods
10.14 Optimum Concrete Mix Design
10.15 Design of High-Strength Concrete Mixes
10.16 Mix Proportioning for High Performance Concrete
10.17 Design of High Workability Concrete Mixes
10.18 Trial Mixes
10.19 Conversion of Mix Proportions From Mass to Volume Basis
10.20 Quantities of Materials to Make Specifi ed Volume of Concrete
10.21 Acceptance Criteria for Concrete
10.22 Field Adjustments
10.23 Generalized Format for Concrete Mix De sign
Review Questions
Multiple-Choice Questions
Answers to MCQ
11. Production of Concrete
11.1 Introduction
11.2 Batching of Materials
11.3 Mixing of Concrete Materials
11.4 Transportation of Concrete
11.5 Ready-Mixed Concrete
11.6 Placing of Concrete
11.7 Compaction of Concrete
11.8 Finishing of Concrete
11.9 Curing of Concrete
11.10 Formwork
11.11 Slip-Forming Technique
Review Questions
Multiple-Choice Questions
Answers to MCQ
12. Concrete Under Extreme Environmental Conditions
12.1 Introduction
12.2 Concreting in Hot Weather
12.3 Cold Weather Concreting
12.4 Underwater Concreting
Review Questions
Multiple-Choice Questions
Answers to MCQ
13. Inspection and Testing
13.1 Introduction
13.2 Inspection Testing of Fresh Concrete
13.3 Non-Destructive Testing of In–Situ Fresh Concrete
13.4 Acceptance Testing of Hardened Concrete
Review Questions
Multiple-Choice Questions
Answers to MCQ
14. Special Concretes and Concreting Techniques
14.1 Introduction
14.2 Lightweight Concrete
14.3 Ultra-Lightweight Concrete
14.4 Vacuum Concrete
14.5 Mass Concrete
14.6 Roller-Compacted Concrete
14.7 Waste Material-Based Concrete
14.8 Shotcrete or Guniting
14.9 Ferrocement
14.10 Fiber-Reinforced Concrete
14.11 Different Types of Fibers
14.12 Polymer Concrete Composites (PCCS)
14.13 Jet (Ultra-Rapid Hardening) Cement Concrete
14.14 Gap-Graded Concrete
14.15 No-Fines Concrete
14.16 High Density Concrete
14.17 Nuclear Concrete
14.18 Heat Resisting and Refractory Concretes
Review Questions
Multiple-Choice Questions
Answers to MCQ
15. Deterioration of Concrete and its Prevention
15.1 Introduction
15.2 Corrosion of Concrete
15.3 Corrosion of Reinforcement
Review Questions
Multiple-Choice Questions
Answers to MCQ
16. High-Perfomance Concretes
16.1 Introduction
16.2 High Performance Concrete
16.3 Classifi cation
16.4 Self-Compacting or Super-Workable Concrete
16.5 Lightweight Foamed or Aerated Concrete
16.6 Low Heat of Hydration Concrete
16.7 General Field Environment
16.8 Durability Performance Grades
16.9 Standard Test Procedures
16.10 Performance Enhancement
16.11 Performance of Fiber-Reinforced Concrete
16.12 Applications of High- Performance Concrete
Review Questions
Multiple-Choice Questions
Answers to MCQ
17. Repair Technology for Concrete Structures
17.1 Introduction
17.2 Symptoms and Diagnosis of Distress
17.3 Evaluation of Cracks
17.4 Selection of Repair Procedure
17.5 Repair of Cracks
17.6 Common Types of Repairs
17.7 Typical Examples of Concrete Repair
17.8 Leak Sealing
17.9 Underwater Repairs
17.10 Distress in Fire Damaged Structures
17.11 Strengthening with Composite Laminates
17.12 Strengthening of Defi cient Structures
Review Questions
Multiple-Choice Questions
Answers to MCQ
Appendix
Bibiliography
Index

Citation preview

CONCRETE TECHNOLOGY Fifth Edition

ABOUT THE AUTHOR Dr M L Gambhir has been Professor and Head of Civil Engineering Department, and Dean Planning & Resource Generation at the Thapar University, Patiala (previously Thapar Institute of Engineering & Technology, Patiala). He obtained his Bachelor’s and Master’s degrees from University of Roorkee (presently Indian Institute of Technology, Roorkee) and PhD from Queen’s University, Kingston, Canada. His major research interests have been in the areas of structural engineering particularly in structural failures and rehabilitation of structures; vibration-based health monitoring of structures; structural reliability; structural stability and dynamics; high performance concrete; steel and reinforced concrete design. He has wide experience in structural design of diverse types of structures in structural steel and reinforced concrete. Dr Gambhir has published over 65 technical papers in archival refereed journals and international conferences and has authored with reputed publishers. He has been a recipient of several prestigious awards. He is a member of Indian Society for Technical Education and the Indian Society for Earthquake Technology. He has also been the Chairman/Member of numerous committees.

CONCRETE TECHNOLOGY Fifth Edition

M L Gambhir Formerly Professor and Head Department of Civil Engineering Dean, Planning and Resource Generation Thapar University, Patiala Punjab

McGraw Hill Education (India) Private Limited NEW DELHI

McGraw Hill Education Offices New Delhi New York St Louis San Francisco Auckland Bogotá Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal San Juan Santiago Singapore Sydney Tokyo Toronto

McGraw Hill Education (India) Private Limited Published by McGraw Hill Education (India) Private Limited, P-24, Green Park Extention, New Delhi 110 016. Concrete Technology, 5e Copyright © 2013 by the McGraw Hill Education (India) Private Limited. No part of this publication may be reproduced or distributed in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers. The program listings (if any) may be entered, stored and executed in a computer system, but they may not be reproduced for publication. This edition can be exported from India only by the publishers, Tata McGraw Hill Education Private Limited. ISBN (13 digits): 978-1-25-906255-1 ISBN (10 digits): 1-25-906255-4 Vice President and Managing Director—MHE: Ajay Shukla Head—Higher Education Publishing and Marketing: Vibha Mahajan Publishing Manager—SEM & Tech Ed.: Shalini Jha Sr. Editorial Researcher: Harsha Singh Manager—Production Systems: Satinder S Baveja Copy Editor: Preyoshi Kundu Production Executive: Anuj K Shriwastava Marketing Manager—Higher Ed.: Vijay Sarathi Product Specialist: Sachin Tripathi Graphic Designer—Cover: Meenu Raghav General Manager—Production: Rajender P Ghansela Production Manager: Reji Kumar Information contained in this work has been obtained by McGraw Hill Education (India), from sources believed to be reliable. However, neither McGraw Hill Education (India) nor its authors guarantee the accuracy or completeness of any information published herein, and neither McGraw Hill Education (India) nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that Tata McGraw-Hill and its authors are supplying information but are not attempting to render engineering or other professional services. If such services are required, the assistance of an appropriate professional should be sought. Typeset at BeSpoke Integrated Solutions, Puducherry 605 008, India. Cover Printer:

CONTENTS Preface

xi

1. Concrete as Construction Material 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

1

Introduction Classification of Concrete Properties of Concrete Grades of Concrete Advantages of Concrete Disadvantages of Concrete Concept of Quality Control Concrete Industry Challenges Faced by the Concrete Industry Review Questions Multiple-Choice Questions Answers to MCQ

1 5 6 6 7 9 9 10 10 15 15 16

2. Concrete Making Materials—I: Cement

17

2.1 2.2 2.3 2.4 2.5 2.6

Introduction Portland Cement Types of Cements International Cement Classifications Storage of Cement Cement Certification and Rejection Review Questions Multiple-Choice Questions Answers to MCQ

17 18 33 50 53 54 54 55 62

3. Concrete Making Materials—II: Aggregate

63

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14

Introduction Classification of Aggregates Characteristics of Aggregates Deleterious Substances in Aggregates Soundness of Aggregate Alkali-Aggregate Reaction (AAR) Thermal Properties of Aggregates Fineness Modulus Maximum Size of Aggregate Grading and Surface Area of Aggregate Testing of Aggregates Aggregate Processing, Handling and Storing Marine-Dredged Aggregate Recycled Concrete Review Questions

63 63 69 75 76 77 78 79 80 81 88 90 91 92 92

vi

Contents

Multiple-Choice Questions Answers to MCQ

4. Concrete Making Materials—III: Water 4.1 Introduction 4.2 Quality of Mixing Water 4.3 Curing Water Review Questions Multiple-Choice Questions Answers to MCQ

5. Chemical Admixtures and Mineral Additives 5.1 5.2 5.3 5.4 5.5 5.6

Introduction Functions of Admixtures Classification of Admixtures Physical Requirements of Admixtures Indian Standard Specifications Mineral or Supplementary Additives Review Questions Multiple-Choice Questions Answers to MCQ

6. Properties of Fresh Concrete 6.1 6.2 6.3 6.4 6.5 6.6 6.7

Introduction Workability Measurement of Workability Factors Affecting Workability Requirements of Workability Estimation of Errors Air Content Test Review Questions Multiple-Choice Questions Answers to MCQ

7. Rheology of Concrete 7.1 7.2 7.3 7.4 7.5

Introduction Representation of Rheological Behaviour Measurement of Rheology by the Modified Slump Test Factors Affecting Rheological Properties Mixture Adjustments Review Questions Multiple-Choice Questions Answers to MCQ

8. Properties of Hardened Concrete 8.1 Introduction 8.2 Strengths of Concrete 8.3 Stress and Strain Characteristics of Concrete

93 95

96 96 96 100 100 100 101

102 102 103 104 129 131 134 143 143 144

145 145 146 148 154 157 160 160 161 161 164

165 165 167 169 174 177 177 177 178

179 179 179 190

Contents

8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13

Dimensional Stability— Shrinkage and Creep Creep of Concrete Permeability of Concrete Durability of Concrete Concrete in Marine Environment Acid Attack Efflorescence Fire Resistance Thermal Properties of Concrete Micro-Cracking of Concrete Review Questions Multiple-Choice Questions Answers to MCQ

9. Quality Control of Concrete 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

Introduction Factors Causing Variations in the Quality of Concrete Field Control Advantages of Quality Control Statistical Quality Control Measure of Variability Application Quality Management in Concrete Construction Review Questions Multiple-Choice Questions Answers to MCQ

10. Proportioning of Concrete Mixes 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 10.13 10.14 10.15 10.16 10.17 10.18 10.19

Introduction Basic Considerations for Concrete Mix Design Factors Influencing the Choice of Mix Proportions Methods of Concrete Mix Design for Medium Strength Concretes Trial and Adjustment Method of Mix Design New European Standards on Concrete British Doe Method of Concrete Mix Design The ACI Method For Mix Proportioning Concrete Mix Proportioning – Is Guidelines Concrete Mix Proportioning using FlY Ash - Is Guidlines Rapid Method for Mix Design Concrete Mix Design Illustration Comparison of Mix Proportioning Methods Optimum Concrete Mix Design Design of High-Strength Concrete Mixes Mix Proportioning for High Performance Concrete Design of High Workability Concrete Mixes Trial Mixes Conversion of Mix Proportions From Mass to Volume Basis

vii

194 198 198 200 206 208 208 208 210 211 212 213 216

218 218 219 220 221 221 223 225 231 236 237 238

239 239 241 241 253 254 255 261 275 283 295 297 301 307 309 313 313 329 333 333

viii

Contents

10.20 10.21 10.22 10.23

Quantities of Materials to Make Specified Volume of Concrete Acceptance Criteria for Concrete Field Adjustments Generalized Format for Concrete Mix Design Review Questions Multiple-Choice Questions Answers to MCQ

11. Production of Concrete 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11

Introduction Batching of Materials Mixing of Concrete Materials Transportation of Concrete Ready-Mixed Concrete Placing of Concrete Compaction of Concrete Finishing of Concrete Curing of Concrete Formwork Slip-Forming Technique Review Questions Multiple-Choice Questions Answers to MCQ

12. Concrete Under Extreme Environmental Conditions 12.1 12.2 12.3 12.4

Introduction Concreting in Hot Weather Cold Weather Concreting Underwater Concreting Review Questions Multiple-Choice Questions Answers to MCQ

13. Inspection and Testing 13.1 13.2 13.3 13.4

334 335 336 337 340 343 349

350 350 350 352 356 361 370 373 381 385 396 401 403 403 408

409 409 409 411 414 420 420 421

422

Introduction Inspection Testing of Fresh Concrete Non-Destructive Testing of In–Situ Fresh Concrete Acceptance Testing of Hardened Concrete Review Questions Multiple-Choice Questions Answers to MCQ

422 423 430 434 458 459 460

14. Special Concretes and Concreting Techniques

461

14.1 14.2 14.3 14.4 14.5

Introduction Lightweight Concrete Ultra-Lightweight Concrete Vacuum Concrete Mass Concrete

461 463 473 474 476

Contents

14.6 14.7 14.8 14.9 14.10 14.11 14.12 14.13 14.14 14.15 14.16 14.17 14.18

Roller-Compacted Concrete Waste Material-Based Concrete Shotcrete or Guniting Ferrocement Fiber-Reinforced Concrete Different Types of Fibers Polymer Concrete Composites (PCCS) Jet (Ultra-Rapid Hardening) Cement Concrete Gap-Graded Concrete No-Fines Concrete High Density Concrete Nuclear Concrete Heat Resisting and Refractory Concretes Review Questions Multiple-Choice Questions Answers to MCQ

15. Deterioration of Concrete and its Prevention

ix

476 482 488 495 506 511 532 541 543 544 544 547 548 554 554 559

560

15.1 Introduction 15.2 Corrosion of Concrete 15.3 Corrosion of Reinforcement Review Questions Multiple-Choice Questions Answers to MCQ

560 560 566 572 573 573

16. High-Perfomance Concretes

574

16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 16.10 16.11 16.12

Introduction High Performance Concrete Classification

574 576 576

Self-Compacting or Super-Workable Concrete

579

Lightweight Foamed or Aerated Concrete Low Heat of Hydration Concrete General Field Environment Durability Performance Grades Standard Test Procedures Performance Enhancement Performance of Fiber-Reinforced Concrete Applications of High- Performance Concrete Review Questions Multiple-Choice Questions Answers to MCQ

617 630 630 633 637 638 667 671 673 673 675

17. Repair Technology for Concrete Structures 17.1 17.2 17.3 17.4

Introduction Symptoms and Diagnosis of Distress Evaluation of Cracks Selection of Repair Procedure

676 676 678 686 690

x

Contents

17.5 17.6 17.7 17.8 17.9 17.10 17.11 17.12

Repair of Cracks Common Types of Repairs Typical Examples of Concrete Repair Leak Sealing Underwater Repairs Distress in Fire Damaged Structures Strengthening with Composite Laminates Strengthening of Deficient Structures Review Questions Multiple-Choice Questions Answers to MCQ

Appendix Bibiliography Index

691 709 715 722 723 727 729 732 735 735 738

739 756 763

PREFACE Introduction In 2009, the fourth edition was completely rewritten, updated and enlarged in the light of revisions in Indian Standards and global developments. Topics such as ready-mixed concrete, pumped concrete and self-compacting concrete, nuclear concrete were introduced. A chapter on high performance concrete was added. In the chapter on special concretes, methods for proportioning high performance concrete were included. In the chapter on repair technology, state-of-the-art technologies of strengthening with composite laminates were included. Since the publication of fourth edition in 2009 a number of significant developments have taken place in the field of Cement and Concrete technology. One of the major developments has been the revision of IS 10262-2009: Concrete Mix ProportioningGuidelines. The revision has followed the format of ACI mix proportioning method, a departure from the traditional similarities with British codes. Although, a common code for European Nations has come into force from January 1, 2004, it does not have a common concrete mix design method because it considers mix design a part of concrete production. However, it exercises control through EN 206-1. It is immaterial whether the concrete mix is proportioned by IS Concrete Mix Proportioning Guidelines or ACI mix design method or British DoE method or DIN, as long as concrete satisfies the requirements/specifications. Although, fast development of infrastructure is taking place in the country, use of high strength and high performance concretes (HPC) is now common practice, but infrastructure developments in India requires adoption of new technologies. For example more than 75 per cent of concrete used worldwide is ready-mixed concrete which is placed by pumping. Concerns have been expressed regarding slow progress in adopting self-compacting concrete (SCC) technology in India; whereas, in Europe, America and in some other parts of the world, it has occupied front seat. In the modern art and science of designing and constructing the infrastructure selfcompacting concrete has carved its prominent place due to its unparalleled surface finish and other high performance qualities. In the past four years alone seven tests for self-compacting concrete have been standardized globally. Keeping in view the above scenario, the book has been revised. Besides presenting large new information, the fifth edition is more user-friendly. New, updated and expanded information has been added in chapter 10 on proportioning of concrete mixes as per IS 10262-2009 guidelines, British mix design procedure is recast in terms of Euro codes. A new section on mix design for conventional fly ash concrete is included. The mix-design procedures have been illustrated with flow charts to enable readers to evolve their own mix proportion calculators. In chapter 16, the section on self-compacting concrete is completely rewritten, updated and enlarged in the light of development of new technologies. The concept of tailoring the properties of self-compacting concrete using fixed cement content (minimum cement content from durability considerations) and the locally available materials to meet the demands of any particular application as a substitute of

xii

Preface

conventional concrete is introduced. The aim has been to bring it in conformity with rapidly changing field of cement and concrete technology and to maintain a state-of-the-art status.

New to the Edition • • • • • •

Section on self-compacting concrete is completely rewritten, updated and enlarged in the light of development of new technologies Updated with revised IS 10262-2009: Concrete Mix Proportioning-Guidelines A new section on mix design for conventional fly ash concrete is included The mix-design procedures have been illustrated with flow charts to enable readers to evolve their own mix proportion calculators Gel-Space Ratio added New section on Exercise Questions added in each chapter.

Salient Features of the Book •

Enhanced and updated discussion on Mix Design Proportioning using IS 10262-2009: Concrete Mix Proportioning-Guidelines • Detailed discussion on Self-compacting concrete • British mix design procedure is recast in terms of Euro codes. • Pedagogy includes 123 Exercise Questions 678 MCQs 237 Figures The following material can be accessed at http://www.com/gambhir/ct5 For Instructors • PowerPoint slides For Students • •

Sample chapter Bibliography

Acknowledgements I wish to acknowledge the contributions made by many individuals and organizations that provided valuable assistance in bringing out this edition. The feedback from the users has been of great help and I express my deep sense of gratitude to them. I am also grateful to my daughter, Ms. Neha Jamwal, M Tech Civil Engineering for her useful discussions and contribution, and my wife for continued cooperation and encouragement in bringing out this edition. I would like to thank the following reviewers for providing their suggestions in improving the manuscript. Nazrul Islam

Jamia Millia Islamia, New Delhi

Archana Bohra Gupta

Mugneeram Bangur Memorial Engineering College, Jodhpur, Rajasthan

Preface

xiii

Amlan Das

National Institute of Technology, Durgapur, West Bengal

N C Shah

Sardar Vallabhbhai National Institute Technology, Surat, Gujarat

Sunil V Desale

Shri Shivaji Vidya Prashashak Sanstha, Dhule, Maharashtra

K Nagamani

College of Engineering, Guindy, Chennai, Tamil Nadu

G Bhaskar

Institute of Road and Transport Technology, Erode, Tamil Nadu

A Jagannathan

Pondicherry Engineering College, Puducherry

Rajesh Kumar

National Institute of Technology, Warangal, Andhra Pradesh

Feedback It is hoped that this revised and expanded fifth edition of the book will be as acceptable to the engineering fraternity and to all those who are interested in concrete construction, as its predecessors have been. To improve and make the book more useful in future reprints and editions, the comments from readers are welcome. The publishers regret to inform the sad and untimely demise of Dr M. L. Gambhir on 4th January 2013. This work is dedicated to the untiring spirit of an academician par excellence who always dreamt of a seamless dissemination of knowledge and strove incessantly towards the same. Dr M L Gambhir

Publisher’s Note We look forward to receiving valuable views, comments and suggestions for improvements from teachers and students, all of which can be sent to tmh.civilfeedback@ gmail.com, mentioning the title and author’s name on the subject line. Report of any piracy related problems/issues would be highly appreciated.

1 1.1

CONCRETE AS CONSTRUCTION MATERIAL

INTRODUCTION

Concrete is the most widely used man-made construction material in the world, and is second only to water as the most utilized substance on the planet. It is obtained by mixing cementing materials, water and aggregates, and sometimes admixtures, (shown in Fig. 1.1) in required proportions. The mixture when placed in forms and allowed to cure, hardens into a rock-like mass known as concrete. The hardening is caused by chemical reaction between water and cement and it continues for a long time, and consequently the concrete grows stronger with age. The hardened concrete may also be considered as an artificial stone in which the voids of larger particles (coarse aggregate) are filled by the smaller particles (fine aggregate) and the voids of fine aggregates are filled with cement. In a concrete mix, the cementing material and water form a paste called cement–water paste which in addition to filling the voids of fine aggregate, coats the surface of fine and coarse aggregates and binds them together as it cures, thereby cementing the particles of the aggregates together in a compact mass.

Fig. 1.1

Basic components of modern concrete: cement, water, fine aggregate, coarse aggregate, mineral additives and admixtures

The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, on the proportions of mix, the method of compaction and other controls during placing, compaction and curing. The popularity of the concrete is due to the fact that from the common ingredients, it is possible to tailor the properties of concrete to meet the demands of any particular situation. The images in

2

Concrete Technology

Fig. 1.2 illustrate the mouldability of concrete in architectural forms. The advances in concrete technology have paved the way to make the best use of locally available materials by judicious mix proportioning and proper workmanship, so as to produce concrete satisfying performance requirements.

Cathedral

Fig. 1.2

Epcot

Architectural use of concrete

The key to producing a strong, durable and uniform concrete, i.e., high-performance concrete lies in the careful control of its basic and process components. These are the following: 1. Cement Portland cement, the most widely used cementing ingredient in present day concrete comprises phases that consist of compounds of calcium, silicon, aluminum, iron and oxygen. 2. Aggregate These are primarily naturally occurring, inert granular materials such as sand, gravel, or crushed stone. However, technology is broadening to include the use of recycled materials and synthetic products. 3. Water The water content and the minerals and chemicals dissolved in it are crucial to achieving quality concrete. 4. Chemical admixtures These are the ingredients in concrete other than Portland cement, water, and aggregates that are added to the mixture immediately before or during mixing to reduce the water requirement, accelerate/retard setting or improve specific durability characteristics. 5. Supplementary cementing materials Supplementary cementing materials, also called mineral additives, contribute to the properties of hardened concrete through hydraulic or pozzolanic activity. Typical examples are natural pozzolans, fly ash, ground granulated blast-furnace slag, and silica fume. After

Concrete as Construction Material

3

concrete is placed, these components must be cured at a satisfactory moisture content and temperature must be carefully maintained for a sufficiently long time to allow adequate development of the strength of the concrete.

Fig. 1.3

Image of a typical modern city with skyscrapers—looks like a concrete jungle

The factors affecting the performance of concrete are shown in Fig. 1.4. The concept of treating concrete in its entity as a building material rather than its ingredients is gaining popularity. The user is now interested in the concrete having the desired properties without bothering about the ingredients. This concept is symbolized with the progress of ready mixed concrete industry where the consumer can specify the concrete of his needs and further in the precast concrete industry where the consumer obtains finished structural components satisfying the performance requirements. The various aspects covered in the following chapters are materials, mix proportioning, elements of workmanship, e.g., placing, compaction and curing, methods of testing and relevant statistical approach to quality control. The discussions on these aspects are based on the appropriate provisions in the Indian Standard Codes. Concrete has high compressive strength, but its tensile strength is very low. In situations where tensile stresses are developed, the concrete is strengthened by steel bars or short randomly distributed fibers forming a composite construction called reinforced cement concrete (RCC) or fiber reinforced concrete. The concrete without reinforcement is termed as plain cement concrete or simply as concrete. The process of making concrete is called concreting. Sometimes the tensile stresses are taken care of by introducing compressive stresses in the concrete so that the initial compression neutralizes the tensile stresses. Such a construction is known as prestressed cement concrete construction.

CEMENT Composition Quality

Fig. 1.4

CHEMICAL ADMIXTURES Properties

Curing

Mixing

SUPPLEMENTARY CEMENTING MATERIALS A Composition Quality

Factors affecting performance of concrete

PERFORMANCE OF HARDENED CONCRETE

Transporting Placing Compacting

PERFORMANCE OF FRESH CONCRETE

AGGREGATE A S Size Shape Grading Quantity Moisture WATER A Quantity

Concrete as Construction Material

5

Fig. 1.5

Typical superhighway with over passes—smooth and efficient traffic movement saves energy

1.2

CLASSIFICATION OF CONCRETE

As mentioned earlier, the main ingredients of concrete are cement, fine aggregate (sand) and coarse aggregate (gravel or crushed rock). It is usual to specify a particular concrete by the proportions (by weight) of these constituents and their characteristics, e.g., a 1:2:4 concrete refers to a particular concrete manufactured by mixing cement, sand and broken stone in a 1:2:4 ratio (with a specified type of cement, water-cement ratio, maximum size of aggregate, etc.). This classification specifying the proportions of constituents and their characteristics is termed as prescriptive specifications and is based on the hope that adherence to such prescripitive specifications will result in satisfactory performance. Alternatively, the specifications specifying the requirements of the desirable properties of concrete such as strength, workability, etc., are stipulated, and these are termed as performance-oriented specifications. Based on these considerations, concrete can be classified either as nominal mix concrete or designed mix concrete. Sometimes concrete is classified into controlled concrete and ordinary concrete, depending upon the levels of control exercised in the works and the method of proportioning concrete mixes. Accordingly, a concrete with ingredient proportions fixed by designing the concrete mixes with preliminary tests are called controlled concrete, whereas ordinary concrete is one where nominal mixes are adopted. In IS:456–2000, there is nothing like uncontrolled concrete: only the degree of control varies from very good to poor or no control. In addition to mix proportioning, the quality control includes selection of appropriate concrete materials after proper tests, proper workmanship in batching, mixing, transportation, placing, compaction and curing, coupled with necessary checks and tests for quality acceptance.

6

Concrete Technology

1.3

PROPERTIES OF CONCRETE

Concrete making is not just a matter of mixing ingredients to produce a plastic mass, but good concrete has to satisfy performance requirements in the plastic or green state and also the hardened state. In the plastic state, the concrete should be workable and free from segregation and bleeding. Segregation is the separation of coarse aggregate and bleeding is the separation of cement paste from the main mass. The segregation and bleeding result in a poor quality concrete. In its hardened state, concrete should be strong, durable, and impermeable and it should have minimum dimensional changes.

Fig. 1.6

Image of a typical concrete bridge which must be specially designed for durability

Among the various properties of concrete, its compressive strength is considered to be the most important and is taken as an index of its overall quality. Many other properties of concrete appear to be generally related to its compressive strength. These properties will be discussed in detail later in the book.

1.4

GRADES OF CONCRETE

Concrete is generally graded according to its compressive strength. The various grades of concrete as stipulated in IS:456–2000 and IS:1343–1980 are given in Table 1.1. In the designation of concrete mix, the letter M refers to the mix and the number to the specified characteristic strength of 150 mm work cubes at 28 days, expressed in MPa (N/mm2). The concrete of grades M5 and M7.5 is suitable for lean concrete bases, simple foundations, foundations for masonry walls and other simple or temporary reinforced concrete constructions. These need not be designed. The concrete of grades lower than M15 is not suitable for reinforced concrete works and grades of concrete lower than M30 are not to be used in the prestressed concrete works.

Concrete as Construction Material Table 1.1 Group Grade designation

Ordinary concrete

7

Grades of concrete Standard concrete

High strength concrete

M M M M M M M M M 10 15 20 25 30 35 40 45 50

M M M M M M 55 60 65 70 75 80

10 15 20 25 30 35 40 45 50

55 60 65 70 75 80

Specified characteristic strength at 28 days, MPa

Fig. 1.7

1.5

Image of monorails—an enjoyable means of city transport

ADVANTAGES OF CONCRETE

Concrete as a construction material has the following advantages: 1. Concrete is economical in the long run as compared to other engineering materials. Except cement, it can be made from locally available coarse and fine aggregates. 2. Concrete possesses a high compressive strength, and the corrosive and weathering effects are minimal. When properly prepared its strength is equal to that of a hard natural stone. 3. The green or newly mixed concrete can be easily handled and molded or formed into virtually any shape or size according to specifications. The formwork can be reused a number of times for similar jobs resulting in economy. 4. It is strong in compression and has unlimited structural applications in combination with steel reinforcement. Concrete and steel have approximately equal coefficients of thermal expansion. 5. Concrete can even be sprayed on and filled into fine cracks for repairs by the guniting process. 6. Concrete can be pumped and hence it can be laid in difficult positions also. 7. It is durable, fire resistant and requires very little maintenance.

8

Concrete Technology

These advantages have resulted in extensive use of concrete in the construction of buildings, skyscrapers (Fig. 1.3), superhighways with over and under passes (Fig. 1.5), bridges (Fig. 1.6), railways, monorails (Fig. 1.7), tunnels (Fig. 1.8), runways of airfields (Fig. 1.9), water-retaining structures, docks and harbors, dams, cross-drainage works (Fig. 1.10), bunkers, and silos.

Fig. 1.8

Fig. 1.9

Fig. 1.10

Concrete used in lining the tunnel

A runway under construction (slip forming, a modern method for concrete paving)

Aqueduct ferry crossing—a unique cross-drainage work with an application

Concrete as Construction Material

1.6

9

DISADVANTAGES OF CONCRETE

The following are the disadvantages of concrete: 1. Concrete has low tensile strength and hence cracks easily. Therefore, concrete is to be reinforced with steel bars or meshes or fibers. 2. Fresh concrete shrinks on drying and hardened concrete expands on wetting. Provision for construction joints has to be made to avoid the development of cracks due to drying shrinkage and moisture movement. 3. Concrete expands and contracts with the changes in temperature. Hence, expansion joints have to be provided to avoid the formation of cracks due to thermal movement. 4. Concrete under sustained loading undergoes creep, resulting in the reduction of prestress in the prestressed concrete construction. 5. Concrete is not entirely impervious to moisture and contains soluble salts which may cause efflorescence. 6. Concrete is liable to disintegrate by alkali and sulphate attack. 7. The lack of ductility inherent in concrete as a material is disadvantageous with respect to earthquake resistant design.

1.7

CONCEPT OF QUALITY CONTROL

Quality in general terms is totality of features and characteristics of a product or service that bear on its ability to satisfy the stated or implied needs. The stated or implied needs are those derived by balanced excellence and equity within the sustainable regime and in the given socio–techno–economic scenario. The quality management has evolved over the period through: 1. Policing quality Acceptance and rejection through inspection and assessment by user, 2. Judging quality Confidence building through third-party judgement, and 3. Fostering quality Ensuring quality of the final product by attending to quality at all intermediary stages such as in Certification Marking Schemes. Concrete, generally manufactured at the site, is likely to have variability of performance from batch to batch and also within the batch. The magnitude of this variation depends on several factors, such as the variation in the quality of constituent materials, variation in mix proportions due to batching process, variations in the quality of batching and mixing equipment available, the quality of overall workmanship and supervision at the site, and variation due to sampling and testing of concrete specimens. The above variations are inevitable during production to varying degrees. For example, the cements from different batches or sources may exhibit different strengths. The grading and shape of aggregates even from the same source varies widely. Considerable variations occur partly due to the quality of the plant available and partly due to the efficiency of operation. Some of the variations in test results are due to variations in sampling, making, curing and testing the specimen even when carried out in terms of relevant specifications.

10

Concrete Technology

The quality control of concrete is thus to reduce this variation and to produce concrete of uniform quality consistent with specified minimum performance requirements which can be achieved by good workmanship and maintainance of the plant at peak efficiency. The concrete industry strives at making ‘quality’, a way of life and a way of management through Quality Systems Approach covering all aspects of ISO 9000 series.

1.8

CONCRETE INDUSTRY

Since concrete is an affordable and reliable material, which is extensively used throughout in the infrastructure of a nation’s construction, industrial, transportation, defense, utility, and residential sectors, it has become a huge industry. India produces about 170 million cubic meters of concrete annually. Slightly more than a tonne of concrete is produced each year for every human being on earth. In addition to cement and concrete manufacturing, the concrete construction industry includes aggregate and material suppliers, designers, haulers, constructors, and repair and maintenance companies. Over 40 million jobs relate to the concrete industry alone. While there is significant diversity of services within this industry, all facets of the concrete industry share a common objective—a sincere desire to deliver a high-quality, long-lasting, competitive, eco-friendly and sustainable product.

Sustainability Sustainable regime may be defined as that regime in which the endeavors are towards meeting the needs of the present generation without compromising the needs of the future generations. So far as construction industry is concerned, it has to work within the following strategic framework if it has to be ‘sustainable’: 1. Responsiveness to environmental regulations, i.e., environmental protection including ecological balancing 2. Material conservation including performance maximization 3. Energy efficiency or conservation 4. Cost effectiveness or cost reduction, both initial and life-cycle costs 5. Safety assurance 6. Durability and serviceability considerations 7. Manpower development and optimization 8. Ergonomic and aesthetic concerns 9. Total quality management 10. Creation of proper interface with computer-integrated knowledge-based systems for technology transfer

1.9

CHALLENGES FACED BY THE CONCRETE INDUSTRY

Portland cement is the most energy-intensive material produced after steel and aluminum. More than seven per cent of world’s carbon dioxide emissions are attributed to Portland cement. In addition to CO2 emissions, the burning of Portland cement at high temperature (1450°C) is costly in terms of fossil fuel usage. Moreover, by some

Concrete as Construction Material

11

estimate concrete industry is largest consumer of natural resources such as water, sand, gravel and crushed rock. Thus for sustainable development, it is recognized that considerable improvements are essential in productivity, product performance, energy efficiency, and environmental performance. To achieve these objectives will require a concerted and focused effort. Research in new materials, processing technologies, delivery mechanisms, and applications of information technology, could transform the industry. Greater materials improvements will enable the industry to demonstrate clearly the full spectrum of performance benefits of concrete. A number of government agencies—NCB, CRI, SERC, CBRI—and CRRI focus on wide variety of concrete research topics. Programmes of several universities and technical institutions are involved in concrete related research. Sponsoring agencies include DST, AICTE, UGC, etc. These agencies sponsor broad-spectrum concrete research— basic and applied—to improve concrete and repair materials technologies. This research is designed to enable cost-effective application of high-performance concrete with extended service life, and to advance concrete technology by providing a sound materials science base. Additionally, there are numerous other state and central programmes that strive to advance the nation’s knowledge of concrete. To make concrete the most efficient and cost-effective material of construction, will require processing improvements throughout the life cycle of concrete including design, production, transportation, construction, maintenance and repair. The concrete industry is unique in that process improvements can crosscut many other industries. Foundry sand, fly ash, silica fume, slag, and other by-products from industries such as aluminum, metal casting, and steel and power generation can be and are used as ingredients in the manufacture of cement and concrete. The concrete industry will have to commit to changes in practices in the materials, design, and construction arenas through the use of materials and systems that improve function, durability and sustainability. There is no central resource for performance data and service life of current concrete products. This limits the ability of designers and constructors to communicate life-cycle benefits of concrete products to the user community. Computer-integrated knowledge systems can provide a practical basis for optimizing concrete for specific applications by taking technical, economic, and environmental factors into account. Advanced systems models must be developed to show the prediction of performance for any mixture design under a range of environmental conditions lasting over a long period, i.e., over decades and even centuries. Aggregates, cement, repair and maintenance, materials transportation, life-cycle analysis and other areas can all be readily addressed under this concept. Due to the fear of failure to meet design criteria, producers, users, and designers are reluctant to shift from tried and proven processes and materials to adopt promising new technologies until long use histories have been substantiated. It is estimated that it takes 15 to 20 years to get a new technology from concept to adoption. Thus the advances in materials and process technologies needed to produce high-performance concrete are advancing slowly, and are not entering the marketplace quickly. Portland cement production is the most energy-intensive phase of the concrete production chain as its production requires high process temperatures to produce the necessary chemical transformations. Cement-manufacturing accounts for about 80 per cent of the total concrete industry’s power consumption. In addition, a large

12

Concrete Technology

amount of energy is required to transport aggregate and other ingredients to manufacturing sites and to deliver finished products to market. The material transportation costs account for 20 per cent to 50 per cent of the final cost of ready-mixed concrete, and delays in material or concrete delivery can create significant labor downtime in concrete construction. Both of these phases of concrete production offer significant opportunities for improvement. Currently, the industry operates in a prescriptive rather than performance-based environment. Thus, the full potential of concrete is often not realized. Accordingly, the procurement process for concrete construction and products typically favors the low-cost bidder because no incentives are provided for improved performance. This forces concrete companies to keep costs down and creates a disincentive to investing in research and development. Improved technology can reduce service life costs, prevent premature repairs and also use less energy.

Process Improvements The industry can achieve significant improvements in process over the next quarter of a century by 1. using a variety of by-products from other industries as well as recycled concrete as constituent materials for concrete production, 2. using a geomimetic approach to tailor the mixture design to specific structural environments, 3. achieving optimal particle size distribution of the constituent materials, 4. manufacturing the cement with less energy and fewer emissions such as nitrous oxide and carbon dioxide, with decreased production of by-product cement kiln dust, 5. using accepted techniques and processes to produce lighter-weight, higherstrength products, thereby reducing volumetric requirements and making transportation easier and less expensive, 6. using advanced systems modeling to predict the performance of concrete for users, and 7. adopting automation as standard practice in concrete placement.

Product Performance As explained earlier concrete is one of the most durable and cost-effective construction materials used in civil engineering. However, more needs to be done to improve its performance, reliability and life cycle costeffectiveness. The diverse applications for concrete have a wide variety of performance requirements. The industry needs critical research to produce high-performance, cost-effective concrete. The industry can pass on the product performance benefits of concrete to the users by 1. using effective, consistent quality assurance/quality control standards throughout the industry, 2. making full use of non-destructive measurements, sensors, intelligent curing techniques, and other technology advances to continuously monitor property performance and to maintain durability, 3. producing concrete products having concrete strengths of 5 to 10 times that of current levels leading to a reduction in the overall volume of concrete required,

Concrete as Construction Material

13

4. having a system of shared, consolidated data such as materials, structures, design, and performance databases and using them with computer-integrated knowledge systems to demonstrate product quality to customers, and 5. making concrete reinforcement more durable through the use of advanced fibers and composites, enhancing the life cycle benefits of concrete.

Fig. 1.11

A typical dam—a multipurpose project —requires use of mass concrete and low-heat Portland cements

Energy Efficiency Energy efficiency can be improved in all stages of the concrete life cycle. The concrete industry should aim at reducing energy consumption from current levels by 50 per cent per unit of output during the next quarter of a century. This can be achieved by 1. 2. 3. 4.

using bio-based raw materials as fuel sources in cement making, using aggregates that are less energy-intensive to produce, using advanced technology to improve heating process for cement making; utilizing cementing materials that require less process heating and produce fewer emissions, and 5. saving energy by making increased use of recycled waste and by-products, from within the concrete industry and from other industries in concrete manufacturing. Recycled aggregates and mineral additives have been extensively used in modern dam construction:

Environmental Performance Approximately seven per cent of the world’s carbon dioxide (CO2) emissions are attributable to Portland cement. Carbon dioxide belongs to the so-called greenhouse gases, which contribute to global warming. Out of 450 million tonnes of fly ash that is suitable for use in cement, only a less than eight per cent is used for cement production. Only a small fraction of the 100 million tonnes of slag produced worldwide each year is utilized as a cement substitute. Nearly 90 per cent of coal ash and metallurgical slag produced

14

Concrete Technology

today end up in low-value applications or simply undergo disposal by ponding and stockpiling. To meet sustainable development and environmental goals, responsiveness to environmental regulations and waste management should be the part of daily operations in the concrete industry. The industry continually should seek to identify how it can increase its use of environmentally friendly practises and processes. An important application of this process is shotcreting as shown in Fig. 1.12. Moreover, the concrete industry must consume a wider range of by-products from other industries to evolve novel concretes for tailored waste isolation.

Fig. 1.12

Shotcrete—an economical means for lining the tunnels, canals, swimming pools, repair of structures, etc. In the image it is being used for stabilization of rock slopes.

Manpower Development Since the construction industry involves multidisciplinary inputs, the manpower development for such an industry is a challenging task. It involves identification of training needs at each level and discipline, the training methodology, preparation of instruction material, mode of training delivery system, hands-on-experience, etc., which are of highly variable nature depending upon the target group considered. The manpower comprises various levels of understanding or education from top management to skilled and semi-skilled workers. Hence, a holistic approach is required for MPD which is currently available through National Council for Cement and Building Materials (NCB) and RTCs. The Construction Infrastructure Development Council (CIDC) dealing with construction, lays emphasis on training of construction craftsman through Construction Traders Training Council (CTTC). CTTC may in turn identifies network agencies like NCB, NAC, ICI, NICMAR, ACC-RCD, etc. In order to achieve desired objectives of securing improved quality, productivity and efficiency in cement and concrete construction, it is necessary to improve quality, content, nature of education and training within sustainable regime. It is also necessary to look into the objective of designing and placing concrete mix and to

Concrete as Construction Material

15

compare this ideal with experience. The aim is to minimize the gap between the ideal and the practice. For the given technical quality of the batching and mixing system, this objective can be achieved by proper training of personnel.

REVIEW QUESTIONS 1.1 Explain the statement with examples: “The popularity of the concrete is due to the fact that from the common ingredients, it is possible to tailor the properties of concrete to meet the demands of any particular situation”. 1.2 What is difference between prescriptive specifications and performance-oriented specifications?

1.3 What are the seven basic advantages and seven basic disadvantages of concrete? 1.4 What are sustainability and the framework for cement industry to be sustainable? 1.5 Briefly explain the challenges faced by the concrete industry in regard to process improvement and energy efficiency.

MULTIPLE-CHOICE QUESTIONS 1.1 Assertion A: Concrete is the most widely used man-made construction material in the world, and is second only to water as the most utilized substance on the planet. Reason R: Locally available materials can be effectively used by judicious control of its basic and process components, so as to produce concrete satisfying performance requirements. (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true (e) Both are false 1.2 Identify the false statement. (a) The classification specifying the proportions of constituents and their characteristics is termed as prescriptive specifications and is based on the hope that adherence to such specifications will result in satisfactory performance. (b) The specifications specifying the requirements of the desirable properties of concrete are termed performanceoriented specifications. (c) The concept of treating concrete in its entity as a building material rather than its ingredients is symbolized

in ready-mixed concrete industry where the consumer can specify the concrete of his needs. (d) Sustainable regime may be defined as that regime in which the endeavors are towards meeting the needs of the present generation without compromising the needs of the future generations. (e) None of the above. 1.3 Identify the false statement(s). (a) Due to the fear of failure to meet design criteria, producers, users, and designers are reluctant to shift from tried and proven processes and materials to adopt promising new technologies until long (15 to 20 years) use histories have been substantiated. (b) Since the construction industry involves multi-disciplinary inputs, the manpower development for such an industry becomes a simple task. (c) Concrete Industry should endeavor to produce concrete products having concrete strengths of 5 to 10 times that of current levels leading to a reduction in the overall volume of concrete required. (d) Energy efficiency can be improved by saving energy by making increased use of recycled waste

16

Concrete Technology and by-products, from within the concrete industry and from other industries in concrete manufacturing. (e) For sustainability, the concrete industry must consume a wider range

of by-products from other industries to evolve novel concretes for tailored waste isolation.

Answers to MCQs 1.1 (a)

1.2 (e)

1.3 (b)

2 2.1

CONCRETE MAKING MATERIALS—I: CEMENT

INTRODUCTION

Cement is a well-known building material and has occupied an indispensable place in construction works. There are a variety of cements available in the market and each type is used under certain conditions due to its special properties as shown in Fig. 2.1. A mixture of cement and sand when mixed with water to form a paste is known as cement mortar whereas the composite product obtained by mixing cement, water and an inert matrix of sand and gravel or crushed stone is called cement concrete. The distinguishing property of concrete is its ability to harden under water.

Gray dry cement

Fig. 2.1

White cement

A fine powder called cement is the delicate link in concrete construction. (The color and properties of cement change with the composition of cement.)

The cement commonly used is Portland cement, and the fine and coarse aggregates used are those that are usually obtainable, from nearby sand, gravel or rock deposits. In order to obtain a strong, durable and economical concrete mix; it is necessary to understand the characteristics and behavior of the ingredients. Portland cement is defined as hydraulic cement, i.e., a cement that not only hardens by reacting with water but also forms a water-resistant product. The ingredients of concrete can be classified into two groups, namely active and inactive. The active group consists of cement and water, whereas the inactive group comprises fine and coarse aggregates. The inactive group is also sometimes called the inert matrix. In this chapter, the ingredients of the active group will be discussed. Although all materials that go into a concrete mixture are essential, cement is by far the most important constituent because it is usually the delicate link in the chain. The function of cement is, first to bind the sand and coarse aggregates together, and

18

Concrete Technology

second to fill the voids in between sand and coarse aggregate particles to form a compact mass. Although cement constitutes only about 10 per cent of the volume of the concrete mix, it is the active portion of the binding medium and the only scientifically controlled ingredient of concrete.

2.2

PORTLAND CEMENT

Portland cement is an extremely ground material having adhesive and cohesive properties, which provide a binding medium for the discrete ingredients. It is obtained by burning together, in a definite proportion, a mixture of naturally occurring argillaceous (containing alumina) and calcareous (containing calcium carbonate or lime) materials to a partial fusion at high temperature (about 1450°C). The basic components of the manufacturing process are shown in Fig. 2.2. The product obtained on burning, called clinker or nodules (5 to 25 mm diameter), is cooled and ground to the required fineness to produce a material known as cement. Its inventor, Joseph Aspdin, called it Portland cement because when hardened, it produced a material resembling stone from the quarries near Portland in England. During the grinding of clinker, gypsum or plaster of Paris (CaSO4) is added to adjust the setting time. The amount of gypsum is about three per cent by weight of clinker. It also improves the soundness of cement.

limestone

blending

kiln

clinker store

cement mill

clay

Fig. 2.2

The basic components of the cement-manufacturing process

Depending upon the location of the cement-manufacturing plant, available raw materials are pulverized and mixed in proportions such that the resulting mixture will have the desired chemical composition. The common calcareous materials are limestone, chalk, oyster shells and marl. The argillaceous materials are clay, shale, slate and selected blast-furnace slag. When limestone and clay are the two basic ingredients, the proportions will be approximately four parts limestone to one part of clay. Certain clays formed during volcanic eruption, known as volcanic ash or pozzolana, found near Italy, have properties similar to that of Portland cement. Since the raw materials consist mainly of lime, silica, alumina and iron oxide, these form the major constituents of Portland cement also. Depending upon the wide variety of raw materials used in the manufacture of cements, the oxide composition of ordinary Portland cement may be expressed as given in Table 2.1.

Concrete Making Materials—I: Cement Table 2.1

19

Oxide composition of ordinary Portland cement

Oxide

Percentage

Average

Lime, CaO

.60–65

63

Silica, SiO2

.17–25

20

Alumina, A12O3

.3.5–9

6.3

Iron oxide, Fe2O3

0.5–6

3.3

Magnesia, MgO

0.5–4

2.4

0.1–2

1.5

0.5–1.3

1.0

Sulfur trioxide, SO3 Alkalis, i.e., soda and/or potash, Na2O + K2O

These oxides interact with each other to form a series of more complex products during fusion. The compound composition will be discussed later in the chapter.

Fig. 2 3

2.2.1

A view of a typical cement plant

Manufacture of Portand Cement

A view of a typical cement plant is shown in Fig. 2.3. The processes used for the manufacture of cement can be classified as dry and wet. When the basic raw material is rock, it is transported to a large gyratory, or jaw crusher for primary reduction in size (to about 150 mm). It then passes through a smaller crusher or hammer mill where further reduction takes place to a 40 mm size aggregate, and from there it goes to a rock storage or stacker. From the stacker, the crushed rock is fed to a vertical ball mill along with clay or crushed shale. In the wet process, water is added at this point to obtain a blended mixture of very finely ground raw materials and water, called slurry. The slurry is stored in tanks under constant agitation and fed into huge firebrick-lined rotary kilns. In the dry process, the raw powdered materials (also called raw meal) which are mixed, and homogenized pass through a series of sophisticated precalcining systems with each system consisting of separate strings of five or six-stage precalcining

20

Concrete Technology

units and fed to the kiln in the dry state. In the recent past, economical, efficient and environmental friendly dry process of cement manufacturing has replaced the more power-oriented wet process. In the recent years, significant advancements have been made in raw material grinding, pyro-processing, controlled clinker cooling, cement grinding and packing technologies with advanced process control and instrumentation system. The coal consumption in the dry process is approximately one-fourth of that in the wet process. Typically, the total consumption of coal in the dry process is 100 kg as against 350 kg in the wet process for producing a tonne of cement.

(a) Schematic diagram

Fig. 2.4

(b) Real view of plant

Schematic diagram and real view of the manufacture of Portland cement by dry process

The kilns are fired with crushed coal or gas from the discharge end under a forced draft so that material being fed in advances against the heat blast as the kiln rotates as shown in Fig. 2.4. The kilns are mounted with the longitudinal axis inclined in such a way that the raw material or slurry is fed at the higher end. At about 425°C, excess water is driven off, and then further along the kiln, at 875°C, limestone breaks down into calcium oxide and carbon dioxide. Finally, at 1400°C to 1450°C, about 10 m from the discharge end, the initial melting stage of material, known as the point of incipient fusion, is reached. Sintering takes place at this point, and a substance having its own physical and chemical properties called clinker is formed. The rate of cooling influences the mineralogy of clinker, i.e., the degree of crystallization, the size of crystals and the amount of amorphous materials. The mineralogy of clinker influences the hydration and strength properties of cement considerably. Various forms of clinkers are shown in Fig. 2.7. A moderate rate of cooling in rotary kiln from 1200°C to 500°C in about 15 minutes and from 500°C to normal atmospheric temperature in about 10 minutes results in high strength cements. The cooled clinker is crushed, mixed with about three per cent crushed gypsum, and fed into a tube mill and processed through closed circuit grinding where proper particle size distribution

Concrete Making Materials—I: Cement

21

is ensured by a cyclonic separator. After initial grinding in a tube mill, the material moves into high efficiency cyclonic separator, which assures that the ground material has the ideal surface and ideal proportion of particles of sizes between 5 to 30 micron (to the extent of 50 per cent). The finished product known as Portland cement is taken to the storage silos where it is finally bagged in high-density polyethylene (HDPE) woven sacks, double Hessian bitumenized sacks, polyethylene lined jute bags, and four-ply paper bags and transported to stockists and construction sites. A typical schematic preheater long cement kiln for manufacture of Portland cement by dryprocess is shown in Fig. 2.5. Whereas, the details of hot end of medium sized modern cement kiln, illustrating tires, rollers and drive gear can be seen in Fig. 2.6.

In addition to primary fuel of crushed coal, new or recycled oil, gas, etc.,, fired from the discharge end, preheaters using waste fuel like rubber tires, containers, by-product fuel like bags, bales and hot gases from kiln heat, provide about 40 per cent calcination before the feed enters the kiln. Schematic diagram

Fig. 2.5

Fig. 2.6

A typical preheater long cement kiln

Hot end of a medium-sized modern cement kiln showing tires, rollers and drive gear

The variations in the chemical and physical properties of cement, especially the strength and fineness can be minimized or consistency in quality can be ensured by installation of proper quality control monitoring systems and modern sophisticated

22

Concrete Technology

instrumentation control systems. Computers are widely used for controlling the operations and quality at various stages in the cement-manufacturing process, i.e., on-line analysers for raw meal quality control and expert systems for controlling operations of raw materials grinding mill, preheaters, precalcinators and clinker coolers, cement grinding and packing sections for achieving smooth operation, uniform quality and maximum output.

Fig. 2.7

Typical forms of clinkers or nodules (5 to 25 mm in diameter)

The composition of Portland cement is rather complicated but basically it consists of the following four main compounds: Tricalcium silicate (C3S), Dicalcium silicate (C2S), Tricalcium aluminate (C3A), Tetracalcium alumino ferrite (C4AF),

3CaO·SiO2 (alite) 2CaO·SiO2 (belite) 3CaO·A12O3 (aluminate) 4CaO·A12O3·Fe2O3 (ferrite)

The symbols in parentheses are the abbreviations generally used. To the above ingredients is added about three per cent gypsum (CaSO4). Depending upon the wide variety of raw materials used in the manufacture of cements, typical ranges of these compounds in ordinary Portland cements may be expressed as given in Table 2.2. Table 2.2

Compound composition of ordinary Portland cement

Compound

Percentage by mass in cement

C3S

25–50

C2S

20–45

C3A

5–12

C4AF

6–12

Differences in the various types of ordinary Portland cements arise due to the variations in the relative proportions of these compounds in the cement. The minerology of clinker is shown in Fig. 2.8.

Concrete Making Materials—I: Cement

23

In the Backscattered SEM Image Light gray crystals are alite (C3S); dark gray crystals are belite (C2S), rounded; bright interstitial material is mainly ferrite (C4AF), small dark inclusions are aluminate (C3A); and black areas are epoxy resin.

Backscattered SEM image

Optical microscope image

Note Belite is not actually blue—it appears blue here because it has been etched to show it more clearly. Fig. 2.8

Typical scanning-electron micrograph (SEM) and optical microscope image of different clinkers [For colored images, visit http://www.mhhe.com/gamdhir/ct4e]

In the Optical Microscope Image (Polished Section) of Clinker Mineral The brown crystals are alite (C3S), blue crystals are belite (C2S), bright interstitial material is mainly ferrite (C4AF), with small dark inclusions of aluminate (C3A). The gray material is the epoxy resin in which the clinker was embedded to make the specimen.

2.2.2

Basic Properties of Cement Compounds

The two silicates, namely C3S and C2S, which together constitute about 70 to 80 per cent of the cement, control the most of the strength-giving properties. Upon hydration, both C3S and C2S give the same product called calcium silicate hydrate (C3S2H3) and calcium hydroxide. Tricalcium silicate (C3S) having a faster rate of reaction (Fig 2.9) accompanied by greater heat evolution develops early strength. On the other hand, dicalcium silicate (C2S) hydrates and hardens slowly and provides much of the ultimate strength. It is likely that both C3S and C2S phases contribute equally to the eventual strength of the cement as can he seen in Fig. 2.10. C3S and C2S need approximately 24 and 21 per cent water by weight, respectively, for chemical reaction but C3S liberates nearly three times as much calcium hydroxide on hydration as C2S. However, C2S provides more resistance to chemical attack. Thus, a higher percentage of C3S results in rapid hardening with an early gain in strength at a higher heat of hydration. On the other hand, a higher percentage of C2S results in slow hardening, less heat of hydration and greater resistance to chemical attack.

24

Concrete Technology 1.0

C4AF

Fraction hydrated

0.8

C3A

0.6 C3S 0.4 0.2 C2S 0.0 1

10

100

180

Log time, days

Fig. 2.9

Rate of hydration of pure cement compounds

70 C3S

Compressive Strength, MPa

60

C2S

50 40 30 20 10

C3A C4AF

0 7

Fig. 2.10

28

90

180 Age, days

360

Contribution of cement compounds to the strength of cement

The compound tricalciumaluminate (C3A) is characteristically fast-reacting with water and may lead to an immediate stiffening of paste, and this process is termed as flash set. The role of gypsum added in the manufacture of cement is to prevent such a fast reaction. C3A reacts with 40 per cent water by mass, and this is more than that required for silicates. However, since the amount of C3A in cement is comparatively small, the net water required for the hydration of cement is not substantially affected. It provides weak resistance against sulfate attack and its contribution to the development of strength of cement is perhaps less significant than that of silicates. In addition, the C3A phase is responsible for the highest heat of evolution, both during the initial period as well as in the long run. Like C3A, C4AF hydrates rapidly but its individual contribution to the overall strength of cement is insignificant. However, it is more stable than C3A.

Concrete Making Materials—I: Cement

25

In terms of oxide composition, a high lime content generally increases the setting time and results in higher strengths. A decrease in lime content reduces the strength of concrete. A high silica content prolongs the setting time and gives more strength. The presence of excess unburnt lime is harmful since it results in delayed hydration causing expansion (unsoundness) and deterioration of concrete. Iron oxide is not a very active constituent of cement, and generally acts as a catalyst and helps the burning process. Owing to the presence of iron oxide the cement derives the characteristic gray color. Magnesia, if present in larger quantities, causes unsoundness.

2.2.3

Hydration of Cements

The extent of hydration of cement and the resultant microstructure of hydrated cement influences the physical properties of concrete. The microstructure of hydrated cement is more or less similar to that of silicate phases. When the cement comes in contact with water, the hydration of cement proceeds both inward and outward in the sense that the hydration products get deposited on the outer periphery and the nucleus of the unhydrated cement inside gets gradually diminished in volume. The reaction proceeds slowly for 2–5 hours (called induction or dormant period) before accelerating as the surface skin breaks. At any stage of hydration, the cement paste consists of gel (a finely grained product of hydration having large surface area collectively called gel), the remnant of unreacted cement, calcium hydroxide Ca(OH)2, and water, besides some other minor compounds. The crystals of various resulting compounds form an interlocking random three-dimensional network gradually filling the space originally occupied by the water, resulting in stiffening and subsequent development of strength as shown in Fig. 2.11. Accordingly, the

Dry cement powder

A dry grain of cement

Hydration starts When water is added

Random 3-D network of crystals of hydration compounds begin to form

Interlocking 3-D network of crystals of hydration compounds develops fast

Until dense network of hydration compunds is created

Fig. 2. 11

Simplistic (microscopic) view of hydration of cement

26

Concrete Technology

hardened cement paste has a porous structure, the pore size varying from very small (4 × 10–4 μm) to a much larger value, the pores being called gel pores and capillary pores, respectively. The pore system inside the hardened cement paste may or may not be continuous. As the hydration proceeds, the deposit of hydration products on the original cement grain makes the diffusion of water to unhydrated nucleus more and more difficult thus reducing the rate of hydration with time. The reactions of compounds of cement and their products may be represented as 2(3CaO.SiO2) + 6 H2O → 3CaO. 2SiO2 .3H2O + 3 Ca(OH)2 or symbolically 2C3S + 6H → C3S2H3 + 3 Ca (OH)2 2 (2CaO.SiO2) + 4 H2O → 3CaO.2SiO2.3H2O + Ca (OH)2 or 2C2S + 4 H → C3 S2 H3 + Ca (OH)2 2C3A + 21 H → C4 A H13 + C2AH8 → 2C3 AH6 + 9H C4AF + 7 H → C3AH6 + CFH Reactions in the presence of gypsum are: –

C3A + 32 H + 3CaSO4 → C3A.3CS H32 → C6AS3H32 (trisulfate hydrate or ettringite) –

–

C3A + 18 H + CaSO4 → C3A.C S H18 → C4A S H18 (monosulfate hydrate) The above equations (with C = CaO; S = SiO2 and H = H2O) only refer to the process in which the cement compounds react with water to form a strong hydrated mass. The hydrated crystals are extremely small, varying from colloidal dimensions (less than 2 μm) to 10 μm or more. The calcium hydroxide, Ca(OH2), liberated during the reaction of silicate phase crystallizes in the available free space. The product C3S2H3 representing calcium silicate hydrate, a gel structure, is normally expressed by hyphenation C-S-H, which signifies that it is not a well-defined compound. The simplistic scanning-electron micrograph of hydration of cement is shown in Fig. 2.11. The hydration of C3S produces a comparatively lesser quantity of C–S–H than that produced by C2S. On the other hand, C3S liberates nearly three times as much calcium hydroxide on hydration as C2S. However, Ca(OH)2 is not a desirable product in the concrete mass as it is soluble in water and gets leached out making the concrete porous. The only advantage of Ca(OH)2 is its being alkaline in nature and maintaining a pH value of around 13 in the concrete. A pH value at this level passivates reinforcing steel against corrosion. In general, the quality and density of C-S-H produced due to hydration of C3S is slightly inferior to that formed by hydration of C2S. The hydration product of C2S is rather dense and its specific surface is higher. On hydration of C3A, a calcium aluminate system CaO–Al2O3–H2O is formed. The cubic compound C3AH6 is probably the only stable product. Hydration of C4AF

Concrete Making Materials—I: Cement

27

is believed to form a system CaO–Fe2O3–H2O. A hydrated calcium ferrite of the form C3FH6 is comparatively more stable. In the presence of gypsum, depending upon the concentrations of aluminate and sulfate ions in the solution phase, the precipitating crystalline product is either calcium aluminate trisulfate hydrate (C6S3H32) or calcium aluminate monosulfate hydrate. The product calcium aluminate trisulfate is known as ettringite, which crystallizes as short prismatic needles on account of high sulfate–aluminate ions ratio in the solution phase during first hour of hydration. When sulfate solution gets depleted, aluminate ions concentration increases due to renewed hydration and the aluminate is gradually converted into monosulfate which is the final product of hydration of Portland cement containing more than five per cent C3A.

Rate of Hydration As mentioned earlier, the reaction of the compound C3A with water is very fast in that flash setting, i.e., siffening without strength development, can occur because the C—A—H phase prevents the hydration of C3S and C2S. However, some of the CaSO4 ground in the clinker dissolves immediately in water and the sulfate ions in the solution react with C3A to form insoluble calcium sulfoaluminate which deposits on the surface of the C3A to form a protective colloidal membrane and thus retard the direct hydration reaction. When all the sulfate is consumed, hydration can accelerate. The amount of sulfate must, therefore, be carefully controlled to leave little excess C3A to hydrate directly. The hardening of C3S appears to be catalyzed by C3A so that C3S becomes almost solely responsible for the gain of strength up to about 28 days by growth and interlocking of C—S—H gel. The later age increase in strength is due to the hydration of C2S. The rate of strength development can, therefore, be modified by changes in the relative quantities of these compounds. Mechanism of Hydration C3A reacts from beneath the thin membrane of calcium sulfoaluminate formed on the C3A surface. Owing to the larger volume of calcium sulfoaluminate, pressure develops and the membrane eventually bursts, allowing the sulfate in solution to come in contact with unreacted C3A to reform the membrane. The cyclic process continues until all the sulfate in solution is consumed, whereupon the C3A can hydrate directly at a faster rate and the transformation of calcium sulfoaluminate into needle like monosulfate crystals leads to the loss of workability and to setting. This gives rise to the induction period which ends when the protective membrane is disrupted. Although the reaction between C3S and water proceeds at the same time, in a properly retarded cement. The end of induction period of C3S hydration coincides with the point at which the sulfate in solution is no longer available for reaction. Setting, now, is due to the simultaneous growth of aluminate hydrate, monosulfate and silicate hydrate in the inter-particle space. The above theory is termed as protective membrane layer theory. Effect of Admixtures on Hydration Some admixtures may reduce the electric repulsion between the individual positively charged hydrating cement particles, so that they approach closer and stick to form agglomerates which grow and eventualy settle out. This process is termed flocculation and the agglomerates floc. The anions may flocculate the colloidal membrane thus making it more permeable. The rapid diffusion of water through the permeable membrane increases hydrostatic pressure

28

Concrete Technology

beneath the membrane till it reaches a level sufficient to rupture it at an earlier stage in hydration, thus accelerating the hydration of cement.

2.2.4 Abram’s Law: Water-to-Cement Ratio and Compressive Strength A cement of average composition requires about 25 per cent of water by mass for chemical reaction. In addition, an amount of water is needed to fill the gel pores. Nearly 100 years ago, Duff Abrams discovered the direct relationship between water-to-cement ratio and strength, i.e., lesser the water used higher the strength of the concrete, since too much water leaves lots of pores in the cement paste. According to Abram’s law, the strength of fully compacted concrete at a given age and normal temperature is inversely proportional to the water-cement ratio. Here the water-to-cement ratio is the relative weight of the water to the cement in the mixture. For most applications, water-to-cement should be between 0.4 and 0.5—lower for lower permeability and higher strength. In concrete, the trade off, of course, is with workability, since very low water content results in very stiff mixtures that are difficult to place. The water-to-cement ratio is a factor selected by the civil engineer.

2.2.5

Gel-Space Ratio

In concrete, as explained earlier, the hardened cement paste is a porous ensemble. Also, the concentration of the solid products of hydration in the total space or volume available (the original water and hydrated cement) is an index of porosity. Like any other porous solid, the compressive strength of cement paste (or concrete) is related to the parameter gel–space ratio or hydrate–space ratio. Gel is the hydrated cement while space is volume occupied by hydrated cement plus capillary pores. Hence, gel/space ratio is the fraction of volume occupied by hydrated cement in the total space occupied by hydrated cement and capillary pores. In other words, it is a measure of capillary pore space. Before hydration, this space is occupied by mixing water, after hydration the space is the sum of the hydrated cement and the remaining capillary pore space. Thus, Gel/space ratio, x =

Volume of gel (including gel pores) Volume of gel + volume of capillary pores

Thus, a decrease in capillary porosity in a hydration product shall increase the gel/ space ratio. The porosity within the gel for all normally hydrated cements is of the order of 0.26. The strength of cement/concrete is primarily governed by its porosity which is affected by the gel/space ratio which depends on the degree of hydration at a given age of the cementitious materials; a higher gel/space ratio that reduces the porosity increases the strength of concrete. On the other hand, the gel/space ratio is itself affected by the water/cement ratio of concrete. A higher water/cement ratio decreases the gel/space ratio, thus increasing the porosity and thereby decreasing the strength of concrete. These observations indirectly validate the Abrams’ law which states that “assuming full compaction, and at a given age and normal temperature, strength of concrete can be taken

Concrete Making Materials—I: Cement

29

to be inversely proportional to the water/cement ratio”. Thus, the water–cement ratio is really an expression of the concentration of hydration products in the total volume at a particular age for the resultant degree of hydration. A typical Power’s gel/space ratio versus strength curve (based on 51 mm cubes) is shown in Fig. 2.12. 120

100

Strength, MPa

80

60

40

20

0

Fig. 2.12

0.2

0.4 0.6 Gel/space ratio

0.8

1.0

Typical Power’s gel/space ratio versus strength curve

However, in contrast to Abrams’ law, in terms of porosity the strength of concrete is directly proportional to the increase in gel/space ratio, regardless of age, w/c ratio, or type of cement. While dealing with the porosity which has a strong influence on strength and durability of concrete, it should be noted that in hardened cement paste, there are several factors contributing to porosity such as trapped or entrained air (air bubbles of 0.1 to several mm in size), capillary pores (0.01 to a few microns) existing in the space between hydration products, and gel pores (several nanometers or below) within the layered structure of the C-S-H. The capillary pores have a large effect on the strength and permeability of the hardened paste itself. Large pores may be more effective than small pores in relieving stress concentrations at crack tips. The relations for total volume of hydration products (cement gel) and the capillary porosity which are based on the degree of hydration reported in literature do not adequately model the strength characteristics. As the hydration reactions of cement and fly ash reaction in high-volume fly ash systems are more complex than plain Portland cement systems, the role played by the gel/ space ratios of fly ash concrete mixes need further investigations. A presumption that the correlation between gel/space ratio and compressive strength for fly ash concrete may be the same as for Portland cement concrete can be good starting point.

30

Concrete Technology

2.2.6

Physical Properties of Portland Cement

The cement to be used in construction must have certain given qualities in order to play its part effectively in a structure. When these properties lie within a certain range, the engineer is confident that in most of the cases the cement performance will be satisfactory. Also, based on these properties, it is possible to compare the quality of cement from different sources. Frequent tests are carried out on the cement either on dry powder or hardened cement paste, and sometimes on the concrete made from the cement, to maintain quality within specified limits. The important physical properties of a cement are as follows. 1. Fineness The fineness of a cement is a measure of the size of particles of cement and is expressed in terms of specific surface of cement. It can be calculated from particle size distribution or one of the air permeability methods. It is an important factor in determining the rate of gain of strength and uniformity of quality. For a given weight of cement, the surface area is more for a finer cement than for a coarser cement. The finer the cement, the higher is the rate of hydration, as more surface area is available for chemical reaction. This results in the early development of strength. The effect of fineness on the compressive strength of cement is shown in Fig. 2.13. If the cement is ground beyond a certain limit, its cementative properties may be adversely affected due to prehydration by atmospheric moisture. As per Indian Standard Specifications, the residue of cement should not exceed 10 per cent when sieved on a 90-micron IS sieve. In addition, the amount of water required for constant slump concrete decreases with the incrase in the fineness of cement. 4.5

Compressive strength, MPa

= 28 days = 1 year 4.0

3.5

3.0

2.5

2.0 1800

2160

2520

2880 2

3240

3600

2

Cement fineness (Blaine), mm //g(x ( 10 )

Fig. 2.13

The effect of fineness of cement on the compressive strength of concrete

Concrete Making Materials—I: Cement

31

2. Setting time Cement when mixed with water forms paste which gradually becomes less plastic, and finally a hard mass is obtained. In this process of setting, a stage is reached when the cement paste is sufficiently rigid to withstand a definite amount of pressure. The time to reach this stage is termed as setting time. The time is reckoned from the instant when water is added to the cement. The setting time is divided into two parts, namely, the initial and the final setting times. The time at which the cement paste loses its plasticity is termed the initial setting time. The time taken to reach the stage when the paste becomes a hard mass is known as the final setting time. It is essential for proper concreting that the initial setting time be sufficiently long for finishing operations, i.e., transporting and placing the concrete. The setting process is accompanied by temperature changes. The temperature rises rapidly from the initial setting to a peak value at the final setting. The setting time decreases with rise in temperature up to 30°C and vice versa. The setting times specified for various types of cements are given in Table 2.4. For an ordinary Portland cement, the initial setting time should not be less than 30 minutes and final setting time should not be more than 600 minutes. A phenomenon of abnormal premature hardening within a few minutes of mixing the water is termed false set. However, not much heat is evolved and remixing the paste without water restores the plasticity and then the cement sets in the normal manner with no appreciable loss of strength. In practice, the length of time for which a concrete mixture will remain plastic is usually more dependent on the amount of mixing water used and atmospheric temperature than on the setting time of cement. 3. Soundness The unsoundness of cement is caused by the undesirable expansion of some of its constituents, sometimes after setting. The large change in volume accompanying expansion results in disintegration and severe cracking. The unsoundness is due to the presence of free lime and magnesia in the cement. The free lime hydrates very slowly because it is covered by the thin film of cement which prevents direct contact between lime and water. After the setting of cement, the moisture penetrates into the free lime resulting in its hydration. Since slaked lime occupies a larger volume, the expansion takes place resulting in severe cracking. The unsoundness due to the presence of magnesia is similar to that of lime. The unsoundness may be reduced by (a) (b) (c) (d)

limiting the MgO content to less than 0.5 per cent, fine grinding, allowing the cement to aerate for several days, and thorough mixing.

The chief tests for soundness are the Le Chatelier and Autocalve tests. The expansion carried out in the manner described in IS: 269–1989 should not be more than 10 mm in the Le Chatelier test and 0.8 per cent in Autoclave test. 4. Compressive strength It is one of the important properties of cement. The strength tests, generally carried out in tension on samples of neat cement, are of doubtful value as an indication of ability of the cement to make concrete strong in compression. Therefore, these are largely being superseded

32

Concrete Technology

by the mortar cube crushing tests and concrete compression tests. These are conducted on standardized aggregates under carefully controlled conditions and therefore give a good indication on strength qualities of cement. Cement mortar cubes (1:3) having an area of 5000 mm2 are prepared and tested in compression testing machine. For ordinary Portland cement, the compression strength at three and seven days curing shall not be less than 16 MPa and 22 MPa, respectively. The graded standard sand used for preparing the cubes should conform to IS: 650–1991. Standard sand A particular variety of sand available at Ennore in Tamil Nadu is used as standard sand which closely resembles the Leighton Buzzard sand (the British Standard Sand) in its properties. The imported Leighton sand has been replaced by Ennore sand. The standard sand has following properties: (a) The standard sand shall be of quartz, of light gray or whitish variety and shall be free from silt. (b) The sand grains shall be angular with shape approximating to spherical forms. (c) The sand shall pass through IS: 850-μm sieve and not more than 10 per cent shall pass through IS: 600-μm sieve. (d) It shall be free from organic impurities. 5. Heat of hydration The silicates and aluminates of cement react with water to form a binding medium, which solidifies into a hardened mass. This reaction is termed hydration, which is exothermic with approximately 120 cal/g heat being liberated. In the interior of mass concrete constructions like dams, etc., the temperature can be as high as 50ºC above the initial temperature of concrete mass at the time of placing the concrete. This high temperature is found to persist for a prolonged period. At the same time, the exterior of the concrete mass loses some heat so that a steep temperature gradient may be established, and during the subsequent cooling of the interior, severe cracking may occur. On the other hand, the heat of hydration may be advantageous in preventing the freezing of water in the capillaries of freshly placed concrete in cold weather. The heat of hydration is defined as the quantity of heat, in calories per gram of hydrated cement, liberated on complete hydration at a given temperature. The different cement compounds hydrate at different rates and liberate different quantities of heats. On adding water to cement, a rapid heat of evolution lasting for few minutes is due to reaction of aluminates. However, this initial heat evolution ceases quickly as solubility of aluminates is restrained by C3S. The total heat generated in the complete hydration process will depend upon the relative quantities of major compounds of cement. A normal cement generally produces approximately 90 cal/g of heat in 7 days and 90 to 100 cal/g in 28 days. It is determined by measuring the quantities of heat liberated by unhydrated and hydrated cements in a mixture of nitric and hydrofluoric acids, the difference between the two values represents the heat of hydration. The heat of hydration for low-heat Portland cement should not be more than 66 and 75 cal/g for 7 and 28 days, respectively.

Concrete Making Materials—I: Cement

33

The heat of hydration increases with temperature at which hydration takes place. For ordinary Portland cement (OPC) it varies from 37 cal/g at 5 ºC to 80 cal/g at 40 ºC. For common types of Portland cements, about 50 per cent of the total heat is liberated between 1 and 3 days, about 75 per cent in 7 days and 83 to 91 per cent in six months. By restricting the quantities of compounds C3A and C3S in cement, the high rate of heat liberation in early ages can be controlled. The rate of hydration and the heat liberation increases with the fineness of cement but the total amount of heat liberated is unaffected by the fineness. 6. Specific gravity The specific gravity of Portland cement is generally about 3.15, but that of cement manufactured from materials other than limestone and clay, the value may vary. Specific gravity is not an indication of the quality of cement. It is used in calculation of mix proportions.

2.2.7

Chemical Properties of Cements

The loss on ignition test is carried on portland cement to determine the loss of weight when the sample is heated to 900–l000°C. The loss in weight occurs as the moisture and carbon dioxide which are present in combination with free lime or magnesia evaporate. The presence of mositure causes prehydration of cement and may be absorbed from atmosphere during manufacturing or afterwards. The carbon dioxide is also taken from the atmosphere. The loss in weight is a measure of the freshness of cement. Since the hydroxides and carbonates of lime and magnesium have no cementing property, they are termed inert substances. Lesser the loss on ignition, lesser is the quantity of these inert substances and better is the cement. The loss on ignition is determined by heating one gram of cement sample in a platinum crucible at a temperature of 900°C–l000°C for minimum of 15 minutes. Normally, the loss will be in the neighborhood of two per cent. Maximum allowable loss is four per cent.

Insoluble Residue The insoluble material is an inactive part of cement. It is determined by stirring one gram of cement in 40 ml of water and adding 10 ml of concentrated HCl. The mix is boiled for 10 minutes maintaining constant volume. Any lump, if present, is broken and the solution filtered. The residue on filter is washed with Na2CO3 solution, water and HCI in the given order and, finally, again with water. The filter paper is dried, ignited, and weighed to give an insoluble residue. The minimum the residue, the better is the cement. The maximum allowable value is 0.85 per cent.

2.3

TYPES OF CEMENTS

By using additives, changing the chemical composition of the Portland cement—by varying the percentage of the four basic compounds through the use of different raw materials—it is possible to obtain several types of cements, each with some unique characteristics for the required performance. A gradual increase in the C3S content and fineness has enabled general-purpose Portland cements to develop very high strength at early ages. The oxide and compound compositions of some of the

34

Concrete Technology

commonly used Portland cements are given in Table 2.3. The compound composition of cements is determined by quantitative microscopy. Table 2.3

Oxide and compound compositions of portland cements

Type of cement

Normal or ordinary cement Rapid-hardening cement

Oxide composition, (per cent)

Compound composition (per cent)

A12O3 Fe2O3 C3S

CaO

SiO2

63

20.6

6.3

3.6

C2 S

C3A

C4 AF

40

30

11

11

64.5

20.7

5.2

2.9

50

21

9

9

Low-heat cement

60

22.5

5.2

4.6

25

45

5

14

Sulfate-resisting cement

64

24.5

3.7

3.0

40

40

5

9

The following are the main types of Portland cement.

2.3.1

General-purpose Portland Cements

The commonly used Portland cement in India is branded as 33-grade (IS: 269–1989), 43-grade (IS: 8112–1989) and 53-grade (IS: 12269–1987) having 28-days mean compressive strengths exceeding 33 MPa, 43 MPa and 53 MPa, respectively. All the three grades of ordinary Portland cement are produced from the same materials as explained earlier. The higher strengths are achieved by increasing the tricalcium silicate (C3S) content and also by finer grinding of the clinker. The fineness of 53-grade cement obtained by Blaine’s air permeability test is specified to be of the order of 350 000 mm2/g. The requirements of the initial and final setting times are same as that of conventional OPC. The conventional OPC, i.e., 33-grade cement has virtually disappeared and has been displaced by high strength 43-grade cement. The minimum compressive strengths of the 43-grade cement are 23 MPa and 33 MPa at the end of three days and seven days, respectively. The use of this cement was originally restricted to the production of railway sleepers and generally referred to as sleeper cement. The railway specifications require that the initial setting time should not be less than 90 minutes. At higher water– cement ratios, the concrete produced with high-strength cement has about 80 per cent higher strength and at lower water–cement ratios, it has 40 per cent higher strength than that of concrete using 33-grade OPC. The cost of high-strength Portland cements is only marginally higher than the OPC. The use of this cement in the usual 1:2:4 nominal mix, with a water–cement ratio of 0.60 to 0.65 can easily yield M25 concrete. Its composition and properties are governed by IS: 8112–1989. Greater fineness of 43 and 53 grade cements increase workability due to reduction of friction between aggregates. Moreover, due to shorter setting time and faster development of strength, the stripping time is shorter. Although cements of grades 43 and 53 are desirable for economical design of high-grade concretes, but they can also be used for lower grade concretes. However, to make high-strength concrete a high-performance concrete, will require extremely careful batching, mixing, transportation, placing, compaction and curing. IS: 10262–1982 has classified the OPC grade-wise from A to F depending upon the 28 days compressive strength as: A (32.5–37.5 MPa), B (37.5–42.5 MPa), C (42.5–47.5 MPa), D(47.5–52.5 MPa), E(52.5–57.5 MPa), F(57.5–62.5

Concrete Making Materials—I: Cement

35

MPa). Accordingly, the 33, 43 and 53 grades of cement correspond to categories A, C and E, respectively. However, most of the 43-grade cements available in the market generally fall in the category D, and the 53-grade cements available are generally in the category F or above. The actual strength of cement must be ascertained either from the manufacturer or through laboratory tests before it is used in concrete mix design to get the maximum benefit of the additional strength and superior quality.

2.3.2

Special-purpose Cements

The special-purpose cements are manufactured for the specific performance requirements. The frequently used ones are the following: 1. OPC-based cements 2. Non-OPC cements These cements have some further classifications, which are described below.

OPC-based Cements 1. Rapid-hardening Portland cement This cement is similar to OPC but with higher C3S content and finer grinding. A higher fineness of cement particles provides greater surface area (not less than 325 000 mm2/g) for action with water. It gains strength more quickly than OPC, though the final strength is only slightly higher. The one-day strength of this cement is equal to the threeday strength of 33-grade OPC with the same water–cement ratio. This cement is used where a rapid strength development is required. The rapid gain of strength is accompanied by a higher rate of heat development during the hydration of cement. This may have advantages in cold weather concreting, but a higher concrete temperature may lead to cracking due to subsequent thermal contraction, and hence should not be used in mass concreting or thick structural sections. The composition, fineness and other properties are governed by IS: 8041–1990. It is only about 10 per cent costlier than OPC. It is recommended for prefabricated concrete construction, road repairs and in applications requiring early stripping of forms. 2. Low-heat Portland cement This cement is less reactive than OPC and is obtained by increasing the proportion of C2S and reducing C3S and C3A. This reduction in the content of more rapidly hydrating compounds C3S and C3A results in a slow development of strength but the ultimate strength is the same. In any case, to ensure a sufficient rate of development of strength, the specific surface of cement must not be less than 320 000 mm2/g. The initial setting time is greater than OPC. The properties and composition are governed by IS: 12600–1989. This cement is recommended for the use in mass concrete construction such as dams where temperature rise by heat of hydration can become excessive. 3. Sulfate-resisting cement A Portland cement with low C3A (less than five per cent) and C4AF contents is very effective against sulfate attack. Such a cement having high silicate content is called sulfate-resisting cement. The

36

Concrete Technology

content of tetra-aluminoferrite C4AF in OPC varies between 6 to 12 per cent. As it is not feasible to reduce the Al2O3 content of raw material, Fe2O3 is added to the raw materials mixture to increase C4AF content at the expense of C3A. IS: 456–2000 limits the total content of C4AF and C3A such that 2C3A + C4AF shall not exceed 25 per cent. Such a cement with low C3A content is effective against sulfate attack whereas the ordinary Portland cement is susceptible to attack of sulfates in solution which permeate in the hardened concrete and react with free Ca(OH)2, hydrate of calcium aluminate and even hydrated silicates to form calcium sulfoaluminate having a volume of approximately 227 per cent of the volume of original aluminates. This expansion within the hardened structure of cement paste results in cracks and subsequent disruption. This phenomenon is called sulfate attack, which is greatly accelerated if accompanied by an alternate wetting and drying as in the case of marine environment. The use of sulfate resisting cement is recommended for concretes to be used in the marine environment, foundations in chemically aggressive soils, for pipes to be buried in marshy regions or sulfate bearing soils, and construction of sewage treatment plants. 4. Masonry cement This cement is manufactured by intimately grinding a mixture of OPC clinker and gypsum with mineral additives (pozzolanas) or inert (non-pozzolanic) materials such as limestone, dolomite, carbonated sludge, etc., and air-entraining agents in suitable proportions, generally to a fineness greater than that of OPC. Masonry cement conforming to the standard requirements can be produced by intergrinding 3 parts of Portland cement clinker, 5 parts of fly ash and 5 parts of granulated blast furnace slag or 1 part of Portland cement clinker, 2 parts of fly ash and 2 parts of hydrated lime with suitable quantity of gypsum and an air-entraining admixture. Masonry cement mortar is considered superior to lime mortar, lime-cement mortar and cement mortar. It combines the desirable properties of cement mortar relating to strength and setting, and lime mortar relating to workability and water-retention. Thus a masonry cement produces, a smooth, plastic, cohesive and strong, yet workable mortar. The cracks due to shrinkage and temperature movement are considerably reduced. Its composition and properties are governed by IS: 3466–1988. The physical requirements of the masonry cement are: (a) Fineness Residue on 45-micron sieve, max 15 per cent (b) Setting times (i) Initial setting time, min 90 minutes (ii) Final setting time, max 24 hours (c) Soundness (i) Le-Chatelier expansion, max 10 mm (ii) Autoclave expansion, max 01 per cent The following properties are measured on the mortar composed of one part of masonry cement and three parts of standard sand by volume:

Concrete Making Materials—I: Cement

5.

6.

7.

8.

9.

37

(d) Compressive strength (i) at 7 days, min. 2.5 MPa (ii) at 28 days, min. 5.0 MPa (e) Air content, min. 6 per cent (f) Water-retention Flow after suction as compared to the original flow, min. 60 per cent Waterproof Portland cement Waterproof cement is manufactured by adding a waterproofing substance to ordinary Portland cement during mixing. The common admixtures are calcium stearate, aluminum stearate and the gypsum treated with tannic acid. White Portland cement The process of manufacturing white cement is the same as of ordinary Portland cement but the amount of iron oxide, which is responsible for grayish color, is limited to less than one per cent. This is achieved by careful selection of raw materials and often by the use of refined furnace oil (RFO) or gas fuel in place of pulverized coal in the kiln. The suitable raw materials are chalk and high purity limestones having 95 per cent CaCO3 and less than 0.1 per cent iron oxide contents, and white clays. Its composition and properties are governed by IS: 8042–1989. Generally white cement is ground finer than the gray cement. Colored Portland cement These are basically Portland cements to which pigments are added in quantities up to 10 per cent during the process of grinding the cement clinker. A good pigment should be permanent, i.e., color should be durable under exposure to light and weather, and chemically inert when mixed with cement. For lighter colors, white cement has to be used as basis. Hydrophobic cement This type of cement is obtained by adding water repellant film forming substances like stearic acid, boric acid, oleic acid and pentachlorophenol to OPC during grinding of cement clinker. These acids form a film around the cement particles, which prevent the entry of atmospheric moisture, and the film breaks down when the concrete is mixed, and then the normal hydration takes place. The film forming materials also entrain certain amount of air in the body of concrete which improves its workability. Its composition and properties are governed by IS: 8043–1991. This cement is useful for the places having high humidity, poor transportation system and perforce storage for long time. In such situations, OPC gets deteriorated and loses significant part of its strength. The physical and chemical requirements for some of the commonly used cements are summarized in the Tables 2.4 and 2.5. Air-entraining cement This cement is manufactured by mixing small quantity of air-entraining agent like alkali salts of wood resins; synthetic detergents of alkyl-aryl sulfate type and calcium lignosulfate with ordinary Portland cement. These agents in powder or in liquid forms are added to the extent of 0.025 to 0.100 per cent by weight of OPC cement clinker at the time of grinding. At the time of mixing, these cements produce tiny, discrete noncoalesceing air bubbles in the concrete mass which enhances workability and reduces tendency to segregation and bleeding.

10.0(5.0)+ 0.80(0.60)+

– –

– 23.0

10.0(5.0)+ 0.80(0.60)+

– –

– 16.0

22.0 33.0 –

168 + 2h, (7 days) 672 + 4h, (28 days) Drying shrinkage, (% ) max.

+ For aerated sample

Note

30 600

30 600

33.0 43.0 –

225000

225000

37.0 53.0 –

– 27.0

– –

10.0(5.0)+ 0.80(0.60)+

30 600

225000

4

3

2

1 Fineness, Specific surface (Blain) (mm2/g) min. Setting Time, (minutes) Initial setting time, min. Final setting time, max. Soundness, Expansion, max. Le-Chatelier method, (mm) Autoclave Method, (%) Heat of hydration (cal/g), max. 7 days 28 days Compressive strength, (MPa), min. 24 + 0.5h, (1 day) 72 + 1h, (3 days) 22.0 33.0 0.15

– 16.0

– –

10.0(5.0)+ 0.80(0.60)+

30 600

300000

5

Portland– Pozzolana (silicate) cement 33 43 53 (IS: 1489 (IS: 269 –1989) (IS: 8112 –1989) (IS: 12269 –1987) –Part 1–1991)

Ordinary Portland cement with grade of

– – –

16.0 27.5

– –

10.0(5.0)+ 0.80(0.60)+

5 30

325000

Rapidhardening Portland cement (IS: 8041 –Part 1–1990) 6

Physical requirements for different types of ordinary Portland cements

Characteristics

Table 2.4

16 35 –

– 10

65 75

10.0(5.0)+ 0.80(0.60)+

60 600

320000

7

Low-heat Portland cement (IS: 12600 –1989)

16.0 22.0 33.0 –

–

– –

30 600

225000

8

Portlandslag cement (IS: 455– 1989)

38 Concrete Technology

Sulfuric anhydride,

(% by mass) max.

0.05

–

Content of Pozzolana, (%)

Total chlorides,

1.00

–

(other than gypsum), max.

Permitted additives, (%)

Slag content, (%)

(3.00)*

6.00

2.50

Magnesia, (% by mass), max.

SO3 (% by mass), max.

4.00

(% by mass), max

Insoluble residue,

(A12O3/Fe2O3), min.

alumina to that of iron oxide,

0.66

0.66 to 1.02

Lime saturation factor, (LSF)

Ratio of percentage of

2

0.05

–

1.00

–

(3.00)*

2.50

6.00

2.00

0.66

0.66 to 1.02

3

0.05

–

1.00

–

(3.00)*

2.50

6.00

2.00

0.66

0.8 to 1.02

4

0.05

10.00 – 25.00

1.00

–

(3.00)*

2.50

6.00

α**

–

–

5

–

1.00

–

(3.00)*

2.50

6.00

2.00

0.66

0.66 to 1.02

6

–

1.00

–

(3.00)*

2.50

6.00

2.00

0.66

–

7

(Continued)

–

1.00

25 – 65

(3.00)*

2.50

6.00

2.50

–

–

8

Portland– RapidLow-heat PortPozzolana hardening Portland land-slag (silicate) cement Portland cement cement cement 33 43 53 (IS: 1489–Part (IS: 8041 (IS: 12600 (IS: 455 (IS: 269–1989) (IS: 8112–1989) (IS: 12269–1987) 1–1991) –Part 1–1990) –1989) –1989)

Chemical requirements for different types of ordinary Portland cements

Ordinary Portland cement with grade of

1

Characteristics

Table 2.5

Concrete Making Materials—I: Cement 39

0.6

5.00

Total alkalies #,

Total loss on ignition, max.

5.00

0.6

3

4.00

0.6

4

5.00

0.6

5

5.00

0.6

6

** Insoluble residue in Portland–pozzolana cement a = p+ # In case of reactive aggregates

LSF-Lime saturation factor = 2.8 SiO + 2

CaO − SO 3 Al 2O3 + 0 65F2O3

5.00

0.6

7

4.00

0.6

8

4.0 (100 − p) wherein p is declared percentage of pozzolana in cement 100

When the content of tricalcium aluminate (C3A) is more than 5 per cent where C3A = 2.65 (Al2O3 )−1.69 (Fe2O3)

Notes

(Na2O) (%), max.

2

Portland– RapidLow-heat PortPozzolana hardening Portland land-slag (silicate) cement Portland cement cement cement 33 43 53 (IS: 1489–Part (IS: 8041 (IS: 12600 (IS: 455 (IS: 269–1989) (IS: 8112–1989) (IS: 12269–1987) 1–1991) –Part 1–1990) –1989) –1989)

Continued

Ordinary Portland cement with grade of

1

Characteristics

Table 2.5

40 Concrete Technology

Concrete Making Materials—I: Cement

41

10. Expansive cement Cement which does not shrink while hardening and thereafter, but expands slightly with time is called expansive cement. This cement does not suffer any overall change in volume on drying. Expansive cement is obtained by mixing about 8 to 20 parts of the sulfoaluminate clinker with 100 parts of the OPC and 15 parts of the stabilizer. In one type of expansive cement called shrinkage compensating cement, the restraint to the expansion induces compressive stress which approximately offsets the tensile stress induced by shrinkage. In another type called self-stressing cement, the concrete induces significant compressive stresses after the occurrence of drying shrinkage. In addition to neutralizing the shrinkage, they provide prestressing effects in the tensile zone of a flexural member. This cement is commonly used for grouting anchor bolts or grouting machine foundations or prestressed concrete ducts wherein drying shrinkage may otherwise defeat the purpose of grout. 11. Oil-well cement The annular space between steel casting and sedimentary rock formation through which oil well has been drilled, is sealed off by cement slurry to prevent escape of oil or gas. The cement slurry also seals off any other fissure or cavities in the rock layer. For this purpose, cement slurry has to be pumped down to points located in the annulus around the casting, at considerable depth where prevailing temperature may be as high as 350 oC under pressure up to 150 MPa. The slurry used for this purpose must remain mobile to be able to flow under these conditions for periods up to several hours and then harden fairly rapidly to give sufficient strength to support the casting. It may also have to resist corrosive conditions from sulfur gases and water containing dissolved salts. The type of cement suitable for above conditions is called oil-well cement. The cement produced by inter-grinding Portland cement clinker, fly ash, gypsum and certain admixtures (retarders) in suitable proportions has been found to conform to the requirements of an oil-well cement. These retarders prevent quick setting and retain slurry in mobile condition to facilitate penetration to all fissures and cavities. The composition and properties are governed by IS: 8229–1986.

Very High Strength Cements The cements of this category can be obtained by improving particle packing density and microstructure of cement pastes as follows. 1. Removing entrapped air In the conventionally mixed cement paste relatively large voids or defects are usually present due to entrapped air which limit the strength. In one of the systems, water soluble polymer is added as a rheological aid to permit cement to be mixed with a very small amount of water and at final processing stage entrapped air is removed by application of modest pressure of 5 MPa. This process has resulted in a strength of 300 MPa for calcium aluminate system and 150 MPa for OPC. This system is called macro-defect free cement. 2. Providing densely packed system OPC and ultra fine silica fume (5 to 20 per cent) are mixed to obtain a densified system containing homogeneously arranged particles. A compressive strength of 270 MPa has been obtained with silica fume substituted paste.

42

Concrete Technology

3. Achieving densification with warm pressing By the method of warm pressing, i.e., applying heat and pressure simultaneously to cement paste results in reduction of porosity and generation of very homogeneous fine microstructure with small porosity. By warm pressing of mixture of Portland and calcium cements has resulted in compressive strength of 650 MPa.

Non-OPC Cements 1. High-alumina cement This cement is basically different from OPC and concrete made with it has properties different from OPC concrete. High-alumina cement (HAC) is very reactive and produces very high early strength. About 80 per cent of the ultimate strength is developed at the age of 24 hours and even at six to eight hours. High-alumina cement has an initial setting time of about four hours and the final setting time of about five hours. Generally no additives are added to alumina cement. For the same water–cement ratio, the alumina cement is more workable than Portland cement. The strength is adversely affected by rise in temperature. HAC is extremely resistant to chemical attack and is suitable for under sea water applications. The raw materials used for its manufacture are limestone or chalk and bauxite which are crushed into lumps not exceeding 100 mm. These raw materials with appropriate proportion of coke are charged into the furnace which is fired with pulverized coal or oil. The fusion takes place at temperature about 1600 °C. The solidified material is fragmented and then ground to a fineness of 250 000–320 000 mm2/g. The very dark gray powder is passed through magnetic separators to remove metallic iron. The alumina cement is considerably more expensive. The pozzolana additives are not useful in concrete made with HAC because it does not produce calcium hydroxide that would react with pozzolanas. Its composition and properties are governed by IS: 6452–1989. The approximate chemical oxide composition is as follows: Alumina (A12O3) Ferric Oxide (Fe2O3) Lime (CaO) Ferrous Oxide (FeO) Silica (SiO2)

39 per cent 10 per cent 38 per cent 4 per cent 6 per cent

During hydration of HAC initially monocalcium aluminate decahydrate (CAH10), dicalcium aluminate octahydrate (C2A H8) and alumina gel (AH3) are formed. However, these compounds of hydration are metastable and at normal temperature convert gradually to a more stable tricalcium aluminate hexahydrate (C3AH6). This conversion is accompanied by a loss in strength and change in crystal form from hexagonal to cubical shape resulting in a release of water with consequent reduction in the volume of solids and an increase in the porosity. The increase in porosity enhances its vulnerability to chemical attack. The rate of conversion increases with the rise in temperature. The hydration and conversion processes can be symbolically represented as CA + 10 H → CA H10

Concrete Making Materials—I: Cement

43

3CA H10 → C3AH6 + 2AH3 + 18 H High alumina cement concrete loses considerable strength when subjected to humid conditions and high temperature. Desiccated high alumina cement concrete when subjected to high temperature, undergoes insignificant conversion and has significant residual strength. A completely desiccated alumina cement has very high resistance to dry heat. A concrete made using this cement and crushed firebricks as aggregate can withstand temperatures up to 1350 °C. A refractory concrete for withstanding temperatures up to 1600 oC may be produced by using aggregates such as dead-burnt magnesite, carborundum, silimanite, etc. Since high alumina cement is slow setting but rapid hardening certain proportions of OPC may be added to reduce setting time. Lithium salts have been effectively used as accelerator in high alumina cement to obtain high early strength cement. This has resulted in strength as high as 4 MPa in one hour, 25 MPa in three hours time and 50 MPa in 24 hours time. 2. Magnesium phosphate cement A very high early strength mortar and concrete developed by CRRI, consists of a pre-packed mixture of dead-burnt magnesite and fine aggregate mixed with phosphate. It sets rapidly and yields durable high strength cement mortar. The dead-burnt magnesite is obtained by calcining MgCO3 at or above 1500 ºC and grinding the product to fineness of 300000–350000 mm2/g (Blains). The ground dead-burnt magnesite is mixed with commercially available crystalline mono-ammonium phosphate after grinding it into a fine powder passing 600 μm sieve, and other ingredients like sodium tri-polyphosphate in the form of fine powder, di-sodium tetra borate (borax), fine aggregate (crushed dolomite sand) and water, mixed for one minute. After application in repair of road and subsequent air curing the traffic can be opened in a short period of about four to five hours.

2.3.3

Composite or Multiple Blended Cements

With the development of Portland cements having very high strength at early ages, there is general trend now to produce correspondingly high early strength concrete mixtures containing large proportions of these cements. These modern concretes tend to crack more easily due to lower creep; higher thermal and drying shrinkages and higher elastic modulus. On the other hand, some mineral industrial by-products when added to the normal concretes are highly effective in reducing the heat of hydration, strength, and elastic modulus of concrete. These concretes when properly cured are generally less crack-prone and therefore less permeable in service. Thus a composite or blended cement can be optimized with a synergistic effect, allowing component ingredients to compensate for any mutual shortcomings. Therefore, resource-efficient cements with tailor-made properties can be developed to achieve the needed balance between the industry’s quest for high-performance concrete and increasingly restrictive environmental regulations. Developing cements involving high-volume replacement of OPC with industrial by-products is perhaps the most promising venture for cement industry to meet its environmental obligations.

44

Concrete Technology

This synergy between modern Portland cements and mineral additives is being systematically exploited by the cement/concrete industry to meet sustainable development and environmental goals by progressively choosing supplementary cementing materials often industrial by-products called pozzolanas such as fly ash (FA), a waste by-product from coal burning thermal power plants; ground granulated blast furnace slag (GBFS), a by-product of iron and steel manufacturing; silica fume (SF), a waste by-product of the manufacture of silicon or ferro-silicon alloys from high purity quartz and coal in a submerged-arc electric furnace; and rice husk ash, a waste by-product from co-generation electric power plants burning rice husk as partial replacements for cement. A pozzolana is a finely ground siliceous glassy material which as such does not possess cementing property in itself, but reacts in the presence of water with lime (calcium hydroxide) at normal temperature to form compounds of low solubility having cementing properties. This action is called pozzolanic action. The pozzolanic activity is due to the presence of finely divided glassy silica and lime which produce calcium silicate hydrate as is produced in Portland cement hydration. The growth and interlocking of this hydrate gives mechanical strength. The lime produced in Portland cement hydration provides the right environment for pozzolanic action to proceed. Since similar hydrates are produced, the combination of two reactions in mixed cement-cementing material concrete results in improved mechanical strength. These cementing materials which participate in the hydration reaction significantly improve the strength, impermeability and durability of concrete. The oxide composition of the typical commonly used cementing materials is compared in Table 2.6. The concrete industry can fulfill its environmental obligations by advantageously using these materials, which would otherwise have to be disposed off in landfill sites, and consequently creating problems with ground water, air and land. Table 2.6 Material

Oxide compositions of cementing materials Oxide composition, per cent by weight

CaO

SiO2

A12O3

Fe2O3

MgO SO3

Na2O

K2O

63.00

20.00

6.30

3.60

2.40

1.50

0.15

0.50

(GGBS)

42.40

32.30

13.30

0.30

6.40

2.10

–

–

Fly ash (FA)

2.50

52.50

28.20

10.50

1.60

0.20

0.04

0.90

Silica fume (SF)

4.15

93.00

0.20

0.05

0.51

0.05

0.20

0.22

Ordinary Portland cement (OPC) Ground-granulated blast-furnace slag

Hydration of Composite Cements As explained earlier about 40 per cent of the Portland cement is composed of the primary mineral tricalcium silicate, which on hydration forms calcium silicate hydrate (C-S-H) and calcium hydroxide, Ca(OH)2. In Portland–pozzolana blended cement, the pozzolana can be represented by silica (SiO2) because non-crystalline silica glass is the principal reactive constituent of

Concrete Making Materials—I: Cement

45

pozzolana. This silica combines with the calcium hydroxide released on the hydration of Portland cement. Calcium hydroxide in hydrated Portland cement as such does not contribute to development of strength, but in composite cements it is utilized with reactive silica. Slowly and gradually it forms additional calcium silicate hydrate which is a binder and fills up the space, and gives impermeability and ever-increasing strength. The hydration process of a composite cement can be expressed as Portland cement only Portland cement +pozzolana

C3S + H2O → C-S-H + Ca (OH)2 SiO2 + Ca (OH)2 → C-S-H (Silica)

The national codes have placed a very heavy emphasis on the chemistry of cementing material or pozzolana, i.e., on the total amount of oxides of silica, alumina, and iron. There is really no direct relation between the chemistry of pozzolana and the properties of cement. Most of the properties of pozzolana in concrete are determined by the pozzolana mineralogy and particle size distribution, and not by chemistry. Except for calcium, pozzolana chemistry has little influence on reactivity.

Particle Size Distribution of Cementing Materials There are two parameters that determine the reactivity of pozzolana, one is the mineralogy, and the second is the size distribution of particles which are mostly glassy, solid and spherical. In case of fly ash, the particles range in size from 1 to 100 microns (0.1 mm). The average size is about 20 microns, which is similar to Portland cement average particle size. More than 40 per cent of the particles which are under 10 microns, regardless of the type of fly ash, are the ones that contribute to the early age (7 and 28 day) strengths. And particles of the size 45 microns or above which do not participate in pozzolanic reactions, even after one year, are considered inert and behave like sand. Particles of size between 10 and 45 microns are the ones that slowly react between the period of 28 days and one year or so. Most of the fly ashes have less than 15 to 20 per cent particles which are above 45 microns. In addition to the physical filler effect, the synergistic action in composite cements continues for the long-term. The calcium hydroxide, Ca(OH)2, produced by early hydration of OPC would be consumed by the hydration of the highly reactive pozzolana in the blend, such as silica fume, rice husk ash, and metakaolin, to yield more desirable C–S–H phase. The calcium hydroxide produced by the later hydration of OPC would be consumed by the less reactive component in the blend, such as fly ash, slag, and natural pozzolana, to provide further refinement of porosity and improvement of microstructure. The overall impact of this sequence of hydration reactions on the decreased permeability and increased durability of concrete is considerable. Some of the significant performance parameters of supplementary cementing materials used in composite cements are the following: 1. No interaction between fly ash and slag occurs when used simultaneously in a composite cement, and each component manifests its own cementing properties as hydration proceeds. 2. Highly reactive pozzolanas enhance the early age strength.

46

Concrete Technology

3. The effectiveness of pozzolana on durability depends on the its characteristics. High calcium pozzolanas provide higher resistance to sulfate attack and chloride-ion penetration or diffusion. 4. Replacement of large volumes of OPC with pozzolanic cementing materials results in significant drop in pH of pore solution and consequent increase in the risk of depassivation of steel in reinforced cement concrete. 5. Pozzolanic activity refines pore structure which increases electrolytic resistances of concrete. 6. The expansion due to alkali–silica reaction can be controlled by high level replacement (as high as 60 per cent) of OPC with high-calcium pozzolana. 7. High replacement cements have higher accelerated carbonation depths compared with OPC. 8. In addition to physical filler effect, the replacement of OPC widens the particle size distribution of the solid suspension and results in better rheological (workability) properties. 9. Finer pozzolanas such as silica fume or rice husk ash can inhibit bleeding problems. 10. Use of pozzolana prevents calcium hydroxide leaching. 11. Large volume of blended cements increase powder content of concrete and thus provide a high colloidal volume that combats segregation without increasing heat of hydration.

2.3.4

Binary Cements

Binary cements are two cementing constituent systems, in which one constituent is OPC and the other is one of the cementing pozzolanas like fly-ash (FA), ground granulated blast furnace slag (GBFS), silica fume (SF), and rice husk ash (RHA). These environment friendly cements have been successfully used in the construction of important projects, meeting demanding design criteria in some of the most hostile environments. However, these cements are often associated with shortcomings, such as the need for extended moist-curing, low-early-age strengths, increased use of admixtures, increased cracking tendency due to plastic shrinkage and as such these cements remain largely underutilized. Most of the current codal practices limit fly ash usage to 15 to 40 per cent which gives about 7 to 15 per cent reduction in water. Using 50 per cent fly ash may give 20 to 25 per cent water reduction. Reduction in the water content controls the cost and ensures good performance at the same time. With 50 per cent replacement of cement with fly ash that has fine particles—mostly less than 45 microns—water requirements are reduced by about 30 per cent as compared to the reduction in the range of 25–30 per cent obtained with an expensive superplasticizer. Fly ash improves workability, apparently due to glass beads acting like ball bearings, but the most important reason for fly ash working as a plasticizer for cement, is that the cement particles are electrically charged due to broken bonds and they tend to flocculate. Like normal plasticizers, e.g., lignosulfonates fly ash particles get adsorbed on the surface of the cement grains and act as a very powerful dispersant to the cement particles.

Concrete Making Materials—I: Cement

47

Addition of mineral additives enhances the intrinsic properties of cement by slow conversion of calcium hydroxide in hydrated cement paste into cementing product. The major advantages currently recognized are: improved and dense pore structure which reduces and micro cracks in the transition zone in concrete; reduced permeability enhances resistance to chemical attack, low diffusivity to chloride ions and hence better resistance to corrosion of steel reinforcement and low heat of hydration. The general-purpose cements of this category are Portland–pozzolana cement (OPC–FA), Portland slag cement (OPC–GBFS) and super-sulfated cement. The early strength is due to cement clinker fraction and later strength is due to FA and slag fractions. Alkali-silica-reaction (ASR) can be minimized by using by using adequate quantities of these additives.

Portland–pozzolana Cement (OPC–FA) Portland–pozzolana cement can be produced either by intergrinding the predetermined quantities of Portland cement clinker and pozzolana (15 to 35 per cent by mass of Portland–pozzolana cement) together with small amounts of gypsum, or by intimately and uniformly blending Portland cement having predetermined fineness and fine pozzolana. While intergrinding two materials together presents no difficulty, blending of dry powders intimately is extremely difficult. The blending should be resorted to only when the intergrinding techniques prove uneconomical in a particular case and requisite machinery to ensure homogeneity or uniformity (±3 per cent) of production is available. If the blending is not uniform, it is reflected in the performance tests. Portland–pozzolana cement produces less heat of hydration and offers greater resistance to the sulfate attack and chloride-ion penetration due to impurities in water than normal Portland cement. Hence it can be conveniently used for sewers and sewage disposal works. It is particularly useful in marine and hydraulic constructions, and other mass concrete structures like dam, bridge piers and thick foundations. The Portland–pozzolana cement can generally be used wherever ordinary Portland cement is usable under normal conditions. However, as explained earlier all the pozzolanas need not necessarily contribute to strength at early ages. IS: 1489–1991 gives the specifications for the production of Portland–pozzolana cement equivalent to 33-grade ordinary Portland cement on the basis of seven-day compressive strength. The compressive strength of Portland– pozzolana cement at 28 days also has been specified to enable the Portland–pozzolana cement to be used as substitute for ordinary Portland cement in plain and reinforced concrete works. The Portland–pozzolana cement should conform to the requirements specified in IS: 1489–1991. The average compressive strength of mortar cubes (area of face 50 cm2) composed of one part of cement, three parts of standard sand (conforming to IS: 650–1991) by mass and ( p*/4) + 3.0 per cent (of combined mass of cement and sand) water obtained in the manner described in IS: 4031 (Part-6)–1988 should be as follows: 1. At 168 ± 2h 2. At 672 ± 4h

22 MPa, minimum 31 MPa, minimum

where p* is the percentage of water to produce a paste of standard consistency.

48

Concrete Technology

The Portland cement/clinker for blending/intergrinding with fly ash should conform to IS: 269–1989. While the fly ash used in the manufacture of Portland–pozzolana cement should conform to IS: 3812–1981. The average compressive strength in lime reactivity test of fly ash should not be less than 4.0 MPa. The fineness of fly ash to be used in blending should not be less than 320000 mm2/g. To achieve almost equal strength at 28 days, a mix with 20 per cent of cement replaced with 27.5 per cent fly ash by weight along with consequential adjustments in fine and coarse aggregates is recommended. On these lines in terms of equivalent strength a nominal mix 1:0.5:2.0:5.0 (C:FA:Sand:CA) can be used in lieu of nominal mix 1:2:4. Similarly the nominal mix 1:0.5:2.0:4.0 is equivalent to 1:1.5:3.0.

High Volume Fly Ash Portland Cement (OPC–HVFA) The HVFA blended cement is produced by intergrinding approximately 55 per cent of a low-calcium fly ash and 45 per cent ordinary Portland cement clinker together with small amounts of gypsum and high-range water-reducing admixture (HRWRA), e.g., sulfonated naphthalene formaldehyde condensate in a dry powder form. The incorporation of HRWRA in the HVFA-blended cement helps the mortars made with the HVFAblended cement achieve the desired compressive strength as a result of the reduction in the water-to-blended cements ratio. The use of HRWRA in HVFA blended cements, however, retards their setting times. The entrained air content of the concrete made with HVFA-blended cements normally maintained between five and seven per cent results in a satisfactory bubblespacing factor in the hardened concrete. The dosage of the air-entraining admixture required for obtaining this air content is also strongly influenced by the type of fly ash used in the blended cement and whether or not the blended cement contains the HRWRA. The concrete made with the blended cements containing a HRWRA usually requires a lower dosage of the air-entraining admixture than that made with the blended cements without the HRWRA. The bleeding of concrete made with the HVFA-blended cements ranges from very low to negligible due to the low water content in the concrete. The setting time of the concrete is generally longer than that of concrete made with Portland cement only. In general, however, the HVFA concrete does not show unacceptable retardation in setting time and demonstrates enough strength development to result in adequate strength at one day. Because of the low cement content, the maximum autogenous temperature rise in concrete made with the HVFA-blended cements is rather low. Thus, HVFA cements are ideal for concrete structures where high heat of hydration is a concern, e.g., mass concrete and thick structural concrete member. The cost of the blended cements depends on the cost of the cement clinker, fly ash, and the energy required for intergrinding. It is believed that the cost of the blended cements should be lower than the cost of normal Portland cement. Portland–slag Cement (OPC-GBFS) This type of cement is made by intergrinding 35 to 65 per cent of ordinary Portland cement clinker and ground granulated blastfurnace slag (GBFS) (an industrial waste product consisting of a mixture of lime, silica and alumina) obtained during the manufacture of pig iron. Its oxide composition is similar to Portland cement so far as oxides of calcium, aluminum and silicon are concerned, but

Concrete Making Materials—I: Cement

49

it contains less calcium oxide. If the slag is cooled rapidly it solidifies in glassy form, which is reactive with water having alkaline medium. The slag can also be used together with limestone as a raw material for the conventional manufacture of Portland cement resulting in clinker which when ground gives Portland–slag cement. This cement is less reactive than OPC and gains strength a little more slowly during the first 28 days, and adequate curing is essential. It has the advantages in generating heat less quickly than OPC. It is suitable for mass concreting but unsuitable in cold weather. Because of its fairly high sulfate resistance it is used in sea-water construction. The composition and properties are governed by IS: 455–1989.

Super-sulfated Cement This cement is manufactured by grinding together a mixture of (80 to 85 per cent) well-granulated slag and 10 to 15 per cent of calcium sulfate with about five per cent of Portland cement clinker. Its specific surface is between 400 000 and 500 000 mm2/g. It has an initial setting time between 2½ to four hours and final setting between 4½ to 7 hours. The total heat of hydration is very low, about 40 to 45 cal/g after seven days and 45 to 50 cal/g at 28 days, which make it suitable for mass concreting. Due to high sulfate resistance, it is particularly useful in the foundations exposed to chemically aggressive conditions, or in the manufacture of RCC pipes to be buried in sulfate bearing soils. As super-sulfated cement has better resistance to sulfate attack than OPC–GBFS cement, it can also be used in marine environment. However, this cement requires great care while concreting in cold weather. Its setting action is different from the other cements and the admixtures should not be used. If cured in air, atmospheric carbon dioxide softens the surface of concrete, and hence water curing is preferable. The super-sulfated cement concrete may expand or contract slightly on setting according to the ambient conditions and hence should be properly cured. The rate of hardening increases with temperature up to about 38 °C but decreases above that. For a normal concrete mix of proportions 1:2:4 with a water–cement ratio of 0.55, the strengths obtained are 35 MPa after seven days, 50 MPa after 28 days, and between 50 to 70 MPa after six months. Its composition and properties are governed by IS: 6909–1990.

2.3.5 Ternary Cements These are three cementing constituent systems. Substantial improvements in concrete performance have been reported with ternary cements, compared with that obtained by using both binary cements and OPC. The ternary and multiple blended cement of OPC–SF–GBFS or FA have shown desired balance of mechanical and durability properties in marine and offshore environments. The need to develop ternary cements and to optimize their blends has been recognized. The ternary blended cements developed and produced are (OPC–SF–FA) and (OPC–SF–GBFS).

2.3.6 Synergistic Actions in Ternary and Quaternary Cements The rationale behind developing ternary and quaternary cements is to evolve blends in which the various mineral additives combine to provide tailor-made properties by

50

Concrete Technology

compensating for their mutual shortcomings. Various aspects of the performance of such composite cements are discussed below.

Particle Packing Density and Microstructure The original size and spatial distribution of cementing particles have a large influence on hydration kinetics, micro structural development, and ultimate properties of concrete. Achieving higher particle-packing density of the cementing blend is believed to enhance the rheological properties and mechanical strength of concrete. By simply adjusting the particle size distribution of cement, a 40 per cent increase in 28-day compressive strength can be obtained.

Durability Performance It is reported that fly ash (FA) has a synergistic action in ternary systems with respect to durability. Combinations of high-calcium fly ash and silica fume (SF) are synergistic. The ternary blend also achieves higher resistance to sulfate attack and chloride penetration. Thus ternary and quarternary cements involving blends of silica fume, rice husk ash, metakaolin, diatomaceous earth, slag, fly ash, and limestone filler could significantly contribute to achieving the needed balance between the industry’s quest for high-performance concrete and the increasingly restrictive environmental regulations. Synergistic effects could allow individual component ingredients in such blends to compensate for their mutual shortcomings, furthering the extent of the use of environmentally efficient cements and promoting their field implementation. Though such composite or blended cements are not a cure for all concrete problems, nor shall they replace all binary cements, composite cements provide a unique opportunity to produce environment-friendly concrete with tailor-made properties and may indeed constitute the next generation of cement products. However, lack of infrastructure for the production, storage, and handling of blended cements, is a big hurdle in popularizing the blended cements. There is a growing need for reliable predictive model for the behavior of high-volume replacement cements and guidelines for their mixture design and field implementation.

2.4

INTERNATIONAL CEMENT CLASSIFICATIONS

The cements meeting ASTM C 150, C 595 and C 1157-08 specifications in the USA for Types I and II, and European EN-197 specifications for Types CEM I, II, III, IV, and V have become commonly available in the world market. Both ASTM Specification C 150 for Portland cement and Specifications C 595 for blended cements deal with prescriptive and performance requirements whereas ASTM C1157-08 standard is confined to the performance specifications. The classification of cements as per these standards is briefly described below for general information.

2.4.1 ASTM Standards for Ordinary and Blended Portland Cements 1. Ordinary Portland cement There are five types of Portland cements with variations of the first three according to Portland cement standards specifications ASTM C 150. Portland cement types are listed in Table 2.7.

Concrete Making Materials—I: Cement

51

Guide to types for ordinary Portland cement/combination and their uses

Table 2.7 Cement type ASTM C150 (C 1157 - 08)

Guidance on type of cement/combination and applications

Type 1 (GU)

Common Portland cement or cement for general use Used for general construction especially in applications where the concrete is not to be in contact with soils or ground water.

Type II1 (MS)

Moderate sulfate resistance This type is for general construction that is exposed to moderate sulfate attack and is meant for use when concrete is in contact with soils and ground water, in drainage structures, etc.

Type III2 (HE)

High-early strength Portland cement This is Type I cement which is ground finer to a specific surface of typically 50-80 per cent higher. Used when high-early strength is required, typically for precast concrete manufacture, where high one-day strength allows fast turnover of moulds.

Type IV3 (LH)

Low heat of hydration Portland cement This is the Portland cement with higher percentages of C2S and C4AF, and relatively lower of C3S and (C3A). This is used for massive concrete structures, such as large gravity dams, retaining walls, and piers, etc., which have a low surface-to-volume ratio, thus requiring a low heat of hydration.

Type V4 (HS)

High sulfate resistance Portland cement This Portland cement with a very low (C3A) composition provides high sulfate resistance. This type of cement is used in concrete that is to be exposed to alkali soil and ground water sulfates which react with (C3A) causing disruptive expansion.

Types IA,

Types I, II, and III + an integral air-entraining agent (i.e., air-entraining 5

IIA and IIIA 1

agent is ground into the mix).

This cement costs about the same as Type I; hence Type II is much used as general-purpose cement. 2 The percentage of gypsum is also increased by a small amount. The concrete using this type of cement gives a three-day compressive strength equal to the seven-day compressive strength of types I and II. Its seven-day compressive strength is almost equal to 28-day compressive strengths of types I and II. However, the long-term strength is lower by a small amount. 3 Portland cement is generally known for its low heat of hydration. Due to a slower rate of heat generation during hydration, the strength development of the concrete is slow. After one or two years the strength is higher than the other types after full curing. This type of cement is not generally used, because Portland-pozzolan cements and ground granulated blast furnace slag offer a cheaper and more reliable alternative. 4 This type of cement is generally replaced by the use of ordinary Portland cement with added ground granulated blast furnace slag or tertiary blended cements containing slag and fly ash. 5 These types of cement are not generally used as the concrete producers prefer to use an air-entraining admixture during concrete manufacture, where they can get better control in obtaining the desired air content. However, this kind of cements can be useful under the conditions in which quality control is poor, particularly when no means of measuring the air content of fresh concrete is available.

52

Concrete Technology

2. Blended Portland cement The use of blended cements in concrete reduces mixing water and bleeding, improves finishability and workability, enhances sulfate resistance, inhibits the alkali-aggregate reaction, and lessens heat evolution during hydration, thus moderating the chances for thermal cracking on cooling. Blended cement types and blended ratios based on blended cement standards Specifications ASTM C 595 and C 1157 are presented in Table 2.8. Table 2.8

Guide to blended cement types and blended ratios

Type (C 595)

Moderate heat of hydration (C 1157)

Moderate sulfate resistance (C 1157)

Blended ingredients

IS

IS (MH)

IS (MS)

25–70 per cent (by weight) of blast furnace slag

IP

IP (MH)

IP (MS)

15–40 per cent (by weight) of pozzolan (fly ash)

I(PM)

I(PM) (MH)

I(PM) (MS)

0–15 per cent (by weight) of Pozzolan (fly ash) (modified)

I(SM)

I(SM) (MH)

I(SM) (MS)

0–25 per cent (by weight) of blast furnace slag (modified)

S

-

-

P

-

P (MS)

70–100 per cent (by weight) of blast furnace slag 15–40 per cent (by weight) of pozzolan (fly ash)

3. ASTM C1157-08 Standard classifications based on performance specifications This performance specification standard covers hydraulic cements for both general and special applications. This classifies the cements into six types based on specific requirements for general use, high-early strength, resistance to attack by sulfates, and heat of hydration. Optional requirements are provided for the property of low reactivity with alkali-silica-reactive aggregates. There are no restrictions on the composition of the cement or its constituents. In terms of performance specifications, the designations GU, MS, HE, LH, HS and MH represent general use, moderate sulfate resistance, high early strength, low heat of hydration, high sulfate resistance and moderate heat of hydration Portland cements, respectively. In addition, these cements can also have an option R, i.e., a cement having low reactivity with alkali-reactive aggregate, specified to control alkali–silica reactivity. For example, GU-R would be a general-use hydraulic cement having low reactivity with alkali-reactive aggregates. These cement types are shown in brackets in the first column of Table 2.7.

2.4.2

European Cement Standards EN:197-1, -3, 2000

EN:197 standard defines five classes of common hydraulic cement that comprise Portland cement as a main constituent. The cement types stipulated are CEM I, II, III, IV and V wherein CEM I is a Portland/Portland-pozzolan cement and CEM II

Concrete Making Materials—I: Cement

53

through V are blended cements. These classes differ from the ASTM classes. EN:197 has also strength classes 32.5, 42.5 and 52.5 MPa. In case of specific suitability established fly ash conforming to EN 450-1/2005 used in combination with CEM I cement, the cement and combinations are treated being equivalent by BS:8500. Thus when specifying the type of cement or combination, letter C or CEM are not added before the II, III or IV. This makes it clear that both are acceptable. The producer will add C or CEM to the delivery document to indicate which one has been used. Table 2.9 provides guidance on the cement/ combination-type designations. Table 2.9 Designation

Guide to cement/combination-type designations

Guidance on cement/combination

CEM 1

Portland cement Comprising Portland cement and up to five per cent of minor additional constituents.

CEM II

Portland cement + up to 35 per cent of other single constituents

CEM III

IIA

Portland cement with 6 to 20 per cent of pulverized fuel ash (fly ash) or ground granulated blast furnace slag or limestone.

IIB

Portland cement with 21 to 35 per cent of pulverized fuel ash (fly ash) or ground granulated blast furnace slag.

Portland cement + higher percentages of blast furnace slag IIIA

Portland cement with 36 to 65 per cent ground granulated blast furnace slag.

IIIB

Portland cement with 66 to 80 per cent ground granulated blast furnace slag.

CEM IV

Portland cement + up to 55 per cent of Pozzolanic constituents Portland cement with 36 to 55 per cent of pulverized-fuel ash (pfa).

CEM V

Composite cement (Portland cement + SR) This applies to cement or a combination of the above types for sulfateresistance. Constituents that are permitted are blast furnace slag, silica fume, natural and industrial pozzolans, silicious and calcareous fly ash, burnt shale and limestone.

2.5

STORAGE OF CEMENT

It is often necessary to store cement for a long period, particularly when deliveries are irregular. Although cement retains its quality almost indefinitely if moisture is kept away from it, but the cement exposed to air absorbs moisture slowly, and this causes its deterioration. Absorption of one or two per cent of water has no appreciable effect, but a further amount of absorption retards the hardening of cement and reduces its strength. The more finely cement is ground the more reactive it is, and consequently more rapidly does it absorb moisture from damp surroundings. Thus cement should be stored in a manner which permits easy access for proper inspection and identification and in a suitable weatherproof structure. Cement, in bulk, can best be stored in bins of depth 2 m or more as shown in Fig. 2.14(a).

54

Concrete Technology

Usually a crust about 50-mm thick forms at the top, and this must be removed before cement is taken out for use. The bagged cement of Fig. 2.14(b) may also be kept safely for many months if stored in a waterproof shed with non-porous walls and floors, the windows being tightly shut. Once the cement has been properly stored it should not be disturbed until it is to be used. The practice of moving and restacking the bags to reduce warehouse pack only exposes fresh cement to air. The transportation should be in vehicles with watertight and properly sealed lids.

2.6

CEMENT CERTIFICATION AND REJECTION

The current trend is to accept certification by the cement manufacturer that the cement complies with specifications. Verification tests are taken by the BIS to continually monitor specification compliance. The cement producer has a variety of information available from production records and quality control records that may permit certification of conformance without much, if any, additional testing of the product as it is shipped.

(a) Cement stored in bins

Fig. 2.14

(b) Cement stacked in paper bags

Storage of cement

However, due to defective storage for long periods, the cement is adversely affected. The cement remaining in bulk storage with manufacturers for more than six months or cement in jute or paper bags or in local storage in the hands of suppliers for more than three months after completion of tests may be retested before use and rejected if it fails to conform to any of the requirements of the relevant code.

REVIEW QUESTIONS 2.1 Describe the dry process of manufacturing of ordinary Portland cement stepwise with the help a flow chart. 2.2 Enlist the major compounds of ordinary Portland cement and briefly describe their importance.

2.3 Discuss the phenomena of hydration of cements. How is the water-cement ratio related to cement paste structure? 2.4 What are the major Bogue’s compounds of cement? Discuss their role in hydration of cement.

Concrete Making Materials—I: Cement 2.5 What is gel-space or hydrate-space ratio and how does it validates Abram’ law of water-to-cement radio? 2.6 What is unsoundness of cement? Explain the testing procedure to determine the unsoundness of cement with a neat sketch of the apparatus? 2.7 Differentiate between (a) Setting and hardening of cement and (b) Quick setting and rapid hardening cements. 2.8 Enlist different types of cements. Discuss the properties and applications of two OPC based cements (e.g., low heat Portland cement and Sulfate-resisting cement) for concrete construction

55

2.9 What are initial and final setting times of cement? Describe the test for determining standard consistency, initial and final setting times of cement. How does the knowledge of these two times help an engineer in construction work? State the IS requirements for setting times for general purpose OPC; rapid hardening and low heat OPC cements. 2.10 What are binary cements; list various types? Describe Portland-pozzolana cement (OPC-FA) or Super-sulfated cement. 2.11 How is cement stored properly? How is it checked on the site? How is the field testing important?

MULTIPLE-CHOICE QUESTIONS 2.1 Cement is an important ingredient of concrete because (a) it is a binding medium for discrete ingredients (b) it is the only scientifically controlled ingredient (c) it is an active ingredient (d) it is a delicate link of the chain (e) Any of the above 2.2 In the manufacture of cement definite proportions of argillacious and calcareous materials are burnt at a temperature of (a) 425 °C (b) 875 °C (c) 1450 °C (d) 1650 °C 2.3 During the manufacturing process of Portland cement, gypsum or Plaster of Paris is added (a) to increase the strength of cement (b) to modify the color of cement (c) to adjust setting time of cement (d) to reduce heat of hydration 2.4 The percentage of gypsum added to the clinker during manufacturing process is (a) 0.2% (b) 0.25% to 0.35% (c) 2.5% to 3.5% (d) 5% to 10% (e) 15% to 25% 2.5 The setting and hardening of cement after addition of water is due to (a) the presence of gypsum

(b) binding action of water (c) hydration of some of the constituent compounds of cement (d) evaporation of water (e) None of the above 2.6 In terms of oxide composition, the maximum percentage of ingredient in the cement is that of (a) lime (b) iron oxide (c) alumina (d) silica (e) magnesium oxide 2.7 In terms of oxide composition, the minimum percentage of ingredient in the cement is that of (a) lime (b) magnesium oxide (c) iron oxide (d) alumina (e) silica 2.8 In terms of oxide composition, in a cement (a) high lime content increases setting time and results in higher strengths (b) high silica content prolongs the setting time and gives more strength (c) presence of iron oxide gives gray color to the cement (d) presence of unburnt lime and magnesia causes unsoundness (e) All of the above

56

Concrete Technology

2.9 The tricalcium aluminate compound present in cement (a) provides weak resistance against sulfate attack (b) is responsible for highest heat of evolution (c) characteristically fast reacting with water (d) All of above (e) None of above 2.10 The constituents of cement which act as binder are (a) tricalcium silicate, dicalcium silicate and sulfur trioxide (b) tricalcium silicate and tetracalcium alumino-ferrite (c) tricalcium silicate, dicalcium silicate and tricalcium aluminate (d) dicalcium silicate, tetracalcium alumino-ferrite, and tricalcium aluminate. 2.11 Which of the following statements in terms of compound composition of cement are incorrect? (a) C3S and C2S together constitute about 70% to 80% of cement (b) Both C3S and C2S give the same product on hydration (c) C2S hydrates slowly and provides much of the ultimate strength (d) C3S having a faster rate of reaction is accompanied by greater heatevolution (e) C3S provides more resistance to chemical attacks 2.12 Following are the statements in terms of compound composition of cement. Identify the incorrect one(s). (a) Tricalcium aluminate (C3A) is fast reacting with water and leads to immediate stiffening of the paste (b) C3A phase is responsible for the highest heat of evolution (c) C3A provides high resistance against sulfate attack (d) Gypsum is added to prevent flash set of C3A 2.13 The constituent compounds of cement in decreasing order of rate of hydration are

2.14

2.15

2.16

2.17

2.18

2.19

2.20

(a) C2S, C3S and C3A (b) C3S, C3A and C2S (c) C3A, C3S and C2S (d) C3A, C2S and C3S Which of the contribution of constituents of cement to the strength of cement is in decreasing order? (a) C3S, C2S, C3A and C4AF (b) C2S, C3S, C3A and C4AF (c) C2S, C4AF, C3A and C3S (d) C3S, C3A, C2S and C4AF Out of the constituents of cement, namely, tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), the first to set and harden is (a) C3S (b) C3A (c) C2S (d) Any of the above (e) all set simultaneously The time taken by dicalcium silicate (C2S) to add to the strength of cement is (a) 1–2 days (b) 2–5 days (c) 5–7 days (d) 7–14 days (e) 14–28 days Dicalcium silicate (C2S) (a) reacts with water only (b) hydrates rapidly (c) hardens rapidly (d) generates less heat of hydration (e) has no resistance to sulfate attack Snowcem is (a) chalk powder (b) powdered lime (c) mixture of chalk powder and lime (d) colored cement (e) None of the above In testing the Portland cement for the loss on ignition, the sample is heated to (a) 100 °C (b) 250 °C (c) 500–800 °C (d) 900–1000 °C (e) 1250 °C In the case of Portland cement, the loss on ignition should be (a) less than 4%

Concrete Making Materials—I: Cement

2.21

2.22

2.23

2.24

2.25

2.26

2.27

(b) less than 10% (c) within 10% to 15% (d) less than 20% (e) more than 20% During the test of OPC for loss on ignition, the loss in weight occurs due to (a) decomposition of silicates (b) chemical reaction (c) burning of constituents (d) melting of tricalcium aluminate (e) evaporation of moisture and carbon dioxide The insoluble residue in cement should be (a) between 10% and 15% (b) less than 10% (c) between 5% and 10% (d) between 1.5% and 5% (e) less than 0.85% Total heat of hydration of cement is independent of (a) ambient temperature (b) composition of cement (c) fineness of cement (d) All of the above The length of time for which the concrete mixture remains plastic predominantly depends on the (a) setting time of cement (b) amount of mixing water (c) atmospheric temperature (d) equally on all of the above Initial setting time is maximum for (a) Portland–pozzolana cement (b) Portland–slag cement (c) low-heat Portland cement (d) high-strength Portland cement The setting time of cement is influenced by (a) percentage of water and its temperature (b) temperature and humidity of air (c) amount of kneading the paste (d) All of the above (e) None of the above For ordinary Portland cement (a) initial setting time should not be less than 5 min. and final setting time should not be more than 24 hr

2.28

2.29

2.30

2.31

57

(b) initial setting time should not be less than 30 min. and final setting time should not be more than 600 min. (c) initial setting time should not be less than 60 min. and final setting time should not be more than 600 min. (d) initial setting time should not be less than 5 min. and final setting time should not be more than 600 min. (e) None of the above Which of the following statements is incorrect? (a) The microstructure of hydrated cement governs the physical properties of concrete (b) The hydrated crystals form an interlocking random three-dimensional network called gel (c) The hydrated paste has a porous structure (d) The size of the gel pores is finer than 4 x 10–4 mm The compressive strength of concrete is basically related to (a) water–cement ratio (b) hydrate-space ratio (c) specific surface of cement (d) None of the above Which of the following Portland cements have specific surfaces in the descending order? (a) Ordinary, rapid hardening, high strength, low heat (b) Rapid hardening, high strength, low heat, ordinary (c) High strength, rapid hardening, low heat, ordinary (d) Low heat, ordinary, rapid hardening, high strength The insoluble material in cement is the (a) active part of cement and it should be kept to a minimum level (b) active part of cement and it should be kept to the maximum permissible level (c) inactive part of cement but it should be kept to the maximum permissible level (d) inactive part of cement and it should be kept to the minimum level

58

Concrete Technology

(e) None of the above 2.32 An excess of free lime in Portland cement (a) results in an increase in strength (b) increases the initial setting time (c) causes unsoundness in the product (d) improves the quality of the product (e) None of the above 2.33 In Portland cement, the quantity of free magnesia should be (a) less than 0.5% (b) less than 5% (c) between 5% and 10% (d) between 10% and 15% (e) less than 20% 2.34 Finer the cement (a) higher is the rate of hydration (b) more is the surface area (c) higher is the possibility of prehydration by atmospheric moisture (d) lesser the amount of water required for constant slump (e) All of the above 2.35 Sieve analysis of Portland cement is performed on IS sieve (a) No. 1 (b) No. 3 (c) No. 5 (d) No. 7 (e) No. 9 2.36 In the air permeability method for testing of Portland cement for fineness, the apparatus essentially consists of (a) permeability cell, sieve, and rotameter (b) permeability cell, manometer, and flowmeter (c) sieve, barometer, and rotameter (d) manometer, sieve, and rotameter (e) sieve, flowmeter, and rotameter 2.37 In the air permeability test, the specific surface (in mm2/g) of cement is of the order of (a) 1000 (b) 2000 to 2500 (c) 2500 to 5000 (d) 225000 to 350000 (e) 750000 to 1000000 2.38 In Vicat’s apparatus, the cement paste is said to be of normal consistency, if the rod penetrates by (a) 3 mm (b) 5 to 10 mm (c) 23 to 25 mm (d) 33 to 35 mm (e) 43 to 45 mm

2.39 To ensure that the concrete product does not undergo a large change in volume after setting (a) add excess quantity of fine aggregate to the mix (b) add minimum quantity of water to the mix (c) add maximum quantity of water to the mix (d) limit the quantities of free lime and magnesia in the cement (e) use proper curing 2.40 The absolute minimum water–cement ratio for concrete of medium strength is (a) 0.25 to 0.30 (b) 0.31 to 0.35 (c) 0.36 to 0.40 (d) 0.41 to 0.45 (e) 0.46 to 0.50 2.41 For complete hydration of 100 kg of cement of average composition, the mass of water required would be (a) 15 kg (b) 25 kg (c) 35 kg (d) 40 kg (e) 42 kg 2.42 The cubes for testing cement in compression are kept at (a) 17 ± 2°C and 100% humidity (b) 27 ± 2°C and 90% humidity (c) 37 ± 2 °C and 80% humidity (d) 100 °C and 70% humidity (e) 0°C 2.43 The compressive strength of OPC after three days is expected to be more than (a) 16 MPa (b) 22 MPa (c) 27.5 MPa (d) 33 MPa 2.44 For rapid-hardening Portland cement (a) initial setting time should not be less than 5 min. and final setting should not be more than 30 min. (b) initial setting time should not be less than 30 min. and final setting time should not be less than 600 min. (c) initial setting should not be less than 60 min. and final setting time should not be more 240 min. (d) initial setting time should not be less than 5 min. and final setting time should not be more than 60 min. (e) None of the above

Concrete Making Materials—I: Cement 2.45 The heat generated during the setting and hardening of cement is called (a) heat of setting (b) heat of evaporation (c) latent heat (d) heat of hydration (e) sensible heat 2.46 Heat of hydration is determinated by an apparatus called (a) hydrometer (b) photometer (c) calorimeter (d) hygrometer (e) None of the above 2.47 Heat of hydration of cement is expressed in terms of (a) calories/cubic centimeter (b) calories (c) farads (d) grams (e) calories/gram 2.48 A warehouse-set cement is (a) cement which is affected by moisture in the warehouse (b) cement which sets due to being stored adjacent to the wall (c) cement which gets compressed due to the load of several bags of cement placed above it (d) cement spoiled in the warehouse (e) there is no such setting 2.49 An ideal warehouse should have (a) waterproof masonry walls (b) waterproof roof (c) windows limited in number and should not allow seepage of water during the raiy season (d) floor of 150 mm thick concrete slab (e) All of the above 2.50 The cement from the warehouse is taken out on the basis of (a) first in, first out (b) first in, last out (c) last in, first out (d) last in, last out (e) Any of the above 2.51 The field test for the quality of cement consists in putting a small quantity of cement in a bucket containing water. A good quality cement will

2.52

2.53

2.54

2.55

2.56

2.57

59

(a) immediately dissolve in the water (b) float on the water surface (c) sink to the bottom of the bucket (d) produce steam (e) produce effervescence In fineness test of rapid hardening Portland cement, the residue on IS sieve No. 9 should not be more than (a) 1.0% (b) 5% (c) 10% (d) 15% (e) None of the above Identify the incorrect statement(s) (a) Expanding cement is used for filling the cracks. (b) White cement is mostly used for decorative work. (c) Portland pozzolana cement produces less heat of hydration. (d) By varying the percentage of four basic compounds of cement, several types of cements can be obtained. (e) High strength Portland cement is produced from the special materials. The compound constituent of cement abbreviated by C3 A represents (a) tricalcium alumino ferrite (b) tricalcium aluminate (c) tricalcium silicate (d) dicalcium silicate (e) None of the above Argillaceous materials are those (a) which have alumina as the main constituent (b) which have lime as the main constituent (c) which evolve heat on the addition of water (d) which easily break when hammered lightly (e) None of the above A sample of cement is said to be sound when it does not contain free (a) lime (b) silica (c) iron oxide (d) alumina (e) All of these Initial setting time of rapid-hardening Portland cement is nearly (a) half a minute (b) 5 min. (c) 30 min. (d) 45 min. (e) 60 min.

60

Concrete Technology

2.58 Low-heat cement is used for (a) repair of roads (b) thin structures (c) thick structures (d) under-water applications (e) All of the above 2.59 For the repair of roads (a) low-heat cement is used (b) rapid-hardening cement is used (c) high-alumina cement is used (d) sulfate-resisting cement is used (e) ordinary Portland cement is used 2.60 Identify the incorrect statement(s). (a) White cement is unsuitable for ordinary work (b) Pozzolana cement is gray in color (c) C3S is tricalcium silicate (d) Strength of cement implies compressive strength (e) Properly stored cement should not be disturbed until it is to be used 2.61 The number of cement bags in a pile of size 4 × 3 × 0.9 m height in a cement store could be (a) 100 (b) 150 (c) 175 (d) 200 (e) 250 2.62 A cement bag stored for two years is likely to result in (a) change in the color of cement (b) increase in the strength (c) loss of strength by 50% (d) formation of lumps (e) swelling by 15% 2.63 In ordinary Portland cement magnesia is restricted to (a) 5% (b) 2.5% (c) 1.5% (d) 1.0% (e) 0.5% 2.64 The cement used in construction of docks and harbors is (a) Blast-furnace-slag cement (b) Waterproof cement (c) Hyrophobic cement (d) Sulfate-resisting Portland cement (e) High-strength Portland cement 2.65 Which cement is used for lining a sulfuric acid plant? (a) Super-sulfate cement (b) High-alumina cement

2.66

2.67

2.68

2.69

2.70

2.71

2.72

(c) Portland-slag cement (d) Rapid-hardening Portland cement (e) Portland–pozzolana cement Which cement is used for lining deep tubewells? (a) High-alumina cement (b) Blast-furnace-slag cement (c) Oil-well cement (d) Sulfate-resisting Portland cement (e) Portland–pozzolana cement The cement used for repair of canal banks during the rainy season is (a) high-alumina cement (b) rapid-hardening Portland cement (c) oil-well cement (d) Portland–pozzolana cement (e) Portland-slag cement The cement generally used for the construction of road pavements is (a) rapid-hardening cement (b) ordinary Portland cement (OPC) (c) low-heat cement (d) blast-furnace-slag cement (e) None of the above For ordinary Portland cement, the maximum expansion by Le Chatelier’s method should not exceed (a) 2 mm (b) 5 mm (c) 7.5 mm (d) 10 mm (e) 12 mm Autoclave method is used to determine (a) residue (b) expansion (c) heat of hydration (d) sulfur content (e) None of the above Le Chatelier’s method can be used to determine (a) unsoundness of cement (b) soundness of cement (c) fineness of aggregate (d) sulfur content (e) All of the above The specific surface of OPC is determined by (a) Le Chatelier’s apparatus (b) air-permeability method (c) autoclave method (d) sieve analysis (e) photo-calorimeter method

Concrete Making Materials—I: Cement 2.73 The specific surface of cement is expressed in (a) mm2 (b) mm2/g (c) g/mm2 (d) mm3/g mm (e) Any of these 2.74 The hydration of concrete ceases at the temperature of (a) 0°F (b) 0 °C (c) 11°F (d) 11°C (e) None of the above 2.75 The average specific surface of cement is closer to (a) 100 000 mm2/g (b) 200 000 mm2/g (c) 300 000 mm2/g (d) 400 000 mm2/g (e) 500 000 mm2/g 2.76 Which of the following statement(s) is are incorrect? (a) Calcium chloride should not be used in prestressed concrete. (b) Strength of concrete increases below freezing point of water. (c) Hardening of concrete takes place rapidly in hot weather. (d) The ingredients of concrete should be mixed within three minutes. (e) All of the above 2.77 While ______ is a calcareous material ______ is an argillaceous material. (a) limestone, shale (b) clay, limestone (c) shale, limestone (d) slate, laterite (e) marl, chalk 2.78 The color of ordinary Portland cement is ______ and that of Portland–pozzolana cement is _____. (a) white, black (b) brown, gray (c) gray, light gray (d) white, gray (e) gray, black 2.79 White cement is the ______ cement and low-heat cement is used in _____ structures.

2.80

2.81

2.82

2.83

61

(a) cheapest, thin (b) costliest, thick (c) costliest, thin (d) cheapest, thick For fineness test of cement IS sieve of ______ is used. (a) 90 mm (b) 9 mm (c) 150 mm (d) 300 mm (e) 600 mm As per Abram’s Law (a) lower the water-to-cement ratio, lower will be permeability (b) the strength of fully compacted concrete at a given age and normal temperature is inversely proportional to the water-cement ratio. (c) a higher gel/space ratio reduces the porosity and increases the strength of concrete. (d) All of the above (e) None of the above Gel–space ratio or hydrate–space ratio contradicts which of the statements given below? (a) The concentration of the solid products of hydration in the total space or volume available is an index of porosity. (b) The strength of concrete is directly proportional to the increase in gel/space ratio, regardless of age, water-cement ratio, or type of cement. (c) The water–cement ratio is an expression of the concentration of hydration products in the total volume at a particular age for the resultant degree of hydration. (d) A decrease in capillary porosity in a hydration product shall increase the gel/space ratio. (e) None of the above Identify the false statement(s). (a) Assuming full compaction, at a given age and normal temperature, strength of concrete can be taken to be inversely proportional to the water/cement ratio.

62

Concrete Technology (b) A higher gel/space ratio reduces the porosity and increases the strength of concrete. (c) A higher water-cement ratio decreases the gel/space ratio, thus

increasing the porosity and thereby decreasing the strength and durability of concrete. (d) None of the above (e) All of the above

Answers to MCQs 2.1 (e)

2.2 (c)

2.3 (c)

2.4 (c)

2.5 (c)

2.6 (a)

2.7 (b)

2.8 (e)

2.9 (d)

2.10 (c)

2.11 (e)

2.12 (c)

2.13 (c)

2.14 (a)

2.15 (b)

2.16 (e)

2.17 (d)

2.18 (d)

2.19 (d)

2.20 (a)

2.21 (e)

2.22(e)

2.23 (c)

2.24 (b)

2.25 (c)

2.26 (d)

2.27 (b)

2.28 (d)

2.29 (b)

2.30 (c)

2.31 (d)

2.32 (c)

2.33 (a)

2.34 (e)

2.35 (e)

2.36 (b)

2.37 (d)

2.38 (d)

2.39 (d)

2.40 (c)

2.41 (b)

2.42 (a)

2.43 (a)

2.44 (a)

2.45 (d)

2.46 (c)

2.47 (e)

2.48 (c)

2.49 (e)

2.50 (a)

2.51 (b)

2.52 (b)

2.53 (e)

2.54 (b)

2.55 (a)

2.56 (a)

2.57 (b)

2.58 (c)

2.59 (b)

2.60 (a)

2.61 (d)

2.62 (c)

2.63 (a)

2.64 (d)

2.65 (b)

2.66 (c)

2.67 (b)

2.68 (a)

2.69(d)

2.70 (b)

2.71 (b)

2.72 (b)

2.73 (b)

2.74 (c)

2.75 (c)

2.76 (b)

2.77 (a)

2.78 (c)

2.79 (b)

2.80 (a)

2.81 (d)

2.82 (e)

2.83 (d)

3 3.1

CONCRETE MAKING MATERIALS—II: AGGREGATE

INTRODUCTION

As explained in Chapter 1, concrete can be considered to be an artificial stone obtained by binding together the particles of relatively inert fine and coarse materials with cement paste. Aggregates are generally cheaper than cement and impart greater volume stability and durability to concrete. The aggregate is used primarily for the purpose of providing bulk to the concrete. To increase the density of the resulting mix, the aggregate is frequently used in two or more sizes. The most important function of the fine aggregate is to assist in producing workability and uniformity in mixture. The fine aggregate also assists the cement paste to hold the coarse aggregate particles in suspension. This action promotes plasticity in the mixture and prevents the possible segregation of paste and coarse aggregate, particularly when it is necessary to transport the concrete some distance from the mixing plant to the point of placement. The aggregates provide about 75 per cent of the body of the concrete and hence its influence is extremely important. They should therefore meet certain requirements if the concrete is to be workable, strong, durable, and economical. The aggregate must be of proper shape (either rounded or approximately cubical), clean, hard, strong and well graded. It should possess chemical stability and, in many cases, exhibit abrasion resistance and resistance to freezing and thawing.

3.2

CLASSIFICATION OF AGGREGATES

The classification of the aggregates is generally based on their geological origin, size, shape, unit weight, etc.

3.2.1

Classification According to Geological Origin

The aggregates are usually derived from natural sources and may have been naturally reduced to size (e.g., gravel or shingle) or may have to be reduced by crushing. The suitability of the locally available aggregate depends upon the geological history of the region. Such an aggregate may further be divided into two categories, namely the natural aggregate and artificial aggregates.

Natural Aggregate These aggregates are generally obtained from natural deposits of sand and gravel, or from quarries by cutting rocks. The cheapest among them are the natural sand and gravel shown in Fig. 3.1(a) which have been reduced to their present size by natural agents, such as water, wind and snow, etc. The river deposits are the most common and are of good quality. The second most

64

Concrete Technology

commonly used source of aggregates is the quarried rock which is reduced to size by crushing. Crushed aggregates shown in Fig. 3.1(b) are made by breaking rocks into requisite graded particles by blasting, crushing and screening, etc. From the petrological standpoint, the natural aggregates, whether crushed or naturally reduced in size, can be divided into several groups of rocks having common characteristics. Natural rocks can be classified according to their geological mode of formation, i.e., igneous, sedimentary or metamorphic origin, and each group may be further divided into categories having certain petrological characteristics in common. Such a classification has been adopted in IS: 383–1970.

(a) A well-graded gravel (rounded) aggregate (b) Graded crushed coarse aggregate

Fig. 3.1

Close up of typical natural gravel (rounded) and crushed aggregates

Within each group, the quality of aggregate may vary to a great extent due to the change in structure and texture of the parent rock from place to place. Aggregates from igneous rocks are highly satisfactory because they are normally hard, tough and dense. They have massive structure with crystalline/glassy texture. The bulk of concrete aggregates are of igneous origin. The aggregate may be acidic or alkaline depending upon silica content and of light or dark color. The quality of aggregates derived from sedimentary rocks vary depending upon the formation history of the rock. Limestones and some siliceous sand stones have proved to be source of good concrete aggregates. Sometimes stratifications in the parent rock show up in the individual aggregates and thereby impair the strength of aggregate. Sedimentary rocks may vary from soft to hard, porous to dense and light to heavy. They may also yield flaky aggregates. The metamorphic rocks show foliated structure. In some cases, individual aggregate may exhibit foliations which is not a desirable characteristic in aggregate. However, many metamorphic rocks particularly quartizite and gneiss have provided good concrete aggregates.

Artificial Aggregate The most widely used artificial aggregates are clean broken bricks and air-cooled fresh blast-furnace-slag. The broken bricks of good quality provide a satisfactory aggregate for the mass concrete and are not suitable for reinforced concrete work if the crushing strength of brick is less than 30 to 35 MPa. The bricks should be free from lime mortar and lime sulfate plaster. The brick aggregate is not suitable for waterproof construction. It has poor resistance to wear and hence is not used in concrete for the road work.

Concrete Making Materials—II: Aggregate

65

The blast-furnace-slag is the by-product obtained simultaneously with pig iron in the blast furnace, which is cooled slowly in air. Carefully selected slag produces concrete having properties comparable to that produced by using gravel aggregate. However, the corrosion of steel is more due to sulfur content of slag, but the concrete made with blast-furnace-slag aggregate has good fire resisting qualities. The other examples of the artificial slag are the expanded shale, expanded slag, cinder, etc.

3.2.2

Classification According to Size

The size of aggregates used in concrete range from few centimetres or more, down to a few microns. The maximum size of the aggregate may vary, but in each case it is to be so graded that the particles of different size fractions are incorporated in the mix in appropriate proportions. The particle size distribution is called the grading of the aggregate. According to size the aggregate is classified as: fine aggregate, coarse aggregate and all-in-aggregate.

Fine Aggregate It is the aggregate most of which passes through a 4.75 mm IS sieve and contains only that much coarser material as is permitted by the specifications. Sand is generally considered to have a lower size limit of about 0.07 mm. Material between 0.06 mm and 0.002 mm is classified as silt, and still smaller particles are called clay. The soft deposit consisting of sand, silt and clay in about equal proportions is termed loam. The fine aggregate may be one of the following types: 1. Natural sand, i.e., the fine aggregate resulting from natural disintegration of rock and/or that which has been deposited by stream and glacial agencies. 2. Crushed stone sand, i.e., the fine aggregate produced by crushing hard stone. 3. Crushed gravel sand, i.e., the fine aggregate produced by crushing natural gravel. According to size, the fine aggregate may be described as coarse, medium and fine sands. Depending upon the particle size distribution, IS: 383–1970 has divided the fine aggregate into four grading zones. The grading zones become progressively finer from grading zone I to grading zone IV.

Coarse Aggregate The aggregates most of which are retained on the 4.75 mm IS sieve and contain only that much of fine material as is permitted by the specifications are termed coarse aggregates. The coarse aggregate may be one of the following types: 1. Crushed gravel or stone obtained by the crushing of gravel or hard stone. 2. Uncrushed gravel or stone resulting from the natural disintegration of rock. 3. Partially crushed gravel or stone obtained as a product of the blending of the above two types. The graded coarse aggregate is described by its nominal size, i.e., 40 mm, 20 mm, 16 mm, and 12.5 mm, etc. For example, a graded aggregate of nominal size 12.5 mm means an aggregate most of which passes the 12.5 mm IS sieve. Since the aggregates are formed due to natural disintegration of rocks or by the artificial crushing of rock or gravel, they derive many of their properties from the parent rocks. These

66

Concrete Technology

properties are chemical and mineral composition, petrographic description, specific gravity, hardness, strength, physical and chemical stability, pore structure, and color. Some other properties of the aggregates not possessed by the parent rocks are particle shape and size, surface texture, absorption, etc. All these properties may have a considerable effect on the quality of concrete in fresh and hardened states.

All-in-aggregate Sometimes combined aggregates are available in nature comprising different fractions of fine and coarse aggregates, which are known as allin-aggregate. In such cases, adjustments often become necessary to supplement the grading by addition of respective size fraction which may be deficient in the aggregate. Like coarse aggregate, the all-in-aggregate is also described by its nominal size. The all-in-aggregates are not generally used for making high quality concrete. Single-size-aggregate Aggregates comprising particles falling essentially within a narrow limit of size fractions are called single-size-aggregates. For example, a 20 mm single-size-aggregate means an aggregate most of which passes through a 20 mm IS sieve and the major portion of which is retained in a 10 mm IS sieve.

3.2.3

Classification According to Shape

The particle shapes of aggregates influence the properties of fresh concrete more than those of hardened concrete. Depending upon the particle shape, the aggregate may be classified as rounded, irregular or partly rounded, angular or flaky.

Rounded Aggregate The aggregate with rounded particles (river or seashore gravel) has minimum voids ranging from 32 to 33 per cent. It gives minimum ratio of surface area to the volume, thus requiring minimum cement paste to make good concrete. The only disadvantage is that the interlocking between its particles is less and hence the development of the bond is poor, making it unsuitable for high strength concrete and pavements. Irregular Aggregate The aggregate having partly rounded particles (pitsand and gravel) has higher percentage of voids ranging from 35 to 38. It requires more cement paste for a given workability. The interlocking between particles, though better than that obtained with the rounded aggregate, is inadequate for high strength concrete. Angular Aggregate The aggregate with sharp, angular and rough particles (crushed rock) shown in Fig. 3.2(a) has a maximum percentage of voids ranging from 38 to 40. The interlocking between the particles is good, thereby providing a good bond. The aggregate requires more cement paste to make workable concrete of high strength than that required by rounded particles. The angular aggregate is suitable for high strength concrete and pavements subjected to tension. Flaky and Elongated Aggregates An aggregate is termed flaky when its least dimension (thickness) is less than three-fifth of its mean dimension. The mean dimension of the aggregate is the average of the sieve sizes through which the particles pass and are retained, respectively. The particle is said to be elongated when its greatest dimension (length) is greater than nine-fifth of its mean dimension.

Concrete Making Materials—II: Aggregate

67

The angularity of aggregate affects the workability or stability of the mix which depends on the interlocking of the particles. The elongated and flaky particles shown in Fig. 3.2(b) also adversely affect the durability of concrete as they tend to be oriented in one plane with water and air voids forming underneath. The presence of these particles should be restricted to 10 to 15 per cent. This requirement is particularly important for crushed-fine aggregate, since the material made this way contains more flat and elongated particles. The angularity of the aggregate can be estimated from the proportion of voids in a sample compacted as prescribed in IS: 2386 (Part-I)–1963. The higher the angularity number, the more angular is the aggregate. The elongation index of an aggregate is defined as the percentage by weight of particles present in it whose greatest dimension (length) is greater than nine-fifth of their mean dimension.

(a) Angular coarse aggregate

Fig. 3.2

(b) Elongated coarse aggregate

Angular and elongated crushed coarse aggregates

Whereas, the flakiness index is the percentage by weight of particles having least dimension (i.e., thickness) less than three-fifth of their mean dimension. The surface texture of the aggregate depends on the hardness, grain size and pore characteristics of the parent rocks, as well as the type and magnitude of the disintegrating forces. Based on the surface characteristics, IS: 383–1970 classifies the aggregates as glassy, smooth, granular, crystalline, honeycombed, porous, etc. The shape and surface texture of aggregate influence the workability of fresh concrete and the compressive strength of hardened concrete, particularly in high strength concrete. The strength of concrete, especially the flexural strength, depends on the bond between the aggregate and cement paste. The bond is partly due to the interlocking of the aggregate and paste. A rough surface results in a better bond. The bond is also affected by the physical and chemical propeties, mineralogical and chemical composition, and the electrostatic condition of the particle surface, e.g., a chemical bond may exist in the case of a limestone aggregate.

3.2.4

Classification Based on Unit Weight

The aggregates can also be classified according to their unit weights as normalweight, heavyweight, and lightweight aggregates.

Normal-weight Aggregate The commonly used aggregates, i.e., sands and gravels; crushed rocks such as granite, basalt, quartz, sandstone and limestone; and

68

Concrete Technology

brick ballast, etc., which have specific gravities between 2.5 and 2.7 produce concrete with unit weight ranging from 23 to 26 kN/m3 and crushing strength at 28 days between 15 to 40 MPa are termed normal-weight concrete. The properties and the requirements of normal-weight aggregate will be discussed in detail in the succeeding sections.

Heavyweight or High-Density Aggregates Some heavyweight or high-density aggregates such as baryte (sg: 4.0–4.6), ferro-phosphorus (sg: 5.8–6.8), goethite (sg: 3.4–3.7), hematite (sg: 4.9–5.3), ilmenite (sg:4.0–4.6), limonite (sg: 3.4–4.0), magnetite (sg: 4.2–5.2), de-greased scrap iron and iron shots (sg: 6.2–7.8) are used in the manufacture of heavyweight concrete which is more effective as a radiation shield. Concretes having unit weight of about 30, 31, 35, 38, 40, 47 and 57 kN/m3 can be produced by using typical goethite, limonite, baryte, magnetite, hematite, ferrophosphorus and scrap iron, respectively. Where high fixed-water content is desirable, serpentine (which has a slightly higher density than normal-density aggregate) or bauxite can be used. Scrap iron is used where concrete with a density more than 47 kN/ m3 is required. The main drawback with these aggregates is that they are not suitably graded and hence it is difficult to have adequate workability without segregation. In general, selection of an aggregate is determined by physical properties, availability, and cost. High-density aggregates should be reasonably free of fine material, oil and foreign substances that may affect either the bond of paste to aggregate particle or the hydration of cement. For good workability, maximum density, and economy, aggregates should be roughly cubical in shape and free from excessive flat or elongated particles.

Lightweight Aggregate The lightweight aggregates having unit weight up to 12 kN/m3 are used to manufacture the structural concrete and masonry blocks for reduction of the self-weight of the structure. These aggregates can be either natural, such as diotomite, pumice, volcanic cinder, etc., or manufactured, such as bloated clay, sintered fly ash or foamed blast-furnace-slag. In addition to reduction in the weight, the concrete produced by using lightweight aggregate provides better thermal insulation and improved fire resistance. The main requirement of the lightweight aggregate is its low density; some specifications limit the unit weight to 12 kN/m3 for fine aggregate and approximately 10 kN/m3 for coarse aggregates for the use in concrete. Because of high water absorption, the workable concrete mixes become stiff within a few minutes of mixing, thus requiring the wetting of the aggregates before mixing in the mixer. In the mixing operation, the required water and aggregate are usually premixed prior to the addition of cement. Approximately, six liters of extra water per cubic meter of lightweight aggregate concrete is needed to enhance the workability by 25 mm. To produce satisfactory strength of concrete, the cement content may be 3.5 kN/m3 or more. Due to the increased permeability and rapid carbonation of concrete, the cover to the reinforcement using lightweight aggregate in concrete should be increased. The other characteristics of concrete using lightweight aggregates are reduced workability due to rough surface texture, lower tensile strength, lower modulus of elasticity (50 to 75 per cent of that of normal concrete) and higher creep and shrinkage. However, the ratio of creep strain to the elastic strain is the same for both the lightweight

Concrete Making Materials—II: Aggregate

69

and normal-weight concretes. They have the tendency to segregate. Some of the important lightweight aggregates are as follows.

Bloated clay aggregate The particles of such an aggregate range from 5 to 20 mm size. They are approximately spherical in shape, hard, light and porous. The water absorption is about 8 to 20 per cent. They produce concrete weighing up to about 19 kN/m3. The bloated clay aggregates are used in places where the cost of crushed stone aggregate is high, and suitable clays, especially silts from waterworks, are easily available. The sintered fly ash aggregate having a unit weight of about 10 kN/m3 produces structural concrete with a unit weight of 12 to 14 kN/m3. Expanded Shale The expanded shale known as herculite or haydite is produced by passing the crushed shale through a rotary kiln at 1100 °C. Gases within the shale expand forming millions of tiny air cells within the mass. The cells are surrounded by a hard vitreous waterproof membrane. The product is carefully screened into commercial sizes. This aggregate is used to a large extent, to replace the stone aggregate in the production of structural concrete because it reduces the weight by about one-third for no loss of strength for comparable cement content. It has high resistance to heat and is used for refractory lining, fireproofing of structural steel, and for the construction of other concrete surfaces exposed to high temperatures. In addition they have better sound absorption. Vermiculite Vermiculite is another artifical lightweight aggregate which produces low-strength and high-shrinkage concrete. It is not used for structural concrete but is widely used for insulating concrete roof decks. The compressive strength of lightweight concrete is comparable to that of normalweight concrete. The concrete produced with artificial and processed aggregates needs extra cover to the reinforcement due to high absorption of the aggregate. The other factor limiting their usefulness is the relatively low modulus of elasticity. The flexural strength of lightweight concrete is of the same order as that of normal concrete at early ages but does not improve significantly with long moist curing. The shear strength on the average is as high as for normal-weight concrete. The shrinkage and creep strains are usually somewhat higher for lightweight concretes. Air-entrainment can be used to make the concrete to perform like good quality normal -weight concrete. IS: 9142–1979 covers the specifications for artificial lightweight aggregate for concrete masonary units, while IS: 2686–1977 covers cinder aggregate for use in lime concrete for the manufacture of precast blocks. The use of bloated clay and sintered fly ash aggregate has been envisaged in IS: 546–2000.

3.3

CHARACTERISTICS OF AGGREGATES

As explained earlier, the properties and performance of concrete are dependent to a large extent on the characteristics and properties of the aggregates themselves. In general, an aggregate to be used in concrete must be clean, hard, strong, properly shaped and well graded. The aggregate must possess chemical stability, resistance to abrasion, and to freezing and thawing. They should not contain deleterious

70

Concrete Technology

material which may cause physical or chemical changes, such as cracking, swelling, softening or leaching. The properties of aggregate for concrete are discussed under the following heads. 1. Strength of aggregate The strength of concrete cannot exceed that of the bulk of aggregate contained therein. Therefore, so long as the strength of aggregate is of an order of magnitude stronger than that of the concrete made with them, it is sufficient. However, in the cases of high strength concretes subjected to localized stress concentration leading to stresses higher than the overall strength of concrete, the strength of aggregate may become critical. Generally three tests are prescribed for the determination of strength of aggregate, namely, aggregate crushing value, aggregate impact value and 10 per cent fines value. Of these, the crushing value test is more popular and the results are reproducible. However, the 10 per cent fines value test which gives the load required to produce 10 per cent fines from 12.5 mm to 10 mm particles, is more reliable. IS: 383–1970 prescribes a 45 per cent limit for the crushing value determined as per IS: 2386 (Part-IV)–1963 for the aggregate used for concrete other than for wearing surfaces and 30 per cent for concrete for wearing surfaces, such as runways, roads and pavements. BS: 882–1965 prescribes a minimum value of 10 tonnes in the 10 per cent fines test for aggregate to be used in wearing surfaces and five tonnes when used in other concretes. The other related mechanical properties of aggregate which are important especially when the aggregate is subjected to high wear are toughness and hardness. The toughness of aggregate which is measured as the resistance of the aggregate to failure by impact, determined in accordance with IS: 2386 (PartIV)–1963 may be used instead of its crushing value. The aggregate impact value shall not exceed 45 per cent by weight for aggregate used for concrete other than those used for wearing surfaces and 30 per cent for concrete for wearing surfaces. The hardness of the aggregate defined as its resistance to wear obtained in terms of aggregate abrasion value is determined by using the Los Angeles machine as described in IS: 2386 (part-IV)–1963. The method combines the test for attrition and abrasion. A satisfactory aggregate should have an abrasion value of not more than 30 per cent for aggregates used for wearing surfaces and 50 per cent for aggregates used for non-wearing surface. The strength of an aggregate as measured by its resistance to freezing and thawing is an important characteristic for a concrete exposed to severe weather. The resistance to freezing and thawing is related to its porosity, absorption, and pore structure. In a fully saturated aggregate, there is not enough space available to accommodate the expansion due to freezing of water resulting in the failure of the particles. An aggregate with higher modulus of elasticity generally produces a concrete with higher modulus of elasticity. The modulus of elasticity of aggregate also affects the magnitude of creep and shrinkage of concrete. The compressibility

Concrete Making Materials—II: Aggregate

71

of aggregate would reduce distress in concrete during its volume changes while a strong and rigid aggregate might lead to the cracking of the surrounding cement paste. Thus the aggregate of moderate or low strength and modulus of elasticity can be valuable in preserving the durability of concrete. 2. Particle shape and texture The physical characteristics such as shape, texture and roughness of aggregates, significantly influence the mobility (i.e., the workability) of fresh concrete and the bond between the aggregate and the mortar phase. As described earlier, the aggregates are generally divided into four categories, namely, rounded, irregular, angular and flaky. The rounded aggregates are available in the form of river or seashore gravel which are fully waterworn or completely shaped by attrition, whereas irregular or partly rounded aggregate (pitsands and gravels) are partly shaped by attrition and have rounded edges. The angular aggregate possessing well-defined edges formed at the intersection of roughly planer faces are obtained by crushing the rocks. The angular aggregates obtained from laminated rocks having thickness smaller than the width and/or length are termed flaky. The rounded aggregates require lesser amount of water and cement paste for a given workability. The water content could be reduced by 5 to 10 per cent, and the sand content by three to five per cent by the use of rounded aggregate. On the other hand, the use of crushed aggregate may result in 10 to 20 per cent higher compressive strength due to the development of stronger aggregatemortar bond. This increase in strength may be up to 38 per cent for the concrete having a water–cement ratio below 0.4. The elongated and flaky particles, having a high ratio of surface area to volume reduce the workability appreciably. These particles tend to be oriented in one plane with water and air voids underneath. The flakiness index of coarse aggregate is generally limited to 25 per cent. The surface texture is a measure of the smoothness or roughness of the aggregate. Based on the visual examination of the specimen, the surface texture may be classified as glassy, smooth, granular, rough, crystalline, porous and honeycombed. The strength of the bond between aggregate and cement paste depends upon the surface texture. The bond is the development of mechanical anchorage and depends upon the surface roughness and surface porosity of the aggregate. An aggregate with a rough, porous texture is preferred to one with a smooth surface as the former can increase the aggregate–cement bond by 75 per cent, which may increase the compressive and flexural strength of concrete up to 20 per cent. The surface pores help in the development of good bond on account of suction of paste into these pores. This explains the fact that some aggregates which appear smooth still bond strongly than the one with rough surface texture. The shape and surface texture of fine aggregate govern its void content and thus affect the water requirement of mix significantly. The use of crushed or manufactured sand with proper shape, surface texture and grading has enabled production of highly workable mix with minimum void content. 3. Specific gravity The specific gravity of an aggregate is defined as the ratio of the mass of solid in a given volume of sample to the mass of an equal volume of water at the same temperature. Since the aggregate generally contains voids, there are different types of specific gravities.

72

Concrete Technology

The absolute specific gravity refers to the volume of solid material excluding the voids, and therefore, is defined as the ratio of the mass of solid to the weight of an equal void-free volume of water at a stated temperature. If the volume of aggregate includes the voids, the resulting specific gravity is called the apparent/bulk specific gravity. As the aggregate generally contains both impermeable and capillary voids (voids between the particles), the apparent specific gravity refers to volume including impermeable voids only. It is therefore the ratio of the mass of the aggregate dried in an oven at 100 to 110°C for 24 hours to the mass of the water occupying a volume equal to that of solids including impermeable voids or pores. The specific gravity most frequently and easily determined is based on the saturated surface dry condition of the aggregate because the water absorbed in the pores of the aggregate does not take part in the chemical reaction of the cement and can therefore be considered as a part of the aggregate. This specific gravity is required for the calculations of the yield of concrete or of the quantity of aggregate required for a given volume of concrete. The specific gravity of an aggregate gives valuable information on its quality and properties. It is seen that the higher the specific gravity of an aggregate, the harder and stronger it will be. If the specific gravity is above or below that which is normally assigned to a particular type of aggregate, it may indicate that the shape and grading of the aggregate has changed. The specific gravity is determined as described in IS: 2386 (Part-I11)– 1963. The specific gravity is given by Specific gravity =

c a b

Apparent specific gravity =

c c b

and

⎛ a c⎞ Water absorption = ⎜⎝ ⎟ × 100 per cent c ⎠ where a = mass of saturated surface dry aggregate in air, b = mass of saturated surface dry aggregate in water, and c = mass of ovendry aggregate in air. The average specific gravity of majority of natural aggregates lie between 2.5 and 2.8. 4. Bulk density The bulk density of an aggregate is defined as the mass of the material in a given volume and is expressed in kilograms/liter. The bulk density of an aggregate depends on how densely the aggregate is packed in the measure. The other factors affecting the bulk density are the particle shape, size, the grading of the aggregate and the moisture content. The shape of the particles greatly affects the closeness of the packing that can be achieved. For a coarse aggregate of given specific gravity, a higher bulk density indicates that there are fewer voids to be filled by sand and cement.

Concrete Making Materials—II: Aggregate

73

The bulk density of an aggregate can be used for judging the quality of aggregate by comparing it with normal density for that type of aggregate. It determines the type of concrete for which it may be used. The bulk density is also required for converting proportions by weight into the proportions by volume. The bulk density is determined as described in IS: 2386 (Part-III)–1963. 5. Voids The empty spaces between the aggregate particles are termed voids. It is the difference between the gross volume of aggregate mass and the volume occupied by the particles alone. The void ratio of an aggregate can be calculated from the specific gravity and bulk density of aggregate mass as follows: Void ratio = 1 −

Bulk density Apparent specific gravity

6. Porosity and absorption of aggregates Due to the presence of air bubbles which are entrapped in a rock during its formation or on account of the decomposition of certain constituent minerals by atmospheric action, minute holes or cavities are formed in it which are commonly known as pores. The pores in the aggregate vary in size over a wide range, the largest being large enough to be seen under a microscope or even with the naked eye. They are distributed through out the body of the material, some are wholly within the solid and the others are open to the surface of the particle. The porosity of some of the commonly used rocks varies from 0 to 20 per cent. Since the aggregate constitute about 75 per cent of the concrete, the porosity of aggregate contributes to the overall porosity of concrete. The permeability and absorption affect the bond between the aggregate and the cement paste, the resistance of concrete to freezing and thawing, chemical stability, resistance to abrasion, and the specific gravity of the aggregate. The pores may become reservoirs of free moisture inside the aggregate. The percentage of water absorbed by an aggregate when immersed in water is termed the absorption of aggregate. The aggregate which is saturated with water but contains no surface free moisture is termed the saturated surface dry aggregate. The method for determining the water absorption of an aggregate is described in IS: 2386 (Part-III)–1963. If the aggregate is previously dried in an oven at 105 °C to a constant weight before being immersed in water for 24 hours, the absorption is referred to as on ovendry basis. On the other hand, the percentage of water absorbed by an air dried aggregate when immersed in water for 24 hours is termed absorption of aggregate (air dry basis). The knowledge of the absorption of an aggregate is important for concrete mix design calculations. 7. Moisture content of aggregate The surface moisture expressed as a percentage of the weight of the saturated surface dry aggregate is termed as moisture content. Since the absorption represents the water contained in the aggregate in the saturated-surface dry condition and the moisture content is the water in excess of that, the total water content of a moist aggregate is equal to the sum of absorption and moisture content. IS: 2386 (Part-III)–1963 describes the method to determine the moisture content of concrete aggregate.

Concrete Technology

The determination of moisture content of an aggregate is necessary in order to determine the net water–cement ratio for a batch of concrete. A high moisture content will increase the effective water-cement ratio to an appreciable extent and may make the concrete weak unless a suitable allowance is made. IS: 2386 (PartIII)–1963 gives two methods for its determination. The first method, namely, the displacement method, gives the moisture content as a percentage by mass of the saturated surface dry sample whereas the second method namely the drying method, gives the moisture content as a percentage by mass of the dried sample. The moisture content obtained by these two methods are quite different. The moisture content given by the drying method will normally be the total moisture content due to free plus absorbed water. The accuracy of the displacement method depends upon the accurate information of the specific gravity of the material in a saturated-surface dry condition. 8. Bulking of fine aggregate The increase in the volume of a given mass of fine aggregate caused by the presence of water is known as bulking. The bulking of fine aggregate is caused by the films of water which push the particles apart. The extent of bulking depends upon the percentage of moisture present in the sand and its fineness. It is seen that bulking increases gradually with moisture content up to a certain point and then begins to decrease with further addition of water due to the merging of films, until when the sand is inundated. At this stage, the bulking is practically nil. With ordinary sands the bulking usually varies between 15 and 30 per cent. The typical graphs shown in Fig. 3.3 give the variation of per cent bulking with moisture content. Finer sand bulks considerably more and the maximum bulking is obtained at a higher water content than the coarse sand. In extremely fine sand, the bulking may be of the order of 40 per cent at a moisture content of 10 per cent but such a sand is unsuitable for concrete. In the case of coarse aggregate, the increase 40

Fi ne

sa

nd

30

M

ed

ium

sa

nd

20

e rs

oa

C

10

nd

sa

Increase in Volume, per cent

74

0

0

Fig. 3.3

5

10 Moisture, content

15

20

Effect of moisture content on the bulking of sand

Concrete Making Materials—II: Aggregate

75

in volume is negligible due to the presence of free water as the thickness of the moisture film is very small as compared with particle size. The precentage bulking is obtained in accordance with IS: 2386 (Part-III)–1963. If the sand is measured by volume and no allowance is made for bulking, the mix will be richer than that specified because for given mass, moist sand occupies a considerably larger volume than the same mass of the dry sand. This results in a mix deficient in sand increasing the chances of the segregation and honey-combing of concrete. The yield of concrete will also be reduced. It is necessary, in such a case, to increase the measured volume of the sand by the percentage bulking, in order that the amount of sand put into concrete the amount intended for the nominal mix used (based on dry sand). If no allowance is made for the bulking of sand a nominal concrete mix 1:2:4, for example, will correspond to 1:1.74:4 for a bulking of 15 per cent. An increase in bulking from 15 to 30 per cent will result in an increase in the concrete strength by as much as 14 per cent. If no allowance is made for bulking the concrete strength may vary by as much as 25 per cent.

3.4

DELETERIOUS SUBSTANCES IN AGGREGATES

The materials whose presence may adversely affect the strength, workability and longterm performance of concrete are termed deleterious materials. These are considered undesirable as constituent because of their intrinsic weakness, softness, fineness or other physical or chemical characteristics harmful to the concrete behavior. Depending upon their action, the deleterious substances found in the aggregates can be divided into three broad categories: 1. Impurities interfering with the process of hydration of cements. 2. Coatings preventing the development of good bond between aggregate and the cement paste. 3. Unsound particles which are weak or bring about chemical reaction between the aggregate and cement paste. The impurities in the form of organic matter interfere with the chemical reactions of hydration. These impurities generally consisting of decayed vegetable matter and appearing in the form of humus or organic loam are more likely to be present in fine aggregate than in coarse aggregate which is easily washed. The effect of impurities is tested as per IS: 2386 (Part-II)–1963. The clay and other fine materials, such as silt and crusher dust may be present in the form of surface coatings which interfere with the bond between the aggregate and the cement paste. Since a good bond is essential for ensuring satisfactory strength and durability of concrete, the problem of coating of impurities is an important one. The soft or loosely adherent coatings can be removed by washing. The well-bonded chemically stable coatings have no harmful effect except that the shrinkage may be increased. However, an aggregate with chemically reactive coatings can lead to serious trouble. The silt and the fine dust, if present in excessive amounts, increase the specific surface of the aggregate and hence the amount of water required to wet all particles in the mix, thereby reducing the strength and durability of concrete.

76

Concrete Technology

The total amount of deleterious material should not exceed five per cent as per IS: 383–1970. The limits of deleterious materials are given in Table 3.1. Table 3.1

Limits of deleterious materials (maximum percentage by mass)

Deleterious substances

Fine aggregate

Coarse aggregate

Uncrushed

Crushed

Uncrushed

Crushed

Coal and lignite

1.0

1.0

1.0

1.0

Clay lumps

1.0

1.0

1.0

1.0

Soft fragments

–

–

3.0

–

Material passing 75 mm IS Sieve

3.0

3.0

3.0

3.0

Shale

1.0

–

–

–

Total of all deleterious materials

5.0

2.0

5.0

5.0

The sand obtained from a seashore or a river estuary contains salt and sometimes its percentage may be as high as 6 per cent of mass of sand. The salt can be removed from the sand by washing it with fresh water before use. If salt is not removed, it absorbs moisture from air and may cause efflorescence; and slight corrosion of reinforcement may also occur. Unsound particles are broadly grouped as (i) the particles failing to maintain their integrity, and (ii) particles leading to disruptive expansion on freezing or exposure to water. The shale and other particles of low density, such as clay lumps, wood, coal, etc., are regarded as unsound as they lead to pitting and scaling. If the percentage of these particles exceeds two to five per cent of the mass of aggregate, they may adversely affect the strength of concrete. The presence of mica in fine aggregate has also been found to considerably reduce the compressive strength of concrete. Hence, if mica is present in fine aggregate, a suitable allowance for the possible reduction in strength of concrete should be made. Likewise, gypsum and other sulfates must not be present in the aggregates. Iron pyrities and marcasite are the most common expansive inclusions in the aggregate. These sulfides react with water and oxygen in the air resulting in the surface staining of concrete and pop-outs. The effect is more under warm and humid conditions. The majority of these impurities are found in natural aggregate deposits, rather than crushed aggregate.

3.5

SOUNDNESS OF AGGREGATE

The soundness indicates the ability of the aggregate to resist excessive changes in volume due to changes in environmental conditions, e.g., freezing and thawing, thermal changes, and alternating wetting and drying. The aggregate is said to be unsound when volume changes result in the deterioration of concrete. This may

Concrete Making Materials—II: Aggregate

77

appear in the form of local scaling to extensive surface cracking or to disintegration over a considerable depth, and thus vary from an imparied appearance to a structurally dangerous situation. IS: 2386 (Part-V)–1963 describes a method to determine the resistance to disintegration of aggregates by saturated solution of sodium sulfate (Na2SO4) or magnesium sulfate (MgSO4). According to IS: 383–1970, the average loss of weight after ten cycles should not exceed 12 and 18 per cent when tested with sodium sulfate and magnesium sulfate, respectively.

3.6

ALKALI-AGGREGATE REACTION (AAR)

The alkali-aggregate reaction (AAR) or alkali-silica reactivity (ASR) is the reaction between active silica constituents of the aggregate and alkalies, i.e., Na2O and K2O present in the cement. The reactive forms of silica generally occur in the aggregates obtained from traps, opaline or chalcedonic cherts, andestite and andesite tuffs, rhyolites and rhylotic tuffs, siliceous limestones and certain types of sandstones. The expansive alkali-silicate gels are formed due to the reaction when conditions are congenial and progressive manifestation by swelling takes place which result in disruption of concrete with the spreading of pattern cracks and eventual failure of concrete structures. However, only such aggregates which contain reactive silica in particular proportion and in particular fineness are found to exhibit tendencies for alkali-aggregate reaction. The factors promoting the alkali-aggregate reaction are: reactive type of aggregate; high alkali content in cement; availability of moisture and optimum temperature conditions. 1. Reactivity of the aggregate The potential reactivity of an aggregate can be determined by petrographic examination of thin rock sections. IS: 2386 (Part-VII)–1963 describes two methods namely the mortar bar expansion test and the chemical test for the determination of the potential reactivity of the aggregate. The reactivity of aggregate depends upon its particle size and porosity as these influence the area over which the reaction can take place. 2. Alkali content in cement The total amount is expressed as Na2O equivalent (Na2O + 0.658 K2O). Many specifications limit the alkali content to less than 0.6. Such a cement is designated as low-alkali cement. The expansion due to reaction also depends upon the fineness of cement. 3. Availability of moisture The progress of alkali-aggregate reaction depend upon the availability of non-evaporable water in the paste. The reaction and hence the consequent deterioration will be more on the surface and insignificant in the interior of the mass concrete. The application of waterproofing agents to the surface of the concrete can reduce deterioration due to alkali-aggregate reaction by preventing additional penetration of water into structure. The reaction is accelerated under the condition of alternating wetting and drying. 4. Temperature conditions The optimum temperature for the promotion of alkali-aggregate reaction is in the range of 10° to 40 °C. 5. Alkali-aggregate reaction mechanism The soluble alkalies in the cement dissolve in the mixing water turning it into a highly caustic liquid which

78

Concrete Technology

reacts with the reactive silica present in the reactive aggregates to form highly expansive alkali–silica gel altering the boundaries of aggregate. The expansive growth due to continuous supply of water and correct temperature results from unabated formation of silica gel. As the silica gel is confined by the surrounding paste the continuous growth of silica gel exerts internal hydraulic pressure generated through osmosis on the surrounding set-cement gel to cause pattern cracking with subsequent loss in strength and elasticity particularly in thinner sections like pavements. The formation of cracks due to alkali-aggregate reaction accelerates other processes of deterioration like carbonation. 6. Control of alkali-aggregate reaction The AAR can be controlled by: avoiding the use of reactive aggregate; in case the use of suspicious reactive aggregate can not be avoided due to economic reasons, the possibility of AAR can be reduced by the use of low-alkali cement with alkali content less than 0.6 per cent or possibly 0.4 per cent; by absorbing the osmotic pressure developed due to the formation of expansive silica-gel in AAR by using air-entraining agent; by controlling the continuous availability of water which is one of the basic requirements of AAR, and by ensuring that the optimum temperature is not available. One of the effective methods of controlling AAR is by turning the aggregate innocuous by disturbing the optimum conditions of silica being in particular proportion and fineness by addition of pozzolanic additives such as crushed stone dust, diatomaceous earth, surkhi, fly ash, etc. The use of pozzolanic additive is an effective and practical solution of inhibiting AAR. The expansion due to alkali-aggregate reaction can also be reduced by adding reactive silica in a finely powdered form to the concrete mix. The addition of fine reactive silica increases the surface area, increasing the calcium hydroxide-alkali ratio of the solution at the boundaries of the aggregate. Under such circumstances, a nonexpanding calcium-alkali-silicate product is formed. It is generally recommended that 20 g of reactive silica be added for each gram of alkali in excess of 0.5 per cent of the mass of the cement.

3.7

THERMAL PROPERTIES OF AGGREGATES

The thermal properties of aggregates affect the durability and other qualities of concrete. The investigations reported to date do not present a clear-cut picture of the effects that might be expected. The principal thermal properties of the aggregate are: (i) coefficient of thermal expansion, (ii) specific heat, and (iii) thermal conductivity. The coefficient of thermal expansion of the concrete increases with the coefficient of thermal expansion of aggregate. If the coefficient of expansion of coarse aggregate and of cement paste differs too much, a large change in temperature may introduce differential movement which may break the bond between the aggregate and the paste. If the coefficients of the two materials differ by more than 5.4 × 10–6 per °C, the durability of concrete subjected to freezing and thawing may be affected. The coefficient of expansion of the aggregate depends on the parent rock. For majority of aggregates, the coefficient of thermal expansion lies between approximately

Concrete Making Materials—II: Aggregate

79

5.4 × 10–6 and 12.6 × 10–6 per °C. For hydrated Portland cement the coefficient varies between 10.8 × 10–6 and 16.2 × 10–6 per °C. Whereas, the linear thermal coefficient of expansion of concrete lies in the range of about 5.8 × 10–6 per °C to 14 × 10–6 per °C depending upon the type of aggregate, mix proportions, degree of saturation, etc. It can be determined by Verbeck’s dilatometer. It is observed that while there is thermal compatibility at higher range, there exists thermal incompatibility at the lower range. This thermal incompatibility at lower range causes severe stress affecting durability and integrity of concrete structure. When concrete is subjected to a high range of temperature difference the adverse effects become acute. The coefficient of thermal expansion also affects the fire resistance of the concrete. The specific heat of the aggregate is a measure of its heat capacity, whereas the thermal conductivity is the ability of the aggregate to conduct the heat. These properties of the aggregate influence the specific heat and thermal conductivity of the concrete, and are important in the case of mass concrete and where insulation is required.

3.8

FINENESS MODULUS

The fineness modulus is a numerical index of fineness, giving some idea of the mean size of the particles present in the entire body of the aggregate. The determination of the fineness modulus consists in dividing a sample of aggregate into fractions of different sizes by sieving through a set of standard test sieves taken in order shown in Figs. 3.4 and 3.5. Each fraction contains particles between definite limits. The limits being the opening sizes of standard test sieves. The material retained on each sieve after sieving represents the fraction of aggregate coarser than the sieve in question but finer than the sieve above. The sum of the cumulative percentages retained on the sieves divided by 100 give the fineness modulus. The sieves that are to be used for the sieve analysis of the aggregate (coarse, fine or all-in-aggregate) for concrete as per IS: 2386 (Part-I)–1963, are 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 μm, 300 μm and 150 μm.

Stack of sieves for manual shaking

Fig. 3.4

Motorized shaking

Coarse aggregate sieving for gradation and size determination

80

Concrete Technology

Stack of sieves on sieve shaker

Fig. 3.5

T Typical sieve

Series of sieves for analysis of fine aggregate

The fineness modulus can be regarded as a weighted average size of a sieve on which material is retained, and the sieves being counted from the finest. For example, a fineness modulus of 6.0 can be interpreted to mean that the sixth sieve, i.e., 4.75 mm is the average size. The value of fineness modulus is higher for coarser aggregate. For the aggregates commonly used, the fineness modulus of fine aggregate varies between 2.0 and 3.5, for coarse aggregate it varies between 5.5 and 8.0, and from 3.5 to 6.5 for all-in-aggregate. The object of finding fineness modulus is to grade the given aggregate for the most economical mix for the required strength and workability with minimum quantity of cement. If the test aggregate gives higher fineness modulus, the mix will be harsh and if, on the other hand, gives a lower fineness modulus it will produce an uneconomical mix. For workability, a coarser aggregate requires less water–cement ratio. The fineness modulus is also important for measuring the slight variations in the aggregate from the same source.

3.9

MAXIMUM SIZE OF AGGREGATE

In general, larger the maximum size of the aggregate, smaller is the cement requirement for a particular water–cement ratio. This is due to the fact that the workability of concrete increases with the increase in the maximum size of the aggregate. In a mass concrete work, the use of a larger size aggregate is beneficial due to the lesser consumption of cement. This will also reduce the heat of hydration and corresponding thermal stresses and shrinkage cracks. Moreover, due to the smaller surface area of the larger size aggregate, the water-cement ratio can be decreased which increases the strength. However, in practice, the size of aggregate is limited depending upon the size of mixing, handling, and placing equipment. The maximum size of aggregate also influences the compressive strength of concrete in that, for a particular volume of aggregate, the compressive strength tends to increase with the decrease in the size of the coarse aggregate. This is due to the fact that smaller size aggregates provide larger surface area for bonding with the mortar matrix. In addition, the stress

Concrete Making Materials—II: Aggregate

81

concentration in the mortar-aggregate interface increases with the maximum size of the aggregate. Thus, for concrete with a higher water-cement ratio, the nominal size of the coarse aggregate must be as large as possible, whereas for high strength concretes a 10 or 20 mm size of aggregate is preferable. In general, for strengths up to 20 MPa aggregates up to 40 mm may be used, and for strengths above 30 MPa, aggregates up to 20 mm may be used. According to IS: 456–2000, the maximum nominal size of coarse aggregate should not be greater than one-fourth the minimum thickness of the member, and should be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum-cover-to-reinforcement distance and 5 mm less than the spacing between the cables, strands or sheathing in case of prestressed concrete. Within these limits, the nominal maximum size of coarse aggregates may be as large as possible.

3.10

GRADING AND SURFACE AREA OF AGGREGATE

The particle size distribution of an aggregate as determined by sieve analysis is termed grading of the aggregate. If all the particles of an aggregate are of uniform size, the compacted mass will contain more voids whereas an aggregate comprising particles of various sizes will give a mass containing lesser voids. Typical aggregate gradations are shown in Fig. 3.6. The particle size distribution of a mass of aggregate should be such that the smaller particles fill the voids between the larger particles. The proper grading of an aggregate produces dense concrete and needs less quantity of fine aggregate and cement paste. It is, therefore, essential that the coarse and fine aggregates be well graded to produce quality concrete.

Well Graded

Combination of large and small aggregate has fewer voids

Fig. 3.6

Poorly Graded

Gap Graded

Continuous grading gives better packing and the fewest voids

Typical packing and grading of coarse aggregate of different sizes

The grading of an aggregate is expressed in terms of percentages by weight retained on or passing through a series of sieves taken in order, 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm for coarse aggregate as shown in Fig. 3.4, and 10 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 microns, 300 microns and 150 microns for fine aggregate as shown in Fig. 3.5. The sieves are arranged in such an order that the square openings are half for each succeeding smaller size. The curve showing the cumulative percentages of the material passing the sieves represented on the ordinate with the sieve openings to the logarithmic scale represented on the abscissa is termed the grading curve. The grading curve indicates

82

Concrete Technology

whether the grading of a given sample conforms to that specified, or is too coarse or too fine, or deficient in a particular size. 1. In case the actual grading curve is lower than the specified grading curve, the aggregate is coarser and segregation of mix might take place. 2. In case the actual grading curve lies well above the specified curve, the aggregate is finer and more water will be required, thus increasing the quantity of cement for a constant water–cement ratio. Therefore, this is uneconomical. 3. If the actual grading curve is steeper than the specified, it indicates an excess of middle-size particles and leads to harsh mix. 4. If the actual grading curve is flatter than the specified grading curve, the aggregate will be deficient in middle-size particles. The grading of the aggregate affects the workability which, in turn, controls the water and cement requirements, segregation, and influences the placing and finishing of concrete. These factors represent the important characteristics of fresh concrete and affect its properties in the hardened state. The main factors governing the desired aggregate grading are: the surface area of aggregate, the relative volume occupied by the aggregate, the workability of the mix, and the tendency to segregate. A number of methods have been proposed for arriving at an ideal grading that would be applicable to all aggregates. None of these has been universally successful because of economic considerations, effect of particle shape and texture of the aggregate, and differences in cement from different sources. Grading specifications have been developed, however, which on the average will produce a concrete of satisfactory properties from materials available in a particular area. The surface area is affected by the maximum size of aggregates. If a sphere of diameter d is taken as representative of the shape of aggregate, the ratio of surface area to the volume is 6/d. This ratio of surface of the particles to their volume is called specific surface. The surface area will vary with the shape but is inversely proportional to the particle size. The smaller the size of aggregate, the greater is the surface area per unit mass or unit volume. The aim must, therefore, be to have as large a maximum aggregate size as possible and to grade it down in such a way that the voids in the coarse aggregate are filled with the minimum amount of fine aggregate as shown in Fig. 3.7. This arrangement, however, cannot be carried too far as an aggregate graded in this way would be too harsh and a slight excess of fines is necessary to prevent this. The greatest contribution to this total surface area is made by the smaller size aggregate and, therefore particular attention should be paid to the proportion and grading of fine aggregate. The mortar consisting of fine aggregate and cement should be slightly in excess of that just required to fill the voids in the coarse aggregate. Too coarse a fine aggregate results in harshness, bleeding and segregation and too fine an aggregate requires too large a water–cement ratio for adequate workability. The surface area of aggregate also influences the amount of mixing water and cement required. Generally, the water–cement ratio is fixed from strength considerations. However, the amount of cement paste should be sufficient to cover the surface of all the particles for proper workability and bond. The drying shrinkage is less with a smaller amount of mixing water, and the temperature rise due to hydration and, therefore, cracking on subsequent cooling is less with the smaller proportion of cement in the mix.

Concrete Making Materials—II: Aggregate

Original image

Aggregate size (S) > 4.75

Boundary for aggregate e size S > 4.75

Aggregate size 4.75 mm < S < 10 mm

Aggregate size 10 mm < S < 16 mm

Aggregate size S > 16 mm

Fig. 3.7

83

Typical grading of thoroughly mixed combined aggregate

Economical and uniform concrete cannot be produced with pit-run or crusher-run aggregate, and it is necessary that the aggregate be separated into its component sizes so that it can be combined in the concrete mix within the limits of variation permitted by the specifications. The grading of fine aggregate has a much greater effect on workability of concrete than does the grading of the coarse aggregate. Experience has shown that usually very coarse sand or very fine sand is unsatisfactory for concrete. Fine grading conforming to the specifications laid by IS: 383–1970 shall be satisfactory for most concretes. In the case of graded aggregate, the grading and the overall specific surface area are related to one another, although there can be many grading curves corresponding to same specific surface. If the grading extends to a larger maximum particle size, the overall specific surface is reduced decreasing the water requirements, but the relation is not linear. As discussed earlier, the maximum size of the aggregate that can be used for a certain job depends upon the size of the member and of the reinforcement used. For reinforced concrete work, the aggregate having a maximum size of 20 mm is generally considered satisfactory. The Road Research Laboratory of the Department of Scientific and Industrial Research, London, has prepared a series of grading curves which are useful for the design of concrete mixes. The grading curves for the aggregates of maximum nominal size of 40 mm, 20 mm and 10 mm are shown in the Figs 3.8 to 3.10, respectively. These are not the ideal or standard curves but represent gradings used in the road research laboratory testing. Higher the number of grading curve, larger will be the proportion of fine particles. The coarsest grading curve No.1 is suitable for harsh mixes, i.e., the most economical mix having highest permissible aggregate–cement ratio.

84

Concrete Technology

120

Percentage Passing

100

77

80 4 65

68 3

60 51 38

40

40

32

31 25 26

20

5 02

7

150 μm

18

16 1 11 7 3 300 μm

26

55

2

44

1

60 49

36

35 22

17 17

13 8

12

600 μm 1.18 mm 2.36 mm 4.75 mm

10 mm

20 mm

40 mm

IS sieve sizes

Fig. 3.8

Recommended grading curves for 40 mm nominal maximum size aggregate

100

80

75

Percentage Passing

65 5 60 55 4 42 40 27 21 20 12 2

14

5

9

35

35

28

28 23

21

50 45

42 3 2

35 30

1

16 2 0 150 μm

3 2 300 μm

600 μm

1.18 mm

2.36 mm 4.75 mm

10 mm

20 mm

IS sieves sizes

Fig. 3.9

Recommended grading curves for 20 mm nominal maximum size aggregate

Concrete Making Materials—II: Aggregate

85

The finest grading curve No.4 is suitable for lean mixes where a high workability is required. The change from one extreme to the other is progressive. The outer curves 1 and 4 represent the limits for the normal continuous gradings shown in Fig. 3.7. The saving in cement affected by using a coarse grading can be considerable. If the locally available aggregate does not conform to the desired grading, the finer and the coarser fractions of aggregates can be suitably combined to obtain the desired grading. This can be achieved either by analytical calculations or graphically as explained in Section 10.3.4 of Chapter 10. 100 90 80

75 4

Percentage Passing

70 60

60

60 3

50

46

46 2

40

37

34

33

30 20

20 14

10

8

0 150 mm

28

1

26

19 16

12

45

30

20

4 300 mm

600 mm

1.18 mm

2.36 mm

4.75 mm

10 mm

IS Sieve Sizes

Fig. 3.10

3.10.1

Recommended grading curves for 10 mm nominal maximum size aggregate

Gap-graded Aggregate

Gap-grading is defined as a grading in which one or more intermediate-size fractions are absent. The term ‘continuously graded’ is used to distinguish the conventional grading from gap-grading. On a grading curve, gap-grading is represented by a horizontal line over the range of the size omitted. Some of the important features of gap-graded aggregate are as follows. 1. For the given aggregate–cement and water–cement ratios the highest workability is obtained with lower sand content in the case of gap-graded aggregate rather than when continuously graded aggregated is used. 2. In the more workable range of mixes, gap-graded aggregates show a greater tendency to segregation. Hence, gap-grading is recommended mainly for mixes of relatively low workability that are to be compacted by vibration. 3. Gap-graded aggregate does not affect compressive or tensile strengths.

86

Concrete Technology

4. Specific surface area of gap-graded aggregate is lower because of higher percentage of coarse aggregate. 5. Gap-graded aggregate requires lesser cement and lower water–cement ratio. 6. The drying shrinkage is reduced in the concrete using gap-graded aggregate.

3.10.2

Grading Limits

There is no universal ideal grading curve. The concrete for satisfactory performance can be obtained with various gradings of aggregate. However, IS: 383–1970 has recommended certain limits within which the grading must lie to produce satisfactory concrete, subject to the fulfilment of certain desirable properties of aggregate, such as shape, surface texture, type of aggregate and amount of flaky and elongated materials. The grading of coarse aggregate may be varied through wider limits than that for fine aggregate since it does not largely affect the workability, uniformity and finishing qualities. The grading limits for coarse aggregate are given in Table 3.3. It is difficult to control the grading of fine aggregate. For bigger jobs it can be effected by combining two or more different kinds of sand from different sources. The sands are generally divided into different zones according to the percentage passing the IS: 600 micron sieve. IS: 383–1970 classifies the sand into four zones, I, II, III and IV so that the range of percentage passing the 600 micron sieve in each zone does not overlap. The grading limits of four zones are given in Table 3.4. From grading zone I to IV, the fine aggregate becomes progressively finer, and the ratio of fine to coarse aggregate should be progressively reduced as suggested in Table 3.6. A fine aggregate is considered as belonging to the zone in which its percentage passing the 600 micron sieve falls and it is allowed to fall outside the limits fixed for other sieves by not more than a total of five per cent. For crushed stone sands the permissible limit on IS:150 micron sieve is increased by 20 per cent. However, this does not affect the five per cent allowance permitted above as applied to other sieve sizes. The following values of fineness modulus may be taken as guidance for making satisfactory concrete: Type of sand

Fineness modulus

Fine sand

2.2–2.6

Medium sand

2.6–2.9

Coarse sand

2.9–3.2

Any sand having fineness modulus more than 3.2 will not be suitable for making satisfactory concrete. It has been noticed that if the specific surface of aggregate is kept constant, wide difference in grading does not affect the workability appreciably. However, the variation of fine to coarse aggregate ratio to keep total surface constant cannot be pushed too far and if a very fine sand is being used this process may result in the mix being under-sanded with a serious risk of segregation, specially in cases of lean mixes. In the case of all-in-aggregate, the necessary adjustments may be made in the grading by the addition of a single-size aggregate without separating into fine and coarse aggregates.

Concrete Making Materials—II: Aggregate

87

The grading limits for various types of aggregates are given in Tables 3.2 to 3.5. Table 3.2 IS sieve designation

Grading limits for single size coarse aggregate (IS: 383–1970) Percentage passing for single size aggregate of nominal size, mm 40

20

16

12.5

10

80 mm

–

–

–

–

–

40 mm

85–100

100

–

–

–

20 mm

0–20

85–100

100

–

–

16 mm

–

–

85–100

100

–

12.5 mm

–

–

–

85–100

100

0–5

0–20

0–30

0–45

85–100

4.75 mm

–

0–5

0–5

0–10

0–20

2.36 mm

–

–

–

–

0–5

10 mm

Table 3.3 IS sieve designation

Grading limits for coarse aggregates (IS: 383–1970) Percentage passing for graded aggregate of nominial size, mm 40

20

16

12.5

80 mm

100

–

–

–

40 mm

95–100

100

–

–

20 mm

30–70

95–100

100

100

16 mm

–

–

95–100

–

12.5 mm

–

–

–

90–100

10–35

25–55

30–70

40–85

4.75 mm

0–5

0–10

0–10

0–10

2.36 mm

–

–

–

–

10 mm

Table 3.4 IS sieve designation

Grading limits for fine aggregates (IS: 383–1970) Percentage passing by weight Grading Zone I

Zone II

Zone III

Zone IV

100

100

100

100

4.75 mm

90–100

90–100

90–100

95–100

2.36 mm

60–95

75–100

85–100

95–100

1.18 mm

30–70

55–90

75–100

90–100

600 μm

15–34

35–59

60–79

80–100

300 μm

5–20

8–30

12–40

15–50

150 μm

0–10

0–10

0–10

0–15

4.0–2.71

3.37–2.10

2.78–1.71

2.25–1.35

10 mm

Fineness modulus

88

Concrete Technology Table 3.5

IS sieve designation

Grading limits for all-in-Aggregate (IS: 383–1970) Percentage by weight passing for all-in-aggregate of 40 mm nominal size

20 mm nominal size

80 mm

100

–

40 mm

95–100

100

20 mm

45–75

95–100

4.75 mm

25–45

30–50

600 μm

8–30

10–35

150 μm

0–5

0–6

Table 3.6

Suggested proportion of fine to coarse aggregate for different size of aggregates

Nominal size of graded coarse aggregate, mm

Fine aggregate: Coarse aggregate, for the sand of zone Zone I

Zone II

Zone III

Zone IV

10

1:1

1:1.5

1:2

1:3

20

1:1.5

1:2

1:3

1:3.5

40

1:2

1:4

1:3.5

–

3.10.3 Crushed Sand The availability of commonly used natural sand which is normally rounded and smooth textured is shrinking at a fast rate and becoming costly. The concrete industry is increasingly going in for crushed or manufactured sand. However, ordinarily crushed sand is flaky, poorly graded, rough textured and hence results in a harsh concrete mix requiring the use of superplasticizers to improve the workability. With the advent of modern crushers specially designed for producing cubical, comparatively smooth textured and well-graded sand, the crushed sand is fast replacing natural sand. New technologies are available for producing coarse and fine aggregates of the desired quality in terms of shape, texture and grading. The dust, i.e., the portion of aggregates consisting of particles of size finer than 75 micron, is limited to 15 per cent in fine aggregate and three per cent in coarse aggregate. Generally, the manufactured sands conform to the grading Zones I and II of fine aggregates as given in the Table 3.7. Table 3.7 IS sieve size Zone I Zone II

3.11

Grading limits for crushed sands

Percentage passing 10 mm 4.75 mm 2.36 mm 1.18 mm 600 μm 300 μm 150 μm 75 μm 100 90–100 60–95 30–70 15–34 5–20 0–20 15, max. 100

90–100

75–100

55–90

35–59

8–30

0–20

15, max.

TESTING OF AGGREGATES

The important characteristics, their significance and test standards of aggregates are summarized in the Table 3.8.

Table 3.8

Summary of aggregate properties

Characteristics

Significance

Test

Specifications

1. Particle shape and texture

Affects workability of fresh concrete.

Visual inspection, flakiness and elongation test IS: 2386 (Part 1)–1963.

Limits on flaky or enlogated particles. Flakiness index not greater than 30 to 40 per cent is desirable.

2. Resistance to crushing

In high strength concrete, aggregate low in crushing value will not give high strength even though cement strength is higher.

Aggregate impact value test IS: 2386 (Part IV)–1963.

30% impact value for pavement; 45% for other applications.

3. Specific gravity

Required in mix design calculations; unit weight of concrete; yield of concrete.

Specific gravity determination IS: 2386 (Part III) –1963.

————–

4. Bulk density

Rodded bulk density is Test for bulk denuseful as a check on the sity IS: 2386 (Part uniformity of aggregate III)–1963 grading; loose bulk density is useful to convert masses into bulk volumes on site or vice versa.

————–

5. Absorption and surface moisture

Affects the mix proportions; to control water content to maintain water–cement ratio constant.

Test for absorption and surface moisture IS: 2386 (Part III) –1963.

————–

6. Deleterious substance

Organic impurities and coatings interfere with hydration of cement.

Test of impurities IS: 2386 (Part III) –1963.

Limits on impurities have been prescribed in IS: 383–1970.

7. Grading

Economizes cement content and improves workability.

IS: 2386 (Part VII) –1963.

Grading limits for coarse, fine and allin-aggregate are laid down in IS: 383-1970

8. Chemical stability

Significant for strength and IS: 2386 (Part VII) durability of all types of –1963. structures specially subjected to chemical attack.

–

9. Resistance to freezing and thawing

Significant to cold countries where frost action deteriorates concrete due to alternate freezing and thawing.

–

Test for soundness IS: 2386 (Part VII) –1963.

90

Concrete Technology

3.12 3.12.1

AGGREGATE PROCESSING, HANDLING AND STORING Aggregate Processing

Aggregate processing consists of two stages: (i) basic processing, and (ii) beneficiation (upgrading) processing. Basic processing includes crushing, screening, and washing to obtain proper gradation and cleanliness. Beneficiation consists in upgrading the quality of the aggregate by specific processing methods such as the following:

Media separation It consists in passing the aggregates through a heavy liquid with specific gravity less than that of the desirable aggregate particles but greater than that of the harmful particles.

Jigging This process separates particles with small differences in density by pulsating water current. Upward pulsations of water through a jig (a box with a perforated bottom) move the lighter material into a layer on top which is then removed. Rising-current classification It separates particles with large differences in specific gravities. Light materials, such as wood and lignite, are floated away in a rapidly upward moving stream of water. Crushing It is used to remove soft and friable particles from coarse aggregates.

3.12.2

Aggregate Handling and Storing

The operations that use minerals in aggregate form require the provisions of outdoor stockpiles as shown in Fig. 3.11. Stockpiles of aggregate shall be constructed on areas that are hard, well drained, and denuded of vegetation. Stockpiles are usually left uncovered, partially because of the need for frequent material transfer into or out of storage. Dust emissions occur at several points in the storage cycle, such as material loading onto the stockpile, disturbances by strong wind, and load out from the pile. The movement of trucks and loading equipment in the storage pile area is also a substantial source of dust. The following factors should be considered in handling and storing the aggregates: 1. Stockpiles should be built in thin layers of uniform thickness to minimize segregation. The truck-dump method of forming aggregate stockpiles is suitable, as in this method the load is discharged in a tightly joined manner. The aggregate is then reclaimed with a front end-loader removing slices from the edges of the pile from bottom to top. Whether the aggregate is handled by dump-truck, bucket loader, clamshell, or conveyor belt, stockpile should not be built up in high, cone-shaped pile since this results in segregation. 2. The stockpiling equipment should not be allowed over the aggregate stockpiles because the aggregate may crush and the gradation may change or foreign particles may be introduced. 3. The crushed aggregates segregate less than the gravel, and large-size aggregates segregate more than the smaller sizes. To avoid segregation of coarse aggregates, size fractions can be stockpiled and batched separately. The

Concrete Making Materials—II: Aggregate

91

stockpiles should be separated from other materials to avoid contamination and to maintain integrity and gradation of aggregate. The partitions between stockpiles should be high enough to prevent intermixing of materials.

(a) Crushed coarse aggregate

Fig. 3.11

(b) Fine aggregate

Stockpiles of coarse and fine aggregates at a construction site

4. Washed aggregates should be stockpiled in sufficient time before use so that they can drain to uniform moisture content. Damp fine material has lesser tendency to segregate than dry material. 5. Dust emissions due to dropping of dry fine aggregate from buckets or conveyors, the wind-blown fines should be avoided as far as possible. 6. Exposure to extreme weather should be taken care off. During extreme heat the stockpile should be misted, covered during freezing and protected from high wind. 7. Aggregates placed directly on the ground shall not be removed from the stockpiles within 300 mm of the ground until final cleanup, and then only clean aggregate shall be used.

3.13

MARINE-DREDGED AGGREGATE

When other aggregate sources are inadequate, marine-dredged aggregate and sand, and gravel from the seashore can be used with caution in limited concrete applications. Aggregates obtained from seabed have two problems: (i) seashells, and (ii) salt.

Seashells 1. The sea shells are hard materials that can produce good quality concrete; however, higher cement content may be required due to angularity of the shells to obtain the desired workability. 2. Aggregate containing complete shells should be avoided as their presence may result in voids in the concrete and lower the compressive strength. 3. Generally, marine aggregates containing large amounts of chloride should not be used in reinforced concrete. Marine-dredged aggregates can be washed with fresh water to reduce the salt content.

Salts The presence of these chlorides may affect the concrete by 1. Altering the time of set

92

Concrete Technology

2. Increasing drying shrinkage 3. Increasing the risk of corrosion of steel reinforcement 4. Causing efflorescence

3.14

RECYCLED CONCRETE

The recycled aggregate results in both material and energy savings. It is primarily used in pavement reconstruction. It has been satisfactorily used as an aggregate in granular sub-bases, lean-concrete sub-bases and soil-cement. The processing involves 1. breaking up and removing the old concrete, 2. crushing in primary and secondary crushers, crushing concrete can be accomplished with a beam crusher, 3. removing reinforcing steel and embedded items, 4. grading and washing, and 5. finally the resulting coarse and fine aggregate are stockpiled. Dirt, gypsum board, wood, and other foreign materials should be prevented from contaminating the final product. Recycled concrete aggregate should be tested for durability, gradation, and other properties.

3.14.1

Properties of Recycled Concrete

1. Recycled concrete aggregate generally has a higher absorption (3 to 10 per cent) and a lower relative density than conventional aggregate. The absorption values increase with decrease in size as coarse particles. 2. The concrete made from recycled concrete aggregate generally has good durability. Carbonation, permeability, and resistance to freeze-thaw action have been found to be the same or even better than concrete with conventional aggregates. 3. Drying shrinkage and creep of concrete made with recycled aggregates is up to 100 per cent higher than the concrete with a corresponding conventional aggregate. This is due to the large amount of old cement paste and mortar especially in the fine aggregate. 4. Frequent monitoring of the properties of recycled aggregates is required because of the variability in the properties of the old concrete. Concrete trial mixtures should be made to check the new concrete’s quality and to determine the proper mix proportions.

REVIEW QUESTIONS 3.1 How does strength of aggregate plays important role in quality and strength of concrete? Discuss briefly the three tests generally prescribed for determination of strength of aggregates. 3.2 How is the aggregate classified according to size, shape and texture?

3.3 What is alkali-silica reactivity (ASR) and how is it avoided? 3.4 What is grading of aggregate and its significance? Describe the process of sieve analysis for determination of fineness modulus of an aggregate in tabular form.

Concrete Making Materials—II: Aggregate 3.5 Determine the fineness modulus of aggregate for the following result of IS sieve size Percentage passing

sieve analysis. What does the result indicate?

10 mm 4.75 mm 2.36 mm 1.18 mm 100

92

74

3.6 Explain the effect of size, shape, texture and grading of aggregate on concrete. What are the factors governing the use of maximum size of aggregate in reinforced concrete? 3.7 Discuss bulking of sand; if the sand is measured by volume and no allowance is made for the bulking of sand what

93

55

600 µm 300 µm 150 µm 75 µm 23

12

9

7

will be its effect on a nominal concrete mix 1:2:4 for a bulking of 15 per cent? 3.8 Write short notes on any three of the following: (a) Maximum size of aggregate, (b) Gap-graded aggregate, (c) Marine-dredged aggregate, (d) Recycled Concrete aggregate (e) Thermal properties of aggregates

MULTIPLE-CHOICE QUESTIONS 3.1 Aggregate is used in concrete because (a) it is a relatively inert material and is cheaper than cement (b) it imparts volume stability and durability to the concrete (c) it provides bulk to the concrete (d) it increases the density of the concrete mix (the aggregate is frequently used in two or more sizes) (e) All of the above 3.2 The function of fine aggregate is (a) to assist in producing workability and uniformity in the mixture (b) to assist the cement paste to hold the coarse aggregate particles in suspension (c) to promote plasticity in the mixture and prevent possible segregation of paste and coarse aggregate (d) All of the above (e) None of the above 3.3 An aggregate should (a) be of proper shape and size (b) be clean, hard and well graded (c) possess chemical stability (d) exhibit abrasion resistance (e) All of the above 3.4 An aggregate generally not preferred for use in concrete is one which has the following surface texture (a) smooth

(b) rough (c) glassy (d) granular (e) honeycombed 3.5 Aggregate can be classified according to (a) geological origin (b) size (c) shape (d) unit weight (e) Any of the above 3.6 The nominal size of particles of graded aggregate is said to be 12.5 mm when most of it passes through a ______ mm IS sieve and is retained in a ______ mm IS sieve. (a) 16, 4.75 (b) 12.5, 4.75 (c) 12.5, 10 (d) 16, 12.5 (e) 20, 12.5 3.7 Identify the incorrect statement(s). (a) Artificial aggregates namely broken bricks and air-cooled fresh blast furnace-slag can be used in concretes. (b) Sand is generally considered to have a lower size limit of about 0.07 mm. (c) Aggregates provide about 75 per cent of the body of concrete. (d) Crushed stone sand is produced by crushing of hard stone. (e) None of the above

94

Concrete Technology

3.8 Classification of aggregate according to size is (a) fine aggregate, coarse aggregate, and all-in-aggregate (b) natural sand, crushed stone sand, and crushed gravel sand (c) coarse, medium and fine sands (d) single size aggregate, coarse aggregate, and all-in-aggregate (e) Any of the above 3.9 Spot the odd one(s). (a) rounded aggregate (b) irregular or partly rounded aggregate (c) angular flaky aggregate (d) single-size-aggregate (e) elongated aggregate 3.10 Which of the following statement(s) are incorrect? (a) Rounded aggregate requires minimum cement paste to make good concrete (b) Irregular aggregate requires more cement paste to make a workable concrete (c) Higher the angularity number, the more angular is the aggregate (d) The shape and surface texture of the aggregate influence the workability of fresh concrete (e) An aggregate is termed flaky when its least dimension is less than nine-fifth of its mean dimension 3.11 The cyclopan aggregate has a size more than (a) 4.75 mm (b) 20 mm (c) 40 mm (d) 60 mm (e) 75 mm 3.12 If the fineness modulus of sand is 2.5 it is graded as (a) very coarse sand (b) coarse sand (c) medium sand (d) fine sand (e) very fine sand 3.13 Bulking of sand is the (a) rodding of the sand so that it occupies minimum volume (b) compacting of the sand (c) increase in the volume of sand due to moisture which keeps sand particles apart

3.14

3.15

3.16

3.17

3.18

(d) segregating sand of particular size (e) None of the above With 4% moisture, the bulking of fine sand may be of the order of (a) 2% to 5% (b) 5% to 10% (c) 10% to 15% (d) 15% to 25% (e) 25 to 30 per cent Bulking of coarse aggregate is (a) less as compared to that of a sand (b) more than that of sand (c) 15% at 4% moisture content (d) 25% at 4% moisture content (e) negligible Which of the following statement(s) are incorrect? (a) Sintered fly ash aggregate produces concrete with a density of 12 to 14 kN/m3 (b) An aggregate with a higher modulus of elasticity generally produces a concrete with a higher modulus of elasticity (c) The strength of bond between the aggregate and cement paste depends upon the surface texture (d) The apparent specific gravity of the aggregate is with respect to void free volume (e) The bulk density is affected by particle shape, size and grading of the aggregate Which of following statement(s) are correct? (a) The surface moisture expressed as a percentage of the weight of saturated surface dry aggregate is termed as moisture content (b) The empty space between the aggregate particles are termed voids (c) The thermal properties of the aggregate affect durability of concrete (d) The alkali-aggregate reaction is a reaction between the active silica constituent of the aggregate and the alkalies in cement (e) All of the above Gap grading is one (a) in which one or more intermediate fractions are absent

Concrete Making Materials—II: Aggregate (b) in which the particles fall within a narrow limit of size fractions (c) which combines different fractions of fine and coarse aggregates (d) in which all the particles are of uniform size (e) Any one of the above 3.19 Which of the following is/are deleterious material in aggregate? (a) Coal (b) Clay lumps (c) Soft fragments (d) Shale (e) All of the above 3.20 Deleterious substances in aggregate are undesirable because they may (a) affect the strength, workability and long-term performance of concrete (b) have intrinsic weakness, softness and fineness (c) interfere with the chemical reaction of hydration

(d) interfere with the bond between the aggregate and cement paste (e) Any one of the above 3.21 The fineness modulus (a) is a numerical index of fineness (b) gives some idea of the mean size of particles present in the entire body of aggregate (c) is a sum of the cumulative percentages retained on the set of specified sieves divided by 100 (d) is regarded as weighted average size of sieve on which material is retained (e) Any one of the above 3.22 Grading of the aggregate (a) affects the workability (b) affects the strength of concrete (c) is dependent on the shape and texture of the particles of the aggregate (d) affects the water–cement ratio (e) All of the above is true

Answers to MCQs 3.1 (e) 3.7 (e) 3.13 (c) 3.19 (e)

3.2 (d) 3.8 (a) 3.14 (d) 3.20 (e)

3.3 (e) 3.9 (d) 3.15 (e) 3.21 (e)

95

3.4 (c) 3.10 (e) 3.16 (d) 3.22 (a)

3.5 (e) 3.11 (e) 3.17 (e)

3.6 (c) 3.12 (d) 3.18 (a)

4 4.1

CONCRETE MAKING MATERIALS—III: WATER

INTRODUCTION

Water is the most important and least expensive ingredient of concrete. A part of mixing water is utilized in the hydration of cement to form the binding matrix in which the inert aggregates are held in suspension until the matrix has hardened. The remaining water serves as a lubricant between the fine and coarse aggregates and makes concrete workable, i.e., readily placeable in forms. Generally, cement requires about three-tenth of its weight of water for hydration. Hence the minimum water–cement ratio required is 0.30. But the concrete containing water in this proportion will be very harsh and difficult to place. Additional water is required to lubricate the mix, which makes the concrete workable. This additional water must be kept to minimum, since too much water reduces the strength of concrete. The water–cement ratio is influenced by the grade of concrete, nature and type of aggregates, the workability and durability, etc. If too much water is added to concrete, the excess water along with cement comes to the surface by capillary action and this cement water mixture forms a scum or thin layer of chalky material known as laitance. This laitance prevents bond formation between the successive layers of concrete and forms a plane of weakness. Excess water may also leak through the joints of the formwork and make the concrete honeycombed. As a rule, the smaller the percentage of water, the stronger is the concrete subject to the condition that the required workability is available.

4.2

QUALITY OF MIXING WATER

The water used for the mixing and curing of concrete should be free from injurious amounts of deleterious materials. The unwanted situations, leading to the distress of concrete, have been found to be a result of, among others, the mixing and curing water being of inappropriate quality. Potable water from the sources shown in Fig. 4.1 is generally considered satisfactory for mixing concrete. In the case of doubt about the suitability of water, particularly in remote areas or where water is derived from sources not normally utilized for domestic purposes, water should be tested.

4.2.1 Effect of Impurities in Water on Properties of Concrete The strength and durability of concrete is reduced due to the presence of impurities in the mixing water. The effects are expressed mainly in terms of difference in the setting times of Portland cement mixes containing proposed mixing water as compared to distilled water, and concrete strengths compared with those of control specimens prepared with

Concrete Making Materials—III: Water

97

distilled water. A difference in 28-days compressive strength up to 10 per cent of control test is generally considered to be a satisfactory measure of the quality of mixing water. IS: 456–2000 prescribes a difference in initial setting time of ± 30 minutes with initial setting time not less than 30 minutes. The effluents from sewerage works, gas works, and from paint, textile, sugar and fertilizer industry are harmful for concrete. The tests show that water containing excessive amounts of dissolved salts reduces compressive strength by 10 to 30 per cent of that obtained using potable water. In addition, water containing large quantities of chlorides tends to cause persistent dampness, surface efflorescence and increases the corrosion of the reinforcing steel. The problem is more in tropical regions, particularly with lean mixes.

The water from these sources may generally be used for mixing in concrete

Fig. 4.1

In case of doubt about the suitability of water for mixing in concrete it should be tested

The adverse effects on compressive strength of concrete due to various dissolved salts are given in Table 4.1. Table 4.1

Effects of dissolved salts in water on compressive strength

Percentage of salt in water

Percentage reduction in compressive strength

0.5 SO4

4

1.0 SO4

10

5.0 NaC1

30

CO2

20

The effect of various impurities on the properties of concrete are summarized below.

Suspended Particles The presence of suspended particles of clay and silt in the mixing water up to 0.02 per cent by weight of water does not affect the properties of concrete. Even higher percentage can be tolerated so far as strength is concerned, but other properties of concrete are affected. IS: 456–2000 allows 2000 mg/liter of suspended matter. The muddy water should, however, remain in settling basins before use.

98

Concrete Technology

Miscellaneous Inorganic Salts The presence of salts of manganese, tin, zinc, copper and lead in water causes reduction in the strength of concrete. The zinc chlorides retard the setting of concrete to such an extent that no strength tests are possible at 2 and 3 days. The effect of lead nitrate is completely destructive. Some salts like sodium iodate, sodium phosphate, sodium arsenate and sodium borate reduce the initial strength of concrete to a very low degree. The carbonates of sodium and potassium may cause extremely rapid setting and in large concentrations, reduce the concrete strength. On the other hand, the presence of calcium chloride accelerates setting and hardening. The quantity of calcium chloride is restricted to 1.5 per cent by weight of cement.

Salts in Seawater Seawater generally contains 3.5 per cent of dissolved salts. The chemical composition of seawater throughout the world is remarkably uniform and all the chloride is associated with sodium except for a very small amount with potassium and all the sulfate is associated with magnesium. The approximate percentages of various ions due to the salts in seawater are: chloride, 51.3; sulphate, 7.2; sodium, 28.5; magnesium, 3.6; calcium, 1.3; potassium, 1.0. However, the total amount of any ion varies widely. For a given mass of seawater the ingress into the concrete of any given ion is proportional to the salinity of that seawater. From the standpoint of chemical effects of seawater on plain or unreinforced concrete, it is the sulfate content which is problematic, hence, the need for sulfate-resisting cement. However, this need is greatly reduced by employing concrete of lower water–cement ratio. The salts present in seawater reduce the ultimate strength of concrete. The reduction in strength of concrete may be of the order of 10 to 20 per cent. However, the major concern is the risk of corrosion of reinforcing steel due to chlorides. In general, the risk of corrosion of steel is more when the reinforced concrete member is exposed to air than when it is continuously submerged under water. The presence of chlorides in water is also responsible for efflorescence. It is advantageous to use cement with as much C3A as can be tolerated without incurring sulfate attack in concrete containing corrodible metal. The more is the C3A in the cement, the more chloride ion will be intercepted by aluminate (precipitated as non-detrimental calcium chloroaluminate), taking longer for the ions to build up at the surface of the steel. There are two sources for the presence of chloride ion in the concrete, the first is calcium chloride added as an accelerating admixture and the second one is the intentional use of seawater as mixing water. For normal cements that are not highly sulfate-resisting, the use of CaCl2 reduces the sulfate-resistance, but not when appropriate sulfate-resisting cement is employed. The use of CaCl2 as an accelerator can be permitted in cold weather with sulfate-resisting cement to the same limited extent as with ordinary cements. However, the codes forbid the use of calcium chloride when sulfate-resisting cement is being used. Under unavoidable circumstances, it may be used for plain concrete when it is constantly submerged in water.

Acids and Alkalies The industrial waste water containing acids or alkalies is usually unsuitable for concrete construction. With reference to acidity, the water having pH value

Concrete Making Materials—III: Water

99

higher than 6 can be used. However, the pH value may not be a satisfactory measure of the amount of acid. The effect of acidity in water is best gaged on the basis of total acidity, the extent of which should satisfy the following requirement: The amount of 0.02 normal NaOH required to neutralize 100 ml sample of water using phenolphthalein as indicator should not be more than 5 ml. This acidity is equivalent to 49 ppm of H2SO4 or 36 ppm of HCl.

Algae Algae may be present in mixing water or on the surface of aggregate particles. It combines with cement and reduces the bond between aggregates and cement paste. The water containing algae has the effect of entraining large quantities of air in concrete and thus lowering the strength of concrete.

Sugar If the amount of sugar present in the mixing water is less than 0.05 per cent by weight of water there is no adverse effect on the strength of concrete. Small amounts of sugar up to 0.15 per cent by weight of cement retard the setting of cement and the early strengths may be reduced whereas the 28-day strength may be improved. When the quantity of sugar is increased to 0.20 per cent by weight of cement, setting is accelerated. When quantity is further increased, rapid setting may result and 28-day strength is reduced.

Oil Contamination Mineral oils not mixed with animal or vegetable oils have no adverse effect on the strength of concrete. If the concentration of mineral oil is up to two per cent by weight of cement, a significant increase in strength has been noticed. For a percentage of mineral oil (more than eight per cent), the strength is slightly reduced. The vegetable oils have detrimental effect on the strength of concrete, particularly at later ages.

4.2.2 Limit of Impurities in Water The amounts of solid impurities in the mixing water shall be restricted as per the Table 4.2. The pH value of water suitable for concrete construction shall generally be between 6 and 8. The water which is fit for drinking purposes will be fit for concrete construction. The test for determination of solid contents shall be conducted as per IS: 3025. Table 4.2 Type of impurities

Limits of permissible impurities Permissible percentage of solids by weight of water

Organic

0.02

Inorganic

0.30

Sulfates (as SO3)

0.04

Alkali Chlorides (as Cl2) (a) Plain concrete

0.20

(b) Reinforced concrete

0.05

100

4.3

Concrete Technology

CURING WATER

The use of water in curing the concrete is intended to penetrate the concrete. If steps are taken to prevent loss of water from the concrete, no added water will be needed as a part of curing process except in the circumstances: (i) when the water–cement ratio is less than 0.4; and (ii) when the concrete is produced using expansive cement. Even at a water–cement ratio of 0.48, empty capillary pores exist, however, there is enough water in the mixture for hydration to proceed to completion, but it is necessary for the water to be uniformly distributed throughout the mass of concrete. In structural members, there is inevitably some loss of water by evaporation from the surface. Consequently, hydration may effectively proceed in the interior of the member but, near the surface, there is an inadequate amount of water in the capillaries so that penetration by curing water is highly desirable. However, if the water used for curing is seawater, chloride ions enter the surface zone and from there move inwards by diffusion. From the standpoint of durability, it is the near surface zone that is much more important than the concrete in the interior of the mass—many durability problems start at the surface or through attack progressing from the surface inwards. In the case of marine structures cast on land, but destined for immersion in the sea, the risk of imbibitions of seawater is high, unless thorough curing with fresh water has been done previously. The water which is satisfactory for mixing concrete can also be used for curing it but should not produce any objectionable stain or unsightly deposit on the surface. Iron and organic matter in the water are chiefly responsible for staining or discoloration and especially when concrete is subjected to prolonged wetting, even a very low concentration of these can cause staining. According to IS: 456–2000, the presence of tannic acid or iron compounds in curing water is objectionable. It is generally recommended that the seawater should not be used as mixing water for hydraulic-cement concrete works containing corrodible embedded ferrous metals, particularly in the tropics. However, under unavoidable circumstances it may be used for mixing and curing in plain concrete after due evaluation of possible disadvantages and consideration of the use of appropriate cement system.

REVIEW QUESTIONS 4.1 What are the essential characteristics of water that can be used for mixing and curing of concrete?

4.2 How does the presence of sugar and oil in water affect the concrete?

MULTIPLE-CHOICE QUESTIONS 4.1 Which of the following statements is incorrect? (a) Water is the most important and least expensive ingredient of concrete.

(b) Mixing water is utilized in the hydration of cement and provides lubrication between fine and coarse aggregates.

Concrete Making Materials—III: Water

4.2

4.3

4.4

4.5

(c) Excess water forms a scum or laitance at the surface. (d) Excess water may make concrete honeycombed. (e) None of the above For mixing water (a) suspended particles of clay and silt should be less than 0.02% (b) the quantity of calcium chloride is restricted to 1.5% (c) the pH value should generally be between 6 and 8 (d) free vegetable oil is harmful but mineral oil up to 2% is beneficial (e) All of the above If sea water is used for preparing concrete (a) it will cause efflorescence (b) it may corrode the reinforcement (c) it will reduce the ultimate strength (d) it may cause dampness (e) All of the above The vegetable oil, if present, in mixing water for concrete (a) improves strength (b) reduces strength (c) gives more slump (d) gives a smooth surface (e) improves workability due to lubrication The mineral oil, if present, in mixing water for concrete (a) reduces strength for all concentrations (b) reduces strength for the concentration of oil up to 8% (c) increases strength for the concentration up to 2%

4.6

4.7

4.8

4.9

(d) increases strength for a concentration beyond 8% (e) does not affect the strength at all The presence of sugar in water for concreting up to ______ per cent has virtually no adverse effect on the strength of concrete. (a) 0.05 (b) 0.15 (c) 0.20 (d) 0.50 (e) 1.0 Presence of 0.20 per cent sugar by weight of cement in the mixing water is likely to (a) retard the setting of cement (b) reduce the early strength of cement (c) accelerate the setting of cement (d) decrease workability (e) None of the above Which of the following impurities in the mixing water is destructive? (a) Calcium chloride (b) Lead nitrate (c) Alkalies (d) Algae (e) Sugar With regard to the curing water, identify the incorrect statement(s). (a) Curing water should not produce objectionable stains on the surface (b) The presence of tannic acid and iron compounds is objectionable (c) Iron and organic matter are responsible for staining (d) Water which is suitable for mixing is also suitable for curing (e) None of the above

Answer to MCQs 4.1 (e) 4.7 (c)

4.2 (e) 4.8 (b)

4.3 (e) 4.9 (e)

101

4.4 (b)

4.5 (c)

4.6 (a)

5 5.1

CHEMICAL ADMIXTURES AND MINERAL ADDITIVES

INTRODUCTION

Admixtures are the chemical compounds in concrete other than hydraulic cement (OPC), water and aggregates, and mineral additives that are added to the concrete mix immediately before or during mixing to modify one or more of the specific properties of concrete in fresh or hardened state. The use of admixture should offer an improvement not economically attainable by adjusting the proportions of water, cement and aggregates, and should not adversely affect the performance of the concrete. Admixtures are no substitute for good concreting practice. An admixture should be employed only after an appropriate evaluation of its effects on the performance of concrete under the conditions in which the concrete is intended to be used. It is often necessary to conduct tests under simulated job conditions in order to obtain reliable information on the performance of concrete containing admixtures. Admixtures that contain relatively large amounts of chloride may accelerate corrosion of prestressing steel. In case of reinforced concrete, to minimize the chances of deterioration of concrete, the total chloride content in the concrete should be limited as specified in IS 456–2000. Superplasticizers are expected to be chloride free. The admixtures have formulated chemical composition and special chemical action, and are used to modify certain properties of concrete. They are used primarily to reduce the cost of concrete construction; to modify the performance of hardened concrete; to ensure the quality of concrete during mixing, transporting, placing, compacting and curing; and to overcome certain emergencies during concreting operations. The properties commonly modified are that the rate of hydration or setting times, workability, dispersion and air-entrainment. The admixture is generally added in a relatively small quantity. A degree of control must be exercised to ensure proper quantity of the admixture, as an excess quantity may be detrimental to the properties of concrete. Most admixtures are supplied in ready-to-use liquid form as shown in Fig. 5.1 and are added to the concrete at the mixing plant or at the jobsite. Certain admixtures, such as pigments, expansive agents, and pumping aids are used only in extremely small amounts and are usually batched by hand from premeasured containers. The effectiveness of an admixture depends on several factors including; type and quantity of cement, water content, mixing time, slump, and temperatures of the concrete and air. The mineral additives or supplementary cementing materials, on the other hand, have no formulated chemical composition nor do they have any special chemical action distinct from pozzolana. These materials do not have any binding property

Chemical Admixtures and Mineral Additives

Fig. 5.1

103

Typical liquid admixtures for concrete, from left to right: anti-washout, shrinkage reducer, water reducer, foaming agent, corrosion inhibitor and air-entraining admixture (Adopted from Portland Cement Association)

by themselves but react with calcium hydroxide liberated on hydration of cement to produce cementing compound with good binding properties. As explained in Chapter 2, these are added in large quantities to improve performance of the concrete and reduce the cost of construction.

5.2

FUNCTIONS OF ADMIXTURES

Some of the important purposes for which the admixtures could be used are the following: 1. To accelerate the initial set of concrete, i.e., to speed up the rate of development of strength at early ages. 2. To retard the initial set, i.e., to keep concrete workable for a longer time for placement. 3. To enhance the workability. 4. To improve the penetration (flowability) and pumpability of concrete. 5. To reduce the segregation in grout and concrete mixtures. 6. To increase the strength of concrete by reducing the water content and by densification of concrete. 7. To increase the durability of concrete, i.e., to enhance its resistance to special conditions of exposure. 8. To decrease the capillary flow of water through concrete and to increase its impermeability to liquids. 9. To control the alkali-aggregate expansion or alkali-silica reactivity (ASR). 10. To inhibit the corrosion of reinforcement in concrete. 11. To increase the resistance to chemical attack. 12. To reduce the heat of hydration. 13. To increase the bond between old and new concrete surfaces. 14. To enhance the bond of concrete to the steel reinforcement. 15. To produce non-skid wearing surfaces.

104

16. 17. 18. 19.

5.3

Concrete Technology

To produce cellular concrete. To produce colored concrete or mortar for colored surfaces. To decrease the weight of concrete per cubic meter. To produce concrete of fungicidal, germicidal and insecticidal properties.

CLASSIFICATION OF ADMIXTURES

The admixtures may be broadly classified as belonging to the general category and the specialty category. According to the functions or characteristic effects produced by them, there are four distinct classes of general category chemical admixtures. 1. General-purpose admixtures The commonly used admixtures of this category are: (a) accelerating admixtures (b) retarding admixtures (c) air-entraining admixtures (d) water-reducing admixtures 2. Specialty category admixtures The admixtures of this category are: (a) grouting admixtures (b) air-detraining admixtures (c) gas-forming admixtures (d) corrosion inhibiting admixtures (e) shrinkage reducing admixtures (f) water or damp-proofing and permeability reducing admixtures (g) bonding admixtures (h) concrete surface hardening admixtures (i) coloring admixtures or pigments (j) fungicidal, germicidal and insecticidal admixtures

5.3.1

Accelerating Admixture or Accelerator

An admixture used to speed up the initial set of concrete is called an accelerator. These are added to concrete either (i) to increase the rate of hydration of hydraulic cement, and hence to increase the rate of development of strength, or (ii) to shorten the setting time. An increase in the rate of early strength development may help in (i) earlier removal of forms, (ii) reduction of required period of curing, and (iii) earlier placement of structure in service. Accelerating admixtures are also used when the concrete is to be placed at low temperatures. The benefits of reduced time of setting may include (i) early finishing of surface, (ii) reduction of pressure on forms or of period of time during which the forms are subjected to hydraulic pressure; and (iii) more effective plugging of leaks against hydraulic pressure. With the availability of powerful accelerators, the underwater concreting, the basement waterproofing operations, the repair work of the waterfront structures in the tidal zones have become easy. With proper proportion these admixtures partly compensate

Chemical Admixtures and Mineral Additives

105

for the retardation of strength development due to low temperatures in cold weather concreting. The general action of accelerators is to cause a more rapid dissolution of compounds of cement, particularly tricalcium silicate, in water and hence facilitate more rapid hydration of these compounds. The mechanism of action is catalytic in nature. The most widely used accelerator is calcium chloride (CaCl2). It is available as flakes (77 per cent CaCl2) or in the fused from (92 per cent CaCl2). It is always dissolved in a part of mixing water before use. It is the solid mass, which is reckoned in the admixture. Calcium chloride can generally be used in amounts up to two per cent by mass of cement, but IS: 7861 (Part-II)–1981 recommends a maximum of 1.5 per cent of CaCl2 for plain and reinforced concrete works in cold weather conditions. However, CaCl2 or admixtures containing soluble chlorides are not permitted to be used in prestressed concrete due to the possibility of stress corrosion. The benefits of the use of calcium chloride are usually more pronounced when it is employed for concreting at temperatures below 25°C. Calcium chloride should not be used in concrete which will be subjected to alkali-aggregate reaction or exposed to soils or to water containing sulfates, in order to avoid lowering of the resistance of concrete to sulfate attack. The use of two per cent calcium chloride by mass of cement can reduce the setting time by one-third and raise the one to seven-day compressive strength by 3 to 8 MPa. An increase in flexural strength of 40 to 80 per cent of one day and up to 12 per cent at 28 days is obtained. The selection of the optimum amount should be based on the type of cement, temperature of concrete and the ambient temperature. Large doses of CaCl2 result in flash set of concrete and also in increased shrinkage. The effect of CaCl2 on the compressive strength of concrete is shown in Fig. 5.2. Calcium formate (a fine powder), which is somewhat less soluble than calcium chloride and is less effective does not have the same adverse effect on corrosion of embedded steel as CaC12. It is added in the same dosages. Some of the accelerators containing fluro-silicates and trietholamine are capable of reducing the period during which concrete remains plastic to less than 10 minutes. An accelerator produced under the trade name ‘Quickset’ which when added to neat cement results in the setting in a matter of seconds. This makes it valuable for making cement plugs to stop pressure leaks. They are added only by a small percentage usually not exceeding 0.2 per cent by mass of cement. The other less commonly used accelerators consist of NaCl, Na2SO4, NaOH, Na2CO3, K2SO4 and KOH. In contrast to CaSO4, the effect of Na2SO4 and K2SO4 is the acceleration of hydration of cement. Rapid hydration can be achieved in the first two hours by the addition of NaOH or KOH. It has been noticed that two per cent of calcium chloride has the same effect on the acceleration of hydration as a rise in temperature of about 11°C. Most accelerating admixtures do not significantly affect rheology ( flowability) and hence the consistency of cement paste at early ages. However, at later ages due to more rapid hydration and consequent stiffening, influence the workability of fresh concrete. It does not result into any adverse chemical effect on cement concrete.

106

Concrete Technology

70 Water–cement ratio = 0.5 With 2% calcium chloride

Compressive Strength, MPa

60

No admixture Rapid-hardening pid-harden Cement

50

40

30

20 No ormal Portland Cem ment 10 0 6

12

18

1

11 2

2

3

Hours

4

5

7

10

14

26

Days Age

Fig. 5.2

5.3.2

Effect of admixtures on the compressive strength of concrete

Retarding Admixtures or Retarder

The set-retarding admixtures slow down the initial rate of hydration of cement or prolong the setting of the cement paste in concrete. They are used primarily to offset the accelerating and damaging effect of high temperature and to keep concrete workable during the entire placing period which should be sufficiently long so that the succeeding lifts can be placed without the development of cold joints or discontinuities in a structural unit. They are also used in grouting oil wells. The speeding up of hydration means that the cement for its hydration uses some of the water usually available to provide workability. Therefore, more water is required to maintain the slump at an appropriate level, which in turn, means lower concrete strength. High temperatures, low humidity, and wind cause rapid evaporation of water from the mix during summer. This drying of concrete leads to the cracking and crazing of the surface. Retarders delay setting of cement either by forming a thin coating on the cement particles and thus slowing down their dissolution in and reaction with water or by increasing the intra-molecular distance of reacting silicates and aluminates from water molecules by forming certain transient compounds in the system. With the formation of silicates and aluminate hydrates, the influence of retarders diminishes and hydration process becomes normal. Thus a retarding admixture holds back the hydration process, leaving more water for workability and allowing concrete to be finished and protected before drying out. Some of the retarding admixtures also reduce the water requirement of the mixture making further reductions possible in the water-cement

Chemical Admixtures and Mineral Additives

107

ratio. They may also entrain some air in concrete. The retarders do not affect significantly the final setting time of cement nor do they have much influence on 28-days strength. Retarders are also added in the concrete that has to be hauled long distances in transit mix trucks, to ensure that it remains in plastic and placeable condition. Ready mixed concrete technology employs retarders with an advantage, i.e., for the purpose of retaining the slump. The materials used as water-reducing and set controlling admixtures, generally called retarding plasticizer, belong to the following groups:

Soluble Carbohydrate Derivatives Admixture like sugar, water-soluble carbohydrates such as soluble starch, dextrin, etc., are effective. Very small dosage of the order of 0.05 to 0.1 per cent of mass of cement is enough. 0.05 per cent sugar can delay initial setting time by about four hours. It virtually kills setting of cement. Inorganic Retarders These are based on hydroxides of zinc and lead, alkalibi-carbonates, calcium borate, etc. Many water-reducing admixtures are also set retarding. Since the use of waterreducing admixture is widespread, set-retarders are not specifically used in many situations.

5.3.3

Air-entraining Admixtures

Air-entraining admixtures help to incorporate a controlled amount of air, in the form of millions of minute non-coalescing bubbles distributed throughout the body of concrete, during mixing, without significantly altering the setting or the rate of hardening of concrete. It is generally recognized that a proper amount of entrained air results in improved properties of plastic concrete like workability, easier placing and finishing, increased durability, better resistance to frost action and reduction in bleeding and segregation. In air-entrained concrete, the air bubbles provide space to relieve the pressure of expanding ice as shown in Fig. 5.3(a). Air-entraining agents are anionic surfactants, which are adsorbed on to the cement particles, forming a sheath of limited solubility. A weak surfactant solution forms bubbles on agitation, which stabilize as microscopic spheres. The entrained air bubbles, ranging approximately from 0.05 to 0.25 mm diameter and spaced 0.003 mm apart reduce the capillary forces in concrete. The capillaries are interrupted by relatively large non-inter-connecting air voids in air-entrained concrete. The air voids present in concrete are classified as: entrained air and entrapped air. Entrained air is intentionally incorporated in the form of minute spherical bubbles referred above. Whereas entrapped air is in the form of voids occurring in the concrete due insufficient or poor compaction. Entrapped air voids may be of any shape and size, non-uniformly distributed along the contours of aggregate surfaces. Their size is large and may range from 0.01 to 1.0 mm or more. They are a source of weakness in the concrete in terms of strength and durability. Air-entrainment, while improving durability and plasticity, may have an adverse effect on the strength of concrete. The decrease in strength is usually proportional to the amount of entrained air. The effect of increase in the percentage of entrained air on the compressive strength is shown

108

Concrete Technology

in the Fig. 5.4. For each per cent increase in air content, the compressive strength reduces approximately 1.4 MPa. Within the normal range of air content, the maximum reduction in compressive and flexural strengths rarely exceeds 15 and 10 per cent, respectively. However, the reduction of water–cement ratio and sand content made possible by air entrainment, for the given workability, compensates fo the most if not all the lost strength.

(a) 5–6 per cent total air voids

Fig. 5.3

Microscopic views of surface of a finely lapped piece of concrete with air voids

33

Maxim mum nominal size of aggregate: 40 mm m Water– –Cement ratio: 0.55 Sand proportions: 38 per cent

30

Compressive Strength, MPa

(b) 17 per cent total air voids

27

24

21

18

15 0

1

2

3

4

5

6

7

8

9

10

Entrained Air (absolute volume), per cent

Fig. 5.4

Effect of entrained air on compressive strength of concrete

The beneficial amount of entrained air depends upon the type and quantity of air-entraining agent; water–cement ratio of the mix; type, grading and maximum size of aggregates; mixing time; extent of compaction of concrete; the temperature and type of cement. The optimum percentage of air for striking a balance between compressive strength and durability is given in Table 5.1.

Chemical Admixtures and Mineral Additives Table 5.1

109

Optimum air content of concrete

Maximum size of aggregate, mm Naturally entrained

Optimal total

Sand–cement mortar

4.0

14 ± 2

10

3.0

8 ± 1.5

12.5

2.5

7.5 ± 1.5

20

2.0

7.0 ± 1.5

25

1.5

6 ± 1.5

40

1.0

4.5 ± 1.5

50

0.5

4.0 ± 1.0

70

0.3

3.5 ± 1.0

150

0.2

3.0 ± 0.5

Water–Cement ratio This is one of the important factors affecting the air content. At very low water-cement ratio, water films developed on the cement grains will be insufficient to produce adequate foaming action. At intermediate water–cement ratio (viz 0.4 to 0.6) abundant air bubbles will be produced. At a higher water-cement ratio although initially, a large quantity of air entrainment is achieved, however, a large proportion of the bubbles is lost progressively with time. The grading of aggregate has significant influence on the quantity of air entrainment. The entrained air content increases with the mixing time up to a certain limit and thereafter with prolonged mixing the air content gets reduced. The temperature of concrete at the time of mixing has significant effect on the amount of air entrainment. The entrained air content decreases with the increase in temperature of concrete. Air content is also reduced by the process of compaction, on account of the movement of air bubbles to the surface and their subsequent destruction. It is estimated that as much as 50 per cent of the entrained air may be lost after vibration for 2–2 ½ minutes and as much as 80 per cent may be lost by vibration for about 10 minutes. Similarly, the use of calcium chloride has the tendency to limit air entrainment. Thus air-entrained concrete is considerably more plastic and workable than nonair-entrained concrete. It is observed that the placeability of air entrained concrete having 75 mm slump is superior to that of non-air entrained concrete having 125 mm slump. This easier placeability of a lower slump concrete should be recognized with respect to concrete construction in difficult situations. The durability of hardened concrete is improved by increased uniformity, decreased absorption and permeability, and by elimination of planes of weaknesses at the top of lifts. Thus there is considerable increase in the resistance to freezing and thawing and to the disruptive action of de-icing salts. During freeze cycles, the pressure exerted by the expanded volume of ice is taken up by the air bubbles acting like tiny springs and during thaw cycles these bubbles revert back to their original size. As suggested by Blanks, the resistance of concrete to freezing and thawing can be measured by means of durability factor which is defined as the number of cycles of freezing and thawing to produce

110

Concrete Technology

–

failure divided by 100. The air-entraining agents also find very useful applications in making cellular concrete and lightweight aggregate concrete. As mentioned above, air-entrained concrete contains microscopic bubbles of air formed with the aid of chemicals called surfactant or surface-active agents. These materials have the property of reducing surface tension of water enabling the water to hold air when agitated, resulting in foam. The structure of air-entrained concrete is shown in Fig. 5.5. A satisfactory air-entraining agent must not react chemically with cement. It must be able to produce air bubbles of a definite size, which must not break too rapidly. These entrained air bubbles constitute a definite part of fine aggregate and lubricate the concrete. The bubbles act like flexible ball-bearings to help increase the mobility of concrete by reducing friction between the particles, i.e., they modify the properties of fresh concrete with regard to its workability, avoidance of segregation and bleeding due to improved cohesion, and finishing qualities of concrete.

Cement particle

–

+ –

– +

+ +

Air

+

Air

+

–

–

–

+

–

–

–

+ –

–

Cement – + particle

–

+

+

–

–

Air

+

–

+

– +

–

–

+

+

– Cement particle

+ – Chain

+

+

–

–

+

–

–

–

+

+ – +

Cement + – particle – + + +

+ –

–

–

–

–

+

+

Negative ion

Fig. 5.5

Structure of air-entrained concrete

Effect on Segregation, Bleeding and Laitance It should be realized that the segregation and bleeding of concrete are different manifestations of loss of homogeneity. Segregation usually implies separation of coarser aggregate from mortar or separation of cement paste from aggregates. Bleeding is the autogenous flow of mixing water within, or its emergence to the surface from freshly placed concrete, usually, as a result of sedimentation of the solids due to compaction and self-weight of the solids. Bleeding results in the formation of a series of water channels some

Chemical Admixtures and Mineral Additives

111

of which may extend to the surface. A layer of water may emerge at the surface of the concrete, often bringing some cement with it. The formation of this layer of neat cement particles in called laitance. Segregation, bleeding and consequent formation of laitance are reduced greatly by air entrainment. These reductions are probably due to the incorporation of the system of air bubbles. Firstly, the bubbles buoy up the aggregates and cement, and hence reduce the rate at which sedimentation occurs in the freshly placed concrete. Secondly, the bubbles decrease the effective area through which the differential movement of water may occur. Thirdly, the bubbles increase the mutual adhesion between cement and aggregate. Lastly, the surface area of voids in the plastic concrete is sufficiently large to retard the rate at which water separates from the paste by drainage.

Effect on Permeability Greater uniformity of air-entrained concrete due to enhanced air content, modified porestructure, reduced water channels due to reduction in bleeding, are some of the reasons for the improvements in permeability characteristics.

Effect on Chemical Resistance In view of the lower permeability and absorption, the air-entrained concrete has greater resistance for chemical attack than the normal concrete. It has been reported that air-entrainment reduces the alkaliaggregate reaction.

Effect on sand, water and cement contents The minute spherical air bubbles act like fine aggregates and enable the reduction of sand. The reduction of fine aggregate further enables the reduction of water requirement without impairing 170 Maxim mum nominal size: 40 mm m Slump p range: 25–125 mm m Avera A age slump: 75 mm

Water Content, kg/m

3

163

156

149

142

135 0

Fig. 5.6

1

2

3 4 5 6 7 Entrained Air (absolute volume), per cent

8

9

Reduction in water content with the increase of entrained air in the concrete

112

Concrete Technology

the workability and slump. The effect of entrained air on the water content for a given slump is shown in the Fig. 5.6. The water requirement of an average concrete mix is reduced approximately by 3.5 kg/m3 with rounded aggregate and 4.5 kg/m3 with angular aggregates for each per cent increase in the air content. The reduction in water–cement ratio naturally affects the basic increase in strength and durability due to the non-availability of excess water for the formation of bleeding channels through the matrix of concrete.

Unit Weight For the same workability and strength, the air-entrained concrete contains approximately five per cent less solid material, and hence has lower density. Usually, the desirable entrained air content in concrete is three to six per cent. The abrasion resistance of concrete pavements with this degree of air-entrainment should be satisfactory. The air content of fresh concrete can be measured by gravimetric; volumetric; and pressure methods. The compounds used for air-entrainment are a number of natural wood resins containing abietic and pimeric acid salts, various sulfonated compounds, and some animal and vegetable fats and oils such as tallow, olive oil and their fatty acids such as stearic and oleic acids. The air-entraining agents made from modified salts of a sulfonated hydrocarbon tend to plasticize a concrete mix. These are particularly useful where aggregates, which tend to produce harsh concrete or natural sand deficient in fines, are used in producing concrete. Another type of air-entraining admixture made from neutralized vinsol resin is used in mass concrete and concrete used in highway pavements. Other admixtures used in conjunction with air-entraining agents significantly affect the amount of entrained air. The use of fly ash and other similar fine materials in concrete also reduce the amount of entrained air. Thus an increased quantity of airentraining agents will be required. When air-entraining agents are used in conjunction with water reducers, a 50 to 60 per cent reduction in quantity of air-entraining agent can be made.

5.3.4 Water Reducing Admixtures When water is added to a grout, mortar or plaster or concrete mixture the cement and other fines in the mix such as, fly ash, silica fume, rice husk ash and stone dust flocculate or clump together. The flocculated fines cause an increase in viscosity by entrapping a part of the water and by physically resisting the flow. To reduce the viscosity (a functional effect of workability) to the desired level, it may be necessary to add more water. Water is added up to a certain point, beyond which the intended plastic and hardened physical properties of the mixture are compromised. To achieve the desired workability and hardened physical properties it is often necessary to add a water reducer to disperse or deflocculate the system and reduce the amount of water to be added during mixing. The organic or combinations of organic and inorganic substances to achieve these objectives are termed as plasticizing admixtures. Water-reducing admixtures enable a given fresh concrete mix to have higher flowability (workability) without increasing the water content which results in faster

Chemical Admixtures and Mineral Additives

113

rate of concrete placement, easy placement in relatively poorly accessible locations without vibration, true shutter finish for highly reinforced concrete members, and reduction in cement content. Benefits of water reduction in hardened state of concrete are increased strength, density, durability, volume stability, abrasion resistance, reduced permeability and cracking. The specific effect of water-reducing and setcontrolling admixtures vary with the type of cement, water–cement ratio, mixing temperature, ambient temperature and other job conditions, and therefore, it is generally recommended that the admixture used be adjusted to meet the job conditions. A good plasticizer fluidises the mortar or concrete in a manner different from that of the air-entraining agents. Many of the plasticizers, while improving the workability, also entrain some air. Since the entrainment of air reduces the mechanical strength, a good plasticizer is the one, which does not entrain air more than one to two per cent.

Action of Plasticizers The action of plasticizers is mainly to fluidify the mix and improve the workability of concrete, mortar or grout. The mechanisms involved are: 1. Dispersion 2. Retarding effect 1. Dispersion Portland cement, being in fine state, will have a tendency to flocculate in wet concrete. This flocculation entraps certain amount of water used in the mix and thereby all the water is not freely available to fluidify the mix. When plasticizers are added, they get adsorbed on the cement particles. The adsorption of charged polymer on the cement particles creates repulsive forces between particles, which overcome the attractive forces. This repulsive force is called Zeta Potential, which depends on the base, solid content, quantity of plasticizer used. The overall result is that the cement particles are deflocculated and dispersed. When cement particles are deflocculated, the water trapped inside the flocs gets released and becomes available to fluidify the mix. Moreover, in the flocculated state there is interparticle friction between particle and particle, and floc and floc. But in the dispersed state due to presence of water in between the cement particles the interparticle friction is reduced. 2. Retarding effect As mentioned earlier, the plasticizer gets adsorbed on the surface of cement particles and forms a thin sheath, which inhibits the surface hydration of cement as long as sufficient plasticizer molecules are available at the particle-solution interface. The quantity of available plasticizers progressively decreases as the polymers get entrapped in hydration products. One or more of the following mechanisms may take place simultaneously: (a) Reduction in the surface tension of water. (b) Induced electrostatic repulsion between particles of cement. (c) Lubricating film between cement particles. (d) Dispersion of cement grains, releasing water trapped within cement flocs. (e) Inhibition of the surface hydration of the cement particles, leaving more water to fluidify the mix.

114

Concrete Technology

(f) Change in the morphology of the hydration products. (g) Induced steric hindrance preventing particle-to-particle contact. Generally an admixture capable of reducing water requirements by more than five per cent is classified as water reducer or plasticizer. Depending upon the degree of water reduction, the water reducers are categorized as: normal water reducer, midrange water reducer and high-range water reducer or superplasticizer. The normal water reducer reduces water content by 5 to 10 per cent. The mid-range water reducers reduce water content by about 10 to 15 per cent and tend to be more stable over a wider range of temperatures. Mid-range water reducers provide more consistent setting times than normal water reducers. Higher water reductions, by incorporating large amounts of these admixtures, result in undesirable effects on setting, air content, segregation, bleeding, and hardening. A new class of water reducers, chemically different from the normal and mid-range water reducers and capable of reducing water content by about 20 to 40 per cent has been developed. The admixtures belonging to this class are popularly known as high-range water reducers (HRWR) or Superplasticizers. These can be added to a concrete mix having a low-to-normal slump and water–cement ratio to produce high-slump flowing concrete. Flowing concrete is a highly fluid but workable cohesive concrete that can be placed homogeneously with little or no vibration or compaction. The effect of superplasticizers lasts only for 30 to 60 minutes, depending on its composition and dosage, and is followed by a rapid loss in workability. As a result of the slump loss, superplasticizers are usually added to concrete at the job site.

Mid-range Water Reducer The mid-range water-reducing admixtures can be categorized as: 1. Derivatives of lignosulfonic acids and their salts (e.g., Ca, Na, NH4 salts) 2. Hydroxylated carboxylic acids and their salts 3. Modifications and derivatives of hydroxylated carboxylic acids and their salts 4. Processed carbohydrates The lignosulfonates and carboxylic acids derivatives and their salts are water reducing and set-retarding admixtures, and they are known to reduce setting times by two to four hours and water requirement by 8 to 15 per cent. The compressive strength at two or three days is usually equal to, or little higher than that of corresponding concrete without the admixture and the strength at 28 days or later may be 10 to 20 per cent higher. These may be used with accelerating or retarding admixtures. Calcium sulfate (gypsum), sugar and carbohydrates also retard the set. The carbohydrate derivatives and calcium lignosulfonate are used in fractions of a per cent by mass of the cement. The dosage of hydroxylated carboxylic acid derivatives ranges from 0.1 to 0.2 per cent by mass of cement. These admixtures are more effective than lignosulfonates in mixes of higher cement contents (say in excess of 350 kg/m3). They are fairly insensitive to variation in cement composition. On the other hand, modified lignosulfonates are more effective in concrete with relatively low cement contents and dosage would vary from 0.1 to 0.3 per cent for sodium

Chemical Admixtures and Mineral Additives

115

lignosulfonate and 0.3 to 0.5 per cent for calcium lignosulfonate. Concretes with lignosulfonate admixture have a tendency to entrap additional air in the range of 0.5 to 2.0 per cent by volume of concrete and is amenable to thorough compaction with a lower tendency for bleeding.

High-range Water Reducers or Superplasticizers These admixtures are principally surface reactive agents (surfactants). They confer negative charge on individual cement particles (and also its hydrated particles) such that they are kept in a dispersed or suspended state due to inter-particle repulsion. Thus they confer high mobility to the particles. Superplasticizers enable the optimization of water content or water–cement ratio and workability. Both the functional effects—providing enhanced plastic and hardened physical properties—are achieved simultaneously by the use of superplasticizer. An ideal superplasticizer is cost effective and reliable dispersant which produces a cohesive low viscosity rheology without increased tendency to segregate, bleed and foam, with little interference with hydration, and compatible with different cement types and with other commonly used chemical and mineral additives. A simple way of utilizing the superplasticizer is to proportion the ingredient of the mixture to produce the required hardened physical properties and then add sufficient superplasticizer to achieve required consistency or workability.

Technical Performance The performance of concrete is normally optimized in one of the following four ways: 1. To produce flowing concrete Superplasticizers when added in small quantities to normal concrete mix impart very high workability of flowing consistency (slump ≥ 200 mm) and produce self-compacting or self-leveling concrete, wherein no attempt is made to either reduce the water–cement ratio or cement content. Instead, the aim is to achieve high slump without causing any segregation or bleeding so that concrete can be efficiently placed in heavily reinforced concrete sections. 2. To produce concrete with very low water–cement ratio The water requirement of given concrete mix can be substantially reduced, while maintaining the workability at the desired level. By this approach, a water reduction up to 30 to 40 per cent can be achieved and concrete with water–cement ratio as low as 0.25 can be produced. This method is used to produce high strength and durable concrete. 3. To produce high performance concrete A very strong composite material consisting of a stable and dense hydrated cement pastes which bonds very strongly to the aggregates and the reinforcing steel can be produced. This is achieved by improving hardened concrete performance by reducing water content and enhancing workability of plastic concrete by adding superplasticizer, i.e., improving both plastic and hardened concrete properties. This concrete mix has better than normal workability and lower than normal amount of water. 4. To produce concrete mix with reduced cement content Superplasticizers can be used to produce concrete with reduced cement content while maintaining the water–cement ratio and workability at the required levels. The strength

116

Concrete Technology

and other properties are not affected due to enhanced performance of cement constituents in the presence of superplasticizers. The water content in this case is reduced in proportion to cement content. This method with reduced cement content in the concrete mixes is pertinent in the context of economy and sustainable development of concrete industry. All the changes listed above are essential to produce a truly high performance concrete (HPC) characterized by low water–cement ratio and high workability level without high cement content.

Classification of Super Plasticizing Admixtures Currently available superplasticizing admixtures are water-soluble micro molecular organic compounds of high molecular weight, some being synthetic and other derived from natural products. These can be grouped broadly into four categories according to their chemical composition, each giving its own characteristics to the concrete as: sulfonated melamine-formaldehyde condensates (SMF), sulfonated naphthalene–formaldehyde condensates (SNF), modified lignosulfonates (MLS), and co-polymers containing sulfonic and carboxylic groups. These admixtures do not entrain a significant amount of air as they do not markedly lower the surface tension of pore water of concrete with respect to conventional normal and mid-range plasticizers and they can, therefore, be used in high proportions. The main characteristics of superplasticizer of different categories are given below.

Category A: Sulfonated melamine–formaldehyde condensates (SMF) Sulfonated melamine-formaldehyde condensates or poly-melamine sulfonates (PMS) having molecular weight in the range of 20 000 are a family of sulfonated superplasticizers that are widely used in the concrete industry. These condensates are usually employed in the form of sodium salts, which are easily soluble in water. This category of superplasticizers is the nearest to ideal one as they do not interfere with hydration of cement, i.e., they have very little effect on set, even at high dosage and do not have any tendency to entrain air. The dosage can be up to three per cent by mass of cement, beyond which beneficial effect is minimized. In water-reduced concrete, this category results in a fairly rapid loss of workability and the set time may be accelerated by about 30 to 40 minutes over that of normal mix of equal workability. 24-hour strength is typically in excess of 150 per cent of that of the normal concrete. The workability of a flowing mix may fall to a 70 mm slump in less than 15 minutes at 40 °C. It is therefore, preferable to add this category of superplasticizer directly into the ready mix truck at the job site and then place the concrete as quickly as possible. If workability is lost before placing, a second dose of this admixture may be added to restore workability without significant loss of mechanical properties of the hardened concrete. This procedure is not normally recommended with other categories of superplasticizer. Category-A superplasticizers, which tend to reduce air entrainment, result in a mix which may be more prone to bleeding, and segregation, so a higher than normal sand content is desirable. Vibration should, therefore, be kept to a minimum. This type of superplasticizers should preferably be used for: low temperature concreting, and where high early strength is required.

Chemical Admixtures and Mineral Additives

117

Category B: Sulfonated naphthalene–formaldehyde condensates (SNF) The most widely accepted compounds of this group are the poly-B-naphthalene sulfonates, having molecular weight of about 2000. These materials have a significant effect on surface tension and are to be used with defoaming materials. These condensates are employed in the form of sodium salts for their easy solubility in water. This category of superplasticizers gives not only slightly greater levels of set retardation and air entrainment than those of category-A, but also gives significantly larger periods of workability retention. This makes it possible for the admixture to be dosed at a ready mix plant prior to trucking to the site. The increase in the level of air entrainment is too low to affect the cohesiveness of the mix so a high sand content is desirable with the high workability mixes to prevent bleeding and segregation. The longer period of workability retention coupled with a set retardation of 20 to 40 minutes in high strength low water content mixes, makes this category of superplasticizers very effective for pre-cast concrete.

Category C: Modified-lignosulfonates (MLS) The lignosulfonates are naturally occurring macromolecular organic compounds. The crude lignosulfonate is the waste liquid product obtained during the process of production of paper making pulp from wood. These liquids contain a complex mixture of various carbohydrates (sugars), free sulfurous acid or sulfates. They are commonly used as plasticizers. These lignosulfonates are refined and modified by removing sugars and other undesirable impurities, which cause excessive, set retardation. The molecular weight varies from few hundred to 100 000. The higher molecular weight lignosulfonates, however, possess very useful properties. Sodium or calcium or alkali metal (Na, Ca, NH4) salts are employed in the admixture. Sodium-based salts may contain less sugar and they dissolve more readily in water even in harsh winter and are more active as surfactants. But calcium based salts are less expensive. The alkali-based salts have superior water reducing capability and are quite efficient at low dosage. This category of superplasticizer gives greatest workability retention and is therefore very effective at high ambient temperatures or where long trucking distances are to be covered or where placing delays may occur. Conversely they give the most set retardation and, therefore, generally give the lowest 24-hour strengths. An increase of one to two per cent in the level of entrained air in the mixture is obtained. This usually obviates the need to increase the sand content of high workability mixes and reduces the likelihood of segregation and bleeding. When category-C superplasticizers are used to produce a water-reduced mixture, the mix may become over cohesive. This prevents the full water reducing potential of the admixture from being realized. In such a situation, the sand content of the mix can often be reduced by three to five per cent. Category D: Carboxylated acrylic ester Co-polymers (CAEC) With traditional superplasticizers of melamine or naphthalene sulfonated formaldehyde formulations the dramatic increase in flowability is not long lasting and point of addition of admixture is important. These factors prove to be major drawbacks in case

118

Concrete Technology

of delay in placement, long haul of concrete and hot weather concreting conditions. CAEC admixtures overcome these placement drawbacks. The acrylic polymers (AP) are normally used as active ingredients of this category of superplasticizers. Besides producing a concrete mix with a lower slump loss, AP-based superplasticizers perform better than the traditional sulfonated polymers even in the terms of higher reduction in water–cement ratio at a given workability or higher slump level for a given mix composition. However, AP- superplasticizers are more expensive than the others and hence not commonly used.

Mechanism of Workability Enhancement Portland cement and fines particles have a strong tendency to flocculate when mixed with water. The flocculation process leads to the formation of an open network of particles. The network voids trap a part of the water, which is then unavailable for surface hydration of cement particles and for the fluidification of the mixture. These effects result in stiffening or increase in apparent viscosity of the cementing system. To achieve a homogeneous distribution of the water and the optimal water cement contact, the cement particles must be properly deflocculated and kept in a state of high dispersion. Due to the dispersion effect, the fluidity in the cement mixture is increased. The water-reducing admixtures perform their function by deflocculating the agglomerations or lumps of cement grains. In the normal stage, the surface of cement grains contain a combination of positive and negative charges. As they are agitated and bump into each other, they are repelled if like charges approach each other and attracted if unlike charges approach. On the other hand, superplasticizers consist of very large molecules (colloidal size), which dissolve in water to give ions with a very high negative charge (anions). These anions are adsorbed on the surfaces of the cement particles in sufficient number to form a complete monolayer around them causing them to become predominantly negatively charged. Thus they repel each other and flocs do not form. In doing so, water trapped within the original flocs is released and can then contribute to the mobility of the cement paste and hence to the workability of the concrete. Representation of superplastisizer molecule and its mode of adsorption on cement grains is shown in Fig. 5 7. Thus the attractive forces existing among cement particles and causing agglomeration would be neutralized by the adsorption of negatively charged anionic polymers such as SMF or SNF, due to the presence of SO–3 groups on the surface of cement particles. Superplasticizers are preferentially adsorbed in substantial amount first by C3A and C4AF, and then by C3S and C2S, i.e., adsorption depends upon the type and grade of cement. The cement that has a higher C3A content requires more dosage of superplasticizer to achieve the same level of workability. The dispersion of Grade 43 cement is maximum when mixed with a sulfonated melamine–formaldehyde liquid superplasticizer as compared to Grades 33 and 53. Ordinary Portland cement results in more workable concrete mix than the Portland–pozzolana cement and the workability retention is also for more time in case of OPC. Increased workability by the addition of superplasticizer depends to some extent on the characteristics of superplasticizer used. However, the mix composition, the variability in cement composition or properties, and other factors such as mixing procedure and

Chemical Admixtures and Mineral Additives

119

equipment used often play an important role. Some of the important factors affecting the efficiency of the superplasticizer are: type of superplasticizer, dosage level of the admixture, cement (type and content), slump loss with time, and mode of addition. 1. Type of superplasticizer The average molecular mass of the superplasticizer is of prime importance for its effectiveness in reducing water content in Portland cement mixes. The higher the molecular mass the higher is the efficiency. The viscosity of concrete mix, a property that greatly influences product performance, i.e., fluidification of cementing system, reflects the average molecular mass of the polymer. It should be noted, however, that there is a maximum value of molecular mass beyond which the dispersion effect is decreased. Chemical nature of the superplasticizer, whether naphthalene or melamine based, can also have an effect on the rheological behavior of a concrete mix.

Organic molecule

+ +

+

Wateryy she ell e + + + +

+

+

– –– – – – – – – – – – – –– – – – – –

+

+ +– + + –– + + + + – + – + + –– + + + –– Cement + + + particle +– + + + – + + – + + + + + – + + + + + + + + Water + + + + Molecule of shell superplasticizer – –– – – – – – – – –

Negative ions + + +

– – – – – – – – – – – – – – –

+ + + atery shell h ll + Wa

Positive ion + + + + + + +

(a) Superplasticizer molecule

– – – –

–

–

–

–

–

Inter particle repulsion

–

–

– –

–

–

–

–

– –

–

Released water

–

Cement grain

Cement particle floc (b) Effect of superplasticizer on cement particle floc

Fig. 5.7

Representation of superplasticizer molecule and mode of adsorption on cement grains

120

Concrete Technology

2. Dosage level of admixture The workability in terms of slump increases with the increase in amount of superplasticizer for the same water–cement ratio. The effectiveness in terms of reduction in viscosity, however, does not continue beyond a particular dosage after which any addition of superplasticizer does not significantly reduce the viscosity of the slurry. This point has been called the saturation point. A typical curve showing the variation of slump with the dosage of superplasticizer is shown in Fig. 5.8. In fact, it could be detrimental to use a higher dosage, as with excessive amount of superplasticizer, the aggregates and cement particles begin to segregate. 240 Cement content: nt: 300 kg/m Water–cement ra atio: 0.6

220

3

Slump, mm

200 180 160 140 120 100 0

0.2

0.4

0.6

0.8

1

Superplasticizer dose, per cent

Fig. 5.8

Superplasticizer dose vs workability in terms of slump

The dosage of plasticizers normally do not exceed 0.25 per cent by weight of cement in case of lignosulfonates, or 0.1 per cent in case of carboxylic acids, the plasticizers of types SMF or SNF require considerably high dosages (0.5 to 3.00 per cent), since they do not entrain air. The modified lignosulfonatebased admixtures, which have effective fluidizing action, at the relatively high dosages, can produce undesirable effects, such as unduly large accelerations or delays in the setting times. Moreover, they increase the air-entrainment in concrete. Plasticizers or superplastizers at nominal dosage can only fluidise a mix with an initial slump of about 20 to 30 mm. A high dosage is required to fluidify no slump concrete. An improvement in slump can be obtained to the extent of 250 mm or more depending upon the initial slump of the mix, the dosage and cement content. The dosage of superplasticizer influences the viscosity of cement matrix and hence the workability of concrete. The optimum dosage can be ascertained

Chemical Admixtures and Mineral Additives

121

from Marsh cone test if the brand of cement, plasticizer and water–cement ratio is predetermined. Simple Marsh test cone can give realistic dosage when instructions given by manufacturers are general in nature. 3. Influence of cement composition The finer the cement, the higher the superplasticizer dosage required to achieve a given workability, i.e., higher the cement fineness, the lower is the fluidizing effect. Among the cement constituents, which exert major influence on the properties of superplasticized mixes are: the C3A content and its morphology, the alkali content, and the form of calcium sulfate added to the clinker. It is not the total amount of SO3– in the cement that is important, but rather the availability or the rate of dissolution of SO4 2– ions, that must be balanced with chemical reactivity of C3A. In the presence of alkali sulfate, the adsorption of superplasticizer on C3A and C4AF is inhibited, leading to increased absorption on C3S and C2S particles. Since the silicate phase adsorbs a much lower amount of polymer than the aluminate phase does, an increase in the alkali content of the cement causes a reduction in the total amount of polymer adsorbed on cement and this results in a higher availability of polymer in the aqueous phase to promote dispersion and reduction of the viscosity of the cement paste. 4. Mode of addition The SMF or SNF-based superplasticizers are able to transform a very low slump concrete into a self-leveling mix with a slump increase of about 200 mm. However, the method of addition of these superplasticizers affects the magnitude of slump enhancement. An immediate addition procedure (superplasticizer introduced with gaging water) produces a less workable mixture than that obtained by the delayed addition of same superplasticizers (after an initial mixing period of say one minute). 5. Mixing procedures Plasticizer must be properly and intimately mixed in the concrete to bring about proper dispersion with cement particles. Therefore, hand mixing is not advised. While using a concrete mixer, generally about 80 per cent of the total water is added to the empty drum before loading the material into the drum by hopper. In case of superplasticizer, it is better to add all the water to drum keeping about one litre of water in spare. The required quantity of superplasticizer is diluted with the spare water and added into the drum in two or three instalments over the well-mixed concrete so that proper dispersion of plasticizer takes place. After adding the plasticizer, the concrete must be mixed for about one more minute before discharging.

Site Problems in the Use of Superplasticizers Some of the commonly encountered site problems in the use of superplasticizers are: lack of knowledge of slump of reference mix (i.e., concrete without plasticizer), inefficient laboratory mixer for premix trials, problem with crusher dust, compatibility with cement, selection of plasticizer and superplasticizer, determination of dosage, and slump loss. As was mentioned earlier, very stiff or zero slump concrete cannot be perceptibly improved at nominal dosage. Although there is improvement in rheology of such a matrix with the use of superplasticizers, but it does not become perceptible and measurable by slump test. If the concrete mix is designed in such a way as to have about 20 to 30 mm initial slump, then only the slump could be enhanced to a high level.

122

Concrete Technology

Generally, available laboratory mixes are inefficient especially when small quantity of plasticizers is used for trial mix. Use of pan mixer gives better results. The dust interferes with plasticized properties of mix and hence the desired results may not be obtained. 1. Slump loss with time The slump loss is unavoidable because of the intrinsic characteristic of cement mixes, which make them to set and harden in a relatively short time. Loss of slump with time for both the normal and superplasticized concretes having same initial slump is compared in Fig. 5.9. When a concrete mixture has to be transported over a long distance, particularly in hot whether, it should maintain as far as possible the initial slump level to avoid the practice of redosing the concrete with water above that required in the mix design. The slump loss can be controlled or managed by taking recourse to any one or more of the corrective actions—providing high initial slump, by using retarders, by using retarding plasticizer or superplasticizer, by repetitive dose, by dosing at final point, by keeping temperature low, and by using superplasticizer which is compatible with the cement. Even if loss of slump occurs for the concretes having very high slump at the mixing point, the residual slump may still be good enough for satisfactory placement. 300 Ambient temperature: 20°C a: Normal mix with water–cement ratio of 0.6 b: Superplasticized mix with water–cement rattio of 0.45 (0.4 % dry SNF)

250

Slump, mm

200

150

100 a 50 b 0 0

30

60

90

120

Time, minutes

Fig. 5.9

Slump loss with time for the normal and superplasticized concrete mixes having same initial slump

Retarders may be used at the time of producing ready mixed concrete (RMC), which will keep the concrete in a plastic condition over a long transit time. Just before discharging, addition of an appropriate dose of plasticizer or superplasticizer will give desired slump for placing requirements. Usually a small dose of superplasticizers is added initially to boost up the slump and

Chemical Admixtures and Mineral Additives

123

when the concrete arrives at the pouring point, if the residual slump is not good enough for placing, an appropriate dose of superplasticizer is added to boost up the slump to the required level. One of the common methods to combat the slump loss is to give repetitive doses at intervals to boost up the slump so that required slump is maintained for long time. Keeping the temperature of the concrete low can retard the hydration process. The commonly encountered problem of incompatibility of plasticizer with the cement can be handled by cross testing of cement with other plasticizers or vice versa. In case of AP-based superplasticizer both the steric hindrance effect and the electrical repulsive force due the negative carboxylic groups would be effective for the dispersion of the cement particles and fluidizing action of the admixture. The low slump loss effect of this superplasticizer is due to the protruding side chains of the acrylic polymer, which prolong the state of dispersion of hydrated cement particles through a steric hindrance effect. 2. Air content In order to improve the durability particularly freezing and thawing resistance of concrete, air voids of proper size and spacing have to be present in the grout. When superplasticizers are used without an airentraining agent, very few air voids are created in concrete. On the other hand, when superplasticizers are used with air-entraining agents, an increase in spacing factor is sometimes observed due to the possibility of escape of air from the fluidized concrete and the coalescence of part of the small air voids. This may destabilize the air-void-system in the flowing concrete significantly. The total air content of air-entrained concrete may decrease with the addition of SNF or SMF-based admixtures and increase with MLS based admixture. 3. Segregation and bleeding After the introduction of an excessive dosage of superplasticizer beyond the saturation point, the cement paste may become too fluid and may no longer maintain the coarse or even the fine aggregates in suspension, causing severe segregation. 4. Compatibility problems In many situations, compatibility of plasticizer with cement becomes primary consideration. Simple Marsh cone test can be used to check the compatibility of plasticizer with the cement and to determine the optimum or economical dosage. Admixtures that modify the properties of fresh concrete may cause problems through early stiffening or undesirable retardation of the time of setting. Early stiffening is often caused by the change in the rate of reaction between tricalcium aluminate (C3A) and sulfate (SO42¯). Retardation can be caused by an overdose of admixture or by lowering of ambient temperature, both of which delay the hydration of calcium silicates of the cement. High performance concrete (HPC) with its requirement of lower and lower water–cement ratio and consequent much higher superplasticizer dose accentuates cement–superplasticizer incompatibility problems in the form of very rapid slump loss. The major cement and admixture factors that influence the rheological behavior in HPC are: tricalcium aluminate (C3A) content, Blaine fineness of cement and the solubility of the gypsum present in the cement; the monomer content and the molecular mass fractions.

124

Concrete Technology

5.3.5 Grouting Admixtures The grouting has become one of the most important operations in civil engineering construction. Grouting below the base plate or machine foundations, grouting of foundation bolt holes in industrial structures, grouting of prestressed concrete ducts, grouting in anchoring and rock bolting systems, grouting of curtain walls, grouting of fissured rocks below dam foundation, grouting the body of the newly constructed dam itself, grouting of deteriorated concrete or fire affected structures for strengthening and rehabilitation, grouting of oil wells are some of the few situations where grouting is extensively used. The grout material should have high early and ultimate strength, should be free flowing even at low water content, should develop good bond with previously set or hardened concrete, essentially it should be non-shrink in nature. The grouting materials can be broadly classified into two categories. One is free flow grout for use in machine foundations, foundation bolts and fixing crane rails, etc. The second category of grout is meant for injection grouting to fill up small cracks, and is normally accomplished under pressure. Some retarders are especially useful in cement grout slurries, particularly where the grouting is prolonged, or in the cases where the grout must be pumped for a considerable distance, or where hot water or high temperature is encountered underground. Cement grouts containing pozzolanic materials are often used in cementing oil wells. Admixtures are also used to prevent the rapid loss of water from cement paste to the surrounding formation. Some of the grouting admixtures are gels, clays, pregelatinized starch and methylcellulose.

5.3.6

Air-detraining Admixtures

These materials are used to 1. dissipate excess air or other gases, and 2. remove a part of the entrained air from a concrete mixture. A number of compounds, such as tributyl-phosphate, dibutylphathalate, waterinsoluble alcohols and silicones have been proposed for this purpose. However, tributyl- phosphate is the most widely used air-detraining agent.

5.3.7 Gas-forming Admixture These admixtures when added to mortar or concrete mixture react chemically with hydroxides present in the cement and form minute bubbles of hydrogen gas of size ranging from 0.1 to 1 mm throughout the cement–water matrix. This action, when properly controlled, causes a slight expansion in plastic concrete or mortar and thus reduces or eliminates voids caused by normal settlement that occur during the placement of concrete. Water films around the gas bubbles prevent bleeding. The gas is beneficial in improving the effectiveness of grout for filling joints, in improving the homogeneity of grouted concrete, and in filling block outs and openings in concrete structures. For example, the voids on the underneath and sides of forms, block outs on reinforcing steel or other embedded parts may interfere with the bond and

Chemical Admixtures and Mineral Additives

125

allow passage of water, and reduce uniformity and strength. The gas largely reduces bleeding, which would cause settlement shrinkage. This improves the intimacy of contact (bond) of the paste with adjacent concrete or aggregate particles as well as embedded steel reinforcing bars. Aluminum powder may be used as the gas-forming admixture. The amount of powder added usually varies from 0.005 to 0.02 per cent by mass of cement. Zinc and magnesium powders are also used for this purpose while hydrogen peroxide and bleaching powder can be used in combination to produce oxygen instead of hydrogen bubbles in the concrete. The effect on the strength of the concrete depends to a large extent on the restraint offered to expansion. With complete restraint imposed, the strength is not affected appreciably with very small amounts of aluminum powder. Larger amounts of powder increase the expansion appreciably resulting in a gas-filled lightweight low strength concrete. These are also called foamed concrete or aerated concrete or cellular concrete. These concretes are very light and are often used for thermal insulation. These concretes are described in Section 16.5.

5.3.8 Corrosion-inhibiting Admixtures Corrosion-inhibiting admixtures are used to slow down corrosion of steel reinforcement in concrete. They are used as a defensive strategy for concrete structures constructed in marine facilities, highway bridges, and in industrial environment where reinforced cement concrete is exposed to high concentrations of chloride. Compounds, such as sodium benzoate, sodium nitrate, etc., can be used as corrosion-inhibiting admixtures. A two per cent benzoate solution in mixing water may be used to prevent corrosion of reinforcement. Sodium nitrate has been found to be effective in preventing corrosion of steel in concrete containing calcium chloride.

5.3.9 Shrinkage Reducing Admixtures The shrinkage reducing, also called expansion-producing admixtures, either expand themselves or react with other constituents of concrete resulting in expansion. This expansion may be of about the same magnitude as the drying shrinkage at later ages or may be little greater. This concept has been used in the development of nonshrinking cement wherein the expansion-producing compound is mixed with cement in appropriate proportion to get the desired expansion or shrinkage compensation. Higher proportion of expansion-producing admixture is employed to produce selfstressing cements. Shrinkage compensating type expansive cement is capable of developing 0.03 to 0.10 per cent restrained concrete expansion. The high expansion self-stressing cement is generally capable of developing up to 0.25 per cent restrained concrete expansion and can attain stress-levels up to 7 MPa. This will be adequate to produce prestressed (precast) concrete members. Expansive cements have greater water demand than OPC. Larger water content gives enhanced workability to fresh concrete, better pumpability and easier finishing characteristics. However, to compensate for serious slump loss in hot weather a small dosage (0.05 per cent) of citric acid can be used as a retarder.

126

Concrete Technology

A number of expansion producing agents have been reported, such as granulated iron and chemicals, and anhydrous sulfoaluminate, etc. Granulated iron and chemicals promote oxidation of iron resulting in the formation of iron oxide, which occupies an increased solid volume. These admixtures are employed in laying heavy machine foundations, patching, production of shrinkage-compensating concrete which is free from shrinkage cracks, and production of self-stressing and pre-stressed concretes, grouting the ducts of post-tensioned members, grouting foundation holes, cast-in-situ joints of precast construction, and for introducing self-stress in the concrete. Shrinkage compensating expansive cements are particularly useful in avoiding cracking in large surface area concrete structures such as taxiways, continuous bridge decks, large parking areas, large slabs, etc.

5.3.10 Water or Damp-proofing and Permeability Reducing Admixtures Water under pressure and in contact with one surface of concrete, can be forced through channels between the two surfaces. The water passing in this manner is a measure of the permeability of concrete. Water can also pass through concrete by the action of capillary forces. The materials used to reduce the water flow by the first method are termed permeability reducer, whereas the materials used to reduce second type of flow are more properly called damp-proofers. A concrete having proper mix proportions, low water–cement ratio and sound aggregate will be impervious and need no additives. However, the resistance of concrete to the penetration of moisture can be improved by adding chemically active water-repelling agents like soda and potash soaps to which are sometimes added lime or calcium chloride. These admixtures prevent the water penetration of dry concrete, or stop the passage of water through unsaturated concrete. The waterproofing admixtures may be grouped into the following four categories: 1. Chemicals which react with hydration products of cement These admixtures react with hydration products of cement and form a thin hydrophobic layer within pores and voids, and on surface of the concrete. This type of admixture is based on liquid fatty acids present in vegetable and animal fats. They may be in the emulsified form or pre-mixed with inert fillers such as talc or silica flour for uniform dispersion in the concrete mix. 2. Chemical which coalesce on contact with hydration product These are finely divided wax emulsions which break down on coming in contact with alkaline environment in cement concrete and form hydrophobic layers in pores, voids and on the surfaces. 3. Finely divided hydrophobic materials Calcium-stearate and aluminumstearate form hydrophobic layers in the concrete pores and widely used in precast industry. 4. Finely divided fillers Mineral additives such as pozzolanas, silica fume, kaolinite when added in lean concrete mix improves water tightness by pozzolanic action and with physical filler effect.

Chemical Admixtures and Mineral Additives

127

In general, waterproofing admixtures have no significant effect on setting times of cement and compressive strength of concrete. However, bleeding is reduced, durability (particularly resistance to chemical ingress, freeze–thaw resistance) of concrete is enhanced, and drying shrinkage is reduced. As explained earlier, the air-entraining agents increase the plasticity of concrete and therefore help place concrete more uniformly. They also reduce bleeding by holding the water in films around the air bubbles, thus reducing the permeability. The small-disconnected voids produced by air-entrainment also break up the capillaries in concrete and, therefore, offer a barrier to the passage of water by capillary action. For these reasons, air-entraining admixtures may also be considered as permeability-reducing and damp-proofing agents. Another type of concrete waterproofer consists of a film applied to the surface, preferably the one adjacent to the water source. The asphaltic products, thick viscous liquids, form an impervious coating over the surface, sodium silicate compounds enter the surface pores and form a gel, which prevent water from entering the concrete.

5.3.11 Bonding Admixtures When fresh concrete is placed over a concrete surface already set and at least partially cured, the fresh concrete shrinks while setting which makes the new concrete pull away from the old surface. Due to this reason, the old surfaces are usually prepared so that the aggregates are exposed and clean which makes the cement paste in the freshly placed concrete, bond the aggregate in the same way as it bonds the aggregates in the new mix. Cement paste slurry is often applied to the prepared old surface immediately prior to pouring new concrete to increase the amount of paste available at the surface for bonding purposes. In situations where such a treatment cannot be applied, the bonding admixtures can be used to join two surfaces. These admixtures increase the bond strength between the old and new concrete. The major applications include: overlay on an existing pavement, provision of screed over roof for waterproofing, repair work, etc. There are two types of bonding admixtures in common use. In the first type, the bonding is accomplished by a metallic aggregate and in the other synthetic latex emulsions are used. The metallic aggregate type of admixture consists of fine castiron particles to which is added a chemical that causes them to oxidize rapidly when mixed with Portland cement and water. The rapid oxidation of the iron particles in the cement slurry applied over the old concrete surface results in the expansion of iron particles. The tiny fingers that thrust out into both the old and the new concrete bind them together. This admixture can also be used as waterproofer by applying additional coats. Successive coats build up a thin but dense watertight film over the surface. There are a number of types of synthetic latex bonding admixtures, which essentially consist of highly polymerized synthetic liquid resins dispersed in water. The commonly used polymer bonding admixtures are made from natural rubber, synthetic rubber or any of a large number of organic polymers or copolymers. The polymers include polyvinyl chloride, polyvinyl acetate, acrylics and butadiene styrene copolymers. These admixtures are water emulsions, which are generally added to the mixtures in proportions equivalent to 5 to 20 per cent by mass of cement depending

128

Concrete Technology

upon the actual bonding requirements. Since these admixtures are emulsions, the bonding agent must lose water for its adhesive ingredients to set. When a bonding agent is sprayed on a concrete surface, the pores in the concrete absorb the water and allow the resin particles to coalesce and bond. When a bonding agent is mixed with cement paste or a mortar, the water is used in the hydration of cement and the resin is left to bind both the surfaces. Many kinds of commercial concrete surface repair materials, mostly modified polymers, are available in the market for effective repairs. They adhere very firmly to the old concrete surface on account of greatly improved bond characteristics. They can be successfully used for repairing the chipped off columns, peeled off ceilings and pitted floors.

5.3.12

Concrete Surface-hardening Admixture

The plain concrete surfaces subjected to heavy traffic or the industrial buildings continuously subjected to wear and tear deteriorate after a period of time. The factory floor, on account of movement of materials, iron tyred trollies, vibrations caused by running machines is likely to suffer damages. Wear resistant and chemical resistant floor must be provided in the beginning itself. Replacing and repairing of old floors will interfere with the productivity and prove to be costly. The hardener commonly used to prevent the destruction of the surface can be divided into two groups, namely, the chemical hardeners and fine metallic aggregates. The liquid chemical hardeners consist of silicofluorides or fluosilicates and a wetting agent. The latter reduces the surface tension of liquid and allows it to penetrate the pores of the concrete more easily. The chemicals combine with free lime and calcium carbonate present in concrete to bind the fine particles into highly wear resistant flint like topping. On the other hand, the metallic hardeners consisting of specially processed grade iron particles are dry-mixed with Portland cement which is spread evenly over to freshly floated concrete surface and are worked into concrete by floating. This gives highly wear resistant and less brittle concrete topping. Sometimes abrasive materials like fine particles of flint, aluminum oxide, silicon carbide, or emery are used in the topping applied as dry shake to obtain water-resistant non-skid surfaces.

5.3.13

Concrete Coloring Admixtures or Pigments

Pigments are the admixtures added to produce colored cements. One of the methods of producing colored concrete surfaces in modern construction is to use concrete paint to be applied after the concrete surface has been neutralized, either through exposure or by using a neutralizing agent like zinc sulfate. The other most commonly used method involves integrating color into the surface of concrete while it is still fresh. This can be accomplished by mixing natural metallic oxides of cobalt, chromium, and iron, etc., called pigments into the topping mix. This is the best way of distributing the color evenly throughout the concrete. The coloring admixture made with synthetic oxides mixed with one or more additional drying ingredients are also available. But the pigments used must be permanent and should not react with free lime in concrete. To obtain a good coloring

129

Chemical Admixtures and Mineral Additives

effect, the pigments should be ground with the cement in a ball mill. Sometimes they are mixed with fillers, like chalk, but the excessive use of fillers may affect strength of concrete. The chief pigments used in concrete are as follows. 1. Black The best permanent black pigment is carbon black, but manganese black gives a brown tint whereas magnetic ferrous oxide has a purple tint. 2. Blue The materials used are barium manganate and ultramarine. Sulfur fumes in polluted atmosphere adversely affect the former. Ultramarine is suitable for concrete used in non-wearing surfaces. 3. Brown Raw umber or burnt umber form satisfactory brown pigments. 4. Green Artificially produced chromium oxide and chromium hydroxide are suitable. 5. Red The most commonly used material is the naturally occurring red oxide or iron. 6. Yellow Hydroxides of iron give yellow color.

5.3.14

Fungicidal, Germicidal and Insecticidal Admixtures

Certain materials like polyhalogenated phenols, dieledren emulsions and copper compounds when added as admixtures impart fungicidal, germicidal or insecticidal properties to the hardened cement pastes, mortars or concretes.

5.4

PHYSICAL REQUIREMENTS OF ADMIXTURES

For assessing an admixture for its suitability or conformance to the specified requirements, the performance of treated concrete mix (with admixture) is compared with the identical untreated control concrete mix (without admixture). The acceptance criteria are given in Table 5.2. Table 5.2

Admixture acceptance test-physical requirements (IS: 9103–1999)

Requirements Accelerating Retarding Water- Air-enSuperplasticized admixture admixture reducing training water-reduced admixture adconcrete mix mixture Accelerating Retarding Percentage reduction in water content, min.

—

—

5

—

Slump

—

—

—

—

–3

+3

±1

—

20

20

Not more than 15 mm below that of control mix

Setting time, deviation from control mix hours Initial max.

–1.5

+4

(Continued)

130

Concrete Technology Table 5.2

Continued

Requirements Accelerating Retarding WaterAirSuperplasticized admixture admixture reducing entrainwater-reduced admixture ing concrete mix admix- Accelerating Retarding ture min. Final max. min.

–1

+1

—

–2 –1

+3 +1

±1

— 125 100 100 90 90

— 90 90 90 90 90

110 100 90

90 90 90

+1

— —

–1.5

+3

— 110 110 110 100 100

— 90 90 90 90 90

140 125 125 115 100 100

— 125 125 115 100 100

100 100 100

90 90 90

110 100 100

110 100 100

Compressive strength, per cent of control mix, min. 1 day 3 days 7 days 28 days 6 months 1 year Flexural strength, per cent of control mix, min. 3 days 7 days 28 days

Notes 1. The percentage increase in the length of a specimen and in the bleeding over the control sample shall not exceed 0.01 and 5, respectively. 2. In case of superplasticized water-reduced concrete mix, the air-content shall not exceed 1.5 per cent over the control sample. 3. The flow of high workability superplasticized concrete mix is generally 510 to 620 mm. 4. The minimum compressive strength of high workability superplasticized concrete shall not be less than 90 per cent of the control mix concrete at the corresponding age. 5. The reduced workability of concrete mix using accelerating and retarding superplasticizers at 45 and 120 minutes standing, respectively, shall not be less than that of control mix concrete at 15 minutes standing.

Chemical Admixtures and Mineral Additives

131

The admixtures are available in liquid and powder forms. Waterproofing agents are normally sold in powder form. The packages contain brand name, its classification (e.g., accelerator, retarder, water reducer), the recommended dosage, safety precautions, user instructions and a reference to the National Code of Practice. The admixtures are dispensed with automatic or semi-automatic or hand dispenser to the desired dispensing accuracy.

Point of addition To optimise the benefit, which is obtained by incorporating an admixture in concrete mix, there is a preferred point of its addition in the mixing cycle of the concrete. It is important to achieve uniform dispersal of admixture (which is added in small quantity) in the bulk of concrete. The preferred points of addition of various admixtures are summarized in Table 5.3. Table 5.3

Preferred point of addition

Admixture type

Point of addition

Remarks

(a) All water reducing admixtures except superplasticizers used for flowing or self-leveling concrete.

1. Admixture to be dissolved in a part of mixing water. 2. All materials including the remaining water to be mixed for at least 30 sec. (preferably for one minute) 3. Aggregates, if moist, can be mixed with cement for 30 sec. to 1 min. before the mixing water containing the admixture is added. Dissolved in mixing water. Added to aggregates and cement in the usual manner.

The purpose is to allow some partial hydration of cement particles before water reducing or high water-reducing agents come into contact with cement. The dispersive action is better and lasts longer.

Pre-mixed with dry aggregates and cement before mixing water is added.

To ensure uniform dispersion the powders may be sprinkled into the mixer as the aggregates and dry cement are being mixed.

After mixing and transporting and just before placing.

Otherwise, the effect will be absent. It is to be noted that for flowing concrete, the dosage will normally be high.

(b) Air-entraining agents. (c) Accelerators (except those in powder form). (d) Emulsified waterproofing agents. (e) Powdered water-proofing agents. (f) Powdered accelerators. (g) Superplasticizer used for flowing or selfleveling concrete.

5.5

Not sensitive to the point of addition, but important to achieve uniform dispersion in the mix.

INDIAN STANDARD SPECIFICATIONS

The Indian standard specifications for admixtures for concrete, IS: 9103–1999, covers the chemical admixtures including superplasticizers, solid or liquid emulsions, to be added to the concrete at the plastic state. The admixtures covered in this standard

Increased workability with faster gain of strength. Increased workability and delayed setting

(c) Water-reducing accelerators

(d) Water-reducing retarders

Delayed setting

(b) Retarding admixtures or retarders

Admixtures Functions (a) Accelerating admix- 1. More rapid gain of tures or accelerators strength or higher early strength. 2. More rapid setting.

3. Extend placing times, e.g., ready-mixed concrete. 4. Prevent cold joint formation. Water reducer with faster strength development.

2. Reduce rate of heat evolution.

1. Maintain workability at high temperatures.

5. Sprayed concreting.

Mixtures of calcium chloride and lignosulfonate. Mixtures of sugars or Water reducer, with slower loss of hydroxylated carboxylic workability. acids and lignosulfonate.

Sugars

Sodium nitrite Sodium sulfate Sodium aluminate Sodium silicate Sodium carbonate Potassium hydroxide Soluble carbohydrate derivatives: starch Hydroxylated carboxylic acids, Inorganic retarders

Applications 1. Normal rate of strength development at low temperature. 2. To counter retarding effects 3. Shorter stripping times. 4. Plugging of pressure leaks.

Details of common type of concrete admixtures

Typical compounds Calcium chloride Calcium formate Triethanolamine (TEA) Soluble inorganic salts

Table 5.4

Risk of corrosion.

(Continued)

May promote bleeding.

Disadvantages 1. Possible cracking due to heat evolution. 2. Possibility of corrosion of embedded effects reinforcement.

132 Concrete Technology

Higher flowability

(h) Superplasticizers Greatly enhanced (Super-water reduc- workability. ers) —15 to 30 per cent water reduction

(g) Plasticizers (Water reducers)—8 to 15 per cent water reduction

Applications Enhanced durability to frost without increasing cement content, improvement in workability, lowered permeability and cellular concrete.

Continued

Sulfonated Melamine formaldehyde resin, sulfonated naphthaleneformaldehyde resin, Mixtures of saccharates and acid amides.

Certain special types of cements like sulfate resistant cement (low C3A content) and expansive cement do not perform well.

1. Not efficient under high hydrostatic pressure. 2. Requires low water– cement ratio and full compaction.

Disadvantages Careful control of air con -tent, water–cement ratio, temperature, type and grading of aggregate and mixing time is necessary.

1. Water reducer, but over a wider 1. Tendency to segregate. range. 2. Flowability is not long lasting. 2. Facilitate production of flowing 3. During hot weather the or self-leveling concrete workability retention period decreases fast.

1. Reduced permeability. 2. Enhanced durability. 3. Increased freeze–thaw resistance. 4. Reduced drying shrinkage. 5. Reduced surface staining. 6. Water tightness of structures without using very low water– cement ratio. Hydroxylated carboxylic 1. Higher workability with acid derivatives strength unchanged. Calcium and sodium 2. Higher strength with workabillignosulfonates. ity unchanged. 3. Less cement for same strength and workability.

Functions Typical compounds Entrainment of air into Natural wood resins, concrete. fats, lignosulfonates, alkyl sulfates, sodium salts of petroleum, sulfonic acids. (f) Damp-proofing or 1. Water-repellent, Potash soaps, calciumwater-proofing agents i.e. prevention of stearate, aluminumwater from enterstearate, ing capillaries of butylstearate, petroleum concrete. wax emulsions. 2. Reduced water permeability of concrete.

Admixtures (e) Air-entraining agents

Table 5.4

Chemical Admixtures and Mineral Additives 133

134

Concrete Technology

are accelerating admixtures, retarding admixtures, water-reducing admixtures, airentraining admixtures, and superplasticizers. The functions, typical compounds, applications and disadvantages of some of the commonly used admixtures are summarized in Table 5.4.

5.6

MINERAL OR SUPPLEMENTARY ADDITIVES

Mineral additives also called supplementary cementing materials (pozzolana) shown in Fig. 5.10 are finely ground siliceous materials which, as such, do not possess cementing property in themselves, but react chemically with calcium hydroxide Ca (OH)2 released from the hydration of Portland cement at normal temperature to form compounds of low solubility having cementing properties. The action is termed pozzolanic action. These materials are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties, which can be used individually or in combination with Portland or blended cement or as a partial replacement of Portland cement. Class C Fly Ash Silica Fume

Portland cement

Class F Fly Ash

Fig. 5.10

GGBF Slag

Supplementary cementing materials

The pozzolanic materials can be divided into two groups namely, natural pozzolanas and artificial pozzolanas. The typical examples of natural pozzolana are: clay, shales, opaline cherts, diatomaceous earth, and volcanic tuffs and pumicites. The commonly used artificial pozzolanas are fly ash, blast-furnace-slag, silica fume, rice husk ash, metakaoline, and surkhi. Other mineral additives, like finely ground marble, quartz, granite powder are also used. They neither exhibit the pozzolanic property nor the cementing properties. They just act as inert fillers. The pozzolanic materials when used as replacement are generally substituted for 10 to 50 per cent of cement. This substitution produces concrete that is more permeable but much more resistant to the action of salt, sulfate or acidic water. Strength gain is usually slower than for the normal concrete. Pozzolanas when added to concrete mixes, rather than substituted for a part of the cement, improve workability, impermeability, and resistance to chemical attack. The overall effect depends on the aggregates used in the concrete. The aggregates deficient in fine material give the best results.

3.8

Al2O3

*High-reactivity-metakaoline

Note

–

–

90

97

–

HRM* Rice husk ash (RHA)

SO3

4–10

–

–

–

–

–

–

–

–

–

2.0–2.5

–

5.0

–

Particle size, lm

85–90

–

2.90

3.1–3.2

Specific Gravity

6–10

1.5

–

2.5

1–100 2.15–2.45 average: 2.3–2.6 10–20 (bottom ash) 0.02–1.0 2.2 average: 1.3–1.4 0.1–0.3 (slurry)

1–80 average:15 85–98 –

–

0.2–5.0 1.5–2.5 0.4–2.6 1.0–1.5 60–90

4–17

–

Alkalis Carbon Glass Na2O + K2O

90–96 0.5–3.0 0.2–0.8 0.5–1.5 0.1–0.4 0.6–1.7 0.5–1.5 85–98

30–60 10–30

0.1– 0.5

1–7

MgO

0.5–6.0 0.1–4.0 1.3–3.0 0.4–1.3

Fe2O3

30–45 25–38 15–32 0.5–2.0

60–67 17–25

SiO2

Silica fume (SF)

OPC Clinker Slag (GBFS) Fly ash (FA)

Material CaO

50000– 100000

–

15000– 20000

350–700

325–600

220–400

–

–

0.7–2.5

1.0–2.0

–

36

Slag > 70

Slag > 70 and C3A < 2

CONCRETE IN MARINE ENVIRONMENT

Concrete used in the marine environment faces simultaneously the physical, the chemical and the mechanical deterioration processes. The marine environment is generally divided into three zones depending upon their effect on the structure. Concrete in each environment zone is subjected to different types of attacks. Different types of attack zones are given in Table 8.2. Table 8.2

Different types of attack zones

Zone description

Type of attack

Atmospheric zone—where the parts of structure are above the highest high tide level or splash zone.

Chemical and physical.

Tidal zone—the located between the highest high tide and lowest low tide zones.

Chemical, physical and mechanical.

Submerged zone—where the parts of structure always remain submerged in sea wate.

Chemical and physical.

Besides physical and chemical reactions, the concrete in the marine structure located in the tidal zone also faces mechanical forces and therefore deterioration is generally observed to be more severe. Moreover, the structure in the tidal zone faces alternate wetting and drying cycles which accelerates chemical action of salts and water on reinforcement steel and concrete around it.

Properties of Hardened Concrete

207

The structures located in tidal and atmospheric zones are more vulnerable to aggressive action of sea than those which are continuously fully submerged in water. This is due to the four main reasons given below. 1. The rate of corrosion of steel is dependent on the availability of oxygen. Dissolved oxygen in sea water is very small and hence corrosion of the reinforcement steel seldom takes place in the totally submerged conditions. In tidal and atmospheric zones where oxygen is present in adequate quantity, corrosion of steel is much faster. 2. In the portion of concrete structure above sea level, the sea water rises upwards by capillary action. This water when evaporates leaves behind crystal salts. With the progressive wetting and drying cycles this crystal-line growth gradually increases causing tensile stresses. When the tensile stresses exceed tensile strength of concrete, disintegration of concrete surface takes place. 3. Due to fluctuation of sea-water level, the leached salts and corroded concrete fragments gets washed away and erosion of concrete takes places which results in the loss of concrete mass. 4. The mechanical impact of sea waves in active tidal zone continuously increases the wear and tear of concrete. Considering the above reasons responsible for deterioration of concrete in marine environment, the following preventive measures are recommended. The type of cement plays a very important role in the structures located in sea water environment. Slag and pozzolanic cements are preferred. Alternatively, mineral additives like ground granulated blast-furnace-slag or fly ash or micro silica can be successfully used with ordinary portland cement. The codal requirements as stipulated in the IS: 456−2000 are given in Table 8.3. Table 8.3

Requirements for reinforced concrete exposed to sea water

Environmental exposure (i) Concrete exposed to coastal environ-ment (excluding tidal and splash zone). (ii) Concrete permanently submerged in sea water. Concrete surface exposed to sea water spray. Surface of members in tidal zone.

Exposure classification

Maximum Minimum Minimum Minimum grade of cover, cement con- Water-ceconcrete mm tent kg/m3 memt ratio

Severe

M 30

45

320

0.45

Very severe

M 35

50

340

0.45

Extreme

M 40

75

360

0.40

Notes 1. *Coastal zone normally extends up to 1 km from the coastal line.

208

Concrete Technology

8.9

ACID ATTACK

Concrete structures are also used for storing liquids, some of which are harmful for concrete. In industrial plants, concrete floors come in contact with liquids which damage the floor as is seen in Fig. 8.26(b). In damp conditions SO2 and CO2 and other acid fumes present in the atmosphere affect concrete by dissolving and removing part of the set cement. In fact, no Portland cement is acid resistant. Concrete is also attacked by water containing free CO2. Sewerage water also very slowly causes deterioration of concrete.

8.10

EFFLORESCENCE

The water leaking through cracks or faulty joints or through the areas of poorly compacted porous concrete, dissolves some of the readily soluble calcium hydroxide and other solids, and after evaporation leaves calcium carbonate as white deposit on the surface as shown in Fig. 8.26(c). These deposits on the surface of concrete resulting from the leaching of calcium hydroxide and subsequent carbonation and evaporation, are termed efflorescence. Unwashed seashore aggregate, gypsum and alkaline aggregate also cause efflorescence. Many kinds of salts have been detected in samples of efflorescences.

8.11

FIRE RESISTANCE

In general, concrete has good properties with respect to fire resistance, i.e., the period of time under fire during which concrete continues to perform satisfactorily is relatively high and no toxic fumes are emitted. The length of time over which the structural concrete preserves structural action is known as fire rating. Under sustained exposure to temperature in excess of 35°C along with the condition that a considerable loss of moisture from concrete is allowed leads to decrease in strength and in modulus of elasticity. The loss of strength at higher temperatures is greater in saturated than in dry concrete. Excessive moisture at the time of fire is the primary cause of spalling as shown in Fig. 8.26(d). In general, moisture content of concrete is the most important factor determining the structural behaviour at higher temperature. Leaner mixes appear to suffer a relatively lower loss of strength than rich ones. Flexural strength is affected more than compressive strength. The loss of strength is considerably lower when the aggregate does not contain silica, e.g., concrete made with limestone, crushed brick and blast-furnace-slag aggregate. Low conductivity of concrete improves its fire resistance, and hence a lightweight concrete is more fire resistant than ordinary concrete. The calcined material aggregate having a low density leads to a good fire resistance of concrete. Due to endothermic nature of carbonate aggregate during calcination at high temperature, heat is absorbed and further temperature rise is delayed. For example, dolomite gravel leads to a good fire resistance of concrete. The data on the variation of strength of concrete upon heating obtained experimentally are generally conditional. The data obtained by generalizing the results

209

Properties of Hardened Concrete

from the fire resistance tests on actual reinforced concrete structures are given in Table 8.4. The variation in the strength with temperature is shown in Fig. 8.27. Coefficient of reduction in compressive strength, gc of dense concrete on heating

Table 8.4 Type of concrete aggregate

Coefficient gc at the temperature, °C. 20

100

200

300

400

500

600

Limestone

1.0

1.0

1.0

1.0

1.0

1.0

0.90 0.67 0.45 0.22

Granite

1.0

1.0

1.0

1.0

1.0

0.92 0.70 0.46

gc

Ec

100

100

30

30

700

800

900

025

1000

0

300 600 Temperature, °C

450 Temperature, °C

(a) Compressive Strength (% Amb)

(b) Initial Modulus (% Amb)

0 0

fc

ac

Ambient

Stress

20

575 °C

6

300 600 Temperature, °C (c) Coefficient of Thermal Expansion ¥ 10

Fig. 8.27

–6

0.004 Strain, E (d) Stress–strain Behavior

Thermo-mechanical properties of concrete at high temperature

The modulus of elasticity of concrete is considerably reduced and thermal creep increases considerably at high temperature. The coefficient of thermal expansion of concrete using different types of aggregates given in Table 8.5 are valid for temperatures up to 100°C. At higher temperature, the values may differ considerably. The variation of thermo-mechanical properties of the concrete with temperature is shown in Fig. 8.27. The values of coefficient of thermal expansion of concrete prepared with different types of aggregates are given in Table 8.5.

210

Concrete Technology Table 8.5

Coefficient of thermal expansion of concrete

Type of aggregate

a c/°C

Type of aggregate

a c/°C

Granite

9.5 × 10−6

−6

Quartz

11.9 × 10

Sandstone

11.7 × 10−6

Basalt

8.6 × 10−6

Gravel

10.8 × 10−6

Limestone

6.8 × 10−6

8.12

THERMAL PROPERTIES OF CONCRETE

The important thermal properties required for the design of structures are thermal conductivity, thermal diffusivity, specific heat, and coefficient of thermal expansion. Thermal conductivity is a measure of the ability of the concrete to conduct heat and is measured in British Thermal Units per hour per square foot area of the body when the temperature difference is 1°F per foot thickness of the body. Thermal conductivity depends upon the composition of concrete. The structural concrete containing normal aggregate conducts heat more readily than lightweight concrete. Lower the water–content of the mix, the higher the conductivity of the hardened concrete. The density of the concrete does not appreciably affect the conductivity of ordinary concrete. The variation of thermal conductivity of concrete with temperature is shown in Fig. 8.28.

Thermal Conductivity, W/M°C

2.0

1.5

1.0

0.5

500

1000

1500

2000

T Temperature, °C

Fig. 8.28

Thermal conductivity of concrete

Thermal diffusivity is a measure of the rate at which temperature change within the mass takes place. Diffusivity can be determined by D=

k Sd

Properties of Hardened Concrete

211

where D, k, S and d are the thermal diffusivity, thermal conductivity, specific heat and density of concrete, respectively. The specific heat gives the heat capacity of concrete. It increases with the moisture content of concrete. The specific heat values of ordinary concrete are between 0.2 to 0.28 BTU/lb/°F. The coefficient of thermal expansion of concrete depends on the composition of the mix and on the values of the coefficient of expansion of cement paste and aggregate. For ordinary cured concrete the coefficient decreases slightly with age but this is not the case in the concrete cured under high pressure steam. For ordinary concrete the value of coefficient of thermal expansion varies from 9 × 10−6 per °C to 12 × 10−6 per °C.

8.13

MICRO-CRACKING OF CONCRETE

Cracking of concrete can be defined as a separation of the individual components of concrete resulting in a discontinuous material as is seen in Fig. 8.29. Depending upon the extent of cracking the cracks can be classified as macro-cracks, micro-cracks and semi-micro-cracks. According to the location, the cracks can be classified as bond cracks, mortar cracks and aggregate cracks. The bond cracks are formed at the interface of the aggregate and mortar, whereas the mortar cracks and aggregate cracks are formed through the mortar and the aggregate, respectively.

Fig. 8.29

Magnified view of micro-cracks

A knowledge of the micro-cracking of concrete contributes considerably to the understanding of its properties, such as its inelastic nature, the descending portion of the stress−strain curve, the strength under combined, repeated and sustained loading, etc. The stress−strain curve is related to the internal cracking. The non-linearity of the stress−strain relation is due to propagation of bond and mortar cracks.

212

Concrete Technology

The progressive cracking in concrete with increasing strain has been indirectly determined by measuring the lateral expansion/contraction, surface cracking and by sonic methods. The exact nature of cracks and the strains at which they occur give precise information pertinent to the mechanism of cracking. Direct microscopic observations help in studying the extent of cracking both quantitatively and qualitatively. The bond between aggregate and mortar plays an important role in controlling the strength characteristics of concrete. The existence of bond cracks prior to loading constitutes one of the weakest links in the heterogeneous concrete system. Due to the settlement of fresh concrete, hydration of cement, and shrinkage of concrete, the bond cracks exist in the hardened concrete near the large aggregates. It has been seen that the bond strength between the aggregate and the mortar is less than the tensile strength of the mortar. Hence it can be deduced that the bond between the aggregate and mortar controls the failure of concrete under uniaxial tensile loading whereas the tensile strength of mortar controls the strength of concrete under uniaxial compressive loading. The failure process in plain concrete is a continuous one and proceeds in two ways. Bond cracks by themselves cannot cause failure, as they are isolated from each other. Failure occurs only when there are sufficient interconnected bond cracks with mortar cracks. The development of a continuous crack pattern does not lead to immediate loss of the load carrying capacity because concrete at this stage behaves as a highly redundant structure. As successive load paths become inoperative through bond cracking, alternative load paths (either entirely through mortar, or partly through mortar and partly through aggregate) continue to be available for carrying additional load. As the number of load paths decrease the intensity of stress and hence the magnitude of strain on remaining paths increases at a faster rate than external load. When an extensive continuous crack pattern has developed and the load paths have been reduced considerably, the carrying capacity of concrete decreases, and from this stage the stress−strain curve begins to descend.

REVIEW QUESTIONS 8.1 What does strength in concrete mean? List the different types of concrete strengths. How are the compressive and flexural strengths determined? What is the relationship between compressive and flexural strength? 8.2 Why is compressive strength usually considered being most important in concrete design? 8.3 What are the fundamental factors influencing the compressive strength of concrete? Explain any one of them. 8.4 What is gel/space ratio? How does it influence the strength of concrete?

8.5 Describe the three phases of concrete that affect the strength of concrete. 8.6 Explain the test parameters related to the specimen and loading which affect the strength of concrete. 8.7 State the destructive tests performed on hardened concrete and describe the split tension test on concrete cylinder and cube for determining the tensile strength of concrete. Enlist its merits of this indirect test. 8.8 Discuss the various aspect of durability of concrete. What measures are suggested by IS:456 2000 to ensure durable structure?

Properties of Hardened Concrete 8.9 List the causes of lack of durability, and explain how chloride-ion penetration affects the durability. 8.10 Write short notes on any three of the following: (a) Durability and its significance, (b) Permeability of concrete, (c) Creep, its importance and factors affecting it and (e) Shrinkage and factors affecting it. 8.11 What is alkali-silica reactivity (ASR)? Enlist the factors that promote the alkali-aggregate reaction. How can

213

the damage from alkali-silica reaction (ASR) in a concrete structure be diagnosed? 8.12 Write short notes on any two of the following: (a) Efflorescence and its prevention, (b) Fire resistance of concrete, (c) Concrete in Marine environment and (d) Factors affecting modulus of elasticity of concrete. 8.13 Describe the phenomenon of microcracking of hardened concrete.

MULTIPLE-CHOICE QUESTIONS 8.1 Concrete may be described as (a) an artificial stone obtained by binding together particles of relatively inert fine and coarse materials with cement paste (b) the most widely used man-made construction material tailored to meet the demands of any particular situation (c) an artificial stone in which voids of larger particles are filled by the smaller particles, and the voids of the finer particles are filled with cement paste (d) a material prepared from locally available materials by judicious mix proportioning and proper workmanship to satisfy performance requirements (e) Any of the above 8.2 The main ingredients of concrete are (a) cement (b) aggregates (c) water (d) admixtures (e) All of the above 8.3 After curing, normal concrete (a) shrinks on drying (b) expands on drying (c) shrinks when still wet (d) may shrink or expand depending upon the proportions of various ingredients (e) neither shrinks nor expands

8.4 The inert ingredient(s) of a concrete mix is/are (a) cement (b) aggregates (c) water (d) entire mix (e) None of the above 8.5 The best way to specify the concrete is by (a) performance-oriented specifications (b) prescriptive specifications (c) degree of control (d) unit weight (e) None of the above 8.6 The most appropriate method to specify the concrete mix is by (a) the nominal mix ratio (b) the designed mix ratio (c) the degree of control (d) the grade of concrete (e) None of the above 8.7 The strength of concrete is influenced by (a) size of test specimen (b) moisture conditions (c) type and rate of loading (d) type of testing machine (e) All of the above 8.8 The strength of concrete depends upon (a) type of cement (b) concrete mix proportions (c) degree of compaction

214

Concrete Technology

(d) type and temperature of curing (e) All of the above 8.9 The compressive strength of concrete (a) decreases with the increase of aggregate−cement ratio (b) increases with the increase in the degree of compaction (c) decreases with entrained air (d) All of the above (e) None of the above 8.10 The stress and strain curve of concrete in compression is obtained by testing the cylindrical specimen under (a) uniform rate of strain (b) uniform rate of stress (c) constant stress condition (d) constant strain condition (e) Any of the above 8.11 As compared to the static tests the dynamic tests on concrete give (a) higher value of Poisson’s ratio (b) lower value of Poisson’s ratio (c) the same value of Poisson’s ratio (d) All of the above depending upon the test conditions (e) None of the above 8.12 The shrinkage in concrete is due to (a) hydration of cement (b) loss of water by evaporation from the surface (c) withdrawal of water stored in unsaturated air voids of concrete (d) All of the above (e) None of the above 8.13 Shrinkage increases with (a) increase in the water−cement ratio (b) increase in cement content (c) decrease in humidity (d) decrease in the maximum size of aggregate (e) All of the above 8.14 Permeability of concrete reduces (a) with the carbonation of concrete (b) with the strength of cement paste (c) with the decrease in the porosity (d) All of the above 8.15 The durability of concrete is due to its resistance to (a) deterioration from environmental conditions

8.16

8.17

8.18

8.19

8.20

8.21

8.22

(b) internal desruptive forces (c) chemical attack (d) All of the above The inelastic behavior of concrete is due to the (a) shrinkage in concrete (b) propagation of bond and mortar cracks (c) presence of macro and micro cracks (d) use of aggregates (e) All of the above The thermal conductivity of concrete decreases with the (a) light-weight concretes (b) increase in the water−cement ratio (c) decrease in the cement content (d) All of the above (e) None of the above For cement concrete, the stress−strain curve is linear approximately up to (a) 1/4 of ultimate stress (b) 1/3 of ultimate stress (c) 1/2 of ultimate stress (d) 5/8 of ultimate stress (e) 3/4 of ultimate stress The modulus of elasticity of concrete improves with (a) age (b) high water−cement ratio (c) shorter curing periods (d) better compaction (e) All of the above Shrinkage of concrete can be reduced by using (a) low water−cement ratio (b) water-tight and non-absorbent formwork (c) presaturated aggregates (d) All of the above (e) None of the above The strength of concrete mainly depends upon (a) quality of fine aggregate (b) quality of coarse aggregate (c) fineness of cement (d) water−cement ratio (e) None of the above The thermal coefficient of expansion of concrete is approximately (a) 3 × 10−8 per °C

Properties of Hardened Concrete

8.23

8.24

8.25

8.26

8.27

8.28

(b) 3 × 10−6 per °C (c) 3 × 10−5 per °C (d) 3 × 10−4 per °C (e) 3 × 10−3 per °C Creep in concrete is undesirable particularly in (a) continuous beams (b) reinforced concrete columns (c) prestressed concrete structures (d) All of these (e) None of the above The knowledge of the flexural tensile strength is useful in the design of (a) reinforced concrete members (b) pavement slabs and airfield runways (c) prestressed concrete structures (d) water-retaining structures (e) All of the above Compressive strength of concrete is the most important property because (a) it depends upon the water−cement ratio (b) it is related to the structure of hardened cement paste and gives the overall quality of concrete (c) it indicates the extent of voids in the concrete (d) it affects the permeability and durability of concrete (e) None of the above The concrete may attain its 100 per cent compressive strength after (a) seven days (b) 14 days (c) 28 days (d) one year (e) three years The strength of concrete is decreased by (a) vibration (b) impact (c) fatigue (d) All of these (e) None of these The permissible stress for concrete subjected to fatigue should be (a) 25% (b) 50% (c) 75% (d) 80% (e) 95%

215

8.29 According to the Indian Standard specifications, the maximum compressive strength of normal strength concrete can be (a) 5 MPa (b) 12.5 MPa (c) 15 MPa (d) 20 MPa (e) 40 MPa 8.30 The tensile strength of concrete is approximately—of compressive strength of concrete? (a) 50 % (b) 20% (c) 10 % (d) 5% (e) 1% 8.31 The standard size of a concrete cube for compressive strength test is (a) 50 mm (b) 100 mm (c) 150 mm (d) 200 mm (e) 250 mm 8.32 As per Indian Standard specifications concrete is designated into (a) 3 grades (b) 5 grades (c) 7 grades (d) 10 grades (e) 12 grades 8.33 The porosity of concrete depends largely upon (a) cement content (b) grading of aggregate (c) quantity of mixing water (d) degree of compaction (e) All of the above 8.34 The concrete for sea water application should not be leaner than (a) 1:2:6 (b) 1:2:4 (c) 1:2:3 (d) 1:3 (e) l:l:2 8.35 In case of plain concrete exposed to sea waves, the grade of concrete should not be lower than (a) M15

216

8.36

8.37

8.38

8.39

8.40

Concrete Technology (b) M20 (c) M25 (d) M30 (e) M40 The concrete in contact with alkaline soil or alkaline water should (a) use a rich mix (b) have a high water−cement ratio (c) have a high alumina content (d) have a low water−cement ratio (e) have a higher percentage of fine aggregate For high frost resistance the concrete should be (a) dense (b) free from cracks (c) air-entrained (d) All of these (e) None of the above For compressive strength determination, the minimum number of cubes required in a sample is (a) 2 (b) 3 (c) 5 (d) 6 (e) 9 The unit weight of plain concrete is generally taken as (a) 20 kN/m3 (b) 22 kN/m3 (c) 24 kN/m3 (d) 25 kN/m3 (e) 16 kN/m3 The unit weight of reinforced cement concrete is generally taken as (a) 18 kN/m3 (b) 22 kN/m3 (c) 24 kN/m3 (d) 25 kN/m3 (e) 26 kN/m3

8.41 Which of the following has the highest unit weight? (a) Common brick work (b) Plain concrete with brick aggregate (c) Plain concrete (d) Reinforced concrete (e) Self-compacting concrete 8.42 Which one of the following does not react with concrete? (a) Sewage water (b) Sulfuric acid (c) Vegetable oil (d) Alcohol (e) None of the above 8.43 Presence of algae in concrete (a) reduces its strength (b) reduces its bond strength (c) causes a large entrainment of air (d) All of the above (e) None of the above 8.44 The direct methods for calculating tensile strength of concrete suffer due to the (a) presence of eccentricity in application of load (b) stress concentration at the jaws (c) difficulty in holding the specimen (d) All of the above (e) Reasons other than the above 8.45 Split tensile strength tests are better than the direct tensile strength tests because (a) the test gives more uniform results (b) the results give values closer to the actual tensile strength values (c) same molds can be used for both compression and tension tests (d) All of the above (e) None of the above

Answers to MCQs 8.1 (e)

8.2 (e)

8.3 (a)

8.4 (b)

8.5 (a)

8.6 (d)

8.7 (e)

8.8 (e)

8.9 (d)

8.10 (a)

8.11 (a)

8.12 (d)

8.13 (e)

8.14 (d)

8.15 (d)

8.16 (b)

8.17 (d)

8.18 (c)

8.19 (a)

8.20 (d)

8.21 (d)

8.22 (d)

8.23 (c)

8.24 (b)

8.25 (b)

8.26 (e)

8.27 (d)

8.28 (b)

8.29 (e)

8.30 (c)

Properties of Hardened Concrete 8.31 (c)

8.32 (c)

8.33 (e)

8.34 (c)

8.35 (d)

8.36 (c)

8.37 (d)

8.38 (b)

8.39 (c)

8.40 (d)

8.41 (d)

8.42 (d)

8.43 (d)

8.44 (d)

8.45 (d)

217

9 9.1

QUALITY CONTROL OF CONCRETE

INTRODUCTION

Concrete is generally produced in batches at the site with the locally available materials of variable characteristics. It is, therefore, likely to vary from one batch to another. The magnitude of this variation depends upon several factors, such as variation in the quality of constituent materials; variation in mix proportions due to batching process; variation in the quality of batching and mixing equipment available; the quality of overall workmanship and supervision at the site. Moreover, concrete undergoes a number of operations, such as transportation, placing, compacting and curing. During these operations, considerable variations occur partly due to quality of plant available and partly due to differences in the efficiency of techniques used. Thus there are no unique attributes to define the quality of concrete in its entirety. Under such a situation concrete is generally referred to as being of good, fair or poor quality. This interpretation is subjective. It is, therefore, necessary to define the quality in terms of desired performance characteristics, economics, aesthetics, safety and other factors. Due to the large number of variables influencing the performance of concrete, quality control is an involved task. However, it should be appreciated that concrete has mainly to serve the dual needs of safety (under ultimate loads) and serviceability (under working loads) including durability. These needs vary from one situation and type of construction to another. Therefore, uniform standards valid for general application to all the works may not be practical. Therefore, the aim of quality control is to reduce the above variations and produce uniform material providing the characteristics desirable for the job envisaged. Thus quality control is a corporate, dynamic programme to assure that all aspects of materials, equipment and workmanship are well looked after. The tasks and goals in these areas are properly set and defined in the specifications and control requirements. The specifications have to state clearly and explicitly the steps and requirements, adherence to which would result in a construction of acceptable quality. Except for compressive strength and appearance there is no early measure of construction performance. Each step in construction procedure is therefore to be specified. The probability based specifications containing allowable tolerances on its attributes is more rational and is preferred. Quality control is thus conformity to the specifications, no more no less. The most practical method of effective quality control is to check what is done in totality to conform to the specifications. An owner will have no right to expect anything more than what is in the specifications. The builder, on the other hand, knows that anything less than what is in the specifications will not be acceptable to the owner. In view of the different processes involved in the manufacture of concrete, the problems of quality control are diversified and their solution elaborated. The factors involved are the

Quality Control of Concrete

219

personnel, the materials and equipment, the workmanship in all stages of concreting, i.e., batching of materials, mixing, transportation, placing, compaction, curing, and finally testing and inspection. It is therefore necessary to analyze the different factors causing variations in the quality and the manner in which they can be controlled.

9.2

FACTORS CAUSING VARIATIONS IN THE QUALITY OF CONCRETE

The main factors causing variation in concrete quality are as follows. 1. Personnel The basic requirement for the success of any quality control plan is the availability of experienced, knowledgeable and trained personnel at all levels. The designer and the specification-writer should have the knowledge of construction operations as well. The site engineer should be able to comprehend the specification stipulations. Everything in quality control cannot be codified or specified and much depends upon the attitude and orientation of people involved. In fact, quality must be a discipline imbibed in the mind and there should be strong motivation to do every thing right in the first time. 2. Material, equipment and workmanship For uniform quality of concrete, the ingredients (particularly the cement) should preferably be used from a single source. When ingredients from different sources are used, the strength and other characteristics of the materials are likely to change and, therefore, they should only be used after proper evaluation and testing. The same type of cement from different sources and at different times from the same source exhibit variations in properties, especially in compressive strength. This variation in the strength of cement is related to the composition of raw materials as well as variations in the manufacturing process. The cement should be tested initially once from each source of supply and, subsequently, once every two months. Adequate storage under cover is necessary for protection from moisture. Set cement with hard lumps is to be rejected. Grading, maximum size, shape, and moisture content of the aggregate are the major sources of variability. Aggregate should be separately stock piled in single sizes. The graded aggregate should not be allowed to segregate. The simple rule of grading is that (a) for fine aggregate, long continuous gradings are preferred and there should be minimum material passing through 300 micron and 150 micron sieves, (b) for fine aggregate, the gradings that are at the coarser end of the range are more suitable for rich mixes and those at the fine end of range should be suitable for lean mixes, (c) a coarser aggregate consistent with the size of the member and the spacing of reinforcement is more suitable, and (d) the aggregate sizes should be so selected that one size fits into the voids left by the next higher size. The aggregate should be free from impurities and deleterious materials; since for every one per cent of clay in sand, there could be as much as five per cent reduction in

220

Concrete Technology

the strength of the concrete. The moisture content of aggregates should be taken into account while arriving at the quantity of mixing water. Bulking of sand is important in several ways. When volume batching is adopted, it gives erroneous results, besides increasing the water–cement ratio which, in turn, enhances the workability but reduces the strength. The aggregates are required to be tested once initially for the approval of each sources of supply. Subsequently, tests should be conducted daily at the site for grading and moisture content. The water used for mixing concrete should be free from silt, organic matter, alkali, and suspended impurities. Sulfates and chlorides in water should not exceed the permissible limits. Generally, water fit for drinking may be used for mixing concrete. The equipment used for batching, mixing and vibration should be of the right capacity. Weight-batchers should be frequently checked for their accuracy. Weightbatching of materials is always preferred to volume batching. When weight-batching is not possible and the aggregates are batched by volume, such volume measures should be frequently checked for the weight–volume ratio. Mixer’s performance should be checked for conformity to the requirements of the relevant standards. Concrete should be mixed for the required time, both under mixing and overmixing should be avoided. The vibrators should have the required frequency and amplitude of vibration. The green concrete should be handled, transported and placed in such a manner that it does not get segregated. The time interval between mixing and placing the concrete should be reduced to the minimum possible. Anticipated targets of strength, impermeability and durability of concrete can be achieved only by thorough and adequate compaction. One per cent of the air voids left in concrete due to incomplete compaction can lower the compressive strength by nearly five per cent. Adequate curing is essential for handling and development of strength of concrete. The curing period depends upon the shape and size of member, ambient temperature and humidity conditions, type of cement, and the mix proportions. Nevertheless, the first week or ten days are the most critical, as any drying out during this young age can cause irreparable loss in the quality of concrete. Generally, the long-term compressive strength of concrete moist cured for only three days or seven days will be about 60 per cent and 80 per cent, respectively, of the one moist cured for 28 days or more.

9.3

FIELD CONTROL

The field control, i.e., inspection and testing, play a vital role in the overall quality control plan. Inspection could be of two types, quality control inspection and acceptance inspection. For repeated operations early inspection is vital, and once the plant has stabilized, occasional checks may be sufficient to ensure continued satisfactory results. The operations which are not of repetitive type would require, on the other hand, more constant scrutiny. Apart from the tests on concrete materials, concrete can be tested both in the fresh and hardened stages. Of these two, the tests on fresh concrete offer some opportunity for necessary corrective actions to be taken before it is too late. These include test on workability, unit weight or air content (where air-entrained concrete is used), etc. Accelerated strength tests by which a reliable idea about the potential 28-day

Quality Control of Concrete

221

strength can be obtained within few hours, are effective quality control tools. In contrast to this, the usual 28-day strength test is, in fact, a post mortem of concrete which has become history by then. It is, therefore, only acceptance tests, which help the decision-maker decide whether to accept or reject the concrete.

9.4

ADVANTAGES OF QUALITY CONTROL

The general feeling that quality control means extra cost is not correct, the advantages due to quality control offset the extra cost. Some of the advantages of quality concrete are the following: 1. Quality control means a rational use of the available resources after testing their characteristics and reduction in the materials costs. 2. In the absence of quality control there is no guarantee that over-spending in one area will compensate for the weakness in another, e.g., an extra bag of cement will not compensate for incomplete compaction or inadequate curing. Proper control at all the stages is the only guarantee. 3. In the absence of quality control at the site, the designer is tempted to overdesign, so as to minimize the risks. This adds to the overall cost. 4. Checks at every stage of the production of concrete and rectification of the faults at the right time expedites completion and reduces delay. 5. Quality control reduces the maintenance costs. It should be realized that if the good quality concrete is made with cement, aggregates and water, the ingredients of bad concrete are exactly the same. The difference lies in the few essential steps collectively known as quality control.

9.5

STATISTICAL QUALITY CONTROL

Probability-based guidelines or specifications are usually laid down to ensure that the concrete attains its desired properties with the minimum expenditure. The specifications allow a certain limits of variability between individual samples. There is little gain in narrowing down the tolerance limits unless the process is capable of operating within these limits. The process of ensuring compliance to specifications which take into account the actual variability of concrete is termed as quality control. The statistical quality control procedures are used to ascertain the range of values that can be expected under the existing conditions. In the production of concrete, the compliance to specifications requires that the mix ingredients, size of aggregate, water–cement ratio, cement content, workability as well as methods of mixing, compaction and curing, to be adopted for a particular work are specified such that they are easy to follow. It should be noted that the usual 28 day cube tests are not quality control measures in the strict sense, they are, in fact, acceptance tests. In situations of site production and placing, the quality of the concrete is to be controlled way ahead of the stage of testing cubes at 28 days. Moreover, the compressive strength, although taken as an index of the quality of concrete, does not satisfy the requirements of durability where impermeability and homogeneity are more important parameters. However, the acceptance criteria of the quality of the

222

Concrete Technology

finished product can be based on the compressive strength of a specified number of 150 mm-cube specimens after 28-day moist curing. The basic parameters of statistical quality control are explained below. 1. Sampling Since the quality of larger mass of the materials or product is based on a few limited samples, it is necessary that samples be as representative as possible of the entire population. A sample should be chosen at random and not in a selective manner, i.e., obviously good or bad samples should not be purposely chosen. 2. Distribution of results The compressive strength test results of cubes from random sampling of a mix, although exhibit variations, when plotted on a histogram are found to follow a bell-shaped curve termed as the Normal or Gaussian distribution curve. The results are said to follow a normal distribution as shown in Fig. 9.1, if they are equally spaced about the mean value. However, some divergence from the smooth curve is only to be expected, particularly if the number of results available is relatively small. The normal distribution curve can be used to ascertain the variation of strength from the mean. The area beneath the curve represents the total number of test results. The proportion of results less than the specified value is represented by the area beneath the curve to the left-hand side of the vertical line drawn through the specfied value.

Number of Results, n

15.9 %

15.9 %

s

s

1.0 % 2.3 %

0.1 %

20s 2.0 23s 2.3 0.14 %

31s 3.1

30s 3.0

Compressive Strength, MPa

Fig. 9.1

Normal distribution of compressive strength results

A normal distribution curve can be defined by two parameters, namely, the mean strength and the standard deviation. The mean strength is defined as the arithmetic

Quality Control of Concrete

223

mean of the set of actual test results. The standard deviation S is a measure of the spread of the results and the formula for computing the standard deviation is given in IS: 456–2000 as explained in Section 9.6.3. Figure 9.2 shows the frequency density versus compressive strength distribution curves of data population of the concrete mixes A and B. The distribution curves follow the normal distribution pattern. The curves are symmetrical about the mean value. Mix B indicates better quality control than that obtained for the mix A although both the mixes have the same average strength. Thus by exercising a better quality control, the standard deviation of the mix can be reduced by giving a lower probability of failure or a higher degree of reliability.

MIX-B

Frequency Density

Mean Value V

MIX-A

28-day Compressive Strength

Fig. 9.2

9.6 9.6.1

Frequency density versus compressive strength distribution curves of mixes A and B

MEASURE OF VARIABILITY Mean

The average or a mean x for a set of n observations x1, x2 ... xn, is expressed as n

∑ xi x=

i =1

n

As the sample size n increases, x approaches the mean of the entire population.

9.6.2

Range

The range is the difference between the largest and the smallest values in a set of observations.

224

Concrete Technology

9.6.3

Standard Deviation

The root mean square (rms) deviation of the whole consignment from the mean x is termed as the standard deviation and is defined numerically as n

∑ ( xi S=

x )2

i =1

n −1

where S = standard deviation of the set of observations, xi = any value in the set of observations, x = arithmetic mean of the values, and n = total number of observations. S has the same units as the quantity x. The square of standard deviation is called variance. Standard deviation increases with increasing variability. It may be appreciated that the value of S is minimum for very good control and progressively increases as the level of control slackens. An important property of standard deviation relating it to the proportions of all the results falling within or outside certain limits, can generally be assumed in the case of concrete work without serious loss of accuracy as long as techniques of random sampling are followed. The spread of the normal distribution curve along the horizontal scale is governed by the standard deviation, while the position of the curve along the vertical scale is fixed by the average value, the limit below or above which the proportion of the results can be expected to fall are set out as (x ± kS), where k is the probability factor. For different values of k, the percentage of results falling above and below a particular value is illustrated in Fig. 9.3, in relation to the area bounded by the normal probability curve. The values of k are given in Table 9.1. Alternatively, the variation of results about the mean can be expressed by coefficient of variation which is a non-dimensional measure of variation obtained by dividing the standard deviation by the average and is expressed as Table 9.1

Probability factor for various tolerances

Percentage 50 20 10 5 2.5 1.0 0.5 0.0 of results (1 in 2) (1 in 5) (1 in 10) (1 in 20) (1 in 40) (1 in 100) (1 in 200) below characteristic strength Probability 0.00 0.84 1.28 1.65 1.96 2.33 2.58 Infinity factor k

S × 100 x With constant coefficient of variation, the standard deviation increases with strength and is larger for high strength concrete. v=

Quality Control of Concrete

225

3.0

2.5

Probability Factor, k

2.0

1.5

1.0

0.5

0

2

3 4 5

8 10

20

30

50

70

100

200 300 500

1000

Number of results from which 1 would be expected to be below minimum strength

Fig. 9.3

9.7

Probability factor k and proportion of results expected to be below the minimum strength

APPLICATION

The standard deviation and the coefficient of variation are useful in the design and quality control of concrete. As the strength test results follow normal distribution, there is always the probability that some results may fall below the specified strength. Recognizing this fact IS: 456–2000 has brought in the concept of characteristic strength. The term characteristic strength indicates that value of the strength of material, below which not more than five per cent of the test results are expected to fall. In the design of concrete mixes, the average strength to be aimed, i.e., the target mean strength, should be appreciably higher than the minimum or characteristic strength if the quality of concrete is to comply with the requirements of the specifications. If, from previous experience, the expected variation in compressive strength is represented by a certain standard deviation or coefficient of variation, it is possible to compute the target mean strength of the

226

Concrete Technology

mix, which would carry with it a predetermined chance of results falling below a specified minimum strength. The target mean strength is obtained by using the following relation: ft = fck + kS where ft = target mean strength, fck = characteristic strength, k = probability factor and S = standard deviation. The value of k where not more than five per cent (1 in 20) of test results are expected to fall below characteristic strength is 1.65 as obtained from Fig. 9.3 or Table 9.1 and the above relation reduces to ft = fck + 1.65 S However, it should be noted that for a given degree of control, the standard deviation method yields higher target mean strengths than the coefficient of variation method for low-strength and medium-strength concretes. For high-strength concrete, the coefficient of variation method yields higher values of target mean strength. The cost of production being dependent on the target mean strength of concrete, the method of evaluation should be consistent with the observed trend of results for different ranges of strength. However, the use of the coefficient of variation is not envisaged in IS: 546–2000. To keep a control on the quality of concrete produced, it is required to cast a number of specimens from random samples and test them at suitable intervals to obtain results as quickly as possible to enable the level of control to be established with reasonable accuracy in a short time. IS: 450–2000 stipulates that random samples from fresh concrete shall be taken as specified in IS: 1199–1959 and the cubes shall be made, cured and tested at 28 days as described in IS: 516–1959. The test result of a sample be the average of the strength of three specimens (constituting the sample). The individual variation should not be more than ±15 per cent of the average. If it is more, the test result of the sample is invalid. The random sampling procedure is adopted to ensure that each concrete batch shall have a reasonable chance of being tested, i.e., the sampling should be spread over the entire period of concreting and cover all mixing units. The code prescribes minimum frequency of sampling of 1, 2, 3 and 4 number of samples, respectively, for 1–5, 6–15, 16–30 and 31–50 m3 of concrete being used in the job. For concrete quantity of 51 m3 and above, the number of samples shall be 4 plus one additional sample for every 50 m3 of concrete or part thereof. At least one sample should be taken from each shift. In case of continuous production unit, e.g., ready mixed concrete plant, the frequency of sampling may be as per agreement. Additional samples may be required for various purposes, e.g., for determination of seven days strength, accelerated strength, time of striking the formwork, etc. As far as the requirements of specifications with regard to the acceptance criteria for concrete is concerned, IS: 456–2000 stipulates that the concrete shall be deemed to satisfy the strength requirements provided the

Quality Control of Concrete

227

mean strength fm of any group of four non-overlapping consecutive test samples satisfies the following:

1. For M15 grade concrete fm ≥ fck + 0.825 times the standard deviation or fck + 3 MPa, whichever is greater with strength of individual test sample being ≥ fck − 3 MPa 2. For M20 or higher grade concrete fm ≥ fck + 0.825 times the standard deviation or fck + 4 MPa whichever is greater with strength of individual test samples being ≥ fck − 4 MPa To establish the value of standard deviation, results of at least 30 samples are used and the standard deviation so obtained is rounded off to the nearest 0.5 MPa. In the absence of established values of standard deviation, values of 3.5 MPa, 4.0 MPa and 5.0 MPa may be assumed for M15; M20–M25; and M30–M50 grade concretes, respectively, for very good quality control. For good quality control these values are increased by 1.0 MPa. Concrete of each grade shall be assessed separately. Table 9.2 gives suggested values of control ratios for various probabilities of results falling below minimum, with four different degrees of control. Table 9.2 Degree of control

Control ratio for different degrees of control for normal supervision Control ratio for probabilities of

Remarks

1 in 25

1 in 40

1 in 100

A

0.82

0.80

0.76

Weight-batching of cement and aggregate by servo-operation.

B

0.79

0.76

0.72

Weight-batching of cement and aggregate by manual operation.

C

0.77

0.74

0.69

Weight-batching of cement and volume batching of aggregate.

D

0.75

0.72

0.67

Volume batching for both cement and aggregate.

Since all the main variations of a job such as in batching, proportions of ingredients, characteristics of aggregates, etc., are reflected in the fluctuations of the water– cement ratio and this ratio is, in itself, closely related to compressive strength, a control-ratio can be applied to reduce the water–cement ratio to take into account the observed variations in the strength. The control-ratio is defined as Control ratio =

Water-cement ratio required to produce average strengt r h Water-cement ratio required to produce minimum strength

Typical values of standard deviation for different manufacturing conditions, i.e., the batching; mixing, placing and curing are given in Table 9.3.

228

Concrete Technology

Table 9.3

Typical values of standard deviation for different working of conditions for batching, mixing, placing, and curing

Concrete manufacturing conditions

Degree of control Standard deviation

Weigh batching of dried and accurately graded aggregate, and cement, exact water-to-cement ratio, proper mixing, placing and temperature controlled curing

Laboratory precision

1.3

Weigh batching of all materials, controlled grading of aggregates (three sizes of coarse aggregate + sand), adjustment for moisture in aggregates and continual supervision.

Excellent

2.8

Weigh batching of all materials, strict control of grading of aggregates, adjustment in water content for moisture in aggregates and continual supervision.

High

3.5

Weigh batching of all materials, control of aggregates grading, control of water added in mix and frequent supervision.

Very good

4.2

Weigh batching of all materials, use of two sizes of aggregates (coarse + fine), water content control by inspection of mix, periodic check of workability and intermittent supervision.

Good

5.7

Volume batching of all aggregates, allowing for bulking of fine aggregates, weigh-batching of cement, water content control by inspection of mix, and intermittent supervision.

Fair

6.5

Volume batching of all materials, use of all in aggregates and little supervision.

Poor

7.0

Volume batching of materials, use of all in aggregates, no correction and no supervision.

(uncontrolled)

8.5

In a construction where the concreting has been completed in three stages, a series of tests were conducted for a given grade of concrete. The specimens were tested at 28 days in each case and the results are represented in Table 9.4. Establish the standard deviation for the grade of concrete.

Example 9.1

Solution The standard deviation of the concrete produced up to the end of Stage I, (samples 1 to 24),

n = 24 Σx = 676.4 x = S x/n = 28.8 MPa

Σ(x − x )2 = 120.61 S=

Σ( xi x ) = 2.30 MPa n −1

Quality Control of Concrete Table 9.4

28-day compressive strengths of sets of cube specimens

Stage I Sample Concrete number strength (MPa) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

28.3 28.1 27.6 26.7 29.2 27.4 26.1 31.2 30.0 25.7 28.6 27.1 28.7 33.6 24.0 30.6 30.5 23.8 29.0 28.0 25.0 29.7 28.1 29.4

229

Stage II

Stage III

Sample number

Concrete strength (MPa)

Sample number

Concrete strength (MPa)

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

27.2 27.6 24.9 26.8 26.4 30.0 29.4 27.1 27.8 30.1 26.8 27.2 27.6 32.7 31.8 30.0 31.3 26.4 37.5 23.3 30.6 26.4 25.3 25.0

49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78

35.3 35.1 33.9 33.2 31.3 35.7 34.6 31.3 30.4 32.2 27.3 28.8 31.3 29.0 33.0 32.7 30.8 33.9 28.1 30.1 27.6 29.0 28.8 36.7 29.2 33.4 27.6 29.7 35.0 33.9

For Stage II (samples 25 to 48), n = 24 Σ = 679.2 x = 28.30 MPa Σ(x – x )2 = 217.64

230

Concrete Technology

S = 3.08 MPa For standard deviation of concrete produced up to the end of Stage II, n = 24 + 24 = 48 Σ x = 1355.6 x = Σ x/n = 28.24 MPa Σ (x – x )2 = 338.42 S = 2.68 MPa For Stage (III), sample 49 to 78 n = 30 Σ = 948.9 x = 31.63 MPa Σ(x – x )2 = 220.40 S = 2.76 MPa To obtain the standard deviation of the concrete produced to date, it is necessary to combine the standard deviations from different stages. n = 24 + 24 + 30 = 78 Σ x = 2304.5 x = 29.5 MPa Σ (x – x )2 = 770.77 S=

770.77 3 16 MPa 77

If in the above construction work, the grade of concrete used is M 30, apply the acceptance criteria of IS: 456–2000 to the following results (of extension work) each representing a day’s production (average strength of three specimens tested at 28 days) expressed in MPa:

28.00, 29.77, 31.10, 27.13; 30.27, 29.80, 27.33, 30.07; 26.57, 27.73, 28.10, 28.03; 30.70, 29.23, 30.47, 25.57; 36.27, 35.40, 34.10, 31.93; 32.60, 34.47, 31.10, 33.50 Arranging the sample test results of concrete into groups of four non-over lapping consecutive samples. The mean strength of resulting six consecutive groups are: 29.00, 29.37, 27.61, 28.99, 34.43 and 32.92 MPa. The established standard deviations is 3.00 MPa. According to IS: 456–2000 the strength requirements are: 1. For individual sample test results (a) fck − 4 = 26.00 MPa 2. For group test results (b) fck + 0.825 S = 32.48 MPa (c) fck + 4.00 = 34.00 MPa In the light of above strength requirements, it may be noted that 1. the individual test values of all samples except the 16th sample are greater than 26.0. Group 4 containing this sample is straightaway unacceptable. 2. the concretes pertaining to the Groups 1, 2 and 3 do not comply with the strength

Quality Control of Concrete

231

requirements stipulated in (b) and (c) above, hence are unacceptable. 3. the concrete produced during sampling of Group 5 is straightaway accepted as it satisfies both the strength requirements specified in (b) and (c). 4. the concrete represented by the Group 6 does not satisfy the strength requirement specified in (c) above but has strength greater than the requirement specified in (b) and hence may be accepted as being structurally adequate without further testing at the discretion of the designer.

9.8

QUALITY MANAGEMENT IN CONCRETE CONSTRUCTION

As explained earlier the quality, meant to measure the degree of excellence, does in fact measure the degree of fulfilment. The quality is thus a philosophy rather than a mere attribute. It is from this philosophy the distinctive culture emanates, guiding the society to attain targets set by it. The presence or absence of this culture makes all the difference which determines the level of acceptability. The constant awareness of this culture amongst other endowments have led many nations where they exist today. In the industrial climate particularly in manufacturing and process industry, the concept of quality management is age old and is extensively used, whereas it is recent in concrete construction industry. Every piece of equipment or product is subjected to quality management in the industrial production as a matter of routine. The quality management ensures that every piece of product keeps on performing over a period of time without heavy maintenance and upkeep. Fortunately in concrete construction even if rigid quality management measures are not followed, it performs, at least for reasonable period of time. On account of this cooperative property of the material, the concrete construction industry has been operating under the misconception that rigid quality management measures which are essential for an industrial product are not that essential for concrete. Thus in concrete industry of most of the developing countries, in spite of best efforts a great deal is yet to be achieved to derive maximum benefit out of this culture. Measures have been devised to enhance serviceable, maintenance and rehabilitation free life of the material and minimize, if not completely eliminate the possibilities of failure. The measures thought of are all related to quality management. Due to well coordinated efforts, a quantum jump has taken place in the design of reinforced concrete. The present day design methods are no longer limited to the earlier deterministic approaches such as working stress methods, but the limit state methods based on semi-probabilistic approaches are now being extensively practised. Today we are interested not only in 28-day cube strength, but also in its variability. The word characteristic has now come to stay in the codes of practice. The characteristic value approach gives insight and underlines the importance of quality assurance. Apart from the strength of concrete, the other important area of concern is the durability of concrete. A great deal of attention has been focussed on this and concrete technologists have come up with many effective suggestions. Some of them are: (i) use of minimum quantities of cement, (ii) drastic reduction in water–cement ratio maintaining the workability by use of plasticizers, (iii) use of pozzolanas, (iv) use of

232

Concrete Technology

low-heat cement, and (v) the most important of all is a good quality control in design, testing and production of concrete. During past decade and so, a good number of concrete structures have shown signs of distress much within their design life and most of these are due to poor durability considerations. The repair of such damages are highly expensive which could have been avoided with the application of quality control measures. The ever-increasing use of concrete in engineering structures, has made a demand of very high order to fulfil the targets or engineering excellence. In some structures the design is not limited to ensure structural integrity, but is based on the axiom that the probability of failure of such structures must be as low as possible and lower than a predetermined value of extremely small order.

9.8.1 Management of Uncertainties Primary Uncertainties All the structures have probabilities of failure in spite of being designed to carry the loads safely because in the probabilistic design approach, the design variables such as loads, material strength, etc., are considered as random variables. Hence, the probability of occurrence of a very large or a very small value of variable is never zero; the probability of such occurrence may, however, be very small. Thus whenever, the load variable exceeds the strength variable a failure situation occurs. If by applying a better quality control the standard deviation of mix is reduced, then the probability of failure will be reduced. Secondary Uncertainties The secondary uncertainties are introduced during both the design and construction phases. Selection of inappropriate design conditions, use of inapplicable site data, injudicious assumptions regarding boundary conditions and other data in design introduces secondary uncertainties. During construction more secondary uncertainties are introduced, e.g., use of inappropriate materials, violation of design conditions and incorrect interpretation of designers’ requirements, etc. Thus the level of confidence which may be viewed as a measure of closeness of the behavior of the actual constructed structure to that of analytical model influences the probability of failure. Although the odds of primary uncertainties can be taken care of by allowing for the randomness of the design variables, no proven analytical approach is available within the present state-of-the-art to increase the level of confidence against the effect of secondary uncertainties. It is, therefore, imperative that a systematic implementation of quality management system in design, manufacture and construction is a must as to produce a safe and reliable structure.

9.8.2 Quality Management System (QMS) QMS is the management and control system document having three elements: Quality Assurance (QA) plans, implementation of Quality Control (QC) process and Quality Audit (QA) system of tracking and documentation of quality assurance and quality control programmes. QMS ensures that the intended degree of excellence is attained. The owner or his representative formulates the policy, determines the scope of quality planning and quality management, establishes the relationship between the

Quality Control of Concrete

233

various participating agencies, and delegates responsibilities and authorities to them so that the quality objectives as set by owner are achieved. It must be understood that QMS cannot be developed in totality at the inception. QMS has to undergo stages of development as various project phases such as design, procurement of materials, construction, inspection, erection and commissioning are entered into with more and more agencies being involved and interfaces take place. The various stages of development of QMS are given in Table 9.5. Table 9.5 Stages

Various stages of development of QMS

QMS Elements

Planning

: Owner formulates QA policy and develops QA plan.

Engineering

: The consultant develops his own design QA programme and that of prospective vendors and contractors.

Procurement

: Suppliers develop and submit their own QA programmes and QC methods.

Construction : Contractors develop and submit their QA programmes and QC methods. Inspection

: The testing agencies develop their QA programmes.

The stage-wise development of QA programme based on owner’s QA plan are required to be reviewed and approved by the owner or by his consultant as the case may be. For continuous improvement, a good quality management system should be based on the full lifecycle approach to quality. This requires accurate, up-to-date reporting tools that provide greater accuracy when it comes to analysis. As discussed above, the quality management system should include a comprehensive list of modules that enable users to streamline a single quality process or an entire management system. Not only should the users be able to choose from a full list of QMS modules, but they should also be able to conceptualize and build their own QMS-forms. A typical lifecycle for quality in general terms is illustrated in Fig. 9.4.

Sharing of Operational Performance Data to Improve Decision Making

Management Review and Setting of Objectives

Status Monitoring and Performance Reporting

“Trickle Down” Functionality Ensures Objectives Are Communicated to All Operating Units

Continual Improvement Planning and Program Implementation Roll-up of Operational Peformance Metrics Operational Implementation of objectives & Tracking/ Recordkeeping and Performance Measurement Management of Activities

Fig. 9.4

A typical lifecycle for a good quality management system

234

Concrete Technology

Quality Assurance (QA) It is a planned and systematic pattern of all actions necessary to provide adequate confidence that a product will conform to established requirements. It is a system of procedures for selecting the levels of quality required for a project or a portion thereof to perform the functions intended and assuring that those levels are obtained. QA is thus the responsibility of the owner/user to ensure that consultants follow codes and sound engineering practices and that contractors and suppliers of materials comply with the contract requirements. QA programme developed by each agency responsible to the extent of its contractual obligation must contain the policies, practices, procedures and method to be followed such that the quality objectives laid down by the owner in his QA plan are fully met. The QA programme must be addressed fully (to the extent applicable) to the following aspects: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Organization set-up Responsibilities and authorities of various personnel involved Identification of coordinating personnel Quality control measures in design including field changes Establishment of control norms, acceptance and rejection criteria for materials Inspection programme for verification of contractual compliance including acceptance and rejection criteria Sampling, testing, documentation and material qualifications Corrective measures during non-complying conditions and non-conformance Resolution of technical differences/disputes Preparation, submission and maintenance of records at all stages

The quality assurance activity has to start right at the planning and design stage. Development of a QA programme for design activities is an art by itself and is beyond the scope of this book. Apart from organizational and administrative aspects, it has to cover procedures of design, conformance to codes, proper detailing and attention to durability and constructability. One important part of quality assurance is Peer Review. It is review of the project including its design, drawings and specifications by an Independent Professional or an agency, with equal or more experience and qualifications than of the professionals engaged for the design of the project.

Quality Control (QC) It implements the quality plan by those actions necessary for conformance to the established requirements. It is the system of procedure and standards by which a contractor, product manufacturer, material processor or the like, monitors the properties of finished work. QC is the responsibility of the contracting organization. The contracting organization is also responsible for QC activities related to its sub-contractors. Quality control starts with the construction. The constructing organization prepares the

Quality Control of Concrete

235

QA programme manual describing and establishing the QA and control system to be used by it in performing design, purchasing, fabrication, production of concrete and other construction activities for the contractual responsibilities assigned to it. Application area, indentification of agencies and personnel responsible for implementing, managing and documenting the QC programmes, their responsibilities and authorities must be well established in the document. The detailed steps in these procedures depend upon the scope and type of work and owner’s policy decision.

Quality Audit (QA) This is a system of tracking and documentation of Quality Assurance and Quality Control programmes. Quality Audit is the responsibility of the owner, and has to be performed at regular intervals through the tenure of the project. Quality Audit covers both the design as well as the construction phases. Thus the concept of Quality Management encompasses a total project and each element of that project. The systems on methodology of implementing concept of Quality Management depend on the available materials and construction technology. As the concrete technology changes, these systems also change. As such the systems of implementing concepts of Quality Management are not universal but regional and not static but dynamic, and ever changing. An integrated systematic implementation of QMS is extremely beneficial, but any attempt to make its piecemeal use will defeat the very purpose for which it is intended. In other words in order to produce a safe, reliable and durable structure, Quality Culture must begin at the beginning and be carried through all the stages of design, procurement, construction and be continued further into the in-service regime. It is only a matter of systematic cultivation and a desire towards increased perfection that can make a complete metamorphosis of a developing construction industry.

9.8.3

Cost Effectiveness of Quality Management

It has been the general experience that whether it is the owner who has to cover the cost of Quality Assurance, Quality Audit and Peer Review or the contractor who has to cover the cost of Quality Control, the expenditure is met out of savings which accrue from the project due to implementation of Quality Management Systems (QMS). On the part of owner the Quality Management ensures a product of assured quality, strength, reliability and maintenance free durable life cycle. This is achieved by eliminating chances of mistakes in planning, overdesign or underdesign and ensures proper detailing and constructability. Any of these items if overlooked can later cost heavily to the owner. It is universally accepted that every project has a Quality Cost Component. Every contractor has a choice as to when he will pay the cost. He can pay the controlled cost of Quality Control during construction, or he can pay the uncontrolled cost of correcting the defective workmanship and materials later. Patched up work, dismantling and re-doing unacceptable work, maintenance and up-keep during performance guarantee period may cost a contractor up to 20 to 25 per cent of his gross income. The unwilling contractors may be motivated to introduce Quality

236

Concrete Technology

Control within their organizations, by fixing the criteria of acceptability of concrete based on statistical control of strength which has a small range with a provision of a bonus for better quality control than stipulated. For example, if the criteria satisfied 100 per cent, the contractor receives 100 per cent payment. If it satisfies the lower limit (say 90 per cent), he gets paid 90 per cent. But if he satisfies the criteria by more than 100 per cent he gets a bonus up to a maximum of two per cent. An intensive dialogue between consultants and contractors on concrete specifications, acceptability criteria, testing procedures, field controls, inspection systems, etc., with the common objective of updating these documents and procedures for definitely attaining the desired quality may be extremely helpful. Since the development of concrete technology is closely linked to general construction industry, the passing away of period of shortage and variance have forced the construction industry to change and modernize. The changed situation is bound to give an impetus to concrete technology to update itself. An alternate criteria for acceptance of concrete based on its durability instead of its loadcarrying ability which helps in prolonging the serviceability life of concrete may be designed. The latter is based on passivity of concrete which can be evaluated by its minimum strength at 28 days. The former is based on active concrete that functions under changing conditions and respond to varying environments and abuses. Its performance is mainly based on water–cement ratio. Thus the designs must not be linked just to the strength of concrete but also to the durability of concrete. The introduction and implementation of Quality Management systems can be successful if the concrete industry directs its efforts towards increas-ing reliability, durability, economy, energy efficiency, versatility, capability, adaptability and aesthetics, as well as towards improvements in materials, material handling, quality control, education of users, construction methods, codes and specifications, disposal and recycling of waste and extension of the environment under which concrete can be used and placed. In addition, efforts should be directed to the development of accurate non-destructive testing procedures, continuous batching and new placing methods, immediate quality control tests, simplified forming methods, simplified reinforcing procedures, simplified methods of joining structural members, new design concepts, performance codes and improved cold and hot weather construction practices. The QMS need be updated to keep pace with advancement in concrete technology.

REVIEW QUESTIONS 9.1 Briefly explain the factors causing variations in the quality of concrete. What parameters are used in measuring this variability? 9.2 What are the advantages of quality control?

9.3 What are the strength requirements which a concrete should satisfy with regard to acceptance criteria as stipulated by IS 456-2000? 9.4 In a project a series of tests were conducted for M30 grade concrete used

Quality Control of Concrete in the construction. Apply the acceptance criteria of IS: 456–2000 to the following sample results (each sample represents the average strength of 3

237

specimens tested at 28 days): 30.70, 29.23, 30.47, 26.57; 31.27, 35.40 MPa. 9.5 Write short note on quality management in concrete construction.

MULTIPLE-CHOICE QUESTIONS 9.1 The quality control of concrete is appropriately defined as the (a) quality of overall workmanship and supervision at the site (b) assurance that all aspects of materials, equipment and workmanship are well looked after (c) conformity to the specifications, no more no less (d) assurance for the safety and serviceability of structure (e) rational use of appropriate materials and reduction in material costs 9.2 The quality control can be exercised by (a) the field controls, i.e., inspection and testing at all the stages (b) adequate compaction and curing (c) by strong motivation to do every thing right the first time (d) by restoring to the acceptance tests (e) ensuring conformity to specifications 9.3 Quality control means (a) extra cost (b) a rational use of the available resources (c) adequate design to minimize cost (d) All of the above (e) None of the above 9.4 Statistical quality control helps (a) in narrowing down the tolerance limits of variability (b) in taking into account the actual variability of concrete (c) to ascertain the range of value that can be expected under existing conditions (d) All of the above (e) None of the above 9.5 The standard deviation is

(a) the measure of spread of compressive strength test results about the mean strength (b) the measure of variability of test results (c) measure of proportions of all the results falling within or outside certain range (d) All of the above (e) None of the above 9.6 The target mean strength of concrete mix is given by (a) ft = k fck + S (b) ft = fck + kS (c) ft = fck + S (d) ft = fck + k (e) ft = fck + 0.85 S 9.7 As per IS:456–2000, a concrete of grade M 25 shall be deemed to satisfy the strength requirements if (i) every test sample has a strength not less than fck − 4 MPa (ii) not more than 5 per cent of samples shall have strength less than fck (iii) the mean of four non-overlapping test results is greater than ( fck + 0.825S) or ( fck + 4) whichever is greater The correct answer is (a) Both (i) and (iii) (b) Both (i) and (ii) (c) Both (ii) and (iii) (d) (i), (ii) and (iii) (e) None of the above 9.8 Larger the value of standard deviation (a) lower will be the variability (b) better will be level of control (c) poorer will be the level of control (d) not related to quality control (e) None of the above

238

Concrete Technology

Answers to MCQs 9.1 (b) 9.7 (a)

9.2 (a) 9.8(c)

9.3 (b)

9.4(b)

9.5 (d)

9.6 (b)

10 10.1

PROPORTIONING OF CONCRETE MIXES

INTRODUCTION

Concrete of different qualities can be obtained by using its constituents namely, cement, water, fine and coarse aggregates, and mineral additives, in different proportions as illustrated in Fig. 10.1. Also, the ingredients of widely varying characteristics can be used to produce concrete of acceptable quality. The common method of expressing the proportions of the materials in a concrete mix is in the form of parts, of ratios of cement, the fine and coarse aggregates with cement being taken as unity. For example, a 1:2:4 mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The amount of water, entrained air and admixtures, if any, are expressed separately. The proportion should indicate whether it is by volume or by mass. The water–cement ratio is generally expressed by mass. The amount of entrained air in concrete is expressed as a percentage of the volume of concrete. The amounts of admixtures are expressed relative to the weight of cement. Other forms of expressing the proportions are by ratio of cement to the sum of fine and coarse aggregates, i.e., aggregate–cement ratio and by cement factor or number of bags of cement per cubic meter of concrete. The wide use of concrete as construction material has led to the use of mixes of fixed proportions, which ensure adequate strength. These mixes are known as nominal mixes. These offer simplicity and, under normal circumstances, have a margin of strength above that specified. However, these do not account for the varying characteristics of the constituents and may result in under or overrich mixes. Generally, a nominal mix is expressed in terms of aggregate–cement ratio. Nominal mix concrete may be used for concrete of grade M20 or lower. The proportions of materials for nominal mix concrete shall be in accordance with Table 10.1. These mixes called standard mixes are by definition conservative, but are useful as off the shelf sets of proportion that allow the desired concrete to be produced with minimum preparatory work. For example, for M15 grade concrete the proportion is 1:2:4. For the ordinary concrete from which quite undemanding performance is expected, the nominal or standard mixes may be used. The concrete making materials being essentially variable result in the production of mixes of variable quality. In such a situation, for high performance concrete, the most rational approach of mix proportioning is to select proportions with specific materials in mind which possess more or less unique characteristics. This will ensure the concrete with the appropriate properties to be produced, most economically. Other factors like workability, durability, compaction equipment available, curing methods

240

Concrete Technology

adopted, etc., also influence the choice of the mix proportions. The mix proportion so arrived at is called the designed mix. However, the method does not guarantee the correct mix for the desired strength, thereby necessitating the use of trial mixes. In the process of mix proportioning, a number of subjective decisions are required on which hinge the important ramifications for the concrete. The designed mix serves only as a guide. For many works it is desirable to go through the process of mix design, for example, where a large volume of concrete is required, a minimization of the cement content may reduce the cost appreciably, or where for technical reasons the type of concrete required necessitates careful selection and proportioning of ingredients. Cement + Water

(Admixture)

Cement paste + Fine Aggregate

Mortar + Coarse Aggregate

Concrete

(a) Schematic stages of concrete production from its constituents

Water 16%

Air 6%

Sand (fine aggregrate) 26%

Portland cement 11%

Gravel or crushed stone (coarse aggregate) 41%

(b) Typical T proportions of constituents for a concrete mix

Fig. 10.1

Typical composition (proportions of constituents) of concrete mix Table 10.1

Proportions of nominal mix concrete

Grade of Total quantity of dry Maximum water content Concrete aggregate per bag of per bag of cement of 50 cement of 50 kg (kg) kg (liters)

Proportions of fine aggregate to coarse aggregate by mass

M10

480

34

Generally I:2 with upper

M15

350

32

limit as 1:1.5 and lower

M20

250

30

limit as 1:2.5

Proportioning of Concrete Mixes

10.2

241

BASIC CONSIDERATIONS FOR CONCRETE MIX DESIGN

The concrete mix design is a process of selecting suitable ingredients for concrete and determining their proportions which would produce, as economically as possible, a concrete that satisfies the job requirements, i.e., concrete having a certain minimum compressive strength, workability and durability. The proportioning of the ingredients of concrete is an important phase of concrete technology as it ensures quality and economy. The proportioning of concrete mixes is accomplished by the use of certain empirical relations which afford a reasonably accurate guide to select the best combination of the ingredients so as to achieve the desired properties. The design of plastic concretes of medium strengths can be based on the following assumptions.

1. The compressive strength of concrete is governed by its water–cement ratio. 2. For the given aggregate characteristics, the workability of concrete is governed by its water content. For high-strength or high-performance concrete mixes of low workability, considerable interaction occurs between the above two criteria and the validity of such assumptions may become limited. Moreover, there are various factors which affect the properties of concrete, e.g., the quality and quantity of cement, water and aggregates; techniques used for batching, mixing, placing, compaction and curing, etc. Therefore, the specific relationships used in the proportioning of a concrete mix should be considered only as a basis for making an initial guess at the optimum combination of the ingredients and the final mix proportion is obtained only on the basis of further trial mixes.

10.3

FACTORS INFLUENCING THE CHOICE OF MIX PROPORTIONS

According to IS: 456–2000 and IS: 1343–1980, the design of concrete mix should be based on the following factors: 1. Grade designation 3. Maximum nominal size of aggregates 5. Water–cement ratio 7. Durability

10.3.1

2. Type and grade of cement 4. Grading of combined aggregates 6. Workability 8. Quality control

Grade Designation

The grade designation gives characteristic compressive strength requirements of the concrete. As per IS: 456−2000, the characteristic compressive strength is defined as that value below which not more than five per cent of the test results are expected to fall. It is the major factor influencing the mix design. Depending upon the degree of control available at the site, the concrete mix has to be designed for a target mean compressive strength which is somewhat higher than the characteristic strength.

242

Concrete Technology

10.3.2

Type and Grade of Cement

The type of cement is important mainly through its influence on the rate of development of compressive strength of concrete. The choice of type of cement depends upon the requirements of performance at hand. Where very high compressive strength is required, e.g., in prestressed concrete railway sleepers Portland cement of grades 43 and 53 conforming to IS: 8112−1989 and IS: 12269−1987, respectively, will be found suitable. In situations where an early strength development is required, rapidhardening Portland cement conforming to IS: 8041−1990 is preferable and for mass concrete construction, low-heat Portland cement conforming to IS: 12600−1989 is preferred. The blended cements such as Portland pozzolana cement and Portland slag cement are permitted for use in reinforced concrete construction. While Portland slag cement is also permitted for prestressed concrete construction, the rate of development of early strength may be somewhat slower with blended cements. A cement of consistent quality which exhibits minimum variation, i.e., minimum standard deviation, in the quality expressed in terms of its compressive strength makes it easier to determine the most economical proportion of cement required to obtain a particular grade concrete mix only by changing the ratio of fine to coarse aggregates. The currently available good brands of cement have been reported to maintain standard deviations as low as 2.5, 1.5 and 1.0 MPa, respectively, for 33, 43 and 53 grades of cement. As explained in Chapter 2 that to account for the inherent variations or inconsistency in the quality of cement, IS: 10262−1982 has classified the cement grade-wise into six continuous ranges designated A to F, depending upon the 28 days compressive strength of the cement, as A (32.5−37.5 MPa), B (37.5−42.5 MPa), C (42.5− 47.5 MPa), D (47.5−52.5 MPa), E (52.5−57.5 MPa) and F (57.5−62.5 MPa). This classification covers the entire spectrum of strengths. The strength of cement to be used in mix design computations is not the mean strength fm of certain number of test results (say ‘n’), but should be the characteristic strength fck given by Eq. (10.1). fck = fm − kS

(10.1)

where k is probability factor, a statistical parameter for not more than five per cent test results to fall below the characteristic strength fck and S is standard deviation. For example, if the mean of ‘n’ compressive strength test results of a cement is 55, the cement would be apparently a grade-53 cement categorized as E (52.5−57.5 MPa). However, if the standard deviation of this particular cement is 4.0 MPa, the characteristic strength would be fck = 55.0 − (1.65 × 4.0) = 48.4 MPa Thus the cement actually comes under grade 43, i.e., D category (47.5−52.5 MPa). It is of prime importance to control the variations in quality of cement to the barest minimum so that cement can be classified for higher grade. If the 28-day compressive strength of cement is considered as an additional parameter influencing the relationship between water−cement ratio and 28-day compressive strength of concrete, the curves of Fig. 10.4 can be used to make more precise estimate of water−cement ratio for the given grade of cement.

Proportioning of Concrete Mixes

10.3.3

243

Maximum Nominal Size of Coarse Aggregate

The maximum nominal size of the coarse aggregate is determined by sieve analysis and is designated by the sieve size higher than the largest size on which 15 per cent or more of the aggregate is retained. The maximum nominal size of the aggregate to be used in concrete is governed by the size of the section and the spacing of the reinforcement. According to IS: 456−2000 and IS: 1343−1980, the maximum nominal size of the aggregate should not be more than one-fourth of the minimum thickness of the member, and it should be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum clear cover to the reinforcement or 5 mm less than the spacing between the prestressing cables. Within these limits, the nominal maximum size of the aggregate may be as large as possible, because larger the maximum size of aggregate, smaller is the cement requirement for a particular water– cement ratio. The workability also increases with an increase in the maximum size of the aggregate. However, the smaller size aggregates provide larger surface area for bonding with the mortar matrix which increases the compressive strength and reduces the stress concentration in the mortar-aggregate interface. For the concrete with higher water−cement ratio, the larger maximum size of aggregate may be beneficial whereas for high strength concrete, 10−20 mm size of aggregate is preferable.

10.3.4

Grading of Combined Aggregate

The relative proportions of the fine and coarse aggregates in a concrete mix is one of the important factors affecting the workability and strength of concrete. For dense concrete it is essential that the coarse and fine aggregates be well graded. Figure 10.2 shows a continuous range of sizes of aggregate used in concrete. In the concrete produced by using a well-graded aggregate smaller size particles and sand fill the voids between larger size particles, reducing the amount of space to be filled by water-cement paste as illustrated in Fig. 10.2(b), wherein magnified image of a piece of concrete shows a well-graded aggregate

(a) Continuous grading of aggregate

Fig. 10.2

(b) Hardened concrete

(a) Continuous grading of aggregate for high-quality concrete, and (b) magnified image of a piece of concrete showing a well-graded aggregate mix locked into a matrix of hardened cement paste. (Adopted from Portland Cement Association)

244

Concrete Technology

mix locked into a matrix of hardened cement paste. This results in improved strength, minimum shrinkage and lower cost of the concrete. Generally, the locally available aggregates do not conform to the standard gradings. In such cases, the aggregates need to be combined in suitable proportions so that the resultant (combined) grading approximates to a continuous grading close to the desired (or standard) grading. The process of combining aggregates is aimed at obtaining a grading close to the coarsest grading of standard grading curves, the most economical mix having highest permissible aggregate–cement ratio. IS: 383−1963 has recommended limits to the coarsest and finest gradings. The aggregates can be combined by analytical calculations. The method is easy to understand and calculations are trivial. Consider two aggregates (designated as aggregate-I and aggregate-II) are to be combined. Let a , b and g represent the percentages of the combined (resultant) aggregate, aggregate-I and aggregate-II, respectively, passing the seive corresponding to the point on standard grading curve taken as criterion, i.e., the point to which the combined aggregate is required to approximate. If x and y are the proportions of two aggregates in the combined state, then the condition that a per cent of combined aggregate passes the criterion sieve results in Eq. (10.2). b x + g y = a(x + y) or i.e., where

x α −γ 1 = = y β α k x:y=1:k k = ( b − a)/(a − g )

(10.2)

Hence the two aggregates have to be combined in the proportions of 1:k. The grading of the resulting combined aggregate is determined by first multiplying the gradings of aggregate-I and aggregate-II by 1 and k, respectively, then dividing the sum of corresponding products of the percentages passing the sieve sizes by (1 + k). The values are rounded off to the nearest percentage. The gradings of fine and coarse aggregates available at a construction site are listed in columns b and c of Table 10.2. These aggregates are to be combined in suitable proportions so as to obtain the specified grading chosen from standard grading curves which is listed in the column d of Table 10.2.

Example 10.1

Solution Let one kilogram of fine aggregate be combined to x kilogram of coarse aggregate to obtain the desired grading. Suppose the percentage passing IS: 4.75 mm sieve is selected as criterion. In the standard grading 42 per cent of the total aggregate passes the IS: 4.75 mm sieve. Hence using Table 10.2, or

96(1) + 3 (x) = 42(1 + x) x = (96 − 42)/(42 − 3) = 1.3846

Therefore the fine and coarse aggregates must be combined in the proportion 1.0:1.3846. The grading of resulting combined aggregate is obtained by multiplying

Proportioning of Concrete Mixes

245

columns (b) and (c) of Table 10.2 by 1.0 and 1.3846, respectively, and dividing the sum of these products by 1.0 + 1.3846 (= 2.3846). The resulting combined grading is listed in column (e) of Table 10.2. Comparing column (e) with column (d), it can be noted that the percentage passing IS: 4.75 mm sieve is same and combined grading is close to the desired grading. Table 10.2

Combining fine and coarse aggregates to a stipulated grading Percentage passing

IS sieve

Fine aggregate

Coarse aggregate

Specified grading

Combined grading

(a)

(b)

(c)

(d)

(e)

40 mm

100

100

100

100

20 mm

100

98

100

99

10 mm

100

43

65

67

4.75 mm

96

03

42

42

2.36 mm

89

0

35

37

1.18 mm

73

0

28

31

600 μm

48

0

20

20

300 μm

20

0

07

08

150 μm

02

0

0

01

In the above problem, there is only one point on the grading curve to which the aggregate is required to approximate. Comparing the grading of resulting combined curve with the selected standard grading curve, the percentage passing the criterion sieve necessarily agree but the other values may not. In some cases variation is very small which may be ignored. If, however, the discrepancies are large, the proportions may be changed by adopting another criterion point. It should be realized that mix proportioning is approximate, and it is extremely doubtful that the result would be better if the grading is further made closer. The method can also be applied if three or more aggregates are to be combined. Example 10.2 will illustrate the procedure. The gradings of fine and two coarse aggregates available at a project site are listed in columns (b), (c) and (d), respectively, of Table 10.3. These aggregates are to be combined so as to approximate the grading listed in column e of Table 10.3.

Example 10.2

Solution It is required to determine fractions x and y of the two coarse aggregates to be combined with unit weight of fine aggregate so as to obtain the specified grading. Two unknowns need two equations for solution. Let the criterion sieve sizes be 10 mm and 2.36 mm. According to the specified grading, the combined aggregate passing the IS: 10 mm sieve is 45 per cent, hence using Table 10.3.

246

Concrete Technology

100(1) + 94x + 18y = 45 (1 + x + y) 49x − 27y = 55

or

The combined aggregate passing IS: 2.36 mm sieve is 23 per cent, hence using Table 10.3. 84(1) + 2(x) + 0( y) = 23(1 + x + y) 21x + 23y = 61

or

Solving the two equations x = 0.2255 and

y = 2.4463

Hence, the fine aggregate, coarse aggregate-I and coarse aggregate-II must be combined in the proportions 1.000:0.2255:2.4463, respectively. The grading of combined aggregate is obtained by multiplying columns b, c and d of Table 10.3 by 1.0, 0.2255 and 2.4463, respectively, and dividing sum of these products by 1 + 0.2255 + 2.4463 (= 3.6718). The resulting combined grading is listed in the column f of Table 10.3. On comparing the resulting grading with the specified grading, it is noticed that the percentage of combined aggregate passing 10 mm and 2.36 mm sieve are the same as in the specified grading. The error is mainly in the percentages passing IS: 1.18 mm and IS: 600 μm sieves. However, since the mix proportioning is only a guide for trial mixes any further effort in approximating it more accurately is not necessary. Table 10.3

Combining two coarse aggregates with the fine aggregate to the stipulated grading Percentage passing

IS sieve

Fine aggregate

Coarse aggregate

Specified grading

Combined grading

Combined grading per cent ( f ) = [(b) + 0.2255(c) + 2.4463 (d)]/3.6718

(a) 20 mm 10 mm 4.75 mm 2.36 mm 1.18 mm 600 μm 300 μm 150 μm

(b) 100 100 100 84 75 51 11 02

(c) 100 94 12 2 0 0 0 0

(d) 95 18 2 0 0 0 0 0

(e) 100 45 30 23 16 9 2 0

(f) 97 45 29 23 20 14 3 0.5

10.3.5

Water-Cement Ratio

The compressive strength of concrete at a given age and under normal temperature depends primarily on the water–cement ratio, lower the water−cement ratio greater

Proportioning of Concrete Mixes

247

28-Day Compressive Strength of Concrete, MPa

is the compressive strength and vice versa. A number of relationships between compressive strength and water−cement ratio are available which are supposed to be valid for a wide range of conditions. In so far as the selection of the water–cement ratio for the target compressive strength at 28 days is concerned, Figure 10.3 is applicable for both ordinary Portland and Portland pozzolana cements with comparable validity. The cement strength or grade specific relations between free water−cement ratio and 28-day compressive strength for cements of grades 33, 43 and 53 are given in Fig. 10.4. However, the 28-day compressive strength of concrete is related to the 7-day compressive strength of cement mortar as shown in Fig. 10.5. These relationships can also be used for the estimation of water−cement ratio. For air-entrained concretes, the compressive strengths are approximately 80 per cent of that of non-air-entrained concretes. 60 Road Note No. 4

50

40 ACI-Cylinder Strength 30

20

BIS

10 0 0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

Water–cement Ratio

Fig. 10.3

Generalized relationship between water–cement ratio and compressive strength of concrete

The cements normally available have 7-day compressive strength between 17.5 MPa to 40 MPa. Thus depending upon the cement strength, an appropriate curve should first be chosen. The steps to be followed in selecting the water−cement ratio are given below. 1. The strength of cement to be used is determined. In India, only those type of cement are officially recognized, which give minimum seven-day strength of 22 MPa. 2. When cement strength data are available, the corresponding curve is chosen for the determination of water-cement ratio. In the absence of such data, the curve corresponding to cement strength of 22 MPa, the minimum permissible as per the Indian Standards may be used.

, MPa

of

Fig. 10.4

28-day

60

40

20

0

20

40

C

0.35

0. ree

.45 ter–Cement

io

5

F

6

.

5 . .5

Relation between free water–cement ratio and concrete strength at 28 days for different cement strengths (IS: 10262–1982)

0.3

G

G G

248 Concrete Technology

Proportioning of Concrete Mixes

7–day cement strength curve as per corresponding IS: 269–with regraded sand

45

A B C D E

40

35

Compressive Strength, MPa

249

17.5 21.0 24.5 28.0 31.5

22.0 26.4 30.8 35.2 39.6

30 E 25

D C

20 B 15 A 10

5 0.4

Fig. 10.5

10.3.6

0.5

0.6 0.7 0.8 0.9 Water–Cement Ratio (by mass)

1.0

Relation between water-cement ratio and compressive strength of concrete as related to 7-day strength of cement (IRC: 44–1972)

Workability

The workability of concrete for satisfactory placing and compaction is controlled by the size and shape of the section to be concreted, the quantity and spacing of reinforcement, and the methods to be employed for transportation, placing and compaction of concrete. The situation should be properly assessed to arrive at the desired workability. The aim should be to have the minimum possible workability consistent with satisfactory placing and compaction of concrete. It should be kept in mind that insufficient workability resulting in incomplete compaction may severely affect the strength, durability and surface finish of concrete and may thus prove to be uneconomical in the long run. For different placing conditions the recommended workabilities are given in Table 10.4. There is no rigid correlation between workabilities of concrete as measured by different test methods. It is desirable that for a given concrete, the test method be identified beforehand and workability be measured accordingly. The workability measured by different test methods for comparable concretes are given in Table 10.5.

250

Concrete Technology Table 10.4

Desirable workability of concrete for different placing conditions

Placing conditions

Degree of workability

Blinding Concrete; Shallow sections; Pavements using pavers Mass concrete; Lightly reinforced sections in slabs, beams, walls, columns; Floors; Hand placed pavements; Canal lining; Strip footings substructure walls Heavily reinforced sections in slabs, beams, walls, columns; pavements Slip form work; Pumped concrete; Trench fill; In-situ piling; Tremie concrete Table 10.5 Degree of a workability

Very low

Compacting factor or slump, mm Strict control required with C.F. of 0.75 to 0.80

Low

25−75

Medium

50−100 75−100

High

100−150

Very high (flowable)

200

Level of workability as measured by different test methods Values of workability in terms of Compacting factor

Slump (mm)

Vee-Bee time (sec)

Drop table revolutions

Extremely low

≤ 0.70*

—

30−20

96−48

(very stiff) Very low (stiff)

0.75−0.80

0−25

20−10

48−24

Low (stiff plastic) Medium (plastic)

0.80−0.85 0.85−0.92

25−50 50−75

10−5 5−2

24−12 12−6

High (flowing)

> 0.92

75–150

2–0

6–0

Notes 1. *Compacting factor test is not used for concrete with aggregate having maximum nominal size of 40 mm and higher.

10.3.7

Durability

The durability of concrete can be defined and interpreted to mean its resistance to deteriorating influences which may reside inside the concrete itself, or to the aggressive environments. The requirements of durability are achieved by restricting the minimum cement content and the maximum water–cement ratio to the values specified by the Ministry of Road Transport and highways (Morth)-IRC specifications for Road and Bridgeworks for bridges and by IS 456-2000 for other structures are given in Table 10.6. The permeability of cement paste increases exponentially

Proportioning of Concrete Mixes Table 10.6

251

Minimum cement content, minimum water–cement ratio and minimum grade of concrete for different exposure conditions [MORTH: IRC Specifiations for Road and Bridgeworks-2000]

(a) For bridges with prestressed concrete or those with individual span lengths more than 30 m or those that are built with innovative design/construction

Structural member

Min. cement content for all exposure conditions, kg/m3

Max. water cement ratio Exposure conditions

Min. grade of concrete Exposure conditions

Normal

Moderate

Severe

Severe

(i) PCC members

360

0.45

0.45

M25

M30

(ii) RPCC members

400

0.45

0.40

M35

M40

(iii) PSC members

400

0.40

0.40

M35

M40

(b) For bridge other than those mentioned in part (a) and for culverts and other incidental construction

Structural member

(i) PCC members (ii) RPCC members

Min. cement content for all exposure conditions, kg/m3

Max. water cement ratio exposure conditions

Min. grade of concrete exposure conditions

Normal

Severe

Normal

Severe

Moderate

Severe

250 310

310 400

0.50 0.45

0.45 0.40

M15 M20

M20 M25

(c) Different exposure conditions Type of exposure Mild Moderate

Severe

very Severe

Extreme

Exposure conditions(IS 456-2000) Concrete surfaces protected against weather or aggressive conditons, except those situated in coastal area. Concrete surfaces sheltered from severe rain or freezing whilst wet. Concrete exposed to condensation and rain Concrete continuosly under water. Concrete in contact or buried under nonaggrressive soil/ground water. Concrete surfaces sheltered from saturated salt air in coastal area. Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immeresed in sea water. Concrete exposed to coasted environment. Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing conditons whilst wet. Concrete in contact with or buried under aggressive sub-soil/ground water. Members in direct contact with liquid/solid aggressive chemicals

Notes (i) The minimum cement content is based on 20 mm aggregrate (nominal maximum size). For 40 mm and larger size aggregates, it may be reduced suitably but the reduction shall not be more man 10 per cent. (ii) For underwater concreting, the cement content shall be increased by 10 percent. (iii) The cement content shall be as low as possible but not less than the quantities specified above. However, in no case shall it exceed 540 kg/m3 of concrete.

252

Concrete Technology

with increase in water-cement ratio above 0.45 or so. Thus from considerations of permeability, the water-cement ratio is usually restricted to 0.45 to 0.55, except in mild environments. For a given water-cement ratio, the cement content in the concrete mix should correspond to the required workability, keeping in view the placing conditions and the concentration of reinforcement. In addition, the cement content is chosen to ensure sufficient alkalinity to provide a passive environment against corrosion of steel, e.g., in concrete for marine environment or sea water a minimum cement content of 350 kg/m3 or more is required. Moreover, the cement content and water-cement ratio are so chosen as to provide a sufficient volume of cement paste to overfill the voids in the compacted aggregates. The blended cements like Portland pozzolana cement and Portland slag cement render greater durability to the concrete in sulfatic environments and sea water. Resistance to alternate freezing and thawing is not so important for Indian conditions, but wherever situations demand, air-entrained concrete could be employed using an air-entraining admixture. Air-entrainment lowers the compressive strength but increases workability which may permit certain reduction in the water content to make up the loss in compressive strength.

10.3.8

Quality Control

The strength of concrete varies from batch to batch over a period of time. The sources of variability in the strength of concrete may be considered due to variation in the quality of the constituent materials, variations in mix proportions due to batching process, variations in the quality of batching and mixing equipment available, the quality of supervision and workmanship. These variations are inevitable during production to varying degrees. Controlling these variations is important in lowering the difference between the minimum strength and characteristic mean strength of the mix and hence reducing the cement content. The factor controlling this difference is quality control. The degree of control is ultimately evaluated by the variation in test results usually expressed in terms of the coefficient of variation. It can be summarized that the aim of mix design is to obtain a most practical and economical combination of materials that will produce a concrete mix of necessary plasticity (workability) and, at the same time, produce hardened concrete of required strength and durability. Most of the mix design procedures are primarily based on the water–cement ratio law and absolute volume system of calculating the amount of materials. As explained earlier, according to Abram’s law, the strength of fully compacted hardened concrete is approximately inversely proportional to the water content per cubic meter of cement, i.e., water-cement ratio. The calculation of the quantities of the aggregates to be used with a given cement paste is based on the absolute volume method. The absolute volume of loose material is the actual volume of the solid matter in all the particles ignoring the space occupied by the voids between the particles. The absolute volume is calculated as given in Eq. (10.3). Absolute volume =

Mass of loose dry material Specific gravity × Mass of unit volume of wate t r

(10.3)

The general process of mix design is outlined in the flowchart given in Fig. 10.6.

Proportioning of Concrete Mixes

253

Degree of quality control envisaged stipulated characteristic strength Type of cement T Type of exposure T (Durability)

Mean target strength Water-Cement ratio Water content Fine aggregate as per cent of total aggregate by absolute volume

Concrete mix proportions

No

• Maximum size of aggregate • T Type and shape of aggregate • Grading of fine aggregate • Required workability

Is trial mixes strength adequate?

Yes Capacity of concrete mixer Weight of ingredients per batch

Fig. 10.6

10.4

Steps involved in mix proportioning

METHODS OF CONCRETE MIX DESIGN FOR MEDIUM STRENGTH CONCRETES

Most of the available mix design methods are based on empirical relationships, charts and graphs developed from extensive experimental investigations. Basically they follow the same principles enunciated in the preceding section and only minor variations exist in different mix design methods in the process of selecting the mix proportions. The requirements of the concrete mix are usually dictated by the general experience with regard to the structural design conditions, durability and conditions of placing. Some of the commonly used mix design methods for medium strength concrete are the following: 1. Trial and adjustment method of mix design 2. British DoE mix design method 3. ACI mix design method 4. Concrete mix proportioning-IS Guidelines 5. Rapid method for mix design The general step-by-step procedure for proportioning of concrete mixes is summarized below. 1. The maximum nominal size of the aggregate, which is economically available, is determined as per the specified requirements. The gradings of different

254

2. 3.

4.

5. 6.

7.

8.

9.

Concrete Technology

size aggregates is determined. The proportions of different size aggregates to obtain a desired combined grading are determined. The mean target strength is estimated from the specified characteristic strength and the level of quality control. A suitable water-cement ratio to obtain a concrete mix of desired strength is selected from the generalized curves given in Fig. 10.3 or cement grade specific curves of Fig. 10.4. The water-cement ratio so chosen is compared with that required for durability, the lower value is adopted. The degree of workability in terms of slump, compacting factor or Vee-Bee time is selected as per job requirements. The water content for the required workability is computed. The cement content is calculated and its quantity is checked for the requirements of durability. The percentage of fine aggregate in the total aggregate is determined from the characteristics of coarse and fine aggregates. Alternatively, the aggregatecement ratio may be determined. The concrete mix proportions for the first trial mix are computed and concrete cubes are cast in the laboratory as per standard codal procedure. After the required period of curing, the cubes are tested for the compressive strength of the mix. The trial batches, obtained by making suitable adjustment in water-cement ratio or aggregate–cement ratio or in proportions of cement, sand and aggregate, are tested till the final mix composition is arrived at. The final proportions are expressed either on mass or volume basis.

Most of the available mix design methods are essentially based on the above procedure and due consideration should be given for the moisture content of aggregate and the entrained air.

10.5

TRIAL AND ADJUSTMENT METHOD OF MIX DESIGN

The trial and adjustment method is based on experimental approach and aims at producing a concrete mix which has minimum voids and hence, maximum density. The fine aggregate is mixed in sufficient quantity to fill the voids in the coarse aggregate; and cement paste is used in sufficient quantity to fill the voids in the mixed aggregate as illustrated in Fig. 10.2(b). The proportion of fine to coarse aggregate which gives maximum mass of combined aggregate can be obtained by trials. The process consists of filling a container of known volume with the two materials in thin layers, the fine being placed over the coarse aggregate and lightly rammed after each layer. If the container is shaken too much, the coarse aggregate will try to come on the top and the fine aggregate will deposit at the bottom without filling the voids of the coarse aggregate. Since the density of the particles of fine and coarse aggregates is nearly the same, the mixture giving maximum weight will have maximum solid matter and hence least voids. Such a combination will need the least amount of cement per cubic meter of concrete and will be most economical for a given water-cement ratio and slump. In an alternate trial mix method, sand is combined with the coarse aggregate in several proportions, such as 20:80, 30:70, 40:60, 50:50 and 60:40, and for each

Proportioning of Concrete Mixes

255

such mixture, the quantity of cement paste of a certain water–cement ratio per unit volume of concrete is determined to give the required workability (expressed in terms of slump). The percentage of sand corresponding to the ratio requiring minimum cement, is termed optimum percentage. If the quantity of sand used is more than the optimum, more cement will be needed to have the same consistency. On the other hand, a smaller quantity of sand will make the mix harsh unless more cement is used for proper consistency. The as optimum percentage of sand is lower for a low water-cement ratio. The step-by-step procedure of mix proportioning is as follows. 1. The target mean compressive strength is determined from the characteristic strength. 2. The water-cement ratio is chosen for the target mean strength computed in Step 1. The water-cement ratio so chosen is checked against limiting water- cement ratio for the requirements of durability and the lower of the two values is adopted. 3. The workability is determined in terms of the slump required for a particular job. 4. The maximum nominal size of the coarse aggregate that is available or desired to be used, is determined. 5. The fine and coarse aggregates are so mixed that either the weight per liter of mixed aggregate is maximum or the sand percentage corresponds to the optimum value. 6. By actual trials the quantity of cement (in the form of cement paste) required per unit volume of aggregate to give the desired slump is determined. 7. The proportions of cement, fine aggregate, coarse aggregate and water to meet the requirements of strength, durability, workability and economy are computed and concrete cubes are cast and tested after the required period of curing for the compressive strength. 8. The trial mix is adjusted, if necessary, by varying the water−cement ratio or the aggregate-cement ratio to suit the actual requirements of the job.

10.6

NEW EUROPEAN STANDARDS ON CONCRETE

In view of the new European Standards having been implemented throughout Europe with effect from December 2003, it is desirable to review briefly these standards for concrete to achieve better understanding of the British DoE mix design method. The new standards for concrete are: 1. 2. 3.

Concrete Design (EN:206) Concrete Service Life (EN:1992) Concrete Repair (EN:1504)

In the process, the British Standard BS:5328 has been withdrawn and replaced by a more comprehensive European Standard EN:206-1 and its complementary British Standard BS:8500. In the UK, EN:206-1 is called the BS EN:206-1:2000 Concrete-Part 1: Specification, performance, production and conformity. This standard gives details of the requirements for specifying and producing fresh concrete to comply with EN:206-1. The complimentary standard BS:8500 is published in two parts:

256

Concrete Technology

1. BS:8500-1, Method of specifying and guidance for the specifier, and 2. BS:8500-2, Specification for the constituent materials and concrete. The standard for fresh concrete, a derived user-friendly document composite of BS EN:206-1 and BS:8500 contains additional guidance and a commentary. An outline of the some of important changes in new code provisions is given in the following sections.

10.6. 1

Terminology

The main changes in BS EN:206-1 are in the terminology used. In this new standard, the word mix has been dropped and the word concrete used in its place, i.e., a designed mix is referred as a designed concrete. Typical examples of some of the new terms and definitions used in the new standards for the common existing terms are listed in Table 10.7. Table 10.7

Typical examples of new terminology

Old standards

New standards

Mix Strength or grade Slump or workability PC/OPC 20-mm single-size aggregate 10-mm single-size aggregate 20-5 mm graded aggregate Medium (M) fine aggregate

Concrete Strength class Consistence (target or class) CEM I 10/20 4/10 4/20 0/4 or 0/2MF

1. Additions This is the term used for constituent materials, such as fly ash, ground granulated blast furnace slag, silica fume, etc. that are added at the concrete mixer. 2. Combinations Refer to cements made in the concrete mixer by combining Portland cement with an addition in proportions that satisfy the criteria given in BS 8500-2. 3. Compressive strength class The grade of concrete is expressed by using letters C for normal and heavyweight concretes and LC for lightweight concrete followed by the minimum characteristic strength of a 150 mm (diameter) × 300 mm (high) cylinder, a back slash ( / ), and the minimum characteristic of 150 mm (and 100 mm in UK) cube strength, e.g., C25/30. 4. Concrete A specifier specifies a concrete and a producer designs a mix that satisfies all the specified requirements for the concrete. 5. Conformity Tests and procedures undertaken by the producer to verify the claims made on the delivery. This replaces the compliance testing procedures of BS:5328. 6. Consistence Indicates the workability. 7. Consistence class A recommended alternative to specifying consistence by a target value.

Proportioning of Concrete Mixes

257

8. Established suitability The concept of established addition suitability allows materials and procedures to be used on a national basis that are not currently covered by European standards, but have a satisfactory history of local use. 9. Execution Refers to workmanship. 10. Fly ash Pulverized-fuel ash (pfa). 11. Identity testing It is acceptance testing in all but name. It identifies whether a particular batch or batches of concrete come from a conforming population. 12. Intended working life European design codes give recommendations as listed in Table 10.8 for intended working lives for various types of structure. For resistance to corrosion of reinforcement, longer intended working lives require higher concrete qualities and/or larger minimum cover to reinforcement. The recommended intended working life for buildings and commercial structures is at least 50 years. The term at least has been used to emphasize that the most structures are expected to perform adequately for a period in excess of the intended working life. Table 10.8

Intended working lives recommended in BS EN:1990

Description of structure Temporary structures

Intended working life (years) 10

Replaceable structural parts

10 to 25

Agricultural and similar structures

15 to 30

Building structures and other common structures

50

Monumental building structures, bridges and other civil engineering structures

100

13. Mix A composition that satisfies all the requirements specified for the concrete. Different producers may have different mixes, all of which satisfy the concrete specification.

10.6. 2

Specifications for Cement

The cement types stipulated in European cement standard EN:197-1, -2, 2000, are CEM I, II, III, IV and V where in CEM I is a Portland cement/ Portland pozzolan cement and CEM II through V are blended cements. EN:197 also has strength classes and ranges (32.5, 42.5 and 52.5 MPa). CEM I cement generally used is of class 42.5 or greater for which specific suitability has been established, i.e. appropriate building inspectorate approval has been obtained. Similarly, the sulfate resisting cement in concrete conforming to EN:206-1 is a specific suitability established cement. Pulverized-fuel ash (fly ash) complying with EN:450 and silica fume with building inspectorate approval may be added to the concrete as concrete additions regardless of the type of cement. In case of specific suitability established addition conforming to EN:450-1 used in combination with CEM I cement, the cement and combinations are treated being equivalent by BS:8500. Thus when specifying the type of cement or combination,

258

Concrete Technology

letter C or CEM are not added before the II, III or IV. This makes it clear that both are acceptable. The producer will add C or CEM to the delivery document/ticket to indicate which one has been used. Table 10.9 provides guidance on the cement/ combination-type designations. Table 10.9 Designation

Guide to cement/combination type designations Guidance on cement/combination

CEM 1

Normal Portland cement

SRPC

Sulfate-resisting Portland cement

IIA

Portland cement with 6 to 20 per cent of pulverized-fuel ash (fly ash) or ground granulated blast furnace slag or limestone

IIB

Portland cement with 21 to 35 per cent of pulverized-fuel ash (fly ash) or ground granulated blast furnace slag

IIIA

Portland cement with 36 to 65 per cent ground granulated blast furnace slag

IIIB

Portland cement with 66 to 80 per cent ground granulated blast furnace slag

IVB

Portland cement with 36 to 55 per cent of pulverized fuel ash (pfa)

+SR

This applies to cement or combination types IIB, IIIB and IVB where the proportions and properties for a sulfate-resisting cement or combination are required.

10.6.3

BS EN:206-1 Specifications for Concrete

BS:8500-1 specifies concrete as either designed concrete or prescribed concrete, each with its respective subset of designated and standardized prescribed concrete. Designed concrete contains a performance requirement for strength and it is specified that the concrete shall be produced in accordance with the relevant clauses of BS EN:206−1/BS:8500−2 and also specify the following: • •

• • • • •

compressive strength class exposure class or limiting values for concrete composition related to durability; it should be noted that in some cases it may not be necessary to specify a maximum water−cement ratio nominal upper aggregate size requirements for aggregates including physical and mechanical characteristics chloride content class; consistence class permitted cement types permitted additions (admixtures)

1. Prescribed concrete Prescribed concrete requires the producer only to batch the specified quantities of constituent materials; the specifier assumes responsibility for concrete performance. 2. Standardized prescribed concrete It is the same as standard mix in BS:5328. The new term correctly identifies the type of concrete and avoids the misunderstanding caused when standard is taken to mean normal.

Proportioning of Concrete Mixes

259

3. Proprietary concrete BS:8500-1 has also introduced proprietary concrete that satisfies a defined performance under standard test conditions, e.g., selfcompaction. Any claims made for proprietary concrete are the responsibility of the producer who assures the performance, subject to good practice in placing, compacting and curing. For proprietary concrete the producer is not required to declare the composition.

10.6.4

Consistence Class

Consistence is a new term introduced by EN:206-1 which covers the workability of concrete. In the new standards, either a class or a target value can be used to specify consistence. Clause 4 of BS EN:206−1 gives classes for slump, flow, Vee-Bee and degree of compactability. Table 10.10 gives the likely target values for a range of slumps and flow classes which are the most commonly used measures of consistence. Table 10.10 Consistence

Likely target values for different consistence classes Consistence in terms of slump

Consistence in terms of flow class Class Likely Maximum Consi- Flow diam- Likely target range target variation stence eter range flow (mm) (mm) value* (mm) allowed (mm) class (mm) S1 10 to 40 20 −20 to +30 F2 350-410 380 S2 50 to 90 65 −20 to +30 F3 420-480 450 S3 100 to 150 120 −20 to +30 F4 490-550 520 S4 160 to 210 180 −20 to +30 F5 560-620 590 S5 > 220 F6 > 630 1. *The design or target slump is lower than the arithmetic mean value of the range to allow for the non-linear relation between slump and water content. 2. The contingence in terms of Vee-Bee class are designated by V0, V1, V2, V3 and V4 and compaction class are C0, C1, C2, and C3 The compacting factor test has been withdrawn.

10.6.5

Concrete Strength/Grade Designation

In mainland Europe, the strengths were referred as cylinder strengths whereas Ireland/Britain standards referred to strengths in terms of cubes. The European Standard EN:206-1 and complementary British standard BS:8500 have addressed the difference between two systems by adopting a dual system for concrete specifications. This dual description combines both the cylinder strength as well as cube strength with the cylinder strength appearing first. New strength classes introduced in the new code have replaced the target strength or grade of concrete of BS:5328. EN:206−1 gives a range of strength classes from C8/10 to C100/115 whereas BS:8500−1 has introduced two additional classes, C28/35 and C32/40, as these are used in the UK durability provisions. The compressive strength classes for normal-weight and heavy-weight concretes are listed in Table 10.11.

260

Concrete Technology

Table 10.11

Specified compressive strength classes or typical grades for normal-weight and heavy-weight concretes

Required grade, i.e., required minimum characteristic cylinder/cube strength fck,cyd /fck,cube (MPa) C8/10

C40/50

C12/15

C45/55

C16/20

C50/60

C20/25

C55/67

C25/30

C60/75

C28/35

C70/85

C30/37

C80/95

C32/40

C90/105

C35/45

C100/115

Typically, C28/35 or C32/40 concretes with minimum binder contents of 325 kg/ m3 and maximum water-cement ratio of 0.55 are used to provide for a good surface finish and ensure adequate abrasion resistance. For further illustration of new terminology, consider the case of concrete mix of 28 day characteristic crushing strength of 35 MPa with minimum cement content of 300 kg/ m3, slump of 90 mm and maximum aggregate size of 20 mm. In the new terminology, this mix shall be quoted as An EN:206-1 concrete is required with strength class: C28/35, 300 kg cement, maximum water cement ratio of 0.60 Exposure classes: Slump class: Upper aggregate size:

XC4, XF3, XA1 (20 year life) S2 (unplasticized) 20 mm.

If plasticized concrete is desired, the slump class shall not exceed S3.

10.6.6

Conformity Testing

Conformity testing has replaced the compliance testing of BS:5328. The guidance on the application of the conformity criteria is available in EN:206-1. The concrete producer shall declare that the concrete conforms to BS:8500-2. A declaration of conformity to BS:8500-2 includes conformity to BS EN:06-1. The EN:206-1 standard stipulates three conformity criteria for compressive strength for the continuous production control of concrete families. For illustration, consider the evaluation of these criteria with respect to a family with two members, composed of 10 test results of a reference concrete and five results of the other member, the criteria are listed in Table 10.12. Criterion #1 checks the conformity of the group mean (based on the transposed test results), while criterion #2 is a minimum value criterion that needs to be applied on each individual (non-transformed) test result. To confirm that each individual member belongs to the family, the mean of all non-transposed test results for a single family member must be assessed against criterion #3.

Proportioning of Concrete Mixes Table 10.12

15

Conformity criteria for concrete strength in EN:206-1

Criterion #1

Criterion #2

xn (MPa)

xi (MPa)

≥ fck, ref + 1.48 s

≥ fck, ref − 4

n

261

ni

Criterion #3

xn

i

(MPa)

2

≥ fck, i − 1

3

≥ fck, i + 1

4

≥ fck, i + 2

5

≥ fck, i + 2.5

6

≥ fck, i + 3

The standard deviation is supposed to be known and is 5 MPa.

10.6.7

Concrete Temperature

The temperature of fresh concrete at the time of delivery shall not exceed 30oC unless otherwise permitted by the specifier.

10.7

BRITISH DOE METHOD OF CONCRETE MIX DESIGN

The British DoE method is a general method developed by the Department of Environment that can be applied to produce designed concrete, using cements and aggregates which conform to the relevant British Standards. The mixes are specified by the mass of the different materials contained in a cubic meter of fully compacted fresh concrete. The method is based on the following assumptions: 1. The volume of freshly mixed concrete equals the sum of the absolute volumes of its constituent materials, i.e., the water, cement, air content and the total aggregate. The method therefore requires that the absolute densities of the materials be known in order that their absolute volumes may be calculated. 2. The compressive strength class of a concrete depends on (a) the free water-cement ratio; (b) the type of coarse aggregate, i.e., whether the aggregate is crushed or uncrushed (gravel); and (c) the type of cement, i.e., whether the cement is normal (ordinary) Portland cement or combined cement. 3. The consistence (workability) of concrete depends primarily on (a) the free water content; (b) the type of fine aggregate and, to a lesser degree, type of coarse aggregate; and (c) the nominal upper (maximum) size of coarse aggregate. 4. The consistence depends secondarily on (a) the fraction of the fine aggregate as a proportion of the total aggregate content; (b) the grading of the fine aggregate; and (c) the free water-cement ratio. EN:206 exerts relatively little influence directly on the process of design of concrete mixtures which is a key part of concrete production. However, it does

262

Concrete Technology

of course have considerable indirect effect through the requirements for specification and conformity. The basic mix design approach is the same. It is based on the characteristic value approach for strength, the target mean for slump and for air, the minimum target mean for cement content and maximum target mean for water−cement ratio.

10.7.1

Design Parameters

Target consistence (workability) of fresh concrete EN:206 permits specification by slump class for the normal working range, i.e., zero slump to 200 mm slump and by other test methods. Where consistence other than slump is specified, it is recommended that a relationship between the two is established. Characteristic compressive strength As discussed earlier, EN:206 classifies strength in terms of 28-day characteristic strengths on the basis of cylinders and cubes, e.g., C25/30, where the first number is the strength of a 150 mm (diameter) × 300 mm (height) cylinder and the second number is the 150-mm cube strength. However, it should not be presumed that by giving both cube and cylinder strengths, a particular relationship is being assumed for purposes of conversion for concrete design or control. The strength margin factor and the standard deviation can be used for calculation of the target mean corresponding to the strength specified and the degree of safety required to take account of the conformity rules stipulated in EN:206 for strength and for production control. It should be noted that the appropriate margin factor and standard deviation for cylinders may differ from those for cubes. Design for tensile strength Design for tensile strength can be performed on the basis of compressive strength by first determining the relationship between tensile and compressive strengths from concrete trials. The relation is generally material sensitive. Target air content of fresh concrete For non-air entrained concrete, air content is not specified but entrapped air is as usual considered in design for EN:206 concrete. For air entrained concrete, EN:206 specifies minimum total air content with a maximum total air content being four per cent higher than the specified minimum. Minimum target cement content and maximum target water–Cement ratio EN:206 requires specification of minimum cement content and maximum water−cement ratio based on durability considerations which include a set of exposure classes related to different mechanisms of deterioration. The main classification is given in Table 10.13. With the exception of X0, each class of exposure is split into a number of subclasses. In practice, there will always be one and, in many cases, more than one relevant exposure class. Exposure class X0 exists on its own and there are no requirements for the water−cement ratio or the minimum cement content. The exposure classes and resistive measures listed in Table 10.14 will provide the planner and designer a rapidly usable basis for identifying relevant exposure classes.

Proportioning of Concrete Mixes Table 10.13

Main exposure classes based upon environmental action

Designation of exposure classes

Description of environmental action or type of attack

X0 XC1, 2, 3, 4

No risk of corrosion or any other attack Corrosion induced by carbonation

XD1, 2, 3 XS1, 2, 3 XF1, 2, 3, 4 XA1, 2, 3

Corrosion induced by chlorides other than from seawater Corrosion induced by chlorides from seawater Freeze-thaw attacks with or without de-icing agents Chemical attack, Abrasion Table 10.14

Exposure classes and provisions for resistance

Exposure classes (environmental effects, i.e., attacks) Class desigType and degree of exposure nation XO

XC

XD / XS

XF

263

No attack

No concrete attack

1 Carbonation Dry Constantly wet 2 H2O + 3 Moderately moist Wet / dry 4 CO2 1 Chloride Moderately moist 2 H2O + Cl Constantly wet 3 Wet / dry 1 Freeze-thaw Moderate water /+ salt saturation (wo.s.)1 2 Moderate water saturation (w.s.) 3

High water saturation (wo.s.) High water

4

Provisions for resistance Maximum Minimum water– cement cement ratio No No requirement requirement 0.75 240 0.75 240 0.65 260 0.60 280 0.55 300 0.50 320 0.45 320

Strength class

C8/10 C16/20 C16/20 C20/25 C25/30 C30/37 C35/45 C35/45

0.60

280

C25/30

0.55 + LP 0.50 0.55 + LP 0.50 0.50 + LP

300 320 300 320 320

C25/30 C35/45 C25/30 C35/45 C30/37

0.60 0.50 0.45 0.55 0.45 0.45

280 320 320 300 320 320

C25/30 C35/45 C35/45 C30/37 C35/45 C35/45

saturation (w.s.) XA

XM

1 2 3 1 2 3

Chemical attack Abrasion (Wear)2

Weakly corrosive Moderately corrosive Strongly corrosive Moderate wear Severe wear Very severe wear

1

Abbreviations: w = with; wo = without; s = de-icing salt

2

BS EN:206-1 does not contain abrasion classes

264

Concrete Technology

An exposure class which requires the greatest resistance in the form of the lowest water-cement ratio along with the highest minimum cement content and the highest concrete strength class is selected. However, the minimum cement contents are independent of the type of cement used. EN:206 specifies design margins in the minimum cement content of minus 10 kg and in maximum water-cement ratio plus 0.02 in trial batch tests.

Additions (admixtures) EN:206 contains provisions for the use of Type 1 (nearly inert) additions and Type 2 (pozzolanic or latent hydraulic) additions. The effect of additions on water demand, strength and on the restrictions placed upon their use in specifications is taken into account. EN:206 specifications for durability allow to count the proportion (k) of addition in the combination with cement towards satisfying specified limits for minimum cement content and maximum water-cement ratio. Here, the factor k called the efficiency or strength factor of the addition, refers to relative strength of addition with respect to the cement. Some additions are allowed to be counted fully towards durability provided special tests of the combinations have been made.

Mean Size of Aggregate A new series of standard sieve sizes for calculating mean sizes of aggregates for concrete has been recommended. The designations are established from the nominal lower and upper sieve sizes for the particular aggregates, the lower size being stated first. For example, an aggregate of maximum nominal size of 10 mm is designated as 4/10. The maximum aggregate sizes recommended are 10 mm; 20 mm and 40 mm.

10.7.2

Procedure for Concrete Design

The method is suitable for the design of normal concrete having 28-day compressive strength as high as 75 MPa for non-air-entrained concretes. The method is also suitable for the design of concretes containing pulverized-fuel ash (fly ash) and GGBFS. The concrete design is carried out in the following six steps described in the flowchart given in Fig. 10.7. 1. Selection of free water-cement ratio (a) The target mean strength is obtained by adding a margin to the stipulated characteristic strength. The margin is either specified or calculated for a given proportion of defectives and statistical standard deviation. (b) If air entrainment is specified, the artificially raised modified target mean strength is calculated. (c) The maximum free water–cement ratio is either specified or selected which will provide the target mean strength for concrete made from the given types of coarse aggregate and cement. The procedure is as follows: For the given type of cement and aggregate, the compressive strength at the specified age corresponding to the reference water−cement ratio of 0.50 is obtained from Table 10.15. For example, when normal Portland cement and uncrushed aggregate are used, the compressive strength is 43 MPa at 28 days. With this pair of data (43 MPa and water−cement ratio = 0.50) as a controlling

Standard deviation

Free water-cement ratio

Type of T cement

Aggregate cement ratio

Water content

B

Type of T aggregate

C Coarse aggregate contentt

Fine aggregate content

B

Grading of fine aggregate

Weight off fine aggregate per cent

Nominal maximum size of aggregate

Batch weights

Moisture content adjustments

Absorption and moisture

Workability

Type of T construction

Concrete mix design flow chart for DoE method

Cement content

A

Water cement ratio curve

Fig. 10.7

Specific gravity of cement

Check for minimum cement content

Check for water cement ratio

T Target mean compressive strength

Stipulated compressive strength

A

Proportioning of Concrete Mixes 265

266

Concrete Technology

or reference point, a strength versus water−cement ratio curve is located in Fig.10.8. In this particular case, it is the fourth (dotted) curve from the top of Fig.10.8 passing the controlling point. Using this curve, the water-cement ratio is determined corresponding to the computed target mean strength. In case an existing curve is not available which passes through the controlling point, the curve is interpolated between two existing curves in the figure. Table 10.15

Approximate compressive strength of concretes with water-cement ratio as 0.5

Type of cement

Type of coarse aggregate

Compressive strength (MPa) Age (days) 3

7

28

91

Uncrushed

22

30

43

49

Crushed

27

36

49

56

Rapid hardening

Uncrushed

29

37

48

54

Portland cement

Crushed

34

43

55

61

Ordinary (CEM 1) or sulfate resisting cement (SRPC)

Compressive Strength, MPa

90 80

Starting ine using data from T 10.15

70

int B is for xample 10.3 oint s for xample 10.6

60 50 40 30 20 10 0 0.3

0.4

0.5

0.6

0.7

0.8

0.9

Water–Cement Ratio

Fig. 10.8

Variation of compressive strength with water-cement ratio (DoE)

(d) The water-cement ratio computed in Step 1(c) is compared with the maximum water-cement ratio specified for the durability, and the lower of the two values is adopted.

267

Proportioning of Concrete Mixes

2. Determination of free water content The water, which is available to react with the cement, is termed the free water content of the concrete and influences the strength, durability and consistence of the concrete. It is the sum of (i) the added water (ii) the surface water of the aggregates and (iii) the water content of admixtures less, (iv) the water absorbed by the aggregate during the period between the mixing and the setting of the concrete. The water to be added is estimated as follows: (a) The minimum free water content is either specified or selected from Table 10.8, which will provide water control for the target consistence (specified in terms of slump or flow diameter or Vee-Bee time) for the concrete made with the given fine and coarse aggregate types and nominal upper size of coarse aggregate. Table 10.16

Approximate water content required for target consistence

Consistence class

S1 / F2

S2 / F3

S3 / F4

S4 /F5

(Very low)

(Low)

(Medium)

(High)

10 to 40

50 to 90

100 to 150

160 to 210

Class range, Flow 350–410 diameter range (mm)

420–480

490–550

560–620

Slump (S) class Flow (F) class

Class range, Slump (mm)

I. Water content Water content ( kg/m3)

Size of aggregate

Type of aggregate

4/10

Uncrushed

150

180

205

225

Crushed

180

205

230

250

10/20

Uncrushed

135

160

180

195

Crushed

170

190

210

225

Uncrushed

115

140

160

175

Crushed

155

175

190

205

20/40

II. Reduction in water content when additives are used Percentage of additive in combination with cement

Reduction in water content ( kg/m3)

10

5

5

5

10

20

10

10

10

15

30

15

15

20

20

40

20

20

25

25

50

25

25

30

30

(b) When the coarse and fine aggregates used are of different types, the water content is estimated by the expression given by Eq. (10.4). W = (2Wf /3) + (Wc /3)

(10.4)

268

Concrete Technology

where

Wf = water content appropriate to type of fine aggregate Wc = water content appropriate to type of coarse aggregate (c) If the free water content has been determined for target consistence, it is adjusted for the specified air entrainment, and further adjusted if a waterreducing admixture is specified. 3. Determination of cement content (a) The minimum cement content is computed by dividing the free-water content obtained in the Step 2 by the free water–cement ratio obtained in the step 1. Cement content (kg/m3) =

water content water-cement ratio

(10.5)

(b) The computed cement content required for strength is compared with the maximum cement content which is permitted. If the calculated cement content is higher than the specified maximum, then the target strength and target consistence cannot be achieved simultaneously with selected materials. In such a situation, the process is repeated by changing the type of cement, the type and upper size of the aggregate. (c) The computed cement content required for target strength is compared with the minimum cement content which is specified for durability and the greater of the two is adopted in the concrete. (d) Divide the free-water content by the cement content adopted in the concrete to obtain a modified free water-cement ratio. Thus, the cement content is the minimum given by a free water-cement ratio that is low enough to provide the target strength and durability. For a concrete designed using EN:206 specifications for durability, the EN:206 specifications allow to count the proportion (k) of addition in the combination with cement towards satisfying specified limits for minimum cement content and maximum water−cement ratio, rather than just the cement content, since generally the presence of Type 2 addition reduces the heat of hydration and improves the durability of a mix. 4. Computation of total absolute volume of aggregates (a) The total aggregate content (kg/m3) can be computed from the wet density of concrete obtained from Fig. 10.9. The wet density of concrete depends on the specific gravity of overall aggregates in the saturated surface dry condition. (b) Alternatively, the absolute volume fraction of the aggregate is calculated by subtracting the proportional volumes of the free water and cement from a unit volume of concrete using Eq. (10.6).

Proportioning of Concrete Mixes

269

W C − (10.6) 1000 Sc 1000 where C and W are the cement and water contents, respectively, and Sc is the specific gravity of cement particles. Therefore, Total aggregate content (kg/m3) = (1000Sa ) × absolute volume of aggregates where Sa is the specific gravity of aggregate particles. If no information is available, Sa may be taken 2.6 for uncrushed aggregate and 2.7 for crushed aggregate, i.e., curves A and B can be used. Absolute volume of aggregates = 1 −

A: Crushed Aggregates

B: ncrushed Aggregates

3

2800 2700

Specific gravity of aggregate

We

ensity of Concrete

2600

2.9

A 2500 B

2.8

A

2400

B

2.7

2300

2.6 2.5

2200

2.4 2100 100

120

140

160

Free Wate

Fig. 10.9

180

200

220

240

ontent, kg/m3

Estimated wet density of fully compacted concrete (DoE)

5. Determination of fine and coarse aggregate contents The percentage of fine aggregate is either specified or obtained from Fig.10.10 expressed as a percentage of total aggregate that will provide the target consistence of the fresh concrete to be made with the given grading of fine aggregate, the nominal upper size of coarse aggregate and the free water-cement ratio obtained in the Step 2. (a) The content of coarse aggregate is calculated from the total aggregate content obtained in the Step 4 as follows: Coarse aggregate content (per cent) = 100 − Content of fine aggregate (per cent) (b) The coarse aggregate can further be divided into different size fractions. The following coarse aggregate fractions can be used as a general guideline.

270

Concrete Technology

15%

60 40%

ggregate (%)

70

a.2—Slump: 10–30 mm or VB: 6–12 s

50

80 70

15%

60

40%

50 60%

40

80% 100%

30

ortion of F

rop

n of F e Aggregate (%)

a.1—Slump: 0–10 mm or VB: >12 s

80

20 0.3 0.4 0.5 0.6 0.7 0.8 0.9

60% 40

80% 100%

30 20 0.3

0.4

ree Water–Cement Ratio or VB: 3–6 s

80 15% 70

60

40%

50

60% 80% 100%

40 30 20 0.3 0.4 0.5

0.6

0.7

a.4—Slump: 60–180

roport n of F e Aggregate (%)

roportion o

ine Aggregate

)

a.3—Slump: 30–60

0.5

0.8

0.9

Free Water–Cement Ratio –3 s

15%

80 70

40%

60 60%

50

80% 100%

40 30 20 0.3

0.6 0.7 0.8 0.9

or VB

90

0.4

Free Water–Cement Ratio

0.5

0.6

0.7

0.8

0.9

Free Water–Cement Ratio

(a) Maximum Aggregate Size—10 mm b.1—Slump: 0–10 mm or VB: >12 s

b.2—Slump: 10–30 mm or VB: 6–12 s

(%)

60

70 60

15% 50

15%

50

40%

40% 40

60% 80% 100%

30

40

60% 80% 100%

30 20

20 10 0.3

of Fine

of ine Aggregate

70

0.4

0.5

0.6

0.7

0.8

ree Water–Cement Ratio

0.9

10 0.3

0.4

0.5

0.6

0.7

0.8

ree Water–Cement Ratio

0.9

271

Proportioning of Concrete Mixes 3–6 s

)

or

15%

60 50

40%

40

60% 80%

30

100%

20 10 0.3

of Fine Aggregate

of Fine Aggregate (%

b.3—Slump: 30–60 70

b.4—Slump: 60–180 mm or VB: 0–3 s 70 15% 60 40% 50 60% 40

80% 100%

30 20

0.4

0.5

0.6

0.7

0.8

0.9

10 0.3

Free Water–Cement Ratio (b)

: >12 s ) Aggregate

c.2

60

40

40%

30

60% 80% 100%

20

fF

15%

Proportio

50

: >12 s

0.8

0.9

lump

0–30 mm or VB: 6–12 s

60 50

15%

40

40%

)

60

60% 80% 100%

30 20

10 0.3 0 0.5 0.6 0.7 0.8 0.9 ree Water ement Ratio

c.2

70

Aggregate

c.1—Slump: 0–10 m

lump

0–30 mm or VB: 6– 2 s

70

50

15%

40

40%

30

60% 80% 100%

20 10 0.3 0 0.5 0.6 0.7 0 ree Water–Cement tio

0.9

fF

Aggregat

0.7

70

10 0.3 0 0.5 0.6 0.7 0 8 0.9 ree Water–Cement Ratio

rtion

0.6

ximum Aggregate Size—20 m

70

Fig. 10.10

0.5

Free Water–Cement Ratio

Proportio

rtion

Aggregat

c.1—Slump: 0–10 m

0.4

60 50

15%

40

40%

30 20

60% 80% 100%

10 0.3 0 0.5 0.6 0.7 0.8 0.9 ree Water ement Ratio

Recommended proportions of fine aggregate for different grading zones (DoE)

272

Concrete Technology Table 10.17

Proportions of different sizes of coarse aggregates

Aggregate size range (mm)

(2.36 /4) - (4 /10)

(4 /10) - (10 /20)

(10 /20)-(20 /40)

Type-I

33

67

−

Type-II

18

27

55

6. Adjustments for aggregate moisture and determination of final proportions Since aggregates are batched on actual weight basis, adjust the amount of mixing water to be added to take into account the aggregate moisture. Prepare and test trial batches having proportions obtained above; and adjust final proportions as per results of the tests. Using the British DoE concrete design method determine the proportions for EN:206-1 concrete with slump class: S2 (unplasticized) for application in structures for 50 years of service life under exposure classes XC3, XC4, XD1 and XA1. The proportions are to be based on (a) CEM-I class cement, i.e., normal Portland cement (42.5N) of specific gravity of 3.15 conforming to EN:197-1 standard without additives, and (b) 30 per cent pulverized fuel ash (i.e., fly ash) conforming to EN:450 category A or B as additive. The materials available are uncrushed fine and coarse aggregates of specific gravity of 2.65. The coarse aggregate class is 4/10 and fine aggregate conforms to the grading zone-III with percentage passing of 600, micron sieve being 70 per cent. The standard deviation as obtained from past records is 5.0 MPa and defective rate at five per cent, i.e., probability factor k =1.65.

Example 10.3

Solution The concrete for application in structures for 50 years of service life under exposure classes XC3, XC4, XD1 and XA1 requires strength of class C30/37 at 28 days. (a) Without additives The steps involved in concrete design are: 1. For the stipulated strength class, the target means compressive strength, ft = fck + k S = 37 + 1.65 × 5.0 = 45.25 MPa 2. For normal Portland cement and uncrushed aggregate used, from Table 10.15 for the reference free water−cement ratio of 0.5, 28-day compressive is 43 MPa. With this pair of data (43 MPa and water-cement ratio = 0.50) as a controlling point, it is the fourth (dotted) curve from the top of Fig.10.8 which passes through the controlling point. On this curve, the point B corresponding to the target strength of 45.25 MPa is marked. This point corresponds to a water-cement ratio of 0.48. For the exposure classes XC3, XC4, XD1 and XA1 (50-year life), the maximum permitted value of free water-cement ratio is 0.55. Therefore, a water-cement ratio of 0.48 can be adopted. 3. For the uncrushed aggregate of class 4/10, the water content for consistence class S2 as obtained from Table 10.16 is 180 kg/m3.

Proportioning of Concrete Mixes

273

4. For water-cement ratio of 0.48, Cement content = 180 / 0.48 = 375 kg/m3 This cement content is satisfactory as it is more than the minimum cement content of 300 kg / m3 recommended in Table 10.14 and less than the maximum prescribed value of 450 kg / m3. 5. Wet density of fully compacted fresh concrete as obtained from Fig. 10.9 is 2400 kg / m3. Proportions of fine and coarse aggregates 6. Total aggregates content: Ca = 24000 × 1 −

375 180 − = 1682 kg/m3 3 15 × 1000 1000

7. For EN:206-1 concrete with consistence class S2, water−cement ratio of 0.48 and fine aggregate conforming to the grading zone III, the proportion of fine aggregate as a per cent of the total aggregate from Fig. 10.10 (a.2) is 35 per cent. Therefore, proportions of saturated surface dry aggregates are

Mass of fine aggregate, Cfa = 0.35 × 1682 = 589 kg/m3 Mass of coarse aggregate, Cca (1 – 0.35) × 1682 = 1093 kg/m3 Hence the concrete proportions by mass can be expressed as

Cement 375 1.0

Water : :

180 0.48

: :

Fine aggregate 589 1.57

Coarse aggregate : 1093 (kg/m3) : 2.91

The final proportions are established by trial batches and site adjustments. 8. The masses of materials for a predetermined trial batch in oven-dry condition can be obtained by multiplying the masses of saturated-surface-dry aggregates by 100/ (100 + w) where w is the percentage of water (by mass) required to bring the dry aggregates to a saturated surface-dry condition. If the absorptions of fine and coarse aggregates are two and one per cent, respectively, then for a trial batch of (say) 0.05 m3 the material contents are Mass of oven-dry fine aggregate = (0.05 × 589) × (100/102) = 28.87 kg Mass of oven-dry coarse aggregate = (0.05 × 1682) × (100/101) = 54.11 kg Water absorbed = [(0.05 × 589) − 28.87] + [(0.05 × 1682) − 54.11] = 1.12 kg Normally, 10 per cent additional quantities of materials are taken to account for any underestimation and wastage. Thus, the quantities for the trial batch are:

Cement: (0.05 × 375) × 1.1 = 20.63 kg, Water: [0.05 × 180) + 1.12] × 1.1 = 11.13 kg, Fine aggregate: 28.87 × 1.1 = 31.76 kg (oven-dry), and Coarse aggregate: 54.11 × 1.1 = 59.52 kg (oven-dry).

274

Concrete Technology

(b) With additives Consider the case of a concrete with cement, pulverized fuel ash, water contents of C, F and W, respectively. The percentage p of pulverized fuel ash in the total cementing material can be expressed in the form of Eq. (10.7): p=

100 F C F

or

F=

pC 100 − p

(10.7)

If the cementing efficiency of pulverized fuel ash is k, the effect of pulverized fuel ash is equivalent to an amount kF of cement. The total cementing material and the free water-cement ratio can, therefore, be expressed as Total cementing material C + k F = C + [k pC/(100 − p)] = C [100 − (1 − k)p]/(100 − p)] Free water−cement ratio = W/(C + kF ) = W(100 − p)/[C{100 − (1 − k) p}] Therefore, the cement content is given by C = W(100 − p)/[{100 − (1 − k)p} {W/(C + kF)}]

(10.8) (10.9)

For cementing efficiency, k = 0.3, Eq. (10.8) reduces to C = W(100 − p)/[{100 − 0.7 p} {W/(C + 0.3F )}]

Design procedure The first three steps are same as in the case of concrete without addition. However, at this stage the free water-cement ratio need not be compared with the maximum prescribed value from durability considerations. The free watercement ratio and water content are 0.48 and 180 kg / m3, respectively. Due to the presence of 30 per cent pulverized fuel ash, a reduction in water content of 15 kg/m3 is recommended in Table 10.16. Therefore,

Water content W = 180 – 15 =165 kg/m3 Cement content C = 165 × (100 – 30)/[{100 – 0.7 × 30} × 0.48] = 305 kg/m3 Fuel ash content, F = (30 × 305)/(100 – 30) = 131 kg/m3 Hence, total cementing material, C + F = 305 + 131 = 436 kg/m3 And free water-cementing material ratio = 165/436 = 0.38 From Fig. 10.9, the wet density of concrete produced with water content of 165 kg/m3 and aggregate with average specific gravity of 2.65, is 2420 kg/m3. Thus, total aggregates content: Ca = 2420 − (305 + 131 + 165) = 1819 kg/m3 For EN:206-1 concrete, with consistence class of S2, water−cement ratio of 0.38 and fine aggregate conforming to the grading zone III, the proportion of fine aggregate as a per cent of total aggregate from Fig. 10.10 (a.2) is 33 per cent. Therefore, proportions of saturated surface dry aggregates are Mass of fine aggregate, Cfa Mass of coarse aggregate, Cca

= 0.33 × 1819 = 1819 − 600

= 600 kg/m3 = 1219 kg/m3

Proportioning of Concrete Mixes

275

The required quantities of materials for one cubic meter of concrete are Cement Water Pulverized fuel ash Fine aggregate Coarse aggregate

305 kg/m3 165 kg/m3 131 kg/m3 600 kg/m3 1219 kg/m3

: : : : :

Hence, the concrete proportions by mass may be expressed as

Cement 375 1.0

Water : :

165 : 0.38* :

Fuel ash 131 0.43

: :

Fine Coarse aggregate aggregate 600 : 1219 1.97 : 4.00

(kg/m3)

The free water-cementing material ratios W/(C + 0.3F ) and W/(C + F )* are 0.48 and 0.38, respectively. The final proportions are established by trial batches and site adjustments.

10.8

THE ACI METHOD FOR MIX PROPORTIONING

In 1991, the American Concrete Institute (ACI) published its guidelines for normal, heavy-weight and mass concrete mix design. The absolute volume method of mix design as described by the ACI method is briefly presented in this section and the design steps for mix proportioning as recommended by ACI Committee 211, are listed in the form of a schematic flow chart shown on Fig. 10.11. The ACI mix proportioning method is suitable for normal and heavy-weight concretes having maximum 28-day cylinder compressive strength of 45 MPa and workability (slump) range of 25 to 100 mm generally used in the applications listed in Table 10.18. The ACI method presumes that the workability of a mix with given maximum size of well-graded aggregate (i.e., an aggregate with suitable particle shape and the grading) is dependent upon the water-content, the amount of entrained air and certain chemical admixtures, but is largely independent of mix proportions, particularly the amount of cementing material. Therefore, ACI has provided a table relating nominal maximum aggregate size, air entrainment and desired slump to the required mixing water quantity. Table 10.18

Slump ranges for specific applications (after ACI, 2000)

Type of construction Reinforced foundation walls and footings; plain footings, caissons, and substructure walls Beams and reinforced walls; building columns Pavements and slabs Mass concrete

Maximum slump (mm)

Minimum slump (mm)

75 100

25 25

75 50

25 25

Maximum slump may be increased 25 mm for consolidation by hand, i.e., roding, etc.

Specific gravity of aggregate and cement

Fig. 10.11

Type of construction

Batch volume

Absorption and moisture

Exposure condition (Air-entrained concrete)

Fineness modulus of fine aggregate

C Coarse aggregate c conten t

Moisture content Adjustment

(nonairentrained concrete) Air content Air-content

A

Concrete mix design flowchart for ACI method

Volume of sand

Volume of all items

Slump or compacting factorr

A

Size of aggregate

Cement content

Water content

Air entrained or non-air entrained

Types of concrete Maximum nominal

Water-cement ratio

Standard Deviation

Check minimum cement content

Target strength

Characteristic strength of concrete

276 Concrete Technology

Proportioning of Concrete Mixes

277

The method further assumes that the optimum ratio of bulk volume of coarse aggregate to the total volume of concrete depends only on maximum size of coarse aggregate and on the grading (fineness modulus) of fine aggregate. For a concrete mix of plastic consistency the bulk volume of coarse aggregate, for the known fineness moduli of fine aggregate to be used, is selected from Table 10.19. Having determined the maximum size of available coarse aggregate, the water-content for specified workability and type of concrete is selected from Table 10.20. This recommendation pertains to concrete with a degree of workability suitable for usual reinforced construction (ACI, 2000). For pavement concrete which is generally stiffer and less workable, ACI permits recommended values of coarse aggregate to be enhanced about 10 per cent. Table 10.19

Bulk volume of coarse aggregate per unit volume of concrete for different fineness moduli of fine aggregate (Adapted from ACI 211.1)

Nominal maximum size of aggregate (after CSA A23.1) (mm) 10 14 20 28 40 56 80 150

Bulk volume of oven-dry-rodded coarse aggregate (m3) Fineness modulus of fine aggregate 2.40

2.60

2.80

3.00

0.50 0.59 0.66 0.71 0.75 0.78 0.82 0.87

0.48 0.57 0.64 0.69 0.73 0.76 0.80 0.85

0.46 0.55 0.62 0.67 0.71 0.74 0.78 0.83

0.44 0.53 0.60 0.65 0.69 0.72 0.76 0.81

Notes (i) The values are for aggregate of specific gravity Sca = 2.68. For an aggregate having specific gravity of S c′ a the value should be multiplied by the ratio Sca/ S′ca. (ii) Since concrete pavements are, in general, stiffer and less workable, the above values can be increased by up to about 10 per cent. (iii) Coarse aggregate volumes are based on oven-dry-rodded weights obtained in accordance with ASTM C 29.

The water-cement ratio is determined as in other methods to satisfy both strength and durability requirements. In general, lower water-cement ratios produce stronger, more durable concrete. If natural pozzolans such as fly ash are used in the mix then the ratio becomes a water-cementing material ratio (cementing material = Portland cement + pozzolonic material). Table 10.20 provides a general estimate of 28-day compressive strength vs. water−cement ratio (or water-cementing ratio). Values in this table tend to be conservative (ACI, 2000). The air content in concrete is taken into account for calculating the volume of fine aggregate. Maximum water-cement ratio for a variety of construction conditions are listed in the Table 10.21. Construction conditions include concrete protection from exposure to freezing and thawing; watertightness of concrete; and exposure of concrete to deicing salts, brackish water, sea water, etc.

278

Concrete Technology

Table 10.20

Approximate mixing water and target air content requirements (after ACI 211.1 and ACI 318)

Slump, mm

Mixing water quantity1 (kg/m3) Specified nominal maximum size of aggregate (after CSA A23.1) (mm) 14

20

28

40

562

802

1502

207

199

190

179

166

154

130

113

228

216

205

193

181

169

145

124

243

228

216

202

190

178

160

-

3.0

2.5

2.0

1.5

1.0

0.5

0.3

0.2

181

175

168

160

150

142

122

107

202

193

184

175

165

157

133

119

216

205

197

184

174

166

154

-

10 I. Non-air-entrained concrete 25−50 (Stiff-plastic) 75−100

(Plastic) 150−175 (Flowing) Approximate entrapped air (per cent) II. Air-entrained concrete 25−50 (Stiff-plastic) 75−100 (Plastic) 150−175 (Flowing)

Recommended average total air content (per cent)

1 2

Mild exposure

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

Moderate exposure

6.0

5.5

5.0

4.5

4.5

4.0

3.5

3.0

Severe exposure

7.5

7.0

6.0

6.0

5.5

5.0

4.5

4.0

Table gives the maximum water content for reasonably well-shaped crushed aggregate. The slump values are based on the slump tests made after removal of particles larger than 40 mm by wet screening.

Table 10.21

Maximum permissible water–cement or water–cementing materials ratio in severe exposure conditions

Type of Structure

Continuously wet structure exposed to frequent freezing and thawing

Thin section (railings, curbs, sills, ledges, ornamental work) and sections with less than 25 mm cover over steel All other structures

10.8.1

Structure exposed to sea water or sulfates

0.45

0.40

0.50

0.45

Mix Design Procedure

The standard ACI mix design procedure can be divided into eight basic steps which are presented in the form of a schematic flow chart shown in Fig.10.11. The steps are the following:

Proportioning of Concrete Mixes

279

1. Selection of slump Generally, the mixes of stiffest consistency that can still be placed adequately should be used. Normally, the consistency expressed in terms of slump is specified depending upon the placing conditions. The generally used slump ranges for specific applications are given in Table 10.18. 2. Selection of maximum aggregate size In general, the maximum size of the coarse aggregate is limited to one-third of the minimum thickness of the member and three-fourth of the minimum clear space between reinforcing bars. Aggregate larger than these dimensions may be difficult to consolidate and compact, and hence may result in a honeycombed structure or large air pockets. The maximum size of the coarse aggregate is determined by sieve analysis. 3. Determination of mixing water and air content Approximate mixing water quantity and air content are selected from Table 10.20 for the desired slump and nominal maximum size of aggregate. 4. Computation of target mean compressive strength The average compressive strength of concrete from trial batch tests must equal or exceed the target mean compressive strength f ′cr in order for the concrete proportions to be acceptable. The target mean compressive strength f ′cr for selecting the mix proportions is larger of the values given by Eqs. (10.10) and (10.11):

f ′cr = f ′c + 1.34 S f ′cr = f ′c + 2.33 S – 3.45

(10.10) (10.11)

f ′c = specified compressive strength of concrete, MPa S = standard deviation, MPa When field data to establish a standard deviation are not available f ′cr can be obtained from Table 10.22. where

Table 10.22

Target mean compressive strength (After ACI 318)

Specified compressive strength f ′c (MPa)

Target mean compressive strength f ′cr (MPa)

Less than 21

f ′c + 7.0

21 to 35

f ′c + 8.5

Over 35

f ′c + 10.0

5. Selection of water-cement ratio In general, a lower water–cement ratio produces stronger and more durable concrete. If natural pozzolans (such as fly ash) are used in the mix then the ratio becomes a water-cementing material ratio (cementing material = Portland cement + pozzolonic material). The water– cement ratio is selected from Table 10.23 for the desired 28-day compressive strength.

280

Concrete Technology

Table 10.23

Water-cementing materials ratio and compressive strength relationship (after ACI 211.1 and ACI 211.3)

28-day compressive strength2 (MPa)

Water-cementing materials ratio by mass1 Non-air-entrained concrete

Air-entrained concrete

45

0.38

0.30

40

0.42

0.34

35

0.47

0.39

30

0.54

0.45

25

0.61

0.52

20

0.69

0.60

15

0.79

0.70

1

Maximum nominal size of aggregate is assumed to be about 20 to 28 mm.

2

Strength is based on moist-cured cylinders.

6. Determination of cement content Cement content computed from the selected mixing water content and water-cement ratio, is compared with specified minimum cement content from durability considerations (generally, 300–360 kg/ m3), if applicable. 7. Determination of coarse aggregate content The coarse aggregate content is estimated from Table 10.19 for the indicated nominal maximum aggregate size and fine aggregate fineness modulus. 8. Determination of fine aggregate content At this stage of mix design, the volumes of water, Portland cement, air and coarse aggregate have been specified. Thus, the fine aggregate volume is just the remaining volume and is determined by subtracting the sum of absolute volumes of water, Portland cement, air and coarse aggregate from the unit volume (1 m3) of concrete. The wet density of fully compacted fresh concrete is given in the Table 10.24. Table 10.24 Nominal maximum size of aggregate (mm) 9.5 12.5 19 25 37.5 50 75 150

Wet density of fully compacted fresh concrete Wet density of concrete (kg/m3) Non-air-entrained concrete 2280 2310 2345 2380 2410 2445 2490 2530

Air-entrained concrete 2200 2230 2275 2290 2350 2345 2405 2435

9. Adjustments for aggregate moisture Aggregate volumes are calculated on the basis of oven-dry unit weights, but are typically batched on actual weight basis.

Proportioning of Concrete Mixes

281

Therefore, aggregate moisture content must be taken into account to ensure correct amount of free water in the mix. The aggregate moisture is monitored to determine if there is any free water in the aggregate or if the aggregate is so dry that it will absorb water that is added to the mix. It is rare to have aggregate at the batch plant that is in the saturated surface dry condition. This causes a net change in the amount of water available in the mix and must be compensated for by adjusting the amount of mixing water added. Trial batches are tested and final proportions are obtained by adjustments. The following mix design example using ACI procedure gives a general idea of the types of calculations and decisions that are typical in concrete mix design. Example 10.4 It is required to proportion a concrete mix for use in a 250 mm thick pavement with mean 28-day flexural strength of at least 4.25 MPa and a slump of the order of 25−50 mm. The coarse aggregate available is well shaped having nominal maximum size of 37.5 mm, specific gravity of 2.66, dry-rodded mass of 1600 kg/m3, moisture content = 1.0 per cent, and absorption = 0.5 per cent. Whereas the fine aggregate to be used has fineness modulus = 2.60, specific gravity = 2.64, moisture content = 5 per cent, absorption = 0.7 per cent, the available Portland cement has specific gravity of 3.15. The other stipulations are: • Air content = 4.0 − 6.0 per cent • Maximum allowable water-cement ratio = 0.44 • Minimum cement content = 335 kg/m3 • Density of water = 1000 kg/m3.

Solution (a) Slump Specified slump range of 25−50 mm is typical for concrete pavements. (b) Maximum aggregate size Specified maximum aggregate size of 37.5 mm is well within the general recommendations for a pavement application. (c) Estimation of mixing water and air content In order to achieve air content above two−three per cent, concrete must be air entrained. Therefore, from Table 10.20, an air-entrained concrete with a target slump of 25−50 mm and a nominal maximum aggregate size of 37.5 mm will require about 150 kg/m3 mixing water. Adequate quantity of air-entraining admixtures will have to be added to achieve 5.0 per cent air content (mean of the specified range 4.0−6.0 per cent). It should be noted that water-reducing admixtures can reduce water requirements by about 5 to 10 per cent and some admixtures may also increase the entrained air content by about 0.5−1.0 per cent. (d) Water-cement ratio Since the specified strength is in terms of flexural strength, the relationship between flexural and compressive strengths expressed by Eq. (10.12) may be used to obtain an approximate equivalent compressive strength in order to use Table 10.23. fb = 0.623

f c (after ACI Code)

(10.12)

where fb and fc represent the flexural and compressive strengths in MPa, respectively. Therefore, for fb = 4.25 MPa, fc = 46.5 MPa. This value does not appear in Table 10.23 for determining water−cement ratio. Hence, either Fig.

282

Concrete Technology

10.3 can be referred or experience with local conditions or a more involved determination of the water−cement ratio can be used. Additionally, for concrete exposed to severe environmental conditions, e.g., freezing and thawing in moist conditions, etc., ACI recommends a maximum water−cement ratio of 0.45 as listed in Table 10.21. In this particular illustrative example, consider that local experience dictates a water−cement ratio of around 0.40. (e) Cement content Based on the mixing water content and water−cement ratio obtained in Steps 3 and 4, respectively, cement content is water content 150 = = 375 kg water-cement ratio 0 40 (f) Coarse aggregate content From Table 10.19 for the nominal maximum aggregate size of 37.5 mm having reference specific gravity Sca = 2.68 and the fine aggregate fineness modulus of 2.60, the recommended volume fraction of rodded coarse aggregate is 0.73. This is to be modified for actual specific gravity S ′ca = 2.66. Thus the volume fraction of rodded coarse aggregate to be used is 0.73 × (2.68/2.66) = 0.735. This means that the coarse aggregate should occupy 73.5 per cent of the total volume. However, this volume of aggregate includes the volume of air entrapped between the aggregate particles. Therefore, the mass of coarse aggregate per cubic meter of concrete is Ca = 0.735 × 1600 = 1176 kg

Cement content =

(g) Fine aggregate content The fine aggregate content can be determined by subtracting the sum of absolute volumes of mix ingredients from the unit volume of concrete. Therefore, Volume of entrapped air = 5/100 Volume of water = W/Sw = 150/1000 Volume of cement = C/Sc = 375/(3.15 × 1000) Volume of C. A. = Ca/Sca = 1176/(2.66 × 1000)

= 0.050 m3 = 0.150 m3 = 0.119 m3 = 0.442 m3

Sum of absolute volumes Thus, volume of fine aggregate per unit volume Mass of fine aggregate = 0.239 × (2.64 × 1000)

= 0.761 m3 = 1 − 0.761 = 0.239 m3 = 631 kg

(h) Adjustments for aggregate moisture Since there is moisture in both the coarse and fine aggregate, their trial batch weights must be adjusted as

Weight of fine aggregate = 631 × 1.05 Weight of coarse aggregate = 1176 × 1.01

= 663 kg = 1188 kg

The amount of mixing water also needs adjustment because both the coarse and fine aggregate are wet and will contribute to free−water available for cement paste. Free-water present in the aggregate: Fine aggregate = 631 × (0.05 − 0.007) Coarse aggregate = 1176 × (0.01 − 0.005) Aggregate free water

= + 27.1 kg = + 5.9 kg = + 33.0 kg

Therefore, the amount of mixing water to be added at the batching plant is

Proportioning of Concrete Mixes

283

= stipulated in the mix design report - aggregate free water = 150 – 33.0 = 117 kg. (i) Summary The final trial batch quantities per cubic meter of concrete are: Water–cement ratio 0.40 -

Water

Cement

117 -

375 1.0

Fine aggregate 663 1.77

Coarse aggregate 1188 3.17

(j) Trial batches Usually, to make trial batches, something less than the unit volume is made—a typical trial batch size is 0.03 m3. Once the trial batch is made, it can be tested for slump, air content, flexural strength, compressive strength and any other required property.

10.9

CONCRETE MIX PROPORTIONING – IS GUIDELINES

The concrete mix proportioning using the IS guidelines is aimed at achieving the specified properties, i.e., workability of fresh concrete, and strength and durability requirements of hardened concrete at specified age with the maximum overall economy. All the requirements of IS 456-2000 are also satisfied in the mix design process. The mix proportioning guidelines are suitable for ordinary and standard concrete grades having maximum 28-day cube compressive strength up to 55 MPa and workability (slump) range of 25 to 125 mm, which are generally used in the reinforced concrete beams, columns and walls in buildings; foundations and footings; caissons and substructure walls, and pavements. The basic data required for proportioning a concrete mix are properties of coarse and fine aggregates (type and maximum nominal size of the coarse aggregate; gradings of fine and coarse aggregates and the gradings zone of fine aggregate); characteristic strength at 28 days (fck); degree of workability; limitations on water−cement ratio and minimum cement content to ensure adequate durability for the given type of exposure; standard deviation (S) for the strength of concrete. According to IS456-2000, the characteristic strength is defined as the value below which not more than five per cent of test results are expected to fall. The design of plastic concrete mixes of medium strength is based on the following two criteria: 1. The compressive strength of concrete is governed by its water-cement ratio. 2. For the given aggregate characteristics (maximum size of well-graded aggregate with suitable particle shape and grading) the workability of concrete mix is dependent on the water content, the amount of entrained air and addition of certain chemical admixtures, but is largely independent of mix proportions, particularly the amount of cementing material.

10.9.1

Mix Design Procedure

First revision of IS 10262-2009: Concrete Mix Proportioning-Guidelines has followed the format of ACI mix proportioning method; the European Nations do

284

Concrete Technology

not have common concrete mix design method because it considers mix design a part of concrete production. However, it exercises control through EN 206-1. It is immaterial whether the concrete mix is proportioned by British DoE method or DIN or some other method, as long as it satisfies the requirements/specifications. The procedure for mix proportioning as stipulated in IS 10262-2009 is presented in the form of a schematic flow chart shown in Fig. 10.12. The steps are outlined below: 1. Determination of basic characteristics of available fine and coarse aggregates The properties required are: (a) (b) (c) (d)

The maximum nominal size of coarse aggregate The gradings of fine and coarse aggregates The gradings zone of fine aggregate The unit weight, specific gravities, and absorption capacities of both the coarse and fine aggregates If necessary, two or more different size coarse aggregate fractions may be combined so that the overall grading of coarse aggregate conforms to Table 2 of IS 383 for the particular nominal maximum size of aggregate.

2. Selection of free water–cement ratio (a) The mean target strength ft is determined from the specified characteristic compressive strength at 28-days fck and the level of quality control using the Eq. (10.13):

ft = fck + k S = fck + 1.65 S

(10.13)

where S is the standard deviation and k the statistical coefficient depending upon the accepted proportion of low results. For the characteristic compressive strength defined in IS 456-2000, k = 1.65. The standard deviation which represents the degree of control can be estimated statistically from the variations in results of tests conducted on trial mixes in the field or laboratory. It shall be based on at least 30 test strength samples. The standard deviation should be calculated as early as possible when the mix is used for the first time. The value shall be updated after every change in mix design. Where sufficient test results for a particular grade of concrete are not available, the value of standard deviation given in Table 10.25 may be adopted. As soon as enough results are available, the mix should be redesigned using actual calculated standard deviation. However, when adequate past records of a similar grade exist, that value of standard deviation may be used. (b) The free water–cement ratio for the mean target strength obtained in the step 2(a) is selected from Fig. 10.3 or 10.4 representing the relationship between the characteristic compressive strength and free water-cement ratio established for the materials to be used in the job. The free water-cement ratio so chosen is checked against the limiting or the maximum water-cement ratio for the requirements of durability given in Table 10.26; the lower of the two values is adopted.

Check for water-cement ratio

Fig. 10.12

Volume of Coarse and fine aggregate

C

Cement content

E

A

C

Weights of all ingrdients

D

Total aggregate vvolume e

Air content

Type of aggregate

A

B

D

Moisture adjustmentss a

Absorption and moisture content

Sand fraction

Coarse aggregate fraction

Nominal maximum size of aggregate e

Schematic flow chart for concrete mix proportioning – IS guidelines

Specific gravity of cement and aggregate

Check for C cement content

A

Water adjustment

Watercement ratio

Workability (Slump)

Water content

Standard deviation

Target mean s strengt h

Characteristic strength

Batch weights

E

fine aggregate grading zone

Proportioning of Concrete Mixes 285

286

Concrete Technology Table 10.25

Assumed standard deviation

Group No.

Grade of concrete

Assumed standard deviation, MPa

1.

M10, M15

3.5

2.

M20, M25

4.0

3.

M30, M35, M40, M45, M50, M55

Table 10.26

5.0

Quality control The values correspond to the site control having proper storage of cement; weigh batching of all materials; controlled addition of water; regular checking of all materials, aggregate grading and moisture content; and periodical checking of workability and strength. Where there is deviation from the above, values given in the above table shall be increased by 1.0 MPa.

Minimum cement content and maximum water-cement ratio of concrete with normal-weight aggregates of 20 mm nominal maximum size subjected to different exposures (Adapted from IS 456-2000)

Sl. Exposure Plain Concrete Reinforced Concrete No. condition Minimum Maximum Mini- Minimum Maximum Minimum cement free watermum cement free watercement grade of content, cement grade of content, ratio concrete kg/m3 ratio concrete kg/m3 1. 2. 3. 4. 5.

Mild Moderate Severe Very severe Extreme

220 240 250 260

0.60 0.60 0.50 0.45

M15 M20 M20

300 300 320 340

0.55 0.60 0.45 0.45

M20 M 25 M30 M35

280

0.40

M25

360

0.40

M40

Adjustments to minimum cement contents for aggregates other than 20 mm nominal maximum size Nominal maximum size, mm Adjustments to minimum cement contents, kg/m3 1. 2. 3.

10 20 40

+40 0 -30

Notes i. Cement content prescribed is irrespective of the grades of cement and it is inclusive of all supplementary cementitious materials. The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolana and slag specified in IS 1489 (Part I) and IS 455, respectively. ii. Minimum grade for plain concrete under mild exposure condition is not specified.

3. Selection of free water content The water content per unit volume of concrete (for aggregates in saturated surface dry condition) is selected from Table 10.27

Proportioning of Concrete Mixes

287

for the standard reference conditions of type of aggregate and workability. This water content is adjusted as per Table 10.27 for any difference in type of coarse aggregate and the workability from the standard reference values. Table 10.27 Sl. No.

Maximum water content for nominal maximum size of aggregate

Nominal maximum size Maximum of aggregate, mm water content, kg

Validity or reference conditions

1.

10

208

Applicable to angular crushed coarse aggregate

2.

20

186

Water content corresponds to saturated surface dry aggregate,

3.

40

165

Applicable to slump range of 25 to 50 mm

Adjustments in the water content for the change in type of aggregate and workability Change in condition stipulated above A. Shape of aggregate 1. Sub-angular aggregates 2. Gravel with some crushed particles 3. Rounded gravel B. Workability 1. For each additional 25 mm slump Alternatively, 2. Required water content may be established by trial 3. Use of chemical admixtures conforming to IS 9103.

Adjustment required in water content -10 kg -20 kg -25 kg + 3 per cent

Water reducing admixtures and superplasticizers usually decrease water content by 5 to 10 per cent and 20 per cent and above, respectively, at appropriate dosages.

4. Selection of cement content (a) The minimum cement and supplementary cementitious material content per unit volume of concrete calculated by dividing the final free water content arrived after adjustments in the step (3) by the free water-cement ratio obtained in the Step 2(b). (b) The cementitious material content so obtained is compared with the minimum value based on the requirements of the durability, and greater of the two values is adopted. 5. Estimation of volume proportion of coarse aggregate in total aggregate The volume proportion p of coarse aggregate of given nominal maximum size is estimated from Table 10.28 for the reference water-cement ratio of 0.5 and grading zone of fine aggregate used; it is adjusted suitably for the selected watercement ratios. For more workable concrete, e.g., pumpable or concrete mixes to be placed around congested reinforcing steel the estimated coarse aggregate content may

288

Concrete Technology

be reduced up to 10 per cent subject to slump, water-cement ratio and strength properties of concrete remaining consistent with the provisions of IS 456 and project specifications. Table 10.28 Sl. No.

Proportion of coarse aggregate to total aggregate for different zones of fine aggregate

Nominal maximum size of aggregate, mm

Volume fraction of coarse aggregate to total aggregate p for different zones of fine aggregate Zone IV

Zone III

Zone II

Zone I

1.

10

0.50

0.48

0.46

0.44

2.

20

0.66

0.64

0.62

0.60

3.

40

0.75

0.73

0.71

0.69

6. Computation of total absolute volume of aggregates The total absolute volume of coarse and fine aggregates (saturated surface dry condition) is computed by subtracting the sum of absolute volumes of cementitious material and water already determined in Steps 2 and 3; the chemical admixture and entrained air (if considered) from unit volume of concrete. Thus, total absolute volume of aggregates Va (m3) is given by Eq.(10.14): Va

C W ⎤ ⎡ 1 0 ⎢v + + ⎥ 1000 S × 1000 c ⎣ ⎦

(10.14)

where W, C, v and Sc are the mass of water (kg), mass of cement (kg), air content (m3) per cubic meter of concrete and the specific gravity of cement, respectively. 7. Determination of absolute volumes of fine and coarse aggregates The volume of total aggregate obtained in Step 6 is divided into coarse and fine aggregate fractions by volume in accordance with coarse aggregate proportion p already determined in Step 5. The absolute volumes of coarse aggregate (Vca) and fine aggregate (Vfa) per unit volume of concrete are determined as Vca = pVa and Vfa = (1 − p) Va

(10.15)

where p represents the ratio of coarse aggregate in the total absolute volume of aggregates. Therefore, contents of fine and coarse aggregates by mass are Cfa = (Sfa × 1000)Vfa and Cca = (Sca × 1000)Vca where Sfa and Sca are the specific gravities of saturated surface dry fine and coarse aggregates, respectively, in kg / liter. Thus the concrete mix proportions for the first trial mix by mass (kg) are: Cement C

:

Water W

:

Fine aggregate VfaSfa(1000)

:

Coarse aggregate VcaSca(1000)

The above concrete mix proportions can be expressed by volume (m3) as where are the bulk densities (kg / m3) of cement, fine and coarse aggregates, respectively.

Proportioning of Concrete Mixes Cement

C γc

:

Water

Fine aggregate

W 1000

V fa S fa (1000) γ fa

:

289

Coarse aggregate

:

Vca Sca (1000) γ ca

where gc, gfa and gca are the bulk densities (kg/ m3) of cement, fine and coarse aggregates , respectively. 8. Adjustments for aggregate moisture and determination of final proportions Since aggregates are batched on actual weight basis, the amount of mixing water to be added is adjusted to take in to account the absorption and the current moisture content to generate equivalent of saturated surface dry condition of the aggregates. 9. Preparation of trial batches and testing (a) The concrete mix proportions for the first trial mix or trial mix no.1 are determined and the workability of the trial mix is measured in terms of slump; the mix is carefully observed for freedom from segregation and bleeding and its finishing properties. If the slump of first trial mix is different from the stipulated value, the water and/or admixture content is suitably adjusted to obtain the correct slump. (b) The mix proportions are recalculated keeping the free water-cement ratio at the pre-selected value; this comprises trial mix No. 2. In addition two more trial mix nos. 3 and 4 are formulated with the water content same as trial mix no. 2 and varying the free water-cement ratio by ±10 per cent of the preselected value. (c) The fresh concrete of each trial batch obtained above is tested for unit weight, yield and air content and three 150 mm cubes are cast. The wet cubes are tested after 28-days moist curing and checked for the strength. 10. Final mix proportions The trial mix nos. 2 to 4 are analyzed for relevant information, including the relationship between compressive strength and water-cement ratio. The water-cement ratio required for the mean target strength using this relationship is computed. The mix proportions for the changed water-cement ratio are recalculated keeping water content at the same level as that determined in trial mix no. 2. For field trials, produce the concrete by actual concrete production method to be used in the field. Example 10.5 It is required to design a M35 grade pumpable concrete mix having a slump of the order of 100-125 mm using G-43 OPC conforming to IS 8112 for a reinforced concrete structure subjected to very severe exposure conditions during its service life. Use IS10262-2009: Concrete mix proportioning- IS guidelines to estimate preliminary mix proportions since final mix proportions will depend upon actual site conditions which vary with location and other factors. The crushed coarse (angular) aggregates available at the site are of nominal maximum sizes of 10 mm and 20 mm with a specific gravity of 2.67, moisture content of 1.0 per cent, and absorption of 0.5 per cent. Whereas the available fine aggregate has fineness modulus of 2.80 (grading zone-I), specific gravity of 2.62, moisture content of 4.0 per cent, absorption of 1.0 per cent; the available G-43 Portland cement has specific gravity of 3.15. The bulk densities of cement, fine and coarse aggregates are 1450, 1700 and 1800 kg/m3, respectively. The other stipulations are

290

• • • • • • •

Concrete Technology

Standard deviation (from past records): 2.0 MPa Air content: 4.0 − 5.0 per cent Maximum allowable free water-cement ratio: 0.45 Minimum cement content: 340 kg/m3 Maximum cement content (IS 1343-1980): 450 kg/m3 Chemical admixture type: Superplasticizer conforming to IS 9103 Density of water: 1000 kg/m3.

Solution Mix proportions Consider the coarsest grading curve No.1 of Fig. 3.9 for the combined aggregate. It is observed that to obtain this grading, the 20-mm and 10-mm size coarse aggregates are to be mixed in the ratio of 55:45. The maximum nominal size for the combined aggregate would be 20 mm. Target mean strength, ft = fck + k S = 35 + 1.65 × 2.0 = 38.3 MPa The corresponding free water−cement ratio from Fig. 10.3 is 0.39. However, based on experience, water-cement ratio adopted is 0.40 [0.40 < 0.45; hence O.K.] (a) Selection of water content From Table 10.27, maximum water content for 25 to 50 mm slump range for 20 mm nominal size aggregate with saturated surface dry condition is 186 liters. Adjustments Since the workability (slump) range of 100−125 mm, is different from the reference value, adjustment is required in the water content for this variation. Estimated water content for 125 mm slump 3 125 − 50 ⎤ ⎡ × = 186 × ⎢1 + = 203 liters 25 ⎥⎦ ⎣ 100 As superplasticizer is used, the water content can be reduced by more than 20 per cent. The desirable reduction in water content is estimated by trials with superplasticizer. For example, consider a superplasticizer with manufacturer recommended dosage @ 2.0 per cent by mass of cementitious material as water−reducing admixture resulting in a reduction in water content of the order of 30 per cent. Therefore, Water content = 203 × 0.70 = 142 liters or kg/ m3 (b) Determination of cement content Hence,

Water-cement ratio = 0.40 cement content = 142./0.40 = 355 kg / m3.

The calculated cement content is more than the minimum cement content of 340 kg /m3 recommended in Table 10.26 and less than the maximum 450 kg /m3 prescribed in Clause 8.1.1 of IS 1343-1980. (c) Volume fractions of coarse and fine aggregates Volume proportion of coarse aggregate (20 mm nominal maximum size) to total aggregate with fine aggregate belonging to Zone I and water-cement ratio of 0.50 as obtained from Table 10.28 is 0.60. Adjustments Since the value of selected water-cement ratio of 0.40 is different from the reference value of 0.50 adjustment is required for this change. Therefore,

Proportioning of Concrete Mixes

291

the volume of coarse aggregate is to be increased to decrease the fine aggregate content. As the selected water-cement ratio is lower by 0.10 from the reference value, the proportion of volume of coarse aggregate is increased by 0.02 (at the rate of ± 0.01 for every ∓ 0.05 change in water-cement ratio). Therefore, corrected volume fraction of coarse aggregate for the water-cement ratio of 0.40, p = 0.62. In case the coarse aggregate is not crushed (angular) one, volume of coarse aggregate need be to be increased suitably, based on experience. For pumpable concrete volume fraction of coarse aggregate should be reduced by 10 per cent. Therefore, Volume fraction of coarse aggregate for pumpable concrete, p = 0.62 × 0.9 = 0.558 Volume fraction of fine aggregate, 1−p = 1− 0.558 = 0.442 Total absolute volume of aggregates Va (m3) is given by Eq. (10.14): Va

C S W ⎡ ⎤ 1 0 ⎢v + + + Sc × 1000 1000 Ss × 1000 ⎥⎦ ⎣

where S and Ss are the mass and specific gravity of chemical admixture (superplasticizer), respectively. Air content, V, for 20-mm maximum nominal size aggregate is two per cent, i.e., 0.02. 355 355 × 0 02 ⎤ 142 ⎡ Va = 1.0 − ⎢0.02 + + + = 0.719 m 3 3 15 × 1000 1000 1.145 × 1000 ⎥⎦ ⎣ For the ratio of coarse aggregate to total aggregate by absolute volume of p, the absolute volumes of coarse and fine aggregates per cubic meter of concrete are

Vca = pVa = 0.558 × 0.719 = 0.401 m3 Vfa = (1 – p)Va = (1 – 0.558) × 0.719 = 0.318 m3 (d) Coarse and fine aggregate contents Therefore, masses of coarse and fine aggregates are: Cca = (Sca × 1000)Vca = (2.62 × 1000) × 0.401 = 1051 kg/m3 Cfa = (Sfa × 1000)Vfa = (2.67 × 1000) × 0.318 = 849 kg/m3 where Sfa and Sca are the specific gravities of saturated surface dry fine and coarse aggregates, respectively, in kg/ liter. The different size fractions of coarse aggregate are

10-mm aggregate = 1051 × 0.45 = 473 kg / m3 20-mm aggregate = 1051 × 0.55 = 578 kg / m3

292

Concrete Technology

Mix proportions (a) Mix proportions by mass Cement 355 1.0

Water : :

142 0.40

Fine aggregate : :

849 2.39

Coarse aggregate : :

Admixture

10 mm 20 mm 473 : 578 : 7.1 1.33 : 1.63 : 00.2

(kg/m3)

(b) Mix proportions by volume Cement

Water

Fine aggregate

Coarse aggregate 10 mm

355 1450

:

1.0

:

142 1000

:

0.40

:

849 1700

:

2.04

:

Admixture

20 mm

473 1800

:

578 : 1800

1.07

:

1.31

:

(m3)

−

00.2

(e) Adjustments for aggregate moisture Since there is moisture in both the coarse and fine aggregates their trial batch masses must be adjusted as Weight of fine aggregate = 849 × 1.04 = 883 kg Weight of coarse aggregate = 1051 × 1.01 = 1062 kg The amount of mixing water also needs adjustment because both the coarse and fine aggregates are wet and will contribute to free water available for cement paste. Free water present in the aggregate: Fine aggregate = 849 × (0.04 − 0.01) Coarse aggregate = 1051 × (0.01 − 0.005) Free-water in aggregate

= + 25.47 kg = + 5.26 kg = + 30.73 kg

Therefore, the amount of water to be added at the batching plant is

= 142 – 30.73 = 111.27 kg The final trial batch quantities per cubic meter of concrete are: Nominal water-cement ratio

Water (kg / m3) effective w/c ratio

Cement (kg / m3)

0.40

111.27

355

-

0.31

1.0

Fine aggregate (kg / m3)

Coarse aggregate (kg / m3)

Admixture (kg / m3)

10 mm

20 mm

883

478

584

7.1

2.49

1.35

1.65

0.02

Using IS10262-2009: guidelines for concrete mix proportioning, design a M25 concrete mix for a reinforced concrete structure to be subjected to mild exposure conditions during its service life for the following requirements:

Example 10.6

Proportioning of Concrete Mixes

(a) Design stipulations: Degree of workability Degree of quality control (b) Characteristics of materials: Cement Type and grade Specific gravity Bulk density

293

Medium (75-100 mm slump) Weigh batching, occasional supervision, no past experience with this grade, S = 5.5 MPa

Ordinary Portland cement, G - 43 3.15 1450 kg/m3

Aggregates Fine aggregate Type River sand (zone II) Maximum nominal size − Specific gravity 2.60 Bulk density (kg/m3) 1700 Fineness modulus 2.3 Free surface moisture (per cent) 2.0

Coarse aggregate Crushed granite 20 mm 2.65 1800 6.0 1.0

Mix Proportions Target mean strength, ft = fck +kS = 25 + 1.65 × 5.5 = 34.1 MPa (a) Free water-cement ratio From compressive strength consideration (from Fig. 10.3) 0.43 From durability consideration 0.55 Hence, water-cement ratio of 0.43 may be adopted; however, based on experience, select water-cement ratio as 0.45 (< 0.55) (b) Determination of water content From Table 10.27, maximum water content for slump range 25 to 50 mm for 20 mm nominal size aggregate with saturated surface dry condition is 186 liters. Adjustments Since the workability (slump) range of 75−100 mm is different from the reference value, adjustment in the value of water content is required for this variation. Estimated water content for 75-100 mm slump 3 100 − 50 ⎤ ⎡ × = 186 × ⎢1 + = 197.2 liters. 25 ⎥⎦ ⎣ 100 (c) Determination of cement content Water-cement ratio = 0.45 Hence, cement content = 197.2./0.45 = 438 kg / m3. The calculated cement content is more than the minimum cement content of 300 kg /m3 recommended in Table 10.26 and less than the maximum of 450 kg / m3 prescribed in Clause 8.1.1 of IS 1343-1980. (d) Volume fractions of coarse and fine aggregates Volume proportion of coarse aggregate (20 mm nominal maximum size) to total aggregate with fine aggregate belonging to grading Zone II, for water-cement ratio of 0.50 as obtained from Table 10.28 is 0.62. Adjustments As the value of selected water-cement ratio of 0.45 is less than the reference value of 0.50 by 0.05; the volume fraction of coarse aggregate is increased

294

Concrete Technology

by 0.01 (at the rate of ± 0.01 for every ∓ 0.05 change in water-cement ratio) to decrease the fine aggregate content. Therefore, corrected volume fraction of coarse aggregate for the water-cement ratio of 0.45, p = 0.63. The volume fraction of fine aggregate content, 1−p = 1− 0.63 = 0.37. (e) Volumes of fine and coarse aggregates Total absolute volume of aggregates from Eq. (10.14): 438 197.2 ⎞ ⎛ 3 Va = 1 − ⎜ 0.02 + + ⎟ = 0.644 m ⎝ 3 15 × 1000 1000 ⎠ Therefore, the absolute volumes of fine and coarse aggregates per unit volume of concrete are

Vfa = 0.37 × 0.644 = 0.238 m3 Vca = 0.63 × 0.644 = 0.406 m3 (f) Mix proportions The quantities of saturated surface dry fine and coarse aggregates are:

Cfa = (2.60 × 1000) = 0.238 = 619 kg/m3 Cfa = (2.65 × 1000) × 0.238 = 619 kg/m3 (i) Mix proportions by (saturated surface dry) mass Cement

Water

Fine aggregate

Coarse aggregate

438

:

197.2

:

619

:

1076

1.0

:

0.45

:

1.41

:

2.46

(kg/m3)

(ii) Mix proportions by volume Cement

Water

Fine aggregate

Coarse aggregate

⎛ 438 ⎞ ⎜⎝ ⎟ 1450 ⎠

:

⎛ 197.2 ⎞ ⎜⎝ ⎟ 1000 ⎠

:

⎛ 619 ⎞ ⎜⎝ ⎟ 1700 ⎠

:

⎛ 1076 ⎞ ⎜⎝ ⎟ 1800 ⎠

1.0

:

0.45

:

1.21

:

1.98

(kg/m3)

(g) Adjustments for aggregate moisture Since there is moisture in both the coarse and fine aggregate their trial batch masses must be adjusted as Weight of fine aggregate Weight of coarse aggregate

= 619 × 1.02 = 1076 × 1.01

= 631 kg = 1087 kg

Free-water present in the fine and coarse aggregates, = 619 × 0.02 + 1087 × 0.01 = 23.2 kg Therefore, the amount of water to be added at the batching plant is = 197.2 − 23.2 = 174 kg The final trial batch quantities per cubic meter of concrete are:

Proportioning of Concrete Mixes Nominal water- Water (kg / m3) cement ratio effective w/c ratio

Cement (kg / m3)

Fine aggregate (kg / m3)

Coarse aggregate (kg / m3)

0.45

174

438

631

1087

-

0.40

1.0

1.44

2.48

10.10

295

CONCRETE MIX PROPORTIONING USING FLY ASH IS GUIDLINES

The IS 10262-2009: Concrete Mix Proportioning Guidelines can be used for mix proportioning of concrete using fly ash. The proportioning procedure is exactly the same as the one used for proportioning non fly ash concretes; simply a part of cement computed is replaced by fly ash. The percentage fly ash to be used is based on project requirements and quality of materials. However, in certain situations an increase in cementitious material (cement + fly ash) content is warranted. The amount of increase in cementitious material content is generally based on experience and field trials. Generally, to achieve approximately equal strength of the fly ash concrete, 22.5 per cent of cement may be replaced with 32.5 per cent fly ash by mass along with consequential adjustments in fine and coarse aggregates. In the procedure the cementitious material (cement + fly ash) content is treated as cement. Using IS10262-2009: Concrete mix proportioning guidelines, it is required to design a M40 grade pumpable fly ash concrete mix having basic slump of the order of 80 -100 mm. Available G-43 OPC having specific gravity of 3.15 conforms to IS 8112 and the fly ash having specific gravity of 2.2 conforms to IS 3812 (Part 1). The concrete mix is to be used for a reinforced concrete structure to be subjected to severe exposure conditions during its service life. The crushed (angular) coarse aggregate available at the site is of nominal maximum sizes of 20 mm with a specific gravity of 2.72 and water absorption of 0.5 per cent. Whereas, the available fine aggregate has fineness modulus of 2.80 (grading zone-I of Table 4 of IS 383), specific gravity of 2.70, and moisture absorption of 1.0 per cent. The other stipulations are:

Example 10.7

• • • • • •

Standard deviation (no past records are available, from Table 10.25): 5.0 MPa Air content: 4.0 − 5.0 per cent Maximum allowable free water-cement ratio: 0.45 Minimum cement content: 320 kg/m3 Maximum cement content (IS 1343-1980): 450 kg/m3 (vi)Chemical admixture type: Superplasticizer conforming to IS 9103 • Density of water: 1000 kg/m3.

Solution Mix proportions Target mean strength, ft = fck + k S = 40 + 1.65 × 5.0 = 48.25 MPa The free water−cement ratio for the target strength from Table 10.23 is 0.38; however, based on experience, water-cement ratio of 0.40 is adopted (0.40 < 0.45, hence, O.K.).

296

Concrete Technology

(a) Selection of water content From Table 10.27, maximum water content for 25 to 50 mm slump range for 20 mm nominal size saturated surface dry aggregate is 186 liters. For workability (slump) range of 80 −100 mm, 3 100 − 50 ⎤ ⎡ Estimated water content = 186 × ⎢1 + = 197.2 liters × 100 25 ⎥⎦ ⎣ As a superplasticizer is used to obtain pumpable concrete, the water content may be reduced up to 30 per cent; the actual reduction in water content can be estimated by trials with superplasticizer. For illustration consider a superplasticizer with manufacturer recommended dosage @ 2.0 per cent by mass of cementitious material resulting in a reduction of the order of 28 per cent. Therefore, Water content = 197.2 × 0.72 = 142 liters or kg/ m3 (b) Determination of cementitious material (cement + fly ash) content Water-cement ratio Hence, cement content

= 0.40 = 142/0.40 = 355 kg / m3.

The calculated cement content of 355 kg / m3 is more than the minimum cement content of 320 kg / m3 recommended in Table 10.6 for ‘severe’ exposure conditions and is less than the maximum prescribed in Clause 8.1.1 of IS 1343-1980. (c) Fly ash content To achieve approximately equal strength of fly ash concrete, 22.5 per cent of cement may be replaced with 32.5 per cent fly ash. Thus, Cement (OPC) content = 355 × 0.775

= 275.1 kg/m3

Fly ash content = 355 × 0.325 Therefore, cementitious material content Effective water-cementitious material ratio Saving of cement while using fly ash

= 115.4 kg/m3 = 390.5 kg/m3 = 142/390.5 = 0.364 = 355 − 275.1 ≈ 80.0 kg/m3.

(d) Volume fractions of coarse and fine aggregates Volume fraction of coarse aggregate (20 mm nominal maximum size) in total aggregate with fine aggregate belonging to Zone I, for water-cement ratio of 0.50 as obtained from Table 10.28 is 0.60. Adjustments As the selected water-cement ratio of 0.40 is less than the reference value of 0.50 by 0.10, volume fraction of coarse aggregate is increased by 0.02 (at the rate of ± 0.01 for every ∓ 0.05 change in water-cement ratio). Therefore, corrected volume proportion of coarse aggregate for the water-cement ratio of 0.40, p = 0.62. For pumpable concrete volume fraction of coarse aggregate should be reduced by 10 per cent. Therefore, Volume fraction of coarse aggregate, p = 0.62 × 0.9 = 0.558. Volume fraction of fine aggregate content, 1−p = 1− 0.558 = 0.442. Total absolute volume of aggregates Va (m3) is given by Eq. (10.14):

Proportioning of Concrete Mixes

Va

297

⎡ ⎤ C F W S 1 0 ⎢v + + + + ⎥ Sc × 1000 S f × 1000 1000 S s × 1000 ⎦ ⎣

where F, Sf, and S, Ss are the mass and specific gravity of fly ash and (superplasticizer, respectively. Air content, V for 20-mm maximum nominal size of aggregate is 2 per cent, i.e., 0.02. For the ratio of coarse aggregate to total aggregate by absolute volume of p, the absolute volumes of coarse and fine aggregates per cubic meter of concrete are Vca = pVa = 0.558 × 0.691 = 0.386 m3 Vfa = (1 − p)Va = (1 − 0.558) × 0.691 = 0.305 m3 Therefore, masses of coarse and fine aggregates are Cca = (Sca × 1000) Vca = (2.72 × 1000) × 0.386 = 1050 kg/m3 Cfa = (Sfa × 1000) Vfa = (2.70 × 1000) × 0.305 = 824 kg/m3 where Sfa and Sca are the specific gravities of saturated surface dry fine and coarse aggregates, respectively, in kg/ liter. (e) Mix proportions by mass Cement 275.1 : 1.0

W/CM

Fly ash

142

: 115.4 :

: 0.364 :

0.42

Fine aggregate Coarse aggregate :

Admixture

824

:

1050

: 7.81

3.00

:

3.82

: 00.28

(kg/m3)

(f) Adjustments for aggregate moisture Aggregates should be used in saturated surface dry condition. As both the coarse and fine aggregates are dry, the amount of mixing water should be increased by an amount equal to the moisture likely to be absorbed by the aggregates as provided by IS 2386 (Part 3). Extra water to be added to provide for absorption in

Fine aggregate = 824 × 0.01 Coarse aggregate = 1050 ×0.005 Extra water to be added

= – 8.24 kg = – 5.25 kg = – 13.49 kg

Therefore, the amount of water to be added at the batching plant is = 142 + 13.49 = 155.49 kg/m3

10.11

RAPID METHOD FOR MIX DESIGN

A more realistic approach to estimate the preliminary water−cement ratio corresponding to the target mean strength is to correlate it with the 28-day compressive strength of cement. In contrast to the usual seven-day strength, the cement is characterized by its 28-day strength because characteristic strength is found to be better related to 28day strength of cement rather than at earlier ages particularly so for blended cements. However, this approach will need 28 days for determining the strength characteristics of cement and at least another 28 days for trial mixes of concrete. The 28 or 56 days time is too long a period for a contractor to wait for trial mix results. There is a tendency of straight away using the mix without waiting for trial

298

Concrete Technology

mixes. In order to cut down the time required for trials, the Cement Research Institute of India (CRI) has developed an alternate rapid method where the compressive strengths of cement and concrete are obtained by using accelerated curing method as described in IS: 9013−1978. The 28-day compressive strength of concrete is found to be statistically significantly related to its accelerated strength, therefore the trial mixes are correlated to the target mean accelerated strength rather than to target mean 28-day strength. This correlation is found to be independent of the type or characteristics of cement, presumably because they affect both the accelerated and normal strengths of concrete in a proportionate manner. On the other hand, results of accelerated compressive strength tests on standard cement mortar (IS: 4932−1968) have been found to be unreliable. In the method suggested by CRI, this problem has been overcome by using accelerated strength of standard or reference concrete mix having water−cement ratio of 0.35 and workability of 0.80 CF (compacting factor) using cement at hand. The strength is determined by using accelerated curing in accordance with IS: 9013−1978. The nominal maximum size of coarse aggregate of reference concrete should be 10 mm and the fine aggregate should conform to zone II given in IS: 383−1970. The mix proportion of reference concrete is 1:0.81:2.07 with a water−cement ratio of 0.35. Using the above proportions, 150 mm cube specimens of reference concrete are made and the accelerated strength is determined by using accelerated curing by the boiling-water method. Corresponding to the accelerated strength of the reference concrete, the water-cement ratio for the required target mean strength of normal concrete is determined from Fig. 10.13.

28-Day Compressive Strength of Concrete, MPa

70 Accelerated strength as per (Tested T IS: 9013–1978) of reference concrete mix. A: 12.5–15.5 MPa B: 15.5–18.5 MPa C: 18.5–21.5 MPa D: 21.5–24.5 MPa E: 24.5–27.5 MPa F: 27.5–30.5 MPa

60 F E 50

D C

40

B A

30

20

10

0 0.30

Fig. 10.13

0.35

0.40 0.45 0.50 0.55 Water–Cement Ratio

0.60

0.65

Water-cement ratio versus compressive strength of concrete for different reference strengths (boiling water method)

Proportioning of Concrete Mixes

299

The accelerated strength of the trial mix using this water-cement ratio is checked against the characteristic target strength using the correlation of accelerated and normal 28-day strengths of concrete given in Fig. 10.14. The step-by-step procedure of mix design is as follows: 1. The accelerated strength of reference or standard concrete using the cement at hand is determined by testing 150 mm cubes cured by the boiling-water method in accordance with IS: 9013−1978. 2. The water−cement ratio for the required target mean strength of normal concrete is determined by using the corresponding accelerated strength of standard concrete obtained in Step 1. 3. The mix proportions are determined by any of the accepted methods of mix design and checked for workability of fresh concrete against the desired value. 4. The accelerated compressive strength of the trial mix is determined on 150 mm cubes cured by the boiling-water method as specified in IS: 9013−1978. 5. The 28-day compressive strength of normal concrete is estimated from its accelerated strength obtained in Step 4., by using the correlation of accelerated test results to the 28-day strengths of normally cured specimens given in Fig. 10.14.

28-Day Compressive Strength F28, MPa

70 60

Regression Equation: F28 = 8.25 + 1.64 Fa

50

Boiling–water Method Regression Equation: F28 = 13 + Fa Warm-water Method

40

30

20

10

0

0

10

20

30

40

50

60

70

Accelerated Strength Fa, MPa

Fig. 10.14

Typical relation between accelerated and 28-day compressive strength

The compressive strength is checked against the target mean strength to judge the suitability of the trial mix. The significant reduction in the time required for the trial mixes will help in the adoption of designed mix concrete and curb the tendency of using the trial mix without waiting for the strength results. The accelerated curing cycles given by IS: 9013−1978 are as follows:

300

Concrete Technology

1. Boiling-water method 1 (a) Specimens are cured for 23 ± h under standard moist conditions at 4 27 ± 2°C. (b) At the end of this period, the specimens are cured in boiling water (100°C) 1 for 3 h ± 5 min. 2 (c) Specimens are cooled to a normal temperature of 27 ± 2 °C in 2 h before the testing. 2. Warm-water method (a) One-and-a-half to three-and-a-half hours after casting, the specimens are immersed in water maintained at 55 ± l °C and cured for 20 h ± 10 min. (b) Demould the specimen and cool at 27 ± 2 °C for one hour before testing. Either method may be adopted as a standard for the prediction of accelerated strength. These are applicable to most test specimens and give results of low variability. The actual correlation of accelerated strength to 28-day strength of the normally cured specimen depends upon the curing cycle adopted, the chemical composition of cement, and the concrete mix proportions. The average correlation shown in Fig. 10.15 is generally used for different concretes, and in the absence of any past records concerning local materials they can be used to predict the 28-day compressive strength within ±15 per cent. 40

Accelerated Strength of Concrete Fa, MPa

35

30

25

20

15

10

5

0

Fig. 10.15

0

10

20

30 40 50 60 70 28-Day Strength of Concrete, MPa

80

90

Relationship between accelerated and 28-day strength of concrete

Proportioning of Concrete Mixes

10.12

301

CONCRETE MIX DESIGN ILLUSTRATION

The methods discussed in the preceding sections are compared by means of concrete mix design problem, where it is required to proportion a concrete mix for M20 grade concrete for a reinforced concrete residential housing colony; the structures are likely to be subjected to moderate exposure conditons during their service for the following requirements:

Example 10.8 1. Design stipulations (a) Characteristic compressive cube strength at 28 days (b) Maximum size of aggregate (c) Type of aggregate (d) Degree of workability (e) Degree of quality control (f) Type of exposure 2. Characteristics of materials (a) Cement (i) Type of cement used (ii) Specific gravity of cement (iii) Bulk density of cement (b) Aggregates (i) Specific gravity Coarse aggregate Fine aggregate (ii) Bulk density Coarse aggregate Fine aggregate (iii) Fineness modulus Coarse aggregate Fine aggregate (iv) Water absorption Coarse aggregate Fine aggregate (v) Free surface moisture Coarse aggregate Fine aggregate (vi) Grading of aggregate Type of aggregate Coarse Fine

20 mm 100 100

20 MPa 20 mm Crushed rock (angular) 0.90 CF (slump: 70−120 mm) weigh batching but occasional supervision and tests, S = 5.0 Moderate

Ordinary Portland cement (OPC) 3.15 1500 kg/m3

2.6 2.6 1600 kg/m3 1700 kg/m3 6.5 2.2 0.5 per cent Nil Nil 2.0 per cent

Percentage passing the IS sieve 10 mm 4.75 mm 2.36 mm 1.18 mm 600 mm 49 1 0 0 0 100 100 98 82 63

300 mm 150 mm 0 0 30 6.5

302

Concrete Technology

3. Target mean strength Target mean compressive strength = (20.0 + 1.65 × 5.0) ≈ 28.0 MPa 4. Maximum water–cement ratio Water–cement ratio for durability under moderate exposure conditions 0.55 5. Water content Maximum size of aggregate 20 mm Workability medium (slump, 70–120 mm) Water content per cubic meter of concrete (as per the method used). 6. Mix proportions

Proportioning of aggregates to obtain standard grading The gradings of available fine and coarse aggregates are to be combined in a suitable proportion so as to obtain the desired standard or specified grading. Adopt the percentage passing IS: 4.75 mm sieve as criterion. Let 1 kg of fine aggregate be combined with x kg of coarse aggregate to obtain desired standard grading and assume that 38 per cent of combined aggregate passes the criterion sieve. The percentage passing IS: 4.75 mm sieve individually must be equal to the total aggregate passing the same sieve, i.e. 100 × (1) + 1 × (x) = 38(1 + x) or x = 1.63 Hence the fine and coarse aggregates must be combined in the proportion 1:1.63. The combined grading obtained is compared with the specified grading in Table 10.29.

10.12.1

British DoE Method of Mix Proportioning

The concrete design specifications in EN:206-1 concrete format are Strength class: Slump range: Exposure classes: Cement: Coarse aggregate: Fine aggregate: Table 10.29

C16/20 at 28 days 70 mm to 120 mm, i.e., slump class: S3 XC1 and XC2 (20 year life) CEM-I class cement i.e. normal Portland cement class 10/20 (nominal maximum size aggregate: 20 mm) grading zone-III. Comparison of combined grading with standard grading

Grading

Percentage passing 20 mm

10 mm

4.75 mm

2.36 mm

1.18 mm

600 μm

300 μm

150 μm

Combined aggregate

100

68

38

37

31

24

11

2

Specified grading

100

65

42

35

28

21

5

1

Proportioning of Concrete Mixes

303

1. Free water-cement ratio For the reference free water−cement ratio of 0.5, 28-day compressive for the normal Portland cement and crushed aggregate obtained from Table 10.15 is 49 MPa. With this pair of data (49 MPa and water−cement ratio = 0.50) as a controlling point, a curve is visually interpolated in Fig.10.8 passing the controlling point. Point B is marked on the interpolated curve corresponding to the target strength of 28 MPa. This point corresponds to a water−cement ratio of 0.72. For the exposure classes: XC1 and XC2, the maximum permitted value of free water−cement ratio 0.55. Therefore, adopt a free water−cement ratio of 0.55. 2. Water and cement contents For the crushed aggregate of class 10/20, the water content for consistence class: S3 as obtained from Table 10.16 is 210 kg / m3. For water−cement ratio of 0.55, Cement content = 210 / 0.55 = 382 kg / m3. This cement content is satisfactory as it is more than the minimum cement content of 240 kg /m3 recommended in Table 10.14 and less than the maximum prescribed value of 450 kg / m3. 3. Proportions of fine and coarse aggregates Wet density fully compacted fresh concrete as obtained from Fig. 10.9 is 2325 kg/m3. Therefore, total aggregates content 382 210 ⎞ ⎛ − Ca = 2325 × ⎜1 − = 1555 kg/m3 ⎝ 3 15 × 1000 1000 ⎟⎠ For a concrete with consistence class: S3, water−cement ratio of 0.55 and fine aggregate conforming to the grading zone III, the proportion of fine aggregate as per cent of total aggregate from Fig. 10.10(b.3) is 34 per cent. Therefore, proportions of saturated surface dry aggregates are: Mass of fine aggregate, Cfa = 0.34 × 1555 = 529 kg/m3 Mass of coarse aggregate, Cca = (1 − 0.34 × 1555 = 1026 kg/m3 4. Proportions of concrete Hence the concrete proportions by mass can be expressed as: Cement

10.12.2

Water

Fine aggregate

Coarse aggregate

382

:

210

:

529

:

1026

1.00

:

0.55

:

1.38

:

2.69

(kg/m3)

ACI Method of Mix Proportioning

1. Coarse aggregate content

Maximum size of coarse aggregate Fineness modulus of fine aggregate Bulk volume of dry-rodded coarse aggregate per cubic meter of concrete from Table 10.19 (by extrapolation) Dry mass, of coarse aggregate = 0.68 × 1600

20 mm 2.2 0.68 m3 1088 kg

304

Concrete Technology

2. Cement content (a) Equivalent cylinder strength = 0.8 × 28 Water–cement ratio From strength considerations from Table 10.23 From durability consideration Water−cement ratio adopted Water content per cubic meter of concrete from workability considerations from Table 10.20 Thus, cement content = 205/0.55 (b) From durability considerations the cement content Cement content adopted

22.40 MPa 0.66 0.55 0.55 205 kg 372.7 kg 290 kg 372.7 kg

3. Air content Air content for a maximum size of aggregate of 20 mm from Table 10.20

2 per cent

4. Estimation of mass of fresh concrete Mass of fully compacted fresh concrete per cubic meter (in kilograms) = 1000 [1 − {v + (C/1000Sc) + (W/l000)}]Sa + C + W where C and W are the cement and water contents in kilograms per cubic meter of concrete, respectively, Sc is the specific gravity of cement, Sa the weighted average specific gravity of fine and coarse aggregate combined. Air content expressed in terms of percentage of volume of concrete is given by v. Mass per cubic meter of fresh concrete. ⎡ ⎛ 372.7 205 ⎞ ⎤ = 1000 × ⎢1 − ⎜ + ⎟ ⎥ × 2.6 + 372.7 + 205 ⎣ ⎝ 1000 × 3 15 1000 ⎠ ⎦

= 2285 kg Total mass of aggregate per cubic meter of concrete = 2285 − 372.7 − 205 = 1707 kg Mass of fine aggregate = 1707 − 1088 = 619 kg, i.e., the percentage of sand is 36. 5. Trial Mix proportions (a) By mass Water

Cement

Fine aggregate

Coarse aggregate

205

:

372.7

:

519

:

1088

0.55

:

1.00

:

1.66

:

2.92

(b) By Volume Cement 1.00

:

Fine aggregate 1.56 :

Water−cement ratio = 0.55.

Coarse aggregate 2.93

Proportioning of Concrete Mixes

10.12.3

305

Concrete Mix Proportioning – IS Guidelines

Target mean strength, ft = Free water-cement ratio: Compressive strength consideration (from Fig. 10.3): Durability considerations: Hence, water cement ratio of 0.50 may be adopted.

28 MPa 0.49 0.50

1. Determination of water content From Table 10.27, maximum water content for 25 to 50 mm slump range for 20 mm nominal size saturated surface dry aggregate is 186 liters. Adjusted water content for 70−120 mm slump 3 120 − 50 ⎤ ⎡ = 186 × ⎢1 + × = 20.16 liters. 25 ⎥⎦ ⎣ 100 2. Determination of cement content Cement content = 201.6/0.50 = 403 kg / m3. The calculated cement content is more than the minimum cement content of 300 kg /m3 and less than the maximum of 450 kg / m3; hence, can be adopted. 3. Proportions of fine and coarse aggregates Volume fractions of coarse and fine aggregates Volume fraction of coarse aggregate (20 mm nominal maximum size) with fine aggregate belonging to Zone III, for water-cement ratio of 0.50 as obtained from Table 10.28 is 0.64. Therefore, volume fraction of fine aggregate content, 1− p = 1− 0.64 = 0.36. Total absolute volume of aggregates from Eq. (10.10): 403 201.6 ⎞ ⎛ 3 Va = 1 − ⎜ 0.02 + + ⎟ = 0.650 m ⎝ 3 15 × 1000 1000 ⎠ Therefore, the absolute volumes of fine and coarse aggregates per unit volume of concrete are: Vfa = 0.36 × 0.650 = 0.234 m3 VCa = 0.64 × 0.650 = 0.416 m3 Therefore, quantities of saturated surface dry fine and coarse aggregates are: Cfa = (2.60 × 1000) × 0.234 = 608 kg/m3 Cfa = 2.60 × 1000 × 0.416 = 1082 kg/m3 4. Mix proportions by (saturated surface dry) mass Cement 403 1.0

Water : :

201.6 0.50

Fine aggregate : :

608 1.51

Coarse aggregate : :

1082 2.68

(kg/m3)

The mix proportions arrived at by different methods is summarized in Table 10.30.

306

Concrete Technology Table 10.30

Method of mix design

Summary of mix proportions obtained by various methods Mix proportions by mass

Cement Fine aggregate Coarse aggregate

Aggregate cement ratio

DoE method

1

1.38

2.69

4.07

ACI method

1

1.66

2.92

4.58

IS Guidelines

1

1.51

2.68

4.19

10.12.4 Actual Quantities of Material Required Per Bag of Cement The weights of materials must be adjusted for moisture content. As an example, adopt the mix proportions obtained by using the Concrete Mix Proportioning-IS Guidelines. Thus, for one bag of cement: = 50 kg = 25 kg = 75.5 kg = 134 kg = 75.5 × 1.02 = 77.01 kg

Cement content Water content Fine aggregate (dry) Coarse aggregate (dry) Weight of wet fine aggregate

The free moisture present in the aggregate must be deducted from the water to be added and extra water is needed to provide for absorption. Surface moisture contributed by Fine aggregate = 75.5 × 0.02 Coarse aggregate Extra water to be added to provide for absorption in

= 1.51 kg = Nil

Fine aggregate Coarse aggregate = 134 × 0.005

= Nil = 0.67 kg

Therefore, Estimated requirement of water = 25.00 − 1.51 + 0.67 = 24.16 kg (or liters) 1. Batch mass per bag of cement Water content Cement content Fine aggregate (wet) Coarse aggregate

= 24.16 kg = 50.00 kg = 77.01kg = 134.00 kg Total = 285.17 kg

Therefore, the net mix proportion by mass is 1.00:1.54:2.68 with free water cement ratio as 0.483. 2. Trial mixes Trial mixes should be prepared using these proportions as explained in Section 10.9 and tested to check if the mix meets the design stipulations. Otherwise, suitable adjustments should be made till it satisfies the design stipulations.

Proportioning of Concrete Mixes

10.13

307

COMPARISON OF MIX PROPORTIONING METHODS

All the methods discussed in this chapter are based on absolute volume concept and the mix design parameters are mostly identical except that ACI method and IS guidelines use volume fraction of coarse aggregates in computation of fine aggregates content, whereas in British DoE the sand content (per cent) is selected directly based on the nominal upper (maximum) size of coarse aggregate and the grading zone of fine aggregate. The ACI method defines fine aggregate grading in terms of fineness modulus (FM), whereas the other two methods use fine aggregate zone as the grading index.

10.13.1

Selection of Materials

The ACI method considers the maximum size of aggregate but does not differentiate between crushed (flaky/elongated) and uncrushed (rounded) coarse aggregates. The British DoE method, on the other hand, takes into account the type (crushed/ uncrushed) and upper size of coarse aggregate for water demand calculation. The IS guidelines consider the maximum size of coarse aggregate as a parameters for calculation of water content and specify adjustments for water requirement when rounded (uncrushed) coarse aggregate is to be used. However, none of the methods treat natural fine aggregate and crushed fine aggregate differently. The shape, i.e., elongation and flakiness of aggregates influences many properties of fresh and hardened concretes thus requiring control over the shape characteristics to optimize the properties of concrete. The flakiness index is higher in smaller size aggregates. For example, for a coarse aggregate of maximum size of 40 mm the average flakiness is generally less than 10 per cent. On the other hand, for coarse aggregates with maximum sizes of 25 mm, 20 mm, and 12.5 and below, the average percentage of flakiness indices generally lie in the ranges 10−15, 15−30 and > 40 per cent, respectively. Thus the problem of flakiness is acute in smaller size aggregates. For a normal mix design, the combined (flakiness + elongation) index for coarse aggregates may be limited to 25 per cent. For a given slump, an increase in the combined index beyond this limit up to 40 per cent may require up to 12 kg/m3 more water. On the other hand, when crushed or manufactured sand is used the mix may require up to 15 kg/m3 more water than natural sand. This additional water demand may increase to 25 kg/m3 in case of stone dust. For economic reasons, the choice is often made in favor of flaky materials, which have the lowest costs. However, to achieve better workability there is increasing trend to replace 10-mm size aggregates (with higher flakiness) by 20-mm size aggregates resulting in gap-graded aggregates. It should be noted that concrete produced with shaped aggregates works out to be economical since the reduction in flakiness of shaped aggregates, say by 15 per cent, may reduce cement consumption up to 30 kg/m3. The stone dust on the other hand is available at less than half the price of crushed sand and there is tendency to use stone dust powder as fine aggregate, which increases the specific surface and hence the water demand, resulting in reduction in the strength and durability of concrete.

308

Concrete Technology

10.13.2

Estimation of Air and Mixing Water Contents

1. Air content In the ACI method, the percentage of air content of the concrete is estimated depending upon the type of concrete, i.e., air-entrained or non-air entrained, exposure conditions, and the nominal maximum size of aggregate, whereas in the IS guidelines, entrapped air is based solely on nominal maximum size of aggregate. The British DoE method considers the influence of air entrainment on workability separately. 2. Water content In the ACI method, the water content is based on the nominal maximum size of coarse aggregate, type of concrete (air entrained or nonair entrained), and specified slump, but is independent of water−cement ratio (target strength). The British DoE method considers type and size of coarse aggregate, and workability in selection of free-water content. It also takes into account the effect of additions (mineral additives). IS guidelines for mix proportioning, on the other hand, computes water content based on the water−cement ratio, workability (in terms of slump), and type and the nominal maximum size of the aggregate.

10.13.3

Selection of Water–Cement Ratio

In the ACI method, the water−cement ratio is based on 28-day target compressive strength and type of concrete (air entrained or non-air entrained). On the other hand, the British DoE method considers the influence of cement, aggregate type, and target mean strength on computation of water-cement ratio. IS guidelines base the selection of water−cement ratio on target mean compressive strength and environmental exposure conditions. All the three methods provide guidance on the use of cementing materials (additions).

10.13.4

Estimation of Cement Content

The provisions for estimation of cement content are almost identical in all the methods.

10.13.5

Estimation of Coarse and Fine Aggregate Contents

All the methods are based on absolute volume concept. In the ACI method and IS guidelines, the coarse aggregate content is estimated directly and that of fine aggregate by subtracting the absolute volume of the known ingredients from a unit volume of compacted fresh concrete. On the other hand, the British DoE method directly selects the required fine aggregate content as a proportion of the total aggregate content, and obtain the total (coarse and fine) aggregate content by subtracting the absolute volumes of the known ingredients (entrained-air, water and cement) from a unit volume of concrete.

Proportioning of Concrete Mixes

10.13.6

309

General Observations

Keeping in view the limitations of the methods in general, all the methods apply to well-shaped aggregates within the range of generally acceptable specifications of relevant codes. However, the following observations may be helpful. 1. Though the British Standard BS:5328 has been withdrawn and replaced by a more comprehensive European Standard EN:206-1 and its complementary British Standard BS:8500 but BS EN: 206 exerts relatively little influence directly on the design process. The basic mix design approach is intact. The main changes in BS EN:206-1 are in the terminology used. The conformity testing has replaced the compliance testing and emphasis is on performance criteria. Compacting factor test has been abandoned. Exposure classes based upon environmental action have been redefined. 2. The ACI method is simple and straightforward. The British DoE method is more involved. The IS guidelines are similar to ACI method. 3. The cement content increases with the targeted strength. The IS method uses higher amount of cement (lowest water-cement ratio) than that used by the other methods. One of the reasons for the high cement demand may be the lower fineness of Indian cements, 225 m2/kg compared to 300−500 m2/kg for American cements. Use of IS guidelines for lower-grade concretes results in over design, e.g., actual strength of 20 MPa concrete achieved is almost double. It indicates wastage of materials, since such a high strength is not desired. 4. The fine aggregate content decreases with the increase in the targeted strength. The fine aggregate content in ACI designed concretes are higher resulting in higher workability. It appears that higher percentage of fine aggregate also contributes to increased strength as the voids are filled, especially in the higher grade concretes. In case of the ACI method, it is generally suggested that calculated coarse aggregate volume sometimes needs to be reduced, so that the fine aggregate content is increased and the cohesiveness and general workability of the mix improved. 5. Mixes (concretes) designed by the ACI and British DoE methods have shown high correlation between targeted and actual strengths which indicates that these are more consistent. On the other hand, the IS guidelines have shown relatively higher strength variations.

10.14 10.14.1

OPTIMUM CONCRETE MIX DESIGN Conventional Design

In case of big projects a comparative analysis of cost of concretes produced using materials from different sources can help to identify the appropriate source. The optimum mix proportions of the ingredients giving the least cost of concrete should satisfy the criteria of strength and durability of hardened concrete, and workability

310

Concrete Technology

of fresh concrete. Since the cost of cement in general is very high in comparison with other ingredients, a leanest mix satisfying the requirements of durability in terms of cement content, and workability is desirable. The process of estimating the optimum relative proportions of ingredients to achieve the most economical mix satisfying the design stipulations regarding strength, durability and workability, can be formulated into a mathematical optimization problem. The formulation consists in minimization of an objective function expressing production cost of unit volume of concrete subject to the constraints of compressive strength, durability, workability and aggregate characteristics.

Design Variables and Constraints The aggregates of different maximum sizes having different properties and unit costs are to be combined in such a way that the grading of resulting aggregate lies within the preselected grading limits and promotes workability. The constraints generally considered are the following: 1. Compressive strength 3. Workability

2. Durability 4. Aggregate characteristics

The objective function to be minimized comprises the total cost of production of unit volume of concrete expressed as the sum of the cost of individual processes. 1. Compressive strength In addition to the water-cement ratio, the other important factor governing the strength of concrete is the type and maximum nominal size of the aggregate. 2. Durability The durability criterion is generally satisfied by limiting the water-cement ratio and minimum cement content. Depending on the type of environmental conditions to which a structure is likely to be exposed, a maximum permissible water-cement ratio constraint is introduced. Sometimes a minimum water-cement ratio constraint is also imposed from practical considerations. 3. Workability The degree of workability designated as very low, low, medium and high can be expressed in terms of range of slump, compaction factor and Vee-Bee time values. For the purpose of formulating constraints in the optimization problem, the workability is generally related to the water−cement ratio, aggregate−cement ratio and standard consistency of cement. 4. Aggregate characteristics The type of aggregate, its maximum size and grading influence the water content to produce a workable concrete mix. Generally, the maximum and minimum permissible grading limits of the combined aggregates are specified for different maximum nominal sizes of aggregates ranging from 10−40 mm. If ten standard sieves—40 mm down to 75 μm—are used, there will be 20 constraints, corresponding to the maximum and minimum grading limits. 5. Solution of optimization problem If the objective function is a linear function of various constraints which themselves are linear, then the resulting formulation is called the linear programming problem. Its solution can be obtained by simplex method. The non-linear programming problem can be transformed into a form which permits application of Simplex Algorithm. Alternatively, the non-linear constrained optimization problem can be solved directly by using the available methods.

Proportioning of Concrete Mixes

10.14.2

311

Design of Concrete Mix as a System

The foregoing procedures of concrete mix design can be expressed almost without qualification in terms of design methodology of Systems Engineering. The term ‘Systems Engineering’ is currently the popular name for engineering processes of planning and design used in the creation of a system or project. In the most general sense, a system may be defined as a collection of various structural and non-structural (e.g., human) components which are so interconnected and organized to achieve a specified objective by the control and distribution of material resources, information and energy. The fundamental characteristic of a properly designed and operated system is that the performance achieved by the whole is beyond the total capability of the separate components operating in isolation. The purpose of this section is to set the scene for development of mathematical model for an efficient and economical concrete mix design. In systems engineering the physical quantities and processes are represented by mathematical models; performance is analyzed and objective measures of costs and benefits are obtained by mathematical operations; and the influence of uncertainty particularly where humans are involved, is modeled by probability distributions where necessary. This principle of systems engineering can be applied to select a most efficient or optimal proposal from a large number of feasible alternatives which may be imperfect to some degree. Sometimes the model as modified and improved in light of preliminary design, tested for its feasibility and optimized for main system parameters may help to reach a reasonably firm decision concerning acceptance or rejection of the proposal. In mix design problems, the mix proportions, water−cement ratio or water content may be selected as decision variables, since effectiveness of the concrete (system) can be evaluated directly or indirectly in terms of these variables. The object of the analysis is to determine the best possible set of values with respect to system effectiveness. This is called optimal proposal. The objective function which is the measure of effectiveness of a particular proposal is expressed as a function of these decision variables. The conditions, which a mathematical model must satisfy before the decision variable values can represent a feasible solution are termed constraints. The process of mix design may be summarized by the following five sequential activities: 1. 2. 3. 4. 5.

Selection of decision variables Definition of objectives and identification of design criteria Generation of design alternatives Testing of feasibility of proposals Optimization and refinement of design to maximize the effectiveness

The objectives should be stated in the most basic and general terms possible. The information provided in the preceding sections may help in building up a picture of the problem environment. In concrete mix design problems, the economy of end product, i.e., the concrete may be the objective. Once the objectives have been determined, design criteria must be identified. In mix design problems the workability, the 28-day compressive strength, and durability are generally taken as the design criteria.

312

Concrete Technology

Testing of Feasibility For each alternative proposal, the first test must be that of feasibility. Technical constraints are normally carried out routinely in the course of preliminary design. The other constraints of economic and interdisciplinary nature also exist, they must be identified and quantified at this stage. As the design proceeds any constraint which is violated will result in the proposal being modified or rejected.

Measure of Effectiveness The most important factor influencing the nature of the final solution is the definition of objective and selection of appropriate measures of effectiveness. In its simplest form, the effectiveness of a mix design may be measured in terms of cost of final product, i.e., the concrete. To illustrate the formulation of objective function and constraints consider the following example. Example 10.9 A concrete mixing plant has to supply M15 grade mass concrete in large quantity to a dam project. The mix proportions have been estimated as 1:1.91:4.46 (by mass). This concrete requires sand and gravel (C.A.) mixture of 30 per cent sand and 70 per cent gravel by mass. The natural deposits at five pits near the dam site are found to have different compositions and their cost including transportation to the site also varies as listed in Table 10.31. However, the constituents satisfy the specifications. Determine the quantities of deposit to be obtained from each source in order to minimize the cost per cubic meter of concrete.

Solution Let xi be the fraction taken from pit i, the cost per cubic meter of concrete can be expressed as Z = 2.0x1 + 3.0x2 + 1.5x3 + 1.0x4 + 2.5x5 The fractions xi should be of the magnitudes such that ratio of sand and gravel in concrete should be 30 and 70 per cent, respectively. Thus 0.45x1 + 0.40x2 + 0.50x3 + 0.55x4 + 0.20x5 = 0.3 0.55x1 + 0.60x2 + 0.50x3 + 0.45x4 + 0.80x5 = 0.7 Table 10.31 Aggregate type

Sand and gravel mixture Mixture composition, per cent Pit No. 1

2

3

4

5

Sand

45

40

50

55

20

Gravel

55

60

50

45

80

2.0

3.0

1.5

1.0

2.5

Relative cost per cubic meter of mixture

The mathematical optimization problem of the system can be stated as:

Minimize Z = 2.0x1 + 3.0x2 + 1.5x3 + l.0x4 + 2.5x5

Proportioning of Concrete Mixes

313

Subject to 0.45x1 + 0.40x2 + 0.50x3 + 0.55x4 + 0.20x5 = 0.3 0.55x1 + 0.60x2 + 0.50x3 + 0.45x4 + 0.80x5 = 0.7 This linear programming problem can be solved easily by SIMPLEX METHOD. The values of fractions obtained by this method are: x1 = 0.0, x2 = 0.0, x3 = 0.0, x4 = 2 / 7, x5 = 5/7 Thus the deposits from the Pits 4 and 5 when mixed in the ratio 2:5 will result in a most economical concrete satisfying all the stipulations. Any deviation from these values will result in an increase in the cost of the material.

10.15

DESIGN OF HIGH-STRENGTH CONCRETE MIXES

The properties of concrete with a compressive strength above 40 MPa or 50 MPa are highly influenced by the properties of aggregate in addition to that of the watercement ratio. To achieve high strength, it is necessary to use the lowest possible water-cement ratio with high cement content which invariably affects the workability of the mix and necessitates the use of special vibration techniques for proper compaction. It should be kept in mind that high cement content may liberate large heat of hydration causing rise in temperature which may affect setting and may result in excessive shrinkage. In the present state-of-the-art, concrete which has a desired 28-day compressive strength up to 70 MPa can be made by suitably proportioning the ingredients and using normal vibration techniques for compacting the mix. A number of methods for designing high-strength concrete mixes are available. Since all the high performance concretes are high-strength concretes, the method described in Section 10.16 can be used for designing a high-strength concrete mix.

10.16

MIX PROPORTIONING FOR HIGH PERFORMANCE CONCRETE

For high performance concrete (HPC), especially when selected mineral additives and chemical admixtures are employed, attainment of a low water-to-cementing material ratio w/(c + p) is considered essential. Many trial mixtures are often required to generate the data necessary to identify optimum mixture proportions. The following procedure for proportioning the high performance concrete mixtures is applicable to normal weight, non-air-entrained concrete having compressive strengths in the range of 40 MPa and 80 MPa. Proper selection and proportioning is required for all material to be used because the performance of high-strength concrete is dependent on the properties of its individual components.

10.16.1

Performance Requirements

1. Age High strength concretes can gain considerable strength after the normally specified 28-days age. To take advantage of this characteristic, many specifications

314

Concrete Technology

for compressive strength have been modified from the typical 28-days criterion to 56 days, 91 days or later ages. 2. Strength To meet the specified strength requirements, the concrete must be proportioned in such a manner that not more than a predefined percentage of the average compressive strength results of field tests shall fall below the specified characteristic strength of concrete, fck. The average target strength, ft used as the basis for selection of concrete proportions is obtained from Eq. (10.8). As explained earlier the expression is based on probability that no more than five per cent (1 in 20) of test results are expected to fall below the fck. However, strength test results under ideal field conditions usually attains only 90 per cent of the strength measured by tests performed under laboratory conditions. Moreover, a high performance concrete mixture in the field generally requires some adjustments in the proportions for the air content and the yield. 3. Workability Because of high coarse aggregate and cementing materials contents, and a low w/(c + p) ratio, high performance concrete can be difficult to place. However, HPC can be placed at very high slumps without segregation with the use of HRWR. 4. Other high performance requirements Factors other than compressive strength influencing the selection of material and mix proportions are: modulus of elasticity, flexural and tensile strengths, heat of hydration, creep and drying shrinkage, durability, permeability, time of setting, and method of placement.

10.16.2

Selection of Materials

Selection of appropriate constituent materials to produce high performance concrete is a critical stage in the process of mixture proportioning, preliminary studies in the laboratory have to be extensive. This situation warrants one to study the mix proportions used successfully in other works as a starting point. These mix proportions are modified with additional laboratory tests to obtain the final mix. The optimum proportions are selected considering the characteristics of cement and mineral additives (such as fly ash and silica fume, etc.), aggregate quality, paste content, aggregate-paste interface, admixture type and its dosage, and mixing. For any given set of materials, there is an optimum cement content beyond which little or no additional increase in strength is achieved by increasing the cement content. The required high cementing materials content and low w/(c + p) ratio can help control the temperature rise in concrete at early ages and may reduce the water demand for a given workability. However, early strength gain of the concrete may be adversely affected.

Cement To produce high performance concrete, use of high-strength cements of Grade 53 is generally recommended. However, the increase of strength with Grade 53 cement over that with Grade 43 cement is only 4 to 8 MPa throughout the practical range of water−cement ratios. At low water−cement ratio the fast reacting Grade 53 cement having highest specific surface is less effective. It consumes a large part of

Proportioning of Concrete Mixes

315

the mixing water, and the pozzolanic and hydration reactions stabilize too soon, and a part of the cement remains unhydrated because of the lack of water. This results in a poor consistency of concrete. On the other hand, due to lower fineness Grade 43 cement provides a better consistency. In HPC, cements having lower initial setting times are recommended to compensate for the delays in setting when high dosages of superplasticizer (up to or over three per cent) are used. This delay is critical in cements with long setting times, such as Grade 43 cement. Compatibility between cement and superplasticizer is one of the major factors affecting the choice of the cement and, consequently of additives for HPC. The chemical composition and fineness of the cement affect the behavior of superplasticizers with cement. There is an optimum cement content that produces the highest strength. Optimum strength is obtained (for the same water-cement ratio) with approxi-mately 450 kg/ m3 of cement. The increase of strength due to larger cement content is slight, usually much less than 10 per cent. However, if cement content is increased there will be a remarkable influence on the consistency of concrete for the same water-cement ratio. Increase of cement content allows addition of more water to the mix for the same water-cement ratio, and thus enhancing the workability. Therefore, on some occasions it may be necessary to increase the cement content in the mix for better workabilities rather than for higher strengths.

Coarse aggregate Usually, high strength concrete is produced with normal weight aggregate which is clean, free from fissures or weak planes, and free from surface coatings. Smaller size aggregates have been found to provide higher strength potential for a given w/(c + p) ratio. A 20−12.5 mm nominal maximum size aggregate is commonly used for producing concretes up to 60 MPa and 16.5−10 mm is used for producing concretes above 60 MPa. Aggregates also influence the consistency of concrete. For good consistency a selected coarse aggregate should have: a low coefficient of absorption (≤ 1 per cent); a shape coefficient ≤0.25; and a maximum size of 16.5 to 10 mm. To improve compressive strength, aggregate shall have: a Los Angeles coefficient ≤15; and a crushing index ≤15. The influence of aggregate grading on workability and strength for the same water-cement ratio is not much. However, the grading has to be maintained within the limits to avoid segregation. Limestone aggregates are especially suitable to produce HPC due to the development of epitaxic adherence which increases strength remarkably. In this case, even aggregates not complying with the preceding limits of the Los Angeles coefficient or crushing index can produce very high strength concretes. Fine aggregate The grading and particle shape of fine aggregate significantly influence the mixing water content and compressive strength. The quantity of cement paste required per unit volume of a concrete mixture decreases as the relative volume of coarse aggregate versus fine material increases. A difference of one per cent in void content in the sand may result in approximately 4.55 kg/m3 difference in water demand. Fine aggregates with a fineness modulus in the range of 2.5 to 3.2 are

316

Concrete Technology

preferable for high performance concretes. Concrete mixtures made with a fine aggregate that has a fineness modulus of less than 2.5 may be sticky and result in poor workability and higher water requirements.

Fly ash The use of fly ash in concrete results in a lowered cost of materials in the finished concrete with improved performance characteristics. It is generally recognized that replacement of Portland cement by fly ash on an absolute volume basis up to 30 per cent, result in lower strengths up to about 28 days, but in greater strengths at 6 months and beyond. Fly ash mixes with 28-day strength equal to that of normal strength mixes can be proportioned by using fly ash quantities in excess of the amount of cement replaced. As a result of this, the fly ash mixes contain a total weight of Portland cement and fly ash greater than the weight of cement used in the comparable normal Portland cement mixes. Volume equivalency Proportioning by the volume equivalency method simply

means that the volume of cementing materials (cement + pozzolan) is the same as the volume of cement in a cement-only mix. When proportioning by the volume equivalency method, w/(cm + p) will always be greater than w/c and can be computed by the following equation: w c × ( w / c) (Gc = = c +p c +p Gc (

) ( w / c) Gc ( w / c) = Fv ) + G p Fv Gc ( Fv ) (G p Fv )

(10.16)

where Fv is the fly-ash fraction by volume, and Gc and Gp are the specific gravities of the cement and pozzolan, respectively. For example, a concrete mix having water-cement ratio of 0.43 and containing 15 per cent, i.e., Fv = 0.15 (by volume) of fly ash with a specific gravity of 2.2 will have a water-cementing materials ratio of w 3.15 × 0.43 = = 0 45 c + p 3.15 × (1 − 0.15) + 2.2 × 0.15 If the percentage of fly ash in a mix is given by weight then the volume equivalency is computed from the relation given by Eq. (10.17): Fv =

1 1 + ((G G p / Gc ) [(1 / Fw ) −1]

(10.17)

where Fv and Fw are the percentages of fly ash by volume and by weight, respectively. For example, if a concrete is composed of 15 per cent fly ash of specific gravity of 2.2 and 85 per cent cement by weight then the percentage of fly ash by volume or volume equivalency is Fv =

1 = 0.2017 or 20.17 per cent 1+ ( 2.2 / 3.15) × [1 / 0.15 − 1]

Quantity of cement by volume Cv = 100 − 20.17 = 79.83 per cent

Proportioning of Concrete Mixes

317

Weight equivalency Proportioning by the weight equivalency method simply

means that the weight of cementing materials (cement + pozzolan) is the same as the weight of cement in a cement-only mix. When proportioning by the weight equivalency method, if the amount of free-water content does not change w/c always equals w/(c + p). If the percentage of fly ash in a mix is given by volume it can be converted to percentage by weight using Eq. (10.18): Fw =

1+ (Gc

1 G p )[(1 / Fv ) − 1]

(10.18)

For example, if a concrete mix is composed of 20 per cent fly ash of specific gravity 2.2 and 80 per cent cement by volume then the percentage of fly ash by weight or weight equivalency is Fw =

1 = 0.1486 or 14.9 per cent 1+ (3.15 / 2.2)(1 / 0.2) − 1]

Quantity of cement by volume Cv = 100 − 14.9 = 85.1 per cent The strength of fly ash or silica fume concrete depends only on the relative proportions of fly ash or silica fume, cement and water. The mix proportioning methods assume that the quantity and grading of the coarse aggregate is the same as in the traditional mixes without fly ash or silica fume and that the difference in yield due to the larger volume of cementing material in the ash mix is balanced by a reduction of the sand content. As the strength requirements decrease, the use of fly ash in concrete becomes more economical. In high performance applications, the fly ash is used at 15 to 35 per cent of cement content. The specific gravity of fly ash usually varies from 2.14 to 2.42. The preferred fly ashes for the use in high performance concrete should have a loss on ignition not greater than three per cent, have a high fineness modulus, and come from a source with a requisite uniformity. The cementing efficiency factor, k, of an ash relative to cement is measured as the number of parts of cement that may be replaced by one part of the ash without changing the property being investigated, generally the compressive strength. k is dependent on the age of the specimen and its value has been found to be of the order 0.40 for mixtures without admixture. The water−cementing material ratio is related to water−cement ratio by the Eq. (10.19): w/(c + p) = (w/c)/[1 + (k − 1) a]

(10.19)

where ‘a’ is the fraction of cement replaced by the ash.

Silica fume It contains 85−95 per cent amorphous silicon dioxide SiO2 in the form of microscopic glassy spherical particles. The average particle size of silica fume is 0.1 to 0.2 micron, which is nearly 100 times smaller than that of a cement grain. Specific surface of silica fume is 15 to 30 m2/g. Large surface area and high content of amorphous silicon dioxide gives silica fume super pozzolanic properties. The specific gravity varies from 2.1 to 2.3 and bulk loose unit weight is 230−300 kg/m3.

318

Concrete Technology

The low bulk density makes it difficult to handle, store and transport. Therefore, it is processed and supplied in densified form. This agglomeration of silica fume formed by densification breaks down during mixing. Silica fume is blended with Portland cement (as percentage of total mass of cementing material in the range 6.7−9.3). Silica fume due to its physical and chemical properties is very efficient as compared to cement and fly ash in enhancing performance of the concrete. The cementing efficiency of silica fume is not constant at all percentages of replacement. Overall efficiency factor k is assessed in two different parts: the general efficiency factor which is constant for all percentages of replacement, and the percentage efficiency factor varying with replacement percentage. The general efficiency factor is usually taken to be 3.0. The value of the efficiency factor is directly related to the mineral additive used. In case of especially active silica fume with an average value of SiO2 of 94 per cent, the efficiency factor is approximately 4.0. For high-performance concrete, it is advisable to select a silica fume with as high as possible a content of SiO2 (not less than 90 per cent). In practice generally, a cementing efficiency factor up to 4.0 is considered. Silica fume is normally not added to mixes with water-cement ratio above 0.40, as this mineral additive is economically advantageous only for HPC.

Admixtures In the production of concrete a reduction in w/(c + p) ratio by decreasing the water content rather than increasing the total cementing materials content, will usually produce higher compressive strengths. For this reason, use of chemical admixtures should be considered when producing high performance concrete. The use of chemical admixtures may improve the rate of hardening, and control the slump loss, and result in accelerated strength gain, better durability, and improved workability. Superplasticizers The use of HRWRs help in dispersing cement particles, and they can reduce mixing water requirements by up to 30 per cent, thereby increasing the concrete compressive strengths. For a given w/(c + p) ratio, the field strength of concrete is greater with the use of HRWR than that without it, and this greater strength is reached within shorter period of time. Air-entraining admixtures are seldom used in high-performance concrete for building applications when there are no freezing−thawing concerns. If entrained air is required because of severe environmental exposure, it will significantly reduce the compressive strength of the concrete.

10.16.3

Water–Cementing Material Ratio, w/(c + p)

The single-most important variable in achieving high strength normal concrete is the water-cement ratio. Since most of high performance concrete mixtures contain other cementing materials, the w/(c + p) ratio must be considered in place of the traditional water-cement ratio. The w/(c + p) ratio, like the watercement ratio, should be calculated on a weight basis. The weight of water in HRWR should be included in the computation of w/(c + p) ratio. The relationship between water-cement ratio and compressive strength, which has been identified for normal

Proportioning of Concrete Mixes

319

strength concretes is reasonably valid for higher strength concretes as well. The use of chemical admixtures and other cementing materials is generally essential for producing placeable concrete with a low water-cementing material ratio. With the use of HRWR, a given target strength can be achieved at a given age using less cementing material than would be required when no HRWR is used. This ratio for high strength concretes typically ranges from 0.20–0.50. However, it should be noted that the validity of step-by-step procedure for proportioning of normal concrete mixes to HPC mixes is limited due to the following: 1. Traditional experimental relationships between water-cement ratio and compressive strength obtained by testing the stiff concrete mixes in the range of low water-cement ratio without chemical admixtures may not be valid for fluid or flowing concretes obtained by using superplasticizers. Moreover, none of these relationships include the effect of mineral additive on strength of concrete. 2. Crushed coarse aggregate of low nominal maximum size of 12.5 to 15.0 mm has to be used. River sand is preferred as a fine aggregate to avoid excessive water demand. 3. In HPC, water content depends on the effectiveness of the superplasticizer, type of cement chosen, and on the content of mineral additive. The relationship between water cementing material ratio and compressive strength of concrete with 15 per cent cement replaced by low calcium fly ash without any admixture can be expressed by Eq. (10.20): fcm (MPa) = A (w/c +p)B

(10.20)

The values of parameters A and B are given in Table 10.32. The relationships between water-cement ratio and mean compressive strength of concrete fcm (MPa) using silica fume can be expressed by Eq. (10.21): fcm = eD−R (w/c)

(10.21)

The above expression contains a constant part eD and a variable part e−R (w/c). The constant part depends upon the type of aggregate. For a coarse aggregate of average properties, D = 4.95 and the relation is expressed by Eq. (10.22): fcm = 140e−R (w/c)

(10.22)

The parameter R, given in Table 10.33, depends on the strength of the cement and on the proportion of silica fume in the mix. If the concept of efficiency factor (expressing the equivalent amount of silica fume which produces the same strength as a fixed content of cement) is introduced, Eq. (10.22) can be written as Eq. (10.23): fcm = 140e−R (w/c)/[1 + (k−1) a]

(10.23)

where ‘a’ represents the fraction of silica fume by weight of cement and the parameter R represents the cementing efficiency factor. The maximum efficiency factor for the silica fume normally recommended is 4.00. The expressions are valid for the concrete mixes containing silica fume and having water-cement ratios less than 0.40.

320

Concrete Technology

Table 10.32 Grade of coarse aggregate

Grade A

Parameters A and B of the relationship between w/(c + p) ratio and f cm Fly ash content, per cent

28 days

56 days

Grade description of coarse aggregate

91 days

−

13.940 1.565

17.950 1.372

21.560 1.204

67 per cent Type−1

15

10.290 1.738

15.310 1.412

22.110 1.088

33 per cent Type−2

−

16.110 1.431

23.020 1.101

23.820 1.125

50 per cent Type−1

15

10.030 1.811

16.450 1.361

21.040 1.164

50 per cent Type−2

−

15.900 1.440

23.030 1.088

23.210 1.144

50 per cent Type−2

15

11.310 1.658

12.890 1.644

20.370 1.204

50 per cent Type−3

Grade B

Grade C

Values of parameters: (A/B) Age of concrete

Notes Aggregate

10.16.4

Type–1: passing 20 mm sieve and retained on 10 mm sieve. Type–2: passing 10 mm sieve and retained on 4.75 mm sieve. Type–3: passing 4.75 mm sieve and retained on 2.36 mm sieve.

Mix Proportioning Procedure

The procedure consists of a series of steps, which when completed provide a mixture meeting strength and workability requirements based on the combined properties of the individually selected and proportioned ingredients. Following are the necessary steps: 1. To obtain the desired workability an initial starting slump of 25−50 mm prior to addition of HRWR is recommended. This will ensure an adequate amount of water for mixing and allow the superplasticizer to be effective. For high strength concretes made without HRWR, a recommended slump range of 50−100 mm may be chosen depending on the type of work to be done. 2. Depending on the level of quality control contemplated on the site, the target mean strength necessary to reach characteristic strength is determined. Based on this target strength the maximum size of the coarse aggregate is selected. 3. The recommended content of the coarse aggregate is selected depending on its strength, potential characteristics and maximum size. The recommended coarse aggregate contents, expressed as a fraction of the dry-rodded unit weight are given in Table 10.34 as a function of nominal maximum size. Using the recommended coarse aggregate fraction value obtained from Table 10.34, the oven-dry weight of the coarse aggregate per cubic meter of concrete is computed. High-strength concrete mixtures, however, have a high content of cementing material, and thus are not so dependent on the fine aggregate to supply fines

Proportioning of Concrete Mixes Table 10.33

Parameter R of relationship between water-cementing material ratio and strength of concrete

Silica fume, per cent

Parameter R Cement Grade 43

Cement Grade 53

0

2.10

1.97

5

1.70

1.60

10

1.41

1.31

15

1.24

1.15

Table 10.34

321

Recommended volume of coarse aggregate* per unit volume of concrete (ACI: 211–1993)

Nominal maximum size, mm

10

12.5

20

25

0.65

0.68

0.72

0.75

25−50 50−75 75−100

184 190 196

175 184 190

169 175 181

166 172 178

Entrapped air content#

3.0 (2.5)

2.5 (2.0)

2.0 (1.5)

1.5 (1.0)

Fractional volume of rodded coarse aggregate (oven dry basis) Mixing water, kg/m3 for slump, mm

Notes *To be used with the sand having fineness modulus between 2.5 and 3.2 # Given values must be adjusted for sands with voids ratio other than 35 per cent.

for lubrication and compactibility of the fresh concrete. Therefore, the values given in Table 10.33 are recommended for use with the sands having fineness modulus values from 2.5 to 3.2. 4. The quantity of mixing water per unit volume of concrete required to produce a given slump is dependent on the maximum size, particle shape and grading of the aggregate, the quantity of cement and type of water-reducing admixture used. Table 10.34 gives estimates of required mixing water for high strength concretes made with 10 to 25 mm maximum size aggregates prior to the addition of any chemical admixture. Also given are the corresponding values for entrapped-air contents. These quantities of mixing water are maximums for reasonably well-shaped, clean, angular and well-graded coarse aggregates. The values for the required mixing water are applicable when a fine aggregate is used that has a void content of 35 per cent. The void content of fine aggregate may be calculated from Eq. (10.24). ⎡ Oven-dry rodedunit weight ⎤ V = ⎢1 0 − × 100 Drybulk specific gravity × 1000 ⎥⎦ ⎣

(10.24)

322

Concrete Technology

When a fine aggregate with a void content V not equal to 35 per cent is used, an adjustment must be made to the recommended mixing water content. This adjustment may be made using Eq. (10.25): adjustment in mixing water, kg/m3 = (V − 35) × 4.55

(10.25)

5. In high performance concrete mixtures using other cementing material such as fly ash, and silica fume, the w/(c + p) ratio is calculated by dividing the net weight of mixing water by combined weight of the cement and fly ash/silica fume. The w/(c + p) values for concretes made without HRWR, and those for concretes made using an HRWR are given in Table 10.35. Necessary w/(c + p) ratio to reach target strength may also be determined by using appropriate relations between compressive strength and water-cementing material ratios. The w/(c + p) may be limited further by durability requirements. Table 10.35

Recommended maximum water-cementing material ratio (ACI: 211–1993) Water–cementing ratio, w/(c + p)

Field strength

Maximum size of coarse aggregate, mm Without HRWR

With HRWR

Strength, MPa

Age, days

10

12.5

20

25

10

12.5

20

25

48.0

28

0.42

0.41

0.40

0.39

0.50

0.48

0.45

0.43

56

0.46

0.45

0.44

0.43

0.55

0.52

0.48

0.46

28

0.35

0.34

0.33

0.33

0.44

0.42

0.40

0.38

56

0.38

0.37

0.36

0.35

0.48

0.45

0.42

0.40

28

0.30

0.29

0.29

0.28

0.38

0.36

0.35

0.34

56

0.33

0.32

0.31

0.30

0.42

0.39

0.37

0.36

28

0.26

0.26

0.25

0.25

0.33

0.32

0.31

0.30

56

0.29

0.28

0.27

0.26

0.37

0.35

0.33

0.32

28

−

−

−

−

0.30

0.29

0.27

0.27

0.33

0.31

0.29

0.29

0.27

0.26

0.25

0.25

0.30

0.28

0.27

0.26

55.0 62.0 69.0 76.0

56 83.0

28 56

−

−

−

−

6. The weight of cementing material required per cubic meter of concrete can be determined by dividing the amount of mixing water per cubic meter of concrete calculated in Step 4 by the w/(c + p) ratio obtained in Step 5. The proportion of silica fume can be obtained from the general relationship of water-cementing material ratio and strength. 7. The optimum mixture proportion is determined by preparing, a number of trial mixtures having different fly ash/silica fume contents. Generally, one basic trial mixture should be made with Portland cement as the only cementing material. The following steps should be taken to obtain the basic mixture proportions:

Proportioning of Concrete Mixes

323

(a) In the basic mixture, since no other cementing material is to be used the weight of cement equals the weight of cementing material calculated.

8.

9.

10.

11.

(b) After determining the weights of coarse aggregate, cement and water, and the percentage of air content, the sand content can be calculated by using the absolute volume method to produce one cubic meter of concrete. The use of fly ash or silica fume in producing high strength concrete can result in lowered water demand, reduced concrete temperature, and reduced cost. However, due to variations in the chemical properties of fly ash or silica fume, the strength-gain characteristics of the concrete might be affected. Therefore, it is recommended that at least two different fly ash or silica fume contents be used for the companion trial mixtures. Trial mixes to determine the workability and strength characteristics of the proportioned mix should be prepared. The weights of coarse aggregate and sand, and water must be adjusted to correct the moisture conditions of the aggregates used. The trial mix proportions should be adjusted to obtain the desired properties of fresh concrete, namely workability in terms of initial slump. The initial slump is obtained by adjusting mixing water content. The weight of cementing materials in the mixture should be adjusted to maintain desired w/(c + p) ratio and the sand content should be adjusted to maintain the yield of the concrete. Different dosages of HRWR should be tried to determine the optimum dosage for strengths and workability of the mixture. In case concrete trial mixture adjusted for the slump is too harsh for the job placement or for finishing, the coarse aggregate content may be reduced and sand content be adjusted accordingly to ensure proper yield. However, this may increase the water demand of the mixture, thereby an increase in the cementing material will be required to maintain given w/(c + p) ratio. If the measured air content is significantly different from the designed value, the admixture dosage should be reduced or sand content should be adjusted to maintain proper yield. In case required compressive strength is not attained, additional trial mixes having lower w/(c + p) ratio should be tested. Once the trial mix proportions have been adjusted to produce desired workability, strength specimens shall be cast from the trial batches, cured and tested as per codal provisions. It is recommended that the cement content should be less than 500 kg/m3. If it is necessary to exceed this limit to get a good workability, it is desirable to reconsider the compatibility of cement−mineral additive to reduce water and cement content. The mix proportioning procedure is illustrated by Example 10.10.

Example 10.10 Proportion a HSC mix of grade M60 with flowable consistency

using fly ash or silica fume, a high range water reducer and retarding admixture for a large-scale construction with closely spaced reinforcement and prestressing cables. Special attention should be paid to split tensile strength of

324

Concrete Technology

concrete. The desirable characteristic split tensile strength is 4.00 MPa. The concrete is to be pumped to a height of 52 meters.

Solution (a) Selection of Materials Cement Considering compressive strength at various ages, fineness, heat of hydration, alkali content, and compatibility with admixtures, Grade 43 Portland cement is selected. Coarse aggregate As the tensile strength of concrete is of prime importance, 20 mm or 12.5 mm maximum size crushed granite stones aggregates are used in the concrete mix. Fine aggregate A properly graded river sand which is washed at the site to remove deleterious materials and chloride contamination is used as a fine aggregate. Admixture A high range water-reducing admixture is used for this concrete of low water−cementing material ratio. The optimum dose of admixture is evaluated by consistency test. Retarder is also added to increase the setting time of the concrete and improve the slump retention properties to avoid cold joints during large construction. Water The potable water available at site is used as mixing water. The thermal stresses can be reduced by restricting the placement temperature of concrete below 23°C. (b) Design Stipulations and Materials Characteristics (i) Characteristic strength (ii) Cement Type of cement Specific gravity (iii) Coarse aggregate Maximum size of aggregate Type of aggregate Bulk specific gravity Absorption (oven dry basis) Dry-rodded unit weight (iv) Sand Type of sand Fineness modulus Bulk specific gravity Water absorption (oven dry basis) Dry-rodded unit weight (v) Fly ash Type Specific gravity Bulk loose unit weight (vi) Silica fume Specific gravity Bulk loose unit weight

60 MPa OPC of Grade 43 3.15 12.5 mm crushed granite stone 2.76 0.75 per cent 1610 kg/m3 Natural river sand 2.87 2.60 1.00 per cent 1650 kg/m3 Low calcium (Grade-1) IS: 3812 2.32 1120 kg/m3 2.2 270 kg/m3

Proportioning of Concrete Mixes

325

(vii) Workability As HRWR and set retarding admixtures will be used, the mix proportions may be based on 25 to 50 mm initial slump prior to addition of HRWR. (viii) Superplasticizer Sulfonated naphthalene-formaldehyde condensate in liquid form with 65 per cent free (non-combined) water is to be used. (c) Mix Design For computation of target mean compressive strength for a construction with excellent quality control the standard deviation for compressive strength is assumed as 5.5 MPa and that for split tensile strength as 0.3 MPa (corresponding to approximately seven per cent coefficient of variation). Target mean compressive strength, ft = fck + kS = 60 + 1.65 × 5.5 ≈ 69.00 MPa Target split tensile strength, fst = 4.00 + 1.65 × 0.3 ≈ 4.50 MPa These values determine the water−cementing material ratio. Since the mix proportioning is through laboratory trial batches, the field trial strength = 69.00/0.9 ≈ 77.00 MPa. Thus the required mean strength of laboratory trial mix specimens is 77.00 MPa. An optimum trial mix is the one which has a 28-day mean compressive strength just exceeding 77.00 MPa. Optimum coarse aggregate content as obtained from Table 10.34 is 0.68 per cubic meter of concrete Dry weight of coarse aggregate Wdry = 0.68 × 1610 ≈ 1095.0 kg/m3 (i) Mixing water Based on an initial slump of 25 to 50 mm and 12.5 mm maximum nominal size of coarse aggregate, water content from Table 10.33 is 175.00 kg/m3. The void content, Vs of sand is given by ⎡ Oven-dry unit weight ⎤ Vs = ⎢1 0 − × 100 Bulk specific gravity 1000 ⎥⎦ ⎣ 1650.00 ⎤ ⎡ = ⎢1 0 − × 100 = 36.5 percent 2 60 × 1000 ⎥⎦ ⎣ Mixing water adjustment, (36.5 − 35) × 4.55 ≈ 6.82 kg/m3 Total mixing water (including that in HRWR) = 175.00 + 6.82 ≈ 182.00 kg/m3 (ii) w/(c + p) ratio Relationship between compressive strength and water−cementing material ratio can be established through various trials using same ingredients. A typical relation is shown in Fig. 10.16. For the strength requirements of 69.00 MPa and 77 mpa, w/(c + p) ratios required are 0.303 and 0.32, respectively. However, for the present case w/(c + p) ratio of 0.31 has been selected. Alternatively, the water−cementing material ratio with HRWR and 12.5 mm nominal size of aggregate for target mean strength of 69.00 MPa can also be obtained from Tables 10.19 and 10.22 for the equivalent target cylinder strength.

326

Concrete Technology

28-day compressive strength, MPa

90

81 80 75 70 0

70

66

6 61

60 0.28

0.32

0.36

Water–cementing materials ratio, w/( /c+ )

Fig. 10.16

Compressive strength versus water-cementing ratio

(iii) Content of cementing material

Weight of cementing material = 182.00/0.31 ≈ 587.00 kg/m3 (iv) Proportions of basic mix with cement only Air content for concrete mix is assumed to be two per cent. Absolute volume method is used to determine the quantities of different ingredients. • Cement content = 587.00 kg/m3 • Fine aggregate content • Volume of sand/m3 of concrete 587.00 1095.00 182.00 2.00 ⎤ ⎡ = ⎢1 0 − − − − 3 15 × 1000 2 76 × 1000 1000 100 ⎥⎦ ⎣ =[ − − − − ] ≈ 0.2149 m 3 Weight of sand •

= 0.2149 × 1000 × 2.6 ≈ 559.00 kg

Fly ash content Recommended limits for replacement are 15 to 35 per cent. Four companion laboratory trial mixes can be proportioned with cement replacements of 15, 20, 25 and 30 per cent. For illustration consider 20 per cent replacement. Fly ash content = 0.20 × 587.00 = 117.00 kg/m3 Cement content = 587.00 − 117.00 = 470.00 kg/m3 Volume of required sand

Proportioning of Concrete Mixes

327

⎡ ⎤ 117.00 ⎤ ⎡ 470.00 = ⎢1 0 − ⎢ + − 0.3967 − 0.1820 − 0.0200 ⎥ ⎥ ⎣ 3 15 × 1000 2 32 × 1000 ⎦ ⎣ ⎦ ≈ 0.2017 m 3

•

Weight of dry sand required = 0.2017 × 2.60 × 1000 ≈ 524.00 kg/m3 Based on trials, HRWR is added at the rate of 1.75 per cent of cementing materials, HRWR content = 0.0175 × (470 + 117) ≈ 10 kg (say 10 litre by volume) Weight of water excluding that in HRWR = 182.00 − 10.00 ≈ 172.00 kg. The mix proportions (oven dry basis) per cubic meter of concrete are: Ingredients Cement Fly ash Sand (dry) Coarse aggregate (dry) Water (including that in the retarder) HRWR

Weight per m3 of concrete, kg 470.00 117.00 524.00 1095.00 172.00 10 litre

Take into account the total moisture present in the coarse and fine aggregates. For example consider that the sand and coarse aggregates have 6.0 and 0.5 per cent of total moisture based on oven dry conditions, respectively. Weight of wet sand = 524.00 × (1 + 0.06) ≈ 555.00 kg Weight of wet coarse aggregate = 1095.00 × (1 + 0.005) ≈ 1100.00 kg Net weight of water =172.00 − 524.00 × (0.06 − 0.01) − 1095.00 × (0.005 – 0.0075) ≈ 148.00 kg. Batch weights cement : FA : sand : CA : water 470.00 : 117.00 : 555.00 : 1100.00 : 148.00 Adjust batch weights for each trial mix to obtain required slump, before and after addition of HRWR. (d) Mix Proportions with silica fume It is generally proportioned on the basis of experience and test results from the similar projects in the past using similar ingredients. In addition, extensive testing of trial batches will be required to arrive at optimum mix proportions. However, trial batch proportions may be estimated from the concrete mix proportions using fly ash. IS: 456−2000 recommends silica fume as replacement of cement in the proportion of 5 to 10 per cent of the cementing materials. In the current example, consider a cementing efficiency factor of silica fume as 3.25. The silica fume content = 117.00/3.25 ≈ 36.00 kg From the yield point of view, Sand content ⎡ ⎤ 36.00 ⎤ ⎡ 470.00 = ⎢1 0 − ⎢ + − 0.3967 − 0.1820 − 0.0200 02 ⎥ ⎥ ⎣ 3 15 × 1000 2.2 × 1000 ⎦ ⎣ ⎦ ≈ 0.2357 m 3

328

Concrete Technology

Weight of sand = 0.2357 × 2.6 × 1000 ≈ 613 kg HRWR at the rate of 1.75 per cent = 0.0175 × (470 + 36) ≈ 8.90 kg (8.9 litre by volume) Net weight of water excluding that in HRWR = 182.00 − 8.90 ≈ 173 kg Percentage of silica fume = 36 × 100/(470 + 36) ≈ 7.11, which is within the optimum range of 6.7 to 9.3 per cent. The mix proportions using silica fume are:

Ingredients Weight per m3 of concrete, kg Cement 470 Silica fume 36 Sand (dry) 613 Coarse aggregate 1095 Water (including that in the retarder) 173 HRWR 8.9 litre Apply corrections for the total moisture present in the coarse aggregate and sand to obtain field batch weights. (i) Trial mixes Correction for total moisture present in coarse aggregate and sand are applied and adjusted weights of wet sand, wet coarse aggregate and water are computed. In proportioning the companion mixes containing different percentages of fly ash or silica fume replacing the cement, though the total content of cementing materials remains constant but the volumes of cement, fly ash or silica fume, and hence of total cementing materials change. However, volumes of coarse aggregate, water and air per cubic meter of concrete are same as in the basic mixture, but the required weight of sand changes. With the increase in the percentage replacement of cement by fly ash or silica fume, the total volume of cementing materials increases and consequently the required weight of dry sand decreases. The dosage of chemical admixtures may or may not change. (ii) Adjustment of trial mixture proportions The batch weight for each trial mixture is adjusted to obtain desired workability (slump), before and after the addition of HRWR. The following cases may arise. • More water is needed to produce desired slump: � � � � � �

Apply correction to dry weights. Calculate actual yield of mix. Adjust mix proportions to obtain yield of one cubic meter of concrete. Calculate new w/(c + p) ratio. Increase weight of cement to maintain constant w/(c + p) ratio. Remove equal volume of sand to maintain yield.

• Change in dosage of HRWR The dosage of chemical admixture is adjusted as per experience/requirements of workability. Additional amount if used would require adjustment in cementing materials content and thus affects the yield of the mix. The variation in yield is corrected by corresponding change in sand content.

Proportioning of Concrete Mixes

329

(iii) Mock-up studies After proportioning the concrete mix and establishing various parameters of fresh and hardened concrete in the laboratory by trial studies, a few full-scale field mock-ups are recommended for big projects to ensure that concrete could be properly placed and compacted under field conditions. Wet-sieve analysis of concrete samples before and after pumping can ensure identical proportions.

10.17

DESIGN OF HIGH WORKABILITY CONCRETE MIXES

The workability of fresh concrete should be suitable for each specific application to ensure that the operations of handling, placing and compaction can be undertaken efficiently. In case of pumped concrete, in addition to its suitability for the particular application the concrete mix must be highly workable, i.e., it must have enough flowability for moving easily through pipes as shown in Fig.10.17.

Fig. 10.17

Pumpable concrete (sheen is seen on the surface)

The workability of concrete mixes can be improved considerably by suitable mix proportioning, the use of cement replacement materials such as pulverized fuel ash or ground granulated blast-furnace slag. Furthermore, the use of admixtures such as water reducers and superplasticizers have beneficial effects on workability without compromizing other concrete properties. None of the national codes has explicitly covered the design procedure for proportioning of pumped concrete and self-compacting concrete (SCC). However, it is possible to specify these concretes within the BS 8500 system using the proprietary concrete category that satisfies a defined performance under standard test conditions. The producer shall assure the performance, subject to good practice in placing, compacting and curing. For proprietary concrete, the producer is not required to declare the composition. Following guidelines may be noted: 1. The mix should satisfy the strength and durability requirements of the specific application, i.e., water-cement ratio.

330

Concrete Technology

2. To maintain cohesiveness of mix during flow though the pipes, i.e., for flow without segregation and bleeding of the type shown in Fig 10.18, and for lubrication film around the concrete plug, a fines content of 350 kg to 500 kg is desirable.

Fig. 10.18

Mix design for pumpable concrete

3. The grading of combined aggregate is a critical factor in mixes for pumpable concrete of high workability. The typical grading curve Nos. 3 and 4 for the combined aggregate of maximum nominal sizes of 40 mm, 20 mm and 10 mm shown in Figs. 3.8 to 3.10, respectively, can be used. The grading should not be coarser than that represented by curve No.3. It is generally recommended that 10 to 20 per cent of fine aggregate should pass through a 250 μm sieve. Minimum fines content (total of cement and fine aggregate passing through 250 μm sieve) of approximately 350 to 500 kg/m3 is required for combined aggregate with maximum nominal sizes of 40 mm to 10 mm. In case of angular, flaky aggregate this quantity is increased by approximately 10 per cent. Limits of fines content for various water contents are listed in Table 10.36. Following example illustrates the procedure for design of pumpable concrete using BS 8500 specifications.

Example 10.11 Determine the mix proportions for pumpable concrete with a

slump of 75 to 100 mm, i.e., slump class S2 for application in structures for 50 years service life under exposure classes XC1, XC2, XC3 and XC4 using CEM-I class normal Portland cement with a specific gravity of 3.1. The materials available are crushed fine and coarse aggregates of specific gravity of 2.65 and 2.55, respectively. The sieve analysis results for the coarse and fine aggregates are given in Table 10.37. The standard deviation as obtained from past records is 5.0 MPa and the probability factor is 1.65.

Proportioning of Concrete Mixes Table 10.36

331

Limits of fines content passing 250 μm sieve

Free water contents (l / m3)

Fines content (kg/m3) Minimum

Maximum

150

260

365

160

280

390

170

295

415

180

315

440

190

330

465

200

350

490

210

365

515

220

385

540

230

400

565

240

420

590

The values are for cement of specific gravity of 3.1.

Solution The concrete for application in structures for 50 years service life under exposure classes XC1, XC2, XC3 and XC4 requires strength of class of C25/30 at 28 days. (a) Target mean compressive strength For the stipulated strength class, the target mean compressive strength, ft = fck + k S = 30 + 1.65 × 5.0 = 38.25 MPa (b) Free water-cement ratio For the reference free water−cement ratio of 0.5, 28day compressive for the normal Portland cement and crushed aggregate obtained from Table 10.15 is 49 MPa. With this pair of data (49 MPa and water−cement ratio = 0.50), the appropriate strength versus water−cement ratio curve in Fig.10.8 gives a free water−cement ratio of 0.60 for the targeted strength of 38.25 MPa. For the given exposure classes the maximum permitted value of free water−cement ratio is 0.60. Therefore, a water−cement ratio of 0.60 can be adopted. (c) The water and cement contents For the crushed aggregate of class 10/20, the water content for consistence class S2 as obtained from Table 10.16 is 190 kg/m3. For the free water−cement ratio of 0.60, Cement content = 190 / 0.60 = 317 kg/m3 This cement content is satisfactory as it is more than the minimum cement content of 280 kg/m3 recommended in Table 10.6 and less than the maximum prescribed value of 450 kg/m3. The entrapped air is 2 per cent. (d) Combined aggregates grading The sieve analysis results for the coarse and fine aggregates are given in Table 10.37. For pumpable concrete of high workability, the finest grading curve No. 4 for the mixed aggregate of maximum nominal size of 20 mm shown in Fig. 3.9 has been selected.

332

Concrete Technology Table 10.37 Percentage passing the IS sieve

Type of aggregate

20 mm

Coarse aggregate grading (c)

100

40

6

1

−

Fine aggregate grading (f )

100

100

97

87

75

Specified/selected grading (s)

100

75

50

42

35

Combined aggregate (f + kc)/(1 + k)

100

69

50

43

36

10 mm

4.75 *mm

2.36 mm

1.18 mm

600 mm

300 mm

150 mm

−

−

−

51

20

5

27

12

2

25

10

2

*Reference sieve size

The available fine and coarse aggregates are to be suitably combined to obtain the desired or selected grading. If fine and coarse aggregates are combined in proportion 1: k, then using IS:4.75 mm sieve size as criteria, the value of k as obtained from Eq. (10.2) is k=

f s 97 − 50 = = 1.068 s c 50 − 6

Therefore, fine and coarse aggregates are to be combined in a mass proportion of 1:1.068. (e) Proportions of fine and coarse aggregates Total absolute volume of aggregates from Eq. (10.14):

Va = 1

317 190 ⎞ ⎛ 3 0.02 + + ⎟ = 0.6877 m ⎝ 3 10 × 1000 1000 ⎠

Therefore, C fa C fa C fa Cca + = + = 0.6877 . 2.65 × 1000 2.55 × 1000 2 65 × 1000 2 55 × 1000

Thus, Cfa = 864 kg/m3 and Cca = 1.068 × Cfa = 1.068 × 864 = 923 kg/m3 (f) Suitability of mixture for pumping The percentages of fine and combined aggregates passing 300 μm sieve are 20 and 10 per cent, respectively, hence can be considered satisfactory. The quantity of fine aggregate passing 250 μm sieve may be obtained by interpolation as ⎛ 20 − 5 ⎞ = 20 − ⎜ × (300 − 250) = 15 per cent ⎝ 300 − 150 ⎟⎠ 15 × 864 = 130 kg/m3 100 Therefore, total fines in the mix = 317 + 130 = 447 kg/m3 Fines in aggregate =

Proportioning of Concrete Mixes

333

For water content of 190 kg/m3 the fines should be with in the range 330 to 465 kg/m3, hence is satisfactory. (g) Concrete mix proportions The concrete proportions by mass can be expressed as Cement

Water

Fine aggregate

Coarse aggregate

317

:

190

:

864

:

923

1.0

:

0.60

:

2.73

:

2.91

(kg/m3)

As usual the final proportions are established by trial batches and site adjustments.

10.18

TRIAL MIXES

The mix proportions arrived at shall be checked by means of trial batches. The quantity of material for each trial batch shall be sufficient for at least three 150 mm concrete cube specimens and concrete required to carry out the workability test. The mix proportions computed by a mix design method shall constitute trial batch No. 1. The workability of this trial batch in terms of slump or compaction factor shall be measured and the mix shall be carefully observed for any tendency for segregation and bleeding, and for its finishing properties. If the measured workability of trial batch no. 1 is different from the stipulated value, the water content shall be adjusted according to Table 10.27 (using appropriate criterion of the method employed) for the required change in workability. For this adjusted water content, the mix proportions shall be recalculated keeping the free water-cement ratio at the preselected value, this will comprise trial mix no. 2. In addition, two more trial mixes no. 3 and 4 shall be made with the water content kept at the level of trial mix no. 2, but varying the free water-cement ratio by ±10 per cent of the pre-selected value. The mix proportions for the trial mixe no. 3 and 4 shall be recalculated for the changed free water-cement ratio by making suitable adjustments in accordance with Table 10.27. The trial batche nos. 2 and 4 will normally provide sufficient information to arrive at the field mix proportions.

10.19

CONVERSION OF MIX PROPORTIONS FROM MASS TO VOLUME BASIS

For volume batch mixing it is desirable to express concrete mix proportions by volume. The mix proportions by mass can be converted into volume proportions by dividing the mass proportions by the corresponding bulk densities. Let the contents of cement, fine aggregate and coarse aggregate per cubic meter of concrete be C, Fa and Ca, respectively, and g c’ g fa and g ca represent the bulk densities of the corresponding materials. Then mix proportion by mass are C : Fa : Ca

(kg)

and mix proportions by volume are given by Eq. (10.26):

334

Concrete Technology

C Fa Ca : : γ c γ f a γ ca

(m3)

(10.26)

The proportions obtained above are based on volume of dry aggregates. If fine aggregate contains moisture, suitable modifications for bulking shall be made.

10.20

QUANTITIES OF MATERIALS TO MAKE SPECIFIED VOLUME OF CONCRETE

When the mix proportions have been determined, the quantities of materials required to produce a specified quantity of concrete can be calculated by absolute volume method. The method is based on the principle that the volume of fully compacted concrete is equal to the absolute volume of all the ingredients. If W, C, Fa and Ca are the masses of water, cement, fine aggregate and coarse aggregate, respectively, used in making the concrete; Sc, Sfa and Sca are the specific gravities of cement, fine aggregate and coarse aggregate, respectively; and v is the percentage of entrained air in the concrete. Then the absolute volume of fully compacted fresh concrete (ignoring air content) is given by Vc =

W C Fa Ca + + 1000 1000 Sc 1000 S fa 1000 Sca

(10.27)

The method is illustrated in Example 10.12 .

Example 10.12 Calculate the quantities of ingredients required to produce

one cubic meter of structural concrete. The mix is to be used in proportions of one part of cement to 1.37 parts of sand to 2.77 parts of 20 mm nominal size crushed coarse aggregate by dry-volumes with a water−cement ratio of 0.49 (by mass). Assume the bulk densities of cement, sand and coarse aggregate to be 1500, 1700 and 1600 kg/m3, respectively. The percentage of entrained air is 2.

Solution The mix proportions of 1:1.37:2.77 by dry volume to be used in the production of structural concrete can be expressed in terms of masses as follows. Water — 0.49

Cement

Sand

Coarse aggregate

: 1 × 1500

:

1.37 × 1700

:

2.77 × 1600 (kg)

:

:

1.55

:

2.95

1

The absolute volume of concrete produced by one bag of cement of 50 kg is Vc = Vc =

0 49 × 50 1 × 50 1 55 × 50 2 95 × 50 3 + + + = 0.127 m 1000 1000 × 3 15 1000 × 2 6 1000 × 2..6

With an entrained air of two per cent, the absolute volume of ingredients in one cubic meter of fully compacted fresh concrete is 1.0 − 0.02 = 0.98 m3. Therefore, Cement content per m3 of concrete is, C = (0.98)/(0.127) = 7.72 bags

Proportioning of Concrete Mixes

or C = 386 kg Therefore, ingredient requirements are Cement Sand Coarse aggregate Water

10.21

335

386 kg/m3 598 kg/m3 1139 kg/m3 189 kg/m3

ACCEPTANCE CRITERIA FOR CONCRETE

In order to ensure proper quality control, IS: 456−2000 requires that a minimum number of random samples from the fresh concrete of each grade should be taken as specified in IS: 1199−1959 and cubes should be made, cured and tested as described in IS: 516−1959. The minimum number of samples of concrete shall be in accordance with Table 10.38. The average of the strengths of three specimens is the test strength of any sample. The acceptance criteria given in IS: 456−2000 stipulates that the strength requirement is satisfied if Table 10.38

Frequency of sampling of concrete

Quantity of concrete in the job, m3

Number of samples*

1−5 6−15 16−30 31−50 51 and above

1 2 3 4 4 plus one additional sample for each additional 50 m3 or part thereof

Notes *At least one sample shall be taken from each shift. Where concrete is produced at continuous production unit, such as ready-mixed concrete plant, frequency of sampling may be agreed upon mutually.

1. Every sample has a test strength not less than (fck − 3) MPa for M15 concrete and ( fck − 4) MPa for M20 or higher grade concretes. 2. For M15 grade concrete, the mean strength of the group of 4 nonoverlapping consecutive test samples is not less than fck + 0.825 S or ( fck + 3) MPa whichever is greater. For M20 or higher grade concrete the mean strength of the group of four nonoverlapping consecutive test results is greater than fck + 0.825 S or ( fck + 4) MPa whichever is greater, where fck is characteristic strength, and S is established standard deviation (rounded to 0.5 MPa). There are many factors that influence the variability of strength measurements in the field. It is generally noticed that test strength under ideal field conditions attains only 90 per cent of the strength measured by tests performed under laboratory conditions. To take into account the variation of individual test sample, the laboratory design strength can be obtained by increasing the average field strength or the target mean strength by 10 per cent or by dividing it by the factor, 0.90.

336

Concrete Technology

10.22

FIELD ADJUSTMENTS

In a concrete mix if W, C, Fa and Ca are the required quantities of water, cement, fine aggregate, and coarse aggregate, respectively, to produce one cubic meter of fully compacted concrete, then based on concept that volume of compacted concrete is equal to the sum of the absolute volumes of all ingredients, the relation given by Eq. (10.27) is obtained Fa Cc W C + + + = 1 00 1000 1000 Sc 1000 S fa 1000 Sca

When entrained air is also present and its content is v per cent of the volume of concrete, the right-hand side of Eq. (10.28) would read as, (1.00 − 0.01v). If specific gravities of fine and coarse aggregates are assumed to be same say Sa, then for a given type of cement the Eq. (10.28) can be written as W 1 ⎛ Fa Ca ⎞ 100 1 + +⎜ − ⎟= C Sa ⎝ C ⎠ C Sc i.e., (Water-cement ratio) +

(Aggregate-cement ratio) 1000 1 = − Sa C Sc

(10.28)

This relation can be used to convert the aggregate–cement ratio into cement content for the given water-cement ratio or vice versa. Figure 10.19 renders such conversion quite simple. Aggregate–Cement Ratio (by mass)

0

Sp

2

ec ific

4

Gr av

ity

of

Ag

gre

ga

6 8

te 2. 2 2 2 .4 2. .6 3. 8 0

10 12

100 200 300

500

0

tR en

m

1.

400

io at

0.6 0 .8 0. 0.24

Cement Content, kg/m

3

0

Ce

r–

e at W

600

Fig. 10.19

Relationship between aggregate-cement ratio and cement content

337

Proportioning of Concrete Mixes

If the aggregate contains free surface moisture whose content is, say, w per cent of the mass of saturated surface dry aggregate then the masses of added water W and of (wet) aggregate must be adjusted. The mass of free water is Ca (w/100). This mass is added to Ca to obtain the mass of wet aggregate required, Ca [l + (w/100)], and is subtracted from W to obtain the mass of water to be added, w – Ca (w/100). Central Road Research Institute (India) has developed curves shown in Fig. 10.20 to adjust the water-cement ratio and aggregate-cement ratio at site to take care of the change in compressive strength due to variation in the quality of cement obtained from different sources. If the source of supply of cement changes during the construction, concrete strength using fresh cement is determined keeping the mix proportions and water-cement ratio same as before. If there is substantial difference, say for example, the new cube strength is 80 per cent of the design strength, then the water–cement ratio should be reduced by 0.09 and aggregate-cement ratio by 1.1 as shown in Fig. 10.20. Increase +1.0

0

0

Adjustment curves for a/c ratio for different water contents in kg/m3

0.6

1

5

2 3 4

0.7 0.8 0.9 1.1 1.2 1.3 1.4 1.0 Ratio of actual field strength/design field strength (at 7 or 28 days)

1.5

1–156 2–168 3–180 4–192 5–204

–0.20

10.23

–2.0

Average adjustment curve for w/c ratio

–0.10

Fig. 10.20

–1.0

Assumptions: (A) sp. gr. of aggregate = 2.65 (B) sp. gr. of cement = 3.15

Decrease

Adjustment in water–cement ratio

Increase

–0.10

Decrease

Field adjustment for variation in cement quality (in terms of compressive strength)

GENERALIZED FORMAT FOR CONCRETE MIX DESIGN

The mix design methods discussed in the preceding sections basically follow the same principles and only minor variations exist in the process of selecting the mix proportions. A generalized proforma applicable to all the methods is suggested in Table 10.39. For the design of a concrete mix using a particular method, only relevant items need to be filled up.

338

Concrete Technology Table 10.39

Proforma for concrete mix design

PART-I: DATA Item A. Design Stipulations A.1 Characteristic compressive strength of concrete ( fck ): A.2 Cement type A.3 Aggregate type (a) coarse (b) fine A.4 Degree of workability A.5 Degree of quality control A.6 Type of exposure B. Characteristics of Materials Cement B.1 Specific gravity of cement B.2 Bulk density of cement Aggregates B.3 Specific gravity Coarse aggregate Fine aggregate B.4 Bulk density Coarse aggregate Fine aggregate B.5 Water absorption Coarse aggregate Fine aggregate B.6 Free surface moisture Coarse aggregate Fine aggregate B.7 Grading of aggregate

Reference (table/figure #)

Value

_____MPa at _____days OPC/RHPC/__________ _____________ _____________ _____________ _____________ mild/moderate/severe

________known/assumed ________kg/m3

_____________ _____________ ________kg/m3 ________kg/m3 ________per cent ________per cent ________per cent ________per cent

Type of Percentage passing the IS sieve aggregate 40 mm 20mm 10 mm 4.75mm 2.36mm 1.18mm 600 μm 300 μm 150 μm Coarse Fine B.8 Maximum size of coarse aggregate B.9 Grading zone of fine aggregate B.10 Fineness modulus Coarse aggregate Fine aggregate C. Mineral Additives C.1 Type of additive C.2 Specific gravity

________ mm ________________ ________________ ________________

fly ash/silica fume ________________

Proportioning of Concrete Mixes

339

________________ kg/m3

C.3 Bulk density C.4 Efficiency factor C.5 Volume (if predetermined)

________________ ________________

PART-II: MIX DESIGN Stage 1

Item

Reference of calculation

1.1Characteristic compressive strength (fck)

Specified (Part-l)

1.2 1.3 1.4 1.5 1.6

Fig./Table given

Standard deviations (S) Probability factor (k) Target mean strength (ft) Free water–cement ratio Maximum free water– cement ratio

ft = fck + kS Table/Fig.

Value __at __days Proportion of defective specimens______per cent ________________MPa ________________ __+ __ ×__= __ MPa —use the lower value

Specified

2 2.1 Compacting factor or Slump or V-B 2.2 Maximum aggregate size 2.3 Free water content

Specified Specified (Part-l) Table

3 3.1 Cement content 3.2 Maximum cement content 3.3 Minimum cement content

________ Specified Specified

3.4 Modified free water–cement ratio 4 4.1 Relative density of aggregate (SSD) Specified (Part-l) 4.2 Concrete density ________ 4.3 Total aggregate content 5 5.1 Grading of fine aggregate Specified (Part-l) 5.2 Proportion of fine aggregate Fig. 5.3 Fine aggregate content 5.4 Coarse aggregate content 6 6.1 Ingredients

6.2 Quantity per m3 (to nearest 5 kg) 6.3 Quantity per trial mix of ____m3 6.4 Ratio

Water (kg) _____ _____ _____

C.F.—Slump—mm or V-B—s ________mm ________kg/m3 _____ / _____= ____kg/m3 ________kg/m3 ________kg/m3. Use if greater than Item 3.1 and use to calculate Item 3.4 ________________ ________________ ________kg/m3 ___–___–___=___kg/m3 ________________ ________per cent ___×___=_____kg/m3 ___–___=_____kg/m3 Fine Coarse Cement aggregate aggregate (kg or 1) (kg) (kg) _____ _____ _____ _____ _____ _____ _____ _____ _____

340

Concrete Technology

REVIEW QUESTIONS 10.1 Enlist the factors governing the design of concrete mix. Briefly describe the influence of workability and durability. 10.2 State the steps involved in mix proportioning for medium strength concretes. 10.3 What is absolute volume and how is it determined for a concrete mix? For a concrete mix using 100 kg of coarse aggregate with Gs = 2.65; 60 kg of fine aggregate with Gs = 2.61; 27 kg of cement, and 12 kg of water, determine the absolute volume of concrete produced. For the same mix proportions but with 4 per cent air, determine of air content in the mix.

10.4 What are the limiting values of water-cement ratios and cement content according to IS456-2000 for some important situations? 10.5 What is water-cementing materials ratio w/(c + p)? 10.6 What are the characteristics of aggregates required for mix design and how are they determined? 10.7 The sieve analysis data of fine and coarse aggregates available at a construction site are listed in the following table. For mix design purposes determine the following characteristics: (a) Fineness modulus and grading of fine aggregate. (b) Fineness modulus and maximum size of coarse aggregate. (c) Proportions of the aggregates in which they should be combined so to obtain the specified grading chosen from standard curves which is also listed in the table.

[Hint: Va, m3 =

27 12 + 3.15 × 1000 1 × 1000 60 100 + + 2.63 × 1000 2.65 × 1000

Volume of air in the mix = 0.04 × Va, m3]. IS sieve size

20 mm 10 mm 4.75 mm 2.36 mm 1.18 mm 600 μm 300 μm 150 μm

Fine aggregate

100

100

92

84

70

46

19

03

Coarse aggregate

99

46

04

0

0

0

0

0

Specified grading

100

65

42

35

38

20

07

0

10.8 List the eight basic steps of ACI Mix Design Procedure. State the limitations of this method. What is the dry rodded unit weight of aggregate as used in ACI mix design method? 10.9 Describe the procedure of concrete mix proportioning using IS102622009: Concrete mix proportioning guidelines with the help of its flow chart. 10.10 Briefly describe design of concrete mixes with fly ash

10.11 What are trial mixes and how do they help in achieving the objectives of mix design? 10.12 What are major differences between ACI mix design method, British DoE method and IS-Concrete mix proportioning guidelines? 10.13 Calculate the quantities of ingredients required to produce one cubic meter of structural concrete. The mix is to be used in proportions of 1 part of cement to 1.26 parts of sand to 2.82 parts

Proportioning of Concrete Mixes of 20 mm nominal size crushed coarse aggregate by dry volumes with a watercement ratio of 0.48 (by mass). Assume the bulk densities of cement, sand and coarse aggregate to be 1500, 1700 and 1600 kg/m3, respectively. The amount of entrained air is two per cent. 10.14 It is required to design a M20 grade concrete mix having a slump of the order of 0−25 mm (0.7 CF) for foun-

341

dations of a structure likely to be subjected to moderate exposure conditions during its service life. Use Indian Standard Recommended Guidelines to estimate mix proportions. The contractor will exercise good quality control, and standard deviation estimated from past records is 4.8 MPa. Test results of the materials available at the site are the following:

(a) Cement Type Specific gravity Average compressive strength at 7 days 28 days

OPC-Grade 43 3.05 39.5 MPa 50.0 MPa

(b) Coarse aggregate Type Specific gravity Water absorption Free surface moisture

Crushed stone aggregate 2.68 1.46 0.00

(c) Fine aggregate Type Specific gravity Water absorption Free surface moisture

Natural river sand 2.60 0.50 1.40

Sieve analysis results for the aggregates are Type of aggregate

Percentage passing the IS sieve 40 mm

20 mm

10 mm

4.75 2.36 1.18 mm mm mm

600 μm

300 μm

150 μm

Coarse

100

99.5

25.6*

3.2

0.0

0.0

0.0

Fine

100

100

100

26.6

2.9

0.0

0.0

95.4 90.6 82.2 54.4*

(d) Durability requirements Maximum allowable water–cement ratio Minimum cement content moderate exposure

0.55 300 kg/m3

[Hint: Cement: fc,7-day > 33.0 MPa and fc,28-day > 43.0 MPa, hence is satisfactory. Maximum size of coarse aggregate is 20 mm+ and grading zone of sand is II*. W/C Ratio = 0.5: W = 180 kg/m3, C = 361 kg/m3; Cfa = 548 kg/m3; Cca = 1224 kg/m3]. 10.15 It is required to design an M60 grade concrete mix having a slump of the order of 25−75 mm for a structure likely to be subjected to extreme exposure conditions during its service life. The coarse aggregate available is wellshaped having nominal maximum

size of 12.5 mm, specific gravity of 2.64, dry-rodded mass of 1640 kg/m3, moisture content = 1.0 per cent, and absorption = 0.5 per cent, whereas the fine aggregate to be used has fineness modulus = 2.60, specific gravity = 2.62, dry-rodded mass = 1725 kg/m3,

342

Concrete Technology

moisture content = 5 per cent, absorption = 0.6 per cent. The available Portland cement has a specific gravity of 3.15. Take the density of water as 1000 kg/m3. Use ACI method of mix proportioning to estimate mix proportions. 10.16 Select a mix design method and develop its calculator using the appropriate flow chart outlining the steps of method in the format given below. [Hint: To determine proportions for a specified concrete follow the general steps: (1) Enter specific gravities for the materials in the right column. (2) Select the water−cement ratio required to achieve design requirements. (3) Determine the water content in per cubic metre of concrete. (4) Divide the water weight by the water−cement ratio and then

divide by 50 to obtain bags per cubic meter of concrete. (5) Enter the entrained-air content (per cent) for the mix. (6) The remaining volume required to make cubic metre of concrete will be filled with coarse and fine aggregates. Enter the fraction of the aggregate volume that will be filled by the coarse aggregate or percentage of fine aggregate as per the method selected. The remaining aggregate volume is filled with fine aggregate or coarse aggregate. Entering 0 in this field would fill 100 per cent of the aggregate volume with fine aggregate or coarse aggregate. Use the following factors for the calculator: Cement: 50 kg = 0.345 m³ loose volume. Water: 1000 kg/m³]

FORMAT I. Concrete Ingredients Calculator Enter materials data in the table below, as determined in accordance with method of mix proportioning (Click for proportion results) CONCRETE MIX DESIGN DATA T ENTRY R TABLE Specific gravity of cement:

3.15

Specific gravity of Pozzolan # 1:

2.32

Specific gravity of Pozzolan # 2:

2.42

0.0

Specific gravity of coarse aggregate:

2.68

Coarse aggregate of total aggregate volume:

0.0

Specific gravity of fine aggregate::

2.64

Entrained air: per cent

3.0

Cement content per cubic meter of concrete:

0.0

W -cement ratio: Water

0.4 .

Pozzolan # 1: per cent

0.0

Pozzolan # 2: per cent

R et Res

Proportioning of Concrete Mixes

343

II. Concrete Batch Weight Calculator

CONCRETE BATC A H WEIGHT Material contents

Material weight (kg/m³)

Material volume (m³)

Chemical admixture

Cement

0.0

0.0

Enter recommended dose in left column

Pozzolan #1

0.0

0.0

ml/50 kg of cement plus pozzolana

Pozzolan # 2

0.0

0.0

Dose

litre/m³

W Water

0..0

0..0

0.0

0.0

Coarse aggregate

0.0

0.0

0.0

0.0

Fine aggregate

0.0

0.0

0.0

0.0

Entrained air

0.0

0.0

Total

0..0

0..0

ml/50 kg of cement plus pozzolana 0.5

0.5

MULTIPLE-CHOICE QUESTIONS 10.1 The approximate strength of concrete at 28 days as a percentage of strength at one year is (a) 98 (b) 90 (c) 80 (d) 75 (e) 60 10.2 The ratio of tensile strength of concrete to the compressive strength is (a) 1/33 (b) 1/25 (c)1/20 (d) 1/10 (e) 1/5 10.3 The cube strength of concrete exceeds the cylinder strength by (in per cent) (a) 10 to 50 (b) 10 to 15 (c) 15 to 20 (d) 20 to 25 (e) 30 to 40 10.4 The permissible diagonal tension of M15 concrete is (a) 1.5 MPa (b) 1.2 MPa (c) 1.0 MPa (d) 0.75 MPa (e) 0.5 MPa 10.5 The proportions of materials in a concrete mix may be expressed in the form of (a) parts (by volume) of cement, fine and coarse aggregates

(b) parts (by weight) of cement, the fine and coarse aggregates (c) ratio of weight of cement to sum of weights of fine and coarse aggregates, i.e., cement-aggregate ratio (d) cement factor (e) Any of the above 10.6 Identify the incorrect statement(s). (a) Nominal mix is a mix of fixed proportions which ensure adequate strength (b) Nominal mixes may result in under or over-rich mixes (c) Standard mixes are useful as off-the shelf sets of proportions that allow the desired concrete to be produced (d) Nominal or standard mixes may be used for high performance concrete (e) Mix design ensures a concrete with the appropriate properties to be produced most economically

344

Concrete Technology

10.7 The choice of mix proportions of a concrete is independent of (a) grade designation (b) maximum nominal size of aggregate (c) minimum water-cement ratio (d) batching, mixing, placing and compaction techniques (e) durability and quality control 10.8 Which of the following statement(s) is correct? (a) The aim of mix design is to produce concrete that satisfies the job requirements, namely of compressive strength, workability and durability as economically as possible (b) Compressive strength is governed by the water-cement ratio (c) For the given aggregates, the workability of concrete is governed by its water content (d) Mix design is a basis for making an initial guess about the optimum combination of ingredients (e) All of the above 10.9 The maximum nominal size of the coarse aggregate is determined by sieve analysis and is designated by the sieve size higher than the largest size on which the material retained is more than (a) 5 per cent (b) 15 per cent (c) 25 per cent (d) 51 per cent (e) None of these 10.10 The larger maximum size aggregate (a) is beneficial for high strength concrete (b) requires a smaller quantity of cement for a particular water−cement ratio (c) results in reduced workability (d) reduces stress concentration in mortar aggregate interfaces (e) All of the above 10.11 Identify the correct statement(s). (a) For air-entrained concretes, the compressive strengths are approximately 80 per cent that of non airentrained concrete (b) Grade designation gives the char-

10.12

10.13

10.14

10.15

acteristic compressive strength requirements of concrete (c) Depending upon the degree of control, mix is designed for a target mean compressive strength of concrete (d) Durability of concrete means its resistance to the deteriorating influences of environment (e) Permeability of cement paste increases exponentially with an increase in water−cement ratio above 0.45 (f) All of the above Water-cement ratio in concrete is the ratio of (a) volume of water to volume of cement (b) volume of water to the weight of cement (c) weight of water to the weight of cement (d) weight of water to the volume of cement (e) weight of water required for chemical reaction to the weight of water required to wet the cement Lower water-cement ratio in concrete (a) increases the compressive strength (b) improves the frost-resistance of concrete (c) reduces the permeability of concrete (d) reduces the shrinkage and creep (e) All of the above Water in excess of that required for chemical reaction in concrete results in (a) bleeding (b) segregation (c) cracks (d) voids on drying (e) honey combing Most of the methods of concrete mix design are based on (a) the water-cement law as a criterion of strength (b) the assumption that workability is solely dependent on the water content

Proportioning of Concrete Mixes

10.16

10.17

10.18

10.19

(c) the assumption that durability is independent of the cement content (d) principle that there is no air-entrainment in the mix (e) All of the above The common mix design method for medium strength concrete is the (a) trial and adjustment method (b) DoE (British) mix design method (c) ACI mix design method (d) mix design according to Indian Standard recommended guidelines (e) Any of the above The trial and adjustment method (a) aims at producing a concrete mix which has minimum voids and hence maximum density (b) requires sufficient quantity of cement paste to fill the voids in the mixed aggregate (c) indicates that the optimum percentage of sand is lower for lower water-cement ratios (d) All of the above (e) None of the above The DoE mix design method (a) determines aggregate-cement ratio (b) uses free water content determined by the size and type of aggregate, and the level of workability (c) uses free water−cement ratio based on target mean compressive strength (d) All of the above (e) None of the above The ACI method of mix proportioning (a) uses bulk volume of coarse aggregate estimated for maximum nominal size of aggregate and fineness modulus of sand (b) takes into account the air-content of concrete (c) is suitable for normal and heavy -weight concretes in the workability range of 25−100 mm slump (d) can be used for the concrete having a target mean compressive strengths of up to 75 MPa (e) All of the above (f) None of the above

345

10.20 Mix design by Indian Standard recommended guidelines (a) is suitable for medium and highstrength concretes (b) requires data on the characteristic strength, degree of workability, limitations on the water−cement ratio, and maximum nominal size of aggregate (c) calculations are based on an absolute volume basis (d) All of the above (e) None of the above 10.21 For a constant aggregate cement ratio, if the coarse aggregate is increased at the expense of sand, the total surface of the aggregate (a) remains constant (b) is reduced (c) is increased (d) depends on other factors (e) None of the above 10.22 In a trial concrete mix, if the desired slump is not obtained, the adjustment in the water content for each 10 mm difference in slump (in per cent) is (a) 0.5 (b) 1.0 (c) 2.0 (d) 5.0 (e) 10.0 10.23 In a concrete mix design, while making adjustments for the air-entrainment of amount e, the quantity of water is reduced by v, then the reduction in the solid volume of sand is given by (a) e–v (b) 12(e–v) (c) e–v/2 (d) 12(e+v) (e) 12e–v 10.24 If the trial mix gives a higher 28-days compressive strength value than the design value, then for the next trial (a) cement content is reduced (b) water content is increased (c) water−cement ratio is increased (d) proportion of sand is increased (e) curing period is decreased 10.25 When water is added in an increasing amount to a fixed mass of dry mortar mix, the volume of mortar (a) initially increases then decreases to minimum value (b) does not change as the water simply fills the voids

346

10.26

10.27

10.28

10.29

10.30

10.31

Concrete Technology (c) decreases (d) increases (e) increases proportionately more than the volume of water added The volume of water which corresponds to a minimum volume of mortar is termed (a) saturation water content (b) basic water content (c) lowest water content (d) highest water content (e) hygroscopic water content The amount of water mixed in mortar should be always (a) more than the basic water content (b) equal to the basic water content (c) less than the basic water content (d) 50 per cent of the basic water content (e) None of the above A water content of 1.25 for a mortar mix means (a) 1.25 liter of water has been added per liter of cement (b) 1.25 liter of water has been added per liter of mortar (c) 25 per cent more water has been added than the basic water content requirements (d) 1 liter of water has been added in 1.25 liter of mortar (e) None of the above The nominal mix corresponding to M20 grade concrete is (a) 1:1:2 (b) 1:l.5:3 (c) 1:2:3 (d) 1:2:4 (e) 1:3:6 The grade of concrete corresponding to nominal mix proportions of 1:3:6 is (a) M35 (b) M25 (c) M15 (d) M10 (e) M7.5 The total number of grades of ordinary concrete stipulated in IS: 456−2000 are (a) 10 (b) 8 (c) 3 (d) 6 (e) 5

10.32 The volume of sand per cubic meter of 1:2:4 (by volume) concrete would be approximately (a) 0.2 to 0.4 (b) 0.4 to 0.6 (c) 0.6 to 0.9 (d) 0.8 to 1.0 (e) None of these 10.33 The number of bags of cement required per cubic meter of 1:2:4 concrete, would be approximately (a) 5 to 6 (b) 4 to 5 (c) 3 to 4 (d) 2 to 3 (e) 1 to 2 10.34 For slabs and beams, the concrete of nominal mix generally used is (a) 1:1:2 (b) 1:1.5:3 (c) 1:2:4 (d) 1:3:6 (e) 1:2:3 10.35 For water retaining structures the nominal mix generally used is (a) 1:1:2 (b) 1:1.5: 3 (c) 1:1.5: 4 (d) 1:2:4 (e) 1:2:6 10.36 To take into account the variation of individual samples, the laboratory design strength can be obtained by increasing the target mean strength by (per cent) (a) 5 (b) 5 to 10 (c) 10 to 15 (d) 15 to 20 (e) 20 to 25 10.37 To ensure proper quality control, the number of cube specimens to be cast for 5 m3 of concrete is (a) 3 (b) 6 (c) 9 (d) 12 (e) 15 10.38 After molding, the test specimens of trial mix are placed at a temperature of (a) 10 ± 2° (b) 15 ± 2° (c) 23 ± 2° (d) 27 ± 2° (e) 100 °C 10.39 It is often difficult to place the whole of the concrete in one operation and hence joints are provided. To have proper joints

Proportioning of Concrete Mixes

10.40

10.41

10.42

10.43

(a) the joint should be provided along the line of minimum shear (b) at the joint the old surface should be treated with a rich cement mortar paste before the new concrete is laid (c) the reinforcement of old concrete should extend into the new one (d) All of the above (e) None of the above Which of the following statements is correct? (a) Bulking of sand always decreases with an increase in the quantity of water (b) While batching by weight, the effect of bulking of sand is not considered (c) For mass concrete in the foundation the mix proportions are 1:2:4 (d) The water content in ordinary concrete is five per cent by weight of cement and 30 per cent by weight of aggregate (e) All of the above For a water-cement ratio of 0.6 the water content per bag of cement is (a) 10 kg (b) 20 kg (c) 30 kg (d) 40 kg (e) 50 kg A concrete has mix proportions of 1:2:4 by dry volume with a water−cement ratio of 0.6. The bulk densities of cement, sand and coarse aggregate are 1500, 1725, 1615 kg/m3, respectively. The mix proportions by weight are (a) 1:1.74:3.78 (b) 1:1.9:3.90 (c) 1:2:4 (d) 1:2.3:4.3 Assertion A: High performance concrete (HPC) is usually placed, at very high slumps without segregation, with the use of high range water reducer (HRWR). Reason R: High coarse aggregate and cementitious material contents, and a low w/(c + p) ratio makes the HPC difficult to place. (a) both (A) and (R) are true, and (R) is the correct explanation of (A)

347

(b) both (A) and (R) are true, and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true (e) Both are false 10.44 Identify the invalid statement(s) for concrete mix proportioning as per IS Guidelines. (a) The mix design is aimed at achieving the stipulated workability of fresh concrete, strength and durability requirements of hardened concrete at specified age with the maximum overall economy. (b) The basic data required for proportioning a concrete mix include the limitations on water−cement ratio and minimum cement content to ensure adequate durability for the given type of exposure. (c) According to IS456-2000, the characteristic strength is defined as the value below which not more than five per cent of test results are expected to fall. (d) The total absolute volume of coarse and fine aggregates (saturated surface dry condition) is computed by subtracting the sum of absolute volumes of cementitious material and water (e) None of the above 10.45 The concrete mix proportioning as per IS Guidelines is based on following criteria: (a) The compressive strength of concrete is governed by its water-cement ratio. (b) For given aggregate characteristics (maximum size of well-graded aggregate with suitable particle shape and grading), the workability of concrete mix is dependent only on water content. (c) Workability of concrete mix is largely independent of mix proportions, particularly the amount of cementing material. (d) All the three given above (e) Only (a) and (b) 10.46 Identify the invalid statement(s).

348

Concrete Technology

(a) First revision of IS 10262-2009: Concrete Mix Proportioning-Guidelines has followed the format of ACI mix proportioning method (b) The European Nations do not have common concrete mix design method because it considers mix design a part of concrete production. However, it exercises control through EN 206-1. (c) The standard deviation represents the degree of control estimated statistically based on at least 30 test strength samples. (d) Cement content prescribed for durability is irrespective of the grades of cement and it is inclusive of all supplementary cementitious materials. (e) None of the above 10.47 Identify invalid statement(s) differentiating ACI and IS Guideline mix proportioning methods. (a) Both the methods are based on absolute volume concept and use volume fraction of coarse aggregates in computation of fine aggregates content (b) The ACI method defines fineaggregate grading in terms of fineness modulus (FM), whereas the IS Guideline method uses fine aggregate zone as the grading index. (c) In the ACI method, the water content is based on the nominal maximum size of coarse aggregate, type of concrete (air-entrained or non-air entrained), and specified slump, but is independent of water-cement ratio (target strength). IS guidelines for mix proportioning, on the other hand, computes water content based on the water-cement ratio, workability (in terms of slump), and type and the nominal maximum size of the aggregate. (d) ACI method does not differentiate between crushed (flaky/elongated) and uncrushed (rounded)

coarse aggregates,; whereas IS guidelines specify adjustments for water requirement when rounded (uncrushed) coarse aggregate is to be used. (e) None of the above 10.48 Identify invalid statement(s) with respect to British DoE method for mix proportioning. (a) The sand content (per cent) is selected directly based on the nominal upper (maximum) size of coarse aggregate an d the grading zone of fine aggregate. (b) The method takes into account the type (crushed/ uncrushed) and upper size of coarse aggregate for water demand calculation. (c) None of the methods consider natural fine aggregate and crushed fine aggregate differently. (d) In contrast to the ACI method and IS guidelines, the British DoE method directly selects the required fine aggregate content and obtains the total (coarse and fine) aggregate content by subtracting the absolute volumes of the known ingredients from a unit volume of concrete. (e) None of the above 10.49 Assertion A: The IS guidelines for mix proportioning uses higher amount of cement (lowest water-cement ratio) than that used by the other methods. Reason R: Fineness of Indian cements is much lower at 225 m2/ kg compared to 300−500 m2/kg for American cements. (a) Both (A) and (R) are true and (R) is partially correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true 10.50 Identify invalid statement(s). (a) The British DoE method is simple and straightforward whereas the ACI method IS guidelines are more involved.

Proportioning of Concrete Mixes (b) Use of IS guidelines for lowergrade concretes results in over design. (c) The fine aggregate content decreases with the increase in the targeted strength. (d) In case of the ACI method, it is generally suggested that calcu-

349

lated coarse aggregate volume sometimes needs to be reduced, so that the fine aggregate content is increased and the cohesiveness and general workability of the mix improved. (e) None of the above

Answers to MCQs 10.1(c)

10.2 (d)

10.3 (d)

10.4 (e)

10.5 (e)

10.6 (d)

10.7 (d)

10.8 (f)

10.9 (b)

10.10 (b)

10.11 (f)

10.12. (c)

10.13(e)

10.14 (d)

10.15 (b)

10.16 (e)

10.17 (d)

10.18 (d)

10.19 (e)

10.20 (d)

10.21 (b)

10.22 (b)

10.23 (a)

10.24 (c)

10.25 (a)

10.26 (b)

10.27 (a)

10.28 (c)

10.29 (b)

10.30 (d)

10.31 (c)

10.32 (a)

10.33 (a)

10.34 (b)

10.35 (a)

10.36 (c)

10.37 (a)

10.38 (d)

10.39 (d)

10.40 (b)

10.41 (c)

10.42 (d)

10.43 (a)

10.44 (e)

10.45 (d)

10.46 (e)

10.47 (e)

10.48 (e)

10.49 (a)

10.50 (a)

PRODUCTION OF CONCRETE

11 11.1

INTRODUCTION

The design of a satisfactory mix proportion is by itself no guarantee of having achieved the objective of quality concrete work. The batching, mixing, transportation, placing, compaction, finishing and curing are very complimentary operations to obtain desired good quality concrete. Good quality concrete is a homogeneous mixture of water, cement, aggregates and other admixtures. It is not just a matter of mixing these ingredients to obtain some kind of plastic mass, but it is a scientific process which is based on some well-established principles and governs the properties of concrete mixes in fresh as well as in hardened state. The aim of quality control is to ensure the production of concrete of uniform strength in such a way that there is a continuous supply of concrete delivered to the place of deposition, each batch of which is as nearly like the other batches as possible. The production of concrete of uniform quality involves the following five definable phases. 1. 2. 3. 4. 5.

Batching or measurement of materials Mixing of concrete Transportation Placing, compaction and finishing of concrete Curing

11.2

BATCHING OF MATERIALS

A proper and accurate measurement of all the materials used in the production of concrete is essential to ensure uniformity of proportions and aggregate grading in successive batches. All the materials should be measured to the tolerances indicated in Table 11.1. Table 11.1 Material

Batching tolerances Accuracy of measurement

Aggregates, cement and water

± 3 per cent of batch quantity

Admixtures

± 5 per cent of batch quantity

For most of the large and important jobs the batching of materials is usually done by weighing. In weigh batching, the weight of surface water carried by the wet aggregate must be taken into account. The factors affecting the choice of proper batching system are: (i) size of job, (ii) required production rate, and (iii) required standards

Production of Concrete

351

of batching performance. The production capacity of a plant is determined by the material handling system, the bin size, the batcher size, and the plant mixer size, and their number available. The batching equipment falls into three general categories, namely, manual, semi-automatic, and fully automatic systems. 1. Manual batching In this sort of batching all operations of weighing and batching of concrete ingredients are done manually. Manual batching is acceptable for small jobs having low batching rates. Attempt to increase the capacity of manual plants by rapid batching often results in excessive weighing inaccuracies. The weighing may also be done by an ordinary platform scale. 2. Semi-automatic batching This batching is one in which the aggregate bin gates for charging batchers are opened by manually operated switches. Gates are closed automatically when the designated weight of material has been delivered. The system contains interlocks which prevent batcher charging and discharging occurring simultaneously. Provision is made for the visual inspection of the scale reading for each material being weighed. All the weighing hoppers should be constructed in a manner facilitating their easy inspection. 3. Automatic batching Automatic batching is one in which all scales for the materials are electrically activated by a single switch and complete autographic records are made of the weight of each material in each batch. However, interlocks interrupt the batching cycle when preset weighing tolerances are exceeded. The batching plant generally comprises two, three, four or six compartment bins of several capacities together with a supporting system. Below the bins are provided the weight batchers discharging over the conveyor belts. The use of separate hoppers as shown in Fig. 11.1 is preferable as it accomplishes some mixing of materials before they enter the mixer. Fine Coarse Cement Medium Sand Gravel Gravel Gravel

Conveyor Belt

Concrete Truck

Fig. 11.1

A typical scheme for a batching plant

352

Concrete Technology

The mobile plant consisting of batching equipment mounted on pneumatic tyred wheels has the advantage that the plant can be kept close to the site where concreting is required. The mobile plant is particularly useful where concrete is required over a very large area, e.g., an aerodrome; the runways and road construction work where the plant can follow the progress of work. Typical batching plants are shown in Fig. 11.2.

(a) Automatic central batching and mixing plant

Fig. 11.2

(b) Mobile batching and mixing plant

Automatic central and mobile concrete batching and mixing plants

In addition to accurate batching of mixing water, the amount of moisture present in the aggregate (particularly in the sand) as it is batched should be taken into account. For most of the small jobs, volume batching is adopted, i.e., the amount of each solid ingredient is measured by loose volume using measuring boxes, wheel barrows, etc. In batching by volume, allowance has to be made for the moisture present in sand which results in its bulking. The proportions by volume are generally specified in terms of the dry-rodded condition of the aggregate; the batch quantities must also be specified in the damp and loose condition. In volume batching, it is generally advisable to set the volumes in terms of whole bags of cement. Fractioned bags lead to variable proportions, resulting in concrete of non-uniform strength in successive batches. Before the batching operations are started, the engineer-in-charge should check the batch box volumes. When filling the boxes, the material should be thrown loosely into the box and struck off, and no compaction is to be allowed. At the end of each day’s work, the boxes should be stacked upside down to prevent any accumulation of rain water.

11.3

MIXING OF CONCRETE MATERIALS

The object of mixing is to coat the surface of all aggregate particles with cement paste, and to blend all the ingredients of concrete into a uniform mass. The mixing action of concrete thus involves two operations: (i) a general blending of different particle sizes of the ingredients to be uniformly distributed throughout the concrete mass, and (ii) a vigorous rubbing action of cement paste on to the surface of the inert

Production of Concrete

353

aggregate particles. Concrete mixing is normally done by mechanical means called mixer, but sometimes the mixing of concrete is done by hand. Machine mixing is more efficient and economical compared to hand mixing. In the mixing process, the cement paste is formed first with simultaneous absorption of water in the aggregates. In the second stage, the cement paste coats the aggregate particles. The mixing process should be continued till a thoroughly and properly mixed concrete is obtained. At the end of this stage the concrete appears to be of uniform colour and grading. The uniformity must be maintained while discharging the concrete from the mixer. As a matter of fact the classification of the mixers is based on the technique of discharging the mixed concrete as follows: 1. Tilting type mixer 2. Non-tilting type 3. Pan or stirring mixer The size of a mixer is designated by a number representing its nominal mix batch capacity in liters, i.e., the total volume of mixed concrete in liters which can be obtained from the mixer per batch. The capacity of a mixer for a particular job should be such that the required volume of concrete per hour is obtained without speeding up the mixer or reducing the mixing time below the specified period and without overloading the mixer above its rated capacity. The standardized sizes of the mixers given in IS: 1971–1985 are listed in Table 11.2. Most of the mixers can handle a 10 per cent overload satisfactorily. If the quantity mixed is much less than the rated capacity of the mixer the resulting mix may not be uniform, and the mixing operation becomes uneconomical. Table 11.2 Type of mixer

Standard sizes of mixer Nominal mixed batch capacity, liters

Tilting (T)

85T, 100T, 140T, 200T

Non-tilting (NT)

200NT, 280NT, 375NT, 500NT, 1000NT

Reversing (R)

200R, 280R, 375R, 500R, 1000R

In the tilting-type mixer, the chamber (drum) which is generally bowl-shaped or double-conical-frustum type shown in Fig. 11.3(a), is tilted for discharging. The efficiency of the mixing operation depends upon the shape and design of the vanes (blades) fixed inside the drum. These vanes direct the concrete into tracing a circulatory path. In addition, there is vertical free falling action due to gravity. The mixed concrete is discharged from the open top of the drum by tilting it downwards. The discharge action is always good as all concrete can be tipped out rapidly under gravity in an unsegregated mass as soon as the drum is tilted. For this reason tilting drum mixers are preferable for the mixes of low workability and for those containing large-size aggregates. The only disadvantage seems to be that a certain amount of mortar adheres to the drum and is left out in the drum itself during discharging operation. Therefore, before the beginning of mixing the first batch of concrete, a certain amount of mortar is mixed in the mixer. This process is called buttering the mixer. The subsequent batches will be as desired.

354

Concrete Technology

(a) Tilting mixer

(b) Non-tilting mixer

Fig. 11.3

(c) Pan-type mixer

Typical concrete mixers

The non-tilting-type mixer shown in Fig. 11.3(b) essentially consists of a cylindrical drum with two circular openings at the ends and blades fixed inside the drum. The drum rotates about a horizontal axis and cannot be tilted. The mixer is loaded through a central opening at one end of drum and, after mixing, the concrete is discharged through the opening at the other end by a chute. Owing to a rather slow rate of discharge, the concrete is sometimes susceptible to segregation. In particular, the larger size aggregate may tend to stay in the mixer, when the other constituents are being discharged. Hence the discharge may initially consist of mortar and then as a collection of a large size coated aggregate. However, it is worthwhile to check the performance of the mixer for a particular type of mix, before it is actually used. The pan mixer shown in Fig. 11.3(c) consists essentially of a circular pan rotating about a vertical axis. One or two stars of paddles also rotate in the pan about a vertical axis not coincident with the axis of the pan. In some types, the pan is static and the axis of star travels along a circular path about the axis of the pan. In other types, the paddles are stationary and the pan rotates about the vertical axis. In either case, the relative movement between the paddles and the concrete is the same, and concrete in every part of the pan is thoroughly mixed. There is another set of blades called scrapper blades, which prevent the sticking of the mortar to the pan sides by continuous scrapping. The paddle height can be adjusted to prevent a permanent coating of mortar forming on the bottom of pan. The mix is discharged through a central hole at the bottom of the pan. The pan mixer is generally not mobile and is, therefore, used either as a central mixing plant on a large concrete project or at a precast concrete factory. This mixer is particularly efficient with stiff and cohesive mixes. Pan mixers are extensively used in the laboratories for mixing small quantities of concrete of consistent quality, because of the efficient scrapping arrangement. A pan mixer consisting of a bowland-stirrer working on the principle of the cake mixer is sometimes used for mixing the mortar. Apart from the three types of mixers given above, another type called dual drum mixer is extensively used for mixing concrete for road or pavement construction. The dual drum mixer consists of a long drum divided into two parts by a central diaphragm. Both parts are operated in series. The concrete is intially mixed up to a certain time in the first compartment of the drum and then transferred to the second compartment for the remaining operation of mixing. In the meanwhile, the first

Production of Concrete

355

compartment is recharged with the constituents of the mix. The dual drum mixers are useful as the mixing capacity can be doubled with the same batching equipment and supervisory staff. The mobile or truck mixers consisting of mixer drum mounted on a conventional truck chassis are powered either from the truck engine or from a separate diesel engine. These mixers are used in ready-mixed concrete industry.

11.3.1

Mixing Time

It is the time required to produce uniform concrete. The mixing time is reckoned from the time when all the solid materials have been put in the mixer, and it is usual to specify that all water has to be added not later than after one quarter of mixing time. The time varies with the type of mixer and depends on its size. Strictly speaking, it is not the mixing time but the number of revolutions of the mixer that are to be considered, because there is an optimum speed of rotation for the mixer. The number of revolutions and the time of mixing are independent of each other. In high-speed pan mixers, the mixing time can be as short as 35s. On the other hand, when light weight aggregate is used, the mixing time should not be less than five minutes, sometimes divided into two minutes of mixing the aggregate with water followed by three minutes with cement added. In general, the length of mixing time required for sufficient uniformity of mix depends on the quality of blending of materials during charging of the mixer. With machine mixing, there is an increase in strength with time of mixing up to about five minutes. The increase in strength is largest in first one minute and after two minutes the increase is very small. A mixing time of not less than one minute after all the materials have been added in the mixer drum is generally recognized as a satisfactory period for mixers up to a capacity of 750 liters. For mixers of larger capacity, the mixing time should be increased at the rate of 20 s or more for each cubic meter or fraction thereof. The recommended minimum mixing times are given in Table 11.3. Table 11.3

Recommended minimum mixing time

Capacity of mixer, m3

Mixing time, minutes

0.8

1.00

1.5

1.25

2.3

1.50

3.1

1.75

3.8

2.00

4.6

2.25

7.6

3.25

The order of feeding the ingredients into the mixer depends on the properties of the mix and those of the mixer. Generally, a small amount of water should be fed first, followed by all the solid materials, preferably fed uniformly and simultaneously into the mixer. If possible, the greater part of the water should also be fed during the same time the remainder of water is added after the solids have been fed.

356

Concrete Technology

The choice of mixer depends upon the size, extent, and the nature of work. The choice between central and site mixing will be governed by local factors, such as accessibility, water supply, transport routes, availability of working space, etc.

11.3.2

Hand Mixing

There may be occasions when the concrete has to be mixed by hand, and because in this case uniformity is more difficult to achieve, particular care and effort are necessary. The aggregate should be spread in a uniform layer on a hard, clean and non-porous base; cement is then spread over the aggregate and the dry materials are mixed by turning over from one end of the heap to another and cutting with a shovel until the mix appears uniform. Turning three times is usually required. The water is gradually added to the trough formed by the uniform dry mix and the mix is turned over until a homogeneous mixture of uniform color and consistency is obtained.

11.4

TRANSPORTATION OF CONCRETE

Concrete from the mixer should be transported to the point where it has to be placed as rapidly as possible by a method which prevents segregation or loss of ingredients. The concrete has to be placed before setting has commenced. Attempts have been made to limit the time lapse between mixing and compaction within the forms. The specifications, however, permit a maximum of two hours between the introduction of mixing water to the cement and aggregates, and the discharge, if the concrete is transported in a truck mixer or agitator. In the absence of an agitator, this figure is reduced to one hour only. All these, however, presume that the temperature of concrete, when deposited, is not less than 5 °C or more than 32 °C. It has now been established that delays in placing concrete, after the so-called initial set has taken place, are not injurious and may give increased compressive strengths, provided the concrete retains adequate workability to allow full compaction. The requirements to be fulfilled during transportation are: 1. No segregation or separation of materials in the concrete. 2. Concrete delivered at the point of placing should be uniform and of proper consistency. The prevention of segregation is the most important consideration in handling and transporting concrete. The segregation should be prevented and not corrected after its occurrence. The concrete being a non-homogeneous composite of materials of widely differing particle sizes and specific gravities, is subjected to internal and external forces during transportation and placing tending to separate the dissimilar constituents. Segregation can be prevented by ensuring that the direction of fall during the dumping or dropping of concrete is vertical. When the discharge is at an angle, the larger aggregate is thrown to the far side of the container being charged and the mortar is collected at the near side, thus resulting in segregation.

Production of Concrete

357

The plant required for transporting the concrete varies according to the size of the job and the level at which the concrete is to be placed. The principal methods of transporting concrete from the mixer are the following: 1. Barrows (a) Wheel barrows and handcarts (b) Power barrows or powered buggies or dumpers 2. Tippers and lorries 3. Truck mixers and agitator lorries 4. Dump buckets 5. The monorail system or trolley or rails The most commonly used method of transporting concrete by the hand pans passing from hand to hand is slow, wasteful and expensive. If concrete is to be placed at or below the mixer level, steel wheel barrows are a better mode of transportation. Concrete can be discharged from the wheel barrow to the required point. When concrete is to be placed much below the general ground level, as in basement slabs, foundations, etc., a wooden or steel chute may be used for chuting the concrete into place. The wheel barrows shown in Figs. 11.4(a) and (b) are suitable for small jobs and where the length of transport is small, and over muddy ground. The average quantity that can be carried in one wheel barrow is about 35 liters (80 kg). Sometimes, for relatively bigger jobs, power barrows which are motorized version of wheel barrows are used.

(a) Wheel barrow or hand (b) Motorized wheel barrow cart

Fig. 11.4

(c) Powered barrow or dumper

Barrows for transporting concrete

Dumpers and ordinary open-steel body tipping lorries shown in Figs. 11.4(c) and 11.5(a) can be used economically for hauls of up to about 5 km. These lorries are suitable only for dry mixes to avoid difficulties caused by segregation and consolidation. The time of journey should be as short as possible. It is essential that the lorry body be watertight to prevent loss of fines. The concrete has to be covered with tarpaulins to prevent the concrete being exposed to sun, wind and rain. If the haul is long, agitators have to be used to prevent segregation. Steel buckets transported by rail or road may be used to transport the concrete for long distances and for large jobs like dams, bridges, etc. While using this method it is necessary to see that (i) the entire mixer batch is placed in the bucket, and (ii) segregation is prevented while filling the bucket.

358

Concrete Technology

For transporting ready-mixed concrete truck mixers and agitator lorries shown in Fig. 11.5(b) are used.The monorail system is useful when the ground conditions are not suitable for normal wheeled traffic. In the monorail system, the rail can easily be provided at such a level that the concrete be tipped directly into the formwork. Basically, the system consists of a power wagon mounted on a single rail capable of a traveling speed of 90 m/min. The engine may be diesel or petrol powered, without a driver.

(a) Tipper truck

(b) Truck mixers or agitator lorries

Fig. 11.5

Typical tippers, truck mixers, and agitator lorries

Conveyor belts have also been used for conveying fairly stiff concrete, but there is a tendency of segregation on steep inclines and at transfer points. When using conveyor belts, it is necessary that the flow of concrete be continuous to minimize the effects of segregation. In the jobs where the concrete is to be lifted up to 5 m, inclined runways with one or two landings for carrying the concrete up to the required level can be built. Another method of lifting concrete to greater heights is by using some sorts of hoists. The various types of hoists are chain hoists, platform hoists or skip hoists. In the chain hoisting, a chain sling is suspended from a pulley and is operated by a power winch at the ground level. The sling is attached to the container, which is then lifted bodily to the working level. Wheel barrows and carts can be elevated by platform hoists operating on vertical steel guides. With some hoists, two platforms are provided, one descends while the other is being raised. The major types of hoists used in tall buildings are tip skip hoists, automatic skip discharge hoists, twin automatic skip, and passenger/concrete hoists. The tip skip hoists, normally fed by direct discharge from the mixer are elevated, and discharged into a receiving hopper at the working level; from that point wheel barrows or other transport can deliver the concrete to the forms. In a modified tip skip model, bottom doors, which open automatically at the required level allowing the concrete discharge via drop chute into the floor hopper, are provided. In tall buildings where the time taken for the traveling of skips and discharging of concrete is large, skips with individual winch units are provided. When one skip travels, the other one is filled, thereby allowing greater efficiency of the whole operation. In certain tall buildings where a passenger or passenger/goods hoist is essential due to mechanical and other permanent services involved, a combined passenger hoist with concrete carrying and discharging abilities can be used. When the hoist is not in use for concrete conveyance, the cage floor is left perfectly free for normal passenger duties.

Production of Concrete

359

In an arrangement consisting of an elevated tower and chutes, concrete is raised in buckets to the central tower and distributed through sloping chutes from the top of the central tower. This system is suitable for large dam jobs. There is a tendency of segregation for dryer mix, it may be necessary to fit vibrators to the chutes.

11.4.1

Pumped Concrete

Pumping of concrete through steel pipelines is one of the successful methods of transporting concrete. Pumped concrete has largely been used in construction of multistory buildings, tunnels, and bridges. Powered by diesel engine, an older version of a concrete pump is a heavy duty, simple two-stroke mechanical reciprocating pump consisting of a receiving hopper, inlet and outlet valves, a piston and a cylinder. The pumping action starts with the suction stroke drawing concrete into cylinder as the piston moves backwards. During this operation, the outlet valve is closed. On the forward or pressure stroke, the inlet valve closes and outlet valve opens to allow concrete to be pushed into delivery pipe. The pipeline is completely filled and concrete moves uniformly. The pump capacity can range from 15 m3/h to 150 m3/h. The normal distance to which the concrete can be pumped is about 400 m, horizontally, and 80 m vertically. Usually 1 m of vertical movement is equivalent to 10 m horizontally. Bends in the pipeline reduce the effective pumping distance by approximately 10 m for each 90 degree bend, 5 m for 45 degree bend, and 3 m for 22.5 degree bend. A modern concrete pump, on the other hand, basically consists of three parts: a concrete receiving hopper, a controlling valve system and concrete transmission system. Typical concrete pumps are shown in Fig. 11.6(a). In the commonly used pump called squeeze pump, the concrete placed in the receiving hopper is fed by rotating blades into the flexible pipe connected to the pumping chamber, which is under vacuum of about 600 mm of mercury. Two rotating rollers progressively squeeze the flexible pipes and force the concrete to move through the delivery pipe in a continuous flow.

(a) Concrete pumps

Fig. 11.6

(b) Concrete is being pumped

Typical concrete pumps and pumping of concrete

360

Concrete Technology

The pipeline of transmission system carrying concrete at high pressure should have correct diameter with adequate wall thickness for a given operating pressure, and well designed coupling system for trouble free functioning and safety. The diameter of the pipe depends on the pumping pressure and the size of aggregate. For long horizontal distance involving high pumping pressure, a larger diameter pipe would be suitable for reduced resistance to flow. On the other hand, for pumping concrete to heights, smallest possible diameter pipelines should be used from gravity consideration. The pipe diameter should be between 3 to 4 times the maximum size of aggregate. As a guide, a pump with an output of 30 m3/h and with length of pipeline not exceeding 200 m may have a diameter of 100 mm, but for lengths in excess of 500 m, a 150 mm diameter could be considered. Generally, 125 mm diameter pipes are used. The pipeline should be carefully laid and well anchored when bends are introduced for trouble free pumping operation as shown in Fig. 11.6(b). The pumps should not be kept very close to the vertical pipe. There must be a starting distance of about 10 to 15 per cent of the vertical distance. The concrete emerging from a pipeline flows in the forms of a plug which is separated from the pipe wall by a thin lubricating layer consisting of cement paste. For continuous plug movement, the flow resistance must not exceed the pump pressure rating. Mix properties for pumpable concrete needs special attention. In general, concrete should be very cohesive and fatty having a slump value of 50 mm to 100 mm or more. A stiff concrete and the concrete with high water–cement ratio are not pumpable. A pumpable concrete is a good concrete proportioned in such a way that is able to bind all constituent materials together under pump pressure and thereby avoiding segregation and bleeding. The mix must also facilitate the radial movement of sufficient grout to maintain lubricating film initially placed on the pipeline wall. The mix should be able to deform while flowing through the bends. To achieve these characteristics the mix proportions should be so chosen that the total quantity of fines, i.e., cement and fine particles passing 250 micron sieve should be between 350 to 500 kg/m3. For obtaining high slump or flowing concrete, superplasticizers are generally used. The design of pumpable concrete is illustrated in example 10.10.

Problems Although the method of transporting and placing concrete by pumps is fast and efficient, a small part of unpumpable mix in hopper can block the pump, leading to delay while the pump is stripped down. The blockage is indicated by an increase in the pressure shown on the pressure gage. Most blockages occur at the tapered sections at the pump end. The reasons include unsuitability of concrete mix, pipeline and joint deficiencies, careless use of hose end, and operator’s errors. High temperatures, may also cause blockage. Chances of blockage are least in continuous pumping. A pipeline not well cleaned after previous operation, uncleaned and worn-out hoses, too many or too sharp bends, and use of worn-out joints add to the problem of blockage. Great attention is required in the design of mix, for a minor variation in the concrete mix is sufficient to make an otherwise pumpable mix completely unpumpable. At the end of the run, the pipeline must be cleared of concrete by inserting a plunger at the pipe end and forcing it through under pressure. After the concrete is cleared, the pipeline is washed out to leave a smooth clean surface ready for next

Production of Concrete

361

day’s work. The minor blockage may be cleared by forward and reverse pumping. Application of unjudicious excess pressure may worsen the problem. Shortening the pipeline (which reduces the pressure) and restarting pump may clear the blockage. Tapping the pipeline with hammer and observing the sound may often help to locate the blockage.

11.5

READY-MIXED CONCRETE

A concrete whose constituents are weight batched at a central batching plant, mixed either at the plant itself or in truck mixers, and then transported to the construction site and delivered in a condition ready to use, is termed ready-mixed concrete (RMC). This enables the places of manufacture and use of concrete being separated and linked by suitable transport operation. The technique is useful in congested sites or at diverse work places and saves the consumer from the botheration of procurement, storage and handling of concrete materials. Ready-mixed concrete is produced under factory conditions and permits a close control of all operations of manufacture and transportation of fresh concrete. Due to its durability, low cost and its ability to be customized for different applications, ready-mixed concrete is one of the most versatile and popular building materials. The concrete quality (in terms of its properties or composition) and quantity or volume required for the particular application is specified by the customer. Quality of ready-mixed concrete is generally specified in terms of performance parameters, i.e., purchaser specifies the strength level and intended use of concrete. It is the best way to order ready-mixed concrete because the ready-mixed concrete (RMC) producer, who is an expert in this field, would design an economical mix with the desired properties. The RMC producer accepts responsibility for the design of the mixture for desired performance. In another system, the quality of ready-mixed concrete is specified in terms of prescriptive specifications, i.e., purchaser specifies aggregate size, slump, air content, cement content or weight of cement per cubic meter of concrete, maximum water content and admixtures required. In this case, the purchaser accepts the responsibility for concrete strength and its performance. In the first system, ready-mixed concrete producer independently selects the material proportions based on previously developed guidelines and experience resulting in an economical and practical mix. Thus, to serve the goal of materials conservation, a paradigm shift is needed from prescriptive-to performance- based standard specification for materials. RMC is ordered and supplied by volume (cubic meter) in a freshly mixed and unhardened state. When ordering concrete 5 to 10 per cent more concrete than estimated from a volumetric calculation is ordered. This will account for the wastage or spillage, over-excavation, spreading of forms, some loss of entrained air, settlement of wet mixture, and change in volume, dry concrete volume is one to two per cent less than that of fresh concrete. It is important not to order too much concrete. The processing and disposal of returned concrete is an expensive proposition for the ready-mixed concrete producer, who has to comply with various environmental regulations.

362

Concrete Technology

11.5.1

Proportioning of Ready-Mixed Concrete

The ready-mixed concrete embodies the concept of treating the concrete in its entity as a building material rather than ingredients. The proportioning of an RMC aims at obtaining an economical and practical combination of materials to produce concrete with the properties desired for its intended use, such as workability, strength, durability and appearance. The following basics of a good concrete mix should be considered while proportioning RMC for the desired performance: 1. Concrete aggregates are required to meet appropriate specifications and in general should be clean, strong and durable. 2. Fly ash or other supplementary cementing materials, which enhance concrete properties, are normally added to RMC. The key to quality concrete is to use the least amount of water that can result in a mixture which can be easily placed, compacted and finished. 3. Admixtures, are commonly used in relatively small quantities to improve the properties of fresh and hardened concrete such as the rate of setting and strength development of concrete, especially during hot and cold weathers. The most common is an air-entraining agent that develops millions of tiny air bubbles in concrete, which imparts durability to concrete in freezing and thawing exposure. Water reducing admixtures while minimizing the water content in the mixture, increase strength and improve durability. A variety of fibres are incorporated in concrete to control cracking or improve abrasion and impact resistance.

11.5.2

Production of Ready-Mixed Concrete

The production of ready-mixed concrete should be carried out in plants where the equipment, operation and materials are suitably controlled under a Quality Assurance Scheme. It is important that all personnel who are involved in the production and delivery of RMC receive adequate training prior to production which may include observing trial batches being produced and tested.

Storage of Constituent Materials For storage of constituent materials attention should be paid to the following points: 1. Aggregates Aggregates should be properly stored to avoid cross-contamination between different types and sizes and protected from weather to minimize the fluctuation of surface moisture content and movement of fines. Ground stock should be stored in specially built partitioned bays, which will allow free drainage of excess moisture in the aggregates and rainwater. There must be adequate storage capacity for aggregates as any significant disruption in the supply that causes a break in placing could cause serious complications. It is desirable that all material stores are filled in advance of a RMC placement.

Production of Concrete

363

2. Cements, additives and admixtures Recommendations of the manufacturer for storage shall always be followed. It is recommended that all material stores are collected in advance to avoid the potential variations in performance following a fresh delivery.

Mixing Equipment and Trial Mixes Ready-mixed concrete can be produced using any efficient concrete mixer including paddle mixers, free-fall mixers and truck mixers, but forced action mixers are generally preferred. However, it is particularly important that the mixer is in a good mechanical condition and that it can ensure full and uniform mixing of the solid materials with sufficient shear action to disperse and activate the superplasticizer. It is important that preliminary trials are carried out to ascertain the efficiency of individual mixers and the optimum sequence for addition of constituents. Ready-made self-compacting concrete may take longer time to achieve complete mixing than for normal concrete due to reduced frictional forces and to fully activate the superplasticizer. Plant-mixing Procedures The high paste content and flowability of modern concretes makes it difficult to achieve a uniform mix than concrete of normal workability. In such cases, unmixed balls of constituents may form during mixing which do not break easily. This balling phenomenon is more likely to occur in freefall mixers (particularly truck mixers) than forced action mixers. This problem can be avoided by first batching the concrete to a lower workability level until it is uniformly mixed. Addition of further water and superplasticizer will increase the workability to the required level while avoiding balling. Time of addition of admixture during the batching is important as it can alter the effectiveness. When using VMA, a late addition to the mix is preferred. A standard procedure should be adopted based on plant trials and this procedure then be strictly followed in order to reduce the variability between batches. Admixtures should not be added directly to dry constituent materials but dispensed together with or in the mixing water. Different admixtures should not be blended together prior to dispensing unless specifically approved by the admixture manufacturer. If air-entraining admixtures are being used, they are best added before the superplasticizer and while the concrete is at a low consistency. Due to the powerful effect of modern superplasticizers, it is important that admixture dispensers are calibrated regularly. During production, there may be a number of factors that individually or collectively contribute to variations in the uniformity. The main factors are changes in the free moisture of the aggregate, aggregate particle size distribution and variations in batching sequence. Because it is normally not possible to immediately identify the specific cause, it is recommended that adjustments to the workability should be achieved by adjusting the level of the superplasticizer.

Methods to Load the Mixer Due to the wide variety of available mixers, the methodology for loading the mixer should be determined by trials before commencing production. Generally, the following methods to load the mixer are preferred:

364

Concrete Technology

1. Free-fall mixers In this method, in the first instance approximately two-thirds of the mixing water is added to the mixer. This is followed by the aggregates and cement. When a uniform mix is obtained, the remaining mixing water and the superplasticizer are added. Where VMA is used, this should be added after the superplasticizer and just prior to final workability adjustment with water. Truck mixers may require additional mixing time for RMC as they are less efficient than plant mixers. Splitting the load into two or more batches can improve mixing efficiency. The condition of the truck mixer drum and mixing blades are particularly important for RMC and should be regularly inspected. The rotational speed of the drum during the mixing cycle should comply with the recommendations of the manufacturer but the mixing speed for RMC will normally be in the range of 10–15 rpm. 2. Forced action mixers In this type of loading, the aggregate is generally added to the mixer first, together with the cement. This is immediately followed by the main mixing water and superplasticizer. Where VMA is used, it is added with the final water. The high shear produced by a forced action mixer improves the flowability and it may be possible to reduce the addition rate of the superplasticizer compared to a free-fall mixer.

11.5.3

Classification of Ready-mixed Concrete

There are three principal categories of RMC. In the first, called the transit-mixed concrete, the materials batched at a central plant are mixed during the period of transit to the site or immediately prior to concrete being discharged. Transit mixing permits a longer haul. Sometimes the concrete is partially mixed at the central plant and the mixing is completed en route; such concrete is known as shrink-mixed concrete. This enables better utilization of transporting trucks. The time of transit after water is added is generally limited from one to one-and-a-half hours. The total number of revolutions during both mixing and agitation are limited to 300. In the third category called the central-mixed concrete, the mixing is done at a central plant and the mixed concrete is delivered generally in an agitator truck, which revolves slowly so as to prevent segregation and undue stiffening of the mix.

Transit-mixed or Truck-mixed Concrete In transit-mixed concrete, also called truck-mixed or dry-batched concrete, all of the raw ingredients are charged directly in the truck mixer. Most of the water is usually batched at the plant. The mixer drum is turned at charging (fast) speed during the loading of the materials. There are three options for truck-mixed concrete: 1. Concrete mixed at the job site While traveling to the job site the drum is turned at agitating speed (slow speed). After arriving at the job site, the concrete is completely mixed. The drum is then turned for 70 to 100 revolutions, or about five minutes, at mixing speed. 2. Concrete mixed in the yard or central batching plant The drum is turned at high speed or 12–15 rpm for 50 revolutions. This allows a quick check of the batch. The concrete is then agitated slowly while driving to the job site.

Production of Concrete

365

3. Concrete mixed in transit The drum is turned at medium speed or about 8 rpm for 70 revolutions while driving to the job site. The drum is then slowed to agitating speed.

Shrink-mixed Concrete Concrete that is partially mixed in a stationary plantmounted mixer and then discharged into the drum of the truck mixer for completion of the mixing is called shrink-mixed concrete. Generally, about two minutes of mixing in truck drum at mixing speed, is sufficient to completely mix shrink-mixed concrete. Central-mixed Concrete Central-mixed concrete batch plants include a stationary, plant-mounted mixer that mixes the concrete before it is discharged into a truck mixer. Central-mix plants are sometimes referred to as wet-batch or pre-mix plants. The truck mixer is used primarily as an agitating haul unit at a central mix operation. Dump trucks or other non-agitating units are sometimes used for low slump and mass concrete pours supplied by central-mix plants. Principal advantages include faster production capability than transit-mix plant, improved concrete quality control and consistency, and reduced wear on the truck mixer drums. As explained earlier in the chapter there are several types of batch plants and plant mixers, including tilt drum mixer, horizontal shaft paddle mixer, dual shaft paddle mixer, pan mixer, and slurry mixer. The tilting drum mixers are fast and efficient, but can prove to be maintenance-intensive since they include several moving parts that are subjected to heavy load. Horizontal shaft mixers have a stationary shell and rotating central shaft with blades or paddles. They have either one or two mixing shafts that impart significantly higher horsepower in mixing than the typical drum mixer. The intensity of the mixing action is somewhat greater than that of the tilt drum mixer. This high-energy mixer produces higher strength concrete by thoroughly blending the ingredients and more uniformly coating the aggregate particles with cement paste. Pan mixers are generally lower capacity mixers at about 4 m3 and are used at pre-cast concrete plants. The slurry mixer is a relatively new feature of concrete mixing technology. It can be added onto a dry-batch plant and works by mixing cement and water that is then loaded as slurry into a truck-mixer along with the aggregates. It benefits from highenergy mixing. Another advantage is that the slurry-mixer reduces the amount of cement dust that escapes into the air. Mix-mobiles or mobile-proportioning plants Mix-mobile plants or plants-onwheels are truck mounted, volumetric batching and continuous mixing units which often supply small volume or speciality pours and offer the convenience of freshly mixed concrete in fairly precise quantities. The unit consists of a truck with bins of sand, coarse aggregate, cement, water, and admixtures.

11.5.4

Production Control

1. Constituents High flowability concrete is more sensitive than normal concrete to variation in the physical properties of its constituents and especially to changes

366

Concrete Technology

in aggregate moisture content, grading and shape, so more frequent production checks are necessary. It is recommended that the aggregates are tested each production day prior to commencing batching. Thereafter, visual checks should be carried out on each delivery of aggregate; any noticeable change should be evaluated prior to accepting or rejecting the delivery. The moisture content of aggregates should be continuously monitored and the mix adjusted to account for any variation. When new batches of cement, additives or admixture are delivered, additional performance tests may be necessary to monitor any significant changes or interactions between constituents. 2. Production The production and supply of ready-mixed concrete shall be in accordance with contractual arrangements between purchaser and producer and the requirements of relevant code. The type of application will determine the specified characteristics and classes that the purchaser has given the producer. The production control must ensure that these are carefully complied with during production. In order to ensure consistent properties, it is desirable that every load is tested for workability until consistent results are obtained. Other mandatory tests may also be needed to confirm compliance with the contract specification. Subsequently, every delivered batch should be visually checked before transportation to the site or point of placing, and routine testing carried out to the frequency stipulated in the specifications.

Transportation and Delivery For efficient placement, it is essential that the production capacity of the plant, journey time and placing capability at site are all balanced to ensure that site personnel can place the concrete without a break in supply and within the workability retention time. Production stops can result in lift lines on the vertical surface. While ready-mixed concrete can be delivered to the point of placement in a variety of ways, the overwhelming majority of it is brought to the construction site in truck mounted, rotating drum mixers. Truck mixers have a revolving drum with the axis inclined to the horizontal. Inside the shell of the mixer drum are a pair of blades or fins that wrap in a helical (spiral) configuration from the head to the opening of the drum. This configuration enables the concrete to mix when the drum spins in one direction and causes it to discharge when the direction is reversed. To load or charge the raw materials from a transit-mix plant or central-mixed plant into the truck, the drum must be turned very fast in the charging direction. After the concrete is loaded and mixed, it is normally hauled to the job site with the drum turning at a speed of less than 2 rpm. The truck mixer having front discharge units are more popular than the traditional ones having rear discharge units. The driver of the front discharge truck can drive directly onto the site and can mechanically control the positioning of the discharge chute without the help of contractor personnel. Fresh concrete is a perishable product that may undergo slump loss depending on temperature, time taken to the delivery point on the job site, and other factors. Water

Production of Concrete

367

should not to be added to the mix unless the slump is less than that specified. If water is added, it should be added at once and the drum of the truck mixer should be turned minimum of 30 revolutions, or about two minutes, at mixing speed. Commonly used specifications for ready-mixed concrete stipulate that the concrete shall be discharged on the job site within 90 minutes and before 300 revolutions after water is added to the cement. In certain situations, air-entraining, water-reducing, set-retarding or high-range water reducing (HRWR) admixtures may need to be added to concrete prior to discharge, to compensate for the loss of air, high temperatures or long delivery times.

11.5.5

Inspection and Testing

Since RMC is a manufactured product specific control tests and evaluations are required during the manufacturing process to produce predictable high quality concrete. Some of the important properties of concrete that are measured by basic quality control tests are strength, temperature, slump, air content, and unit weight. When there are no formal job specifications, such as with a homeowner or small contractor, it is important for the concrete producer to agree to supply concrete in accordance with relevant national codes. Any agreement between the producer and a purchaser should include definition of the basis of purchase, i.e., unit of concrete and its measurement, acceptable material specifications, and acceptable industry practice and tolerance. The agreement should also include strength testing procedures and acceptance criteria, laboratory personnel’s qualifications and assurance of compliance with relevant codal provisions.

Site Requirements and Preparation Prior to delivery of the concrete, the contractor/user must ensure that appropriate site preparations have been made. These preparations should include that 1. 2. 3. 4. 5.

the specified RMC mix is appropriate for the job, the site can utilize/place the concrete at the agreed delivery rate, acceptance procedures for the RMC are agreed and documented, site personnel are trained in the specific requirements for placing RMC, and formwork is properly prepared.

These requirements are discussed in detail in Section 11.6.

Site Control A quality control procedure shall be documented and followed on the job site for acceptance of concrete. It is recommended that every batch of RMC delivered should be tested for slump until uniformity of supply is confirmed.

11.5.6

Mix Adjustment

In general, modification of RMC on site is undesirable as the producer should be capable of supplying the specified mix with the required properties for the job. However, if special circumstances exist or if some experimentation is expected/planned

368

Concrete Technology

in order to optimize the mix for specific formwork configurations and surface finish, it may be prudent to establish a further documented procedure for minor adjustment of the concrete, under supervision at site.

11.5.7

Supervision and Skills

It is essential that the site personnel used to place RMC have been trained/instructed in the specific requirements for placing a particular type of concrete. Site personnel should be made aware of the general guidelines and particular emphasis should be placed on effect of vibration on mix stability, rate of placing, effect of a break or stoppage during placing, actions to be taken if a break or stoppage occurs, observations for segregation or air release, requirements for placing by pump, skip or chute, including positioning to induce flow and the requirements for finishing the top surfaces and curing.

11.5.8

Discharging of Ready-mixed Concrete

Discharge should not take place before control checks have taken place. RMC can be placed by pumping shown in Fig. 11.7(b) or by direct discharge from truck mixers via a chute as shown in Fig. 11.8(a). Alternatively, it can be first discharged into a skip shown in Fig. 11.7(c) (with tremie pipe) or to a pump. A receiving hopper or holding vessel with agitator may be used if necessary if the RMC is to be held on site for any length of time before placing.

(a) Concrete mixer-cum-pump

(c) Typical T bottom opening concrete skips

Fig. 11.7

(b) RMC discharged directly in forms

Discharging ready-mixed concrete directly in to formwork

Production of Concrete

369

1. Placing by pump As discussed earlier, pumping is the most common method of placing RMC. If the pump has not been primed with a cement mortar the first part of the load (100 to 150 liters) should be run through the pump and recycled back into the truck. This lubricates the pump lines, while the residual coarse aggregate is remixed into the bulk of the RMC. High flowability RMC mixes like self-compacting concrete, discussed in Section 16.5 in Chapter 16 is well suited to pumping through a valve from the bottom of the formwork provided it has good segregation resistance. This method gives a smooth and clean surface finish for any vertical concrete surface and has proved to be very successful when casting thin section walls in buildings with system formwork. It takes less air into concrete and allows faster casting rates than pumping from the top. The hopper and pump line must be kept completely full of concrete to ensure that air is not introduced at the bottom. It must also be remembered that restarting after a stop can lead to an increase in pressure on formwork. After pumping from the bottom, the valve is closed and locked. When pumping from the top, and when surface finish needs to be optimized, RMC should be placed with a submerged hose in order to minimize the possibility of entrapped air. Casting should start at the lowest part of the form, and at a place where the pumping hose can be located as close as possible to the bottom of the form. As soon as sufficient depth has built up, the hose should then be submerged into the concrete. The end of the pump hose should, if possible, be maintained below the concrete surface at all times, including when changing its location so that air is never allowed into the hose. The pumping should be controlled to produce a continuous and even rate of rise of the concrete in the formwork, with as few breaks in delivery as possible. 2. Placing by chute or skip Although casting of RMC by a pump is recommended, both concrete chute and skip have been successfully used. When discharging with a chute, the outlet from the chute should be directed towards the farthest end of the casting and withdrawn as the casting proceeds as shown in Fig. 11.8(a).

(a) RMC discharged through a chute

Fig. 11.8

(b) RMC discharged through a skip

Discharge of RMC through a chute and a skip or concrete bucket

370

Concrete Technology

The skip method shown in Fig. 11.8(b) is useful only for relatively small units or short walls with limited casting requirements (typically casting rate of 12 to 20 m3/hour). The casting rate depends on the size of the concrete skip and the maneuverability of the crane. When discharging RMC from a crane and skip, the following points should be considered: (a) The skip has to be watertight to prevent loss of mortar or paste during transport. (b) The skip should not be subject to vibration or excessive shaking to avoid segregation of the concrete. (c) A prolonged stagnation of the mix in the skip may stiffen the mix so that it will not run freely and smoothly when opened for discharge.

11.6

PLACING OF CONCRETE

The methods used in placing concrete in its final position have an important effect on its homogeneity, density and behavior in service. The same care which has been used to secure homogeneity in mixing and the avoidance of segregation in transporting must be exercised to preserve homogeneity in placing. To secure good concrete it is necessary to make certain preparations before placing. The forms must be examined for correct alignment and adequate rigidity to withstand the weight of concrete, impact loads during construction without undue deformation. The forms must also be checked for tightness to avoid any loss of mortar which may result in honeycombing. Before placing the concrete, the inside of the forms are cleaned and treated with a release agent to facilitate their removal when concrete is set. Any coating of the hardened mortar on the forms should be removed. The reinforcement should be checked for tightness and clean surface. It should also be freed of all loose rust or scales by wire brushing or any other method. Coatings like paint, oil, grease, etc., are removed. The reinforcement should be checked for conformity with the detailing plans for size, spacing and location. It should be properly spliced, anchored and embedded to a given minimum distance from the surface. Anchor bolts, pipe sleeves, pipe conduits, wiring and other fixtures should, in general, be firmly fixed in position before the concrete is placed. Rubbish, such as sawdust shavings and wire, must be blown out with compressed air. The concrete should be placed in its final position rapidly so that it is not too stiff to work. Water should not be added after the concrete has left the mixer. The concrete must be placed as closely as possible to its final position. It should never be moved by vibrating it and allowing it to flow, as this may result in segregation which will show on the surface of the finished work. When placing the concrete, care should be taken to drop the concrete vertically and from not too great height. Segregation, if it occurs, should be eliminated by taking remedial measures. The surfaces against which the fresh concrete is to be placed must be examined as to their possible effect in absorbing mixing water. For example, subgrades should be compacted and thoroughly dampened to prevent loss of moisture from concrete.

Production of Concrete

371

Where fresh concrete is required to be placed on a previously placed and hardened concrete, special precautions must be taken to clean the surface of all foreign matter and remove the laitance or scum before the fresh concrete is placed. For securing a good bond and watertight joint, the receiving surface should be made rough and a rich mortar placed on it unless it has been poured just before. The mortar layer should be about 15 mm thick, and have the same water–cement ratio as the concrete to be placed. In all cases, the base course should be rough, clean, and moistened. The surface can be cleaned by a stiff or steel broom a few hours after placement when the concrete is still soft enough to allow removal of scum but hardened enough not to permit loosening of aggregate particles. It is becoming increasingly more economical to place concrete in deep lifts. This technique saves time and reduces number of horizontal joints. For placing in deep lifts to be successful, the mix must be designed to have a low risk of segregation and bleeding. The concrete should be introduced into the forms through trunkling, as this reduces impact damage to the forms and reinforcement, and enables the layer of concrete to be built up evenly. The actual procedure depends largely upon the type of structure, the quality of concrete and of the receiving surface. In mass concrete construction, as in dams, two principal methods are employed in preparing the surface to receive the fresh concrete. For the surface having excessive laitance, it has been a common practice to remove all laitance and inferior surface concrete and to wash the mortar from the protuding aggregate by means of a high-velocity jet of air and water as soon as concrete has hardened sufficiently to prevent the jet revealing the concrete below the desired depth. Ordinarily, the surface is cut to a depth of about 3 mm. The time interval between placing and clean up operation may range from 4 to 12 hours depending upon the temperature, humidity, and the setting characteristics of concrete. This surface is thereafter protected and cured by covering it with a layer of about 40 mm wet sand until concreting is resumed, when it receives a final clean up. The final clean up is most effectively accomplished by wet sand blasting and washing. While concreting in walls, footings and other thin sections of appreciable height, the concrete should be placed in horizontal layers not less than 150 mm in depth, unless some other thickness is specified. The concreting should start at the ends or corners of forms as shown in Fig. 11.9(b) and continue towards the center. In large openings, concrete should be placed first around the perimeter. On a slope, concreting should begin at the lower end of slope to avoid cracking due to settlement. The concrete in columns and walls should be allowed to stand at least for two hours before concrete is placed in slabs or beams which they are to support. Haunches and columns capital are a part of the floor or roof and should be concreted integrally with them. Concrete in cast-in-situ piles and deep caisson footings has to be dropped from a considerable height. The concreting should be as nearly continuous as possible, because the consolidation in the lower portion of footing depends upon the impact of succeeding increments of concrete. Plastic consistency of about 10 mm (slump) is adequate. While concreting a slab, the batches of concrete should be placed against or towards preceding ones, not away from them. Batches should not be dumped in separate, individual piles.

372

Concrete Technology

(a) Pumping of concrete with a boom

(b) Concreting starts from the edges

(c) Placing high quality concrete with a highway paver

Fig. 11.9

Placing the concrete by pumping and highway paver

In placing the high quality concrete in highway or runway pavement construction, the concrete pavers of the type shown in Fig. 11.9(c) are extensively used. There are two basic methods of placing concrete pavement, namely, the fixed-form paving and slip-form paving. After the paving equipment has passed through, hand tools are used to further finish the slab. These operations are called finishing, floating or straight edging. The entire set of paving and placing machines and activities is called the paving train. On a highway project, the typical paving train, consists of a spreader or belt placer, slip-form paver, and curing and texturing machine. Smaller paving projects may use only the slip-form paver. In some paving machines the placing, compaction and finishing operations are performed by the same machine. These machines usually have electrically driven vibrators, automatic height control unit and they steer by themselves which results in

Production of Concrete

373

a high quality construction. In general, slip-form paving is preferred by contractors for large paving areas where it can provide better and economical productivity than fixed-form paving. The slip-form systems are discussed in detail in Section 11.10.

11.6.1

Construction Joints

Construction joints are a potential source of weakness and should be located and formed with care and their number is kept to a minimum. As the construction proceeds, water sometimes collects on horizontal surfaces. If this occurs, a drier mix should be used for the layer to be poured to avoid the formation of laitance. Any laitance so formed should be removed by spraying the surface with water and brushing it to expose the coarse aggregate. Preferably this should be done an hour or so after the concrete has been placed. The best joints are obtained by light brushing soon after pouring. Water bars are often installed across construction joints to provide a positive barrier against movement of water through the joint. Great care is needed when placing concrete around water bars because the space is often congested. If the concrete is not properly compacted and is honeycombed, water can pass round the water bar and its object is defeated. Insufficient care in placing may even displace the bars.

11.6.2

Effect of Delay in Placing

It is now generally recognized that there is a gain in compressive strength with delay in placing provided the concrete can still be adequately compacted. The limits imposed by the latter requirement varies with the type of mix. Only a short delay can be allowed for a dry mix in hot weather, a delay of several hours is possible with very wet mix in cold weather. According to the current specifications in general the delay between mixing and final placing of concrete is limited to between half an hour and one hour. Brook has suggested a sliding scale of half an hour for ambient temperature exceeding 20°C, three-quarters of an hour for temperature between 15°C and 20°C and one hour for temperature below 15°C. The effect of delay in placement of concrete varies with the richness of the mix and the initial slump. A low slump concrete could be compacted satisfactorily for only up to one-and-a-half hours, but high slump concrete could be compacted satisfactorily even after five hours in agitation. According to the ASTM specification C94–71, the environmental and other handling conditions are automatically taken into account by controlling the uniformity of the concrete as delivered for placement.

11.7

COMPACTION OF CONCRETE

During the manufacture of concrete a considerable quantity of air is entrapped and during its transportation there is a possibility of partial segregation taking place. If the entrapped air is not removed and the segregation of coarse aggregate not corrected, concrete may be porous, non-homogeneous and of reduced strength. The process of removal of entrapped air and of uniform placement of concrete to form a homogeneous

374

Concrete Technology

dense mass is termed compaction. Compaction is accomplished by doing external work on the concrete. The density and, consequently, the strength and durability of concrete depend upon the quality of this compaction. Therefore, thorough compaction is necessary for successful concrete manufacture. The concrete mix is designed on the basis that after being placed in forms it will be thoroughly compacted with available equipment. The presence of even five per cent voids in hardened concrete left due to incomplete compaction may result in a decrease in compressive strength by about 35 per cent. Compaction is necessary for the following reasons: 1. The internal friction between the particles forming the concrete, between concrete and reinforcement, and between concrete and formwork, makes it difficult to spread the concrete in the forms. The friction also prevents the concrete from coming in close contact with the reinforcement, thereby leading to poor bond between the reinforcement and surrounding concrete. The compaction helps to overcome the above frictional forces. 2. Friction can also be reduced by adding more water than can combine with cement. The water in excess to that required to hydrate the cement fully forms water voids which have as harmful an affect in reducing strength as air voids. Nevertheless, it is preferable to use slightly more water than run the risk of securing inadequate compaction. The compaction reduces the voids to minimum. The voids due to inadequate compaction can be readily seen when they are at the surface. The patching done to hide surface honeycombing is regarded as a very poor substitute for properly compacted concrete since it can never improve concrete which may honeycombed right through. Furthermore, badly honeycombed concrete does not allow necessary bond to be developed between concrete and reinforcement, and over a period of time the moisture may penetrate to corrode the steel.

Compaction Methods The compaction of the concrete can be achieved in four ways: (i) hand rodding, (ii) mechanical vibrations, (iii) centrifugation or spinning, and (iv) high pressure and shock. The choice of a particular technique of compaction of concrete depends upon the following factors: 1. The type of structural element. 2. The properties of the concrete mix, particularly its water–cement ratio. 3. The desired properties of the hardened concrete, i.e., strength, durability and watertightness, etc. 4. The duration of the production process and the rate of the output in the case of precast concrete products. The different methods are compared in Table 11.4.

11.7.1

Hand Rodding

Rodding is the process of ramming the concrete manually with a heavy flat-faced tool in an effort to work it around the reinforcement, the embedded fixtures, and corners of the form work. The rodding action is effective for a depth of concrete equal to five times the maximum size of aggregate and hence the depth of each layer has to be restricted to this value. The rod should penetrate to the full depth of the concrete layer

Production of Concrete Table 11.4 Method of compaction

375

Different methods of compaction

Limiting characteristics of concrete Workability

Type of concrete

Typical applications

Hand rodding

Mixes of all workabilities except very fluid and very plastic mixes.

All grades including lightweight concrete.

Flat elements like slab, etc.

Mechanical vibration

All mixes except fluid and very plastic mixes.

All grades of concrete.

All elements

Centrifugation or spinning

Plastic mix

All grades of concrete; dense and rich mixes.

Precast products having radial symmetry like poles and pipes.

Only dense concrete.

Precast elements.

Other methods like high All mixes pressure and shock

and into underlying layer if it is still plastic to ensure proper bonding of the layers. The compaction should continue until the cement mortar spreads on the surface of the concrete. Fast rodding can be done by using rodding or tamping equipment. Fast rodding produces better compaction than hand rodding. The main disadvantage of rodding is that it produces large pressures on the form work. However, such a system though better than no compaction, cannot assure a thoroughly dense and compacted concrete free of air pockets.

11.7.2

Mechanical Vibrations

Vibration is the commonly used method of compaction of concrete, which reduces the internal friction between the different particles of concrete by imparting oscillations to the particles and thus consolidates the concrete into a dense, and compact mass. The oscillations are in the form of simple harmonic motion. The mechanical vibrations can be imparted by means of vibrators which are operated with the help of an electric motor or diesel engine or pneumatic pressure. The vibration, in general, is caused by the rotation of an eccentrically loaded shaft at high speed usually greater than 2800 rpm. The tendency at present is to use higher frequencies beyond 6000 rpm (up to 15000 rpm), such a vibration being termed as highfrequency vibration. The lower frequencies cause oscillations mainly of coarse aggregate particles which transfer the oscillations to the other particles. On the other hand, the higher frequencies affect mainly the fine aggregate particles which in turn transfer the vibrations to the other particles. However, in both cases, the vibrations are communicated rapidly to the particles of concrete making it fluid and enabling it to flow around the reinforcement and enter into the corners. Any entrapped air is forced to the surface and the particles occupy a more stable position making the concrete considerably denser. The acceleration produced on the particles in the case of high frequency vibrations is of the order of 4g to 7g, where g is the acceleration due to gravity. The amplitude of oscillation is very small, of the order of 0.5 mm. The kinetic energy imparted to the concrete to cause compaction is found to depend upon the squares of the amplitude and the frequency.

376

Concrete Technology

The optimum frequency of vibration of concrete depends on the size of the particles and on the mobility or stiffness of concrete. For the concrete mix containing relatively coarser fractions of aggregate, a lower frequency of vibration with greater amplitude, and for concrete containing finer fractions, a higher frequency with lower amplitude are necessary. For all practical purposes, vibration can be considered to be sufficient when the air bubbles cease to appear and sufficient mortar appears to close the surface interstices and facilitate easy finishing operations. The period of vibration required for a mix depends upon the workability of the mix. Plastic mixes need less time of vibration than harsh or dry mixes, since the latter need more compacting energy to form dense masses. Every mix has an optimum period of vibration depending upon the characteristics of the mix. This optimum period can be estimated by conducting trials with different periods of vibration to obtain compaction without segregation and then choosing the period which gives maximum strength of concrete cubes.

Choice of Vibrators Since concrete contains particles of varying sizes, the most satisfactory compaction would perhaps be obtained by using vibrators with different speeds of vibration. Polyfrequency vibrators for compacting concrete of stiff consistency are available. The vibrators used in practice have frequency suitable for average particle size of concrete. Vibrators for compacting concrete are manufactured with frequencies of vibration from 2800 to 15000 rpm. The various types of vibrators used are described in the following subsections: 1. Immersion or needle vibrators Of the several types of vibrators, this is perhaps the most commonly used. It essentially consists of a steel tube (with one end closed and rounded) having an eccentric vibrating element inside it. This steel tube called poker is connected to an electric motor or a diesel engine through a flexible tube as shown in Fig. 11.10. They are available in sizes varying from 40 to 100 mm in diameter. The diameter of the poker is decided from the consideration of the spacing between the reinforcing bars in the formwork.

(a) Typical T needle vibrator

Fig. 11.10

(b) Compaction by a needle vibrator

Concrete compaction by needle vibrator or vibrating poker

Production of Concrete

377

The frequency of vibration varies up to 15 000 rpm. However, a range between 3000 to 6000 rpm is suggested as a desirable minimum with an acceleration of 4g to 10g. The normal radius of action of an immersion vibrator is 0.50 to 1.0 m. However, it would be preferable to immerse the vibrator into concrete at invervals of not more than 600 mm or 8 to 10 times the diameter of the poker. The period of vibration required may be of the order of 30 s to 2 min. The concrete should be placed in layers not more than 600 mm high. The vibrator can be placed vertically or at a small inclination of not more than 10° to the vertical to avoid flow of concrete due to vibration and consequent scope for segregation. The vibrator should be allowed to penetrate the concrete under its own weight during vibration. The vibrator should be removed while still running at a rate of 75 mm/s so that the hole left by the vibrator closes without any air being entrapped. The vibrator should be immersed through the entire depth of freshly placed concrete and into the layer below if this is still plastic or can be brought into plastic state (by revibration) to avoid the plane of weakness at the junction of the two layers. Internal vibrators are comparatively more efficient since all energy is utilized to vibrate the concrete unlike other types of vibrators.

External or Shutter Vibrators These vibrators called form vibrators are clamped rigidly to the formwork at the predetermined points so that both the form and concrete are vibrated. They consume more power for a given compaction effect than internal vibrators. These vibrators can compact up to 450 mm from the face but have to be moved from one place to another as concreting progresses. These vibrators operate at a frequency of 3000 to 9000 rpm at an acceleration of 4g. If external vibrators are to be used, the shuttering must be stronger and more rigid than for other types of vibrators. The formwork should also be absolutely watertight. In case parallel forms are used for the casting of a structural element, the distance between parallel shutters should not be more than 750 mm. The use of an immersion vibrator along with the form vibrator can be considered for vibration of top layer concrete, if the spacing of the reinforcement allows the pocker. This will ensure more uniform compaction of concrete in the case of wide sections. The external vibrators are more often used for the precasting of thin in-situ sections of such shape and thickness as cannot be compacted by internal vibrators.

Surface Vibrators Surface vibrators are placed directly on the concrete mass. These are best suited for the compaction of shallow elements and should not be used when the depth of the concrete to be vibrated is more than 250 mm. For example, these are used for compacting plain concrete or one-way-reinforced concrete floors, and road surfaces where immersion vibrator is impracticable. Surface vibrators can also be used as supplementary compacting equipment for vibrating the top layer of concrete when the concrete underneath is subjected to the action of immersion or form vibrators. Very dry mixes can be most effectively compacted with surface vibrators, since the vibration acts in the direction of gravity, thereby minimizing the tendency for segregation. Surface vibrators cause movement of finer material

378

Concrete Technology

to the top and hence aid the finishing operation. However, the movement of a large amount of fine material in case of plastic mixes should be avoided. The surface vibrators commonly used are pan or trowel vibrators and vibrating screeds shown in Fig. 11.11(d). The pan vibrator consists of a flat steel pan of approximate size of 400 mm × 600 mm on which an electric motor is mounted. The main application of this type of vibrator is in the compaction of small slabs, not exceeding 150 mm in thickness, and patching and repair work of pavement slabs. A vibrating screed on the other hand consists of a steel beam of 4 to 5 m length over which one or more vibrators are mounted. The operating frequency is about 4000 rpm at an acceleration of 6g to 9g. The screeds are useful for compacting flat slabs or pavements whose depth is not more than 150 mm.

Vibrating Table The vibrating table consists of a rigidly built steel platform mounted on flexible springs and is driven by an electric motor. The normal frequency of vibration is 4000 rpm at an acceleration of about 4g to 7g. The springs are so designed that they cause resonance. The moulds are rigidly clamped on the platform to enable the system to vibrate in unison. Vibration is considered adequate when the concrete develops a smooth level surface. Large vibrating tables are fitted with more than one vibrator. All vibrators should produce oscillations which are perfectly synchronized. The compaction is thorough since the vibrations are in the direction of gravity. The vibrating tables are very efficient in compacting stiff and harsh concrete mixes required for the manufacture of precast elements in the factories and test specimens in the laboratories.

rammer (a) Tamping T

(b) Mechanical trowel

(c) Vibratory plate concrete compactor

(d) Concrete vibratory screeds

Fig. 11.11

Typical surface vibrators used to compact the concrete

Production of Concrete

11.7.3

379

Prolonged Vibration and Revibration

The vibration of concrete with low water–cement ratio can be continued beyond two minutes (the time required for the satisfactory compaction of concrete). Prolonged or overvibration has been found to increase the strength appreciably from 30 s to 3 min and marginally after that for a vibration frequency of 5000 rpm. At higher frequencies of the order of 8000 rpm the strength is found to increase appreciably even after 12 min. The increase in strength due to prolonged vibration can be attributed to a decreased water–cement ratio of the concrete mass. Generally, the concrete is vibrated immediately after placement to complete its consolidation before it has stiffened. However, in order to ensure a good bond between layers, the upper part of the underlying layer should be revibrated provided the layer can still regain the plastic state. If the concrete has not already set, the mass once again becomes plastic due to the revibration and any residual air is forced out. If the concrete is at the point of the initial set of cement, the revibration disrupts the setting mass slightly and causes reconsolidation of concrete with possible expulsion of free water. If the concrete has already set, it becomes so stiff that it cannot be revibrated. Concrete can be successfully revibrated up to about four hours from the time of mixing. Revibration up to three hours after initial vibration is found to increase the 28-day compressive strength by as much as 25 per cent and bond strength of plain reinforced bars by about 30 to 50 per cent. The bond strength at first slip is increased by almost 100 per cent compared to the unvibrated reinforced concrete. If retarders are used, the concrete can be revibrated up to 10 hours after placing. Revibration is preferred when watertightness is required. It can also be advantageously used for the manufacture of precast and pre-stressed elements. The vibration technique is suitable only for properly graded or designed concretes. While using vibrators for compacting concrete mixes, the following general points should be kept in mind: 1. Vibration should not be used as a means of spreading the concrete in forms, as this may result in the segregation of coarse aggregate. 2. The prolonged vibration of concrete mixes with a slump of more than 100 mm entails their being segregated causing smaller and lighter constituents of the mix to rise to the surface causing a layer of mortar or even laitance on the surface. The resulting concrete may have honeycombing at the bottom and a dusty surface at the top. This causes planes of weakness is succeeding layers. In the case of a single layer, the surface lacks resistance to abrasion. 3. Vibration may reduce the entrained air in the air entrained concrete to about 50 per cent. Hence air entrainment should be doubled if the mix is to be vibrated. Undervibration should not be resorted to for fear of expelling the entrained air. 4. When concrete is compacted by internal vibrators, the thickness of the layers placed should not exceed the depth of the operating part of the vibrator by more than 25 per cent. 5. The period for which a vibrator is kept in one position should be such as to ensure adequate compaction, taking into account the stiffness of the mix

380

Concrete Technology

and the thickness of the compacted layer. The adequacy of the compaction is indicated by no further settlement of concrete, appearance of slurry on surface, and disappearance of rising bubbles. 6. On completion of compaction at one place, the vibrator is transferred to another place. The distance between successive positions of the vibrator must not exceed one-and-a-half times its radius of operation. When an internal vibrator is transferred, it must be removed slowly by switching off the meter. 7. The internal vibrator must be set up at a distance not exceeding 50 to 100 mm from the wall of the form. The reinforcing bars must not be touched by the operating vibrator as the bond between reinforcement and concrete may be disturbed by vibration. 8. The compaction of concrete with surface vibrator is carried out in straight continuous strokes with a 100 to 200 mm overlapping on the previously compact area. The vibration time at a position may be approximately 30 to 60 s depending upon the mobility of concrete and as verified by external indications discuss in point (5). 9. The surface vibrator should be withdrawn by an upward jerk and not pulled slowly through the concrete. 10. The external vibrator should be firmly clamped to the form as otherwise, its efficiency is reduced drastically. The time of vibration using external vibrators varies from 60 to 90 s. 11. The vibrators must be switched-off at regular intervals to allow cooling of the meters. 12. The total number of vibrators on the site should be about 30 to 50 per cent more than the calculated number. In oversanded concrete mixes, a phenomenon called rotational instability occurs during compaction using vibration technique. The coarse aggregate particles coated with cement–mortar form nearly ball-like particles. These particles, during compaction, do not consolidate and settle in a dense mass but continue to rotate about a horizontal axis passing through them. This phenomenon is found to occur in cases where low-frequency (below 6000 rpm) and high-amplitude (above 0.13 mm) vibrations are employed as a means of compaction. Under this condition, the air is sucked into concrete and entrapped, causing reduction in strength. This phenomenon is found to occur particularly when vibrating low slump concrete in narrow sections on a vibrating table.

11.7.4

Centrifugation or Spinning

The method is used in the production of elements which are circular in cross-section, such as concrete pipes, concrete lamp posts, etc. It comprises feeding the concrete into the horizontal mold spinning at a low speed. After the predetermined amount of concrete is fed into the mold, the spinning speed is increased to a high value. The water is forced out of the mix which flows out of the mould. At the end of the spinning process, the speed is slowly reduced and dry cement is sprinkled in small quantity such that

Production of Concrete

381

any free water on the surface does not increase the local water-cement ratio. A round rod is held against the two end-rings to finish the surface. The initial water-cement ratio for effective compaction without segregation should be between 0.35 and 0.40. The final water-cement ratio after spinning reduces to about 0.30. High speed of rotation and prolonged centrifugation may cause segregation of concrete. The coarser particles, due to their higher mass, consolidate on the outer face of the product. The optimum speed and duration of spinning depend upon the diameter of the pipe and quality of mix. The segregation can be minimized by adopting a continuous grading curve for the aggregate. The centrifugation results in a watertight product and hence is used in the manufacture of both pressure pipes for water-supply and non-pressure pipes for sewerage disposal and storm water drains.

11.7.5

Vibropressing

The method comprises applying external pressure from the top and vibration from below the mold. The vibration tables can be used for this purpose. The excess water added during the mixing is forced out due to large pressure. The water-cement ratio at the end of vibropressing can be reduced to a value as low as 0.30. The product obtained by this process is of extremely good quality and durability. The technique has been successfully used for mass manufacturing of concrete kerbs, etc.

11.7.6

Other Methods

Jolting This method of compaction consists of subjecting the mould containing the concrete to a series of jolting actions at a frequency of 100 to 150 jolts per minute. This jolting is in effect a vibrating action of a low frequency and high amplitude. The cams used for this purpose raise the mould by about 12 mm and then allow it to fall to its original position under gravity. The method is quite effective for dry mixes and is used for the manufacture of precast concrete products.

Rolling It is a continuous pressing operation for compacting the soft and plastic concrete obtained by previbration. The previbrated concrete is fed continuously in between rubber roller employing pressures up to 50 atmospheres which force out the excess water in the concrete. The continuity of production makes it best suited for automated factory production of very thin concrete products like concrete tiles.

11.8

FINISHING OF CONCRETE

The requirements of finishing concrete depend on the type of structural element and its intended service use, e.g., minimal finishing is required for a beam, whereas careful finishing is required for flat surfaces like roads and airport runway pavements; domestic and office floors. The shortcomings in concrete appearance observed during casting are rectified and concrete is made to exhibit a pleasant surface finish using special techniques. The surface finishes are generally grouped in three categories, namely, formwork finishes, surface treatments and applied finishes.

382

Concrete Technology

Concrete slabs can be finished in many ways, depending on the intended service use. Options include various colors and textures, such as exposed aggregate or a patterned-stamped surface. Some surfaces may require only strike off or screeding to proper contour and elevation, while for other surfaces a broomed, floated or troweled finish may be specified. In slab construction, screeding is the process of cutting off excess concrete to bring the top surface of the slab to proper grade. A straight edge is moved across the concrete with a sawing motion and advanced forward a short distance with each movement as explained in Fig. 11.12.

(a) Concrete being placed and screeded in forms

Fig. 11.12

(b) Screeding surface level with the form

Placing the concrete and screeding the surface level with the forms

Vibratory screeds shown in Figs. 11.13(a) and (b) strike off slab surface level with the forms, eliminate high and low spots, and embed large aggregate particles immediately after strike off. They consist of one or more straight edges or beams fitted with mechanical vibrators at their topes which are pulled across the concrete.

(a) Single-beam vibrating screed

Fig. 11.13

(b) Heavy-duty two-beam vibrating screed

Finishing slab surface with manual vibratory screeds

Control joints are required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the fresh concrete. Saw cut joints can be made after the concrete is sufficiently hard enough to prevent raveling. After making the grooves, the concrete should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the sur-

Production of Concrete

383

face; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations. Where a smooth, hard, dense surface is desired, floating should be followed by steel troweling. Troweling should not be done on a surface that has not been floated. A slip-resistant surface can be produced by brooming, as illustrated in Fig. 11.15(d), before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression. Some of the typical basic hand tools used for finishing the small concrete slabs are shown in Fig. 11.14. EDGING TOOL

DARBY

GROOVER

The derby

Fig. 11.14

The edger

The groover

Hand tools used for finishing of small slabs

The fundamentals of finishing operations for concrete slabs are described below in terms of hand finishing. Each step in the finishing procedure requires a different tool.

11.8.1

Steps for Manual Finishing of Concrete Surface

1. Concrete placed in the form is leveled with the top of the forms with a screed board, a straight edge of 50 mm × 100 mm, about 300 mm longer than the width of the slab. Leveling of the concrete is started as soon as pouring is finished. The screed board is placed on the forms, and pushed and pulled with a sawing motion toward the end of the pour. Screeding is repeated over the same area to remove excess concrete. 2. To smoothen or flatten the concrete surface, draw a bull float or sweep the darby across the concrete in overlapping arcs as shown in Fig. 11.15(a) over the fresh concrete immediately after screeding to force down the lumps and fill the lower spots left during screeding with fresh concrete. This process raises scum (gravel-free concrete) near the surface for finishing. Two passes over the surface with the darby are enough. Overworking the concrete will draw too much cement and fine sand to the top and result in a weak surface. 3. The surface finish is commenced, after the bleed water and sheen has disappeared entirely, with a hand float using sweeping motions, starting at the beginning of the pour. Working the concrete before the surface bleed water disappears will weaken the surface of the slab when it dries. The concrete is ready for surface finish when pressing hard with the gloved thumb onto the surface near the perimeter leaves about a 5-mm deep impression.

384

Concrete Technology

(a) Flattening concrete surface by durby

(c) Cutting control joints by a groover

Fig. 11.15

(b) Rounding corners with an edging tool

(d) A broomed non-slip surface

Various hand-finishing operations

4. The outside edges and corners are rounded and compacted with an edging tool as illustrated in Fig. 11.15(b). The edging tool is worked back and forth around the perimeter, using the edge of the form as a guide. If the edger leaves a path deeper than about 3 mm then the concrete is allowed to set a little longer and less downward pressure is applied. 5. The slabs need control joints or grooves at an interval to control cracking due to drying and base movement. The groove creates a weakened spot for the crack to develop where it would not be seen. To be effective, the depth of the groove must be at least one-fourth the thickness of the slab. In small slabs, grooves are added at about every 1.15 m. However, in bigger slabs evenly spaced control joints are cut in every 1.5 to 2 m with a groover. 6. To add a groove, a straight board is placed along the predetermined mark. The groover moves back and forth against the straightedge until the bed of the tool is in contact with the concrete surface as illustrated in Fig. 11.15(c). 7. When the edging and grooving are completed, the concrete is floated to blend in the marks left by the edger and groover. The edging and grooving steps are repeated after floating and troweling to refine the groove and edges. Troweling step is repeated two or three times with the concrete hardened a bit between each pass. For a rougher, non-slip surface or in case of air-entrained concrete, instead of troweling, the surface is broom finished.

Production of Concrete

385

Dragging a broom across partially hardened concrete leaves a rough texture that gives better traction in slippery conditions. The bristles are washed off as soon as brooming is finished. If concrete starts to pile up in front of the bristles, the broomed area re-smoothened with a float or trowel, and the step is repeated after the lapse of a little longer period. For brooming the concrete, the broom is gently placed on the far side of the slab and slowly pulled in and off the edge of the form as illustrated in Fig. 11.15(d). The brooming is carried down to the end of the slab, overlapping the previous sweep by about 150 mm. If clumps of concrete start gathering or the texture is too rough, the concrete is still too wet to broom, broomed-over areas are refloated to smoothen out the marks, and brooming may be carried out again in about 15 minutes.

11.9

CURING OF CONCRETE

The physical properties of concrete depend largely on the extent of hydration of cement and the resultant microstructure of the hydrated cement. Upon coming in contact with water, the hydration of cement proceeds both inwards and outwards in the sense that the hydration products get deposited on the outer periphery of cement grains, and the nucleus of unhydrated cement inside gets gradually diminished in volume. At any stage of hydration, the cement paste consists of the product of hydration (called gel because of its large area), the remnant of unreacted cement, Ca(OH)2 and water. The product of hydration forms a random three-dimensional network gradually filling the space originally occupied by the water. Accordingly, the hardened cement paste has a porous structure, the pore sizes varying from very small (4 × 10−10m) to very large and are called gel pores and capillary pores. As the hydration proceeds, the deposit of hydration products on the original cement grains makes the diffusion of water to the unhydrated nucleus more and more difficult, and so the rate of hydration decreases with time. Therefore, the development of the strength of concrete, which starts immediately after setting is completed, continues for an indefinite period, though at a rate gradually diminishing with time. 80 to 85 per cent of the eventual strength is attained in the first 28 days and hence this 28-day strength is considered to be the criterion for the design and is called characteristic strength. As mentioned above, the hydration of cement can take place only when the capillary pores remain saturated. Moreover, additional water available from an outside source is needed to fill the gel pores which will otherwise make the capillary empty. Thus, for complete and proper strength development, the loss of water in concrete from evaporation should be prevented, and the water consumed in hydration should be replenished. Thus the concrete continues gaining strength with time provided sufficient moisture is available for the hydration of cement which can be assured only by creation of favorable conditions of temperature and humidity. This process of creation of an environment during a relatively short period immediately after the placing and compaction of the concrete, favorable to the setting and the hardening of concrete, is termed curing. The desirable conditions are: a suitable temperature, as it governs the rate at which the chemical reactions involving setting and hardening take place; the provision of ample moisture or the prevention of loss of moisture; and the avoidance

386

Concrete Technology

of premature stressing or disturbance. All the care taken in the selection of materials, mixing, placing and compaction, etc. will be brought to nought if curing is neglected. The curing increases compressive strength, improves durability, impermeability and abrasion resistance.

11.9.1

Curing Conditions

Proper curing practice is one of the important steps in making high-quality concrete. A good mix design with low water–cement ratio alone cannot ensure good concrete. The favorable conditions to be set up at early hardening periods for best results are: 1. Adequate moisture within concrete to ensure sufficient water for continuing hydration process. 2. Warm temperature to help the chemical reaction. In addition, the length of curing is also important. The first three days are most critical in the life of Portland cement concrete. In this period the hardening concrete is susceptible to permanent damage. On an average, the one-year strength of continuously moist cured concrete is 40 per cent higher than that of 28-day moist cured concrete, while no moist-curing can lower the strength to about 40 per cent. Moist curing for the first 7 to 14 days may result in a compressive strength of 70 to 85 per cent of that of 28 days moist curing as shown in Fig. 11.16. 42

In ai a r after 28

In air after 14 days

35

Compressive Strength, MPa

days

In air after 7 days 28

In air after 3 days Continuously moist curing

21

Stored continuously in air Mix data: W/C—0.50, Slump—85 mm Cement content—330 kg/m3 Sand—36 per cent Air content—4 per cent

14

7 037 14

Fig. 11.16

28

90 Age, days

180

Strength of concrete dried in air after preliminary moist curing

It has been observed that the hydration takes place only when the vapor pressure in the capillaries is more than 80 per cent of the saturation pressure. The rate of hydration is maximum at the saturation pressure and is minimum at three times the saturation pressure. The vapor pressure in capillaries reduces with the passage of time

Production of Concrete

387

resulting is a reduction of rate of hydration and hence of development of strength. It is necessary to prevent even a small loss of water during the process of hardening. If the concrete is left in air, i.e., without any method of curing being adopted, there is a continuous loss of moisture due to evaporation and self-desiccation. The rate of evaporation depends upon the temperature and the relative humidity of the surrounding air and on the velocity of wind. An air-cured concrete develops considerably less strength compared to the moist-cured concrete as is seen in Fig. 11.16. The rate of development of strength with curing period is given in Fig. 11.17. 100 Grade e of concrete: M15 M e ratio: 0.55 Water–cement

Strength, per cent

80

Ord dinary Portttland Cement

60 Portland–Pozzolana –P Cement 40

20

0

0

3

7

14

28

Age, days

Fig. 11.17

Development of strength with curing period

The rate of development of strength not only depends on the period of curing but also on the temperature during the period of curing. The influence of temperature on the strength is shown in Fig. 11.18. It can be seen that the optimum temperature during the curing period is 15°C to 38°C. The ambient temperature in most parts of India provide warmth required for satisfactory hydration.

11.9.2

Maturity of Concrete

Since the strength of concrete depends on both the period of curing (i.e., age) and temperature during curing, the strength can be visualized as a function of period and temperature of curing. The product (period × temperature) is called the maturity of concrete. Here the temperature is reckoned from −10°C which is a reasonable value of the lowest temperature at which an appreciable increase in strength can take place and the period in hours or days. The maturity of concrete is measured in °C hours or °C days. The strength of concrete is found to increase almost linearly with its maturity as shown in Fig. 11.19. The strength of concrete at any maturity can be expressed as the percentage of strength for the maturity of concrete cured at 18°C for 28 days, i.e., (18 + 10) × (28 × 24) = 18800°C-hrs.

388

Concrete Technology 200

Temperatures A:A = 49 °C B:B = 40 °C C:C = 32 °C D:D = 13 °C E:E = 4 °C F:F = − 4 °C

Percentage of 23 °C Compressive Strength

A

160

B C

120

D E F C B A

100 23° 80

D

40

E 0

F 1

0

Fig. 11.18

7 28 90 Age of T Test, day (log scale)

3

365

Effect of curing temperature on compressive strength of concrete

The requisite maturity factor recommended for minimum curing for OPC is 4200°C-Hrs (preferably 6000°C-Hrs). In the case of high early-strength cement, a maturity factor of 2400°C-Hrs is recommended.

Compressive Strength, MPa

30

41h 20

24h 20h

23h

18h 10 Water–C Cement Ratio:: 0.41 Coarse Aggregate: Crushed Limestone Lim Li

0 400

Fig. 11.19

600

800 1000 Maturity, y h °C

1200

1400

Relationship between maturity and compressive strength

Production of Concrete

389

The increase in strength with increased curing temperature is due to the speeding up of the chemical reactions of hydration. This increase affects only the early strengths without affecting ultimate strengths. Hence, curing of concrete and its gain of strength can be speeded up by raising the temperature of curing, thereby reducing the curing period. This type of curing called accelerated curing has many applications in the manufacture of precast concrete products.

11.9.3

Curing Periods

To develop design strength, the concrete has to be cured for up to 28 days. As the rate of hydration, and hence the rate of development of strength, reduces with time, it is not worthwhile to cure for the full period of 28 days. IS: 456–2000 stipulates a minimum of seven-day moist-curing, while IS: 7861 (Part-1)–1975 stipulates a minimum of 10 days under hot weather conditions. Highearly-strength cements can be cured for half the periods suggested for OPC. For Pozzolana or blast-furanace-slag cements, the curing periods should be increased. There are many opinions on the length of curing period. Periods varying from 13 to 30 days are specified for highway pavements. There cannot be a definite mandate on this matter as there are too many variables involved, such as the type of cement, ambient temperature, nature of the product, method of curing adopted, etc. Generally, increased curing periods are desirable for high-quality concrete products, concrete floors, roads and airfield pavements. The variation of compressive strength with the curing period is given in Fig. 11.20. 60

Compressive Strength, MPa

Water–Cement Ratio: 0.6 50 Blast Furnace rn Slag Cement 40 Ordinary Portland Cement

30 20 10 0

0

7

28

91 Age, Days

Fig. 11.20

11.9.4

Variation of compressive strength with curing period

Methods of Curing Concrete

There are various methods available for curing. The actual procedures used vary widely depending on the conditions on the site, and on the size, shape and position of the member. The methods can be broadly classified as:

390

Concrete Technology

1. The methods which replenish partly the loss of water by interposing a source of water, or prevent the evaporation, viz. (a) Ponding of water over the concrete surface after it has set This is the most common method of curing the concrete slab or pavements and consists of storing the water to a depth of 50 mm on the surface by constructing small puddle clay bunds all around as illustrated in Fig. 11.21(a). Ponding may promote efflorescence by leaching. (b) Covering the concrete with wet straw or damp earth In this method, the damp earth or sand in layers of 50 mm height are spread over the surface of concrete pavements. The material is kept moist by periodical sprinkling of water. (c) Covering the concrete with wet burlap The concrete is covered with burlap (coarse jute or hemp) shown in Fig. 11.21(b) as soon as possible after placing, and the material is kept continuously moist for the curing period. The covering material can be used a number of times and, therefore, tends to be economical. The effectiveness of the method as compared with the ponding is shown in Fig. 11.22.

(a) Curing of concrete slab by ponding

(c) Slab covered with waterproof paper

Fig. 11.21

(b) Curing of concrete columns

(d) Liquid membrane with hand spraying

Typical methods of curing concrete

Production of Concrete 70

Compressive Strength, MPa

60

Ponded ed Hessian a

391

Age at Tes Ag st s in days

50

28

40 30 3 20 1 10 0 0.28

Fig. 11.22

0.40 0.32 0.36 Water–Cement Ratio

0.44

Effect of curing condition on the compressive strength of concrete

(d) Sprinkling of water This is a useful method for curing vertical or inclined surfaces of concrete wherein the earlier methods cannot be adopted. The method is not very effective as it is difficult to ensure that all the parts of concrete be moist all the time. The spraying can be done in fine streams through nozzles fixed to a pipe spaced at set intervals. Flogging is done in the same way except that the flogging nozzles produce a mist-like effect, whereas spraying nozzles shed out fine sprays. 2. The methods preventing or minimizing the loss of water by interposing an impermeable medium between the concrete and the surrounding environment are as follows. (a) Covering the surface with waterproof paper Waterproof paper prevents loss of water in concrete and protects the surface from damage. The method is satisfactory for concrete slabs and pavements. A good quality paper can be often reused. The paper is usually made of two sheets struck together by rubber latex composition. Plastic sheeting is a comparatively recent innovation as a protective cover for curing concrete. Being light and flexible, it can be used for all kinds of jobs, effectively covering even the most complex shapes. Several types of sheets, which are guaranteed to give excellent results consistent with economy and can be used over and over again, are available. Most plastic sheetings used in the concrete industry are milky or white in appearance as shown in Fig. 11.21(c), and this helps keep the, concrete temperature at a reasonable level. Plastic sheeting can be welded at the site instead of resorting to large overlaps and made airtight to prevent moisture evaporation from concrete.

392

Concrete Technology

(b) Leaving the shuttering or formwork on The thick watertight formwork also prevents the loss of moisture in concrete and helps in curing the sides and the base of the concrete. (c) Membrane curing of the concrete The process of applying a membrane forming compound on concrete surface is termed membrane curing. Often, the term membrane is used not only to refer to liquid membranes but also to a solid sheeting used to cover the concrete surface. The curing membrane serves as a physical barrier to prevent loss of moisture from the concrete to be cured. A curing liquid membrane should dry within three to four hours to form a continuous coherent adhesive film free from pinholes and have no deleterious effect on concrete. Curing with a good membrane for 28 days would give strengths equivalent to two weeks moist curing. Membrane curing shown in Fig. 11.21(d) may not assure full hydration as in moist curing, but is adequate and particularly suitable for concrete members in contact with soil. Following are the different sealing compounds used: (i) Bituminous and asphaltic emulsion or cutbacks (ii) Rubber latex emulsions (iii) Emulsions of resins, varnishes, waxes, drying oils and water-repellant substances (iv) Emulsions of paraffin or boiled linseed oil in water with stabilizer Sealing compounds are used only after testing for their efficiency. For effective sealing of the surface, two uniformly applied coats of the compound may be necessary. These are generally applied to the interior surfaces not directly exposed to the sun. The quantity of emulsions required per square meter is about 0.1 gallon. Application of membrane should be started immediately after the water sheet has disappeared from the concrete. The solid membranes have been found to be superior to the liquid membranes. Membrane curing should not be adopted if the water–cement ratio is less than 0.5, lest the phenomenon of self-desiccation would weaken the concrete by progressively reducing the space available for hydrated products. (d) Chemical curing It is accomplished by spraying the sodium silicate (water glass) solution as shown in Fig. 11.21(d). About 500 g of sodium silicate mixed with water can cover 1 m2 of surface and forms a hard and insoluble calcium silicate film. It actually acts as a case hardener and curing agent. The application of sodium silicate results in a thin varnish like film which also fill pores and surface voids, thus sealing the surface and preventing the evaporation of water. 3. Methods involving the application of artificial heat while the concrete is maintained in a moist condition are used in–plant curing where the curing of concrete is accelerated by raising its temperature. The accelerated process of curing has many advantages in the manufacture of precast concrete products since; (i) the molds can be reused within a shorter time; (ii) due to reduced period of curing the production is increased and the cost reduced, and (iii) storage space in the factory is reduced.

Production of Concrete

393

The temperature can be raised in practice by (a) Placing the concrete in steam. (b) Placing the concrete in hot water. (c) Passing an electric current through the concrete.

11.9.5

Steam Curing

For concrete mixes with water–cement ratio ranging from 0.3 to 0.7, the increased rate of strength development can be achieved by resorting to steam curing. The mixes with low water–cement ratio respond more favourably to steam curing than mixes with higher water–cement ratio. In steam curing, the heating of the concrete products is caused by steam either at low pressure or high pressure. The method ensures even heating of products all over, even if the space between the stacked precast concrete products is very small. A number of considerations govern the choice of steam curing cycle, namely, the precuring period, the rate of increase and decrease of temperature, and the level and time of constant temperature. An early rise in temperature at the time of setting of concrete may be detrimental to concrete because the green concrete may be too weak to resist the air pressure set up in the pores by the increased temperature. Too high a rate of increase or decrease in temperature introduces thermal shocks and the rates should generally not exceed 10 to 20°C per hour. The higher the water–cement ratio of concrete, the more adverse is the effect of an early rise in temperature. Therefore, to meet the requirement of compressive strength of concrete, the temperature and/ or time required for curing can be reduced by having a lower water–cement ratio. While in identical time cycle, the higher the maximum temperature, the greater is the compressive strength. The advantages of curing above 70°C are negated by dilatational tendencies due to the expansion of concrete. All the above-mentioned factors lead to the conclusion that for a concrete of specified composition and curing period, there is an optimum curing temperature which will result in maximum compressive strength at the end of the curing cycle. It has been suggested that the steam curing of concrete should be followed by water curing for a period of at least seven days. This supplementary wet curing is found to increase the later age strength of steam-cured concrete by 20 to 35 per cent. In the case of concretes with high water–cement ratios, a rapid rate of temperature rise during steam curing may result in lesser 28-day strength than that of normally cured concrete, even though the initial rate of development of strength is higher than that for normal curing. The rapid temperature rise may also result in the reduction of bond strength. On the other hand, if a slow rate of temperature rise is adopted, the 28-day strength will almost be equal to that of normally cured concrete and there is no deleterious effect on bond. In most cases, steam curing is employed only for achieving 50 to 70 per cent of specified strength in a short period instead of full treatment for two to three days required to obtain specified strength. This would result in the economy in the reuse of molds and equipment by achieving stripping strength which is normally about 50

394

Concrete Technology

per cent of the specified strength. The stripping strength will be sufficient to take care of any impacts which may be produced during their demolding and transportation to the stackyard. The strength which the concrete should develop to enable the despatch and transport of the products from the storeyard should be about 75 per cent of the specified strength. This strength is termed delivery strength.

Low Pressure Steam Curing Steam curing at atmospheric pressure can be continuous or intermittent. In the continuous process, the products move on conveyor belts from one end of a long curing chamber to the other end, the length of the chamber and the speed of movement of the conveyors being so designed that the products remain in the curing chamber for the required time. On the other hand, in the intermittent process, the concrete products are stacked in the steam chamber and the steam is allowed into the closed chamber. The steam curing cycle can be divided into three stages: (i) heating stage, (ii) steam treatment, and (iii) cooling stage. In the normal steam curing procedure, it is advisable to start the steam curing a few hours after casting. A delay of two to six hours—called the presetting or presteaming period—depending upon the temperature of curing, is usual. The presteaming period helps to achieve a 15 to 30 per cent higher 24-hour strength than that obtained when steam curing is resorted immediately. The rate of initial temperature rise after presteaming period is of the order of 10 to 20°C per hour and the maximum curing temperature is limited to 85 to 90°C. A temperature higher than this does not produce any increase in the strength of concrete and in fact, as discussed earlier, a temperature of 70°C may be sufficient. For a particular product, the maximum desired temperature is raised at a moderate rate and then the steam is cut off, and the product is allowed to soak in the residual heat and moisture of the curing chamber. The product after steamcuring and cooling off to 30°C, should be kept in a warm room at a temperature of about 25°C before being exposed to the outside atmosphere of lower temperature. By adopting a proper steam-cycle, more than 70 per cent of the 28-day compressive strength of concrete can be obtained in about 16 to 24 hours. The steam curing cycle depends upon: 1. the type of cement, 2. the aimed stripping and delivery strengths, and 3. the accelerator. A typical steam curing cycle is given below:

Presteaming period Temperature rise period Period of maximum temperature Cooling off period

3 hours 4 hours 4 hours 5 hours

High-Pressure Steam Curing In the case of normal steam curing at atmospheric pressure, the ultimate strength of concrete may be adversely affected if the temperature is raised rapidly. This difficulty can be overcome by employing the steam at a pressure of eight atmospheres. The process is termed high-pressure steam curing. High-pressure steam curing is done in the cylindrical steel chambers called autoclaves. The concrete products, after a suitable presteaming period, are wheeled

Production of Concrete

395

on racks into the autoclaves. The steam is let in gradually until the prescribed pressure or temperature (generally about 1 MPa or 185°C) is reached. This heating stage should be completed and the prescribed pressure reached in about three hours. The increase in temperature allowed is up to 50°C in the first hour, up to 100°C in second hour and up to 185°C in the third hour. The period of treatment under full pressure depends upon the strength requirements. This period is 7 to 10 hours for hollow block products and 8 to 10 hours for slab or beam elements, the period increasing with the thickness of concrete. The steam is cut off and the pressure is released after the completion of this stage and the products are left in the autoclaves for two hours for cooling off gradually. High-pressure steam curing is usually applied to precast products when any of the following characteristics is desired: 1. High-early strength With high-pressure steam curing, the compressive strength at 24 hours is at least equal to that of 28-day normally cured products. 2. High durability High-pressure steam curing results in an increased resistance to sulfate action and other forms of chemical attack, and to freezing and thawing. During the hydration of cement at higher temperatures, the calcium hydroxide released as the result of primary reaction, reacts with finely divided silica, which is present in the coarse and fine aggregates, forming a strong and fairly unsoluble compound. This results in higher concrete strengths. Leaching and efflorescence are minimized due to reduction in free calcium hydroxide content. The hydrating dicalcium silicates and tricalcium aluminates react together at high temperatures to form sulfate-resisting compounds. Hence autoclaved products show higher resistance to sulfate attack. The initial drying shrinkage and moisture movements are also considerably reduced. On the debit side, high-pressure steam curing reduces the bond strength to 50 per cent of that obtained with normally cured concrete. Hence steam curing of reinforced concrete members is not recommended.

11.9.6

Curing of Concrete by Infrared Radiation

The curing of concrete by infrared radiation has been used in Russia. It is claimed that a much more rapid gain of strength can be obtained than even with steam curing. The rapid initial rise of temperature does not result in a decrease in the ultimate strength as it does in the case of steam curing. The system is described as particularly applicable to the manufacture of hollow concrete products, where the heaters are placed in the hollow spaces of the product. The normal operative temperature is 90°C.

11.9.7

Electrical Curing of Concrete

Concrete products can be cured by passing alternating current of low voltage and high amperage through electrodes in the form of plates covering the entire area of two opposite faces of concrete. The potential difference generally adopted is between

396

Concrete Technology

30 and 60 V. Evaporation is prevented by using an impermeable rubber membrane on the top surface of the concrete. Initially up to three hours, the resistance of concrete to flow of current decreases due to rise in temperature. There is rise in resistance afterwards, due to decrease in the quantity of free water available in the concrete due to hydration and evaporation. This period of rise in temperature should be about 12 hours. The duration of electrical curing should be about 48 hours at the temperature of 50°C or 36 hours at the temperature of 70°C. The concrete products are cooled gradually in heat insulated chambers for a minimum period of 24 hours. By electrical curing, concrete can attain the normal 28-day strength in a period of 3 days. The technique is expensive and is not used in India.

11.9.8

Effects of Delayed Curing

The concrete specimens which were placed in laboratory air for varying periods after casting before being moist cured, have indicated that the strength at 7 to 28-days decreases progressively as the period of air curing is increased. An exposure for three days to air at a temperature of 23°C and having a relative humidity of approximately 60 per cent before being moist cured at 23°C has been found to reduce the seven day strength by 12 per cent and the 28 day strength by about 10 per cent. The specimens left in air at 23°C for the entire curing period have shown a reduction of 25 per cent in the strength at 7 and 28 days as compared with standard moist curing. The reduction under field conditions would probably have been greater. Similar adverse curing causes greater relative reduction in strength when Portland blast-furnace-slag cement and the cements blended with fly ash are used.

11.10

FORMWORK

Though formwork generally forms a part of concrete construction practice, but as it influences the performance of hardened concrete appreciably, its salient features are described in brief in the following sections. The formwork or shuttering may be defined as molds of timber or some other material into which the freshly mixed concrete is poured at the site and which hold the concrete till it sets. The formwork includes the total system of support of freshly placed concrete, i.e., form lining and sheathing plus all necessary supporting members, bracings, hardware and fasteners. Concrete construction practices directly affect formwork requirements. It is more than simply making forms of right size. A good formwork should be strong, stiff, smooth and leakproof. As the cost of formwork may be of the order of 20 per cent of the cost of project, it is essential that the forms be properly designed and detailed to effect economy without sacrificing strength and efficiency. It must be realized that smoothness of the external surface is not the main objective.

11.10.1

Requirements of Formwork

Quality The formwork is designed and built accurately so that the desired shape, size, position and finish of cast concrete is obtained, and thus

Production of Concrete

397

1. all lines in the formwork should be true and the surface be plane, so that the cost of finishing the surface of concrete on removal of shuttering is the least, and 2. the formwork should be leakproof.

Safety The formwork is built substantially so that it is strong enough to support the dead and live loads during construction without collapse or danger to workmen or the structure. The joints in the formwork should be rigid to minimize the bulging, twisting or sagging due to dead and live loads. Excessive deformations may disfigure the surface of the concrete. Economy The formwork should be built efficiently to save time and money for the contractor and owner alike. After the concrete has set, the formwork should be easily strippable without damage so that it can be used repeatedly.

11.10.2

Formwork Planning

The formwork is planned in such a way that it becomes an integral part of the total job plan. The above objectives are usually emphasized in the planning. i.e., the planning for maximum reuse, economical form construction, efficient setting and stripping practices, and safety from all causes of formwork failure. Generally, the butted-and-cleated types of joints are preferable in the construction of formwork. All the formwork should be so designed and constructed that it can easily be stripped in the desired order after the setting of concrete, and no piece of formwork gets keyed into the mass of concrete. The shuttering must be chamfered at the junctions to facilitate its free and easy withdrawal. If nails are used they should be driven until they are in the concrete surface and their heads should be slightly projected outside for easy removal. The nails should preferably be driven at an angle both ways. To prevent the concrete from sticking to the forms, the interior surface of forms should be coated with a thin layer of mineral oil, soft soap or be white washed. Sometimes linings of oil paper or canvas, etc., are also employed. The formwork should be properly inspected by the engineer in-charge. Only after making sure that the formwork is properly made should the concreting be allowed. During concreting the formwork should be continuously observed for bulges and other signs of failure. Small cleats, wedges and bolts, etc., should be put in separate boxes and should not be thrown indiscriminately.

11.10.3

Types of Formwork

The material used in the formwork largely depends upon the availability and cost. Usually, the timber scantlings consisting of softwood planks and joists are very suitable. However, for big projects where the forms are to be used repeatedly, steel formworks are commonly used.

Timber Formwork The timber used for formwork should be cheap, easily available and easy to work manually and on machines. A good timber for formwork should be light for easy handling and lifting, stiff for not giving excessive deflections, usually free from knots, knot holes, bad flaws, etc., which may cause failure. Sawn timber is preferable

398

Concrete Technology

for rough surfaces to be rendered afterwards. Planned timber gives smooth surface. The timber used should not be green, as it would then become dry and shrink, and at the same time not too dry as it would absorb water from concrete. Partially seasoned timber is the most suitable. In case dry or green timber is used, suitable allowances for bulging and shrinkage should be made in preparing the surface. The face of timber that would be in contact with concrete should be properly dressed and its sides should be truly plane for providing water tight joints with adjoining pieces. To take care of any sags in beams, the forms are given a camber of 1:1500 along the length. Timber sheathing can either be square-edged type which is easily strippable and sturdy but liable to leakage, or tongued-and-grooved which does not allow leakage. The latter type, where stripping and cleaning take more time is suited for high class work. For timber formwork, due to its temporary nature, higher stress may be allowed in design than that used in permanent timber works. The soft timber used for the formwork can be assumed to have a linear stress–strain relation with the modulus of elasticity as 9.8 × 103 MPa. The allowable stresses for Chir of density of 575 kg/m3 are as follows: Bending stress Compressive stress parallel to grains Compressive stress across the grains Shear stress parallel to grains

8.4 MPa 6.4 MPa 2.6 MPa 0.92 MPa

The stresses are based on the assumption that the timber will remain dry. In case it is subjected to alternate wetting and drying, the stresses should be reduced to 0.85 times the value given above and, if continuously wet, to 0.65 times these values. In practice, it would be economical to standardize the size of timber used in formwork so that their repeated use is possible. This would necessarily entail proper planning, and great care is to be exercised by the designer in adjusting the parameters in such a way that the standard scantling can be used. Sometimes it would result in a bit of over-expenditure on concrete but, in the long run, especially in large projects, the saving in formwork will offset this loss. It is better to use clamps and screws, rather than nails, in the formwork to facilitate its stripping and reuse.

Plywood Formwork Plywood sheets bound with synthetic resin adhesive are being widely used nowadays. The thickness of ply varies from 3 to 18 mm. Sizes less than 6 mm thick are used for lining the timber formwork to get neat and smooth surface finish and as a formwork for curved surfaces. The common sizes are 1200 × 1200 mm to 3000 × 3000 mm. The main advantage is that large panel surfaces are available. The fixing of forms is rapid and economical. It does not warp, swell and shrink during the setting of concrete. Moreover, it has high impact resistance. Steel Formwork Steel formworks are commonly employed for big projects where the forms are to be repeatedly used. The steel forms can be easily fabricated and do not require many adjustments as the units are standardized. They give smooth surfaces needing very little finishing. These prove to be economical and are best suited for circular columns and flat slab construction. Joists can be used from wall to

Production of Concrete

399

wall to support the steel beams used for stiffening the plates. Square steel plates of 500 mm size are generally used. Light steel sheet panels of 500 mm size and stiffened with angles are also available. Typical formworks for stairs and floor slab are shown in Fig. 11.24(b) and (c), respectively.

11.10.4

Design Loads on the Formwork

The formwork used in the construction of roofs and floors has to carry its own weight, the weight of wet concrete, the live load due to labor, and the impact owing to the pouring of concrete, etc. The surfaces of the formwork should be so dressed that after the deflection due to concrete weight, etc. the surface takes the shape desired by the designer. In the design of formwork for columns shown in Fig. 11.23 and walls shown in Fig. 11.24(a), the hydrostatic pressure of concrete should be taken into account. This pressure depends upon the water content in the concrete, rate of pouring and the temperature. The hydrostatic pressure of concrete increases with the increase in water content, rate of pouring, and with the reduction in the size of the aggregate and temperature. It is possible to adjust the rate of pouring with the rate of setting of concrete in large and tall structures, so that the formwork can continually move upwards. Therefore, the movable forms need not be very high. A similar procedure can be adopted for columns.

(a) Rectangular columns

Fig. 11.23

(b) Circular column

Adjustable steel formworks for rectangular/square and circular columns

The lateral pressure decreases rapidly after the initial set of concrete, and, therefore, only the height of concrete poured in the preceding half to three-fourths of an hour is considered for calculating the lateral pressure. It may be calculated by considering the concrete as a liquid with a density of 2300 to 1200 kg/m3 for heights of concrete from 1.5 to 6 m, respectively. Due to large number of variables involved this is only a rough estimate. The experience alone should determine the size of various parts.

400

Concrete Technology

(a) Formwork for a wall

(b) Formwork for stairs

(c) Steel formwork for a slab

Fig. 11.24

11.10.5

Typical temporary concrete formworks for wall (aluminum beams), stairs and slab

Stripping of Form

The removal of forms after the concrete has set is termed stripping of forms. The stripping or striking of forms should proceed in a definite order. The formwork should be so designed and constructed as to allow them to be stripped in the desired order. The period up to which the forms must be left in place before they are stripped is called stripping time. The factors affecting the stripping time are the position of the forms, the loads coming on the elements immediately after stripping, temperature of the atmosphere, the subsequent loads coming on the element, etc. Using ordinary portland cement with temperatures above 20°C, the stripping times normally required are given in Table 11.5. For rapid hardening Portland cement, the stripping period can be reduced to three-sevenths of that given in Table 11.5 except for the vertical sides of slabs, beams and columns where the forms are to be retained for 24 hours.

Production of Concrete Table 11.5

Stripping time for different conditions

Element and supporting conditions

Stripping time, Days

Walls, columns, vertical sides of beams Slabs with props left in position Beam soffits with props left in position Slabs: removal of props (a) Span up to 4.5 m (b) Span over 4.5 m Beam and arches: removal of props (supports) (a) Span up to 6 m (b) Span over 6 m

1 to 2 3 7

11.11

401

7 14

14 21

SLIP-FORMING TECHNIQUE

The slip-form method, developed by Swedish Technologies for constructing chimneys, cooling towers, etc., refers to the continuously moving form, moving along the project at such a speed that the previously poured concrete has already achieved enough strength to support the vertical pressure from the concrete still in the form and to withstand lateral pressure caused by wind, inclination of walls, and so on. The vertical slip-form relies on the quick-setting properties of concrete requiring a balance between early-strength gain and workability. Concrete needs to be workable enough to be placed to the formwork and strong enough to develop early strength so that the form can slip upwards without any disturbance to the freshly placed concrete. Slip forming differs from conventional fixed-form concrete placement in that concrete is placed or pumped in slip forms and the forms act as moving molds to shape the concrete. The rate of movement of slip forms is regulated so that forms leave the placed concrete only after it is strong enough to retain its shape while supporting its own weight. The form ties are not used. Slip-forming utilizes a mechanized moving work platform system enabling semicontinuous placement of concrete. All concreting operations like the placing of reinforcement, installation of all block-outs and fixtures, pouring of concrete as well as finishing and inspection of concrete surfaces are performed gradually from work platforms shown in Fig. 11.25 which are attached permanently to the slip forms and thus move together with the forms. The slip forms and work platforms are raised by hydraulic jacks spaced at equal intervals which climb on vertical steel rods or tubes. All jacks are operated simultaneously and they lift the slip form in increments of 25 mm every three to 12 minutes, depending on the required sliding speed. An average sliding speed of 250 mm an hour may be achieved during construction. Since in the slip form construction, the concrete is poured continuously, there are no cold joints and surface colour is uniform. This results in a strong and esthetically attractive surface. Once the forms have been built, a 50 m tall slip form can take approximately five days to pour.

402

Concrete Technology

slip forming (a) Vertical V

(i) Slip forming a wall

(ii) Slip - forming canal paving

(b) Horizontal slip forming

Fig. 11.25

Vertical and horizontal slip-forming techniques

A vertical slip forming illustrated in Figs. 11.25(a) and (b) is the fastest and most efficient method of casting vertical reinforced concrete walls in many types of structures. The construction of large concrete towers begins with the construction of a fixed form of desired geometry/shape on top of a foundation, with a back-up support and bracing system to ensure that the form maintains its shape during movement. Inside and outside forms create the cavity for casting the wall, and inside this cavity reinforcing steel is tied together vertically and horizontally to reinforce the concrete wall. The form is then connected to hydraulic jacks, which automatically move the form vertically in minute increments as the concrete is being poured. Once pouring begins, it continues until the top of the structure is reached resulting in a monolithic concrete structure. Horizontal slip forming technique shown in Fig. 11.9(c) has been very successful in concrete highway and runway pavement construction. There are two basic methods of slip forming concrete pavement, namely, the fixed-form paving and slip-form paving. Fixed form paving requires the use wooden or metal side forms that are set up along the perimeter of the pavement before paving. Slip-form paving does not require any steel

Production of Concrete

403

or wooden forms. As the slip-form paving machine moves forward, the attached sliding forms also move along, thus fixing of forms beforehand and removing them afterward as paving is not required in fixed form. After the fixed-form or slip-form equipment has passed through, the hand tools are used to further finish the slab. Smaller paving projects may use only the slip form. In general, slip-form paving is preferred by contractors for large paving areas where it can provide better and economical productivity than fixed-form paving. Figure 11.25(c) shows the slip forming for a wall. The combination of a well-designed slip-form system and quality concrete pump performance has enabled this method of construction used typically on large scale storage silos and other vertical concrete structures, such as nuclear shield walls, high-rise buildings, chimneys, caissons, shafts, bridge piers, dams, elevator service cores, etc. Notable applications of the technique are the Skylon Tower in Niagara Falls, and CN Tower in Toronto. The method is commonly used for construction of tall buildings in Australia.

REVIEW QUESTIONS 11.1 Which are the different stages of manufacturing of concrete? Describe in detail the compaction of concrete. 11.2 What is the effect of vibration on the strength and durability of concrete? Explain the different types of vibrators. 11.3 Assuming that concrete is made from the correct ingredients and in the correct proportions, what other requirements must be met to ensure a durable structure, that is, a durable structure with long life? 11.4 Differentiate between volume batching and weigh batching in concrete mix procedure which one is superior. 11.5 Explain methods of transporting concrete.

11.6 What is pumped concrete and what are the precautions taken in pumping the concrete? 11.7 State the different types of special concreting techniques. Explain the ready mixed concrete with its classification and merits 11.8 What is curing of concrete and its objectives? Enlist the various methods for curing and describe the membrane curing. 11.9 What are the basic requirements of formwork for concrete construction? Describe slip-forming technique. 11.10 Write short notes on the following: (a) Maturity of concrete (b) Prolonged vibration and re vibration (c) Effect of delay in placing

MULTIPLE-CHOICE QUESTIONS 11.1 Which of the following statement(s) is incorrect? (a) The design of a satisfactory mix proportions ensures quality concrete work (b) The production of concrete consists of mixing ingredients to obtain a plastic mass (c) Batching, mixing, transportation, placing, compaction, finishing and

curing are independent operations for the production of concrete (d) All of the above (e) None of the above 11.2 Identify the incorrect statement(s). (a) Batching, mixing, transportation, placing, compaction, finishing and curing are complimentary operations for the production of quality concrete

404

11.3

11.4

11.5

11.6

Concrete Technology (b) Good quality concrete is a homogeneous mixture of ingredients obtained by a scientific process based on well established principles (c) The aim of quality control is to ensure the production and continuous supply of concrete of uniform strength (d) A proper and accurate measurement of ingredients is essential to ensure uniformity of proportions and aggregate grading in successive batches (e) None of the above In the batching of materials, the ingredients should be measured to a tolerance (as a percentage of batch quantity) of (a) ±1.0 (b) ±2.0 (c) ±3.0 (d) ±5.0 (e) 0.0 In weight batching the weight of surface water carried by the wet aggregate (a) can be ignored (b) must be taken into account (c) may or may not be taken into account depending upon the type of job (d) is taken care of by drying the aggregate (e) for coarse aggregate it may be taken in to account while for sand it may be ignored The choice of a proper batching system depends upon (a) size of job (b) required production rate (c) required standard of batching performance (d) availability of resources (e) All of the above A mobile mixing plant is particularly useful (a) as it can be kept close to the site where concreting is required (b) where concrete is required to be laid over a very large area (c) as it can also be used to carry the materials

11.7

11.8

11.9

11.10

11.11

(d) All of the above (e) None of the above Identify the incorrect statement(s). (a) In volume batching it is generally advisable to set the volumes in terms of whole bags of cement (b) In volume batching, allowance has to be made for the moisture present in the sand (c) While filling the measuring boxes, no compaction is to be allowed (d) Volume batching is adopted for small jobs (e) None of the above Mixers are normally classified on the basis of (a) the technique of discharging the mixed concrete (b) capacity of batch handled (c) the number of drums (d) number of revolutions (e) Any of the above The capacity of a concrete mixer is expressed in terms of (a) total volume of concrete produced per day (b) total volume of concrete produced in 8 hours (c) total volume of concrete produced per hour (d) volume of concrete mix handled per batch (e) weight of aggregate per batch A mixer designated 400 NT indicates that (a) it is a non-tilting type mixer (b) its nominal mix batch capacity is 400 liters (c) Both (a) and (b) (d) it is a non-tilting type mixer requiring 400 revolutions for proper mixing (e) None of the above The objectives of mixing concrete materials are the following except (a) coat the surface of all aggregate particles with cement paste (b) blend all the ingredients into a uniform mass (c) obtain concrete of uniform colour and grading

Production of Concrete

11.12

11.13

11.14

11.15

11.16

(d) obtain concrete of desired workability (e) None of the above Identify the incorrect statement(s). (a) The size of the mixer is designated by a number representing its nominal mix batch capacity in liters (b) Most of the mixers can handle a 15 per cent overload satisfactorily (c) In the tilting-type mixer the chamber is tilted for discharging (d) The efficiency of the mixing operation depends upon the shape and design of vanes fixed inside the drums (e) A non-tilting type mixer rotates about a horizontal axis and cannot be tilted. The pan mixer consisting of a circular pan rotating about a vertical axis is suitable (a) as a mobile mixer (b) as a central mixing plant (c) for ready- mixed concrete (d) Any of the above (e) None of the above The mixing time (a) is the time required to produce uniform concrete (b) is reckoned from the instant when all the solid materials have been put in the mixer (c) is independent of the number of revolutions (d) may be ignored in favor of number of revolutions (e) All of the above In machine mixing, the recommended minimum mixing time for mixers up to 750 litre capacity reckoned from the time when all the materials have been added is (in minutes) (a) 1.0 (b) 1.5 (c) 2.0 (d) 2.25 (e) 5.0 Identify the incorrect statement(s). (a) Delays in laying the concrete after the initial set has taken place are not injurious provided the concrete retains adequate workability for compaction

11.17

11.18

11.19

11.20

405

(b) The specifications permit a maximum of two hours between introduction of mixing water to the dry mix and the discharge if concrete is transported in a truck mixer or agitator (c) During transportation of concrete segregation should be prevented and the concrete should remain uniform (d) All of the above (e) None of the above The freshly mixed concrete can be transported by (a) barrows (b) trippers and lorries (c) truck mixers or agitator lorries (d) dump buckets (e) Any of the above Pumpable concrete (a) is transported through completely filled delivery pipelines (b) should be very cohesive and fatty having a slump of 50 to 100 mm (c) should have mix proportions with total fines passing 200 mm sieve not less than 350 kg/m3 (d) is high slump, flowing concrete obtained by using super-plasticizers (e) All of the above While pumping concrete (a) care should be taken to reduce the number of bends in the delivery pipe (b) the pipe should be cleaned immediately after use (c) initially a 1:3 cement–sand mortar should be pumped to lubricate the pipeline (d) All of the above (e) None of the above (i) When concrete is pumped by a pump of 60 hp, the maximum horzontal distance that can be covered would be (a) 150 m (b) 200 m (c) 300 m (d) 350 m (e) 400 m

406

Concrete Technology

(ii) A 90° bend in the pipeline reduces the effective pumping distance by approximately (a) 10 m (b) 5 m (c) 3 m (d) 2 m (e) 1.5 m 11.21 Ready-mixed concrete (RMC) (a) is weigh batched and mixed in a centrally located plant, transported in a track mixer or agitator and delivered in a condition ready to use (b) is produced under site conditions (c) does not require control of all operations of manufacture and transportation of fresh concrete (d) All of the above (e) None of the above 11.22 Sometimes when the concrete is partially mixed at the central plant and mixing is completed en route the concrete is known as (a) transit-mixed concrete (b) ready-mixed concrete (c) shrink-mixed concrete (d) Any of the above (e) None of the above 11.23 Identify the incorrect statement(s) with respect to placing of concrete. (a) Concreting should begin at the ends or corners of forms and continue towards the center (b) In large openings concreting should end around the perimeter (c) On a slope concreting should begin at the lower end of slope (d) The concrete in columns should be allowed to stand for at least two hours before concrete is placed in slab or beams (e) None of the above 11.24 The effect of delay in placing of concrete (a) is the gain in compressive strength provided concrete can still be adequately compacted (b) varies with the richness of the mix and the initial slump

11.25

11.26

11.27

11.28

11.29

(c) is automatically taken into account by controlling the uniformity of concrete as delivered for placement (d) All of the above (e) None of the above Identify the incorrect statement(s). (a) The process of removal of entrapped air and uniform placement of concrete to form a homogeneous dense mass is termed as compaction (b) Compaction is accomplished by doing external work (c) Presence of even five per cent voids in hardened concrete due to incomplete compaction may reduce compressive strength by about 40 per cent (d) All of the above (e) None of the above Compaction by mechanical vibrations is suitable for (a) all the grades of concrete (b) all the structural elements (c) all the mixes except very plastic mixes (d) All of the above (e) None of the above The acceleration imposed on the particles during compaction of concrete by high frequency vibrations is of the order (a) up to g (b) g to 2g (c) 4g to 7g (d) 7g to 9g (e) None of the above Which type of vibrator is generally used for compaction of concrete? (a) form vibrator (b) needle vibrator (c) surface vibrator (d) screen vibrator (e) None of the above For compacting thin reinforced concrete slabs following vibrator is recommended (a) immersion vibrator (b) surface vibrator (c) vibrating table (d) Any of the above (e) None of the above

Production of Concrete 11.30 Surface vibrator is effective only when the thickness of concrete member does not exceed (a) 100 mm (b) 125 mm (c) 150 mm (d) 200 mm (e) 500 mm 11.31 A surface vibrator for compaction of concrete is preferred for all of the following except (a) raft footings (b) columns (c) RCC slab (d) road pavements 11.32 While using vibrators for compacting concrete mixes (a) vibrations are used for spreading concrete in the form (b) vibrations reduce entrained air (c) vibrations cause smaller and lighter constituents to rise to the surface and give better finish (d) prolonged vibrations reduces chances of segregation (e) All of the above 11.33 Rotational instability occurring during compaction of concrete using vibration technique is due to (a) mix being under-sanded (b) mix being oversanded (c) entrapped air (d) All of the above (e) None of the above 11.34 Curing of concrete (a) governs the resultant microstructure of the hydrated cement (b) provides adequate moisture within concrete to ensure sufficient water for continuing hydration process (c) provides warm temperature to help chemical action (d) All of the above (e) None of the above 11.35 Maturity of concrete is the (a) 28-day strength of concrete (b) 365-day strength of concrete (c) product of period of curing and temperature of curing

11.36

11.37

11.38

11.39

11.40

407

(d) percentage of strength of concrete cured at 18 °C for 28 days (e) None of the above Membrane curing of the concrete is the (a) process of providing plastic sheeting as a protective cover for curing concrete (b) process of applying a membrane forming compound on the concrete surface (c) process of spraying the sodium silicate on the concrete surface (d) Any of the above (e) None of the above In steam curing of concrete (a) mixes of high water-cement ratio respond more favorably than mixes with low water-cement ratio (b) the heating of concrete products is caused by steam at low pressure or at high pressure (c) the steam curing is followed by water curing for a period of at least 21 days (d) All of the above (e) None of the above The following methods may be used for the curing of concrete except (a) membrane curing (b) electrical curing (c) mechanical curing (d) infrared radiation curing (e) chemical curing The following sealing compounds can be used for the membrane curing of concrete except (a) rubber latex emulsions (b) asphaltic emulsion or cutbacks (c) sodium silicate solution (d) emulsions of paraffin (e) varnishes The standard moist curing of concrete for the fi rst 7 to 14 days may result in a compressive strength of _____ per cent of 28-day moist curing. (a) 60 to 70 (b) 70 to 80 (c) 80 to 90 (d) 90 to 95

408

Concrete Technology

(e) None of the above 11.41 The timber formwork for concrete should be made of (a) teak wood (b) Shisham wood (c) soft wood planks (d) green timber (e) hard wood 11.42 For a concrete slab for a 3.75 × 4.75 m room, the stripping time of form should be (a) 3 days (b) 7 days (c) 14 days (d) 21 days (e) 28 days 11.43 To take care of any sag in the beams, the forms are given a camber of (a) 1:200 (b) 1:300 (c) 1:500 (d) 1:650 (e) 1:750 11.44 For a medium income group big housing project which type of formwork is recommended? (a) Timber formwork (b) Plywood formwork (c) Steel formwork (d) Slip forms (e) Other type 11.45 Identify incorrect statement(s). (a) The formwork or shuttering may be defined as moulds of timber or some other material into which the freshly mixed concrete is poured at the site and which hold the concrete till it sets. (b) The formwork includes the total system of support of freshly placed concrete, i.e., form lining and sheathing plus all necessary supporting members, bracings, hardware and fasteners.

(c) The main objective of formwork is the smooth and esthetically attractive external surface of the cast concrete member (d) In addition to forms being of right size, a good formwork should be strong, stiff, smooth and leakproof. (e) As the cost of formwork may be of the order of 20 per cent of the cost of project, it is essential that the forms be properly designed and detailed to effect economy without sacrificing strength and efficiency. 11.46 Identify the false statement. (a) Slip forming differs from conventional fixed-form concrete placement in that concrete is placed or pumped in slip-forms and the forms act as moving molds to shape the concrete. (b) The rate of movement of slip forms is regulated so that forms leave the placed concrete only after it is strong enough to retain its shape while supporting its own weight. (c) Slip-forming utilizes a mechanized moving-work platform for concreting operations which is attached permanently to the slip-forms and thus moves together with the forms. (d) The slip-forms and work platforms are raised by hydraulic jacks spaced at equal intervals which climb on vertical steel rods or tubes. (e) There are cold joints and surface colour is non-uniform. This results in a strong but esthetically not so attractive surface.

Answers to MCQs 11.1 (d) 11.7 (f) 11.13 (b) 11.19 (d) 11.24 (d) 11.30 (d) 11.36 (b) 11.42 (c)

11.2 (e) 11.8 (a) 11.14 (e) 11.20i. (e) 11.25. (d) 11.31 (b) 11.37 (b) 11.43 (c)

11.3 (c) 11.9 (d) 11.15 (a) 11.20ii. (a) 11.26 (d) 11.32 (b) 11.38 (c) 11.44 (c)

11.4 (b) 11.10 (c) 11.16 (e) 11.21 (a) 11.27 (c) 11.33 (b) 11.39 (c) 11.45 (c)

11.5 (e) 11.11 (d) 11.17 (e) 11.22 (c) 11.28 (b) 11.34 (d) 11.40 (b) 11.46 (e)

11.6 (b) 11.12 (b) 11.18 (e) 11.23. (b) 11.29 (b) 11.35 (c) 11.41 (c)

12 12.1

CONCRETE UNDER EXTREME ENVIRONMENTAL CONDITIONS

INTRODUCTION

Whenever the concrete is to be placed in extreme weather conditions or underwater, its performance is adversely affected unless appropriate measures are taken to control it. Extreme weather conditions include situations where environmental temperatures during concreting and subsequent curing periods are markedly different from those in normal conditions, i.e., either the temperature is too high or too low. The properties and performance of concrete are affected under these situations unless appropriate precautions are taken. In general, an increase in temperature accelerates the rate of hydration and therefore, leads to an accelerated development of strength. The accelerated growth of hydrates under higher temperature may result in a less uniform microstructure of gel than could be expected, were the reactions to proceed at the normal rate. On the other hand, a decrease in temperature retards the rate of hydration and hence of strength development, but the microstructure of the gel formed is perhaps more orderly and compact. The situation may become further aggravated by decrease of humidity in the atmosphere, increase of wind or a combination of these. This may result in a rapid loss of water due to evaporation which may affect the workability of fresh concrete and cause plastic shrinkage and cracking that accompany the rapid drying. The subsequent cooling due to evaporation may introduce tensile stresses.

12.2

CONCRETING IN HOT WEATHER

Any operation of concreting done at atmospheric temperature above 40 °C or where the temperature of concrete at the time of placement is expected to be beyond 40 °C may be categorized as hot weather concreting. Concrete is not recommended to be placed at a temperature above 40 °C without proper precautions as specified in IS: 7861 (Part-I)–1975. The climatic factors affecting concrete in hot weather are a high ambient temperature and reduced relative humidity, the effects of which may be more pronounced with the increase in wind velocity. The effects of hot weather may be summarized as follows. 1. Accelerated setting A higher temperature results in a more rapid hydration leading to accelerated setting, thus reducing the handling time of concrete and also lowering the strength of hardened concrete. The workability of concrete decreases and hence the water demand increases with the increase in the temperature of concrete. The addition of water without proper adjustments in mix proportions adversely affects the ultimate quality of concrete. It has been

410

2.

3.

4.

5.

Concrete Technology

reported that an approximately 25 mm decrease in slump has resulted from 11 °C increase in concrete temperature. Reduction in strength Concrete produced and cured at elevated temperature generally develops higher early strength than normally produced concrete, but the eventual strengths are lower. Regarding the influence of simultaneous reduction in the relative humidity, it is seen that specimens molded and cured in air at 23 °C and 60 per cent relative humidity, and at 38 °C and 25 per cent relative humidity attained strengths of only 73 and 62 per cent, respectively, in comparison with the specimens which are moistcured at 23 °C for 28 days. High temperature results in greater evaporation and hence necessitates increase of mixing water, consequently reducing the strength. Increased tendency to cracking Rapid evaporation leads to plastic shrinkage cracking, and subsequent cooling of hardened concrete introduces tensile stresses. The rate of evaporation depends on the ambient temperature, relative humidity, wind speed and concrete temperature. Rapid evaporation during curing As the hydration of cement can take place only in water-filled capillaries, it is imperative that a loss of water by evaporation from the capillaries be prevented. Furthermore, water lost internally by self-desiccation has to be replaced by water from outside. A rapid initial hydration results in a poor microstructure of gel which is probably more porous, resulting in a large proportion of the pores remaining unfilled. This leads to lower strength. Difficulty in controlling the air content At higher temperatures it is more difficult to control the air content in air-entrained concrete. This adds to the difficulty of controlling workability. For a given amount of airentraining agent, hot concrete entrains less air than does concrete at normal temperatures.

12.2.1 Recommended Practices and Precautions Temperature Control of Concrete Ingredients The temperature of the concrete can be kept down by controlling the temperature of the ingredients as shown in Fig.12.1. The aggregates may be protected from direct sunrays by erecting temporary sheds or shelters over the aggregate stockpiles. Water can also be

(a) Use of ice as mixing water

Fig. 12.1

(b) Cooling with nitrogen

(c) Cooling truck

Temperature control of ingredients for hot-weather concreting

Concrete under Extreme Environmental Conditions

411

sprinkled on to the aggregate before using them in concrete. The mixing water has the greatest effect on lowering the temperature of concrete, because the specific heat of water (1.0) is nearly five times that of common aggregate (0.22). Moreover, the temperature of water is easier to control than that of other ingredients. Under certain circumstances, the temperature of water can most economically be controlled by mechanical refrigeration or mixing with crushed ice. The precooling of aggregates can be achieved at the mixing stage by adding calculated quantities of broken ice pieces as a part of mixing water, provided the ice is completely melted by the time mixing is completed. The cooling of concrete can also be achieved by nitrogen gas.

Proportioning of Concrete Mix The mix should be designed to have minimum cement content consistent with other functional requirements. As far as possible, cement with lower heat of hydration should be preferred to those having greater fineness and heat of hydration. Use of water-reducing or set-retarding admixtures is beneficial. Accelerators should not be used under these conditions. Production and Delivery The temperature of aggregates, water and cement should be maintained at the lowest practical levels so that the temperature of concrete is below 40°C at the time of placement. The temperature of the concrete at the time of leaving the batching plant should be measured with a suitable metal clad thermometer. The period between mixing and delivery should be kept to an absolute minimum by coordinating the delivery of concrete with its rate of placement. Placement and Curing of Concrete The formwork, reinforcement and subgrade should be sprinkled with cool water just before the placement of concrete. The area around the work should be kept wet to the extent possible to cool the surrounding air and increase its humidity. Speed of placement and finishing helps minimize problems in hot weather concreting. Immediately after compaction, the concrete should be protected to prevent the evaporation of moisture by means of wet (not dripping) gunny bags, hessian, etc. After the concrete has attained a degree of hardening sufficient to withstand surface damage, moist-curing should begin. Continuous curing is important because the volume changes due to alternate wetting and drying promote the development of surface cracking. On the hardened concrete, the curing shall not be much cooler than the concrete because of the possibilities of thermal stresses and resultant cracking. High velocity winds cause higher rate of evaporation, and hence wind breakers should be provided as far as possible. If possible, the concreting can be done during night shifts.

12.3

COLD WEATHER CONCRETING

Any concreting operation done at a temperature below 5 °C is termed cold weather concreting. Most codes do not advocate concreting to be done at an atmospheric temperature below 5 °C without special precautions. Due to low temperature, the problems are mainly due to the slower development of concrete strength; the concrete

412

Concrete Technology

in the plastic stage can be damaged if it is exposed to low temperatures which cause ice lenses to form and expansion to occur within the pore structure, and subsequent damage may occur due to alternate freezing and thawing when the concrete has hardened. The effects of cold weather concreting may be summarized as follows. 1. Delayed setting At low temperatures, the development of concrete strength is retarded as compared with the strength development at normal temperatures. The setting period necessary before removal of formwork is thus increased. Although the initial strength of concrete is lower, the ultimate strength will not be severely affected provided the concrete has been prevented from freezing during its early life. 2. Early freezing of concrete When plastic concrete is exposed to freezing temperature, it may suffer permanent damage. If the concrete is allowed to freeze before a certain prehardening period, it may suffer irreparable loss in its properties so much so that even one cycle of freezing and thawing during the prehardening period may reduce compressive strength to 50 per cent of what would be expected for normal temperature concrete. The prehardening period depends upon the type of cement and environmental conditions. It may be specified in terms of time required to attain a compressive strength of the order of 3.5 to 7.0 MPa. Alternatively it can be specified in terms of period varying from 24 hours to even three days depending upon the degree of saturation and water–cement ratio. 3. Stresses due to temperature differential A large temperature differential within the concrete member may promote cracking and has a harmful effect on durability. Such situations are likely to occur in cold weather at the time of removal of formwork.

12.3.1

Recommended Practice

As per IS: 7861 (Part-II)–1981, the following measures should be taken:

Temperature Control of Ingredients The temperature at the time of setting of concrete can be raised by heating the ingredients of the concrete mix. It would be easier to heat the mixing water. The temperature of the water should not exceed 65°C as the flash set of cement will occur when the hot water and cement come in contact in the mixers. Therefore, the heated water should come in direct contact with the aggregate first, and not the cement. The aggregates are heated by passing steam through pipes embedded in aggregate storage bins as shown in Fig.12.2. Another precaution taken along with the heating of ingredients is to construct a temporary shelter around the construction site. The air inside is heated by electric or steam heating or central heating with circulating water. The temperature of ingredients should be so decided that the resulting concrete sets at a temperature of 10 to 20°C. Use of Insulating Formwork and Blanket Covers A fair amount of heat is generated during hydration of cement. Such heat can be gainfully conserved by having insulating formwork covers capable of maintaining concrete temperature above the

Concrete under Extreme Environmental Conditions

413

desirable limit up to the first three days (or even seven days) even though the ambient temperatures are lower. The formwork covers can be of timber, clean straw, blankets, tarpaulines, plastic sheeting, etc., and are used in conjunction with an air gap as insulation. The efficiency of the covers depends upon the thermal conductivity of the medium as well as ambient temperature conditions. For moderately cold weather, timber formwork alone is sufficient.

(a) Heating the ingredients

(b) Insulated blanket covers (c) Covering with blankets

Fig. 12.2

Cold-weather concreting

Proportioning of Concrete Ingredients The important factor for coldweather concreting is the attainment of suitable temperature for fresh concrete. Since the quantity of cement in the mix affects the rate of increase in temperature, an additional quantity of cement may be used. It would be preferable to use high alumina cement for concreting during frost conditions, the main advantage being that a higher heat of hydration is generated during the first 24 hours. During this period, sufficient strength (approximately 10 to 15 MPa) is developed to make the concrete safe against frost action. No accelerator should be used if high alumina cement is used. Alternatively, the rapid hardening Portland cement or accelerating admixtures used with proper precautions can help in getting the required strength in a shorter period. Air-entraining agents are generally recommended for use in cold weather. Air-entrainment increases the resistance of the hardened concrete to freezing and thawing and normally, at the same time, improves the workability of fresh concrete. The calcium chloride used as accelerating admixture may cause corrosion of reinforcing steel. In any case, calcium chloride should not be used in prestressed concrete construction. Placement and Curing Before placing the concrete, all ice, snow and frost should be completely removed. Care should be taken to see that the surface on which the concrete is to be placed and eminent parts are sufficiently warm. During the periods of freezing or in near-freezing conditions, water curing is not applicable.

Delayed Removal of Formwork Because of slower rate of gain of strength during the cold weather, the formwork and props have to be kept in place for a longer time than in usual concreting practice.

414

Concrete Technology

The problem of concreting in cold weather can be minimized by adopting precast construction of structures. Precast members are manufactured in the factories where adequate precautions can be taken and concreting can be done in the controlled conditions.

12.4

UNDERWATER CONCRETING

Special precautions need be taken whenever the concrete is to be placed underwater. In regard to the quality of concrete, the recommendations of the Portland Cement Association are as follows. “The concrete should be plastic and cohesive but should have good flowability. This requires a fairly high slump, usually 150 to 180 mm. A richer mix than generally used for placing under normal conditions is required; usually the cement requirement is not less than eight sacks per cubic metre of concrete. The proportions of fine and coarse aggregates should be adjusted to produce the desired workability with a somewhat higher proportion of fine aggregate than used for normal conditions. The fine aggregate proportion can often be from 45 to 50 per cent of the total aggregate, depending on the grading. It is also important that the aggregate contain sufficient fine material passing the 300 and 150 micron sieves to produce a plastic and cohesive mixture. ASTM standard specifications for concrete aggregate require that not less than 10 per cent of fine aggregate pass the 300 micron sieve and not less than 2 per cent pass the 150 micron sieve. The fine aggregate should meet the minimum requirements and somewhat higher percentage of fines would be better in many cases. For most works coarse aggregate should be graded up to 20 mm or 40 mm.” In addition the coarse aggregate should not contain loam or any other material which may cause laitance while being worked. The demands on the formwork are usually higher than in normal concreting under dry conditions. The formwork not only has to impart the required shape to the structure or its elements, it must also protect the concrete mix during placing until it matures from the direct action of current and waves. Thus, the formwork also serves as a temporary protective casing which during concreting prevents possible washing out of cement and the leakage of cement mortar from the concrete mix. After completion of concreting, it will protect the soft concrete from the impact and abrasive action of the water currents. If necessary, coffer dams are to be constructed to reduce the velocity of flow through the construction zone.

12.4.1 Concreting Methods Following are the principal techniques which have been used for placing concrete underwater:

Concrete under Extreme Environmental Conditions

1. 2. 3. 4. 5.

415

Placing in de-watered caissons or coffer dams Tremie method Bucket placing Placing in bags Prepacked concrete

1. The placing in de-watered caissons or coffer dams follows the normal inthe-dry practice. 2. Tremie method A tremie is a watertight pipe, generally 250 mm in diameter, having a funnel-shaped hopper at its upper end and a loose plug at the bottom or discharge end as shown in Fig. 12.3. The valve at the discharge end is used to de-water the tremie and control the distribution of the concrete. The tremie is supported on a working platform above water level, and to facilitate the placing it is built up in 1 to 3.5 m sections. Crane for raising pipe Concrete supplied by skip or pump

Hopper

Tremie pipe Water level

Smooth bore pipe with quick release, water-tight joints Loose plug

Driven sheet pile formwork

Immersion depth controls output Bed

Fig. 12.3

Typical arrangement for a tremie pipe

During the concreting, air and water must be excluded from the tremie by keeping the pipe full of concrete all the time; and for this reason the capacity of the hopper should be at least equal to that of the tremie pipe. In charging the tremie a plug formed of paper is first inserted into the top of the pipe. As the hopper is filled the pressure of fresh concrete forces the plug down the pipe, and the water in the tremie is displaced by concrete. For concreting, the tremie pipe is lowered into position and the discharge end is kept as deeply submerged beneath the surface of freshly placed concrete as the head

416

Concrete Technology

of concrete in tremie permits. As concreting proceeds, the pipe is raised slightly and the concrete flows outward. Care should be taken to maintain continuity of concreting without breaking the seal provided by the concrete cover over the discharge end. Should this seal be broken, the tremie should be lifted and plugged before concreting is recommended. The tremie should never be moved laterally through freshly placed concrete. It should be lifted vertically above the surface of concrete and shifted to its new position. For placing concrete underwater a tremie should be set up as shown in Fig. 12.4(a). This will prevent the larger size aggregate being washed out of the concrete mix as shown in Fig. 12.4(b). The tremie is gradually pulled up as the pipe gets filled with concrete. The mix for underwater application should contain much larger amount of cement, i.e., the mix should be richer. The following procedure can be adopted for placing the concrete in water-filled forms:

(i)

(ii)

(iii)

(a) Components of tremie pipe: (i) A 900-mm tall tremie section, (ii) Spreader bar, and (iii) Super-chute tremie and funnel support (shown over a manhole)

Metal hopper

Fiexible pipe Formwork

Large size aggregate washed out

(b) Placing the concrete in a water-filled formwork

Fig. 12.4

Components and arrangement of tremie pipe for underwater concerting

Concrete under Extreme Environmental Conditions

417

(a) The formwork is generally a hollow steel piling driven to a depth h’ meter below the bed, i.e., the level of concreting. This additional depth h’ depends upon depth of concreting level. (b) An auger can be used to remove filled material from inside the piling to a depth of concreting or bed. (c) As the filled material is removed, the subsurface water will fill the piling. (d) The reinforcing steel skeleton is placed in position. (e) The tremie is lowered into the piling to the bed. (f) As the tremie is open ended, it will get filled up with water. (g) A soccer ball or a paper plug is placed in the top of the tremie. (h) The concrete is pumped into the tremie. (i) The descending ball will prevent the concrete from mixing with the water. (j) The ball will exit the bottom of the tremie and shoot to the surface. (k) As concrete exits the tremie, the piling will start to be filled up with concrete. (l) Water displaced by the concrete will gush out of the top of the piling. (m) The tremie is slowly raised so that the lower end of the tremie always stays in the concrete mass. When large quantities of concrete are to be placed continuously, it is preferable to place concrete simultaneously and uniformly through a battery of tremies, rather than shift a single tremie from point to point. It has been recommended that the spacing of tremies be between 3.5 and 5 m and that the end tremies should be about 2.5 m from the formwork. The risk of segregation and non-uniform stiffening can be minimized by maintaining the surface of concrete in the forms as level as possible and by providing a continuous and rapid flow of concrete. 3. Dump bucket placing This method has the advantage that concreting can be carried out at considerable depths. The dump buckets are usually fitted with drop-bottom or bottom-roller gates which open freely outward when tripped as shown in Fig. 12.5. The bucket is completely filled with concrete and its top covered with a canvas cloth or a gunny sack to prevent the disturbance of concrete as the bucket is lowered into water. Some buckets are provided with a special base which limits the agitation of the concrete during discharge and also while the empty bucket is hoisted away from the fresh concrete. The bucket is lowered by a crane up to the bottom surface of concrete and then opened either by divers or by a suitable arrangement from the top. It is essential that the concrete be discharged directly against the surface on which it is to be deposited. Early discharge of bucket, which permits the fresh concrete to drop through water, must be avoided. The main disadvantage of the bucket method is the difficulty in keeping the top surface of the placed concrete reasonably level. The method permits the use of slightly stiffer concrete than does tremie method. 4. Placing in bags The method consists in partially (usually about two-third) filling of cloth or gunny sacks with concrete, and tying them in such a way

418

Concrete Technology Lifting crane for aising

ket

Overlapping canvas flaps to prevent washout

Concrete T pping lock on chain

Drop bottom gates

Steel skirt to resist intermixing with water

Fig. 12.5

Typical arrangement for a bottom opening dump bucket

that they can readily be accommodated in a profile of the surface on which they are placed. The properly filled bags are lowered into water and placed carefully in a header-and-stretcher fashion as in brick masonry construction with the help of divers. The method has advantages in that, in many cases, no formwork is necessary and comparatively lean mixes may be used provided sufficient plasticity is retained. On the other hand, as the accurate positioning of the bags in place can be only accomplished by the divers, the work is consequently slow and laborious. Voids between adjacent bags are difficult to fill, there is little bonding other than that achieved by mechanical interlock between bags. The bags and labour necessary to fill and tie them are relatively expensive; and the method is only suited for placing the concrete in rather shallow water.

Concrete under Extreme Environmental Conditions

419

5. Prepacked concrete This technique, also called grouted concrete, consists of placing the coarse aggregate only in the forms and thoroughly compacting it to form a prepacked mass. This mass is then grouted with the cement mortar of the required proportions. The aggregate should be wetted before being placed in position. The mortar that grouts the concrete displaces water and fills the voids. The aggregate should be well graded to produce a dense and compact concrete. Aggregates up to a maximum size of 80 mm can be conveniently used. Only shutter vibrators can be used for compacting the coarse aggregate. The coarse aggregate may also be allowed to fall from heights of up to 4 meters, without causing any appreciable segregation. The mortar consists of fine sand, pozzolanic filler material and a chemical agent, which serves (i) to help the penetration, (ii) to inhibit early setting of cement, (iii) to aid the dispersion of the particles, and (iv) to increase the fluidity of mortar. An air-entraining agent is also added to the mortar to entrain about four per cent of air. A small variation of the procedure of preparation of the cement mortar for grouting leads to a process called colcrete. In this process, the mortar grout is prepared in a special high-speed mixer. No admixtures are used in this process. The high-speed mixing produces a very fluid grout which is immiscible with water. The maximum size of sand used is 5 mm and the sand should be well graded. The mix ratio ranges from 1:1.5 to 1:4 with a water–cement ratio of about 0.45. Rich cement mortar is used for underwater construction and grouting of prestressing cables in post-tensioned bonded construction. The grouting of prepacked aggregates can be done in any of the following methods: (a) The mold can be filled partially with grout, and the coarse aggregate can then be deposited in the grout. (b) The grout can be poured on the top surface of aggregate and allowed to penetrate to the bottom. The method is particularly useful for grouting thin sections. (c) Pumping the grout into the aggregate mass from bottom at carefully designed positions through a network of pipes. The formwork should be constructed at the top of the coarse aggregate in this method. The quantity of grout in any of these methods should be estimated from the void contents of the coarse aggregates. The grout pressure employed will be of the order of 0.2 to 0.3 MPa. This technique is very much suited for underwater construction and repair work of mass concrete structures, such as dams, spillways, etc. The prepacked concrete is known to exhibit lower drying shrinkage and higher durability, especially the freezing and thawing resistance compared to ordinary concrete of the same proportions. The rate of development of strength is comparatively slow for the first two months and the eventual strengths are about the same as for normal concrete. In USA and USSR, the tremie method is most commonly used. In Holland, where large volumes

420

Concrete Technology

of concrete have been placed under water, the usual method is that of placing by bucket. The bag method is nowadays seldom used for important works overseas, but has found some applications in the building up of permanent underwater forms.

REVIEW QUESTIONS 12.1 What are the effects of cold weather on concreting? Briefly describe the recommended practices. 12.2 Enlist the principal techniques for under water concreting. Briefly describe the Tremie method for underwater concreting.

12.3 Write short note on pre-packed concrete. 12.4 What are the effects of hot weather on concreting? Briefly describe the recommended practices.

MULTIPLE-CHOICE QUESTIONS 12.1 The following conditions of concrete placement are termed as extreme environmental conditions. (a) When concreting operations are carried out at temperature beyond 40°C (b) When concreting operations are done at temperature below 5 °C (c) Underwater concreting (d) Any of the above (e) None of the above 12.2 Concreting in hot weather (a) reduces handling time of fresh concrete and strength of hardened concrete (b) increases tendency to cracking (c) make it difficult to control air content (d) All of the above (e) None of the above 12.3 In hot weather concreting it is recommended to (a) use cold mixing water (b) have minimum cement content consistent with other functional requirements (c) use cements with lower heat of hydration and use water reducing admixtures (d) reduce period between mixing and placement to an absolute minimum (e) All of the above

12.4 Concreting in cold weather (a) reduces rate of development of strength (b) delays removal of formwork (c) temperature differential within the concrete mass may promote cracking (d) freezing and thawing during the prehardening period may reduce strength by 50 per cent (e) All of the above 12.5 In cold weather concreting it is recommended to (a) heat the water for mixing (b) use insulating formwork and delay its removal (c) use additional quantity of cement (d) use air-entraining agents (e) All of the above 12.6 In cold weather curing of concrete should be continued for (a) 7 days (b) 14 days (c) 21 days (d) 28 days (e) 45 days 12.7 For placing the concrete underwater the principal technique(s) used are (a) tremie method (b) bucket placing (c) placing in bags (d) prepacked concrete (e) Any of the above

Concrete under Extreme Environmental Conditions

Answers to MCQs 12.1(d) 12.7 (e)

12.2 (d)

12.3 (e)

12.4 (e)

12.5 (e)

12.6 (d)

421

13 13.1

INSPECTION AND TESTING

INTRODUCTION

The principal aim of conventional in-situ testing is to ensure that proper materials are used in desired proportions and correct steps of workmanship are followed. Recent trends are towards switch-over to the performance-oriented system approach and quality control where a number of items and operations have to be controlled at the right time and in the right measures. From this point of view, the testing of representative concrete does not represent the quality of the actual in-place concrete, and quality control cannot be regarded as a mere testing of three concrete cubes at 28 days. In fact, to avoid inferior concrete being placed, the control is to be carried out much before any cubes become available for testing. The cube tests relate to the concrete specimens specially prepared for testing. What is really needed is to carry out tests on concrete in the structure, so that the influence of workmanship in actual placing, compaction and curing are also reflected. However, a complete switch-over to performance-oriented specifications has not been possible, because of difficulties involved in defining what constitutes the satisfactory performance, in setting appropriate performance limits, and in monitoring the performance in the absence of suitable tests. For in-situ testing, the aims of investigation should be clearly established at the outset to avoid misleading test results and consequent future disputes over results. By carefully formulating the test programme the uncertainties can often be minimized. Since in-situ testing of existing structures involve engineering judgement, a complete knowledge of the range of tests available, and their limitations and the accuracies that can be achieved is important. There are three basic categories of concrete testing, namely, 1. Quality control It is normally carried out by the contractor to indicate adjustments necessary to ensure an acceptable supplied material. 2. Compliance testing It is performed by, or for, the engineer according to an agreed plan, to check compliance with the specifications. 3. Secondary testing This test is performed on the hardened concrete insituations where there is a doubt about the reliability of control and compliance results or they are unavailable or inappropriate as in an old structure. Quality control and compliance tests are normally performed on standard hardened specimens from the sample of fresh concrete being used in construction. However, these tests may misrepresent the true quality of concrete actually used in the structure. This is due to differences of compaction, curing and general workmanship. The modern trend is to perform compliance testing, using methods

Inspection and Testing

423

which are either non-destructive or cause only limited damage. Such a test may be used as a back up for conventional testing. The principal usage of in-situ tests is, nevertheless, as secondary testing for checking compliance with specifications and in assessing the in-situ quality.

13.2

INSPECTION TESTING OF FRESH CONCRETE

The inspection testing of fresh concrete includes workability test, analysis of fresh concrete, accelerated testing, and non-destructive testing.

13.2.1

Workability Tests

The workability of concrete should be measured at frequent intervals during the progress of work, by means of slump test, compacting factor test or Vee-Bee consistency test as per IS: 1199−1959 specifications or the ball-penetration test (ASTM: C−360). Additional tests should be carried out whenever a change in the materials or mix proportion occurs. The slump test shown in Fig.13.1 is of real value as a field control of the mix to maintain the uniformity between different batches of supposedly similar concrete. By control of uniform workability, it is easier to ensure a uniform quality of concrete and hence uniform strength. The advantage of the ball penetration test lies in its simplicity and the speed at which the test is carried out; there is no necessity of sampling as the test can be performed on the concrete in a wheelbarrow or as placed in the forms. The compacting factor test is more accurate than the slump test and the results are reproducible. This test may be performed for a wide range of workability, i.e., for concrete mixes of high to very low workabilities (CF of 0.92 to 0.68). The Vee-Bee test is suitable for low and very low workabilities. In the absence of definite correlation between different measures of workability under different conditions, it is recommended that for a given concrete, the appropriate method should be decided beforehand and the workability of concrete should be expressed in terms of such a test, rather than being interpreted from the results of other tests. If the proportions of the materials are properly maintained and workability is satisfactory, the results should not differ by more than the tolerance indicated in Table 13.1.

13.2.2

Air-Void Analysis Testing

On-site pressure meter testing described in Section 6.7, which is routinely used to measure the total air content of fresh concrete, does not provide information about bubble size and air distribution throughout the volume of concrete which is vital for adequate freeze thaw performance. On the other hand, the only available standardized method to determine the air-void system is to perform tests on hardened concrete which typically takes several weeks to complete, which is too late for corrective measures to be taken. To overcome these problems an air-void analyzer (AVA) can be used to evaluate the air void system parameters, i.e., the volume of entrained air, spacing factor

424

Concrete Technology 100

A standard steel testing cone Hollow steel cone with an open top to allow concrete to be placed inside Fill and compact with metal rod and level off f the top

300 0

200 straight edge

This is a This measured distance, the average height good shape concrete of the top of the concrete that sags but is the slump. stays together

Empty cone

straight edge This is bad concrete, far too wet. Anything over 125 mm total slump should not be used.

Fig. 13.1 Table 13.1

empty cone

Slump test and its measurement

Permissible variations in different workability measurements

Workability measurement

Tolerance or allowable variation

Slump

±25 mm or ± one-third of required value, whichever is less

Ball penetration

±12 mm

Compacting factor (CF)

±0.03 for CF values of 0.90 or more ±0.04 for CF values between 0.90 and 0.80 ±0.05 for CF values of 0.80 or less

Vee-Bee time

±3s or ± one-fifth of the required value, whichever is greater

Inspection and Testing

425

(distribution) and specific surface (size) of air voids in the fresh concrete (PCC) on the jobsite in about 30 minutes. This information allows the concrete manufacturer to take quality control measures by making appropriate adjustments in concrete batching, mixing and admixture dosages in real time, to ensure a proper air-void system. This technology offers many advantages over current practices of evaluating air content in concrete.

Air-Void System Parameters

In the air-void analysis, the number and size distribution of air voids in concrete is measured by the spacing factor parameter. Spacing factor is a measure related to the distance between the peripheries of air voids in the cement paste. Usually, a spacing factor of less than 0.20 mm is preferable. The size of air voids in concrete, on the other hand, can be measured directly or expressed in terms of specific surface. Specific surface is the ratio of the surface area of air voids to their volume. Smaller voids have higher specific surface. Specific surface is an important factor in determining freeze-thaw durability. The basic methodology of the AVA involves expelling all air (bubbles) present in a given concrete sample, collecting the air bubbles and recording their quantities and size distribution. The test method involves retrieving a sample of fresh paste using a vibrating drill, and injecting it into a column of a glycerin solution. The viscosity of the solution allows the individual air bubbles to retain their original size, i.e., neither coalesce nor collapse while the sample is injected into the solution. The air bubbles which rise through the viscous solution enter a column of overlaying water. They rise through the water column and collect under a submerged buoyancy recorder. The rising speed of the air voids through the liquids is dependent on their size (according to Stoke’s law), as large bubbles rise faster than small ones. The viscosity of the glycerin solution slows the initial rise of the bubbles and provides a measurable separation in time between the arrivals at the top of the column of bubbles of different sizes. From this data, the air-void parameters (air content, spacing factor, specific surface) can be calculated. Figure 13.2 shows an image of the typical AVA apparatus.

Significance of Air-Void System As water in concrete expands during freezing, the pressure increases in relation to the distance it must travel to reach the nearest air void. The more closely the air voids are spaced, the less likely it is that the pressure of freezing water will damage the concrete. Ensuring that concrete has an adequate air-void system with closely spaced air voids can ensure concrete freeze-thaw durability and also improve sulfate and scaling resistance. With adequate air void distribution, the ice formed in capillary pores in concrete during freezing will expand into adjacent voids without causing spalling and deterioration of the concrete. Airentraining agents are added to concrete mixtures to stabilize very small air bubbles in the concrete mixture in an attempt to minimize freeze-thaw damage. However, the air-void structure can be adversely affected during the construction cycle, due to the use of some high-range water-reducing admixtures and over-vibration. The AVA machine is a sensitive machine. It has been considered usable only in buildings, not in the field, because vibrations, such as those caused by wind or people movements, can have a significant effect on the AVA results. The AVA isolation base can allow AVA testing on the jobsite.

426

Concrete Technology

Fig. 13.2

13.2.3

Air-void analysis test equipment

Microwave Water-Content Test

Water content of as-delivered concrete has always been a source of speculation and uncertainty. Water in excess of the mix design amount can have a direct, negative impact on the strength, quality, and durability of concrete. Since better quality control, optimization of materials in the mix design, and high-performance concrete are specified for construction projects, it has become critical that accurate control of the water content of a concrete mix be enforced.

Test Apparatus The procedure outlined in this section requires a high-power microwave (900 W) of the type shown in Fig. 13.3 which is equipped with a turntable

Fig. 13.3

A typical microwave oven for water-content test

Inspection and Testing

427

to provide uniform drying of concrete sample; Pyrex (heat-resistant glass) tray and fiberglass cloth (a container/wrapper for the concrete sample); a balance to obtain the mass of the sample; a metal scraper and a porcelain pestle for grinding the sample as it is dried.

Testing Procedure (Refer to AASHTO T 318 for comprehensive guidance) 1. 2. 3. 4. 5.

6.

7.

8. 9.

The fiberglass cloth and glass tray are weighed together and the total mass is recorded. A sample of approximately 1500 g of freshly mixed concrete is wrapped in the fiberglass cloth placed in the glass tray. The glass tray, fiberglass cloth and concrete sample are weighed together. The tray and wrapped sample are placed inside the microwave at a power of 900 W and heated for a period of approximately five minutes. After the first drying cycle, the sample is taken out from the microwave, weighed, and broken up separating the coarse aggregate from the mortar with the scraper and the mortar is ground into powder using the pestle. The sample is rewrapped for a second drying cycle and placed in the microwave and heated for another five minutes in the microwave. The sample is taken out from the microwave stirred with the scraper and the mass recorded. The weighing, breaking, rewrapping and heating cycle at two-minute intervals are repeated until the sample loses less than 1 g of mass between reheating cycles. A minimum of three drying cycles are required. When stoppage criterion is met, i.e., the mass change is less than 1g, the final mass is recorded. The total water in the concrete sample is calculated as the difference between the recorded masses of the wet and dry concrete samples. It can be expressed as a percentage:

Total water content =

(Wet sample mass − dry sample mass) × 100(per cent) wet sample mass

The total water content can be monitored and used as a relative indicator of potential variability in concrete strength. 10. The water absorbed in the aggregate is subtracted from the total water, and the remainder is used to calculate the free water content. 11. The water-to-cementing materials ratio (microwave) is obtained from the relative cement content given in the batch document.

Advantages 1. The test is simple to perform in the field. The simplicity of the test is inherent in its lack of mechanical parts and user-dependent variables (any field worker can use both a microwave and scale accurately). 2. This test can be performed anywhere a powered microwave can be managed. The microwave oven test is inexpensive as all required apparatus and materials are readily available. T318-02 stipulates the use of a microwave with a 900 watt power setting.

428

Concrete Technology

3. The test can be completed in total test time of 15 to 20 minutes, including time taken for sample preparation and check weighing twice during the drying process. 4. The test is accurate within a 95 per cent confidence level. However, the confidence level is highly dependent on proper testing procedures and is highly susceptible to human error. 5. The results are on an average of 15 per cent in error when testing the samples where no specific aggregate moisture information is available, i.e., typical values are used. On the other hand, when the exact moisture content of the aggregate is known, the method is accurate within five per cent for computing the water-to-cementing materials ratio of hand mixed fresh concrete. This testing method can be used as a quality-control tool in calculating the waterto-cementing materials ratio for an as-delivered mix as long as the cement content, moisture content and absorption of the aggregates are known. Since the latter two of these requirements are difficult to know accurately on a truck-by-truck basis and the aggregate moisture content greatly affects the accuracy of the results, the application of this test for normal usage is limited.

13.2.4

Analysis of Fresh Concrete

The field variations in the actual mix proportions of the concrete can be determined by analyzing the composition of fresh concrete. The quality of concrete can be controlled if a rapid analysis of fresh concrete is carried out allowing the engineer to take the necessary remedial measures, if required. The density and composition of in-place concrete may be determined by analyzing the representative samples taken from the forms. The density values would indicate the degree of compaction achieved. The cement and water contents of the concrete may be determined according to the method given in IS: 1199−1959. The method involves separating the constituents of fresh concrete by wet sieving and determining their proportions after weighing in water. The mass of cement in the sample of concrete is determined from the difference between the mass of concrete in water, and the masses of coarse and fine aggregates in water. The water content is next determined as the difference between the mass of concrete and the combined mass of cement and aggregates. The method requires about two hours and a fairly high degree of experience and skill. There are many other rapid methods for determining the cement content of a sample of fresh concrete. Important among these are: the rapid analysis machine (RAM) method; the EDTA titration method; the HCl heat of solution method, and accelerated strength method. The rapid analysis machine method is based on the principle of elutriation. A weighed sample of concrete is fed into the machine which separates the cement and fine particles as slurry, by elutriation. This is followed by subsampling, flocculation and measurement of cement by mass. The sequence of procedure is built into the machine. The determination of cement content takes about 10 minutes. The EDTA titration method, involves the separation of a sample of mortar from the fresh concrete sample by sieving. The mortar sample is dried at 105 °C for one hour. The silica-free acid solution of the sample is titrated against a standard EDTA

Inspection and Testing

429

solution. The percentage of CaO present in the sample is estimated, which gives the cement content of the original sample of concrete. In the HCl heat of solution method, a sample of concrete is diluted by adding a fixed quantity of water. To the diluted solution is added hydrochloric acid, resulting in an exothermic reaction which decomposes the cement contained in the sample. The heat of reaction reaches a steady temperature quickly. The temperature rise is related to the cement content of sample of concrete. This method has been developed by the Cement Research Institute of India. A comparison of the four analysis methods is given in Table 13.2. Table 13.2

Comparison of various analysis methods

Method of analysis

Variation of estimated cement content from the actual values, per cent

Approximate time required for sample preparation

Approximate time required for conducting the test

IS: 1199−1959 method RAM method EDTA method HCl heat of solution method

−11.3 to 2.2 −10.3 to 6.7 −6.6 to −3.1 −10.4 to +10

Nil Nil 1.5 h Nil

2h 10 min 30 min 10 min

The accelerated strength tests give a reliable idea about the potential 28-day strength of concrete. The details of accelerated strength tests for the purpose of quality control of concrete are available in IS: 9013−1978. Either of the following two methods may be adopted as a standard for the accelerated curing of concrete. 1. Warm water method 1 1 to 3 hours after molding. 2 2 Curing water temperature 55 ± l °C Curing period 20 h ± 10 min Demould and cool at 27 ± 2 °C for 1 h before test. 2. Boiling water method Standard moist curing 23 h ± 15 min Water temperature 100 °C 1 Curing period 3 h ± 5 min 2 Cooling period 2h The specimens are immersed in water 1

The actual correlation of accelerated test results to 28 days, normally cured specimens depends upon the curing method adopted, the chemical composition of cement and the concrete mix proportions. Typical relationships are shown in Fig. 10.14. It is recommended that the actual relationship under given site conditions should be established using local concrete making materials and such relationships be continuously improved upon as more and more data become available progressively. In the absence of past records with local materials, the relation suggested can be used to predict the 28day compressive strength of the normally cured concrete, within ±15 per cent limits.

430

Concrete Technology

13.3 13.3.1

NON-DESTRUCTIVE TESTING OF IN–SITU FRESH CONCRETE Maturity of Concrete

As discussed in Section 11.9.2 of Chapter 11, the strength of a concrete mix that has been properly placed, consolidated, and cured is a function of its age and temperature history. Longer curing periods and higher curing temperatures lead to increase in strength development. The maturity method of testing recognizes this combined effect of time and temperature and provides a basis for estimating the in-situ strength gain of concrete by monitoring its temperature over a period. The maturity is thus an indicator of the time−temperature history of the concrete mixture and is often taken as the summation of product of age and curing temperature. Here for ordinary concrete the temperature is reckoned from datum (generally between −10°C to −12°C) which is a reasonable value of the lowest temperature, at which an appreciable increase in strength can take place and the period in hours or days. Below this datum temperature the water crystals (ice) do not react with cement, as such, time periods during which temperatures are at or below this datum temperature do not contribute to strength gain. This technique for estimating in-situ concrete strength is based on the assumption that samples of a given concrete mixture exposed to different time and temperature histories will have same strength if they attain equal values of maturity. This concept is illustrated in Fig. 13.4(a) (97), which shows that a sample exposed to colder

(a) Maturity concept (Nelson 2003)

(b) Maturity meter

(c) Pavement instrumented with maturity meter

Fig. 13.4

Maturity concept, maturity meter, and instrumentation

Inspection and Testing

431

temperatures takes longer to reach maturity (M1), whereas a sample exposed to a hotter temperature takes less time to reach maturity (M2). If M1 = M2, then these two samples have equal strengths even though the individual curing conditions (time and temperature) are different. The maturity is represented by 3 days to 28 days of curing at normal temperatures. The most common expression used for the maturity or maturity index or the maturity temperature-time factor, commonly referred to as the Nurse−Saul maturity function, is given by t

M = ∑(T − T0 ) Δt

(13.1)

0

where M T T0 t Δt Σ

= maturity in °C-hours (or °C-days), = average concrete temperature, °C, during each time interval, Δt, = datum temperature (typically taken to be -11°C), = elapsed time (hours or days), and = time interval (hours or days). = summation of all the intervals of time multiplied by temperature.

It should be noted that the maturity function, based on the temperature history of the concrete, represents the area under the temperature-time curve for a given concrete above a datum temperature. The amount of cement hydrated depends on how long the concrete has cured and at what temperature, i.e., the maturity is a measure of how far hydration has progressed. The datum temperature below which cement hydration is assumed to cease may depend on admixture type and dosage, cement type, and temperature range that the concrete experiences while hardening. Concrete is considered to be fully matured when it is cured at 18°C for 28 days. For this curing condition, the maturity of concrete is [18 − (−11)] × (28× 24) = 19488°C-hours which is taken as19800°C-hours. Thus, for ordinary concrete maturity should not be less than 19800°C hours. The most common way of expressing the maturity index is in metric units of °C-hours (usually shown by the shorthand notation ‘C-Hours’). In case of variation in temperature, the period of curing can be broken into smaller intervals of constant temperature and maturity of concrete computed for each interval is summed up to obtain the maturity of concrete for entire period of curing.

Strength-maturity Relationships Based on the temperature history of the concrete, one of the popular strength-maturity relationships proposed by Plowman (1956) is f = a + blog10 (M × 10−3) where f = strength for maturity M, per cent b = slope of line, and

(13.2)

432

Concrete Technology

Equation (13.2) is popular because of its simplicity; it is a straight line when a log scale is used for the maturity axis, but it has its limitations. It does not provide a good representation of the relationship between strength and maturity for low or high values of the maturity. As per this relation, the predicted strength keeps on increasing with maturity, i.e., there is no limiting strength. The values of coefficients a and b depend on strength level of concrete as listed in Table 13.3. Table 13.3

Plowman’s coefficient for maturity equation

Strength at M = 19800°C-Hrs, MPa

a

b

6 mm), and honeycombing when the voids are interconnected. 2. Strength tests The following strength tests may be performed on the cores: (a) Compressive strength The cores are tested in saturated surface dry condition after capping and immersion in water for at least two days. The core length and mean diameter (average of diameter measured at quarter and mid-points along the length) are determined to the nearest 1 mm. Compression testing is carried out at a rate within the range 12−24 MPa/min on a suitable testing machine and mode of failure is noted. In case the cap cracks or gets separated from the core, the results should be considered as being of doubtful accuracy. (b) Tensile strength The tensile strength may be measured by the splitting test on the core as explained in Section 8.2.3.

Factors Influencing Core Compressive Strength The significant factors are outlined below. 1. Moisture and voids The moisture condition of the core influences the measured strength; a saturated specimen has a value of 10 to 15 per cent lower than comparable dry specimen. It is therefore important that the relative moisture conditions of core and in-situ concrete are taken into account while estimating the actual in-situ concrete strengths. Voids in core will reduce the measured strength. 2. Length/diameter ratio of core As l/d ratio increases, the measured strength will decrease due to the effect of specimen shape, and stress distribution during test. For establishment of relation between core strength and standard cube strength, a ratio l/d = 2.0 is regarded as the datum of computation. 3. Diameter of core The diameter of the core may influence the measured strength and variability. Measured concrete strength decreases with the increase in the diameter of specimen; for sizes above 100 mm this effect will be small, but for smaller sizes this effect may become significant. 4. Direction of drilling As a result of layering effect the measured strength of specimen drilled vertically relative to the direction of casting is likely to be greater than that for a horizontally drilled specimen from the same concrete; an average difference of eight per cent has been reported in the literature. 5. Reinforcement Due to the presence of reinforcement, the measured strength of concrete is underestimated up to 10 per cent. Reinforcement must therefore be avoided wherever possible, but in the case where it is present, the measured core strength may be corrected using Eq. (13.4). ⎛ φ h⎞ Corrected strength = measured strength × 1.0 + 1.5 ⎜ r ⎟ ⎝ φc l ⎠

(13.4)

452

Concrete Technology

where

fr = reinforcement bar diameter, fc = Core diameter, h = distance of bar axis from the nearer end of core, and l = core length (uncapped). For the case of core having multiple bars, the corrected strength may be computed by, Eq. (13.5) ⎡ ⎛ ∑ φr h ⎞ ⎤ Corrected strength = measured strength × ⎢1.0 + 1.5 ⎜ . ⎟ ⎥ ⎝ φc l ⎠ ⎦ ⎣

(13.5)

If the spacing of two bars is less than the diameter of the larger bar, only the bar with the higher value of (fr h) should be considered.

Estimation of Cube Strength The equivalent cube strength can be estimated in two steps. In the first step a correction for the effect of length/diameter ratio is applied to convert the core strength to an equivalent standard cylinder strength. In the second step, appropriate relationship between strength of cylinders and cubes is used to convert the equivalent standard cylinder strength obtained in the step 1 to equivalent cube strength. This conversion to a cube strength may be based on the generally accepted average relationship given by Eq. (13.6): 1 ⎛ ⎞ Cube strength = 1.25 × cylinder strength ⎜ for = λ = 2 0⎟ ⎝ ⎠ d

(13.6)

The corresponding relations taking into account the strength differential of six per cent between a core with cut surface relative to cast cylinder, and strength reduction of 15 per cent for weaker top surface zone of a corresponding cast cylinder, as adopted by BS: 1881 are given by (Eqs. (13.7) to (13.10)). 1. For vertically drilled core Estimated in-situ cube strength =

2 3 fc 1.5 (1 / λ )

(13.7)

where fc is the measured strength of a core with length/diameter = l. For horizontaly drilled core 8 per cent difference between vertically and horizontally drilled cores is incorporated resulting into expressions. Estimated in-situ cube strength =

1.08 2.3 f c 25 f c � 1.5 (1 / λ ) 1.5 1 / λ

(13.8)

However, the target (potential) strength of a standard specimen made from a particular mix is about 30 per cent higher than the actual fully compacted in-situ strength. The expressions for cube strength are given by Eqs. (13.9) and (13.10). 2. For vertically drilled core

Estimated potential cube strength =

300 f c 1.5 1 \ λ

(13.9)

Inspection and Testing

453

3. For horizontally drilled core

Estimated potential cube strength =

3 25 f c 1.5 1 \ λ

(13.10)

If smaller diameter cores are drilled, then to obtain comparable accuracy, at least thrice the number of standard cores should be used in testing. The method of preparation of cores after drilling and the procedure of test are described in IS: 516−1959. As per IS: 456−2000 the concrete in the member represented by a core test shall be considered aceptable if the average equivalent cube strength of the cores is equal to at least 85 per cent of the cube strength of the grade of concrete specified for the corresponding age, and no individual core has a strength less than 75 per cent. The main drawback of this test is the difference in the intrinsic quality of concrete in structure and control specimens in the laboratory. In case the core test results do not satisfy the above requirements, the load test is resorted to. The core tests are performed when 1. The standard 28-day cube strength test gives lower results than acceptable and the primary aim of the core test is to ascertain whether the structural element is of adequate strength. 2. It is essential to estimate the load-carrying capacity of the structure for its safety under change of loading or usage contemplated for the structure. The typical values of coefficient of variation and maximum accuracies of expected in-situ strength prediction for a single site-made unit constructed from a number of batches as reported in literature are given in Table 13.5. The values offer only an approximate guide and are applicable under ideal conditions with specific calibrations for the particular concrete mix. If any factor varies from this ideal condition the accuracies of prediction will be reduced. Table 13.5

Typical values of coefficient of variation of test results and maximum accuracies of prediction

Test method

Windsor prob Rebound hammer Ultrasonic pulse velocity Pull-out method Concrete cores (i) standard (ii) small (non-standard)

Co-efficient of variation for Best 95 per cent individual member with good confidence limits on degree of control, per cent strength estimate, per cent 4 4 2.5 8

±20 (3 tests) ±25 (12 tests) ±20 (1 test) ±20 (6 tests)

10 15

±10 (3 specimens) ±15 (9 specimens)

A typical 50 mm non-standard diameter core drilled vertically from a transmission tower foundation contained one 18 mm f reinforcement bar normal to the core axis which was located 27 mm from one end. The crushing load on the core of length (after capping) of 80 mm was 35 kN. Determine the in-situ cube strength.

Example 13.4

454

Concrete Technology

Solution (a) In-situ concrete strength as per BS: 1881−Part 120: Measured concrete core strength, fc length ratio of the core diameter Esimated in-situ concrete strength

−3 = 35 × 10 = 17.8 MPa π × 50 2 / 4

= 80/50 = 1.6 =

2.3 × 17.8 [1.5 (1 \ 1.6)] = 19.2 MPa

⎛ 18 27 ⎞ Correction factor for the reinforcement = 1 + 1.5 ⎜ × ⎟ = 1.18 ⎝ 50 80 ⎠ Corrected in-situ concrete strength ±12 per cent for an individual result

= 19.2 × 1.18 = 22.66 MPa = 22.66 ± 2.7 MPa

(b) Potential strength: Estimated potential cube strength (for vertically drilled core) 3.0 × 17.8 = 1.5 (1 \ 1.6) = 25.13 MPa [ ] Corrected potential cube strength

= 25.13 × 1.18 = 29.65 MPa

13.4.7 Load Tests In the cases where member strength cannot be adequately determined from the results of in-situ concrete strength tests, load testing may be necessary. These are aimed at checking the structural capacity, and are hence concentrated on suspect or critical locations. Except in the cases where variable loading dominates, the static load tests are conducted. Loading tests may be divided into two main categories: 1. In-situ load testing The principal aim is to demonstrate satisfactory performance under an overload above the service load conditions. The performance is usually judged by measurement of deflection under this load sustained for a specified period. The need for the test may arise from the doubts about the quality of construction or design, or where some damage has occurred, and the approach is particularly valuable where public confidence is involved. In some international codes, the static load tests are an established component of acceptance criteria. These tests are normally accompanied by some sort of monitoring of structural behaviour under incremental loads. The in-situ tests should not be performed before the characteristic strength of concrete has been reached (i.e., concrete is 28 days old). Preliminary work is always required to ensure safety in the event of a collapse under test conditions, and that the members under test are actually subjected to the calculated test load. Scaffolding must be provided to support at least twice the total load from any member liable to collapse together with the test load. This

Inspection and Testing

455

should be set to catch falling members after a minimum drop, but at the same time should not interfere with expected deflections. Test loads IS: 456−2000 requires that the structure should be subjected to a total design dead load of structure plus 1.25 times the imposed design load for a period of 24 hours, and then the imposed load should be removed. Thus

Test load = design dead load + 1.25 × (design imposed load)

The performance is based initially on the acceptability of the measured deflection and cracking in terms of the design requirements coupled with the examination of unexpected defects. The deflection due to imposed load only shall be recorded. If within 24 hours of removal of imposed load, the structure does not recover at least 75 per cent of the deflection under superimposed load, the test may be repeated after a lapse of 72 hours (second load cycle). If the recovery under second load cycle is less than 80 per cent, the structure shall be deemed to be unacceptable. If significant deflections occur, the deflection recovery rates after removal of load should also be examined. On the other hand, if the maximum deflection in mm is less than 40 l2/D (where l is the effective span in metres and D is the overall depth in mm), it is not necessary for the recovery to be measured. Load application The load should be provided as cheaply as possible. The rate of application and distribution of load must be controlled and the magnitude must be easily assessable. Bricks, bags of cement, sand bags, steel weight, and water are amongst the materials which may be used and the choice will depend upon the nature and magnitude of the load required as well as the availability of materials and ease of access. Care must be taken to avoid arching of the load as deflection increases. If loading is to be spread over a larger area, ponding of water is the most appropriate method providing load. Slabs may be pounded by providing suitable containing walls and water proofing. Water is particularly useful in the locations with limited space or difficult access. However, leakage should be minimized to avoid damage to the finishes. Loads should always be applied in predetermined increments and in a way which will cause minimum lack of symmetry or uniformity. Similar precautions should be taken during unloading. Deflection dial gages must be carefully observed throughout the loading cycle, and if there are signs of deflections increasing with time under constant load, further loading should be stopped and the load be reduced as quickly as possible. The potential speed of load removal is thus an important safety consideration. Non-gravity loading offers advantages of greater control, which can also be affected from a distance from the immediate test area. However, it is restricted to specialized or complex loading arrangements. The deflection measurements are made by mechanical dial gages which must be clamped to an independent rigid support. Gages are normally located at midspan and quarter points to check symmetry of behavior. The gages must be set so that they can be easily read with a minimum risk to personnel and chance of

456

Concrete Technology

disturbance during test is small. Telescopes may often be convenient for this purpose. Readings should be taken at all incremental stages throughout the test cycle. Measurement accuracy of ± 0.1 mm is generally possible with dial gages. A plot of load deflection curve is recommended. An examination of the plot can yield valuable information about the behavior of the test member. In-situ load test may be used in the following circumstances: (a) Where the structure is suspected to be substandard due to the quality of design and construction and it is required to check the adequacy of the structure. (b) Where non-standard design concept has been used and it is required to demostrate the validity of the concept. (c) Where structural modification has been carried out for change in occupancy which may require increased loading. (d) Where a proof of improved performance is required following major repairs. 2. Ultimate load testing It is frequently used as a quality control check on standard precast elements. Ultimate load testing is an important approach where in-situ overload tests are inadequate. These are generally carried out in laboratory where carefully controlled hydraulic load application and recording system are available. The results of a carefully monitored test provide conclusive evidence regarding the behavior of the component examined.

13.4.8

Chemical Testing

Chemical analysis of hardened concrete may be used to check the specification compliance involving cement content, aggregate−cement ratio or alkali content determination. Water−cement ratio, and hence strength, are difficult to assess to any worthwhile degree of accuracy. Thus the analysis may be used only in cases of uncertainty, or in resolving disputes, rather than as a means of quality control of concrete. Specialized laboratory facilities are required for most forms of chemical testing. One of the major problems of basic chemical testing is the lack of a suitable solvent which will dissolve hardened cement without affecting the aggregates, and if possible samples of aggregates and cement should also be available for testing. The basic steps involved are outlined as follows.

Sampling Sufficient samples are taken to represent the body of concrete under examination at a particular location in the structure. The basic requirements for a sample for chemical analysis are that 1. the concrete sample should preferably be in a single piece with the minimum linear dimension being at least five times the maximum aggregate size (weighing about 2 kg), several samples are taken from different points 2. the sample should be free from reinforcement and foreign matter The sample shall be clearly labelled giving all relevant details and sealed in a heavy-duty polythene bag which should also be labeled.

Chemical Analysis The method is based on the fact that the lime compounds and the silicates in Portland cement are readily decomposed by, and soluble in dilute

Inspection and Testing

457

hydrochloric acid (HCl), than the corresponding compounds in aggregate. The quantity of soluble silica or calcium oxide is determined by a simple analytical procedure, and if the composition of cement is known, the cement content of the original volume of the sample can be calculated. Allowance must be made for any material which might have been dissolved from the aggregate. The representative sample of aggregate should, therefore, be analyzed by identical procedures to determine the correction to be made. 1. Preparation of sample The sample is initially broken into lumps not larger than 40 mm, taking care as far as possible to prevent aggregate fracture. These lumps are dried in an oven at 105 °C for 15−24 hours, allowed to cool to room temperature, and divided into subsamples. A portion of the dried sample is crushed to pass a 4.75 mm sieve and a subsample of 500−1000 g is obtained which is then crushed to pass a 2.36 mm sieve and quartered to give a sample which is pulverized in a ball mill to pass a 600 μm sieve. This is also quartered and further ground to a powder to pass 150 μm sieve. This final sample is freed from particles of metallic iron abraded from pulverizer ball mill, by means of a strong magnet. 2. Determination of calcium oxide content A portion of the prepared sample weighing 5 + 0.005 g is treated with boiling dilute hydrochloric acid. Triethanolamine, sodium hydroxide and calcein indicators are added to the filtered solution which is then titrated against a standard EDTA solution. The CaO content may be calculated to the nearest 0.1 per cent. If the CaO content of the aggregate is less than 0.5 per cent, this analysis may be considered adequate. However, additional determination of soluble silica content is recommended. 3. Determination of soluble silica content Soluble silica is extracted from a 5 ± 0.005 g portion of the prepared sample by treatment with hydrochloric acid and its insoluble residue collected by filtration. The filtrate is reduced by evaporating and treating with hydrochloric acid and polyethylene oxide, before being filtered again and diluted to provide a stock solution. The filter paper containing the precipitate produced at the last stage, is ignited in a weighed platinum crucible at 1200 ± 50 °C until constant mass is achieved, before cooling and weighing. The soluble silica content can be calculated to the nearest 0.1 per cent from the ratio of the mass of the ignited residue to that of prepared sample. The calcium oxide content is determined from the stock solution using the procedure given in 1. above. The insoluble residue is determined from the material retained during the initial filtration process by repeated treatment with hot ammonium chloride solution, hydrochloric acid and hot water followed by ignition in a weighed crucible to 925 ± 25 °C.

Calculation of Cement and Aggregate Contents The cement content should be calculated separately from both the measured calcium oxide and soluble silica contents, unless the calcium oxide content of the aggregate is less than 0.50 per cent or greater than 35 per cent in which case results based on CaO are not recommended. In the latter case, if the soluble silica content of the aggregate is greater than 10 per cent the analysis should be undertaken to determine some other constituent present in a larger quantity in the cement.

458

Concrete Technology

The calculation of cement content is based on the assumption that the combined water of hydration is 0.23 times the cement content, and that 100 g of oven dried concrete consists of C g of cement + A g of aggregate + 0.23C g of combined water of hydration as given by Eq. (13.11). 100 = C + A + 0.23C = 1.23C + A

(13.11)

Thus if, a = calcium oxide or soluble silica content of cement, per cent, b = calcium oxide or soluble silica content of aggregate, per cent, and c = measured calcium oxide or soluble silica content of the analytical sample, per cent then

c b aC bA = + a . 3 b 100 100

or

(c − b) A = (a − 1.23c)C

(13.12)

Then from Eqs. (13.11) and (13.12), the cement content C=

c( .23c + A) × 100 per cent (to nearest 0.1 per cent) 100

and aggregate content, A=

a 1.23c × 100 per cent (to nearest 0.10 per cent) a . 3b

Thus the aggregate−cement ratio A/C may be obtained to the nearest 0.10. The cement content by mass is given by C × oven dry density of concrete kg/m3 to nearest 0.10 kg/m3 100 The above calculations require that an analysis of both the cement and aggregate to be available. If an analysis of cement is not available, OPC or RPC complying with the relevant Indian Standard Code may be assumed. If the two estimated cement contents are within 25 kg/m3 or one per cent by mass, the value is adopted. Thus, the method of analysis suffers from the drawback that it cannot be used for concretes which contain aggregates or admixtures or additives such as fly ash or pozzolanas, which liberate soluble silica under the conditions of test.

REVIEW QUESTIONS 13.1 What is non-destructive testing of insitu fresh and hardened concretes? Discuss the pulse velocity method. 13.2 Which tests are included in acceptance testing of Hardened Concrete? Describe ultrasonic pulse velocity test.

13.3 Explain the basic principle on which Schmidt’s rebound hammer works. What are its limitations? 13.4 What is surface hardness test method for assessing the strength of concrete? Describe the rebound hammer test

Inspection and Testing procedure. State the factors influencing the test results and the applications where this method is useful. 13.5 What are partially destructive strength Tests? Enlist the partially destructive strength tests and describe briefly the concrete core tests. 13.6 What is maturity of fresh concrete? Describe its advantages and limitations. 13.7 Concrete mix of laboratory strength of 35 MPa, in fully matured condition, is used in a construction at a site where average temperature is 10°C.

459

How long the contractor will have to wait after the concrete is placed before removing the forms, if the stripping stress is 21 MPa? 13.8 The strength of a sample of fully matured ordinary concrete is determined to be 34 MPa; using maturity concept estimate the strength of identical concrete at 7 and 14 days when cured at an average day temperature of 22°C and night temperature of 15°C for the entire period. For this concrete, the Plowman’s coefficients are: a = 21 and b = 61.

MULTIPLE-CHOICE QUESTIONS 13.1 Identify the incorrect statement(s) (a) The testing of representative concrete does not give the quality of actual in-place concrete (b) Quality control can be exercised by testing three concrete cubes at 28 days (c) The quality control is carried out much before any cubes become available for testing (d) Cube tests relate to concrete specimens specially prepared for testing (e) The influence of workmanship in placing, compaction and curing can be judged by testing the concrete in the structure. 13.2 The concept of performance oriented specifications suffers due to difficulty in (a) defining what constitutes satisfactory performance (b) setting appropriate performance limits (c) the absence of tests to monitor the performance (d) All of the above (e) None of the above 13.3 Which of the following statement(s) are incorrect? (a) Uniform workability ensures uniform strength (b) The ball-penetration test can be performed on concrete as placed in the forms

13.4

13.5

13.6

13.7

(c) Slump test is more accurate than compacting factor test and the results can be reproduced (d) Vee-Bee test is suitable for low and very low work-abilities (e) None of the above The permissible variation in the compacting factor measurement is (a) ±0.02 for CF values of 0.90 or more (b) ±0.04 for CF values between 0.90 and 0.80 (c) ±0.06 for CF values of 0.80 or less (d) ±0.07for CF values below 0.70 (e) None of the above The allowable variation in the slump measurement is (a) ±25 mm (b) one-third of the required value (c) lesser of the above values (d) greater of the above values (e) None of the above The cement content in a sample of fresh concrete can be determined by (a) rapid analysis machine (b) EDTA titration method (c) HCl heat of solution method (d) accelerated strength method (e) Any of the above Identify the incorrect statement(s). (a) The accelerated strength test results are of doubtful nature as far as potential 28-day strength of concrete is concerned

460

Concrete Technology

(b) The maturity test gives valid results provided concretes have initial temperature between 15 °C to 26 °C (c) The ultrasonic pulse velocity method can assess the quality of in-place fresh concrete (d) At the time of initial setting the fresh concrete has ultrasonic pulse velocity of the order of 2000 m/s (e) All of the above 13.8 The quality and strength of concrete in a structure can be assessed by (a) the concrete core test (b) the pull out test (c) the ultrasonic meth (d) the Schmidt test hammer method (e) Any of the above 13.9 In ultrasonic test for hardened concrete good quality of concrete is indicated if the pulse velocity is (a) below 3.0 km/s (b) between 3.0 to 3.5 km/s (c) above 3.5 km/s (d) above 4.5 km/s (e) None of the above 13.10 The ultrasonic pulse velocity test is based on the assumption that

(a) the time taken by a pulse in passing through a concrete mass is proportional to the modulus of elasticity of the concrete (b) the frequency of pulse is proportional to the compressive strength of concrete (c) the amplitude of the pulse is proportional the compressive strength of the concrete (d) due to internal flaws the pulse velocity is reduced (e) None of the above 13.11 The resonant frequency method is based on the assumption that (a) pulse velocity depends primarily upon the materials and mix proportions of the concrete (b) the modulus of elasticity of concrete improves with the quality of concrete (c) resonant frequency is directly proportional to the square of strength of concrete (d) strength of concrete increases with the age (e) All of the above

Answers to MCQs 13.1(b) 13.7 (a)

13.2 (d) 13.8 (e)

13.3 (c) 13.9 (c)

13.4 (b) 13.10 (a)

13.5 (c) 13.11 (b)

13.6 (e)

14 14.1

SPECIAL CONCRETES AND CONCRETING TECHNIQUES

INTRODUCTION

Notwithstanding its versatility, cement concrete suffers from several drawbacks, such as low tensile strength, permeability to liquids and consequent corrosion of reinforcement, susceptibility to chemical attack, and low durability. Modifications have been made from time to time to overcome the deficiencies of cement concrete yet retaining the other desirable characteristics. Recent developments in the material and construction technology have led to significant changes resulting in improved performance, wider and more economical use. The improvements in performance can be grouped as: 1. Better mechanical properties than that of conventional concrete, such as compressive strength, tensile strength, impact toughness, etc., 2. Better durability attained by means of increased chemical and freeze−thaw resistances. 3. Improvements in selected properties of interest, such as impermeability, adhesion, thermal insulation, lightness, abrasion and skid resistance, etc. The mechanical properties can be improved by using one or more of the following approaches: 1. 2. 3. 4.

Modifications in microstructure of the cement paste. Reduction in overall porosity. Improvements in the strength of aggregate-matrix interface. Control of extent and propagation of cracks.

14.1.1 Modification in the Microstructure Considerable improvements in inter-particle cohesive forces can be realized by reducing the inter-particle spacing of the hydrate phase. Perhaps the most notable attempt to modify the microstructure is the application of the hot pressing technique. By application of pressures of up to 350 MPa during molding at temperatures up to 150 °C, compressive strengths of the order of 520 MPa have been obtained. The electron micrographs of such hot pressed cement plates have revealed a marked improvement in the microstructure in comparison to those cured at ordinary temperatures in that they show dense and relatively homogeneous structures. Though not yet used in construction, this method reveals the potential of future concrete.

462

Concrete Technology

14.1.2

Reduction in Porosity

The mechanical properties and durability of concrete can be improved by filling the pores, voids and cracks by incorporating or impregnating the concrete with polymers. In polymer-impregnated concrete (PIC), the pores in conventional concrete after normal curing are emptied under vacuum and then a monomer is sucked in, which is later polymerized by the application of heat or radiation. Considerable increase in tensile and compressive strengths and modulus of elasticity and hardness results. Compressive strengths, of the order of about 280 MPa have been obtained. Commercial applications of polymer-impregnated concrete include piles, tunnel liners, precast prestressed bridge deck panels and in wide ranging repairs. Sulfur-impregnated concretes (SIC), in a similar manner, have resulted in high strength concretes from lean conventional concrete mixes. A typical value of compressive strength of sulfur-impregnated concrete has been reported to be 55 MPa from a reference moist-cured ordinary concrete having a strength of 5.5 MPa, i.e., a ten-fold increase. In India, the applications of sulfur-impregnated concrete are limited due to high cost of sulfur.

14.1.3

Stronger Aggregate-Matrix Interface

The mechanical properties of cement concrete which consists of a relatively inert aggregate bounded by hydrated cement binder or matrix, depend upon the strength of aggregate and the stability of concrete through the matrix. In particular, the interface between the aggregate and the matrix must be capable of transferring the stresses due to loads to aggregate. This is generally achieved in cement concrete through the strong Van der-waals bonds between the micro-crystalline components of hydrated cement paste and the aggregate. However, the bonds are not so strong as to transfer tensile or shear stresses, and hence the composite, i.e., cement concrete is relatively weak in tension and shear. Only the compressive stresses are effectively transmitted. As the aggregate is usually very strong, the aggregate strength can be fully exploited by achieving greater force transfer capability. Beyond a level, the conventional cement matrix is unable to accomplish this. It is possible to supplement the cement matrix in the composite with another matrix or, if the cement matrix is replaced by a more efficient matrix, it should be possible to obtain concrete of much higher strength. If the binder or the matrix exhibits ionic or covalent bonds with aggregate at the interface, the resulting composite will also be sufficiently strong to transmit large tensile forces. Efforts in this direction have resulted in the use of polymers, either as sole matrix or supplement to the cement matrix. With the addition of polymers, the failure of concrete specimens does not occur through the aggregate-mortar interface, but through the aggregates themselves, thereby showing improvement in bond strength at the interface. With an improvement in bond strength at the interface, the aggregate strength can be fully exploited, i.e., the concrete strength is limited by the mechanical strength of the aggregate.

Special Concretes and Concreting Techniques

14.1.4

463

Control of Extent and Propagation of Cracks

The most notable development in this direction is the use of ferrocement and fiber-reinforced concretes. In ferrocements, meshes of thin steel wires of various configurations and sizes are incorporated as reinforcement in cement-mortars. However, in fiber-reinforced concrete (FRC), steel, glass or polymeric fibers of suitable mechanical and chemical properties and having optimum aspect ratios are incorporated with other concrete materials at the mixing stage. In a way both can be viewed as reinforced concretes. The wire-mesh or fibers hold the matrix together after localized cracking, and provide improved ductility and post-cracking loadcarrying capacity. The compressive strength improves slightly (say by 25 per cent), but the tensile strength, first-crack tensile strength, impact strength and toughness or shock absorption capacities show a two-to-four fold improvement. Ferrocement has found wide applications in boat-hull building, construction of shells, and similar structural components of thin sections. Applications of fiber-reinforced concrete include pavements and runways, industrial floors, hydraulic structures, breakwater, armour units, pile foundations, etc. The combinations of fiber-reinforced concrete and polymer impregnation technique are seen as the potential method of utilizing the advantages of both, i.e., a ductile material of high toughness equal to 228 times that of normal mortars. Similarly, a fibrous ferrocement composite can be regarded as a future composite of high potential.

14.2

LIGHTWEIGHT CONCRETE

The conventional cement concrete is a heavy material having a density of 2400 kg/m3, and high thermal conductivity. The dead weight of the structure made up of this concrete is large compared to the imposed load to be carried, and a relatively small reduction in dead weight, particularly for members in flexure, e.g., in highrise buildings, can save money and manpower considerably. The improvement in thermal insulation is of great significance to the conservation of energy. The reduction in dead weight is normally achieved by cellular construction, by entraining large quantities of air, by using no-fines concrete and lightweight aggregates which are made lighter by introducing internal voids during the manufacturing process. The term no-fines indicates that the concrete is composed of cement and coarse aggregate (commonly 10 or 20 mm grading) only, the product has uniformly distributed voids. Suitable aggregates used are natural aggregates, blast-furnace slag, clinker, foamed slag, sintered fly ash, expanded-clay, etc. Lightweight aggregate is a relatively new material. For the same crushing strength, the density of concrete made with such an aggregate can be as much as 35 per cent lower than the normal weight concrete. In addition to the reduced dead weight, the lower modulus of elasticity and adequate ductility of lightweight concrete may be advantageous in the seismic design of structures. Other inherent advantages of the material are its greater fire resistance, low thermal conductivity, low coefficient of thermal expansion, and lower erection and transport costs for prefabricated members. For prefabricated structures a smaller crane is required or the same crane can

464

Concrete Technology

handle larger units due to reduction in dead weight. For cast in-situ structures, its smaller dead weight makes foundations less expensive. Moreover, continued extraction of conventional dense natural aggregate from the ground is bound to be accompanied by severe environmental problems leading to deterioration of the countryside and its ecology. On the other hand, use of manufactured aggregates made of industrial wastes (slags, etc.,), preferably those containing sufficient combustible materials (pulverized fuel ash) which provide all or most of the energy for their production, may help in alleviating the problem of disposal of industrial waste.

14.2.1

Lightweight Aggregates

Lightweight aggregates may be grouped in the following categories: 1. Naturally occurring materials which require further processing, such as expanded clay, shale and slate, etc. 2. Industrial by-products, such as sintered pulverized fuel ash (fly ash), foamed or expanded-blast-furnace slag. 3. Naturally occurring materials, such as pumice, foamed lava, volcanic tuff and porous limestone.

Aggregates Manufactured from Natural Raw Material The artificial lightweight aggregates are mainly made from clay, shale, slate or pulverized fuel ash, subject to a process of either expansion (bloating) or agglomeration. During the process of expansion the material is heated to fusion temperature at which point pyroplasticity of material occurs simultaneously with the formation of gas. Agglomeration on the other hand occurs when some of the material fuses (melts) and various particles are bonded together. Thus to achieve proper expansion a raw material should contain sufficient gas-producing constituents, and pyroplasticity should occur simultaneously with the formation of gas. The gas may form due to decomposition and combustion of sulfide and carbon compounds; removal of CO2 from carbonates or reduction of Fe2O3 causing liberation of oxygen. The common examples of natural minerals suitable for expansion are clay, shale, slate and perlite and exfoliated vermiculite. 1. Expanded or bloated-clay Bloated-clay aggregates are made from a special grade of clay suitable for expansion. The ground clay mixed with additive which encourages bloating, is passed through a rotary or vertical shaft kiln fired by a mixture of pulverized coal and oil with temperature reaching about 1200 °C. The material produced consists of hard rounded particles with a smooth dense surface texture and honeycomb interior. 2. Expanded shale The crushed raw material such as colliery waste, blended with ground coal is passed over a sinter strand reaching a temperature of about 1200 °C. At this temperature, the particles expand and fuse together trapping gas and air within the structure of the material with a porous surface texture.

Special Concretes and Concreting Techniques

465

3. Expanded slate The crushed raw material is fed into a rotary kiln with temperature reaching 1200 °C. The material produced is chemically inert and has a highly vitrified internal pore structure. This material is then crushed and graded. 4. Exfoliated vermiculite The raw material resembles mica in appearance and consists of thin flat flakes containing microscopic particles of water. On being suddenly heated to a high temperature of about 700−1000 °C, the flakes expand (exfoliate) due to steam forcing the laminates apart. The material produced consists of accordion granules containing many minute air layers.

Industrial By-product Lightweight Aggregate These include sinteredpulverized fuel ash, foamed-blast-furance slag and pelletized slag. 1. Sintered-pulverized fuel ash The fly ash collected from modern power stations burning pulverized fuel, is mixed with water and coal slurry in screw mixers and then fed onto rotating pans, known as pelletizers, to form spherical pellets. The green pellets are then fed onto a sinter strand reaching a temperature of 1400 °C. At this temperature, the fly ash particles coagulate to form hard brick-like spherical particles. The produced material is screened and graded. 2. Foamed-blast-furnace slag It is a by-product of iron production formed by introducing water or steam into molten material. The material produced after annealing and cooling is angular in shape with a rough and irregular glassy texture, and an internal round void system.

Naturally Occurring Lightweight Aggregates The common examples are pumice and diatomite. Pumice is light and strong enough to be used in its natural state, but has variable qualities depending upon its source. It is chemically inert and usually has a relatively high silica content of approximately 75 per cent. Diatomite, on the other hand, is a semiconsolidated sedimentary deposit formed in cold water environment. Production In India, raw lightweight aggregates are produced by using any of the following: 1. Bloated-clay aggregates by bloating suitable clays with or without additives 2. Sintered-fly-ash aggregates by sintering the fly-ash. 3. Lightweight aggregate from blast-furnace slag. In one of the processes for manufacturing lightweight concrete, the cement and pulverized sand are first mixed in a certain proportion (1 : 1 for insulation and 1 : 2 for partitioning purposes). The mixture so formed is then made into slurry with the addition of a predetermined quantity of water. The sand-cement slurry is next foamed to the extent of predetermined volume with the help of a foaming compound. The foam product is thereafter poured into molds. The molded blocks are finally

466

Concrete Technology

cured under elevated hydrothermal conditions in autoclaves which imparts strength, reduces drying shrinkage and gives the block a creamy color. In another product, lime and sand are used as raw materials. Both are first ground to fine powder in huge ball mills. The mixture is then made into slurry with the addition of water. Adding aluminum powder and gypsum to the slurry triggers a chemical reaction, and the hydrogen gas evolved gives the cellular concrete its lightness. After initial hardening, it is cut into convenient sizes and the molded blocks are finally cured under elevated hydrothermal conditions (under a pressure of 12 atmospheres and temperature of 196 °C). The suitability of a particular lightweight aggregate is determined by the specified compressive strength and the density of concrete.

14.2.2 Properties of Lightweight Aggregates The properties of the manufactured lightweight aggregates depend mainly on the raw material, and the process of manufacture. The properties of aggregates manufactured from materials which occur as industrial by-products can be altered to a limited extent only by the processes of bloating, foaming, sintering, agglomerating and crushing. Since the aggregate make up approximately 75 per cent of the total volume of the concrete, it influences the workability, strength, modulus of elasticity, density, durability, thermal conductivity, shrinkage, and creep properties of concrete. The structural concrete should have a high strength with low density, high modulus of elasticity, and low rate of shrinkage and creep. On the other hand, a lightweight aggregate concrete should possess low thermal conductivity. The thermal conductivity decreases with decreasing density, therefore the density of the concrete must be as low as possible. The most suitable aggregates for structural lightweight concrete are expanded-clay, shale and slate, fly ash and colliery waste. Adequate strength for structural lightweight aggregate concrete can be obtained with foamed and expanded-blast-furnace slag. For lightweight aggregate concrete for thermal insulation, the suitable aggregates are pumice, perlite, vermiculite, diatomite and expanded-polystyrene. A surface texture with tiny and uniformly distributed pores is preferred. Particle size and shape as well as surface condition of aggregates influence properties of fresh concrete. Crushed and angular lightweight aggregate requires high mortar content resulting in a higher density than that with rounded aggregate. The strength of the lightweight aggregate particles decreases with decreasing density. The density, bulk density and water absorption capacity of some of the commonly used lightweight aggregates are given in Table 14.1. The compressive strengths and unit weights of typical concretes produced by these aggregates are also given in the table.

14.2.3

Mix Proportions

Due to large variations in the characteristics of lightweight aggregates, it is difficult to seek a single approach to mix design for structural lightweight aggregate concrete. However, following points should be considered:

Particle shape, and surface texture

Similar to expanded clay.

Irregular angular

particles with rough and open pored surface.

Angular with open-pored surface.

Expanded shale and slate

Fly ash

Foamed-blast-

furnace slag

Sintered-colliery waste

Rounded particles with open-textured but rather smooth surface.

Rounded and of angular shape and rough surface.

Cubical

Perlite

Vermiculite

C. Aggregate for low strength concrete (0.5 to 3.5 MPa)

Pumice

B. Aggregate for low-medium strength concrete (3.5 to 15 MPa)

Rounded and slightly rough particles. Often angular and slightly rounded, smooth surface.

Expanded clay

A. Aggregate for structural concrete ( fck > 15 MPa)

Aggregate Type

100 to 400

100 to 400

550 to 1650

1000 to 1900

1000 to 2200

1300 to 2100

Coarse 600 to 1600 Fine 1300 to 1800 Coarse 800 to 1400 Fine 1600 to 1900

60 to 200

40 to 200

350 to 650

500 to 1000

400 to 1100

600 to 1100

400 to 1200

300 to 900

Bulk density (kg/m3)

—

—

50

15

10 to 15

1.2 to 3.0

1.2 to 3.0

5 to 15

10 to 40

10 to 45

30 to 60

20 to 50

5 to 15 20

10 to 60

Compressive strength (MPa)

300 to 700

400 to 500

1200 to 1600

1400 to 1600

1800 to 2000

1500 to 1600

1300 to 1600

1000 to 1700

Unit weight (kg/m3)

Typical concrete

5 to 30

24-hour water absorption capacity (per cent)

Physical properties of lightweight aggregate

Density (kg/m3)

Table 14.1

Special Concretes and Concreting Techniques 467

468

Concrete Technology

As in case of normal weight concrete, the lightweight aggregate concrete can attain the strength of mortar matrix only if the strength and stiffness of the aggregate are at least as high as those of the mortar. Below this limit, internal stress transfer takes place in the same way as in normal weight concrete. In this case, concrete strength is approximately equal to the strength of mortar. The water-cement ratio and mix proportions applicable to ordinary concrete, can be adopted, however instead of total water only the effective water must be taken into account. It is, of course, also possible to manufacture lightweight concretes with higher strength than the limit strength mentioned above by using a stronger mortar (having greater stiffness) with a higher density. In this concrete, the mortar matrix will transmit higher stress at the same deformation. For economic reasons, it is preferable to select a stronger aggregate such that the required concrete strength can be attained with the mortar of lower strength. For a concrete of given compressive strength, a strong aggregate requires a low mortar strength and a weak aggregate requires a high mortar strength. Since aggregate strength and its modulus of deformation is not usually available, the suitability of a lightweight aggregate for a specific application is generally assessed by means of the particle density or bulk density. For structural lightweight concrete, the maximum nominal size of the aggregate is limited to 20 mm since the modulus of deformation, strength and density of aggregate particles decrease as particle size increases. On the other hand, a lower maximum size and a large proportion of fines may lead to higher strength but the concrete density will increase. Natural sand is often used to improve the workability and reduce the shrinkage of fresh concrete and increase its strength, but it will increase the density of concrete. The conventional water-cement ratio rule is not suitable for lightweight concrete. In lightweight concrete, the water content to be taken into account for calculation of water-cement ratio is not the total quantity of water present but only the effective or free water. The relationship between strength and water-cement ratio varies from aggregate to aggregate. The effect of cement strength on the strength of concrete is not linear. The main problem of lightweight concrete mix design lies in the advance determination of effective water and air contents of cement matrix at the moment of completion of compaction of concrete. The prediction is difficult to make since throughout the mixing process the effective (free) water content is progressively reduced through absorption by aggregate, except when completely saturated aggregate is used. The combined free-water and air contents can be approximately estimated as the residual absolute volume, when the density of fresh concrete, the mix proportions and particle density are known. The residual absolute volume, Vres, is obtained by subtracting the volume of the solid cement and aggregates from the total volume of concrete. ⎡ ⎛C A ⎞⎤ Vres = Vw + Vair = 1000 ⎢1− ⎜ S − S ⎟ ⎥ a ⎠⎦ ⎣ ⎝ c where, Vw and Vair are the free-water and air contents in litre/m3, respectively. C and A denote cement and aggregate contents in kg/m3, respectively. Sc and Sa are the density of cement and mean particle density of aggregates in kg/m3, respectively. In

Special Concretes and Concreting Techniques

469

contrast to normal concrete, the relationship between residual absolute volume-cement ratio and the strength of lightweight concrete vary from aggregate to aggregate. As the residual absolute volume-cement ratio decreases the concrete strength increases, however the increase is less than that for normal concrete. For every type of lightweight aggregate the compressive strength of the concrete bears a definite relationship to the residual absolute volume-cement ratio and to the cement strength. This characteristic can facilitate the design of lightweight concrete mixes. The optimum cement content may be determined by trial mixes. In general, for first trial, the cement content required for ordinary sand and gravel concrete may be used, but more cement is normally required for most lightweight aggregate concretes. There are several methods to determine the aggregate content. In this section, a method using effective water-cement ratio for the calculation of aggregate content is described. This method of lightweight aggregate concrete mix design which is based on water-cement rule and is an adaptation of the well-known British mix design method has been suggested by F.I.P. The steps involved to obtain the mix proportions for the stipulated 28-day strength of concrete are the following: 1. The target mean strength of the concrete is determined from the characteristic strength. 2. The water-cement ratio for the required target strength is read off from Fig. 14.1. 28-Day Cube Compressive Strength, MPa

50

40

30

20

10 0.4

0.5

0.6

0.7

0.8

0.9

1.0

Effective f Water–Cement Ratio by Mass

Fig. 14.1

Typical relationship between effective water-cement ratio and compressive strength of lightweight aggregate concrete

3. For the water-cement ratio determined in step 2. aggregate-cement ratio (by volume), cement content in kg/m3 and optimum percentage of fine aggregate for the desired workability are selected from Table 14.2.

470

Concrete Technology

Table 14.2 Water– cement ratio

Variation of cement content, aggregate-cement ratio, and fine aggregate content with water-cement ratio for the lightweight aggregate concrete Degree of workability High: Compacting factor = 0.95 slump = 75–150 mm

Medium: Compacting factor = 0.90 slump = 15–75 mm

Cement content (kg/m3)

Aggregatecement ratio

Fine aggregate (per cent)

Cement content, (kg/m3)

aggregate cement ratio

Fine aggregate (per cent)

0.40

525

3.00

28

591

2.65

27

0.45

468

3.50

30

533

3.03

28

0.50

419

4.04

32

474

3.50

30

0.55

372

4.63

35

425

4.01

32

0.60

332

5.35

40

383

4.55

34

0.65

293

6.05

49

342

5.23

38

0.70

260

6.83

58

306

5.99

44

0.75

230

8.02

69

273

6.75

53

4. The cement and aggregate volumes are converted to mass contents by multiplying with their dry bulk densities. 5. The water content is adjusted for the absorption and moisture content of aggregates to obtain effective free-water content for the mix. 6. A trial mix is prepared and water content is adjusted to maintain the desired workability. The density of fresh wet compacted concrete is calculated and cement content checked. If it is not correct, minor corrections are made by adding or subtracting cement and subtracting or adding the same volume of fine aggregate as follows: Volume of fresh concrete = Absolute volume of ingredent Weight of cement Weight of fine aggregate + Density of cement Saturate t d surface dry *Density of fine aggregate Weight of coarse aggregat ae + + Air + Free water e Saturated surface dry *Density of coarse aggrate

1000 litres =

For other lightweight aggregates, necessary reference curves of effective watercement ratio against 28-day cube crushing strength, and tables should be prepared with the help of experimental results on laboratory mixes using varying fine-coarse aggregates ratios. Proportion a lightweight concrete mix for a 28-day characteristic compressive strength of 30 MPa. The degree of workability envisaged is 0.95 CF and the degree of quality control available at the

Example 14.1

* mean particle density after 1/2 hour soaking.

Special Concretes and Concreting Techniques

471

site may be termed as good. The unit weights of cement, fine and coarse aggregates available are 1425 kg/m3, 1100 kg/m3 and 850 kg/m3, respectively. The moisture contents of fine and coarse aggregates (by mass) are 10 and 6 per cent, respectively, and short term absorptions (by mass) are 5 and 11 per cent, respectively.

Procedure: = 30 + 1.65 × 6 _~ 40 MPa

Target mean strength, ft (i) From Fig. 14.1

= 0.53

Water-cement ratio (ii) From Table 14.2

= 3.794 _~ 3.8 = 31.2 _~ 31.0 = 444.6 _~ 445 kg/m3

Aggregate−cement ratio Fine aggregate, per cent Cement content

(iii) Dry volumes per cubic meter of concrete Cement = 445/1425 Aggregate = 3.8 × 0.312 Fine aggregate = 31% of 1.186 Coarse aggregate = 1.186 − 0.368 (iv) Batch masses of ingredients for 1.0 m3 of concrete, kg Ingredient

water

: cement :

fine aggregate

0.53 × 445

445

0.368 × 1100

Apparent masses Adjustment for moisture content Absorption of aggregate Fine 5% × 405 Coarse 11% × 695 Corrected dry batch masses

14.2.4

236 (−)40.5 (−)41.5

445

405 (+)40.5

:

= 0.312 m3 = 1.186 m3 = 0.368 m3 = 0.818 m3 coarse aggregate 0.818 × 850 695 (+)41.5

+20.0 +76.0 250 0.56

445 1.0

445.5 1.0

736.5 1.65

Properties of Structural Lightweight Concrete

With suitable lightweight aggregates, a concrete can be produced with densities which are 25 to 40 per cent lower but with strengths equal to the maximum normally achieved by ordinary concrete. The fracture mechanism of lightweight aggregate concrete under compression differs from that of a normal weight concrete. In normal weight concrete, the tensile stress is generated on the interface between aggregate and mortar matrix directed transversely to the direction of external loading. With lightweight aggregate

472

Concrete Technology

concrete, the tensile stress also acts transversely to the direction of external loading, but immediately above and below the aggregate due to diversion of greater part of force around the aggregate because of its lower modulus of deformation. The internal cracks which lead to fracture are initiated at all the places in concrete where tensile stress exceeds the actual tensile strength. The important characteristics of lightweight concrete are: 1. Low density The density of the concrete varies from 300 to 1200 kg/m3. The lightest grade is suited for insulation purposes while the heavier grades with adequate strength are suited for structural applications. The low density of cellular concrete makes it suitable for precast floor and roofing units which are easy to handle and transport from the factory to the sites. 2. High strength Cellular concrete has high compressive strength in relation to its density. The compressive strength of such concrete is found to increase with increasing density. The tensile strength of aerated cellular concrete is about 15 to 20 per cent of its compressive strength. Due to a much higher strength-to-mass ratio, the cellular concrete floor and roof slabs are approximately of one quarter of the weight of normal reinforced concrete slabs. 3. Thermal insulation The insulation value of lightweight concrete is about three to six times that of bricks and about ten times that of concrete. A 200 mm thick wall of aerated concrete of density 800 kg/m3 has the same degree of insulation as a 400 mm thick brick wall of density 1600 kg/m3. 4. Fire resistance Lightweight concrete has excellent fire resisting properties. Its low thermal conductivity makes it suitable for protecting other structures from the effects of fire. 5. Sound insulation Sound insulation in cellular concrete is normally not as good as in dense concrete. 6. Shrinkage Lightweight concrete is subjected to shrinkage but to a limited extent. The autoclaving of cellular concrete reduces drying shrinkage to one-fifth of that occurring during air curing. 7. Repairability Lightweight products can be easily sawn, cut, drilled or nailed. This makes construction easier. Local repairs to the structure can also be attended to as and when required without affecting the rest of the structure. 8. Durability–Aerated concrete is only slightly alkaline. Due to its porosity and low alkalinity it does not give rust protection to steel which is provided by dense compacted concrete. The reinforcement used, therefore, requires special treatment for protection against corrosion. 9. Speed of construction With the adoption of prefabrication, it is possible to design the structure on the concept of modular coordination which ensures a faster rate of construction. 10. Economy Due to light weight and high strength-to-mass ratio of cellular concrete products, their use results in lesser consumption of steel. Composite floor construction using precast unreinforced cellular concrete blocks and reinforced concrete grid beams (ribs) results in appreciable

Special Concretes and Concreting Techniques

473

saving in the consumption of cement and steel, and thereby reduces the cost of construction of floors and roofs considerably. A saving of as much as 15 to 20 per cent in the cost of construction of floors and roofs may be achieved by using this type of construction compared to conventional construction. 11. Quality control A better quality control is exercised in the construction of structure with lightweight concrete products owing to the use of factory made units.

14.2.5

Applications

Different uses of lightweight (aerated) concrete can be summarized as follows: 1. As load bearing masonry walls using cellular concrete blocks. 2. As precast floor and roof panels in all types of buildings. 3. As a filler wall in the form of precast reinforced wall panels in multi- storeyed buildings. 4. As partition walls in residential, institutional and industrial buildings. 5. As in-situ composite roof and floor slabs with reinforced concrete grid beams. 6. As precast composite wall or floor panels. 7. As insulation cladding to exterior walls of all types of buildings, particularly in office and industrial buildings. The Bureau of Indian Standards has published several codes for controlling the production of autoclaved cellular concrete in India. The relevant code of practice for ordinary concrete, viz. IS: 456−2000, is also applicable to the design of reinforced lightweight concrete structural elements.

14.3

ULTRA-LIGHTWEIGHT CONCRETE

The ultra-lightweight concrete, with unit weights or densities ranging from 600 to 1000 kg/m3, is made from a mixture of cement, sand (omitted for concrete having unit weight or density less than 600 kg/m3) and expanded-polystyrene beads one to six millimeters in diameter. This concrete has a high thermal insulation efficiency and is mainly used for prefabricated non-load bearing panels, hollow and solid blocks, lightweight sandwich panels and in highway construction as a part of sub-base where frost could endanger the stability of the subgrade. Polystyrene beads or foam manufacture is essentially a polymerization process which utilizes liquid styrene monomer dispersed in an aqueous medium containing foaming or expanding agent and polymerization catalyst. The foam or expanded bead products may be treated with bromine solutions for improving the fire retardance and self-extinguishing characteristics. The expanded beads normally have a density between 12 to 25 kg/m3, however sheet form produces densities up to 30 to 75 kg/m3. The commonly used size range of expanded beads is 1 to 3 mm. When exposed to sunlight, the foam or beads deteriorate producing a characteristic yellowing. The expanded-polystyrene concrete mixes can be designed to have compressive strengths up to 15 to 20 MPa at densities around 1600 kg/m3. The mix design aims

474

Concrete Technology

at achieving an economic and optimum balance between density, thermal insulation and strength. The expanded-polystyrene beads become electrostatically charged during processing which makes them difficult to wet (hydrophobic) during mixing. Proneness to segregation can be overcome by using a bonding agent (normally water-emulsified epoxies, and aqueous dispersions of polyvinyl propionate) and by controlling the fluidity of the paste or mortar. Compressive strength and thermal insulation properties of expanded-polystyrene concrete increase with its density. Due to reduced specific thermal capacity the heat of hydration of the cement causes greater and more rapid increase in temperature in this concrete than in the concrete made with conventional aggregate resulting in accelerated rate of setting and hardening. The setting and hardening rates can be controlled by selecting the appropriate cement and using water-reducing admixtures. The conventional workability tests namely slump test, compacting factor test. Vee-Bee test, and flow table test are unsuitable in case of expanded-polystyrene concrete. Conventional techniques can be used for casting and placing expanded-polystyrene concrete. The mechanical properties of expanded-polystyrene concrete are a function of density as in the case of other lightweight aggregate concretes, but have comparatively lower values. The elastic and shrinkage deformations are considerably greater than for normal-weight concrete.

14.4

VACUUM CONCRETE

In concreting thin sections like slabs and walls, a fluid mix with water–cement ratio of 0.50 to 0.65 is required to facilitate the placing and compaction. Such a mix will lead to relatively low strength and poor abrasion resistance. In such situations, the vacuum treatment of concrete, involving the removal of excess water and air by using suction can be helpful. An arrangement for vacuum dewatering of concrete using suction through a surface mat connected to a vacuum pump is shown in Fig. 14.2. The duration of treatment depends upon the water–cement ratio and the quantity of water to be removed. It generally ranges from 1 to 15 minutes for slabs varying in thickness from 25 mm to 125 mm. The effect of treatment is more pronounced in the beginning and falls off rapidly. Hence, it is of no advantage to prolong the periods of treatment beyond these values. The sequence of operations for vacuum dewatering the concrete are illustrated in Fig. 14.3. The vacuum treatment is not very effective for water−cement ratios below 0.4. The suction pressure on the concrete is about one-third the atmospheric pressure. The vibration of concrete before vacuum treatment can assist the process. The application of vibration simultaneously with vacuum treatment after initial vibration is very effective. Continued vibration beyond 90s may damage the structure of concrete, and hence the vibrations should be stopped beyond this period and only vacuum needs to be applied for the remaining duration of the treatment. The vacuum treatment has been found to considerably reduce the time of final finishing of floor and stripping of wall forms. The strength of concrete and its resistance to wear and abrasion increases and total shrinkage is reduced. Vacuum-treated concrete provides a good bond with the underlying concrete.

Air-tight seal all round Filter screen ose pipe

Wooden frame

alve Expanded metal

Fine muslin cloth

Concrete (a) Schematic diagram showing vacuum dewatering process To vacuum pump Atmospheric pressure

Floor to be dewatered

Capillaries that bring out excess water

(b) Vacuum process

Fig. 14.2

Typical arrangement for vacuum dewatering of concrete

1. A typical dewatering system

ewatering the floor by vacuum

Fig. 14.3

2. Screeding floor before dewatering

Finishing the floor by power toweling

Sequence of operations for vacuum dewatering the concrete

476

Concrete Technology

Vacuum concrete attains its 28-day compressive strength in 10 days and has a 25 per cent higher crushing strength. The details of structural properties enhancement are: 1. Compressive strength of floor increases by up to 60 per cent. 2. Tensile strength increases approximately by 70 per cent. 3. Cement consumption is reduced to the extent of 40 per cent; no cement is required separately for finishing the surface. 4. Abrasion resistance of the floor increases nearly by 60 per cent. 5. Shrinkage of concrete and wrapping of floor are reduced. The vacuum-processed concrete is extensively used for factory production of precast plain and reinforced concrete units. The other important application is in the construction of horizontal and sloping concretes slabs, such as floor slabs, road and airport pavements, thin load-bearing and partition walls. Vacuum treatment can also be effectively used in the resurfacing and repair of road pavements.

14.5

MASS CONCRETE

The concrete placed in massive structures like dams, canal locks, bridge piers, etc., can be termed mass concrete. A large-size aggregate (up to 150 mm maximum size) and a low slump (stiff consistency) are adopted to reduce the quantity of cement in the mix to about five bags per cubic meter of mass concrete. The mix, being relatively harsh and dry, requires power vibrators of immersion type for compaction. The concrete is generally placed in open forms. Because of the large mass of the concrete, the heat of hydration may lead to a considerable rise of temperature. Placing the concrete, in shorter lifts and allowing several days before the placement of the next lift of concrete can help in the dissipation of heat. Circulation of cold water through the pipes buried in the concrete mass may prove useful. Alternatively, where possible concreting can be done in the winter season such that the peak temperature in concrete can be lowered, or the aggregates may be cooled before use. The high temperature of mass concrete due to the heat of hydration may lead to extensive and serious shrinkage cracks. The shrinkage cracks can be prevented by using low heat cement and by continuous curing of concrete. The mass concrete develops high early age strength but the later age strength is lower than that of continuously cured concrete at normal temperatures. The volume changes of mass concrete during setting and hardening are small, but the concrete is susceptible to large creep at later ages.

14.6

ROLLER-COMPACTED CONCRETE

Roller compacted concrete (RCC) is a mixture of aggregates, cement with or without supplementary cementing materials, water and in some cases water-reducing admixture, proportioned to support external compaction equipment. This stiff (dry and lean), zero-slump concrete mixture has the consistency of damp gravel. The low water content requires it to be mixed in a continuous flow system, typically in a pug mill mixer, spread with a modified asphalt paver and compacted with a roller. The resulting product is a construction material with the strength and characteristics of conventional concrete.

Special Concretes and Concreting Techniques

477

A pug mill plant stationed at or near the site is used because it is easily portable and allows a steady and continuous flow of materials for high productivity. The pug mill provides a vigorous mixing action, which is necessary to disperse the small amount of mixing water throughout the RCC. The process requires no forms, finishing, surface texturing, or joint sawing and sealing. In comparison to conventional concrete, RCC requires only 60 to 75 per cent cement and avoids the undesirable conditions created by the high heat of hydration in mass concrete. Water content is so low that the freshly mixed RCC looks like damp gravel, if the site conditions warrant, it can be transported in dump trucks or paver or loaders to the construction site and spread with bulldozers or conventional asphalt spreaders or graders in 200−300 mm thick layers, called lifts. Lifts are then compacted using vibratory steel-wheel and pneumatic tire rollers, shown in Fig. 14.4, to 95−100 per cent of the specified density. Immediately after completion of compaction, water is sprayed as a fine mist to cure the concrete.

Multipurpose road paver

Vibratory steel wheel and pneumatic tire rollers

Fig. 14.4

Multipurpose road paver, and vibratory steel-wheel, and pneumatic tire rollers

Since the pavement roller compacted concrete can be sized and shaped with conventional paving machines; the construction of gravity dams using roller compacted concrete employs methods and equipment similar to those used for earth dams, thus considerably less labour is required.

478

Concrete Technology

A major concern in roller concrete design is to obtain an adequate bond between the lifts. The drier consistency of RCC mix, particularly mixtures containing aggregates larger than 40 mm nominal size, creates problems in bonding fresh lift to the hardened lift. Improved bond may be obtained by restricting the time interval between placement of lifts, by providing supplemental joint treatment or by increasing the paste content of the mixture.

14.6.1

Construction Process

For effective consolidation, concrete mixture must be dry enough to support the weight of the vibratory equipment but wet enough to permit adequate distribution of paste binder throughout the mass during mixing and vibration process. Roller compacted concrete construction techniques for mixing, transporting, and placement are controlled primarily by material characteristics in relation to moisture retention, segregation, and compaction. In the wet stage, it exhibits the material handling properties of a moist, granular soil; hence standard earthmoving equipment can be used for transportation, placement and compaction. After the Roller compacted concrete hardens, it is a concrete with the physical properties and a finished appearance of conventional concrete. The construction process for RCC is a three-step cycle that results in continuous placement of mix to eliminate the delay in laying the subsequent lifts. The three steps are: (i) mix proportioning, (ii) transportation to site, and (iii) spreading, compaction and curing. Following are the factors affecting mix proportioning: 1. Cement The cement and cementing material content in RCC normally varies approximately between 12 and 15 per cent of dry materials, i.e., from 250 to 350 kg/m3 in the wearing course, and in the base course it ranges from 100 to 150 kg/m3. 2. Pozzolana Modern RCC is in fact fly ash modified lean concrete containing high volume of fly ash and very low water content. Use of fly ash in RCC mixtures is an effective technique of increasing paste volume and improving compactability. It also results in low heat of hydration, lower shrinkage and delay in start and finish of the setting process, ease in spreading and compacting operations. Fly ash improves long-term strength and the durability under adverse conditions and chemical action of chlorides and sulfates. 3. Aggregate The aggregate comprises approximately 75 to 85 per cent of the volume of RCC mixture and therefore, significantly affects both fresh and hardened concrete. The use of continuously graded aggregates is desirable for best results, as it affects the relative compactability of the concrete and influences the minimum number of vibrating roller passes required for full consolidation of a given lift thickness. It also affects the water and cementing material requirements to fill the voids in the aggregate and coat the aggregate particles to produce a solid concrete volume. The ideal grading for minimum paste requirements would be one, which produces the maximum dry rodded density with the least surface area. Depending upon the thickness of placement the coarse aggregates of maximum nominal size ranging from 10 to 225 mm can be used. To avoid risk of segregation and non-bonding of lifts, and to obtain a better surface quality of pavement, aggregates with a maximum size of 20 mm are preferred. However,

Special Concretes and Concreting Techniques

479

for low traffic application, aggregate with maximum size up to 40 mm may be used and it is necessary that two-third of the aggregate used is of the crushed type. 4. Admixture Air entraining, water reducing and set controlling admixtures are effective in reducing the vibration time required for full consolidation of the RCC. Full consolidation lowers the entrapped air-void content, increases strength, and lowers the permeability of the concrete. The proportions of RCC mixture primarily differ from that of conventional concrete mixture for its lower water content; a lower paste content; larger fine aggregate content in order to produce a combined aggregate that is well graded and stable under the action of a vibratory roller; and maximum aggregate size not greater than 20 mm in order to minimize segregation and produce a relatively smooth surface texture. The concrete for RCCP is continuously graded with special emphasis on the grading of fine aggregates in order to attain high density and a smooth texture of slab surface. Moreover, unlike in plain cement concrete (PCC) the water−cement ratio law does not hold good in case of RCC which is a no-slump concrete, it holds only in case of workable mixes. In RCC mix design, water is selected from optimum moisture content (OMC) at maximum dry density (MDD). Chemical reaction of water with cement is a secondary effect. By adding cementing mineral additives ( pozzolanas), RCC mix can be redesigned and modified as per performance requirements. As compared to conventional concrete, typical RCC mixes contain lower cement and cementing material paste (250−350 kg/m3) and significantly higher fly ash proportion ranging from 25−70 per cent by weight of cementing material. RCC mixes usually attain 28-day strengths similar to, or even greater than the conventional concrete. The trend of growth of flexural strength of RCC pavement with curing period is compared with typical conventional concrete pavement in Fig. 14.5. 1.1

Fraction of 28-day Flexural Strength

a. Roller compacted concrete pavements b. Typical T of conventional concrete pavements 1.0

0.9

0.8 a 0.7

0.6

0.5

Fig. 14.5

b

3

7 Curing Period, days

28

Trend of growth of flexural strength of RCC pavement with curing period

480

Concrete Technology

Typical mix proportions, by mass, of roller compacted concrete (RCC) and conventional concretes (PCC) for a 91-day strength of 25 MPa are compared in Table 14.3. Table 14.3 Type of Water- Cement Concrete binder ratio RCC PCC

0.440 0.475

0.78 1.00

Typical mix proportions of RCC and PCC Fly ash

Fine aggregate

0.22 −

1.78 1.75

Coarse aggregate, mm 10–4.75

20–10

Water content, kg/m3

1.33 1.05

2.0 1.58

160 190

After mixing, RCC is transported to the site, and is placed and compacted in lifts in final position, with vibratory rollers to maximum density. First and second compaction is done by vibrating roller (first without vibration, second with vibration) and finishing compaction is done by pneumatic tire roller. After the compaction, the paved surface is covered by curing mat and cured by water spray. In comparison to construction with conventional mass concrete, the RCC construction is economical due to number of factors such as: reduction in quantity of cement, reduction in manual work, elimination of formwork for transverse joints, easier clearing of joints, and lesser rigidity in the specifications for concrete aggregates. Fines to a greater extent are permitted, which reduces the requirement of processing. One of the most significant advantages of RCC is the speed of construction, which in turn, may lead to cost savings of the order of 15 to 35 per cent. Compared to conventional concrete, RCC exhibits lower shrinkage. Smallest value of dry shrinkage is achieved with medium range graded aggregates and optimum moisture content. Due to low cement content it results in lesser thermal stresses and hence lesser cracks. The creep is considerably less, as fly ash reduces the creep strain considerably. The maximum dry density of RCC increases and optimum moisture content decreases with increasing compacting efforts in accordance with soil compaction theory. Unit weight of Roller compacted concrete is typically equal or slightly higher than that of conventional concrete.

14.6.2

Applications

Due to the low water-cement ratio and good aggregate interlocking, RCC pavement is generally very strong and can carry heavy loads. As the same equipments as for asphalt pavement are used, rapid construction is done in short time, and since the strength growth is fast enough the opening for traffic in a short time can be permitted. Due to small drying shrinkage the interval between joints can be maximized. RCC has adequate resistance to freezing and thawing. It has established itself as a fast economical construction method for dams, off-highway pavement projects, heavyduty parking and storage areas, and as a base for conventional pavement. Other common applications are in ordinary roads, roads in factories, temporary roads for construction works, parking areas, service areas, container yards, material-handling yards, apron and carriageway of airports, for binder course of expressway and heavily trafficked roads.

Special Concretes and Concreting Techniques

481

Construction of Roller Compacted Concrete Pavement (RCCP) RCC is weigh-batched and mixed in a continuous mixing pug mill or a normal mixer such as used for soil−cement treated base or asphalt concrete construction. The pavement is initially constructed in lifts of 150−200 mm for a pavement thickness of more than 400 mm with an elapse time of 30 minutes to 2 hours between the lifts. A common paver shown in Fig. 14.6 can place and compact a 150 mm lift to 95 per cent of the specified density in only two or three passes of roller. The lower lift requires more

Fig. 14.6

Construction of concrete pavement with a common paver

compactive effort with more passes of the roller, to achieve the desired density. If a smooth pavement surface is not obtained, a layer of asphalt may be used to cover the surface and smooth out the roadway. Vee-Bee consistometer apparatus with some modification can be used to evaluate RCC consistency. Curing of RCCP is accomplished by keeping the surface of the pavement wet for seven days. Water spray or fine mist is most appropriate and is commonly used. Wet sand can also be used for curing RCC. Due to very low drying shrinkage of RCC as compared to PCC, the contraction joints are provided at a spacing of 10 to 20 meter whereas the spacing between contraction joints is 4.5 meter in PCCP. RCCP as a result of the high stability achieved by the mineral skeleton formed by the aggregates after compaction can be opened to traffic almost immediately. Furthermore, the stability of the mineral structure allows high volume of mineral binders to be used than that used in the vibrated concrete pavements. The total thickness of an RCCP structure is much less than an asphalt/gravel pavement of the same loadcarrying capacity. Due to its advantages as a comparatively low cost and durable paving material, RCC is also emerging as a common base for conventional highway and street pavements. Thus RCC is a material that has the longevity of concrete at the price of asphalt.

482

Concrete Technology

Dam Construction Applications of RCC technology to dam construction has enabled the concrete to play an increasingly important role in the construction and rehabilitation of dams. Typically, in RCC dam construction, no-slump concrete mix is spread in 300 mm thick lifts. The RCC mixture is usually mixed at a temporary plant erected near the dam site and transported by conveyor belt, front-end loader, or dump truck to the placement site. The newly placed lift of RCC, is compacted with a vibratory roller. Continuous placement of RCC is desirable on dam projects to minimize cold joints between the horizontal lifts that could inhibit bonding of the concrete lifts to each other. RCC can be used to overlay the downstream slope of the existing embankment dam to protect the dam from erosion if the structure is overtopped by water. The RCC construction is less expensive than conventional methods of dam construction, in part, because RCC dam projects can be completed faster than embankment dam projects since they require lesser volume of material.

14.6.3

Limitations

Unlike concrete pavement, RCCP surface is uneven as in the case of flexible pavement. It is therefore, necessary to add a wearing course several centimeters thick made of bituminous concrete or, at least, a surface treatment to provide a non-skid finish for heavy traffic moving at high speeds. Due to the fact that the RCC is highly sensitive to loss of strength, the compaction has to be carried out energetically with a sufficient number of passes of the rollers and that the densities required are to be carefully controlled. To achieve high density, material to be compacted must be on a solid base. RCC placed on resilient sub-grade such as clay cannot achieve high level of compaction. In case the cement used in RCC has a high content of pozzolanas they develop their strength rather slowly. This makes RCC sensitive to freezing temperature during setting in cold weather.

14.7

WASTE MATERIAL-BASED CONCRETE

Recent investigations have made it possible to make concrete using agro, urban and industrial waste materials. Successful utilization of a waste material depends on its use being economically competitive with the alternate natural materials. These costs are primarily made up of handling, processing and transportation. The stability and durability of products made of concrete using waste materials over the expected life span is of utmost importance, particularly in relation to building and structural applications. The forms in which they are used are wide and varied: they may be used as a binder material, as partial replacement of conventional Portland cements or directly as aggregates in their natural or processed states. For discussion waste materials may be classified as organic waste (agro-wastes), inorganic wastes (urban wastes) and industrial wastes.

14.7.1

Organic Wastes

The waste materials included in this category are of plant origin, namely sawdust, coconut pith, rice husk, wheat husk, groundnut husk, etc. It must be appreciated that

Special Concretes and Concreting Techniques

483

development of concrete using such aggregates is still in early stages and published data are limited. Before using organic wastes on a large scale as constituents in concrete, careful investigations regarding their structural properties and durability, need be carried out. Natural organic waste materials are used for making lightweight concrete. However, they often contain substances (cement poisons) which retard the hydration and hardening of cement which need be neutralized appropriately. Moreover, it is difficult to obtain a waste material which is not a mixture of several species. Consequently, there is a considerable variation in results from batch to batch. The lightweight concrete produced using organic wastes have comparatively high moisture movement and show relatively higher percentage of volume changes. A very high shrinkage limits its use to designs where freedom of movement is possible.

Rice Husk Huge quantities of rice husk are generated in rice mills. Each tonne of paddy produces about 200 kg of husk. Because of its very low density, rice husk requires large space for storage and hauling. In India, it is disposed of by burning, thus reducing the bulky waste to manageable volumes of ash of less than 50 per cent of its initial volume but open burning creates severe pollution problems. Rick husk contains only very small quantities of water-soluble cement poisons as compared to saw dust. It has a low bulk density of only 100 to 150 kg/m3. The lightweight concrete (bulk density 600 kg/m3) produced using rice husk as an aggregate is suitable mainly for making precast blocks and slabs for walls and partitions. The modern trend is to incinerate the rice husk under the conditions which results in ash containing a highly reactive form of silica. The ash produced can be easily pulverized to the required fineness. Typically rice-husk ash contains 80−90 per cent of amorphous SiO2, 1 to 2 per cent of K2O and the rest being unburnt carbon. The ground reactive rice-husk ash can be blended with ordinary Portland cement to produce satisfactory hydraulic acid resistance cements. The compressive strengths of some of the blends are given in Table 14.4. Hydration of Portland cement produces Ca(OH)2 which quickly combines with highly reactive silica of rice-husk ash to form additional calcium silicates. Thus the ricehusk ash does not fall in the category of ordinary reactive silica materials commonly Table 14.4

Strengths of blended cement made from rice-husk ash and Portland cements

Proportions of blends (by mass), per cent

Compressive strength, MPa

Portland cement (IS : 8112–1976)

Rice-husk ash

Quicklime

3 days

7 days

28 days

100

—

—

80

20

—

23

33

43

28

46

61

70

30

—

32

46

60

50

50

—

26

40

59

30

70

—

24

36

43

0

80

20

10

24

35

484

Concrete Technology

known as pozzolanas which add to the strength only at later stages. If controlled burning of rice husk cannot be carried out, it is still possible to obtain pozzolanic material from rice husk. Central building Research Institute, Roorkee (India) has developed a cheap cementing material from rice husk and waste lime sludge available from the sugar and paper industries. The dried cakes of mixture of sludge and rice husk are burnt, and the burnt material on grinding yields a fast-setting gray-colored cementing material which can be used in place of cement or lime in mortars for brick masonry work, plastering and foundation concreting. Ordinary Portland cement contains approximately 60 to 65 per cent of CaO, part of which is released upon hydration as free Ca(OH)2 and it is this product which makes Portland cement concrete prone to deterioration in acidic environments. On the other hand, the rice-husk ash cement though has similar strength characteristics contains as little as 20 per cent CaO. Upon hydration none of free lime would be present as Ca(OH)2, the products of hydration being mainly calcium silicate hydrates and silica gel, thus the rice-husk ash concrete is more resistant to acid environments.

Rice-husk Ash Cement Concrete The compressive strengths of concretes made with rice-husk ash cement using siliceous gravel and crushed limestone are given in Table 14.5. The 28-day compressive strength using crushed limestone aggregate is about 23 per cent higher than that obtained using gravel aggregate, probably due to the formation of a stronger interfacial bond between the cement paste and aggregate. Because of black color of rice-husk cements, these cements can be used to make permanent black concrete for glare-free pavements and architectural applications. These concretes show better long-term color stability than that obtained by using coloring pigments. Table 14.5

Compressive strength of rice-husk cement concrete

Aggregate type

Compressive strength, MPa 3 days

7 days

28 days

Crushed aggregate

29

39

47

Gravel aggregate

24

33

38

Rice-husk ash when mixed with sand and lime in suitable proportions with an appropriate quantity of water can be used to cast bricks. These will require some air curing followed by wet curing before being used. The rice-husk ash bricks have a density of 1400−1600 kg/m3 and compressive strength of 5−6 MPa. The water absorption is 15−20 per cent.

14.7.2

Inorganic Waste (Urban Wastes)

The inorganic wastes which are hard, particularly, the demolition waste such as broken concrete, broken bricks and crushed glass can be used to produce concretes of requisite strength and durability.

Special Concretes and Concreting Techniques

485

Broken or Recycled Concrete Huge quantities of building rubble become available each year by the way of demolition of old structures to make way for new and modern ones due to rapid urbanization. The quantities increase tremendously during massive reconstruction after devasting earthquakes. Disposal of such materials is difficult in view of the scarcity of suitable dumping grounds, and meeting the environmental requirements. Hence, the broken concrete is increasingly being recycled. Recycled concrete is simply the old concrete that has been removed from buildings, foundations, pavements and other structures, and crushed to the specified size. The basic requirement for recycled aggregates for concrete construction is that the original concrete shall be sound, hard, normal weight concrete. As a rule, recycled concrete aggregate of distinctly different qualities shall be used separately. The cement mortar attached to the recycled aggregate primarily determines the performance of recycled concrete. Finishing materials, reinforcing bars and other embedded material, if any, in the original concrete shall be removed in the best possible way. Recycled aggregates shall not contain excessive amounts of dirt, plaster of Paris or gypsum and other injurious foreign matter like wood and asphalt which may adversely affect recycled aggregate concrete and steel used therein.

Production The basic method of the recycling is one of crushing the debris to produce a granular product of given particle size and then reprocessing and screening, the degree of which depends on the level of contamination and the application for which recycled aggregate is produced. Recycled aggregates normally have more angular shape and more coarser surface and exhibit more or less similar particle size distribution as that for natural aggregate. It has been reported that the properties of aggregates from demolished concrete are affected more by the method of crushing than by the properties of the original concrete. The specific gravity and bulk density of the recycled aggregate are lower than the original aggregate and water absorption is five per cent higher, due to the presence of low density mortar. The relative densities of crushed concrete fine and coarse aggregates are 2.1 and 2.3, respectively. Recycled aggregates produced from good quality concrete can be expected to fulfil the requirements for the Los Angeles abrasion loss percentage, crushing and impact values. Before using the crushed concrete as an aggregate in concrete for roads and buildings, investigations should be carried out for establishing its suitability and feasibility especially in helping the solid-waste disposal problem.

Properties of recycled aggregate concrete The concrete produced with recycled aggregate loses its workability more rapidly than the conventional concrete, because recycled aggregate is more porous than natural aggregate. Thus concrete with recycled concrete aggregate may require 5 to 10 per cent more mixing water to achieve the same workability as the gravel concrete. If both fine and coarse aggregates are recycled aggregates, around 15 per cent more free water is required. An air entraining and water-reducing admixture shall be incorporated into fresh recycled aggregate concrete mix. The air content of recycled aggregate concrete may be slightly higher than that of conventional aggregate concretes, it shall be between three

486

Concrete Technology

and six per cent. The slump of recycled coarse aggregate concrete shall not exceed 200 mm. The smallest possible water content and fine to coarse aggregate ratio shall be used which will produce a concrete with the required slump and having proper cohesion. The basic water-cement ratio law is applicable to recycled concretes. Water−cement ratio shall not exceed 0.65. Cement content shall not be less than 260 kg/m3. To achieve comparable strength, recycled aggregate concretes requires approximately 8 to 15 per cent higher cement contents. The rate of development of strength is similar to that for original concrete. Workability and mix proportions being the same, the compressive strength of recycled aggregate concrete is in the range of about 75 per cent, and the modulus of elasticity about 65 per cent of conventional concrete with natural aggregates. The tensile and flexural strengths are approximately 10 per cent lower. The damping capacity, expressed in terms of logarithmic decrement, has been reported to be between 15 to 20 per cent higher. The creep and drying shrinkage are 30 to 60 per cent higher. The abrasion resistance for concrete has been found to reduce as compared to original concrete.

Durability Permeability, water absorption, rate of carbonation and thus risk of reinforcement corrosion in recycled aggregate concrete are somewhat higher and the frost resistance is lower than that for original concrete. However, such undesirable effects can be offset if recycled aggregate concretes are produced with slightly lower water-cement ratios than corresponding conventional concrete. Applications Utilization of recycled concrete in the form of aggregate is widely accepted for pavements, base and sub-base courses and to some extent for foundation purposes. The lean concretes produced using recycled aggregate are called econo cretes and have resulted in a saving in the cost of construction of order of 30 per cent.

Broken-brick Aggregate Concrete Broken brick is a waste product obtained as rejected overburnt or damaged bricks in brick works and at construction sites. Brokenbrick aggregate is obtained by crushing waste bricks and has a density varying between 1600−2000 kg/m3. It is used in concrete for foundation in light buildings, flooring and walkways. Broken-brick aggregate may also be used in lightweight reinforced concrete floors. Brick aggregate gives a very low slump even when reasonably workable and may thus require use of plasticizers. Broken-brick concrete can be designed to obtain the compressive strength (between 15 MPa to 35 MPa) generally required in practice and such a concrete possesses satisfactory structural properties.

14.7.3

Industrial Wastes

Some of the industrial by-product wastes can be profitably used in the concrete construction industry which requires large quantities of low cost raw materials. This utilization offers triple benefits, namely, conservation of fast-declining natural resources, planned gainful exploitation of waste materials and release of valuable land for more profitable use. The most influential factor that dictates the utilization of industrial by-products is the economic cost in comparison to the conventional

Special Concretes and Concreting Techniques

487

materials that would have been otherwise used. A brief description of waste byproducts is given here.

Blast-furnace Slag Large quantities of slag are generated during the production of iron and steel. Granulated or foamed or dense blast-furnace slag can be produced depending on the rate and manner of cooling the molten slag. The granulated slag can be used in the manufacture of slag cements. Blast-furnace slag cements contain slag up to 60 per cent, hence there is considerable reduction in the rate of heat evolution and a significant increase in the resistance to chemical attack. The low rate of heat evolution and the fact that the early strength is less affected in hot weather make blastfurnace slag cements attractive for use in tropics where thermal contraction cracking often poses problems. For the same reasons, these cements can be advantageously used in mass concrete, and for high chemical resistance in marine structures. The dense air-cooled slag aggregate may be used as a replacement of natural aggregate in concrete. On the other hand, foamed blast-furnace slag, a lightweight aggregate, is mainly used for blockmaking and insulating roofs and floor screeds, and is suitable for structural reinforced concrete.

Coal Ash from Power Stations The main by-product is fly ash or pulverized fuel ash which is the fine dust carried upward by combustion gases and collected in cyclones or wet scrubbers, and electrostatic precipitators. The bulk ash which is greyish in color becomes darker with increasing proportions of unburnt carbon. It is used as a cement replacement. The contribution of fly ash to the strength of concrete has been attributed to: (i) direct water reduction, (ii) increase in the effective volume of paste in the mix, and (iii) pozzolanic reaction. However, fly ash reduces the rate of development of strength and increases drying shrinkage and creep strains. Since the early strength of fly ash concrete is less than that of Portland cement, its proportion is generally limited to 30 per cent in the situations where early strength is important. The low rate of heat evolution makes fly ash useful in mass concrete. The fly ash concrete has high resistance to sulfate attack. Red Mud Aggregate The red-colored waste by-product resulting from the production of alumina from bauxite by Bayer’s process is known as red mud. The compressive, tensile and flexural strengths of concrete made with red mud aggregate have been reported to be considerably higher than those of concrete made with gravel. lightweight aggregates may be manufactured from mixtures of red mud and fly ash, and blast-furnace slag and some types of pumice. The addition of two−five per cent red mud to Portland and slag cements increases strength at an early age but gives lower strength at a later age as compared to that obtained with pure cements. Silica-fume Concrete Silica-fume is a by-product of the reduction of high purity quartz with coal in electric arc furnaces in the production of ferro-silicon metal. Because of its extreme fineness (about 2 00 00 000 mm2/g) and high glass content, silica-fume is a very efficient pozzolanic material, i.e., it is able to react efficiently with the hydration products of Portland cement in concrete.

488

Concrete Technology

Silica-fume is generally more efficient in concretes having high water-cement ratios. Investigations indicate that in concretes with a water-cement ratio of about 0.55 and higher, the silica-fume has an efficiency factor of 3 to 4. This means that (within the usual 0−10 per cent range of replacement) 1 kg of silica-fume can replace 3 to 4 kg of cement in concrete without changing the compressive strength of concrete. Ultra high strength concrete (of the order of 80 to 125 MPa) is now possible for field placeable concrete with silica-fume additive. Such high strength concrete has increased modulus of elasticity, low creep and drying shrinkage, excellent freeze−thaw resistance, low permeability and increased chemical resistance. Silica-fume in concrete can be used for the following purposes: 1. 2. 3. 4. 5.

To conserve cement To produce ultra high strength concrete To control alkali-silica reaction To reduce chloride associated corrosion and sulfate attack To increase early age strength of fly ash/slag concrete

Recently the attributes of silica-fume found their use in shotcrete applications in the pulp, mining, and chemical industries. The high strength concrete made with silica-fume and local aggregates provided greatest abrasion-erosion resistance for the eroded stilling basin of Kinzua Dam in Pennsylvania.

14.8

SHOTCRETE OR GUNITING

Shotcrete is mortar or very fine concrete deposited by jetting it with high velocity (pneumatically projected or sprayed) on to a prepared surface as shown in Fig.14.7. The system has different proprietory names in different countries such as Blastcrete, Blowcrete, Guncrete, Jet-crete, Nucrete, Pneukrete, Spraycrete, Torkrete, etc.,

Swimming in pool construction

T Typical shotcreting machine

Fig. 14.7

Slope stabilization

Typical shotcreting machine and examples of shotcrete

Special Concretes and Concreting Techniques

489

though the principle is essentially the same. Shotcrete offers advantages over conventional concrete in a variety of new construction and repair works. Shotcrete is frequently more economical than conventional concrete because of less formwork requirements, requiring only a small portable plant for manufacture and placement. It is capable of excellent bonding with a number of materials and this may be an important consideration. Shotcrete has wide applications in different constructions, such as thin over-head vertical or horizontal surfaces, particularly the curved or folded sections; canal, reservoir and tunnel lining; swimming pools and other water-retaining structures and prestressed tanks. Shotcrete is very useful for the restoration and repair of concrete structures, fire damaged structures and waterproofing of walls. Shotcrete has been successfully used in the stabilization of rock slopes and temporary protection of freshly excavated rock surfaces. Its utility has been proved for protection against long-term corrosion of piling, coal bunkers, oil tanks, steel building frames and other structures, as well as in encasing structural steel for fireproofing. Special shotcrete has been developed for high temperture applications, such as refractory lining of kilns, chimneys, furnaces, ladles, etc.

14.8.1 Types of Shotcreting There are two basic types of shotcreting processes which are described here.

Dry Mix Process In the dry mix process, the mixture of cement and damp sand is conveyed through a delivery hose pipe to a special mechanical feeder or gun called delivery equipment. The mixture is metered into the delivery hose by a feed wheel or distributor. This material is carried by compressed air through the delivery hose to a special nozzle. The nozzle is fitted with a perforated mainfold through which water is introduced under pressure and intimately mixed with other ingredients. The mortar is jetted from the nozzle at high velocity on to the surface to be shotcreted. In this process any alteration in the quantity of water can be easily accomplished by the nozzleman (i.e., the worker in charge of the nozzle). If the water content is more, then concrete tends to slump when jetted onto the vertical surface. On the other hand, in the case of deficiency of water the material which will rebound from the surface will be excessive. Alteration of water content can be made accordingly. The amount of water should be so adjusted that wastage of material by rebounding is minimum. The water-cement ratio should be between 0.33 and 0.50. Several forms of equipment are available for shotcreting by this technique. A common layout includes an air compressor, a material hose, air and water hoses, a nozzle gun and a pressure tank or pump for water supply, and transporting equipment. The equipment ensuring continuous supply of the mortar can convey the material to a distance of 300 to 500 m horizontally and 45 to 100 m vertically.

Wet Mix Process In this process, all the ingredients, i.e., cement, sand, smallsized coarse aggregate and water, are mixed before entering the chamber of delivery equipment. The ready-mixed concrete is metered into the feeding chamber and conveyed by compressed air at a pressure of 5.5 to 7 atmosphere to a nozzle. Additional air is

490

Concrete Technology

injected at the nozzle to increase the velocity and improve the gunning pattern. Equipment capable of placing concrete at the rate of 3 to 9 cubic meters per hour is available. The phenomenon of falling back of a part of mortar or concrete jetted on to surface to be treated, due to high velocity of jet, is called rebound and depends upon the watercement ratio, and the nature and position of the surface treated. The rebound decreases with higher water-cement ratio, and has higher values for vertical and overhead surfaces. The approximate range of rebound is 5 to 15 per cent for horizontal slabs, 15 to 30 per cent for sloping and vertical surfaces and 20 to 50 per cent for the treatment of overhead surfaces and corners. The rebounding material mostly consists of sand or/ and coarse particles and very little quantity of cement falls back. The rebound material falling on surface is cleaned before being treated by shotcreting. The vertical surfaces should be treated from bottom to top. The rebound also depends upon the angle of jetting with respect to the surface being treated; it is minimum when the nozzle is held at right angles to the surface. The nozzle should be kept at a distance of 900 mm from the surface. Addition of pozzolanic material to the mix reduces the rebound by improving its plasticity. Since rebounded material is a wastage, it is economical to reduce the rebound by adding as much water at the nozzle as conditions permit. The dry-mix process is preferred in case the lightweight concrete is used. The lower water-cement ratio used results in higher strengths, less creep and drying shrinkage, and higher durability. Whereas in the wet process, the higher durability can easily be achieved by using air-entraining agents. The water-cement ratio can be very accurately controlled in the wet process. Further, the wet process does not cause dust problems. The larger capacity available in wet mix process results in higher rates of placing of concrete. The procedure of shotcreting a surface involves the following steps: 1. Preparation of surface to receive shotcrete Where the shotcrete is to be placed against earth surfaces as in canal linings, the surfaces should first be thoroughly compacted and trimmed to line and grade. Shotcrete should not be placed on any surface which is frozen, spongy, or where there is free water. The surface should be kept damp for several hours before applying shotcreting. For repairing deteriorated concrete it is essential to remove all unsound material. Chipping should be done to remove all the offsets in the cavity which may cause abrupt change in thickness of the repair work. No square shoulders should be left at the perimeter of the cavity, and all edges should be tapered. After ensuring that the surface to which the shotcrete is to be bounded is sound, it should be sand blasted. The nozzleman usually scours clean the area before applying the shotcrete with an air-water jet, and then the water is shut off and all free water is blown away by compressed air. 2. Construction of forms The forms are usually of plywood sheeting, true to line and dimension. They are adequately braced to ensure protection against excessive vibration. The forms should be constructed to permit the escape of air and rebound during the gunning operation. They should also be oiled or dampened. Adequate and safe scaffolding is necessary so that the nozzleman can hold the nozzle at a distance of 1 to 1.5 m from the surface. 3. Placement of reinforcement Sufficient clearance should be provided around the reinforcement to permit complete encasement with the shotcrete.

Special Concretes and Concreting Techniques

491

The minimum clearance between the reinforcement and the form may vary between 12 mm for the case of a mortar mix and wiremesh reinforcement to 50 mm for the case of concrete and reinforcing bars. 5. Preparation for succeeding layers The receiving layer should be allowed to take its initial set before applying a fresh layer of shotcrete. All laitance, loose material and rebound should be removed by brooming. Any laitance which has been allowed to take final set should be removed by sand blasting and the surface cleaned with an air-water jet. 6. Finishing of the surface Natural gun finish is preferred from both structural and durability standpoints. There is a possibility that further finishing may disturb the section, harming the bond between shotcrete and underlying material, and creating cracks in the shotcrete. Where greater smoothness or better appearance is required special finishes must be applied. Sometimes, for finer finish, a flash coat consisting of finer sand than normal, and with the nozzle held well back from the surface, is applied to the shotcrete surface as soon as possible after the screeding.

14.8.2

Properties of Shotcrete

The properties of the shotcrete are essentially the same as for conventional concrete of same materials, proportions and void system. However, the following points should be borne in mind: 1. In shotcrete, generally, a small maximum size aggregate is used and cement content is high. These should enhance durability in most cases. 2. Whereas conventional concrete is consolidated by vibration, shotcrete is consolidated by the impact of a high-velocity jet impinging on the surface. This process not only increases the cement content due to rebound but also brings about different air-void systems affecting the durability of shotcrete. 3. The application procedures have a greater effect on the in-place properties of shotcrete than the mix proportions. 4. Shotcrete specimens are usually sawed from test panels of about one meter square and 75 mm thick made by gunning out a plywood form.

14.8.3

Durability of Shotcrete

The low water-cement ratio enhances the durability of shotcrete, for most types of exposures. The preferred range for most shotcretes is 0.30 to 0.45. The wetter mixes give a poorer quality of shotcrete, and tend to sag or fall out during application. On the other hand, the drier mixes have hardly enough water for hydration and cause excessive rebound which makes it difficult to get sound shotcrete under field conditions and increases the waste and aggravates the problems of disposing it. The properties of cement and aggregate affect the durability of shotcrete to the same extent as that of the ordinary concrete. Exposure conditions may make it advisable to specify a particular type of cement or restrict its alkali content. Shotcrete is somewhat vulnerable to sand pockets and laminations which adversely affect its durability. There are doubts regarding its ability to withstand severe freezing and thawing.

492

Concrete Technology

14.8.4

Air-entrainment in Shotcrete

It is generally believed that it is impossible to entrain air with dry-mix shotcrete, because of the absence of the usual concrete mixing action and the high velocity of impingement of the material on to the application surface. However, in wet process, the addition of air-entraining admixtures has been found to reduce the size of air voids, thereby increasing the durability marginally. Some investigators believe that air entrainment is helpful from the application standpoint; the mix is a little stickier and the rebound is reduced. The simplest procedure, however, is to use air-entraining cements.

14.8.5

Nature of Failures in Shotcrete

Most shotcrete failures involve the peeling off of sound shotcrete because of bond failure. This occurs in spite of the fact that one of shotcrete’s greatest attributes is its excellent ability to form a bond with concrete or another shotcrete layer. The other type of failure is the delamination between shotcrete layers where the surface preparation has not been good. In repair technology applications, shotcrete has been found to spall because of the corrosion of reinforcement. The drying shrinkage is somewhat higher than for most of the low slump conventional concretes, but generally falls within the range of 0.06−0.10 per cent.

14.8.6

Special Shotcretes

Steel Fibrous Shotcrete The plain unreinforced shotcrete like unreinforced concrete is a brittle material, with little capacity to resist pronounced tensile stresses or strains without cracking and disruption. If reinforced with steel fibers, its strength increases considerably. This reinforcement of shotcrete also improves its ductility, energy absorption and impact resistance. Steel fibers control the cracking and hold the material together even after excessive cracking. A fibrous shotcrete containing up to two per cent of steel fibers (by volume), has shown an increase in flexural strength of an order of 50 to 100 per cent and in compressive strength by 50 per cent. The toughness and impact resistance are found to increase by ten times or more. An important improvement is evident in the mode of failure which requires large deformations to cause failure and the material continues to carry a significant load even after cracking. This large increase in strain capacity provides post-crack resistance which is advantageous in applications such as tunnel and mine linings, where there may be large deformations. It has been noticed that a greater percentage of fibers makes the aggregate rebound from the surface. High-speed photography has shown that many steel fibers were in the outer portion of air stream and that many of them were blown away radially from near the point of intended impact shortly before or after they hit the surface. If less air is used, the amount and velocity of remnant air currents is reduced and the rebound of fibers is less. Other measures include jetting at the wettest stable consistency. It has been noticed that hooked steel fibers can be used in the field with a conventional shotcrete machine. They have a substantial higher load carrying capacity after the

Special Concretes and Concreting Techniques

493

development of cracks. These shotcretes have a very high toughness index indicating excellent energy-absorbing capacity. Due to the excellent anchorage established between the hooked fibers and the matrix, the composite has high ductility and flexural strength. Shotcretes with hooked fibers show a tremendous ability to absorb impact loading. For higher fiber contents, the impact resistance increases dramatically. The reduction in drying shrinkage is proportional to the quantity of fibers added.

Refractory Shotcrete Shotcrete containing hydraulic cement (i.e., calcium aluminate cement) as binding agent which is suitable for use at high temperatures, is termed refractory shotcrete. This shotcrete utilizes aggregates and binders suitable for use up to a temperature of 1900 °C. It exhibits variable properties throughout its thickness after firing due to the temperature gradient from the hot to the cold surface. In addition to this property, other factors, such as thermal cycling, thermal shock, chemical attack, abrasion and erosion should be considered in the design of refractory shotcrete. Another characteristic is that its 24-hour strength is similar to the 28-day strength of Portland cement shotcrete.

Ingredients Hydraulic binders are available in three types: (i) low purity (ii) intermediate purity, and (iii) high purity. The higher the purity, higher is the aluminum oxide content and lower is the iron oxide content. The higher the service temperature, the higher is the aluminum content required. Among the aggregates used in increasing order of service temperature are slag, limestone, trap rock, expanded-shale, perlite, calcined fire clay, calcined bauxite, kaolin, etc., The water used should be potable and free from acids. Portland cement and calcium aluminate cement combined in a mix will accelerate the hardening process in the shotcrete.

Curing, drying and firing Refractory shotcrete achieves its ultimate strength in 24 hours. Therefore, it is important that the curing procedure be instituted immediately after placing and continued for 24 hours to achieve complete hydration and control drying shrinkage. The usual methods of curing like covering with wet burlaps, fine spraying or resin type curing membrane are effective. After curing and before placing the refractory shotcretes in service, it is essential that the lining be dried to eliminate both free and combined water. Thorough drying minimizes any chances of explosive spalling resulting from the internal formation of steam. A well-executed heating procedure will assure the integrity of a lining, thereby assuring a longer service life. Refractory shotcreting is particularly effective where forms are impractical, access is difficult, thin layer and one of variable thickness are required, normal casting techniques cannot be employed from considerations of economy.

14.8.7

Disadvantages with Shotcretes

The method in which raw materials, aggregate and cement are handled may be objectionable to environmentalists. Dust from either the fine aggregate and/or cement can settle on the ground around the application area. This potential hazard must be considered when designing or applying a shotcrete coating. Sometimes special type

494

Concrete Technology

of enclosures may be necessary to confine the area designed for batching, mixing or charging the gun. In addition to the dusting problem, rebound also has to be cleaned up and hauled to an approved waste area. The high cost of shotcrete combined with wastage due to rebound has to be weighted with the difficulties involved with other techniques before shotcreting is adopted for any particular situation. In spite of all these problems, shotcrete has its own merits, such as the need of formwork only on one side of the work, its suitability for concreting thin sections and in sites where access is difficult. Moreover, shotcrete bonds perfectly with the existing old concrete masonry, exposed rock or suitably prepared steel surface, and hence is very effective in the repair of the structures concerned.

14.8.8 Guniting Gunite also known as a dry process shotcrete, uses air pressure to convey dry material from machine through hose to nozzle where water is added. The technique of depositing very thin layers of mortar in each pass of the nozzle than that available with the shotcrete, is termed guniting. A typical arrangement in the gunite system is shown in Fig. 14.8(a). In addition to the general requirements Nozzle Flexible Pipe for Carrying Water Water Control Valve Water Supply Pressure Tank T

Flexible Pipe Carrying Dry Cement and Sand

Air Compressor Cement Gun

Moisture Extractor (a) A typical arrangement for gunite system

(b) Gunite machine

Fig. 14.8

(c) Guniting the prepared surface

Gunite system and guniting of a prepared surface

Special Concretes and Concreting Techniques

495

for quality control of normal concrete, guniting requires careful and skilful handling of nozzle for high quality finished work. The surface to be gunited should be cleaned thoroughly of grease or oil or any other loose or defective material by applying either air blast or high pressure water jet. If necessary, the surface can even be sand-blasted by omitting the cement using the same gun and reducing the velocity of jet. The sand blasting can also help in removing the loose rust on the reinforcement. The surfaces likely to absorb water should be kept wet up to six hours before guniting. The mix generally used for guniting is 1:3 to 1:4.5 with a water-cement ratio of about 0.30. The maximum size of sand is limited to 10 mm. A seven-day strength of the order of 70 MPa can be achieved with a 1:3 mix. This high-quality mortar with low water-cement ratio results in low permeability, good resistance to weathering and chemical attack, etc. The resistance to the chemical attack can be increased by using sulfate-resisting Portland cement or high-alumina cement. Addition of pozzolana up to five per cent by a mass of cement can improve plasticity and reduce the rebound. Due to good bonding between reinforcement and gunite, the gunite acts as a part of the structure and not merely as an added cover. Each layer of the gunite is usually provided with a spot-welded steel-wiremesh fabric of five mm diameter wires, to reduce initial shrinkage and to prevent cracks in the freshly placed material. The meshes should overlap each other by one mesh to maintain continuity whenever the meshes are joined. The reinforcing fabric should be maintained at a distance of 6 to 10 mm from the original concrete surface during guniting. The guniting can effectively be used for the repair of dams, spillways, bridge piers, sewerage pipes and water mains, and for protection of canal banks. It has been extensively used for the protection of steel girders from corrosion, etc., and waterproofing of reservoirs and tunnels. Another important application is in the repair of all types of concrete structures where the concrete has spalled off due to corrosion of reinforcement.

14.9

FERROCEMENT

The concept of use of fibers to reinforce brittle materials dates back to ancient constructions built in India using mud walls reinforced with woven bamboo mats and reeds. In the present form, ferrocement may be considered as a type of thin reinforced concrete construction where cement mortar matrix is reinforced with many layers of continuous and relatively small diameter wire meshes as shown in Fig. 14.9. While the mortar provides the mass, the wire mesh imparts tensile strength and ductility to the material. In terms of structural behavior ferrocement exhibits very high tensile strength-to-weight ratio and superior cracking performance. The distribution of a small diameter wire mesh reinforcement over the entire surface, and sometimes over the entire volume of the matrix, provides a very high resistance against cracking. Moreover, many other engineering properties, such as toughness, fatigue resistance, impermeability, etc., are considerably improved. Sometimes, conventional reinforcing bars in a skeleton form are added to thin wire meshes in order to achieve a stiff reinforcing cage. The commonly used composition and properties of ferrocement made with steel wire mesh reinforcement are summarized in Table 14.6.

496

Concrete Technology Wire mesh

Construction of typical sections of ferrocement

Fig. 14.9 Table 14.6

Construction of typical sections of ferrocement

Normal ranges of composition and properties of ferrocement

Parameter

Range

Wire mesh Wire diameter Type of mesh

0.5 ≤ f ≤ 1.5 mm Chicken wire or square woven- or welded-wire galvanized mesh, expanded metal

Size of mesh openings Distance between mesh layers Volume fraction of reinforcement

5 ≤ s ≤ 25 mm Distance between 2 layers ≥ 2 mm Up to 8 per cent in both directions corresponding to 650 kg/m3 of concrete

Specific surface of reinforcement

Up to 4 cm2/cm3 in both directions

Skeletal reinforcement (if used) Type Diameter Grid size

Wires; wire fabric; rods; strands 3 ≤ d ≤ 10 mm 50 ≤ g ≤ 100 mm

Typical mortar composition Portland cement Sand-cement ratio Water-cement ratio Fine aggregate (sand)

Composite properties Thickness Steel cover Ultimate tensile strength Allowable tensile stress Modulus of rupture Compressive strength

Any type depending on application 1.0 ≤ S/C ≤ 2.5 (by mass) 0.35 ≤ W/C ≤ 0.6 (by mass) Fine sand all passing IS: 4.75 mm sieve and having 5 per cent by mass passing IS: 1.18 mm sieve, with a continuous grading curve in between 10 ≤ t ≤ 60 mm 1.5 ≤ c ≤ 5 mm 34.5 MPa 10.0 MPa 55.0 MPa 27.5 to 60.0 MPa

Special Concretes and Concreting Techniques

14.9.1

497

Materials

Cement Mortar Matrix As described above, the ferrocement composite is a rich cement-mortar matrix of 10 to 60 mm thickness with a reinforcement volume of five to eight per cent in the form of one or more layers of very thin wire mesh and a skeleton reinforcement consisting of either welded mesh or mild steel bars. Normally, Portland cement and fine aggregate matrix is used in ferrocement. The matrix constitutes about 95 per cent of the ferrocement and governs the behavior of the final product. This emphasises the need for proper selection of constituent materials, their mixing and placing. The choice of cement depends on the service conditions. To maintain the quality of cement, it should be fresh, of uniform consistency and free of lumps and foreign matter. Cement should be stored under dry conditions and for as short duration as possible. The fine aggregate (sand) which is the inert material occuping 60 to 75 per cent of the volume of mortar must be hard, strong, non-porous and chemically inert. The aggregate should be free from silt, clay and other organic impurities. The particle sizes of 2.36 mm and above, if present in substantial quantities, may cause the mortar to be porous. On the other hand, very fine particles, if present in a substantial amount, will require more water to achieve the required workability, thereby adversely affecting the strength and impermeability. The fine aggregates conforming to grading zones II and III with particles greater than 2.36 mm and smaller than 150 μm removed are suitable for ferrocement. Therefore, sands with maximum sizes of 2.36 mm and 1.18 mm with optimum grading zones II and III are recommended for ferrocement mixes. Use of fine sand in ferrocement is not recommended. The water content which governs the strength and workability of mortar primarily depends upon the maximum grain size, the fineness modulus, and the grading of the sand. The water used for making mortar should be free from impurities such as clay, loam, acids, salts, vegetable matter, etc. Plasticizers and other admixtures may also be added for achieving: (i) an improved workability, (ii) water reduction for increase in strength and reduction in permeability, (iii) water proofing, (iv) increase in durability. In addition, admixtures (containing chromium trioxide) may be used to prevent galvanic-corrosion of galvanized steel reinforcement. Pozzolanas such as fly ash may be added as cement replacement materials (up to 30 per cent) to increase the durability.

Mix Proportions The mix proportions in terms of sand-cement ratio (by mass) normally recommended are 1.5 to 2.5. The water-cement ratio (by mass) may vary between 0.35 and 0.6. In order to reduce permeability, the water-cement ratio must be kept below 0.4. The moisture content of the aggregate should be taken into account in the calculation of required water. The amount of water can be reduced by the use of appropriate admixtures. The slump of fresh mortar should not normally exceed 50 mm, and 28-day compression strength of moist cured cubes should be around 35 MPa for most applications. Sand being the principal constituent of ferrocement, its properties have a

498

Concrete Technology

major influence on the amount of water and hence on the mix design. Improvements in the grading composition of sand may allow considerable reduction of water requirement. Sand with maximum nominal size less than 2.36 mm or 1.18 mm should be avoided in ferrocement mixes. The mixes should have compositions such that the total absolute volume of cement and fines is about 300 cm3 per litre of mortar. A change in the amount of cement must be accompanied by a corresponding change in that of fines.

Reinforcement As explained earlier, the reinforcement used in ferrocement is of two types, viz. skeleton steel and wire mesh. The skeleton steel frame is made conforming exactly to the geometry and shape of structure, and is used for holding the wire meshes in position and shape of the structure.

Skeleton Steel The skeleton steel comprises relatively large diameter (about 3 to 8 mm) steel rods typically spaced at 70 to 100 mm. It may be tied-reinforcement or welded wire fabric. The welded-wire fabrics normally contain larger diameter wires spaced at 25 mm or more. Welded-wire fabrics of 3 to 4 mm diameter wires welded at 80 to 100 mm center to center have been successfully used for making skeleton frames for the cylindrical or other ferrocement surfaces where these meshes can be bent easily. They provide better and uniform distribution of steel and save time in fabrication but may cost a little more when compared to mild steel bar frames. In the case of structures where higher stresses may occur, as in case of boats, barges, etc., the mild steel bars provided to act as skeletal steel are also counted as reinforcement imparting structural strength, stiffness and durability. However, a minimum possible size of bars should be used in order to obtain the effect of wire meshes and hence the composite effect. The spacing of the skeletal transverse and longitudinal steel bars of diameter of 5 to 7 mm depends upon the type and shape of structure. In the case of boathulls, a spacing of 75 to 100 mm is adequate whereas in water tanks, bins, etc., the spacing may vary between 200 and 300 mm. The bars are mostly tied with binding wires but can also be welded. The reinforcement should be free from dust, loose rust, coatings of paint, oil or similar undesirable substances.

Wire Mesh The wire mesh consisting of galvanized wire of diameter 0.5 to 1.5 mm spaced at 6 to 20 mm center to center, is formed by welding, twisting or weaving. Specific mesh types include woven or interlocking mesh, woven cloth, and welded mesh. The welded-wire mesh may have either hexagonal or square openings as shown in Fig. 14.10. Meshes with hexagonal openings are sometimes referred to as chicken wire meshes. The hexagonal wire mesh is cheaper but structurally less efficient than the mesh with square openings because the wires are not oriented in principal (maximum) stress directions. Moreover, the rectangular meshes have better rigidity when placed or tied over the skeleton frame, and do not sag during placing the mortar. Meshes with square openings are available either in the form of welded-wire mesh or in the woven form. The welded wire meshes have a higher Young’s modulus and

Special Concretes and Concreting Techniques

Plan

Plan

Section (a) Square woven wire mesh

Section (b) Square welded wire mesh

(c) Hexagonal wire mesh

(d) Expanded metal lath

Fig. 14.10

499

Different types of welded wire meshes

hence provide a higher stiffness and less cracking in the early stages of loading. On the other hand, woven-wire meshes are a little more flexible and easy to work with than the welded meshes. In addition, welding anneals the wires and limits the tensile strength. Generally, the square woven meshes consisting of 1.0 or 1.5 mm diameter wires spaced at about 12 mm are preferable. Wire meshes are also available in the galvanized form. Galvanizing, like welding, reduces the tensile strength. However, to control cracking the welded wire fabric should be used in combination with wire meshes. The minimum yield strength of wire used in fabric should be 415 MPa for plain wires and 500 MPa for deformed wires. The wire diameter should be less than 12 mm except in case of very thick plates. Mechanical properties of steel wire meshes and reinforcing bars are given in Table 14.7. Table 14.7 Property

Mechanical properties of steel wire meshes and reinforcing bars Woven square mesh

Welded Hexagonal Expanded square mesh mesh metal lath

Logitudinal bars

Yield strength, fy, MPa

450

450

310

380

410

Effective modulus, ERL, GPa

140

200

100

140

200

Effective modulus ERT, GPa

160

200

70

70

—

Notes ERL = value of modulus in the longitudinal direction. ERT = value of modulus in the transverse direction.

500

Concrete Technology

Expanded metal lath is formed by slitting thin gage sheets and expanding them in the direction perpendicular to the slits. Expanded metal offers strength approximately equal to that offered by welded-wire mesh. However, they result in a stiffer composite resulting in reduced crack widths at the early stage of loading and provide better impact resistance. It is unsuitable in the applications involving sharp curves. Reinforcing bars may be used in combination with wire meshes for relatively thick ferrocement elements. The minimum possible size of bars should be used in order to obtain the effect of wire meshes and hence the composite effect. Addition of steel fibers to ferrocement seems to enhance the properties considerably. They assist in distributing cracks and hence may allow the use of heavier meshes.

14.9.2

Construction in Ferrocement

The construction in ferrocement can be divided into four phases: (i) fabrication of skeleton framing system, (ii) fixing of bars and mesh, (iii) application of mortar, and (iv) curing. The quality of mortar and its application is the most critical phase. Mortar can be applied by hand or by shotcreting. Since no formwork is required as in conventional reinforced concrete construction, ferrocement is suitable especially for structures with curved surfaces such as shells and other free-form shapes. The required number of layers of wire mesh are fixed on both sides of the skeleton frame. First, the external mesh layers are fixed and tied to the frame bars. The meshes should be fixed by staggering the hold positions in such a manner that the effective hold size is reduced. A spacing of at least 1 to 3 mm is left between two mesh layers. Wherever two pieces of the mesh are joined, a minimum overlap of 80 mm should be provided and tied at a close interval of 80 to 100 mm center to center. The weighed quantities of the ingredients, namely, cement, good quality graded sand, waterproofing and antishrinkage compounds are dry mixed. The liquid additives are added to the mixing water taken in the required quantity. About half of the mixing water is put in the mixer before charging the mixer with dry mixed mortar ingredients. The mixing is carried out and the remaining water is then added gradually. The cement-aggregate ratio is generally kept between 1:1.75 to 1:2.5 (by mass) and water-cement ratio may be 0.35 to 0.40 depending upon the required workability. Generally, a 3 minute mixing time is enough. The mortar should be mixed in batches of such a quantity as can be utilized in one hour of working, so that mortar can be placed before its setting starts. The placing of the mortar is termed as the impregnation of meshes with matrix. This is the most critical operation in ferrocement casting. If the mortar impregnation is not proper the structure is bound to fail in its performance. A sufficient quantity of mortar is impregnated through mesh layers so that the mortar reaches the other side and there are no voids left in the surface. A wooden hammer of about 100 mm diameter with a 150 mm long wooden handle can be used for mild hammering over the temporarily held form. This will give sufficient vibrations for compacting the mortar. As soon as it is ensured that the mortar penetration through the mesh is satisfactory, the form is shifted to the next position.

Special Concretes and Concreting Techniques

501

For structures like boat hulls shown in Fig. 14.11 and shells, the mortar is placed using a technique called the two-operation mortar impregnation. In this system, the outside of the mesh is plastered first and the inner layer left exposed. The excess mortar is scrapped using trowels and wire brushes. The mortar is left for setting till it attains enough strength for carrying the load from inside during the application of a second layer of mortar. Cement slurry is sprayed or brushed over the entire inner surface and the second layer of mortar is applied from inside.

(a) Ferrocement boat hull

Fig. 14.11

(b) Completed ferrocement boats

Construction of ferrocement boat hull

In structures like shelters and houses shown in Fig. 14.12 where many layers are used as reinforcement and the thickness is more than 20 mm, it is advisable to do the casting in three layers. The core or middle layer is applied first covering the skeleton steel and one layer of wire mesh. This core provides a firm surface for mortar application on top and bottom. The core is cured for at least three days before the other two layers of mortar are applied. Cement slurry may be brushed over the middle layer for getting a good bond between old and new mortars.

(a) Non-toxic ferrocement shelters

Fig. 14.12

(b) A ferrocement house under construction

Construction of ferrocement shelters and houses

For normal applications, the mortar provides adequate protection against corrosion of reinforcement, but where the structure is subjected to chemical attack by

502

Concrete Technology

the environment as in sea water, it is necessary to apply suitable protective coatings on the exposed surface. These coatings should be such that they do not react with either the mortar or the reinforcement, and at the same time not be susceptible to the environmental attack. Vinyl and epoxy coatings have been found to be satisfactory especially on structures exposed to sea water and also in most other corrosive environments. For protection against a less severe environment, cheaper asphaltic and bituminous coatings are generally satisfactory.

14.9.3

Properties of Ferrocement

Though ferrocement is often considered to be just a variation of conventional reinforced concrete which may be true for the ferrocement with small quantities of reinforcement, however, it is not true for the quantity of reinforcement provided in most of the applications. Moreover, a system of construction using layers of closely spaced wire mesh separated by skeleton bars and filled with cement mortar presents all the mechanical characteristics of a homogeneous material. Tensile strength of ferrocement depends mainly on the volume of reinforcement in the direction of force and the tensile strength of the mesh. The tension behavior may be divided into three regions, namely, pre-cracking stage, post-cracking stage and post-yielding stage. A ferrocement element (member) subjected to increasing tensile stresses behaves like a linear elastic material till the development of first crack in the matrix. Once the cracks have developed the material enters the stage of multiple cracking and this stage continues up to the point where wire meshes start to yield. In this stage number of cracks keep on increasing with an increase in tensile stress without any significant increase in crack width. With the yield of reinforcement, the composite enters the stage of crack widening. The number of cracks remains essentially constant and the crack widths keep increasing. The behavior is primarily controlled by the reinforcement bars. In the elastic pre-cracking stage, the modulus of ferrocement composite Ec can be expressed in terms of modulii of mortar and reinforcement Em and Er, respectively, and volume fraction of reinforcement in longitudinal direction, Vr1: Ec = (1− Vr)Em + VrEr ô Em + VrEr = Em(1 +hVr)

where h = Er /Em. During the multiple cracking stage, the contribution of mortar to the stiffness of composite is negligible. Hence, the stiffness of composite is approximately represented by Ec = VrEr The value of Er may be substantially different for woven mesh from that for a welded mesh. It has been noticed that higher the volume of reinforcement and smaller the diameter of wires, longer is multiple cracking stage with a larger number of cracks developed in the same gage length. An inverse relationship between the first crack strength and average wire spacing based on linear elastic fracture mechanics has been established. The load-carrying capacity of ferrocement is correlated with the specific surface area of reinforcement, S,

Special Concretes and Concreting Techniques

503

which is defined as the total surface area of the wires in contact with cement mortar divided by the volume of composite. However, it should be noted that some investigators have used the surface area of the wires in the load resisting direction SL only. The specific surface area has been found to influence the first crack load in tension, as well as the width and spacing of cracks. For example, a 12-mm thick ferrocement section with five layers of a 12 mm square welded or woven 1.0 mm diameter wire mesh reinforcement has about 10 times as much specific surface areas as the conventional reinforcement. This results in a considerably increased load-carrying capacity. Consequently, ferrocement has tensile strength as high as its compressive strength, i.e., 27 MPa, and the widths of cracks are very small even at failure (about 0.05 mm). Ferrocement structures can be designed to be watertight at service loads. The maximum composite stress at first crack increases in direct proportion to the specific surface. A specific surface area SL equal to 1 cm2 /cm3 has been suggested as the lower limit for a composite to be the ferrocement. The other parameter which is a direct measure of the ultimate strength of ferrocement is the percentage of reinforcement, p, defined as either the volume of wires per unit volume of composite in the loaded direction or the area of wires per unit cross-sectional area of composite in the loaded direction. There is a unique geometric relationship between S and p, but their relationships to the physical properties are quite different. S is mostly associated with the cracking behavior whereas p is a direct measure of the ultimate strength of ferrocement because the ultimate load is resisted entirely by the wire mesh. Thus depending upon the cracking stage a typical tension stress-strain curve for ferrocement exhibits three distinct regions namely, elastic, quasi-elastic or elastoplastic, and plastic regions. In Region I, the material is linearly elastic because both the reinforcement and matrix deform elastically. The cracking of cement mortar is the beginning of Region II and the slope of the stress-strain curve decreases. The point of decrease of the slope of the stress−strain curve indicates the first crack visible to the naked eye or with special lighting arrangement. In Region III, the wire reinforcement supports the total load and the ultimate capacity can be estimated from the maximum load capacity of the wire reinforcement alone. The boundaries of the elastoplastic region are found to shift with the specific surface of the mesh, mesh size, geometry and orientation of the mesh, yield and ultimate strengths of wire. The behavior of thin ferrocement element under compression is primarily controlled by the properties of the cement-mortar matrix, i.e., thin ferrocement plate elements can be treated as plain mortar plates for most practical applications. Like in the reinforced and prestressed concrete beams, the fatigue behavior of ferrocement flexural element is governed by the tensile fatigue properties of the mesh. The ferrocement beams show poor resistance to fatigue under cyclic loading. Impact tests on ferrocement slabs show that the impact resistance increases almost linearly with the increase in specific surface (volume fraction) and ultimate strength of mesh reinforcement. For the same reinforcement fraction, ferrocement using welded-wire meshes offers highest impact resistance and the one reinforced by chicken-wire meshes offers the lowest. Woven mesh reinforcement provides an impact strength higher than that obtained by chicken-wire meshes but lower than that by welded-wire meshes.

504

Concrete Technology

14.9.4

Applications

Due to the very high percentage of well distributed and continuously running steel reinforcement, the ferrocement behaves as steel plates. As discussed earlier, its cracking resistance, ductility, impact and fatigue resistance are higher than those of concrete. In addition, the impermeability of ferrocement products is far superior to that of ordinary reinforced concrete products. Ferrocement combines easy moldability of concrete to any desired shape, lightness, tenacity and toughness of steel plates. Due to very high tensile strengthto-weight ratio and superior cracking behavior, the ferrocement is an attractive material for light and water-tight structure and other portable structures such as mobile homes. The other specialized applications include water tanks as detailed in Fig. 14.13, silos and bins, boat hulls, biogas holders, pipes, folded plates and shell roofs, floor units, kiosks, service core units, wind tunnels, modular housing, swimming pool, and permanent forms of concrete columns. 100 mm

L

B

100 mm

h Fold 100 mm Welded mesh piece before bending

Welded mesh after bending

Fig. 14.13

Fold

Welded mesh after assembling (for side walls)

Details of welded mesh for rectangular ferrocement tank

The major advantages of the ferrocement can be summarized as follows: 1. Ferrocement structures are thin and light. Therefore, a considerable reduction in the self-weight of structure and hence in foundation cost can be achieved. A 30 per cent reduction in dead weight on supporting structure, 15 per cent

Special Concretes and Concreting Techniques

2. 3. 4. 5.

505

saving in steel consumption and 10 per cent in roof cost has been estimated in USSR. Ferrocement is suitable for manufacturing the precast units which can be easily transported. The construction technique is simple and hence does not require highly skilled labour, even for complicated forms shown in Fig. 14.14. Partial or complete elimination of formwork is possible. Ferrocement construction is easily amenable to repairs in case of local damage due to abnormal loads (such as impact).

(a) Natural rock-shaped, stone colored, ferrocement tank

Fig. 14.14

(b) 3500 gallon ferrocement rainwater storage tank shaped like an urn

Construction of ferrocement water tanks of different architectural forms

14.9.5 Fibrous Ferrocement Fiber-reinforced concrete possesses higher compressive strength, toughness, increased resistance to wear and tear, and higher post-cracking strength. Its permeability is very low, and the tensile and flexural strengths are lower than those of ferrocement. Another major practical difference is that fiber-reinforced concrete must be cast in forms whereas ferrocement can be shaped into surfaces of desired configuration. In the case of ferrocement, a very fine wire mesh is required to control the cracking and skeleton steel to support the wire mesh. Use of fine meshes with thin wires at closer spacings for effective crack control is the limitation of ferrocement. The strength of conventional thin ferrocement mortar panels is also limited, and therefore prone to localized damage resulting from impact, etc. On the other hand, fiber-reinforced concrete possesses better impact resistance. To improve some of the mechanical properties of ferrocement, such as toughness and impact resistance, a new composite material known as fibrous ferrocement has been developed using fibers in plain mortar and meshes with larger diameter wires at longer pitches. Atcheson and Alexander fabricated fibrous ferrocement panels using 25 mm long steel fibers of 0.4 mm square section in plain mortar with high-strength tensile wire meshes of thrice the usually accepted maximum diameter at twice the normal spacing which withstood stresses up to 57 MPa. This is very high compared to the stresses of 10 to 20 MPa in conventional ferrocement panels. Further intensive research is required in exploiting the potential of the new product obtained by combining two modern economical composites.

506

Concrete Technology

14.10 14.10.1

FIBER-REINFORCED CONCRETE Introduction

The presence of microcracks at the mortar−aggregate interface is responsible for the inherent weakness of plain concrete. The weakness can be removed by inclusion of fibers in the mix. The fibers help to transfer loads at the internal microcracks. Such a concrete is called fiber-reinforced concrete. Thus the fiber-reinforced concrete is a composite material essentially consisting of conventional concrete or mortar reinforced by fine fibers.

Discrete Fiber Reinforced Concrete In this system, the concrete is reinforced by the random dispersal of short, discontinuous, and discrete fine fibers of specific geometry. The fibers can be imagined as an aggregate with an extreme deviation in shape from the rounded smooth aggregate. The fibers interlock and entangle around aggregate particles and considerably reduce the workability, while the mix becomes more cohesive and less prone to segregation. The fibers suitable for reinforcing the concrete have been produced from steel, glass and organic polymers. Naturally occurring asbestos fibers and vegetable fibers, such as jute, are also used for reinforcement. Fibers are available in different sizes and shapes. They can be classified into two basic categories, namely, those having a higher elastic modulus than concrete matrix (called hard intrusion) and those with lower elastic modulus (called soft intrusion). Steel, carbon and glass have higher elastic moduli than cement mortar matrix, and polypropylene and vegetable fibers are the low modulus fibers. High modulus fibers improve both flexural and impact resistances simultaneously whereas low modulus fibers improve the impact resistance of concrete but do not contribute much to flexural strength. The major factors affecting the characteristics of fiber-reinforced concrete are: water-cement ratio, percentage (volume fraction) of fibers, diameter and length of fibers. The location and extent of cracking under load will depend upon the orientation and number of fibers in the cross section. The fibers restrain the shrinkage and creep movements of unreinforced matrix. However, fibers have been found to be more effective in controlling compression creep than tensile creep of unreinforced matrix. In contrast to reinforcing bars in reinforced concrete which are continuous and carefully placed in the structure to optimize their performance, the fibers are discontinuous and are generally randomly distributed throughout the concrete matrix. As a result, the reinforcing performance of steel fibers, for example, is inferior to that of reinforcing bars. In addition, the fibers are likely to be considerably more expensive than the conventional steel rods. Thus, fiber-reinforced concrete is not likely to replace conventional reinforced concrete. However, the addition of fibers in the brittle cement and concrete matrices can offer a convenient, practical and economical method of overcoming their inherent deficiencies of poor tensile and impact strengths, and enhances many of the structural properties of the basic materials such as fracture toughness, flexural strength and resistance to fatigue, impact, thermal shock or spalling. Thus the provision of small-size reinforcement as an integral part (or ingredient) of fresh concrete mass enhances its potential in the manufacture of thin sheet products and fabrication of structural components.

Special Concretes and Concreting Techniques

507

Essentially, fibers act as crack arrestor restricting the development of cracks and thus transforming an inherently brittle matrix, i.e., Portland cement with its low tensile and impact resistances, into a strong composite with superior crack resistance, improved ductility and distinctive post-cracking behavior prior to failure. Steel fibers are probably the best suited for structural applications. Due to superior properties like increased tensile and bending strengths, improved ductility, resistance to cracking, high impact strength and toughness, spalling resistance, and high energy absorption capacity, fiber-reinforced concrete (FRC) has found special application in hydraulic structures, airfield and highways pavements, bridge decks, heavy duty floors and tunnel linings.

Preplaced or Slurry Infiltrated Fiber Concretes In general, the superior toughness and energy absorption properties of FRC in comparison to conventional concrete improve as volume fraction of fibers increases. Techniques for achieving high fiber volumes include the strategy of pre-placing dry fibers in the framework and infiltrating the bed of fibers with a cementing slurry. This composite is called slurry infiltrated fiber concrete (SIFCON). Recently, another form of slurry infiltrated fiber composite called slurry infiltrated mat concrete (SIMCON) has been developed. SIMCON is a new generation of high performance fiber reinforced concrete (HPFRC), made by infiltrating continuous steel fiber-mats with a specially designed cement-based slurry. Thus instead of reinforcing concrete with steel bars, it is reinforced with sheets of stainless steel fibers injected with a mixture of cement, aggregates and water, called slurry. The fiber mats (available in rolls) are shaped and wrapped around existing columns and beams, and injected with concrete slurry for repairing or strengthening existing structures. The mats are made of recycled stainless steel fibers. They add tensile strength and ductility, energy absorbing properties, to the concrete. Since continuous fiber mats are used, SIMCON differs from other high performance fiber reinforced concrete in at least two aspects: (i) it requires smaller fiber volume fraction to achieve substantial increases in mechanical properties, and (ii) it is delivered in pre-packed rolls that can be easily cut, handled, and installed in the field. SIMCON can be used in new construction or to reinforce existing structures. Unlike conventional concrete (where reinforcement is designed to fail before the concrete and where at failure large slabs chunks of concrete break apart from the reinforcement and fall from the structure) in SIMCON at failure, the mass of fibers and concrete does not collapse. Instead of large chunks breaking and falling from a structure, the material crumbles into small harmless flakes which pose little danger to people or property below. This controlled form of failure is a key advantage of SIMCON. In conventional concrete, the cracks are large and connected, allowing water to seep into the concrete and further compromise the integrity of the structure. On the other hand, in HPFRC the cracks are small and disconnected hairlines discounting the possibility of water seepage. As the concrete slurry uses very little water to allow it to be packed tightly into the mat, some of the cement powder remains unhydrated. With the passage of time as water seeps in through the small cracks, it mixes with the unhydrated cement and causes it to hydrate, essentially making the HPFRC

508

Concrete Technology

system self-healing. The HPFRC system is designed to use conventional concrete construction equipment with minimal modifications, adding to the already lowered construction costs.

Process Technology Since SIMCON uses a manufactured continuous mat of interlocking discontinuous steel fibers, and flowable cement-based slurry it controls corrosion in very thin members, and permits development of high flexural strengths and very high ductility. Even with comparatively lower fiber volume fraction fiber-mats, SIMCON exhibits improved properties in tension, compression and shear. Furthermore, since fiber-mats are pre-packed in the plant, distribution and orientation of fibers can be more accurately controlled, than is the case with short randomly distributed discrete fiber HPFRCs. This allows the manufacture of high performance cement-based fiber composite that can have different yet easily controllable properties in the longitudinal and transverse directions. These material characteristics are desirable in the repair/retrofit of structural elements such as columns, which require a high increase in strength and toughness in the transverse direction while increasing only ductility but not strength in the longitudinal direction, i.e., moment-carrying direction. In a retrofit situation continuous SIMCON fibermats, delivered in large rolls, can be easily installed by wrapping around members to be rehabilitated. SIMCON has tremendous potential in seismic retrofit. The presence of SIMCON layer leads to both improved performance and durability of the member. The member dimensions, amount of reinforcement and weight of member can be optimized. In contrast to the behavior observed in short fibers HPFRC, SIMCON is insensitive to the angle of fiber-mat.

Properties of SIMCON The advantage of steel fiber mats over a large volume of discrete fibers is that the mat with predecided configuration provides inherent strength and can utilize fibers with much higher aspect ratios. The fiber volume is less than half that required for slurry infiltrated fiber concrete (SIFCON), while achieving similar flexural strength and energy absorption capacity. The typical aspect ratios for FRC range from 40 to 100, although special handling procedures may be required as the aspect ratio approaches 100. SIMCON utilizes fibers with aspect ratios exceeding 500. Since the mat is already in a pre-formed shape, handling problems are minimized and balling does not become a factor. The superior performance of the SIMCON over SIFCON is related to the bonding of the mat fibers in the composite. In the standard SIFCON, the relatively short embedment length of 25 mm results in fiber pullout as the primary failure mode. In the SIMCON composites, the failure mode comprises multiple cracks and ultimate failure occurs through fiber breakage in the high tensile stress zones of one or more of the crack planes. In the mat reinforced composites, the yield strength of the steel is fully utilized. The elastic modulus, ultimate strength, strain at ultimate strength, toughness up to 0.15 per cent strain are all increased with an increase in the fiber volume fraction of the composite. Apart from the elastic modulus, an increase in all the other parameters seems to be linearly related to the increase in the fiber volume fraction. The injection of the slurry from the bottom of structural member provides a better and more uniform slurry infiltration, leading to a higher density of the composite.

Special Concretes and Concreting Techniques

509

Simcon vs. Sifcon Compared to SIFCON, SIMCON exhibits the same or improved mechanical properties with markedly lower fiber volume fraction. SIMCON is also easier to handle and construct than with SIFCON. Hence the best design solution can be achieved only synergy of SIMCON and SIFCON. SIMCON is better suited for applications where one dimension is much smaller than the remaining two, such as in the case with bridge deck overlays. On the other hand, SIFCON is better suited for three-dimensional application, such as zones of reinforcing bar anchorage or of beam column joint.

14.10.2

Mechanism of Fiber-Matrix Interaction

In contrast to fiber composites in resin and metal matrices where the fibers are aligned and constitute to about 60 to 80 per cent of composite volume, FRC contains much less fibers which are randomly oriented. The tensile cracking strain of cement matrix (less than 0.02) is very much lower than the yield or ultimate strain of steel fibers. As a result, when a fiber reinforced composite is loaded, the matrix will crack long before the fibers can be fractured. Once the matrix is cracked, the composite continues to carry the increasing tensile stress; the peak stress and the peak strain of the composite are greater than those of the matrix alone. During the inelastic range between first cracking and the peak, multiple cracking of matrix occurs as indicated in Fig. 14.15. Until the initial cracking of the matrix, it is reasonable to assume that both the fibers and the matrix behave elastically and there is no slippage between the fibers and the matrix. After initial cracking has occurred, the composite will carry increasing load only if the pull-out resistance of fibers is greater than the load at the initial cracking. In the post-cracking stage, the failure of composite is generally due to fiber-pullout rather than fiber yielding or fracture. In FRC, the fracture is a continuous process wherein the cracking occurs over a wide range of loading and the debonding of fibers occurs over several stages. The bond or the pull-out resistance of fibers depends on the average bond strength between the fibers and the matrix, the number of fibers crossing the crack, and the length and diameter of the fibers. The ratio l/d is called the aspect ratio where l is the length and d the diameter of the fibers. Improvement in the structural performance of FRC depends on the strength characteristics of the fibers themselves, volume of fiber reinforcement, dispersion and orientation of fibers, and their shape and aspect ratio. Higher strength, larger volume, larger length, and smaller diameter of fibers have been found independently to improve strength of the composite. The orientation and dispersion effects may depend, among other things, on loading conditions. Unidirectional fibers uniformly distributed throughout the volume are most efficient in uniaxial tension. While flexural strength may depend on a unidirectional alignment of fibers dispersed away from the neutral plane, flexural shear strengths may call for random orientation. A proper shape and higher aspect ratio are also needed to develop adequate bond between the concrete and the fibers so that the fracture strength of fibers may be fully utilized. For steel fiber reinforced concrete (SFRC), the idealized stress-strain relation is shown in Fig. 14.15.

510

Concrete Technology

Composite specimen

Tensile Stress

Peak Fiber pull-out B

A

No

sli be ppa ha ge vio el r a

st ic

First cracking

g kin r ac vio r c ha e ipl c be t l i u M last e n i

Strain

Fig. 14.15

Behavior of fiber reinforced concrete under tensile load

In an FRC member subjected to flexure, the load at the first crack will increase due to the crack arresting mechanism of the closely spaced fibers. After the concrete cracks in tension, the fibers continue to take the load, provided the bond is good. When the fiber strain reaches its breaking strain, the fibers fracture resulting in load transfer to the fibers of adjacent layers which on reaching their breaking strain fracture and result in the shifting of the neutral axis. Failure occurs when the concrete in compression reaches the ultimate strain. The most important factors affecting the ultimate load are the volume of fibers and their aspect ratio.

14.10.3

Concrete Matrix

The cement required is OPC or PPC conforming to IS: 269−1989 or IS: 1489−1991, respectively. The aggregates are usually crushed quartz conforming to IS: 383−1970. A fiber-reinforced concrete requires a considerably greater amount of fine material than plain concrete so that it may be conveniently handled and placed. To be fully effective, each fiber needs to be completely embedded in the matrix and this determines the proportion of fine to coarse aggregate. The effect of aggregate size on the workability is shown in Fig. 14.16. Fiber concrete, therefore, generally requires a greater proportion of cement paste than conventional concrete for handling and placing by using the equipment meant for ordinary concrete.

Special Concretes and Concreting Techniques

511

Normal concrete contains 25 to 35 per cent of cement paste of the total volume of concrete, and fiber-reinforced concrete requires paste content of the order of 35 to 45 per cent of the total volume of concrete, depending upon the fiber geometry and fiber volume. 300 Concrete with maximum size of aggregate A = 20 mm B = 10 mm C = 4.75 mm D = Cement paste Aspect Ratio of Fibers = 100

250

Vee-Bee Time, s

200

150 A

B

C

D

100

50

0

2

0

1

Fig. 14.16

14.11

4

6

8

10

12

14 (By weight)

2 3 Content of Fibers, per cent

4

(By volume)

Effect of aggregate size on workability of fiber reinforced concrete

DIFFERENT TYPES OF FIBERS

The most commonly used man-made fibers have been steel and polypropylene, principally in concrete, and glass, principally in cement mortar for thin section applications. Properties of some of the commonly used fibers are given in Table 14.8.

14.11.1

Steel-Fiber Reinforced Concrete

A number of steel-fiber types are available as reinforcement. Round steel fibers, the commonly used type, are produced by cutting round wires into short lengths. The typical diameters lie in the range of 0.25 to 0.75 mm. Steel fibers having a rectangular cross section are produced by slitting the sheets about 0.25 mm thick. For improving the mechanical bond between the fiber and matrix, indented, crimped, machined and hook-ended fibers are normally produced. The aspect ratio (=length/ diameter) of fibers which have been employed vary from about 30 to 250. Typical examples of shape are shown in Fig. 14.17.

Table 14.8

Physical properties of various types of fibers and matrices

Material

Specific gravity

Effective Modulus, GPa

Tensile Strength, MPa

Elongation at breaking, point, per cent

Acrylic

1.10

2.1

210−420

25.0−45.0

Asbestos (Chrysotile) Carbon

2.55

8.4−14

200−1800

2.0−3.0

(i) high modulus

1.9

380

1800

0.5

(ii) high strength

1.9

230

2600

1.0

1.5

10−40

500

−

Cellulose Cotton

1.5

5

420−700

3−10

Glass (Cem-FIL filament)

2.7

80

1050−3870

1.5−3.5

Nylon

1.1

4.2

780−850

16.0−20.0

Polyester

1.4

8.5

750−880

11.0−13.0

Polyethylene (high modulus)

0.96

15−40

300−700

3.0−10.0

Polypropylene

0.91

3−15

560−780

8.0

Rayon

1.50

7.3

420−630

10−25

Steel

7.86

200

280−420

3.5

2.0−2.2

10−20

2−6

0.01−0.05

2.30

10−35

1−4

0.005−0.015

OPC paste OPC concrete

(a) Flat steel sheet fibres

(b) Two-dimensional steel (c) Three-dimensional steel fibres fibres

(d) Fiber reinforced concrete slab

Fig. 14.17

Examples of steel fibers and fiber reinforced concrete slab

Special Concretes and Concreting Techniques

513

Fibers made from mild steel drawn wire conforming to IS: 280−1976 with the diameter of wire varying from 0.3 to 0.5 mm have been practically used in India. Round steel fibers are produced by cutting or chopping the wire, flat sheet fibers having a typical cross section ranging from 0.15 to 0.41 mm in thickness and 0.25 to 0.90 mm in width are produced by slitting (shearing) flat sheets. Deformed fibers which are loosely bonded with water soluble glue in the form of a bundle are also available. Since individual fibers tend to cluster together, their uniform distribution in the matrix is often difficult. This may be avoided by adding fiber bundles which separate during the mixing process. The properties of various types of fibers are compared in Table 14.8.

Properties of Fresh Steel-Fiber Reinforced Concrete For satisfactory performance in the hardened state, fiber reinforcement should be uniformly distributed and fresh concrete be well compacted. Before adding fibers during mixing, it is essential that the clumps of tightly bound fibers be broken up. For bulk steel-fiber mixes, a mixing sequence is recommended which is to blend fiber and aggregate before charging the mixer, e.g., by combining fiber and aggregate on a conveyor belt or chute. The ease with which the fiber concrete can be compacted during construction depends on the nature and amount of the fiber used and, most importantly for short fibers, on their aspect ratio. The slump test has been judged to be a poor indicator of relative workability of steel-fiber concretes, since the addition of fibers to the mix changes the slump out of proportion to the workability change. The Vee-Bee test which incorporates the effects of vibration has been found to give a realistic assessment of workability of fiber concretes. The unsuitability of conventional workability tests for fiber concrete is essentially because of the fact that internal structure and flow characteristics of fiber-reinforced concrete are distinctly different from those of conventional concrete due to the presence of fibers. The composite forms a relatively stable system due to the interlocking of fibers which resists the flow of fresh concrete. This makes the tests like slump and compacting factor ineffective for fiber concrete because the mobilizing force in these tests (self-weight) is inadequate to overcome the effective cohesion in the presence of fibers. Typical relationships between Vee-Bee time, fiber content and aspect ratio for fiber-reinforced mortars are shown in Fig. 14.18 which indicate that the workability of mix decreases with an increase in fiber concentration and aspect ratio. There is a critical fiber content for each aspect ratio beyond which the response to vibration decreases rapidly. Figure 14.16 indicates that a reduction of maximum aggregate size facilitates the inclusion of fibers, although little is gained by using aggregate size smaller than 4.75 mm. Use of pulverized fuel-ash as a partial replacement of cement (30 % by mass of cement) and a water-reducing admixure may be recommended to facilitate compaction.

Measurement of Workability ACI Committee: 544 (1978) has recommended the use of inverted slump-cone test for the measurement of workability. The test measures the time to empty the steel-fiber concrete mix from an inverted slump-cone

514

Concrete Technology 70 60 l/d 253

152

100

73

66

Vee-Bee Time, s

50 40 30 20 10 0

1 0

Fig. 14.18

2

2 4

3

6 8 10 12 Content of Fibers, per cent

4

5 14

(By volume) (By weight)

Effect of fiber aspect ratio on workability of fiber reinforced concrete

resting 75 mm above the bottom of a nine-liter (yield) bucket, after a 25−30 mm diameter vibrator prob has been inserted. The prob is allowed to fall and touch the bottom of the bucket. The time recorded in the range of 11 to 28 seconds indicates a steel-fiber concrete of good workability. This test has not been fully evaluated and is somewhat cumbersome. In the workability measurement by conventional tests it is basically the cohesion of the mix which is indirectly measured. This cohesion of mix results in shear strength of the mix in the fresh state. It has been observed that the resistance to penetration by a cone of plastic material is dependent on the shear strength of the fresh concrete. Based on this observation, a cone penetration test has been suggested to measure the workability of fiber-reinforced concrete wherein a standard cone penetrates by its own weight through a mass of fresh mix. The depth of penetration in millimetres may be taken as a measure of workability. The penetration depth of a metallic cone with an apex angle of 30° and having a weight of 40N has been reported to give the representative workability. The choice of cone with 30° apex angle and 40N weight is based on the observation that the penetration depths obtained with this cone are neither too large nor too small, and are suitable for the normal range of mixes. For normal mixes the depth of penetration has been found to vary from 200 mm to 50 mm. The cone penetration test is easy to conduct and can be conveniently adopted in the field conditions. The test has the comparative simplicity of a slump test while being suitable even for low workability mixes for which conventional tests fail. The test data have a consistent relationship with the other measures of workability given by slump, Vee-Bee time, compacting factor and ACl inverted cone method. The relationships between workabilities measured by different methods are given in Figs. 14.19 to 14.21.

Special Concretes and Concreting Techniques 140

70

Cone-penetration Depth d0, mm

120

60

80

100

Slump V Vee-Bee Time Cone-Penetration Depth d0

80 60

40

40

Slump, mm

Vee-Bee Time, s

50 60

40 30 20

20 20

10 0

0

0

Fig. 14.19

0 0.0 0.5 1.0 1.5 2.0 Fiber Content (by volume), per cent

Variation of workability values with fiber content

280

Fibre content, per cent Vf = 0.0 Vf = 0.5 Vf = 1.0 Vf = 2.0

Cone-Penetration Depth d0, mm

240

200

160

120

80

40

0 0

Fig. 14.20

20

40 60 80 V Vee-Bee Time, s

100

Relation between Vee-Bee time and cone penetration depth

515

516

Concrete Technology 250

Cone-penetration Depth d0, mm

200

150

100

50

0.6

0.7

0.8

0.9

1.0

Compacting Factor

Fig. 14.21

Relation between compaction factor and cone penetration depth

Factors Affecting Workability The factors having a predominant effect on the workability are aspect ratio (l/d ) and fiber volume concentration. Long thin fibers (l/d > 100) tend to mat together while short stubby fibers (l/d < 50) cannot interlock and can be dispersed by vibration. A minimum fiber volume concentration called critical concentration is needed to increase the strength. The critical concentration is generally inversely proportional to the aspect ratio. For l/d = 100, a volume concentration of 0.5 per cent for flexural strengthening and 1.7 per cent for tensile strengthening is required. However, for a 1.7 per cent concentration, an adequate workability can be obtained only with cement paste, and cement-sand mortar; whereas a 0.5 per cent concentration can perfectly be provided in the concrete. Thus there is a restricted range of practical fiber reinforced concrete with improved

Special Concretes and Concreting Techniques

517

strengths. The performance of hardened concrete depends upon the specific fiber surface (SFS) which is defined as the total surface area of all the fibers present within the unit volume of the composite. The specific fiber surface depends upon the fiber volume concentration, fiber size and aspect ratio. For a fiber volume concentration of Vf per cent, the specific fiber surface in a unit volume of composite is given by SFS = n (p dl ) where n, l and d are the number, length and diameter of the fibers, respectively, and p dl is the surface area of each fiber. The number of fibers is given by

n= Thus,

SFS =

v f ×100

π d 2l / 4 400 V f d

=

400 V f A l

where A = l/d is the aspect ratio. The above expression indicates that for the given fiber volume concentration and aspect ratio, the specific fiber surface is inversely proportional to the fiber length.

Behavior of Hardened Steel-Fiber Concrete The crack-arrest and crackcontrol mechanism of SFRC results in the improvement of all properties associated with cracking, such as strengths (tensile, flexural, shear, torsional, bearing strengths), stiffness, ductility, energy absorption, and the resistance to freeze−thaw damage, impact, fatigue and thermal loading. The crack controlling property of fibers has three major effects on the behavior of concrete composite: 1. Fibers delay the onset of flexural cracking, the increase in tensile strain at the first crack being as much as 100 per cent. The ultimate strain may be as large as 20 to 50 times that of plain concrete. 2. The fibers impart a well-defined post-cracking behavior to the composite. 3. The crack-arrest property and consequent increase in ductility imparts a greater energy absorbing capacity to the composite prior to failure. With a 2.5 per cent fiber content the energy absorbing capacity is increased by more than 10 times as compared to unreinforced concrete. The range of improvement in the mechanical properties of steel-fiber-reinforced concrete are given in Table 14.9. The fiber concretes reinforced by conventional steel bars have substantially improved serviceability conditions obtained by crack and deflection control, besides increasing flexural strength marginally. These conditions are as follows. 1. Tensile strength The failure in tension of cement-based matrices is rather brittle and the associated strains are relatively small in magnitude. The addition of fibers to such matrices, whether in continuous or discontinuous form, leads to a substantial improvement in the tensile properties of the FRC in comparison with the properties of the unreinforced matrix.

518

Concrete Technology Table 14.9

Improvement in the properties of fiber-reinforced concrete

Property

Maximum improvement over plain reference concrete, per cent

Optimum fiber parameters Volume fraction, Vf

Aspect ratio, l / d

Compressive strength at failure (M 20 mix)

25

1.5

−

Tensile strength (direct)

45

1.0

80

Tensile strength (split cylinder)

40

1.5

80

Modulus of elasticity

15

1.5

80

Ultimate strain

300

−

−

(i) at first crack

40

1.5

80

tensile strain

100

−

−

60

1.5

80

−

−

Flexural strength

(ii) at failure tensile strain Modulus of rupture Energy absorption

20 to 50 times 10 500

1.5

80

1000

2.5

100

400−900

−

−

125

−

−

(i) Non-reversal

90 of static

−

−

(ii) Full-reversal

70 of static

−

−

10−30 of similar beams of nonfatigue histories

−

−

Impact resistance (due to explosive charges and dropped weight) Flexural fatigue Static load Endurance to 2 × 106 cycles at a strain rate equal to that in reference specimens subjected to static load

Post-flexural fatigue, flexural strength

The stress-strain or load-elongation response of fiber composites in tension depends mainly on the volume fraction of fibers. In general, the response can be divided into two or three stages, respectively, depending on whether the composite is FRC (fiber volume less than about three per cent) or Slurry Infiltrated Concrete (SIFCON) where the volume of fibers normally varies between 5 and 25 per cent. Before cracking, the composite (both SIFCON and FRC) can be described as an elastic material with a stress-strain response very similar to that of the unreinforced matrix. After cracking, i.e., in the stage of bridging the cracked

Special Concretes and Concreting Techniques

519

surface, the fibers tend to pull out under load resulting in a sudden change in the load-elongation or stress-strain curve. If the maximum post-cracking stress is larger than the cracking stress, such as in SIFCON, then a second stage of behavior can be identified as the multiple cracking stage. This corresponds to the portion of the load-elongation curve that joins the cracking stress point to the maximum post-cracking stress point (peak point on the curve). Beyond the peak point, a third stage of behavior exists which is characterized by failure and/ or pull out of the fibers across a single critical crack. The post cracking strength increases with increasing bond strength, aspect ratio and volume fraction of fibers. It is now generally accepted that the type and amount of fibers currently used do not significantly enhance the first cracking tensile strength of the fiber reinforced composite. Many of the current applications of fiber-reinforced concrete involve the use of fibers ranging around 1.0 per cent by volume of concrete. In SIFCON and SIMCON with large volume of aligned fibers, there is substantial enhancement of the tensile load-carrying capacity of the matrix. This may be attributed to the fact that fibers suppress the localization of microcracks into macro-cracks and consequently the apparent tensile strength of the matrix increases. 2. Compressive strength The presence of fibers in normal strength concrete produces only modest increase in compressive strength, although the increased ductility resulting from the addition may be advantageous, particularly in over-reinforced concrete beams where a brittle failure can be changed into a ductile one. On the other hand, the use of steel fibers in lower strength concrete mixtures increases their compressive strength significantly compared to plain unreinforced matrices and is directly related to volume fraction of steel fiber used. fibers improve the compressive behavior by enhancing the toughness. The magnitude of the increase is dependent on the fiber shape and the content. This increase is more for hooked-end steel fibers in comparison with straight steel fibers, glass or polypropylene fibers. 3. Flexure As in the case of tension response shown in Fig. 14.8 there are three stages of the load-deflection response in flexure: (a) A more or less linear response up to point A The strengthening mechanism in this portion of the behavior involves a transfer of stress from the matrix to the fibers by interfacial shear. The imposed stress is shared between the matrix and fibers until the matrix cracks at what is termed as the first cracking strength or the proportional limit. This is called the process zone, the distributed region in front of an advancing crack due to the stress concentration field. (b) A transition non-linear portion between point A and the maximum load capacity point B (assuming that the load at B is larger than the load at A) In this portion (after cracking) the stress in the matrix is progressively transferred to the fibers. With increasing load, the fibers tend to gradually pull out from the matrix leading to a non-linear load-deflection response until the ultimate flexural load capacity point B is reached. This point is

520

Concrete Technology

termed as peak strength. This zone is called the pseudo-plastic zone where matrix has cracked but fibers bridging the crack provide some resistance to pullout. The pseudo-plastic zone provides the main contribution to the fracture energy of fiber-reinforced cement composites. (c) A descending portion following the peak strength until complete failure of the composite The load-deflection response in this portion, i.e., the degree at which loss in strength is encountered with increasing deformation is an important indication of the ability of the fiber composite to absorb large amounts of energy before failure. It is a characteristic that distinguishes fiber-reinforced concrete from plain concrete. This characteristic is referred to as toughness. This zone is also called stress free zone because the fibers have either completely pulled out or failed. Because of the linear dependence of the ultimate flexural strength of FRC on the volume fraction of fibers and their aspect ratio, it could be stated that the ultimate flexural strength generally increases with the fiber-reinforcing index, defined as the product of fiber volume fraction and aspect ratio (Vf L/df ). Based on this observation, following general equation for predicting the ultimate flexural strength of the fiber composite has been proposed. fc = C fm (1 − Vf) + D (Vf L/df) where fc is the ultimate strength of the fiber composite, fm is the maximum strength of the plain matrix (mortar or concrete), and C and D are constants which can be determined experimentally. For plain concrete C = 1 and D = 0. The constant C accounts for the bond strength of the fibers and randomness of fiber distribution. The values for the constants C and D have been proposed as 0.95 and 4.95 for the ultimate flexural strength of steel fiber-reinforced concrete and 0.85 and 4.25 for its first cracking strength. The increasing fiber-reinforcing index (Vf L/df) has a positive influence on performance because of the improved resistance to pull-out of the fibers from the matrix. The maximum quantity of hooked-end fibers that can be added without causing balling is limited to 1.0 per cent by volume. Compared to plain concrete, the addition of fibers increase the first cracking strength by 15 to 75 per cent and static flexural strength (characterized by modulus of rupture) by 15 to 30 per cent for the values of Vf L /d from 40 to 120 (a practical limit from workability consideration). Compared on equal basis of 1.0 per cent by volume, the hooked-end steel fibers contribute the highest increase, and the straight fibers provide the least increase in the above-mentioned properties. The ultimate load carrying capacity of fiber-reinforced concrete beam depends mainly on the adequacy of bond. In the absence of excellent interfacial bond, the fibers are debonded as soon as the load is transferred to them immediately after cracking of the matrix and the ultimate load will not be greater than the ultimate load of beams without fiber reinforcement. If the bond is excellent, the fibers can withstand loads even after the cracking

Special Concretes and Concreting Techniques

521

of the matrix, and this results in an increase in the ultimate strength. An improvement in bond can be achieved by the introduction of indented, crimped, or bent fibers. The polyester and polypropylene fibers significantly increase the flexural toughness and the post-peak resistance of concrete. These improvements continue as fiber volume increases, except in ultimate strength, for which it starts to decrease beyond fiber volume of 0.35 per cent. The addition of silica fume enhances toughness and post-peak strength of plastic fiber concrete. Slurry infiltrated concrete (SIFCON) when used over the reinforced concrete beams leads to ductility indexes exceeding three times those obtained without it. Crack widths and spacing are more than an order of magnitude smaller than in conventional reinforced concrete. There is no need for stirrups in flexural members with SIFCON matrix. 4. Shear strength The enhancement of shear strength of fiber reinforced high strength concrete is of the order of 60 per cent with steel and 15 per cent with polypropylene fibers, whereas fibers reinforced normal strength concrete attains an enhancement of 35 per cent with steel fibers and no increase with polypropylene fibers when compared to the strengths of their respective unreinforced plain concretes. The enhancement of performance of fibers in high strength concrete is attributed to the improved bond characteristics associated with the use of fibers in conjunction with high-strength concrete. For the concrete with steel fibers, significant increases in ultimate load and ductility is achieved. With polypropylene fibers, a lower increase in ultimate load is obtained when compared to the increase due to steel fibers. Ductility of the polypropylene fiber reinforced specimens is greater than that of steel fiber reinforced concrete. Combination of fibers and conventional stirrups, results in slight increases in the ultimate load but offers major improvements in ductility as compared to the corresponding plain concrete with conventional stirrups. 5. Modulus of elasticity The dynamic modulus of elasticity of FRC containing steel fibers up to about two per cent by volume of concrete varies within five per cent of the unreinforced matrix. Hence, the conventional solutions for the static elastic modulus can also be applied for the dynamic modulus of fiberreinforced concrete. 6. Creep and shrinkage The factors that influence the shrinkage strain in plain concrete also influence the shrinkage strain in fiber reinforced concrete; namely, temperature and relative humidity, material properties, the duration of curing and the size of the structure. The addition of fibers, particularly steel, to concrete have beneficial effects in counterbalancing the movements arising from volume changes taking place in concrete, and tends to stabilize the movements earlier when compared to plain concrete. The primary advantage of fibers in relation to shrinkage is their effect in reducing the adverse width of shrinkage cracks. Shrinkage cracks arise when the concrete is restrained from shrinkage movements. The presence of steel fibers delays the formation of first crack, enables the concrete to accommodate more than one crack and reduces the crack width substantially. Polypropylene

522

Concrete Technology

fibers are much less effective in reducing crack widths than steel fibers. High strength concretes with silica fume undergo early cracking when deformation is restrained. This phenomenon, which occurs even when concrete is protected against any evaporation, is attributed to autogenous shrinkage because of the exceptionally low water–cement ratio. This phenomenon can be corrected by the use of fibers. 7. Strain capacity The ability to accomodate relatively large strains before failure, the superior resistance to crack propagation, the ability to withstand large deformations and the enhanced ductility are characteristics that distinguish fiber-reinforced concrete from plain concrete. These characteristics are generally described by toughness, which is the main reason for using fiber-reinforced concrete in most of its applications. Unlike plain concrete the presence of fibers imparts considerable energy absorption capacity to stretch and debond the fibers before complete fracture of the material occurs. Thus the toughness is a measure of the ability of the material to mobilize large amounts of post-elastic strains or deformations prior to failure. The area under the complete load-deflection curve (or under a prescribed part of the curve) can be described as a measure of toughness or energy absorption capability of the material. The variables that affect the ultimate flexural strength of FRC beams also influence the flexural toughness; namely, the type of fiber, volume fraction of fiber, the aspect ratio, the fiber’s surface deformation, bond characteristics and orientation. The steel fibers are very effective in improving the flexural toughness of rapid-set materials. Considerable ductility and toughness can be achieved by using SIFCON and SIMCON. The increase in silica fume content renders the fiber-reinforced concrete more brittle as compared to concrete without silica fume. 8. Impact resistance Impact resistance is essential for applications such as the bridge piers. It is well recognized that the addition of fibers to concrete enhances the impact resistance. Improvements in impact strength for fiberreinforced concretes are highly dependent on the type of fiber and the method of test. It is estimated by using falling weight method or explosives or pendulum-type impact machine. The impact strength against dynamic tensile and compressive loads due to dropped weights or explosives is 8 to 10 times that of plain concrete. The fiber-concretes incorporating hooked-end and corrugated steel fibers have excellent impact resistance. Furthermore, addition of silica fume at the rate of 5 to 10 per cent by mass increases the impact resistance even more due to the improvement in fiber dispersion and enhancement in bond between fibers and concrete caused by silica fume. However, it should be realized that the adverse effects on workability, caused by high contents of silica fume or fibers, results in the reduction in the impact resistance of the material. 9. Fatigue In many applications, particularly in pavements, bridge deck overlays, and off-shore structures, the flexural fatigue strength and endurance limit are important design parameters. Fatigue strength can be described as the maximum flexural stress at which FRC composites can withstand a prescribed

Special Concretes and Concreting Techniques

523

number of fatigue load cycles before failure. Alternatively, it can be defined as the maximum number of fatigue load cycles needed to fail a beam under a given maximum flexural stress level. However, the fatigue strength is often evaluated on the basis of endurance limit. The endurance limit of FRC in flexural bending is defined as the maximum flexural stress at which the beam can withstand a prescribed number of loading cycles (usually two million cycles), expressed as a percentage of either: (a) its virgin static flexural strength (first cracking strength or modulus of rupture), or (b) the maximum static flexural strength of similar plain unreinforced matrix. The flexural fatigue strength of steel FRC is about 80 to 90 per cent of its static flexural strength at two million cycles when non-reversed loading is applied and about 70 per cent of its static flexural strength when full reversed loading is used. The addition of collated hooked-end steel fibers results in a considerable increase in the flexural fatigue strength of concrete. The flexural fatigue strength increases by 200 to 250 per cent, and endurance limit (to achieve two million cycles) by 90 to 95 per cent, relative to plain concrete. The fatigue strength and endurance limit increase with the addition of fibers and increasing volume fraction of fibers. The improved bond characteristics of fibers improves the fatigue strength of fiber composites. The highest increase in fatigue strength was with hooked-end steel fibers and the lowest increase is with straight steel fibers and polypropylene fibers. 10. Durability As in case of conventional reinforced concrete, steel fibers will be protected from corrosion provided the alkalinity of the matrix is maintained in the vicinity of the fibers. Carbonation of concrete matrix may lead to corrosion of the fibers, and any deterioration may be accelerated if the concrete is cracked. Since fiber-concrete normally fails due to fiber pull-out rather than fiber fracture the uncorroded fiber strength is not fully utilized, a considerable reduction in diameter due to corrosion could be tolerated provided that corrosion does not reduce the interfacial bond strength. The studies have indicated a greater rate and extent of chloride penetration for fiber-reinforced concrete than for conventional plain concrete. This suggests that the fibers extending from the surface may create an entry for the chlorides in addition to normal capillary system and make fiber reinforced concrete more vulnerable to corrosion damage than conventional steel reinforcement.

Application of Steel-fiber Reinforced Concrete Steel-fiber reinforced concrete (SFRC) provides additional strength in flexure, fatigue, impact and spalling. These properties lead to smaller concrete sections, improved surface quality and reduced maintenance. The main applications of SFRC are in highway and airfield pavements, hydraulic structures, tunnel linings, industrial floors, bridge decks, repair works, etc. SFRC can be applied in the following areas: 1. Highway and airfield pavements The steel-fiber concrete can be used in new pavement constructions or in the repair of existing pavements by the use of bonded or unbonded overlays to the slab beneath. The major advatages are: a higher flexural strength results in the reduction of required

524

2.

3.

4.

5.

6.

Concrete Technology

pavement thickness; the resistance to impact and repeated loading is increased. The transverse and longitudinal joint spacings may be increased. Under conditions of restrained shrinkage, the greater tensile strain capacity of steel-fiber concrete results in lower maximum crack widths than in plain concrete. SFRC gives a smooth riding surface without irregular depressions. The overlays for the rehabilitation of runways, taxiways, bridge decks, and the strengthening of existing runways and taxiways to comply with the rigid requirements of the newer generation heavy-duty jet aircrafts, are extensively used. SFRC can be advantageously used in the repair of damaged patches in existing runways, and highway pavement slabs. The thickness of pavements constructed with concrete having a cement content of 410 kg/m3, water−cement ratio of 0.6, maximum size of aggregates as 20 mm using 1.4 per cent (by volume, i.e., 106 kg/m3) trough type steelfibers could be 25 per cent less than normal concrete pavements. Hydraulic structures The major advantage of using steel-fiber concrete in hydraulic structures is its resistance to cavitation or erosion damage by high velocity water flow. The steel-fiber concrete has been successfully used in the repair of spilling basin at Tarbela Dam in Pakistan. The fiber concrete contained about one per cent (by volume) of 25 × 0.25 × 0.55 mm slit steel fibers. Fiber shotcrete fiber shotcrete has been used in rock slope stabilization, tunnel lining and bridge repair. A thin coating of plain shotcrete applied monolithically on top of the fiber shotcrete, may be used to prevent surface staining due to rusting. The conventional sprayed concrete techniques can be used by including fiber mixing with the pneumatic conveying of fibers from a rotary fiber feeder to the nozzle via a 75 mm diameter flexible hose. In addition to usual shotcrete advantages, the fibers are aligned in two dimensions (in a plane) by the mode of application of relatively thin coating. The fiber shotcrete can be used in the protection of structural steel work particularly in the support structure. Refractory concrete Steel-fiber reinforced refractory concretes have been reported to be more durable than their unreinforced counterpart when exposed to high thermal stress, thermal cycling, thermal shock or mechanical abuse. The increased service span is probably due to combination of crack control, enhanced toughness, and the spall and abrasion resistance imparted by the steel fibers. Through the use of shotcrete technique, the material can be used for lining ash hoppers and flame exhaust ducts. Precast applications They include manhole covers, concrete pipes, machine bases and frames. Improved flexural and impact strengths may allow the use of steel-fiber concrete components in rough handling situations. Structural applications Structural applications of steel-fiber concrete are rare. However, the following possibilities may be considered: (a) Fiber reinforcement can provide an increased impact resistance to conventionally reinforced beams, and thus an enhanced resistance to local damage and spalling.

Special Concretes and Concreting Techniques

525

(b) Fiber reinforcement can inhibit crack growth and crack widening, this may allow the use of high strength steel without excessive crack widths or deformations at service loads. (c) Fiber reinforcement provides ductility to conventionally reinforced concrete structures, and hence enhances their stability and integrity under earthquake and blast loading. (d) Fiber reinforcement increases the shear strength of concrete. As a consequence punching shear strength of slabs is increased and sudden punching failure may be transformed into gradual ductile one.

Mix Design for Steel-fiber Reinforced Concrete The mix should contain minimum fiber content and maximum aggregate for the specified strength and workability. The cement paste content depends upon three factors: 1. Volume fraction of fibers 2. Shape and surface characteristics of fibers, i.e., specific fiber surface 3. Water-cement ratio For the commonly encountered SFRC mixes, the following range of parameters is associated: Cement content 300 to 500 kg/m3 Water−cement ratio 0.45 to 0.60 Ratio of sand to total aggregate, per cent 50 to 100 Maximum size of aggregate 10 and 20 mm fiber content 1.0 to 2.5 per cent fiber−aspect ratio 50 to 1000

Mix design procedure Following are the steps involved in the mix design of fiber reinforced concrete: 1. Corresponding to the required 28-day field flexural strength of steel fiberreinforced concrete, the design strength for laboratory mix is determined. 2. For fibers of known geometry and for stipulated volume fraction, the watercement ratio is selected between 0.45 and 0.60. 3. Depending on the maximum size of aggregate and fiber concentration, the paste content is determined by mass. 4. The fine-to-coarse aggregate ratio varies from 1:1 to 1:3, a ratio of 1:1.5 is a good start for a volume percentage of fiber up to 1.5 and length of fiber up to 40 mm. 5. For the water-cement ratio and paste content determined in Steps 2 and 3, respectively, the cement and water contents may be worked out. 6. The fiber content (by mass) is calculated by taking the density of fibers as 7850 kg/m3. 7. The total quantity of the aggregate is determined from WA = WFRC − (WW + WC + WF)

526

Concrete Technology

where WA, WFRC, WW , WC and WF are the masses of total aggregate, fiber reinforced concrete, water, cement and fibers, respectively. 8. The quantities of fine and coarse aggregates are worked out by using Step 4. 9. The trial mix is prepared and the paste content adjusted if the mix shows any tendency to segregate. 10. The workability of the mix is checked using appropriate test. For the illustration of the above procedure, select a fiber concentration of 1.5 per cent (by volume) of trough-shaped fibers with aspect ratio of 80 with 0.45 mm diameter. For the maximum nominal size of aggregate of 20 mm, consider Paste content 40 per cent Fine-to-coarse aggregate ratio 1:1.5 Water-cement ratio 0.55

Example 14.2

mass of fibers per cubic metre of SFRC = 7850 × 0.015 = 117.7 kg (say 118 kg) For 1 kg (= 1/3.15 = 0.317 litre) of cement, the water content required is 0.55 lite rgiving a total paste content of 0.867 liter. Therefore, for a cement paste content of 40 per cent, i.e., 400 liters per cubic meter of concrete, the cement and water contents are 461(= 400/0.867) and 254(=0.55 × 461) kg, respectively. The total aggregate content = 2400 − (254 + 461 + 118) = 1567 kg. For the assumed fine-to-coarse aggregate ratio of 1:1.5, the coarse and fine aggregates are 940 and 627 kg, respectively.

Practical mix proportions Though the high fiber content brings about large improvements in mechanical properties, it makes the concrete unworkable. On the other hand, a low fiber content in workable concretes show no significant improvements in the desirable properties. Thus a practical concrete is a compromise between these situations. Typical mixes using fiber volume concentrations of 0.75 to 1.50 per cent with water-reducing admixtures and/or fly ash have been extensively used. With steel fibers, the typical mix proportions by mass are: Cement 1

: Water-cement ratio : (0.4 to 0.6)

: :

Sand : (2 to 3) :

10 mm aggregate (0.8 to 3)

Similar mixes have also been used for polypropylene fibers.

14.11.2

Non-Steel fibers

The examples of commercilly available non-steel fibers are given in Fig. 14.22.

Polypropylene Fiber Reinforced (PFR) Cement–Mortar and Concrete Polypropylene is one of the cheapest and abundantly available polymers. Polypropylene-fibers are resistant to most chemicals and it would be the cementing matrix which would deteriorate first under aggressive chemical attack. Its melting point is high (about 165°C), so that a working temperature as high as 100°C may be sustained for short periods without detriment to the fiber properties. Polypropylene short fibers in small volume fractions between 0.5 to 1.0 per cent have been commercially used in concrete to achieve considerable improvement in impact

Special Concretes and Concreting Techniques

(a) Polypropylene fibers

Glass fibers

Fig. 14.22

(b) PTE fibers

527

(c) Super short fibers

Carbon fibers

Examples of commercially available of non-steel fibers

strength of the hardened concrete. They have low modulus of elasticity. Polypropylene fibers are available in two forms: monofilaments produced from spinnarets, and film fibers produced by extrusions. The film fibers are commonly used and are obtained from fibrillated film twisted into twine and chopped, usually into 25−50 mm lengths for use in concrete. The fibrillated film may also be opened to produce continuous networks for use in thin sheet manufacture. A typical machine for production of polypropylene filament is shown in Fig. 14.23. Fibrillated film may also be woven to produce flat meshes which may be used as thin cement sheet reinforcement.

Fig. 14.23

A typical complete line machine for production of polypropylene filament

528

Concrete Technology

Polypropylene fibers being hydrophobic can be easily mixed as they do not need lengthy contact during mixing and only need to be evenly dispersed in the mix. These are therefore added shortly before the end of mixing the normal constituents. Prolonged mixing may lead to undesirable shredding of fibers. There is no physicochemical bond between fiber and the matrix, only a mechanical bond is formed as cement paste penetrates the mesh structure between individual fabrics of chopped length or continuous network.

Properties of fresh PFR concrete The compacting factor test has been reported to be most suitable. The inclusion of polypropylene fibers reduces the workability considerably, e.g., a normal concrete mix of medium workability (CF about 0.88) may reduce to a low workability mix (CF about 0.75) following the addition of one per cent of chopped 35 mm polypropylene fibers. Polypropylene monofilaments can be used in small volume fractions of about 0.1 to 0.2 per cent to alter rheological properties of the material, e.g., highly air-entrained concretes can be stabilized by fibers.

Properties of hardened PFR concrete The tensile strength of concrete is essentially unaltered by the presence of a small volume of short polypropylene fibers. Although the change in flexural strength of polypropylene reinforced-concrete is marginal, the post-cracking behavior has shown its ability to continue to absorb energy as fibers-pullout. The energy absorbing capacity has been found to increase with the length of fibers, e.g., the 75 mm polypropylene fibers may result in an energy absorption comparable to that of the less efficient of steel fibers; and at a considerably lower cost.

Durability Polypropylene may deteriorate under attack from ultraviolet radiation or by thermal oxidation process. The cement matrix appears to prevent the former. To combat thermal oxidation, sophisticated stabilizers have been developed to delay degradation, and enhance durability. Applications of PFR mortar and concrete 1. Clading panels Inclusion of polypropylene fibers instead of steel mesh reinforcement may allow reduction in panel thickness. 2. Shotcreting Surface coatings of polypropylene reinforced-mortar may be provided by shotcreting using normal equipment. The fibers of about 20 mm length enable smooth transport of the dry mix through air hoses and nozzles. Water is then added at the gun orifice. Shotcreting can be advantageously used in wet environments where polypropylene fibers can eliminate the need for steel (corrodable) mesh on which spray of mortar is required. 3. Polypropylene concrete can be advantageously used in the energy dissipating blocks. The potential market for polypropylene reinforced-cement is principally as a substitute for absestos-cement roofing and cladding panels.

Special Concretes and Concreting Techniques

529

Glass-fiber Reinforced Concrete (GFR) Glass fibers are made up form 200 to 400 individual filaments which are lightly bonded to make up a strand. These stands can be chopped into various lengths or combined to make cloth, mat or tape. Using the conventional mixing technique for normal concrete, it is not possible to mix more than about two per cent (by volume) of fibers of up to a length of 25 mm. The major application of glass fiber has been in reinforcing the cement or mortar matrices used in production of thin-sheet products. The commonly used varieties of glass-fibers are E-glass used in the reinforcement of plastics, and AR-glass. E-glass have inadequate resistance to alkalies present in Portland cements whereas AR-glass have improved alkali-resistant characteristics. Sometimes polymers are also added in the mixes to improve some physical properties such as moisture movement. The process of manufacture of glass-fiber cement products may involve spraying, premixing or incorporation of continuous rovings. In the spray-suction process, the glass-fiber strand is chopped into lengths between 10 and 50 mm and blown in spray simultaneously with the mortar slurry on to a mold or flat bed followed by suction to remove excess water. On the other hand, in the technique involving premixing, short strands (about 25 mm in length) are mixed into mortar paste or slurry before further processing by casting into open molds, pumping into closed molds, etc. Care must be taken to avoid fiber tangling and matting together, and to minimize the fiber damage during mixing. In the process incorporating continuous rovings, the rovings are impregnated with cement slurry by passing them through a cement bath before they are wound on to an appropriate mandrel. Additional slurry and chopped fibers can be sprayed on to the mandrel and compaction can be achieved by the application of roller pressure combined with suction. Properties of hardened GFR concrete The behavior of glass-fiber cement sheets under tensile force is typified by multiple cracking of the matrix. Longer fibers improve the ultimate failure stress. In wet environments, significant reduction in strength takes place. The material may become brittle on ageing. One of the most important improvements in the property achieved by glass fiber is the spectacular improvement in impact strength. With the addition of just 5 per cent glass fibers, an improvement in the impact strength of up to 1500 per cent can be registered as compared to plain concrete. With a two per cent fiber content (up to 25 mm in length), the flexural strength is almost doubled. The second important improvement is in the resistance to thermal shock. Ductility also improves with an increase in strength and modulus of rupture. The flexural strength of water stored and weathered specimens reduces with time and nearly equals that of the matrix alone. The reduction in energy absorption is similar to that in flexural strength. The long-term durability of glass fiber-reinforced cement can be improved by the addition of 15 per cent polymer to the mortar matrix. The increase in matrix cost is balanced by the use of cheaper E-glass fibers.

Applications The glass fiber-reinforced cement finds its use in formwork systems, ducting, roofing elements, sewer lining, swimming pools, fire-stop partitioning,

530

Concrete Technology

tanks and drainage elements, etc. Sometimes it is used in combination with polymer impregnated in-situ concrete.

Asbestos Fibers The naturally available inexpensive mineral fiber, asbestos, has been successfully combined with Portland cement paste to form a widely used product called asbestos cement. Asbestos fibers have thermal, mechanical and chemical resistance making them suitable for sheet products, pipes, tiles and corrugated roofing elements. Asbestos-cement products contain about 8 to 16 per cent (by volume) of asbestos-fibers. The flexural strength of asbestos cement board is approximately two to four times that of unreinforced matrix. However, due to relatively short length (10 mm), the fibers have low impact strength. There are health hazards associated with the use of asbestos cement. Its use is banned in most of countries. In the near future, it is likely that glass fiber-reinforced concrete will replace asbestos completely.

Carbon Fibers Carbon fibers form the most recent and probably the most spectacular addition to the range of fibers available for commercial use. Carbon fibers come under the high E-type fibers. These are expensive. Their strength and stiffness characteristics have been found to be superior even to those of steel. But they are more vulnerable to damage than even glass fibers, and hence are generally treated with resin coating.

Organic Fibers Organic fibers, such as polypropylene or natural fibers may be chemically more inert than either steel or glass fibers. They are also cheaper, especially if natural. The polypropylene-fiber concrete has been described earlier. A large volume of vegetable fibers (7 per cent, 50 mm length) may be used to obtain a multiple cracking composite. The problem of mixing and uniform dispersion may be solved by adding a superplasticizer. Polypropylene, nylon and other organic fibers due to their low modulus of elasticity are not effective in crack control, and also the organic fibers may decay. However, these fibers improve impact resistance. Vegetable Fibers The commonly used fibers are jute, coir and bamboo. They possess good tensile strength in their natural dry state. Their tensile strengths do not suffer significantly even after being immersed in 10 per cent normal solution of sodium hydroxide for up to 28 days. However, long-term durability is doubtful. In contrast to glass fibers, steel and polypropylene fibers are chemically stable in a cement paste matrix. The high alkalinity of cement paste protects steel from being corroded. The corrosion of steel fibers can however become a problem when the matrix has cracked. Irrespective of the type, size and shape of fibers to be used in a mix, the fundamental requirement of fiber-reinforced concrete is that all the individual fibers should be uniformly distributed throughout the matrix. The mix should have sufficient paste content to coat the fibers and aggregate, so that the ingredients can be placed and compacted in the final position without any segregation.

Special Concretes and Concreting Techniques

531

The mix proportions generally depend on the intended applications of the composite. The prime considerations are uniform dispersion of fibers, adequate workability for placing and compaction with the available equipment. The workability of fiber-reinforced concrete is influenced by maximum size of aggregate as can be seen in (Fig. 14.16), volume fraction, geometry and aspect ratio of fibers as shown in Fig. 14.18. As the size of aggregate increases, it becomes more difficult to achieve uniform fiber dispersion, since the fibers are bunched into mortar fraction which can move freely past the aggregate during compaction. To obtain a better dispersion the coarse aggregate content is kept lower than in a normal mix and the maximum size of aggregate is preferably limited to 10 mm. The mortar matrix (consisting of particles less than 4.75 mm) should be around 70 per cent, and aggregate−cement ratio as low as 3:1. A fine-to-coarse aggregate ratio of 1:1 is often a good starting point for a mix trial. Water-cement ratio between 0.4 and 0.6, cement-content of 250 to 430 kg/ m3 are recommended for providing adequate paste content to coat large surface of fibers. Beyond a certain optimum content of fibers the workability of the composite decreases rapidly.

14.11.3

Batching, Mixing, Placing, Compaction and Finishing

The fibers are usually added to the aggregates before the introduction of cement and water into the mixer. For laboratory testing, fibers can be added in small amounts to the rotating drum charged with cement, aggregate and water. For large batches, the fibers are blown into the previously charged rotating drum. A fiber mix generally requires more time and vibration to move the mix and to compact it into the forms. Surface vibration of forms and exposed surface is preferable to prevent segregation. The properties of fiber reinforced concrete depend upon fiber alignment. More energy is required to compact fiber concrete than conventional concrete. Some of the precautions taken while mixing, placing and compacting fiberreinforced concrete are as follows: 1. While mixing small quantities of fiber reinforced concrete by hand, there is a possibility of steel fibers shooting up and hitting the eyes of the worker or even pricking the hand. To avoid these hazards, the hands should be protected by gloves and the eyes with safety glasses. 2. A pan mixer of the counter-flow type should be used for mixing fiber reinforced concrete. 3. For uniform distribution of steel fibers, a dispenser should be used. While dispensing the fibers into concrete, the rate at which the fibers are fed to the mixer should be synchronized with rate of mixing. 4. Forks and rakes can prove helpful for handling low slump mixes. 5. Standard screeding methods and trowels can be used for finishing fiber concrete. A textured surface can be obtained by using a stiff brush. Standard workability tests, such as the slump, compacting factor and Vee-Bee tests are suitable for conventional concrete but not for mixes containing fibers. For instance, the slump of a mix, even with a low fiber content, can be zero though the mix responds well when vibrated. A workability test should provide the condition of

532

Concrete Technology

flow on vibration, because FRC responds well to conventional vibrating table as it does not easily segregate from the mix due to its low specific gravity.

14.12

POLYMER CONCRETE COMPOSITES (PCCS)

Polymer concrete composites are obtained by the combined processing of polymeric materials with some or all of the ingredients of the cement concrete composites. Depending on the process by which the polymeric materials are incorporated, polymer concrete can be classified as follows.

14.12.1

Polymer-Impregnated Concrete (PIC)

In polymer-impregnated concrete, low viscosity liquid monomers or prepolymers are partially or completely impregnated into the pore systems of hardened cement composites and are then polymerized. The partial or surface impregnation improves durability and chemical resistance. Overall improvements in the structural properties are modest. On the other hand, total or in-depth impregnation improves structural properties considerably. The hardened concrete, after a period of moist curing, contains a considerable amount of free water in its voids. The water-filled voids form a significant component of the total volume of concrete ranging from five per cent in dense concrete to 15 per cent in gap-graded concretes. In polymer-impregnated concrete, it is these waterfilled pores that are sought to be filled with polymers, i.e., the major parameters affecting monomer loading are the moisture and the air in the voids in concrete. The total or in-depth polymer impregnation of concrete, therefore, involves the following states: 1. Construction of element with well-designed cement concrete, which is adequately moist cured with optimum strength. 2. Removal of moisture by drying the concrete by heating to develop surface temperatures of the order of 120 to 150 °C. The small elements can be heated in an air oven. For large cast–in-situ surfaces a thick blanket of sand (usually 10 mm thick) can be used to prevent a steep thermal gradient. Infrared heaters may be used. About six to eight hours of heating is required to expel a large part of the free water in the concrete. 3. Cooling of concrete surfaces to safe levels (about 35 °C) to avoid flammability. 4. Removal of air by subjecting the dry concrete to vacuum. The degree of vacuum applied and the duration have significant influence on the quantity of monomer that can be impregnated and therefore, on the depth of impregnation. 5. Application of monomer by soaking the concrete surface in it for a sufficiently long time to achieve the desired depth of penetration. The soaking time depends on the viscosity of monomer, preparation of the surface prior to soaking and the characteristics of the concrete. To reduce the time required to achieve a desired depth of monomer penetration, external pressure using nitrogen gas or air is generally employed.

Special Concretes and Concreting Techniques

533

6 Covering the surface with a plastic sheet to prevent evaporation of monomer. 7. Polymerization by heating the catalyzed monomer to the required temperature levels (usually between 60 and 150 °C depending upon the type of monomer) also called thermal catalytic technique. The heating can be done, by infrared heaters or in an air oven. Depending on the polymer, two to six hours are required for this stage. The heating decomposes the catalyst and initiates the polymerization reaction. This reaction is called a thermal catalytic reaction. When monomer has penetrated into concrete, polymerization can also be initiated using ionizing radiation such as gamma rays. The polymers, when fully polymerized or cross-linked, are solids occupying the volume in which they have been impregnated. As such, at the impregnation stage, the polymer has to be in a prepolymer liquid form, generally called monomer. The state of polymerization of monomers, or of prepolymer resins, is brought about also by adding initiators, and crosslinking agents. Polymers can be broadly categorized as thermoplastics and thermosetting resins. Thermoplastics soften at an elevated temperature (usually between 100 and 150 °C and called glass transition temperature), and as such the advantage of using thermoplastic impregnated concrete is lost at such temperatures. Thermoplastic monomers have a low viscosity and are able to penetrate hardened concrete well and fill a large part of the pores. Their polymerization is accomplished by addition reactions not leading to low molecular weight by-products. Thermosetting resins, on the other hand, are more viscous and difficult to impregnate into concrete. However, they can withstand higher temperatures without softening. But the condensation reactions which occur may lead to the formation of low molecular weight by-products which would occupy some of the space. It is necessary that a monomer or its polymer is chemically compatible with the compounds of cement and the constituents of hydrated cement paste to prevent their adverse effects. Monomer/resin systems used for polymer impregnated concrete are styrene, polyester, methylmethacrylate, butylacrylate, acrilonitrile, epoxies and their copolymer combinations. The types and strength properties of some of the commonly used systems are given in Table 14.10. The applications of polymer impregnated concrete are as follows: 1. Surface impregnation of bridge decks The aim of impregnating the bridge decks is to render them impervious to the intrusion of moisture, deicing chemicals and chloride ions. 2. Applications in irrigation structures The effect of cavitation and erosion in dams and other hydraulic structures can be catastrophic. Conventional repairs of the damage are expensive and huge losses may be caused due to loss of benefits from irrigation, power generation, flood control, etc. In such cases, the polymer impregnated treatment may be cost effective. The concrete may be removed from the place of severe damage and the damaged area patched, dried and treated by impregnation.

534

Concrete Technology

Table 14.10 Polymer type

Styrene

Types of polymers and strength properties of polymer impregnated concrete Technique employed

Polymer Compreloading, ssive per cent strength, (by mass) MPa

Strength improvement ratio

Polymeriza-tion method

• Specimens vacuum \ and pressure impregnated

4 to 6

60−90

2.6−3.0

Thermal catalytic

• Predried specimens just soaked in monomer

1 to 2

25−30

1.5−2.0

Thermal catalytic

60% styrene + 40% trimetholpropane trimethacrylate (TMPTMA)

• Vacuum treated and pressure impregnated

6 to 7

50−60

1.5−2.0

Thermal catalytic

Methyl-methacrylate (MMA) MMA

• Vacuum treated and pressure impregnated 5 to 7

100−125

3.5−4.0

Thermal catalytic

5 to 7

120−140

4.0−4.5

Radiation

5.5 to 7.5

150

5

Radiation

2

70

2.3

Thermal catalytic

• High pressure steam cured concrete, dried, vacuum treated and impregnated under pressure

6 to 8

170−190

5.7−6.3

Radiation

• Vacuum treated and pressure impregnated

3.5 to 5.5

80

2.7

Thermal catalytic

• Vacuum treated and pressure impregnated

5 to 6.5

130

4.3

Thermal catalytic

• Vacuum treated and pressure impregnated

3 to 5

70

2.3

Thermal catalytic

• Vacuum treated and pressure impregnated

−

105

3.5

Thermal catalytic

MMA + 10% TMPTMA

Acrylonitrile

10% polyester + 90% styrene Vinyl chloride

Epoxy

• Vacuum treated and pressure impregnated • Vacuum treated and pressure impregnated • Predried specimens just soaked with monomer from one face only

Special Concretes and Concreting Techniques

535

3. Structural members Polymer-impregnated concrete has potential as a structural material. Polymer-impregnated prestressed concrete beams have shown remarkable improvements over conventional concrete. The maximum tendon force could be enhanced to four times that in unimpregnated concrete. The creep deflection is of the order of 1/19 to 1/16 that of static deflection. Shear strength improves by the same factor as compressive strength. The stress patterns and strain curves of polymer concretes are shown in Fig. 14.24. 120

Compressive Strength, MPa

(MMA)

80

(MMA–BA)

40

Cement concrete

0

0

4 Strain

8

(a) Polymer Impregnated Concrete

80 (MMA) (MMA–BA)

40

0

Compressive Strength, MPa

Compressive Strength, MPa

80

PAE (P/C = 0.15)

0

0 Strain (b) Polymer Concrete

Fig. 14.24

Cement concrete

40

0

4 Strain

8

10

–3

(c) Latex Modified Concrete

Stress–strain relationship for polymer concretes

4. Marine and underwater applications Greatly improved structural properties and negligible water absorption and permeability make polymer-impregnated

536

Concrete Technology

concrete an excellent material for marine and underwater applications, such as in desalination plants and sea floor structures. Even a partial impregnation of concrete piles in sea water reduces the corrosion of reinforcing bars by 24 times. 5. Repair of structures Polymer impregnation has a very good potential for the repair of damaged structures. Restoration and preservation of stone monuments is an interesting application.

14.12.2

Resin or Polymer Concrete

Polymer concrete is a composite wherein the polymer replaces the cement−water matrix in the cement concrete. It is manufactured in a manner similar to that of cement concrete. Monomers or pre-polymers are added to the graded aggregate and the mixture is thoroughly mixed by hand or machine. The thoroughly mixed polymer concrete material is cast in molds of wood, steel or aluminum, etc., to the required shape or form. Mold releasing agents can be added for easy demolding. This is then polymerized either at room temperature or at an elevated temperature. The polymer phase binds the aggregate to give a strong composite. Polymerization can be achieved by any of the following methods: 1. Thermal-catalytic reaction 2. Catalyst-promoter reaction 3. Radiation In the first method, only the catalyst is added to the monomer (thermoplastic) and polymerization is initiated by decomposing the catalyst by the application of elevated temperatures up to 90 °C. Typical catalysts used for different monomer systems include, benzoyl peroxide, methyl-ethyl-ketone peroxide, benzenesulphonic acid, etc. In the second method, a constituent called promoter or accelerator is also added, which decomposes the catalyst or accelerates the reaction, at the ambient temperature itself. Typical promoters include cobalt naphthanate, dimethyl-p-toluidine, ferric chloride, etc. Some promoters ensure polymerization at the ambient temperature within an hour. Gamma radiation is applied in the radiation polymerization method. Depending on the method of polymerization and the other conditions, polymerization takes place within a period ranging from a few minutes to a few hours. Special precautions are to be taken in handling and cleaning because the monomers are highly inflammable. Fire safety precautions are to be observed. A thoroughly dry aggregate system is to be used as the monomers may not polymerize in the presence of moisture. Moreover, the catalyst and promoter should never be added to each other as it will result in an explosion. Some of these materials are toxic and are carcinogenic, and have to be handled with extreme care. The polymer systems which have been successfully used for polymer concrete include methyl-methacrylate, polyester-styrene, epoxy-styrene, styrene and furfuryl acetone. Others are furane, acrylic, polyurethane, urea formaldehyde and phenol formaldehyde, etc. The design considerations for polymer concrete are: 1. Smaller the binder ( polymer) content to fill the voids of the aggregate system content greater is the economy.

Special Concretes and Concreting Techniques

537

2. Workability for easy mixing and placing of cement concrete without bleeding and segregation. 3. Film forming ability of the polymer, and bonding with the aggregate surface to transmit load forces. 4. Economic curing (cross-linking) times and temperatures. 5. Durability in environments to which the polymer concrete composite is exposed. Polymer concretes can be reinforced with steel, nylon, polypropylene or glass fibers in a manner similar to cement concrete. In general, polymer concrete exhibits a fairly linear stress-strain curve nearly up to failure; the failure is characterized as brittle. Concretes made of thermoset polymers show a decrease in strength by 30 to 40 per cent at higher temperatures, such as 90 °C. The elastic limit may also be substantially lowered at higher temperatures. Use of microfillers, such as finely powdered CaCO3 and silane coupling agents, improve the compressive and tensile strengths of polymer concretes. Table 14.11 shows that wide range of strengths are possible depending upon the resin system used. Table 14.11

Compressive and tensile strengths of polymer concrete

Type of polymer system

Compressive strength, MPa

Isophthalate or Orthophthalate polyester

50−140

7−10

9−30

114

7−9

−

45−130

6−16

7−31

propane trimetha-acrylate

60−80

8−9

36

Furane

70−80

5−8

20−32

Methyl-metha-acrylate (MMA)

60−120

8−9

15−18

130

30

−

40−60

25−50

10−12

Vinylester Epoxy

Tensile Modulus of strength, MPa elasticity, (×102) MPa

Methyl-metha-acrylate + trimethol-

Acrylic Carbamide

Thermosetting polymers, such as polyester and epoxy exhibit significant shrinkage during the polymerization of the resin. This shrinkage can be reduced by shrinkage reducing agents, however, at some cost to the strength. Well-cured or fully cross-linked polymer concrete has excellent resistance to acids, salts, common solvents and petroleum products. Fatigue strength of polymer concrete, with or without fibers, is excellent. Polymer concrete having up to five per cent steel fibers has better ductile and impact-resistant properties. While the general purpose polyester-styrene systems require an elevated temperature of about 60 to 70 °C for complete polymerization; resins or monomer systems are available which can cross-link at low temperatures such as 0 °C within one or two hours. Condensation polymers which are relatively inexpensive, like phenol formaldehyde have been used successfully to develop polymer concrete.

538

Concrete Technology

Polymer concretes have good potential as repair material and for overlays. Thin sand-filled overlays (12 to 30 mm thick) reduce water permeability and chloride penetration. Polymer concrete can be used for rapid repair of damaged airfield pavements and industrial structures. Polymer concrete can be used for treating the sluiceways and stilling basin of the dam. Polymer concrete pipes have been used for transporting a variety of chemicals, for carrying effluents and wastewater, etc. Polymer concrete can be used in rock bolts. It provides necessary corrosion protection to ground anchors. Polymer concretes possess good electrical properties and can be used for high voltage insulator application. Electrical structures such as poles for electrical transmission lines have been manufactured from polymer concrete.

14.12.3

Polymer Modified Concrete

Polymer modified concrete (PMC), more specifically called polymer cement concrete, is a composite obtained by incorporating a polymeric material into concrete during the mixing stage. However, the polymer so added should not interfere with the hydration process. Since many polymers are insoluble in water, their addition can only be in the form of emulsion or dispersion or latex. The composite is then cast into the required shape in the conventional manner and cured in a manner similar to the curing of cement concrete. The hydrated cement and the polymer film formed due to the curing of the polymeric material constitute an interpenetrating matrix that binds the aggregate. The polymeric materials in the form of lattices and prepolymers may be added to modify cement concretes. Depending upon the type of modifier, polymer modified cement concretes can be subdivided as: 1. Latex-modified cement concrete (LMCC) 2. Prepolymer-modified cement concrete (PMCC) In general, the quantities of polymers required for polymer-modified cement concretes are relatively small, being in the range of one to four per cent by mass of the composite. In contrast polymer-impregnated concretes require five to eight per cent and polymer concretes 8 to 15 per cent of polymer. Polymer modified cement concretes, are therefore, the least expensive. The processing of PMCC is also simplest. Conventional plant and equipment could be adopted. However, the improvements in mechanical properties have not been as high as observed in PIC or PC. 1. Latex-modified cement concrete Lattices are white milk like suspension consisting of very small-sized polymer particles suspended in water with the help of emulsifiers and stabilizing agents. It contains about 50 per cent of polymer solid by mass. Both elastomeric and glassy polymers have been employed in lattices for modifying cement concrete. The elastomeric polymers are characterized by their rubber-like elongation and by their relatively low modulus of elasticity at ambient temperatures. Some of the commonly used elastomeric lattices are:

Special Concretes and Concreting Techniques

539

natural rubber latex, styrene-butadiene rubber latex, acrilonitrile-butadiene rubber latex and neoprene. Glassy polymers are characterized by high modulus of elasticity, higher strength, and relatively brittle type of failure. Common examples are polyvinyl acetate, polyvinylidene chloride, styrene-butadiene copolymer latex, and acrylic polymers. The use of polyvinyl acetate latex due to its sensitivity to the moisture is discontinued. Polyvinylidene copolymer latex, due to its residual chloride and possible corrosion of reinforcement, is used only in unreinforced concrete applications. The latex systems for modifying cement concrete are not available in India. The optimum curing procedure involves the moist curing of composites for one to seven days, followed by dry curing at room temperature. At 28 days, the latex modified composites reach about 80 per cent their final strength. 2. Prepolymer-modified cement concrete Some of the prepolymer systems used are polyester-styrene-based system, epoxy systems and furane systems. With exception of epoxies, prepolymers (unlike lattices) do not improve the workability of cement concrete. The strength improvement of PMC over conventional concrete is of the order of 50−100 per cent. Its adhesion to plain concrete is good. The ductility is significantly improved and early micro-cracking is avoided. Consequently, the tensile strength and modulus of rupture are more than twice those of control concrete. There is considerable improvement in durability over conventional concrete due to lower water-cement ratio and filling of pores with polymer. Further research is required since the high cost of polymer addition has not been commensurately reflected in improved strength. The excellent bond of latex concrete to existing concrete, superior shear bond strength, good freeze−thaw resistance, resistance to the penetration of chloride ions, improved ductility, and superior tensile and flexural strengths makes latex modified concrete an eminent material for overlays and resurfacing applications for bridge decks, industrial flooring, food processing factories, fertilizer stores, damp resistant floors, for railway platforms, and nuclear processing areas. Surface deterioration is a major problem in marine and irrigation structures. Excellent resistance to salt water makes LMCC very effective repair material. LMCC are used for fixing ceramic tiles, lining effluent ducts, reservoirs, and sewerage and industrial waste handling structures. Latex and fiber-reinforced composites have a great potential in cement com-posites due to their synergistic behavior and improvement in matrix fiber bond.

14.12.4 Prepolymer Cement Concrete (PCC) PCC is used for flooring in food processing and chemical industries, in wear-resistant floors, and in decks over steel bridges. Due to the early development of strengths, it is suited for repair of sea defence structures.

540

Concrete Technology

The development of polymer-concrete composites has opened up the possibility of extending the very range of applicability of concrete-like composites. It has become possible to tailor a polymer-concrete composite to meet the requirements of any given application. Polymer-concrete composites are far superior to cement concretes in their resistance to chemicals, such as acids, and salt solutions. Polymer-impregnated ferrocement, a thin, lightweight and highly durable composite has a great potential for applications in coastal, off-shore and chemical industrial structures. Polymer-concrete composites are very cost effective in applications requiring high degrees of durability and chemical resistance and where so far, costlier materials and composites have been employed. In such situations developing nations could ill-afford either the use of inefficient material of construction or the employment of costlier conventional alternatives. The improved durability of polymer-impregnated concrete is shown in Fig. 14.25. 60

15% H2SO4 Cement Concrete

Weight Loss, per cent

PIC 40

20

0

0

14 Exposure, days

15 5% HCl

Weight Loss, per cent

Cement Concrete PIC

10

5

0

0

Fig. 14.25

14 Exposure, days

Durability of polymer-impregnated concrete

Special Concretes and Concreting Techniques

14.13

541

JET (ULTRA-RAPID HARDENING) CEMENT CONCRETE

The jet cement which has entered the market in the early 1970s, has many characteristics superior to those of ordinary Portland cement. Due to very short setting time, it develops super high initial strength making it suitable for use in a wide range of placing and curing temperatures. The development of strength under low temperatures is excellent. Jet cement is also called one hour cement as it is easily possible to obtain high early strength within an hour. It contains about 20 per cent of reactive calcium fluoroaluminate which is the source of high early strength. The setting time of mortars and concretes made with jet cement can be freely controlled by adding the required amount of retarder. Jet cement shows stable strength development extending over a long time and has high ultimate strength. In contrast to aluminous cement, there is no loss of strength with age. It has low drying shrinkage and low permeability. The jet cement is manufactured by mixing mainly specially selected anhydrite (II-CaSO4) and cement clinker powder integround with sodium sulfate and calcium carbonate (about one per cent), and boric acid (about 0.2 per cent). The jet cement clinker is usually made from a ground homogenized mixture of limestone, clay, bauxite and fluorite by burning at a fairly low temperature of 1250−1350 °C in order to prevent the formation of a tricalcium aluminate phase. Clinker and anhydrite are ground to fineness 400−450 m2/kg and 600−800 m2/kg, respectively. The specific gravity of jet cement is about 3.03−3.05 and specific surface area is about 500−550 m2/kg. Thus the specific gravity of this cement is lower than that of ordinary Portland cement and the specific surface area is considerably higher. The setting time of jet cement is extremely short, the final setting time being from 10 to 15 minutes. The initial setting time can be prolonged in proportion to the amount of retarder added. The anhydrite is usually manufactured by burning by-product gypsum and desulfurization waste from power plants. The one day compressive and flexural strengths of jet cement mortar with cement:sand ratio of 1:2 and water−cement ratio of 0.65 are approximately equal to seven-day and three-day strengths of ordinary Portland cement mortar having the same mix proportions. With the use of jet cement, improved workability of freshly mixed concrete is obtained due to enhanced cohesiveness and resistance to segregation. However, it is necessary to increase the water content by 1.25 to 1.75 per cent in order to increase the concrete slump by approximately 10 mm. The Vee-Bee time of jet cement concrete is higher than that of ordinary Portland cement concrete of same water content. There is an optimum fine aggregate percentage for each type of cement, at which the Vee-Bee time reaches a minimum value. The jet cement generally reduces the optimum value by four to five per cent because of higher fineness of the cement. The setting time of concrete can be regulated by controlling the amount of retarder added. It is necessary to adopt an optimum amount of retarder based on the temperature and working conditions in order to retain sufficient handling time for the fresh concrete. At the job site having high temperature, site mixing of concrete materials is preferable to the use of ready-mixed concrete. The bleeding of fresh

542

Concrete Technology

concrete made with jet cement is insignificant in mixes having slump values lower than 150 mm (used for normal concrete work). Consequently, concrete surface must be finished as soon as possible after placing the concrete. The concrete made with jet cement shows good strength development at low temperature, and hence is suitable for winter concreting. The rate of strength development of jet cement is quite different from that of ordinary Portland cement. The moist curing of concrete at early ages is important, since the concrete cured in dry state immediately after stripping yields a lower strength development. The 28day strength of jet cement concrete is about 20 per cent higher than that of ordinary Portland cement concrete at the same water-cement ratio, and a curing temperature of 20 °C. The ratio of tensile strength to compressive strength varies from 1/10 to 1/14, and the value is almost the same as that of regular concrete using ordinary Portland cement. Bond strength between reinforcing bars and concrete using jet cement is considerably higher than that of ordinary Portland cement concrete. The adhesive strength of concrete construction joints of jet cement concrete is 1.5 to 1.8 times higher than that of regular concrete when the concrete surface is treated carefully. The jet cement concrete yields high modulus of elasticity at early ages. The relationship between the dynamic modulus of elasticity, Ed and compressive strength of concrete fck (MPa) is given by the following equations. Jet cement concrete Ordinary Portland cement

: :

Ed = 8920 fck 0.376 MPa Ed = 11980fck0.320 MPa

When the concrete strengths are same the modulus of elasticity of concrete using jet cement is slightly lower than that of regular concrete because of the lower specific gravity of cement. The jet cement concrete gives lower values of drying shrinkage than that of concrete made with ordinary Portland cement. However, the creep values are higher at early ages and lesser after two months. The watertightness at early ages is considerably higher than that of ordinary Portland cement concrete. This can further be improved by extending curing period and by increasing the cement content. The rise in the temperature of concrete, caused by hydration of cement, is considerable.

14.13.1 Application of Jet Cement Concrete Jet cement has been found to be most suitable for urgent repair work, and winter concreting. The cost of this cement is about five times that of ordinary Portland cement. 1. Building construction The jet cement concrete can be used for the purpose of urgent building construction at low temperature. The surface should be finished immediately after placing, and concrete slab may be cured under canvas sheets. The column and wall forms may be removed after one day, and slab forms after two days. Since the handling time is short and the slump loss tends to be higher, quick handling is required in construction work. 2. Concrete pavements The jet cement concrete may enable the road to be used within hours after placing with little or no curing.

Special Concretes and Concreting Techniques

543

3. Repair work The cracks in reinforced concrete piers, and damage in expansion joints in railway or highway bridges may be repaired with jet cement concrete during the period when no train or traffic passes over the bridge. The jet concrete has been used satisfactorily in renewing the concrete pavings on an earth sub-base, repair of machine bases and concrete sleepers in Japan. 4. Winter concreting The jet cement has found major applications in winter concreting. At very low temperatures concrete may be cured with heaters to obtain the required strength. 5. Concrete products To increase production efficiency by allowing early removal of form or stripping and early transportation, the jet cement can be used for the manufacture of concrete blocks, precast concrete panels, concrete curtain walls, reinforced concrete pipes, etc. 6. Grouting Grouting mix consisting of jet cement, water, sand and an admixture may yield a strength of 1.5 to 2.5 MPa at one hour. The grouting may be used in stiffening the construction, consolidation of earth, etc.

14.14

GAP-GRADED CONCRETE

This type of concrete is obtained when a gap graded aggregate is used in the production of concrete. In case of gap grading certain undesirable sizes of aggregates are omitted from the conventional continuous gradings. The undesirable sizes are those which prevent the efficient packing of the other sizes. Sometimes available single-sized aggregate only is used. The gap-grading is normally aimed at achieving strength from the efficient packing of the aggregate. A well-packed aggregate will require minimum cement paste to fill the minor voids. For discussion consider the coarse aggregate to be mathematically modeled as spheres of diameter D called major spheres. A multitude of these spheres will have a rhombohedral form of packing. The voids between the major spheres can be fitted with spheres of diameter 0.414 D, known as major occupational spheres. The fine aggregate would then mathematically consist of minor occupational spheres of diameter 0.225 D which would fit into the remaining voids. The remaining minor voids can now be fitted by admittance spheres of diameter 0.155 D, and these could also be provided by the fine aggregate. Cement paste would then occupy the remaining voids and a mathematically perfect compact mix would result. Such a mix, however, cannot be cast in practice and consequently only the major, and admittance spheres are considered to be of practical value in a mix design. Mixes, therefore, are often designed with single-sized aggregate and a sand, all the particles of which can pass through the voids in the compacted coarse aggregate. However, the particles of sand must not be smaller than necessary to restrict the surface area to be coated with cement paste. Irrespective of the calculation suggested above, the sand content should be sufficient to distribute itself uniformly throughout the mix under practical conditions. The workability can be increased be reducing the surface area of all ingredients in a unit volume. This can be achieved by using largest size aggregate consistent with other constraints.

544

Concrete Technology

Gap grading enables leaner and drier mixes than conventional concrete of equivalent strength to be used resulting in lesser shrinkage. However, a leaner mix makes the vibration almost essential. Compressive forces on gap-graded concrete are transmitted from particle to particle of the coarse aggregate and not through cementsand matrix. Consequently the creep associated with such concrete is low. Due to the use of single-sized aggregate the segregation tendency is checked. A number of investigators have recommended the use of two single-sized coarse aggregates with sand and cement in a gap-graded mix. Because of efficient packing of aggregates in gap-graded concrete, vertical shuttering can often be removed shortly after casting. However, the gap grading is very sensitive to undesirable particles and the mix obtained will be of reduced efficiency.

14.15

NO-FINES CONCRETE

As the name suggests, this concrete does not contain fine aggregate. The coarse aggregate particles have been found to possess a cement paste coating of up to 1.3 mm around them. Hence no-fines concrete contains a multitude of voids which is responsible for its low strength. However, large voids give good thermal insulation, and these voids being large enough prevent the movement of water through the concrete by capillary action. The compressive strength of no-fines concrete is considerably lower than that of conventional concrete and depends on the cement content and grading of aggregate. The strength generally varies from 1.5 MPa to 15 MPa. In lean mixes, cement content may be as little as 70 to 130 kg per cubic meter of concrete, this is due to the absence of large surface area of fine aggregate particles which would have otherwise to be coated with cement paste. Thus the cost of no-fines concrete is lower than that of conventional concrete. It does not segregate, hence can be dropped from a considerable height. However, it should be vibrated for a very short period otherwise cement paste would run off. The water-cement ratio does not seem to be the controlling factor in this case. It varies from 0.38 to 0.52. The density of no-fines concrete depends on grading of aggregate, and with normal aggregate it varies from 1600 to 2000 kg/m3. Shrinkage is generally lower than in the ordinary concrete. Normally no-fines concrete is not suitable for reinforced concrete work. However, due to good thermal insulation, no-fines concrete walls have been used in cold countries for housing. It has been found that rain beating on a wall penetrates only a short horizontal distance before falling down to the bottom of the wall, there being no capillary paths to conduct the water completely through it. It is, however, often desirable to paint exposed no-fines concrete walls. High absorption of water makes no-fines concrete unsuitable for use in foundation and in situations where it may be in contact with water.

14.16

HIGH DENSITY CONCRETE

Concrete having unit weight of 30 kN/m3 to 64 kN/m3 is called high density or heavy weight concrete. Thus the unit weight of high density concrete is more than about 25 per cent higher than that of conventional concrete which is in the range of

Special Concretes and Concreting Techniques

545

24 kN/m3. High density concrete can be produced by using different types of heavy weight aggregates. High density concrete is used for construction of nuclear radiation shield walls, ballast blocks, counterweights, sea walls and other applications where high density is important. As a shielding material, high-density concrete protects the users against the biological hazards of penetrating radiaion from nuclear reactors, production facility of radioactive materials, particle accelerator, industrial radiography, and X-ray and gamma-ray therapy. The shielding against biological hazards of radiation mainly involves protection against X- and gamma rays, and neutrons. For the shielding to be effective radiations must be attenuated sufficiently so that they do not damage the body cells of the user exposed to it. In addition to the biological hazards, nuclear reaction also generates very high temperature (resulting in cracks on outer face of concrete) necessitating shielding to protect the electronic and other sensitive equipment in the vicinity. Selection of concrete for radiation shielding is based on space requirements, and on the type and intensity of radiation. Where there are no space restrictions, normal-density high-performance concrete will generally provide the most economical shield; where space is limited, high-density concrete will allow for reductions in shield thickness without sacrificing shielding effectiveness.

14.16.1

High-Density Aggregates

As discussed earlier in Section 3.2.4, high-density aggregates such as baryte, ferrophosphorus, goethite, hematite, limonite, magnetite, and de-greased scrap steel and steel shot having specific gravities ranging from 3.4 to 7.8 are used to produce highdensity concrete with a unit weight of about 30 to 60 kN/m3.

14.16.2

Properties of High-Density Concrete

As in the case of normal-weight concrete, the properties of high-density concrete in both the freshly mixed and hardened states can be tailored to meet the application requirements by proper selection of materials and mixture proportions. Except for density, the physical properties of high-density concrete are similar to those of normal weight concrete. As usual, strength is a function of the water-cementing materials ratio; thus, for any particular set of materials, strengths comparable to those of normal weight concretes can be achieved. As in the case of conventional concrete, high modulus of elasticity, low thermal expansion and low elastic and creep deformations are the desirable properties high weight concrete. High-density concrete may contain higher cement content; in that case it may exhibit increased creep and shrinkage. When only smooth cubical pieces of steel or iron are used as coarse aggregate, the compressive strength may not exceed about 21 MPa, regardless of the grout mixture or water-to-cement ratio. If the pieces of sheared reinforcing bars are used as aggregate, with good grout, normal strength may be produced. The grout used in high-density preplaced aggregate concrete should be somewhat richer than that used in normal-density preplaced concrete. Typical densities of concretes made with some commonly used high-density aggregates are given in Table 14.12.

546

Concrete Technology

Table 14.12

Densities of typical high-density aggregates and concrete (Adopted from PCA)

Type of aggregate

Specific gravity

Bulk density, (kg/m3)

Concrete density, (kg/m3)

Goethite

3.4−3.7

2080−2240

2880−3200

Limonite*

3.4−4.0

2080−2400

2880−3360

Barite

4.0−4.6

2320−2560

3360−3680

Hematite

4.9−5.3

2880−3200

3850−4170

Magnetite

4.2−5.2

2400−3040

3360−4170

Ferro-phosphorus

5.8−6.8

3200−4160

4080−5290

Scrap steel

6.2−7.8

3860−4650

4650−6090

*Water retained or chemically bound in aggregates per cent (by mass): Goethite (10−11), Limonite (8−9), Ferro-phosphorus (0) and Steel scrap (0). The aggregates may be combined with limonite to produce fixed-water contents varying from about 0.5 to 5 per cent.

14.16.3

Proportioning, Mixing, and Placing

The procedures for selecting mix proportions for high-density concrete are the same as those for normal-density concrete. The cement-aggregate ratio generally varies from 1:5 to 1:9 with a water-to-cement ratio from 0.5 to 0.65. They produce dense and crack-free concrete. The following are the most common methods of mixing and placing high-density concrete: Conventional concreting practice with respect to mixing, transporting, placing as adopted for normal concrete may also be adopted to heavy-weight concrete but care must be taken to avoid overloading the mixer, especially with very high-density aggregates such as scrap steel and shots. Batch sizes should be reduced to about 50 per cent of the rated mixer capacity. Because some high-density aggregates are quite friable, excessive mixing should be avoided to prevent aggregate breakup with resultant detrimental effects on workability and bleeding. To prevent segregation of heavier aggregates from the rest of the ingredients, a higher cement content may be required; better workability may help reducing segregation. Wear and tear of the mixer drum may be high. The formwork is required to be made stronger to withstand higher load. Preplaced aggregate methods of concreting can be used for placing high-density concrete in confined areas and around embedded items; this will minimize segregation of heavy-density coarse aggregate, especially scrap steel. The method also reduces drying shrinkage and produces concrete of uniform density and composition. With this method, the coarse aggregates are preplaced in the forms and grout made of cementing material and sand, and water is then pumped through pipes to fill the voids aggregate. Pumping of high-density concrete through pipelines may be advantageous in locations where space is limited, but high-density concretes cannot be pumped as far

Special Concretes and Concreting Techniques

547

as normal-density concretes because of their higher densities. Puddling is a method whereby a 50-mm or more layer of mortar is placed in the forms and then covered with a layer of coarse aggregate that is rodded or internally vibrated into the mortar. Care must be taken to ensure uniform-distribution of aggregate throughout the concrete.

14.17

NUCLEAR CONCRETE

Due to its excellent characteristics for neutron and gamma-ray attenuation, the ease of construction and a relatively low initial as well as maintenance costs, make concrete a most suitable material for radiation shielding. The concrete primarily used for radiation shielding may be called nuclear concrete. To design nuclear concrete for effective radiation shielding, it is desirable to understand the types of radiation and the resulting hazards. The general types of radiation considered in the design of biological shields are electromagnetic waves and nuclear particles. In the electromagneticwaves category, the high-energy, high-frequency waves known, as X- and gammarays are the only ones that require shielding for the users. These waves are similar to light rays but have higher energy with greater penetrating power. Although both X-rays and gamma-rays are highly penetrating, they can be adequately absorbed by an appropriate thickness of specially constructed nuclear concrete shield. Nuclear particles, on the other hand, include neutrons, protons, alpha and beta particles of the nuclei of atoms. Of all these, the neutrons are uncharged and continue unaffected by electrical fields, until they collide with a nucleus. On the other hand, protons, and alpha and beta particles carry electrical charges which interact with the electrical field surrounding the atom of the shielding material, and they lose their energy considerably. Though the accelerated protons at high energy levels are most penetrating, their energy is eventually degraded or is lost in the process that creates additional particles, and thus they do not constitute a separate shielding problem. The type and intensity of radiation usually determine the requirements for density and water content of shielding concrete. The effectiveness of a concrete shield against gamma rays is approximately proportional to the density of the concrete, i.e., the higher the density, the more effective the shield in absorbing neutrons by inelastic collisions or scattering. On the other hand, an effective shield against neutron radiation requires both high and low atomic weight elements. The hydrogen in water provides an effective light atomic weight material in concrete shields to slow down fast neutrons. Some aggregates contain crystallized water, called fixed water, as a part of their structure. For this reason, high-density aggregates with high fixed-water contents often are used if both gamma rays and neutron radiation are to be attenuated. This can be accomplished by the use of hydrous ores. These materials contain a high percentage of water of hydration. On heating the concrete, some of this fixed water in the aggregate may be lost. Lemonite and goethite are reliable sources of hydrogen as long as the shield temperature does not exceed 200°C. Serpentine aggregates may be used, because of their ability to retain water of crystallization at an elevated temperature of up to about 400°C. This assures a source of hydrogen, which is not necessarily available in all heavy weight aggregates. Boron glass (boron frit) is also added to neutrons.

548

Concrete Technology

14.17.1

Additions

At times materials such as colemanite, boron glass (boron frits), and borocalcite are added to improve the neutron attenuation properties of concrete. However, they may adversely affect the setting and early strength of concrete; therefore, trial mixes should be made under field conditions to determine the suitability of the addition. Admixtures such as pressure-hydrated lime can be used with coarse sand sizes to minimize any retarding effect.

14.17.2

Radiation Shielding

Radiation shielding walls are constructed to prevent radiation in the user areas. As discussed above the major contribution to the environmental radiation is due to neutrons. However, neutrons can be stopped by inelastic collision or scattering and absorption in thick materials like high-density concrete composed of cement, water and typically iron ore substituting the sand and gravel. The non-magnetic coarse and fine haematite (fe2O3) aggregate with an iron content of more than 60 per cent and water (which is stabilized in to the mix as the cement is hydrated) are most effective in nuclear shielding. The iron nuclei lowers the neutron energy spectrum by inelastic scattering at energies above 1 m eV, while the hydrogen nuclei (from water) further degrades the energy and ultimately absorbs neutron with wall thickness that increases inversely with square root of energy below several eV. The nuclear shielding effect of the high-density concrete can be improved by using artificially enriched iron oxide pellets manufactured from haematite ore (iron content 30−35 per cent) for the steelmaking industry as aggregate. The high strength and somewhat lower density (due to increased porosity with connected pores of typical size from 1−10 microns) will help improve the nuclear shielding of high-density concrete. The cured normal concrete contains about five per cent water, whereas enriched iron oxide pellets as aggregate can hold much more water which helps in neutron attenuation. The nuclear shielding of concrete mixture can further be improved by impregnating it with a good neutron absorber.

14.18

HEAT RESISTING AND REFRACTORY CONCRETES

One of the advantages of concrete is that it is non-combustibl, i.e., it neither burns nor supports combustion, so it can be used wherever non-combustible construction is permitted. Any concrete which does not disintegrate when exposed to constant or cyclic heating at the temperature below which a ceramic bond is formed is called heat resistant concrete. As Portland cement is not suitable for this application, the heat resistant concretes are normally composed of hydraulic cement (calcium aluminate cement) as a binding agent combined with heat resistant, refractory aggregates and or fillers. There is a more or less continuous spectrum of high temperature resistant concretes, extending from 300−400 oC (the limit of concretes bound with Portland cements) to 2000 oC or more, using high range calcium aluminate cements (CAC) containing 80 per cent alumina. Somewhat arbitrarily, the boundary between

Special Concretes and Concreting Techniques

549

heat-resistant and refractory concretes is taken as 1000 oC, although some definitions start refractory concretes from 1500 oC.

14.18.1

Fire Rating

As discussed in Section 8.11, a fire rating, or more correctly fire resistance rating as used in building codes, refers to the ability of concrete to withstand fire or to provide protection from fire. As defined in the 2000 edition of the International Building Code (IBC-2000), fire resistance rating means the period of time (in hours) a building or building component retains the ability to confine a fire or continues to perform a given structural function or both, as determined by prescribed tests.

14.18.2

Materials for Heat Resistant Concrete

Binding Material The behavior of Portland cement concretes subjected to high temperatures is complex. If the concrete is dry or the heat is applied slowly, relatively little permanent damage is done with concrete temperatures up to 200 to 250oC. At concrete temperatures of about 500oC, hydrated lime Ca(OH)2 which forms a significant portion of the hydrated Portland cement loses water to form quicklime (CaO): Ca(OH)2 ⇔ CaO + H2O This reaction is reversible. At concrete temperatures of about 540oC compressive strength loss can be 55 to 80 per cent of the original strength. At the time of heating, the degree of saturation of the concrete influences the severity of strength loss; and repetitions of heating and cooling cycles further degrade the concrete. The moisture present in the atmosphere leads to rehydration of quicklime which is an expansive reaction resulting in disruption of concrete. Thus OPC is not suitable for the application which involves cyclic heating to high temperatures and then cooling to ambient temperatures. Furthermore, near the service temperature of concrete, the silica and lime present in the Portland cement undergo a chemical change to form a low melting point compound. Thus ordinary Portland cement has limited use at high temperatures. On the other hand, calcium aluminate cement (CAC) hydrates do not contain hydrated lime and thus are not subjected to disruption caused by rehydration of quicklime. The progressive dehydration of CAC hydrates with increasing temperatures above 300 oC forms stable compounds. These compounds at still higher temperatures (>1000 oC) react with refractory aggregates to form new stable phases. The higher the alumina contents in CAC, the more refractory the concrete. The refractoriness can be further extended by adding free alumina to 70 per cent A12O3. CAC to increase the alumina contents to 80 per cent. This is generally the upper limit of alumina contents in modern CACs. Generally, grey CAC with 39 per cent alumina will have sufficient temperature resistance for most heat-resisting applications up to 1000 oC. In refractory concretes, the higher refractoriness of the aggregate will extend the temperature range of the CAC.

550

Concrete Technology

Aggregates for Heat Resistant Concrete Fire resistance of concrete is influenced by aggregate type, moisture content, density, permeability and thickness. Dolomite and limestone aggregates called carbonate aggregates which consist of calcium or magnesium carbonate or combinations of the two, calcine during exposure to high temperatures, i.e., carbon dioxide is driven off and calcium (or magnesium) oxide remains intact. Since calcining requires heat, the reaction absorbs some of the heat generated by the fire. The reaction begins at the fire-exposed surface and slowly progresses toward the opposite face. Thus, carbonate aggregates behave somewhat better than other normal-weight aggregates in a fire. Moisture content has a complex influence on behavior of concrete in fire. Concrete that has not been allowed to dry may spall, particularly if the concrete is impermeable, such as concretes made with silica fume or latex, or if it has an extremely low water-cement ratio. Concretes that are more permeable will generally perform satisfactorily, particularly if they are partially dry. In general, dried lightweight concrete performs better in fire than normal-weight concrete. The thicker or massive the concrete, the better will be its behavior when exposed to fire. lightweight concretes and carbonate aggregates are suitable for heat resisting and refractory concretes. Thus, reduction in lime content and increase alumina content in cement are the keys to high performance of concrete at high temperatures.

Thermal insulation The thermal insulating properties of the concretes are primarily associated with their density which is mainly controlled by the density of aggregate. The density of lightweight aggregates varies from extremely light, e.g., Perlite: bulk density 1.0 to 1.1 kN/m3 to moderately light, e.g., sintered PFA or expanded clay: bulk density 6 to 8 kN/m3. The insulating properties (thermal conductivity) of concretes made with these aggregates will be in the range 0.15−0.5 W/m°K as shown in Fig. 14.26.

Thermal conductivity at 540°C, W/m °K

0.6 Expanded clayy

0.5

HT Insulating brick 0.4 Pumice 0.3 Diatomite 0.2

Perlite Vermiculite V

0.1 0

4

6

8

10

12

14

16

Fired density, kN/m3

Fig. 14.26

Relationship between fired density and thermal conductivity of heat resisting concretes at about 540oC

Special Concretes and Concreting Techniques

551

Concrete Mixes for Heat Resistant Concrete Refractory concrete mixtures consist of suitably graded aggregates and hydraulic cements in proportions formulated to achieve certain desired properties for the particular end use. As explained earlier in Section 14.8.6, among the aggregates used in increasing order of service temperatures are slag, limestone, expanded-shale, calcined fireclay, perlite, vermiculite, etc. Aluminum powder is quite often used in refractory concrete either to minimize explosive spalling during castable de-watering or to inhibit the oxidation of coke/ graphite at high temperatures in carbon-containing materials. In the first case, the aluminum powder is expected to increase the permeability of castables by generating H2 gas during reaction with H2O and forming open porosity within the microstructure. In the latter application, on the other hand, it is desirable that a minimum amount of aluminum reacts with H2O during castable processing, so that most of the metal remains in the microstructure to prevent carbon oxidation. Ingredients for typical example mix River gravel or crushed fire bricks, sand, calcium aluminate cement, and water. A small amount of standard fireclay can be added. For normal heat resistant concretes, half the cement may be replaced with the hydraulic lime and fireclay can also be added. The cement holds the mixture together when it is drying but when the heat gets into the cement and burns it out, the lime holds it all together. A typical mixture (by volume) for heat resistant concrete is 1: 1: 1.5 + 0.25 lime + water

14.18.3

Placing and Compaction

The methods used are identical to those used for conventional concrete, thus no specialist equipment or skills are required. However, as explained in Section 14.8.6, refractory shotcreting or gunning is commonly undertaken in special circumstances.

14.18.4

Curing, Drying, and Firing

The curing is of utmost importance, and the methods of curing are similar to those for conventional concrete. However, due to rapid hardening and high heat evolution of CAC concretes, it is important to start curing three−four hours after placing and continued until at least for 24 hours to achieve complete hydration and to control drying shrinkage. The curing may be done by spraying the concrete with a fine spray of water and covering with plastic sheeting to prevent rapid loss of water on edges and the surface. After 24 hours, remove coverings and let the air dry the concrete without strong sun for 48 hours. The free water left in green or unfired concrete after curing must be allowed to escape at the start of heating to prevent spalling. Natural or forced drying at up to 100oC is generally used to drive off as much free water as possible before exposure to higher temperatures.

552

Concrete Technology

After drying, the concrete is heated gradually from 100oC to 350oC, to drive off the combined water or water of hydration from the concrete. The heating cycle varies with the application, thickness and type of the concrete product. However, for conventional castables, generally the temperature may be raised at the rate not exceeding 25oC per hour to 500oC with a hold of 12 hours at this temperature. After the hold the temperature continues to be raised at a slightly higher rate to the service temperature. For thick sections (>100 mm) a hold at different temperatures is advisable. Hold at a temperature is for the period until the heat balance through the material is established to obtain the ultimate ceramic bond. Heat-up is to be continuous and uninterrupted. All temperatures are to be measured at the surface face of the refractory. Cooling should not exceed a rate of 35oC per hour.

Properties Usually refractory concretes exhibit cracks after first firing. These cracks are due to dehydration shrinkage and ceramic reaction between the cement and aggregate at high temperatures. In normal service conditions, these cracks will close down when concrete is reheated to its service temperature due to thermal expansion.

14.18.5

Applications of Heat Resisting Concrete

In addition to structural applications, heat resistant concrete is commonly used in runway pavements. Concrete pavement exposed to high temperatures from an aircraft jet blast or from auxiliary power units can suffer damage. If the concrete is wet when the heat is suddenly applied, the production of steam within the concrete can cause spalling. Typical concrete pavement damage resulting from high temperatures of jet blast includes spalling, aggregate popouts, scaling, cracking, and loss of joint sealant. The time that the concrete is exposed to the jet engine or auxiliary power unit exhaust is critical. Since there is considerable thermal lag in concrete, properly designed pavements generally do not suffer heat damage from aircraft.

14.18.6

Applications of Refractory Concretes

Refractory concretes are subjected to high temperatures, thermal and mechanical stresses, chemical and abrasive attacks. They are normally designed for specific applications predominately in the metal industry, but are also used extensively in the chemical, cement and glass industries. Refractory concretes rely on a complicated mix of aggregate and binder, the most common binder being High Alumina Cement (HAC). Aggregates used vary depending on the intended application. Refractories come in two general types: preformed and monolithics. Preformed includes bricks and large-scale monoliths shown in Fig. 14.27. Monolithics are generally obtained as shotcrete or gun mixes and castables used as linings and coatings normally fired in situ. Typical heat resistant products are shown in Fig. 14.27. Refractory concrete sheets also have the potential for use as heat resistant wall claddings and decorations. Manufacture of thin sheets of refractory concrete may not suffer from any of the difficulties associated with fired clay thin sheets even when cast in large size with only 6 mm thickness. The fire clay thin sheets are:

Special Concretes and Concreting Techniques

Fig. 14.27

553

Different refractory concretes products

1. Liable to warp during drying 2. Brittle and delicate prior to firing 3. Liable to further warping during firing and often require a support structure

14.18.7

Advantages of Heat Resistant Concretes

Shrinkage The refractory concretes do not warp during drying and firing, as they will set with a chemical reaction, which is subsequently sintered to create ceramic bonds.

Green Strength Because refractory concrete has a similar strength to conventional concrete even before it has been fired, it can be maneuvered far more easily than large fragile clay pieces.

Fired Strength and Toughness Once fired, refractory concretes are substantially harder than conventional concrete and are generally tougher than ceramics due to the aggregate’s ability to arrest crack propagation.

Drying Time Once set, they require a short drying cycle to drive off any free water. In addition, the high alumina cement used as a binder in many refractory concretes has a far shorter setting time than conventional Portland cement.

554

Concrete Technology

Thermal Shock Refractory concretes are specifically engineered to cope with rapid and substantial changes in temperature during normal industrial application, therefore fast firing is needed.

14.18.8

Disadvantages

Reduced Workability The refractory concretes cannot be molded in the same way as plastic clay and therefore require molds.

Limited Glaze Compatibility The chemical composition of refractory concretes is different from clay bodies and therefore the interaction between glaze and refractory concretes is different.

REVIEW QUESTIONS 14.1 Enlist the approaches generally considered for improvement of mechanical properties of concrete. Describe the approach for achieving stronger aggregate-matrix interface. 14.2 Discuss the properties of structural lightweight concrete and its applications. 14.3 What is vacuum concrete, what properties does it improve over conventional concrete? 14.4 Describe briefly the roller compacted concrete. 14.5 What is Shotcrete? Explain the procedure of shotcreting a surface. What are its disadvantages? 14.6 Write short note on two of following: (a) Guniting and its applications, (b) Waste material based concrete, and (c) Mass concrete

14.7 What is fiber reinforced concrete and what are its advantages? 14.8 What is ferrocement? Describe briefly its properties and applications. 14.9 Explain the factors affecting properties of fiber reinforced concrete. 14.10 Enlist polymer concrete composites. Describe polymer concrete and its applications. 14.11 Write short notes on two of following: (a) Jet cement concrete, (b) Gap-graded concrete, (c) High density concrete and (d) Nuclear concrete. 14.12 What is the recycled concrete? Compare the behavior and properties of recycled-aggregate concrete with the one made with natural aggregates? Can concrete exhibiting ASR damage be used as recycled concrete aggregate?

MULTIPLE-CHOICE QUESTIONS 14.1 Special concretes are obtained by improving the properties of concrete by (a) modification in the microstructure of the cement paste (b) reduction in the overall porosity (c) improvements in the strength of aggregate-matrix interface (d) control of extent and propagation of cracks (e) All of the above

14.2 The microstructure of the concrete can be improved by (a) application of high pressure during molding (b) molding at a temperature up to 150 °C (c) application of high pressure during molding at high temperature (d) prolonged steam curing (e) None of the above

Special Concretes and Concreting Techniques 14.3 Sulfur-impregnated concrete is obtained by (a) emptying the pores in the conventional concrete under vacuum and sucking the liquid sulfur in the pores (b) mixing sulfur powder as an ingredient of normal concrete and heating the cured concrete at high temperature (c) applying a coating of molten sulfur on the surface of the concrete (d) Any of the above (e) None of the above 14.4 The extent and propagation of cracks in concrete can be controlled by (a) providing reinforcement bars (b) incorporating fibers in the concrete (c) polymer impregnation (d) All of the above (e) None of the above 14.5 Lightweight concrete is used (a) for reducing the dead weight of structures (b) for improving thermal insulation (c) in filler wall panels in multistorey buildings (d) for non-load bearing and partition walls (e) Any of the above 14.6 The lightweight concrete may be produced by (a) incorporating air in its composition (b) using lightweight aggregates (c) omitting the finer sizes from the aggregate grading that is using no fines concrete (d) formation of air voids in the cement slurry by the addition of substances causing sponge-like cellular forms (e) Any of the above 14.7 Aerated concrete is produced by addition of (a) copper sulfate (b) aluminum powder (c) sodium silicate (d) zinc sulfate (e) None of the above 14.8 Lightweight concrete has all the following beneficial characteristics except

14.9

14.10

14.11

14.12

555

(a) high strength-to-mass ratio (b) high thermal insulation (c) high sound insulation (d) excellent fire resistance (e) reduced drying shrinkage Lightweight aggregate are produced by (a) bloating clays with or without additives (b) sintering fly ash (c) using blast furnace slag (d) Any of the above (e) None of the above In mass concrete (a) a large size aggregate and low slump is adopted (b) mix being harsh and dry requires immersion-type power vibrators (c) heat of hydration may lead to a considerable rise of temperature (d) there is early high strength but lower later age strength (e) All of the above Vacuum concrete (a) is obtained by vacuum treatment of fresh concrete involving the removal of excess water and air by suction (b) is the normally cured hardened concrete involving removal of air from the voids of the concrete by suction (c) is no-fine-concrete where finer sizes are omitted from the aggregate grading producing uniformly distributed voids in the concrete mass (d) has a low wear and abrasion resistance (e) None of the above The ferrocement is a composite material obtained by (a) random dispersal of short, discontinuous fibers in the conventional concrete (b) reinforcing the cement mortar with steel fibers in the form of wire mesh (c) blending ferrous compounds in the ordinary Portland cement (d) Any of the above (e) None of the above

556

Concrete Technology

14.13 The cement−sand ratio in the ferrocement matrix should not be leaner than (a) 1:1.5 (b) 1:2.0 (c) 1:3.0 (d) 1:4.0 (e) 1:6.0 14.14 The volume of reinforcement in ferrocement (per cent) normally varies between (a) 1−2 (b) 2−5 (c) 5−8 (d) 8−10 (e) None of these 14.15 The aggregate, i.e., the sand recommended for ferrocement mixes is (a) with maximum sizes of 2.36 mm and 1.18 mm with optimum grading zones II and III, respectively (b) with maximum size 4.75 mm and grading zone I (c) with maximum size of 600-micron and grading zone IV (d) Any of the above (e) None of the above 14.16 The water-cement ratio for ferrocement mix should be (a) less than 0.35 (b) between 0.35 to 0.40 (c) between 0.40 and 0.50 (d) between 0.50 and 0.60 (e) greater than 0.60 14.17 For ferrocement structures exposed to corrosive environments (a) apply asphaltic and bituminous coatings on the exposed surface (b) apply rustproof paint on the wire mesh (c) apply vinyl and epoxy coatings on the wire mesh (d) Any of the above (e) None of the above 14.18 Identify false statement(s) (a) The cracking resistance, ductility, flexibility, impact and fatigue resistances of ferrocement are higher than those of concrete. (b) The impermeability of ferrocement products is far more superior than ordinary RCC product.

14.19

14.20

14.21

14.22

(c) Ferrocement has high resistance to cyclic loading. (d) Ferrocement is suitable for manufacturing the precast units. (e) None of the above Fibrous ferrocement (a) is obtained by adding short fibers to plain mortar matrix of ferrocement (b) has improved toughness and impact resistance over conventional ferrocement (c) panels can withstand very high stresses compared to those in conventional ferrocement (d) All of the above (e) None of the above Identify the true statement(s). (a) Fresh fiber concrete has reduced workability. (b) Fiber reinforced concrete is more cohesive and less prone to segregation. (c) High modulus fibers improve both flexural and impact resistance simultaneously. (d) Low modulus fiber improve only the impact resistance of the concrete. (e) All of the above Fiber reinforced concrete (a) is used for precast products, airport runways, blast and impact resistant structures, tunnel lining and hydraulic structures (b) has superior crack resistance, improved ductility, high impact resistance and toughness (c) uses indented, crimped or bent fibers for improved bond (d) is more vulnerable to corrosion damage than conventional steel reinforcement (e) All of the above Which of the following statement(s) are incorrect? (a) The ratio of diameter of fiber to its length is called aspect ratio. (b) The improvement in structural performance of fiber reinforced concrete depends on strength characteristics, volume, shape and aspect ratio of fibers.

Special Concretes and Concreting Techniques

14.23

14.24

14.25

14.26

14.27

(c) The ultimate strain of steel fiber reinforced concrete is 20 to 50 times that of plain concrete. (d) With four per cent of steel fibers, the flexural strength increases by 2.5 times the strength of unreinforced composite. (e) None of the above Identify the incorrect statement(s). (a) Asbestos fibers cause health hazards. (b) With vegetable fibers long-term durability is doubtful. (c) Organic fibers may decay. (d) Carbon fibers are very expensive and are more vulnerable to damage than glass fibers, hence are treated with resin coating. (e) Glass fibers are chemically stable in cement paste matrix. Which of the following fibers give highest improvement in the impact strength of fiber reinforced concrete? (a) Polypropylene, nylon and other organic fibers (b) Glass fibers (c) Carbon fibers (d) Asbestos fibers (e) Vegetable fibers The fundamental requirement of fiber reinforced concrete is (a) uniform distribution of fibers throughout the mix (b) mix should have sufficient paste to coat the fibers and aggregate (c) mix should have optimum content of fibers for workability (d) All of the above (e) None of the above The performance of hardened fiber reinforced concrete basically depends upon the (a) specific fiber surface (b) critical concentration of fibers (c) aspect ratio (d) All of these (e) None of the above The following range of parameters is associated with commonly encountered steel fiber reinforced concrete mixes except

(a) (b) (c) (d) (e)

14.28

14.29

14.30

14.31

14.32

557

water−cement ratios: 0.45 to 0.60 cement content: 300−500 kg/m3 fiber aspect ratio: 10 to 100 fiber content: 1.0 to 2.5 per cent fine-to-total aggregate ratio: 0.5 to 1.0 Polymer-impregnated concrete is obtained by (a) impregnating low viscosity prepolymers or monomers into the pore systems of hardened concrete and polymerizing it by heating (b) replacing the cement−water matrix in cement concrete by prepolymer and polymerizing it (c) incorporating a polymeric material into concrete during the mixing state (d) Any of the above (e) None of the above The partial or surface polymer impregnation of bridge deck concrete mainly improves (a) its structural properties (b) durability and chemical resistance (c) the riding of the surface (d) All of the above (e) None of the above Factors affecting the monomer loading are the (a) extent of moisture in concrete and air in the voids (b) degree of vacuum applied and its duration (c) viscosity of monomer and external pressure (d) All of the above (e) None of the above The polymerization by heating the catalyzed monomer to the required level can be done by (a) heating under water (b) low pressure steam injection (c) infra-red heaters (d) heating in an air-oven (e) Any of the above Identify the incorrect statement(s). (a) Thermoplastic monomers lose their effectiveness at high temperatures.

558

14.33

14.34

14.35

14.36

14.37

Concrete Technology (b) Thermosetting resins are more viscous and difficult to impregnate into concrete, but can withstand higher temperatures without softening. (c) The heating decomposes the polymer before polymerization by cross-linking. (d) All of the above (e) None of the above The repair of damaged concrete structures by polymer impregnation is (a) cost effective for normal structures (b) suitable for restoration and preservation of stone monuments (c) suitable for strengthening piles in sea water (d) All of the above (e) None of the above In polymer concrete (resin concrete) the polymerization can be achieved by (a) thermal−catalytic reaction (b) catalyst−promoter reaction (c) radiation (d) Any of these (e) None of the above Polymer concrete can be used (a) for overlays (b) rapid repair of damaged airfield pavements and industrial structures (c) treating sluiceway and stilling basin of the dam (d) for the manufacture of electrical transmission poles, etc. (e) All of the above Polymer modified concrete (a) is obtained by incorporating a polymeric material into concrete during the mixing stage (b) constitutes an interpenetrating matrix that binds the aggregate (c) is least expensive and the processing is simplest (d) All of the above (e) None of the above Shotcrete (a) is mortar or very fine concrete deposited by jetting it with high velocity on to the prepared surface

14.38

14.39

14.40

14.41

(b) is frequently more economical than conventional concrete (c) is very useful for restoration and repair of fiber damaged concrete structures (d) is used for stabilization of rock slopes, etc. (e) All of the above Shotcrete differs from conventional concrete with regard to (a) materials, proportions and void system (b) consolidation or compaction (c) application procedure (d) nature of failure (e) All of the above Identify the false statement(s). (a) If the water content is more than the concrete, then it tends to slump when jetted on to the vertical surface (b) If the water is deficient, the material which will rebound from the surface will be excessive (c) The water−cement ratio should be between 0.45 and 0.60 (d) In the wet mix process, all the ingredients are mixed before entering the chamber of delivery equipment (e) Dry mix process is preferred in case of lightweight concrete Shotcretes suffer due to (a) environmental hazards because of dust problem (b) necessity of cleaning and hauling of rebound material to the approved waste area (c) high cost of shotcrete and wastage due to rebound (d) spalling of shotcrete due to corrosion of reinforcement and peeling of sound shotcrete because of bond failure due to lack of surface preparation (e) All of the above Special shotcretes can be obtained (a) by adding up to two per cent of steel fibers (by volume) (b) by using calcium aluminate cement (hydraulic cement) as binding agent

Special Concretes and Concreting Techniques (c) by using air-entraining cements (d) Any of the above (e) None of the above 14.42 While repairing deteriorated concrete by shotcreting it is essential that (a) all unsound material be removed (b) at the perimeter of the cavity square shoulders should be provided (c) shotcrete is applied on the moist surface (d) the fresh layer of shotcrete is applied before the receiving layer takes it initial set (e) All of the above

14.43 Guniting (a) is the technique of depositing very thin layers of mortar in each pass of nozzle than that available with the shotcrete (b) mix is 1:3 to 1: 4.5 with a water−cement ratio of about 0.30 (c) requires careful and skilful handling of nozzle for high quality finish work (d) reduces permeability and enhances resistance to weathering and chemical attack (e) All of the above

Answers to MCQs 14.1 (e) 14.7 (b) 14.13 (c) 14.19 (d) 14.25 (d) 14.31 (e) 14.37 (e) 14.43 (e)

14.2 (c) 14.8 (c) 14.14 (c) 14.20 (e) 14.26 (a) 14.32 (c) 14.38 (e)

14.3 (a) 14.9 (d) 14.15 (a) 14.21 (e) 14.27 (c) 14.33 (b) 14.39 (c)

559

14.4 (b) 14.10 (e) 14.16 (b) 14.22 (a) 14.28 (a) 14.34 (d) 14.40 (e)

14.5 (e) 14.11 (a) 14.17 (a) 14.23 (e) 14.29 (b) 14.35 (e) 14.41 (d)

14.6 (e) 14.12 (b) 14.18 (c) 14.24 (b) 14.30 (d) 14.36 (d) 14.42 (a)

15 15.1

DETERIORATION OF CONCRETE AND ITS PREVENTION

INTRODUCTION

Though concrete is quite strong mechanically, it is highly susceptible to chemical attack, and thus concrete structures get damaged and even fail unless some measures are adopted to counteract deterioration of concrete and thereby increasing the durability of the concrete structure. The durability of concrete can be defined as its resistance to the deteriorating influences of both external and internal agencies. The external or environmental agencies causing the loss of durability include weathering, attack by natural or industrial liquids and gases, etc. Whereas the internal agencies responsible for the lowering of durability are harmful alkali-aggregate reactions, volume changes due to noncompatible thermal and mechanical properties of aggregates and cement paste, presence of sulfates and chlorides from the ingredients of concrete, etc. In the case of reinforced concrete the ingress of moisture or air may lead to corrosion of steel, and cracking and spalling of concrete cover. A durable concrete is dense, workable having as low a permeability as possible under the given situation. The recommendations for making durable concrete usually envisage limits for maximum water–cement ratio, thickness of cover, type of cement and the amounts of chlorides and sulfates in the concrete.

15.2

CORROSION OF CONCRETE

The gradual deterioration of concrete by chemically aggressive agents is called concrete corrosion. Basically corrosion of concrete is a physico-chemical process and the extent of deterioration caused to it by the aggressive agents is dependent upon the properties of the constituents of concrete and the corrosive media. Any factor which may help in the development of cracks in concrete will promote the penetration of the aggressive solution and gases, and will lead to the faster deterioration of concrete structure.

15.2.1

Cracking-Corrosion Interaction

The two main factors responsible for the loss of durability of concrete structures are 1. deterioration of concrete, and 2. corrosion of steel reinforcement. These two phenomena cannot be separated out because the deterioration of concrete cover over the steel reinforcement leads to the corrosion of steel. On the other hand, corrosion of steel reinforcement promotes destruction of concrete due to the

Deterioration of Concrete and Its Prevention

561

development of internal stresses on the formation of voluminous corrosion products of iron. Thus the durability of concrete is greatly dependent on the interaction of cracking and corrosion. Microcracks, which are always present at the aggregate–cement interface and at the reinforcement–cement interface, do not affect the durability of concrete as long as they are limited in number and size, and are discontinuous. But when these microcracks become continuous and enlarged under the influence of stresses or due to the leaching of cement paste, they facilitate the transport of aggressive ions and gases, thereby affecting the durability of concrete to a great extent.

15.2.2

Types of Deterioration

Deterioration of concrete is caused not only by acids in the form of water solutions or acidic gases which form acids on dissolving in water, but by salt solutions and even by alkalies. A large number of other substances, such as fertilizers, insecticides and certain organic compounds are harmful to concrete. Deterioration of concrete due to corrosion caused by the various aggressive chemicals can be classified into three categories as shown in Fig. 15.1. This classification will help in developing the methods to increase durability of concrete and protection to reinforcement. The principal forms of destruction due to corrosion are as follows. 1. Decomposition of concrete In this form the decomposition of concrete is caused by action of liquids (aqueous solutions) which are able to dissolve the ingredients of hardened cement. Water percolating through the mass of concrete can greatly speed up decomposition by increasing the ionic strength of solution. A common example of this type of destruction is the leaching action as shown in Fig.15.2(a). 2. Chemical reaction In this form of destruction a chemical interaction between hardened cement constituents and a solution takes place. The easily soluble reaction products are removed from the internal structure of concrete by diffusion or percolation. This happens when concrete is attacked by a solution of acids and certain salts. 3. Crystallization This form of destruction involves accumulation, crystallization, and polymerization of reaction products which increase the volume of solid phase within the pore structure of the concrete. In addition to the principal forms of destruction cited above there are other specific influences, such as destruction due to the interaction between the hardened cement and the aggregate; attack by biological agents, etc. The above principal processes are explained in following sections.

Alkali–Silica Reaction Alkali–silica reaction (ASR) takes place when free lime (alkali) in concrete interacts with reactive silica present in many types of aggregates. The reaction forms a gel that absorbs moisture, and expands and creates tensile stresses that can crack concrete as shown in Fig. 15.2(b). Some chemical compounds can prevent ASR by consuming the free lime essential for ASR to occur and by reducing penetration of the moisture necessary for ASR.

aggressive environment

Exposure to

Concrete

T Type C

Fig. 15.1

Increase in internal stress

Removal of Ca a++ ions as slouble product

Removal of Ca a++ ions as non-expansive insoluble product

Deterioration of concrete due to aggressive agencies

Reactions involving formations of expansive products

Reactions involving exchange T Type B of ions between aggressive fluid and components of hardened cement

T Type A

Reactions involving hydrolysis and leaching of the components of hardened cement paste

Increase in porosity and permeability

Deformation

Cracking and spalling

Loss of strength and rigidity

Increase in deterioration processes

Loss of mass

Loss of alkalinity

562 Concrete Technology

Deterioration of Concrete and Its Prevention

(a) Deterioration by leaching

Fig. 15.2

563

(b) Deterioration by alkali-aggregate reaction

Deterioration by alkali-aggregate reaction and leaching

Leaching This type of deterioration may be caused by the dissolution of the ingredients of hardened cement by the aqueous solution, i.e., by the leaching process. Since calcium hydroxide is a readily soluble ingredient of hardened cement, the destruction of concrete by the leaching action is also called lime leaching. It is greatly dependent upon the permeability of the concrete. When the free lime of concrete is leached out, hydrolysis of calcium silicates and aluminates takes place to release more lime for further leaching action. Out of the silicate hydrates, dicalcium hydrosilicate (2CaO·SiO2·aq.) which is most unstable in the absence of a saturated solution of calcium hydroxide dissociates at a faster rate to liberate more of lime. Whereas, among the aluminate hydrates, tetra calcium aluminate hydrate (4CaO. Al2O3.H2O) is least stable in the absence of calcium hydroxide. Therefore, when the concentration of lime inside the concrete is reduced on account of leaching action, more of it will dissociate to produce additional amount of lime. The presence of salts in a solution has a marked bearing in the solubility of calcium hydroxide, e.g., similar ions such as Ca 2+, OH– tend to reduce it, while others, such as SO42 –, Cl–, Na+, K+, etc. produce the opposite effect. Table 15.1 depicts the variation in the solubility of lime with the nature of salts and their concentration in a solution. Increased solubility of lime accelerates the destruction of concrete and after a 10 per cent loss in lime in terms of initial cement, concrete starts rapidly losing its strength. Table 15.1 Chemicals

Effect of various salts on the calcium hydroxide solubility Concentration, per cent

Solubility, gm/litre

–

1.18

Na2SO4

1.0

2.14

Na2SO4

2.0

3.00

NaOH

0.5

0.18

Distilled water

564

Concrete Technology

When the leached out lime reacts with atmospheric carbon dioxide gas, the concrete surface gets covered with white residue of calcium carbonate. This is called the white death of concrete due to the leaching action.

Chemical Interaction Deterioration may be caused by the chemical reaction between the hardened cement constituents of concrete and the chemicals of a solution. The reaction products formed may be either water soluble and may get removed from the internal structure of concrete by a diffusion process, or the reaction products if insoluble in water may get deposited on the surface of concrete as an amorphous mass having no binding properties, with the result that it can be easily washed out from the concrete surface. Acids first react with free lime of concrete forming calcium salts and later on attack the hydrosilicates and hydroaluminates forming the corresponding calcium salts, whose solubility will govern the extent of deterioration caused to the concrete. As can be observed from Fig. 15.3, the hydrochloric acid corrodes the concrete to a greater extent in comparison to the sulfuric acid at low concentrations because H2SO4 forms a less soluble CaSO4 on reacting with lime of concrete, which seals the pores of concrete for further permeation and offers resistance to acid corrosion. But at higher concentrations of H2SO4, the concrete strength is reduced due to the accumulation of CaSO4 in the pores and the development of internal stresses. 4

T Tensile strength, MPa

O4

H 2S

l

HC

3

2

1.5 0.01

Fig. 15.3

0.05 0.1 0.5 1 Concentration, per cent

5

Deterioration of concrete due to acid attack

Crystallization Concrete may get deteriorated by the accumulation or crystallization of salts in its pores, which leads to the development of internal stresses and formation of cracks. These salts in the pores of concrete may be either formed

Deterioration of Concrete and Its Prevention

565

as a result of chemical reaction between the corrosive media and the constituents of hardened cement or may be brought from outside by the penetration of salt solution and released there on the evaporation of water. Alkaline solutions of low concentration are less harmful to concrete; however, the concrete gets deteriorated on exposure to concentrated solutions of alkalies, as they combine with atmospheric carbon dioxide producing crystallizable carbonates. Alternate wetting and drying of structural members with salt solutions increase the deleterious effect of salt due to the phenomena of accumulation or crystallization of salts. The same holds good for partially immersed structural members. Magnesium ions are particularly damaging in combination with sulfate ions, since magnesium corrosion is enhanced by the crystallization of gypsum which increases the permeability of concrete. This effect is revealed from the data of Table 15.2. In the case of MgCl2, the resulting colloidal products may seal the concrete before the hydrated cement components are decomposed and thus prevent the inward diffusion of Mg ions from the outside. Table 15.2

Effect of MgSO4 concentration on the strength of concrete

Concentration of MgSO4 solution, per cent

Compressive strength as percentage of original value

0.0

100

0.25

80

0.50

66

1.0

62

2.0

60

3.0

56

4.0

54

15.2.3 Prevention of Concrete Deterioration A durable structural concrete requires the satisfaction of two criteria, namely that of a suitable binding agent of adequate chemical resistance and that of thorough compaction to a high density. Thus, the making of a denser concrete having the least porosity is a most effective means of reducing the deterioration of concrete. A quantitative information regarding the effects of the range of parameters like water-cement ratio, cement content, curing conditions together with effects of cement admixtures and replacements on the corrosion of concrete helps determine the durability of concrete empirically. The effects of these parameters are described below. An increase in the water-cement ratio above 0.45 or 0.50 is found to increase the permeability of cement paste exponentially. Thus from the considerations of permeability the water-cement ratio is usually limited to 0.45 to 0.55 except in mild environment. For a given water-cement ratio, the cement content is governed by the required workability. In addition, the cement content should be such that to ensure sufficient alkalinity (pH value of concrete) to provide passive environment against corrosion of reinforcement.

566

Concrete Technology

In the concrete for marine environment or in sea water applications, a minimum cement content of 350 kg/m3 or more is required. Moreover, the water-cement ratio and cement content must provide enough paste to overfill the voids in compacted aggregate. The void content of aggregates depends upon the type and nominal maximum size of aggregate used, e.g., crushed rock and rounded river gravels of 20 mm nominal size have approximately 27 and 22 per cent of aggregate voids. A cement content of 400 kg/m3 and water–cement ratio of 0.45 will produce paste volume of 30 per cent which is sufficient to overfill the voids of crushed rock. On the other hand, a cement content of 300 kg/m3 and water–cement ratio of 0.50 will result in 25 per cent paste volume which may be suitable for rounded gravels aggregate. A further increase in cement content will result in higher workability. The concrete in sea water or exposed directly should be at least of M20 grade in case of plain concrete and M25 in case of reinforced concrete. The use of Portland slag or pozzolana cement is advantageous under such conditions. The ordinary Portland cement having C3A content less than five per cent has got the maximum resistance against sulfatic environment. The supersulfated cement is supposed to provide an acceptable durability against acidic environment, when concrete is dense with a water–cement ratio of 0.40 or less. As the addition of hydraulic additives reduces the rate of leaching considerably, their addition will also be helpful in the prevention of deterioration of concrete. Since the carbonated layer on the surface of concrete increases the resistance of concrete to deterioration by leaching, it is possible to attain a marked improvement in the quality of concrete by encouraging natural or artificial carbonation of surface layer. Deterioration of concrete can also be prevented by treating concrete with solutions of salts or even acids in minor concentration to attain on the surface of hardened cement a layer of calcium salts less soluble than calcium hydroxide. This can be accomplished by treating the surface with solutions of three per cent fluosilicic acid, five per cent oxalic acid and saturated solution of mono calcium phosphate. Durability of concrete can also be increased by impregnating the pores of concrete with a suitable polymer. As the destructive processes in concrete are complex, a clear understanding of the destructive mechanism may help the selection of an appropriate technique to protect or improve the resistance of structural concrete to the aggressive agents.

15.3

CORROSION OF REINFORCEMENT

Concrete normally provides a high degree of protection against corrosion to embedded steel reinforcement. This is because concrete inherently provides a highly alkaline environment for the steel which protects and passivates the steel against corrosion. In addition, concrete of low water-cement ratio and well cured has a low permeability which minimizes penetration of corrosion inducing agents like oxygen, chloride ion, carbon dioxide and water. With concrete of suitable quality, corrosion of steel can be prevented provided the structure is properly designed for the intended environmental exposure. In instance of very severe exposure, the use of other protective measures such as corrosion inhibitor coatings on steel or concrete or cathodic protection may be utilized. However,

Deterioration of Concrete and Its Prevention

567

corrosion of steel and accompanying distress can result if the concrete is not of suitable quality, the structure is not properly designed for the anticipated environment or the actual environment or other factors were not as anticipated or changed during the life of the structure. Sometimes, the first evidence of distress is the brown staining of concrete around the embedded steel. This brown staining resulting from rusting or corrosion of the steel permeating to the concrete surface without cracking of the concrete but usually is accompanied by cracking. Sometimes, cracking of concrete occurs shortly thereafter. Concrete cracking occurs because the corrosion products of steel, an iron oxide or rust, has a volume twice as much as that of metallic iron from which it is formed. The forces generated by this expansive process can far exceed the tensile strength of the concrete with resulting cracking. Steel corrosion not only causes distress because of staining, cracking and ravelling of the concrete but may also cause structural failure resulting from the reduced cross section and hence reduced tensile force capacity of the steel, this normally being more critical with thin prestressing steel tendons than with larger reinforcing bars. To understand the corrosion phenomenon of embedded reinforcement, it is imperative to study the corrosion of steel itself. Steel corrosion can take place by several mechanisms, but corrosion of steel by direct oxidation or due to stress corrosion is of little concern in concrete structures. Indirect oxidation of steel in concrete resulting because of the existence of difference in metals or non-uniformities of the steel or non-uniformities in the chemical or physical environment provided by the surrounding concrete, is believed to be the main cause of corrosion distress in concrete. This type of corrosion is termed electro-chemical corrosion.

15.3.1

Mechanism of Electro-chemical Corrosion

The metals have a tendency to oxidize to a metal ion in an aqueous solution of normal ionic activity at standard temperature. This ionization of metal, i.e., oxidation of metal at the anode is often referred to as the primary stage of the corrosion reaction called anodic reaction and can be represented by Fe→Fe++ + 2e This reaction results in the anodic region of the metal to have an excess of electrons. Therefore, to maintain equilibrium of electric charges an equivalent quantity of hydrogen is plated out at adjacent surfaces of the metal. This thin invisible protective film of hydrogen around the cathode inhibits further progress of corrosion reaction, unless the hydrogen film is removed in some manner. The destruction of hydrogen film may take place in one of the two ways: (i) oxygen depolarization at the cathode, and (ii) evolution of hydrogen as a gas. These processes called cathodic reactions are usually represented by 1 O + H2O + 2e → 2OH− 2 2 2H+ + 2e− → H2 These cathodic reactions which are often called the secondary reactions control the rate of corrosion of the structural steel. The chemical reactions are depicted in

568

Concrete Technology

Fig. 15.4. Therefore, any environmental condition which influences these reactions will influence the rate of corrosion. Since cathodic depolarization is dependent on the concentration of dissolved oxygen next to the metal, it is influenced by the degree of aeration, temperature, salt concentration, etc. The secondary reactions permit the primary reaction to proceed with the accumulation of ferrous ion in the solution which in the presence of water and oxygen are oxidized and precipitated as rust. However, two stages of oxidation may exist depending upon the availability of oxygen. The products of the first stage, namely ferrous hydroxide is more soluble than the second-stage product, i.e., hydrated ferric hydroxide. The first is usually formed directly at the metal surface and is converted to the latter at a little distance away from the surface where it is in contact with more oxygen as shown in Fig. 15.4(a).

(a) Schematic representation of electrochemical process for corrosion of steel

(b) Typical T examples of deterioration of concrete by corrosion of reinforcement

Fig. 15.4

Cracks due to corrosion of steel by electrochemical process

Deterioration of Concrete and Its Prevention

569

The structure and composition of the rust varies considerably with the conditions prevailing during its formation and the structure of rust plays an important role in the subsequent corrosion process, e.g., if the rust layer is hard, dry and fairly adherent to the metal surface, it may retard corrosion by forming a protective coating. On the other hand, if the rust layer is spongy and readily detachable, it will absorb oxygen and moisture from the surrounding media and consequently add to further corrosion.

15.3.2 Corrosion of Steel Reinforcement Embedded in Concrete The corrosion of the reinforcement embedded in concrete is extremely complicated and is influenced by numerous factors, both external and internal. The effect of these factors can be studied under two heads.

Factors Associated with Steel The non-homogeneities in the metal surface due to difference of chemical composition over the surface, discontinuous surface layers or differences in texture tend to increase the probability of corrosion due to the development of potential difference. But from the total corrosion standpoint these factors are not as important as the external conditions that may exist. The difference in potential may also be due to variation of stresses in the reinforcement. A crystalline structure in the strained area has a somewhat different configuration from that in the unstrained areas and is anodic to it, thereby setting up an electrochemical cell.

Factors Associated with Concrete In the basic concrete-steel system, electrochemical cells are set up by heterogeneities of the concrete media. Reaction variables influencing electro-chemical corrosion are the moisture content, pH at the concrete-steel interface and the amount of available oxygen. When moisture is present, concrete medium becomes an electrolyte containing mainly calcium hydroxide.

Effects of pH Corrosion is more rapid in acidic solutions than in neutral solutions (pH = 7). Steel becomes passive in alkaline solutions due to the formation of an impervious layer of ferric products on the steel surface. However, if for some reason, the hydroxyl ion concentration is reduced, the protective layer is destroyed and corrosion proceeds. There are two general mechanisms by which the highly alkaline environment and accompanying passivating effect may be destroyed: (i) reduction of alkalinity by leaching of alkaline substances like lime with water or partial neutralization by reaction with carbon dioxide or other acidic materials, and (ii) by electro-chemical action involving chloride ions in the presence of oxygen.

Influence of oxygen Oxygen is primarily responsible for the corrosion. Oxygen acts as a depolarizer at the cathode and consequently tends to increase the rate of corrosion. Dissolved oxygen alone will accelerate corrosion in acid, neutral or slightly alkaline electrolytes.

570

Concrete Technology

Though the main action of oxygen is as a depolarizer at the cathode, but at the anode it may lead to the formation of protective layer of insoluble ferric hydrates, which influences the rate and probability of corrosion. Thus oxygen may play a dual role: as a depolarizing agent it tends to enhance corrosion. On the other hand, it may produce protective layers on the anodic regions prohibiting further corrosion. The cracking of concrete and its permeability allow the penetration of oxygen to local areas of the reinforcement and in the unequal distribution of oxygen over the steel surface, which results in the setting up of anodic and cathodic regions. Thus the presence of oxygen in varying concentrations along the reinforcement will tend to increase the probability of corrosion.

Influence of moisture As the corrosion reactions occur only if moisture is present, all corrosive factors become ineffective in its absence. In addition, the moisture penetration is the means whereby any exterior substance, such as chloride salts, carbon dioxide and dissolved oxygen may gain access to the reinforcement. Influence of chloride ion concentration The presence of salts provides two opposing effects: (i) it increases the conductivity of the electrolyte, thus raising the corrosion rate, and (ii) at high concentrations it diminishes the solubility of oxygen, thereby lowering the corrosion rate. However, there is a critical or threshold concentration of chloride ions which must be exceeded before the initiation of corrosion. Any increase in chloride ions concentration beyond this critical value results in an increased rate of corrosion up to some limit at which the availability of oxygen necessary for corrosion to occur may be significantly reduced. The increased concentration of chloride ions which are destructive to the protective film, can be tolerated without any resulting corrosion provided the alkalinity is increased, thereby promoting the formation of a protective oxide film. Chlorides may be present in the concrete from several sources, e.g., soluble chlorides. Calcium chloride may be introduced in fresh concrete by the use of aggregates containing chlorides, or using saline water as a concrete mix water. Calcium chloride may be used as an accelerator or a concrete admixture containing chlorides may be used. Some cements may also contain small amounts of chlorides. Chlorides may also enter the concrete from the environment, deicing salts, etc. Influence of carbonation The carbon dioxide absorbed into the concrete may convert the calcium hydroxide into calcium carbonate thereby reducing the pH value and, consequently, the protective value of the concrete. Carbonation also tends to increase the shrinkage of concrete and thus promotes the development of cracks. The increase in the permeability of concrete due to shrinkage cracks allows the penetration of moisture and other external chemicals which may promote corrosion. Influence of the quality of concrete Permeability of concrete is probably the most important single factor affecting the corrosion of reinforcement. Concrete of high permeability will have a high electrical conductivity and allow the penetration

Deterioration of Concrete and Its Prevention

571

of deleterious substances to the reinforcement. Concrete permeability depends on numerous factors: some of which are water–cement ratio, cement content, nominal size of aggregate and its grading, methods of compaction, curing, etc. Low-quality concrete is characterized by voids adjacent to the reinforcement, which may retain high moisture content, leading to rapid corrosion attack.

Effect of thickness of concrete cover over steel The thickness of concrete cover over steel is of great importance as this cover protects the steel from the factors that promote corrosion. The diffusion of chloride ions into the cement paste results in the formation of partially insoluble calcium chloroaluminate. This reaction reduces the concentration of chloride ions at any particular location and hence the tendency for inward diffusion is further reduced. Influence of humidity In the regions having a relative humidity of 50 per cent or less, corrosive actions may be negligible. Likewise structures permanently immersed in water exhibit little or no corrosion of the reinforcement. The concrete of submerged structures generally maintains a high pH value and uniform salt concentration, thus reducing the formation of corrosion cells. Probably the main reason for this protection is absence of air at the concrete surface and thus oxygen cannot penetrate to the reinforcement. Influence of inhibitors A number of admixtures, both organic and inorganic have been suggested as specific inhibitors of iron corrosion. Inorganic inhibitors are potassium dichromate, stannous chloride, zinc and lead chromates, calcium hypophosphite and sodium nitrite, while organic inhibitors are sodium benzoate, ethyl aniline and mercaptobenzothiazole. Some of the admixtures may retard setting of concrete or be detrimental to later-age strength. With some inhibitors, inhibition may occur at an optimum percentage of the inhibitor, whereas at lower or higher ratios the inhibitor may actually promote corrosion.

15.3.3

Effects of Corrosion

In most cases, the corrosion rate is extremely slow and the normal life span of a structure is not largely affected. However, if the external and internal conditions, are such that a corrosive environment exists, a destructive action may take place at an increased rate and create serious problems. The distress due to corrosive action may be in the form of deep pitting and a severe loss of cross section of the reinforcement. This is particularly serious if the reinforcement is subjected to high stresses as in the case of structures carrying heavy loads. A combination of high stress and intense corrosion will produce stress concentrations that may result in rupture of the reinforcement. The corrosion of embedded steel can be minimized by using the following recommendations: 1. For the reinforced-concrete members totally immersed in sea water, the cover should be increased by 40 mm beyond that specified for normal conditions. However, for the members periodically immersed in the sea water, this

572

Concrete Technology

increase in cover should be raised to 50 mm. In the case of high-strength concretes of grade M 25 or above, the additional thickness of cover specified above may be reduced to half. 2. The additional cover thickness ranging from 15 to 50 mm beyond the values for normal conditions may be provided when the surfaces of the concrete members are exposed to the action of harmful chemicals (e.g., concrete in contact with soil contaminated with such chemicals), saline atmosphere, acid vapors, sulfurous smoke, etc. However, the total cover is limited to 75 mm. 3. To reduce the corrosion of reinforcement, the chloride ions in the concrete should be limited to its threshold or critical value. IS: 456–2000 has prescribed the limit of total amount of chloride in concrete at 0.15 per cent by mass of cement. However, for prestressed concrete, the total amount of chloride ions in concrete should be limited to 0.06 per cent. 4. In the case of an excessively aggressive environment, or where for practical reasons it is not possible to meet the requirements of cover and quality of concrete recommended above, special protection systems should be considered. Corrosion inhibitors may be added to concrete to prevent the corrosion of embedded steel. They may be either anodic or cathodic or mixed type. Anodic inhibitors form an extremely insoluble film adhering firmly to the surface of the bar at the anodic areas. Alkalies, phosphates and chromates are typical examples. In the same way, cathodic inhibitors stifle the reaction at the cathode by depositing a non-conductive film on the steel bar. Calcium carbonate, aluminum oxide, calcium nitrite are common examples of cathodic inhibitors. The use of special steels to overcome the problem of corrosion of reinforcement is a costly solution. However, the sacrificial protection provided by coating the reinforcement with either a metallic or non-metallic material has been found to be satisfactory. Irrespective of the type, it is essential that the coating should completely envelope the bar and should remain unbroken. In addition, it should remain passive under all conditions. Hot-dip galvanizing providing zinc coating is effective as it is metallurgically bonded to the reinforcing bar and it does not affect its bond with concrete, yield and ultimate strengths, and ductility of the bar. Organic coatings have also been used as an alternative to metallic coatings. The epoxy coated steel bars have shown significant increase in protection and reduction in cracking of cover. The concrete surface coatings providing a barrier at the surface which penetrate and seal the pores of concrete are popular for this type of protection. Waterproof membranes are also being extensively used.

REVIEW QUESTIONS 15.1 What is corrosion of concrete? State the principal forms of destruction of concrete, and explain alkali-silica reaction.

15.2 Discuss briefly the corrosion of reinforcement embedded in concrete.

Deterioration of Concrete and Its Prevention

573

MULTIPLE-CHOICE QUESTIONS 15.1 Identify the incorrect statement(s). (a) The durability of concrete is its resistance to the deteriorating influences of both external and internal agencies (b) the gradual deterioration of concrete by chemically aggressive agents is called corrosion of concrete (c) corrosion of steel reinforcement results in internal stresses which promote destruction of concrete (d) micro-cracks present at the aggregate-cement interface do not affect durability as long as they are small in size and discontinuous (e) None of the above 15.2 Following are the principal forms of destruction of concrete except (a) decomposition of concrete due to leaching action (b) chemical reaction between hardened cement constituent of concrete and chemicals of a solution (c) grinding action of sand storms (d) crystallization resulting in the increase of volume of solid phase within the pore structure 15.3 Deterioration of concrete can be prevented by (a) making a denser concrete (b) ensuring sufficient alkalinity to provide passive environment against corrosion of reinforcement (c) limiting water–cement ratio (d) impregnating the pores of concrete with a suitable polymer (e) All of the above 15.4 The corrosion of reinforcement can be prevented by the following except (a) the use of corrosion inhibitors, coating on steel or concrete (b) proper design of concrete for the intended environmental exposure (c) use of stainless steel

15.5

15.6

15.7

15.8

(d) impregnating the pores of concrete by suitable polymer (e) increase in the thickness of concrete cover over reinforcement The corrosion of steel reinforcement embedded in concrete is rapid when the member is immersed in (a) acidic solution (b) alkaline solution (c) water with dissoved oxygen (d) sea-water (e) water with chloride ion concentration To reduce the corrosion of reinforcement the chloride ions should be limited to its threshold or critical value of (per cent by mass of cement): (a) 0.05 (b) 0.10 (c) 0.15 (d) 0.20 (e) 0.25 Identify the false statement(s). (a) In the regions having relative humidity of 50 per cent or less corrosive action may be negligible (b) Structures permanently immersed in water exhibit little corrosion of the reinforcement (c) The distress due to corrosive action may be in the form of deep pitting and a severe loss of cross section of reinforcement (d) For prestressed concrete, the total amount of chloride ions in concrete should be limited to 0.10 per cent by mass of cement (e) For reinforced concrete members totally immersed in sea water additional cover of 40 mm should be provided The following compounds may be used as inhibitors of iron corrosion (a) potassium dichromate, zinc and lead chromates (b) calcium and sodium chlorides (c) calcium sulfate (d) fly ash (e) molasses

Answers to MCQs 15.1 (e) 15.7 (d)

15.2 (c) 15.8 (a)

15.3 (e)

15.4 (c)

15.5 (a)

15.6 (c)

16 16.1

HIGH-PERFORMANCE CONCRETES

INTRODUCTION

Over the years, to produce concrete with improved properties such as higher early strength, there have been gradual increase in the fineness and C3S content in the general-purpose Portland cement. An increase in C3S content as compared to the C2S content has resulted in a more rapid hydration and more rapid development of strength. Since concrete mixtures are proportioned on the basis of 28-day compressive strength, this change has gradually resulted in decreasing the cement content and increasing the water-cement ratio of concrete mixtures with a given consistency. On the other hand, a reduction in particle size generally results in increased hydration and a higher compressive strength. However, the greater fineness leads to an increased water demand and a more rapid heat generation in concrete structures. In recent years, further improvements in concrete properties have been achieved in the so-called high performance concrete (HPC) by improvements involving a combination of improved compaction, improved paste characteristics and aggregate-matrix bond, and reduced porosity. In these systems, a substantial reduction in water-to-cement ratio is achieved through the use of superplasticizers, further enhancements of some properties have been obtained through the addition of mineral micro fillers (supplementary cementing or pozzolanic materials such as silica fume and fly ash). One consequence of lower water-cement ratio is that not all of cement in the concrete mixture participates in the hydration reactions. It is generally accepted that for water-cement ratios less than approximately 0.42, unreacted cement will be present regardless of the particle size distribution of cement. Compared to conventional or normal concrete high-early-strength concrete containing high cement content tends to crack more easily due to lower creep, higher thermal and drying shrinkages, and higher elastic modulus. There is a close inverse relationship between high-strength and early-age cracking in concrete. The deterioration, on the other hand, is closely associated with cracking of concrete structures exposed to severe environmental conditions. To overcome various problems encountered in the field and to achieve better and better performance, a number of further process improvements have been suggested. It is to be realized that durability of concrete cannot be enhanced without a holistic approach considering the cracking-durability relation. The root causes of many durability problems can be traced to this kind of reductionistic approach ignoring the cracking-durability relationship and over emphasizes on the strength-durability relationship. A change-over to a holistic approach to control cracking in concrete

High-Performance Concretes

575

structures is necessary to create a much closer working relationship between the structural designer, materials engineer, and construction personnel. There has been significant interest and development in the use of short discrete or continuous fiber reinforcement for improving the pre- and post-cracking behavior of cementing composites and/or concrete. Fiber reinforced polymers (FRP) or sometimes also referred to as fiber reinforced plastics are increasingly being accepted as an alternative for uncoated and epoxy-coated steel reinforcement for prestressed and non-prestressed concrete applications. Although high-performance concretes are made with the same basic components as the normal concrete, their much higher qualitative and quantitative performances make them new materials. On the basis of their use, they offer different advantages such as enhanced durability, reduced permeability, higher strength, etc., at an economical cost. Nevertheless, usually the end-product characteristics do not stand alone but are supplemented by other clauses or sections. The lack of adequate HPC provisions in various national codes of practice is the greatest deterrent in its extensive use. The development of cost-effective state-of-the-art procedures for producing, evaluating, and designing with HPC will instil confidence that the user stated level of performance for each performance characteristic can be reliably achieved in the field. There is no bad concrete; it is just that the concrete is inappropriate for the particular application. The goal is to use concrete with characteristics at appropriate levels to ensure satisfactory performance for the intended service life. A concrete does not perform as desired, when either the specifications are inadequate or they are not followed. Modern QC/QA procedures greatly increase the likelihood that specifications are met when followed. A concrete produced to comply strictly with code requirements should be an HPC. The intent of HPC is not to produce a high cost product, but simply to provide the means for producing concrete that will perform satisfactorily with only a reasonable maintenance cost for the intended service life. In spite of increasing emphasis being placed on the performance of concrete, the construction industry still operates in a prescriptive rather than performancebased environment where there is no need for prescribing the mixture ingredients and proportions in detail. To achieve the goals of materials conservation, major paradigm shift is needed from prescriptive to performance-based codal specifications for the selection of materials, mixture proportions, and construction practice. However, the trend is shifting slowly but surely towards performance-based environment as described in Chapter 1. In certain applications, such as pavements, bridges and high-rise buildings, the concrete industry has made significant strides in performance enhancement by process improvement to tailor mixture to specific structural environments, to achieve optimal particle size distribution of constituent materials, etc. Computer-integrated knowledge systems can provide a practical basis for optimizing concrete for specific applications by taking technical, economic and environmental factors into account. The motto of concrete construction industry is to have quick and low-cost construction. There is no longer a market for slow construction. The emphasis is on the production of concrete having high early strength but crack resistant to reduce premature deterioration. To build environmentally sustainable concrete structures, it is clear that instead

576

Concrete Technology

of strength, the 21st Century concrete practice must be driven by considerations of durability. The industry envisions an extraordinary expansion of activities to advance the HPC technology in response to the need for the massive civil infrastructure renewal and construction around the world. In this chapter, the mechanisms of enhancement of concrete performance and durability have been emphasized.

16.2

HIGH PERFORMANCE CONCRETE

A performance enhanced concrete or high-performance concrete (HPC) is a specialized series of concrete designed to provide several benefits in the construction of concrete structures that cannot always be achieved routinely using conventional ingredients, normal mixing and curing practices. In other words, a high performance concrete is a concrete in which certain characteristics are developed for a particular application and environment, so that it will give excellent performance in the structure in which it will be placed, in the environment to which it will be exposed, and with the loads to which it will be subjected during its design life. It includes concrete that provides either substantially improved resistance to environmental influences (durability in service) or substantially increased structural capacity while maintaining adequate durability. It may also include concrete, which significantly reduces construction time without compromising long-term serviceability. It is, therefore, not possible to provide a unique definition of HPC without considering the performance requirements of the intended use of the concrete. Examples of characteristics that may be considered critical in an application requiring performance enhancement are: ease of placement and compaction without segregation, early-age strength, long-term mechanical properties, permeability, density, heat of hydration, toughness, volume stability, and long life in severe environments, i.e., durability. Concretes possessing many of these characteristics often achieve higher strength. Therefore, HPC is often of high-strength, but high-strength concrete may not necessarily be of high performance. Thus, in practical applications of this type of concrete, the emphasis has in many cases gradually shifted from the compressive strength to other properties of the material, such as a high modulus of elasticity, high density, low permeability, and high resistance to some forms of attacks. The cost and other benefits derived may include less material, light and fewer structural elements, reduced maintenance, extended life cycle and aesthetics.

16.3

CLASSIFICATION

A suitable classification of HPC according to different levels of performance requirements would enable design engineers to select appropriate performance criteria of HPC for different applications in different environmental conditions.

16.3.1

Based on Characteristic Strength

Since there is significant improvement in concrete durability resulting from an increase in the strength, a performance definition of HPC includes adequate durability and strength parameters. Based on the characteristic (28-days) strength of concrete

High-Performance Concretes

577

following classification has been suggested. This characterization does not really describe different properties of HPC. Classification Ordinary concrete Standard/normal concrete High-performance concrete Very high-performance concretes Exceptional concretes

OC NC HPC VHPC EC

28-day compressive strength 10 to 20 MPa 25 to 55 MPa 60 to 100 MPa 100 to 150 MPa > 150 MPa

High Performance Concretes This class is further divided into two subclasses according to whether or not they contain ultrafine mineral additives. They need high-strength or high grade Portland cement with its content in the range of 400 to 450 kg/m3. High range water reducing (HRWR) admixtures must be added in order to maintain the water-cementing ratio between 0.35 and 0.40 without impairing the concrete workability. No special aggregates are required.

Very High Performance Concretes These are obtained by providing a further reduced water-cementing ratio between 0.20 and 0.35 with high dosages of HRWR admixtures. All the concrete components should be of high quality. It is necessary to use ultrafines such as silica fume or/and rice husk ash. They generally need retarding admixtures. The aggregates must have strength and Young’s modulus not too different from that of hardened cement paste in order to minimize differential deformations between the aggregates and the matrix. Generally, the use of aggregates with a maximum nominal size of 10 to 12 mm is recommended. Exceptional Concretes These are still laboratory concretes with characteristic strengths as high as 250 MPa with water-binder ratios of the order of 0.16.

16.3.2

Based on Durability and Target Strength

Under SHRP (The Strategic Highway Research Programme), high performance concrete has been defined in terms of certain target strengths and durability criteria. In this definition, the target minimum strength should be achieved in the specified time after water is added to the concrete mixture. The water-cement ratio is based on all cementing materials. The minimum durability factor should be achieved after 300 cycles of freezing–thawing. SHRP has defined HPC into four categories as the concrete with 1. a maximum water-cementing ratio of 0.35, 2. a minimum durability factor of 80 per cent, 3. a minimum strength criterion of either (a) 20 MPa within six hours after placement (Very Early Strength, VES), or (b) 35 MPa within 24 hours (High Early Strength, HES), or (c) 70 MPa within 28 days (Very High-strength, VHS), and 4. Fiber reinforcement, i.e., HES + steel or polyfibers.

578

Concrete Technology

High Early Strength Concrete Traditionally, interest in the strength and other properties of concrete has been focussed on those at the age of 28 days and beyond. In the recent past, there has been an increasing interest in the properties of concrete at ages less than 28 days. Any strength measured at ages less than the standard 28 days is regarded as early strength. There are at least three factors which have contributed to this increased interest in early strength: (i) the fast-paced construction schedules that expose concrete to significant structural loads at early ages, (ii) the development of specialty cements or admixtures which enable the achievement of higher strength at early ages, and (iii) the recognition that long-term performance of concrete is greatly affected by its early-age history. Since the properties of concrete depend closely on the degree of cement hydration, one definition of early strength could be the strength at the age corresponding to 50 per cent hydration of cement. For concrete made with ordinary Portland cement and cured at a standard temperature of 27 ± 2 °C, approximately 50 per cent of the cement will hydrate within three days. As described earlier, the compressive strength of the concrete increases with decreasing water–cementing material ratio and with increasing amounts of fly ash. However, a major concern with concrete using composite or blended cements in which high volumes of OPC are replaced with FA or GGBFS is its low early-age strength development. The incorporation of highly reactive pozzolanas, such as silica fume, rice husk ash, metakaolin, or fillers known to accelerate the early-age hydration of cement provides a solution to this problem. Compressive strength of a concrete using OPC–FA–SF ternary binder cement is lower than that of concrete obtained by using OPC at seven days and surpasses it at 91 days for all binder contents. The optimum amount of fly ash is about 48 per cent of the total cementing materials. These concretes have adequate early-age strength and satisfactory freezing–thawing resistance, but poor scaling resistance. In HPC due to the greater accessibility of cement grain surfaces, the greater early hydration results in up to 24-hours strength of concrete being generally higher than that in the case of normal concrete of the same water-cement ratio. Frequently, the greater rate of cement hydration in the well-dispersed system concretes containing superplasticizer show even higher compressive strengths at one, three and seven days than the normal concrete having the same water-cement ratio. Based on the target minimum strength achieved in the specified time after addition of water to the concrete mix SHRP has classified high early strength HPC as VES, HES and VHS concretes as mentioned above.

Fiber-reinforced High Performance Concretes High-performance fiber-reinforced concrete results from the addition of either short discrete fibers or continuous long fibers to the cement-based matrix. Due to the superior performance characteristics of this category of HPC, its use by the construction industry has significantly increased. The normal fiber reinforced concretes (FRC) contain short discrete fibers up to three per cent by the volume of concrete. For using very high volume percentages of steel fibers, slurry infiltrated fiber concrete (SIFCON) and slurry infiltrated (fiber) mat concrete (SIMCON) systems are used.

High-Performance Concretes

579

Cementing composites based on the use of prepacked discontinuous steel fibers with higher volume percentages (in the range of 10 to 15 per cent) termed as slurry infiltrated concrete (SIFCON) when used over reinforced concrete beams lead to ductility indexes exceeding three times those obtained without it. Crack widths and their spacing are very much smaller than in conventional reinforced concrete. There is no need for stirrups in flexural members with SIFCON matrix. SIMCON using a manufactured continuous mat of interlocking discontinuous steel fibers, placed in a form, and then infiltrated with a flowable cement-based slurry produces concrete components with extremely high flexural strength. The advantage of steel fiber mats over a large volume of discrete fibers is that the mat configuration provides inherent strength and can utilize fibers with much higher aspect ratios. The use of continuous mats, typically made with stainless steel to control corrosion in very thin members, permits development of high flexural strengths and very high ductility with a reduced volume of fibers than SIFCON. The use of SIMCON appears to be very promising for at least two reasons. First, the very high volume of fibers required to provide significant increases in mechanical properties with SIFCON may not provide economic justification in a large number of practical applications. Secondly, in situations where normal FRC may be economically justified, such as in the pavements, the addition of fibers to the mix and the placement of the fiber-reinforced mix require special care, and considerable extra time and expense. SIMCON overcomes many of these limitations since the fiber mat, normally delivered in large rolls, can be laid out by hand and the slurry simply pumped into place. The use of SIMCON permits fabrication of thin, complex shapes with very high ductility and flexural strength. Another interesting and useful development in FRC construction has been to provide non-metallic fibers in small, cylindrical bundles, approximately 50 mm high (the length of the fiber) and 55 mm in diameter, wrapped in a water soluble compound. This permits easy addition of the fibers, by hand, into the mixing drum of a mixer, either during charging or at the job site. The wrapper disintegrates, allowing the fibers to disperse into the concrete mixture with little balling or segregation. Quality control is improved since this makes the quantity of fibers added easy to determine and easy to check, and minimizes the problems in dispersion in the mixer. Further, production rates are maintained with little additional effort.

16.4

SELF-COMPACTING OR SUPER-WORKABLE CONCRETE

This type of HPC has been developed for the use in situations where vibration is difficult and reinforcing steel is highly congested as in the case of typical prestressed beam example shown in Fig. 16.1. There are well-established procedures for selection of materials, mix proportioning and quality curing for production of high performance concrete. However, much less control is exercised on the concrete placement. In this regard, the development of self-compacting non-segregating concrete utilizing HRWR is an important mile stone towards achieving high performance concrete through automation.

580

Concrete Technology

(a) Dense reinforcement in a beam

Fig. 16.1

(b) Surface quality of beam

Example of a beam with dense reinforcement and complex forms

Self-Compacting Concrete (SCC) technology is based on increasing the amount of fine material like fly ash, lime stone filler, etc., without changing the water content compared to the conventional concrete. Figure 16.2 shows an example of mix proportions used in self-compacting concrete as compared to conventional concrete mix. These variations in proportions change the rheological behavior of the concrete. The self-compacting or super-workable concrete, also referred to as self-consolidating concrete is a highly flowable or self-leveling cohesive concrete that can spread readily into place (low viscosity) through and around dense reinforcement under its own weight. It adequately fills formwork without segregation or bleeding, and without the need for significant vibration. SCC mix has a low yield stress and an increased plastic viscosity. The mix requires minimal force to initiate flow yet have adequate cohesion to resist aggregate segregation and excessive bleeding, i.e., coarse aggregate can float in the mortar without segregating. The yield stress is reduced by using an advanced synthetic high-range water-reducing admixture (HRWR), while the viscosity of the paste is increased by using a viscosity modifying admixture (VMA) or by increasing the percentage of fines incorporated into the SCC mix design.

Fig. 16.2

Comparison of proportions of constituent materials of self-compacting and conventional concretes

High-Performance Concretes

581

Modern application of self-compacting concrete (SCC) is focused on high performance; better and more reliable quality, dense and uniform surface texture, improved durability, high-strength, and faster construction. In Japan and Europe, self-compacting concrete (SCC) technology has been extensively used in bridges, buildings and tunnel construction, where as in U.S.A. SCC technology has been used in precast concrete industry, tanks, bridge decks and architectural concretes. Typical examples of applications of self-compacting concrete in casting of a slab and narrow wall section are shown in Fig. 16.3.

(a) Casting of SCC slab

Fig. 16.3

16.4.1

(b) Pouring in narrow wall section

Placing the super-workable or self-compacting concrete (SCC)

Materials for SCC

The materials are the same as used in the conventional concrete. However, to transform the conventional concrete into self-compacting concrete, aggregate shape, size, grading, cement and water contents, and admixture dosage all have to be carefully selected and proportioned to ensure the self-consolidating properties. Well-graded cubical or rounded coarse aggregates are desirable as they minimize cement paste content as well as admixture dosage. The maximum size of aggregate is generally limited to 20 mm. For mass concrete, the maximum size of coarse aggregates may be as large as 50 mm. However, the aggregate should be of uniform shape and grading. Fine aggregate can be natural or manufactured sand, but it should be of uniform grading. The particles finer than 150 μm sieve are considered as fines. To achieve a balance between deformability or fluidity and stability, the total content of fines has to be high, usually about 520 to 560 kg / m3 In addition to chemical admixtures including high range water reducers and viscosity modifying admixtures mentioned earlier, traditional mineral additives including silica fume, fly ash, blast furnace slag, and limestone powder help to achieve the balance between fluidity and cohesion. These ingredients when added in appropriate quantity improve rheological properties and durability of SCC along with the other parameters discussed above. Finely crushed lime stone, dolomite or granite finer than150 μm may be added to increase the powder content. The use of fly ash as filler seems to be advantageous compared to lime stone filler; it results in higher strength and higher chloride resistance.

582

Concrete Technology

High-range water-reducing admixture (HRWR) based on polycarboxylate ethers are typically used to plasticize the mix. Due to very low water-cement ratio, SCC is very sensitive to moisture fluctuations in the manufacturing process; therefore, stabilizers such as polysaccharides are added. The overall concept for experimental proportioning of the SCC mixes is illustrated in Fig.16.4 and listed in Table 16.1. Fluidity/Deformability I. Increase paste deformability use of HRWR balanced w/c ratio

II. Reduce inter-particle friction low coarse aggregate volume use continuously graded powder

Easy flow/low blockage I. Enhance cohesiveness low w/c ratio use of VMA

II. Ensure compatible flow space and aggregate size low coarse aggregate volume low max, size aggregate

Homogeneity/stability Fluidity/stability Trade-off f

Fluidity

High fluidity Low viscosity High viscosity Low fluidity viscosity

Fig. 16.4

I. Reduce solid separation Limit aggregate content reduce max, size aggregate increase cohesion & viscosity -low w/c ratio -use of VMA

II. Minimize bleeding low w/c ratio increase VMA use finer powder

Concepts for achieving self-compacting concrete characteristics

The self-compacting concretes are generally categorized in to three different types based on the composition of the mortar: 1. Powder-type SCC This mix achieves the fluidity requirements through the reduction of the coarse aggregate volume and the use of High Range Water Reducer (HRWR). The stability comes from a low water to cement ratio with high sand to paste ratio, i.e., larger amounts of fine aggregates are necessary to resist segregation in the mix for no or low VMA level. 2. Stabilizer or VMA-type SCC In VMA-type SCC, the fines content can be in range required for conventional vibrated concrete, but the required viscosity to inhibit segregation is ensured by using a viscosity-modifying admixture (VMA). This mix uses a high water to cement ratio with little to no HRWR to achieve the fluidity requirements, allowing for a moderate volume of coarse aggregate, while the stability is achieved through the use of VMA and moderate sand to paste ratios. 3. Combination-type This mix is obtained by adding a small amount of stabilizer to the powder type SCC to balance the moisture fluctuations.

583

High-Performance Concretes Table 16.1 Type of mix NCC

Concepts for experimental development of SCC mix

Water-to-cement ratio

HRWR

Moderate Normal As needed Low

Powder-type SCC

0.35

Combination- Moderate type SCC High

VMA-type SCC

0.40 0.45

High

VMA

Coarse aggre- Sand-togate content paste ratio

None

Moderate

Moderate

Low

Low

More

Moderate

Moderate

Moderate

Low

Moderate Moderate Low

High

Typical mixes used in Japan, Europe and USA are listed in Table 16.2. This will give the reader an idea of the possible proportions of the mix. Table 16.2

Typical self-compacting concrete mixes used in Japan, Europe, and the U.S.A

Country

Japan J1

Water Portland Cement Fly Ash Limestone Powder GGBFS Silica Fume Fine Aggregate Coarse Aggregate II. Admixtures HRWR

2

Europe

U.S.A.

J2

J3

E1

E2

E3

U1

U2

U3

175 165 530 220

175 298

190 280

192 330

S200 310

174 408

180 357

154 416

70 -

0 -

206 -

0 245

0 0

190 0

45 -

0 -

0 -

0 0 751

220 0 870

0 0 702

0 0 865

200 0 870

0 0 700

0 0 1052

119 0 936

0 0 1015

789

825

871

750

750

750

616

684

892

9.0

4.4

10.6

4.2

5.3

6.5

I. Mix Ingredient (kg/m3) 4

3

VMA 0 4.1 0.0875 0 0 7.5 III. Flow characteristics Slump Flow 625 600 660 600-750 600-750 600-750 Test -Diam. of Spread, mm Mix-J1, E1, U1 are Powder type Mix-J2, E2, U2 are Stabilizer (VMA) type Mix-J3, E3, U3 are Combined type 1 ml 2 HRWR = High-range water reducing admixture. 3 VMA = Viscosity-modifying admixture 4 Mix-J1 uses low-heat-type Portland cement

16021 25001 26161 0

0

5421

710

660

610

584

Concrete Technology

16.4.2

Characteristics of Fresh Self-Compacting Concrete

Self-compacting concrete is characterized by its special properties in fresh state namely flowability, viscosity, blocking tendency, self-leveling and stability of mixture. These workability parameters are grouped into three key properties, namely, 1. Filling ability or deformability This is the ability to flow into and completely fill intricate and complex forms under its own weight. 2. Passing ability This is the ability to pass through and bond to dense reinforcement under its own weight. 3. Stability This represents high resistance to aggregate segregation. While adequate flow is generally the main parameter used to assess SCC, a highly flowable concrete may not always be acceptable. To be of high quality, SCC must not only fill formwork and consolidate under its own weight, it must also remain homogenous throughout the entire construction process, i.e., it must be stable. These workability parameters can be measured by a combination of tests that give an indication of the quality of the SCC. SCC mixtures are designed and tested in both a laboratory and a construction site to confirm performance prior to its use in a job. While it may be convenient to use several test methods during the design of SCC, it is often not practical to carry out the same tests during production. It should be noted that the dimensions of the apparatus and spacing of the obstacle bars, if any, may vary depending on the type of job. However, if the results of a particular test are used to qualify an SCC mix design, then the same test with an apparatus having same dimensions should be used during construction QC testing. The visual stability index (VSI) recorded while performing the inverted slump flow test illustrated in Figs. 16.6 and 16.7 may identify significant segregation problems, especially inadequate paste volume and severe bleeding. The rating is based on the visual inspection of the slump flow patty immediately after it stops flowing. The rating is given to the concrete based on the appearance of the patty in terms of the amount of water bleeding from the edge of the concrete patty, the amount of aggregate piled in the center of the patty, or the presence of an aggregate free mortar halo at the edge of the patty as shown in Fig. 16.5. The VSI test ranks the stability of the SCC on a scale of 0-3, with 0 indicating highly stable SCC and 3 indicating unacceptable SCC. However, the short duration of the test may not indicate segregation that occurs over a longer period of time. The subjective nature of the VSI determination further limits the precision of the test. Though visual stability index (VSI) which is a qualitative measure of the fresh concrete’s ability to resist segregation does not quantify any single property of the concrete, but it can be used as a measure of the relative quality of several similar SCC mixes. The fresh property tests also help to assess and determine consistent quality control of the mixes from batch to batch. Thus, VSI = 0 is for high-quality SCC obtained by a mix that produces a concrete patty with aggregate dispersed to the edge of the concrete with no visible free

High-Performance Concretes

585

water bleeding beyond the edge of the patty. Concrete with a good VSI should not be considered resistant to segregation. The VSI = 1 is for acceptable SCC; whereas, VSI = 2 is a case of marginal or borderline quality and qualified QC personnel should evaluate this mix further before advising acceptance or rejections and corrective action on batches that are to follow. For the SCC mix exhibiting a poor VSI = 3 rating further investigation is warranted or it is rejected. The visual stability index ratings vary from 0 to 3 are summarized in Table 16.3. Table 16.3

Visual stability index (VSI) ratings criteria (ASTM C 1611)

VSI Ratings

Visual stability index criteria

0 = Highly Stable

No evidence of segregation or bleeding; very good aggregate distribution and materials carried to the outer edge of the slump flow without bleeding

1 = Stable

Mix starts to exhibit a mortar halo and possibly some bleed water/ separation observed as a sheen on surface of concrete

2 = Unstable

Mix exhibits more separation, i.e., slight mortar halo ≤ 10 mm, and/ or uneven distribution of aggregate, e.g., an aggregate pile in the center of the concrete mass.

3 = Highly Unstable

Mix showing all the signs of segregation, separation, bleeding, and instability; typically, segregation is indicated by a large mortar halo >10 mm and/or a large aggregate pile in the center of the concrete mass.

VSI = 0

VSI = 1

VSI = 2

VSI = 3

Fig. 16.5

Visual stability index ratings

586

Concrete Technology

16.4.3 TEST PROCEDURES FOR SELF-COMPACTING CONCRETE (SCC) This section describes the various tests generally performed on self-consolidating concrete (SCC). The physical characteristics of SCC as determined using these tests are critical for ensuring quality structures that are safe, durable and economical. 1. Testing fresh concrete As discussed earlier self-compacting concrete is characterized by its special properties in fresh state namely flowability, viscosity, blocking tendency, self-leveling, and stability of mixture. However, the tests for filling and passing abilities of self-compacting concrete are generally sufficient to monitor production quality at the site. Because SCC flows so readily, the flowability is measured in terms of spread instead of slump, i.e., instead of measuring the slumping distance vertically; the mean spread of the resulting concrete patty is measured horizontally as shown in Fig. 16.6. This number is recorded as the slump flow.

(a) Slump flow test

Fig. 16.6

(b) Spread of concrete patty

Measurement of flowability or spread of the resulting concrete patty

A number of tests have been suggested to evaluate the properties of fresh SCC but the commonly used field methods are: Slump flow/inverted slump flow which also includes visual stability index (VSI) and T500 test methods. ASTM has standardized three tests for SCC namely the J-Ring test for passing ability, the slump flow test for flow ability, and the column segregation technique for segregation resistance. The European Guidelines for Self-Compacting Concrete Specification, Production and Use, 2005, has standardized four tests namely, slump flow test with T500, V-funnel and T5 Tests, L-box test and sieve stability test. In evaluating the workability of SCC, tests should measure filling ability, passing ability, and segregation resistance independently. Such an approach is preferred to pass/fail-type tests that measure multiple aspects of workability. Measuring each property individually provides a more direct insight into the performance of the concrete and allows more effective troubleshooting. These advantages outweigh the need to conduct multiple tests.

High-Performance Concretes

587

To evaluate the workability of SCC, the slump flow test (with T500 and VSI) should be used for filling ability, the J-Ring test for passing ability, and the column segregation test or sieve stability test for segregation resistance. For quality control measurements in the field, only the slump flow test is needed in most cases. The slump flow spread should be used to adjust the HRWRA dosage to achieve proper slump flow for self-flow, T500 should be used to measure indirectly plastic viscosity and to detect changes in materials and mixture proportions, and VSI should be used to identify significant segregation. The nine common test methods currently available are divided in to two categories, standardized and nonstandardized tests.

Standardized Tests This category of tests include following tests which are described in this section. 1. Slump flow test with T500 and VSI tests (European Guidelines-2005) and (ASTM C 1611/C 1611M-05) 2. J-Ring test (ASTM C 1621/C1621M-06) 3. V-funnel and T5 tests (European Guidelines-2005) 4. L-box test (European Guidelines-2005) 5. Sieve stability test (European Guidelines-2005) 6. Column segregation test (ASTM C 1610/C 1610M-06)

Non-standardized Tests There are variations in the test apparatus, test procedure, and measurement of test results that are important to interpreting results consistently. The test apparatus mainly varies in the dimensions. The test procedures mainly vary in the amount of time from filling the mould to releasing the concrete for free flow. This period can be lengthened to measure segregation. Whatever period is chosen, it should be consistent for all tests. The following tests fall in this category; except U-box test other tests are not described in this section: 1. U-box test 2. Fill-box test 3. Orimet test 1. Slump flow test (European Guidelines-2005) and (ASTM C 1611/C 1611M-05) This test which is similar to the conventional slump test using the Abram’s cone can be performed at the site and gives good assessment of filling ability (deformability) and also indicates the resistance to segregation, i.e., stability. A slump cone having base diameter, top diameter and height of 200 mm, 100 mm and 300 mm, respectively, is placed centrally on a stiff square non-absorbent plate which shall not to be readily attacked by cement paste or liable to rusting (coated plywood, plastic, metal or similar material) of at least 900-1000 mm side with a minimum thickness of 2 mm and with concentric circles (one with 500 mm diameter) marked on it. The cone is filled with SCC with a scoop in one lift without tamping/rodding, taking care that the sample is well mixed and not segregated in the sampling process. The concrete is struck off level with a trowel.

588

Concrete Technology

After removing the surplus concrete lying on the base plate around the cone, the cone is then raised vertically in 3 ±1 seconds to a height of 230 ±75 mm, allowing the fluid concrete to flow onto the base plate. The slump flow in mm is the average of diameters of the resulting spread concrete or patty measured along the greatest diameter and in the direction perpendicular to it. Large differences between the two diameters indicate a non-level surface, which must be corrected. The result is reported to the nearest 10 mm. SCC generally has slump flow of 560 to 760 mm. Absence of water or cement paste or mortar without coarse aggregate at the edge of spread concrete indicates absence of segregation. This test can be used for SCC with coarse aggregates having the maximum size of less than 40 mm.

(a) Slump flow test set up

Fig. 16.7

(b) Spread concrete patty

Flowability by slump flow test- spread of the resulting concrete patty

Some professionals prefer the inverted orientation of the Abram’s cone as shown in Fig. 16.7. Due to the fluid nature of SCC, the cone when it is in the normal orientation must be very firmly held down to prevent the concrete from leaking out the bottom during filling. This is not the case with the inverted orientation and, therefore, a single person can more easily complete the test. However, the energy acting on the SCC and the friction between the cone and the concrete will be different for the two orientations. Since the difference in results is small, both procedures can be used, but the procedure (normal or inverted) used to qualify the mix design should be used during construction QC testing. The slump flow test can be used in both the laboratory and field. For many cases, the slump flow test is the only test needed in the field for quality control. The slump flow spread should be used to adjust the HRWRA dosage to ensure the ability of the concrete to flow under its own mass. T500 should be used in the laboratory for developing and qualifying mixtures to assess plastic viscosity and should be used in the field to detect unexpected changes in materials and mixture proportions. The VSI can be used to identify the cases

High-Performance Concretes

589

of severe segregation; however, it is not reliable as an assurance of adequate segregation resistance. Mixtures with high VSI should be investigated further but not necessarily rejected. T500 Test This is another slump flow test, which measures the rate at which the SCC flows, i.e., the time taken by the slump flow patty to reach 500 mm mark. This time, called T500 time, provides a relative measure of the plastic viscosity of the SCC, for similar mix designs. Higher T500 values indicate greater viscosity and better stability and a lower time indicates greater flowability. The T500 is determined during the slump flow measurement. A stopwatch with least count of 0.05 seconds is started as the Abram’s cone is first lifted. When the SCC flow reaches 500 mm diameter circle marked on the base plate, the stopwatch is stopped. The time is recorded to the nearest 0.1 second. This time should be between two to five seconds. Visual stability index (VSI) test The visual stability index ranking on a scale of 0 to 3 recommended by ASTM provides an approximate visualization of concrete flow; however, it is not adequate to evaluate segregation resistance. This rating is based on the visual inspection of the slump flow patty immediately after it stops flowing. The VSI rating mainly reflects the ability of the concrete to flow laterally. More specifically, it characterizes whether the paste exhibits adequate rheology to move aggregates to the periphery of the slump flow patty and to prevent a mortar halo and whether the concrete is susceptible to severe bleeding. The rank 0 indicates highly stable SCC and 3, the unacceptable SCC. The comparison of appearance of the patty with standard descriptions or representative pictures of the surface bleed, mortar halo, and aggregate distribution can be useful. In terms of rheology, yield stress is the fundamental difference between the workability of SCC and conventionally placed concrete. The static yield stress must be sufficiently high to prevent segregation while the dynamic yield stress must be sufficiently low for self-flow. The visual stability index ratings are summarized in Table 16.3. 2. J-ring test (ASTM C 1621/C1621M-06) This simple modified slump test measures the filling and passing abilities of SCC with aggregate of nominal maximum size of 25 mm. In addition to slump cone, the test uses an open rigid steel ring of rectangular cross section 30 × 25 mm of 300 mm diameter, called J-Ring. Vertical holes drilled in the ring allow threaded plain or deformed reinforcing bars to be attached to the ring to support it. Bar can be of different diameters and spacing of the bars can be adjusted in accordance with normal reinforcement considerations, i.e., three times the maximum aggregate size might be appropriate. However, the arrangement of reinforcing bars of constant size placed at spacing stipulated in ASTM C 1621 is more practical because it allows the same J-Ring apparatus to be used in all cases without adjustment. Typically, the ring may be supported on sixteen 16 mm diameter bars of 100 mm height spaced uniformly along the circumference of the ring. The J-Ring shown in Fig. 16.8 fits around the base of a standard slump cone. The SCC mix has to pass through the bar obstacles in the J-ring

590

Concrete Technology

without segregation of paste and coarse aggregates. The slump flows, with and without J-ring, are measured and the difference calculated. The spread diameter of the well-proportioned SCC is approximately the same with and without the J-ring. Typical spread values range from about 500 to 750 mm.

(a) J-Ring test set up

Fig. 16.8

(b) Result of J-ring test

Filling and passing ability of self-consolidating concrete by J-ring

The test is performed in a similar manner as the cone test with cone placed centrally inside the J-ring. After the initial slump flow measurement (d1), the J-Ring is placed in the center of the slump flow base plate. The Abram’s cone (using the same cone orientation, i.e., upright or inverted as used in normal slump flow test) is placed inside the J-ring and is filled in one lift. The cone is raised to a height of 230 ±75 mm in 3 ±1 seconds. The SCC flows through the reinforcing bars of the J-ring and onto the slump base plate. The largest diameter dm of the slump flow patty and the one at right angles to it dp are measured to the nearest 5 mm. The average of these two diameters of the slump flow patty provides the J-ring flow, d2. When a halo is observed in the slump flow patty, it shall be included as part of the diameter of the slump flow patty. The tests with and without the J-ring is completed within six minutes. In addition to the determination of average diameter of slump flow patty, the difference in heights of the concrete just inside and outside J-ring bars is measured in mm at four locations. The average of differences between the heights of concrete just inside the and outside of the J-ring measured at four equally spaced locations is calculated to assess the passing ability of SCC. An average difference less than 25 mm indicates good passing ability with no visible blocking and a difference greater than 50 mm indicates poor passing ability with noticeable to extreme blocking. In the average difference range of 25 to 50 mm, the blocking is minimal to moderate. Generally, an average difference in heights of the concrete inside and outside of J-ring bars should be less than 10 mm. The J-ring test is a simple and effective test for independently measuring the passing ability and should preferably be performed with the slump cone in the inverted position. The difference in height between the concrete inside and

High-Performance Concretes

591

outside the J-ring is an indication of passing ability or the degree to which the passage of concrete through the bars is obstructed. Based on the appearance of the concrete after the test visual blocking index (VBI) rating can be assigned in accordance with Table 16.3. The J-ring can distinguish the ability of concrete to flow through obstacles better than the L-box or U-box. The J-ring slump flow values can be used for quality control instead of using both the J-ring slump flow and the unrestricted slump flow. However, the L-box test is generally favored because of the availability of more field experience with the L-box. 3. V-funnel time test (European Guidelines-2005) The V-funnel flow time is defined as the time taken by a standard volume of SCC to pass a narrow opening in a V-shaped metal funnel (not liable to rusting or cement attack) shown in Fig. 16.9. Since the test measures the flow time, i.e., the ease of flow of the concrete; shorter flow times indicate greater flowability. The V-funnel test is used to measure the filling ability of self-compacting concrete with a maximum aggregate size of 20 mm and can also be used to judge its stability, i.e., the resistance to segregation. The test which gives an indication of the filling ability (flowability) is suitable for SCC with aggregate of maximum size of up to 20 mm provided that blocking and/or segregation do not take place. The flow time of the V-funnel test is to some degree related to the plastic viscosity. The V-funnel is made to the dimensions (tolerance ±1 mm), fitted with a quick release, watertight gate at its base and supported so that the top of the funnel is horizontal. With the apparatus placed on a firm ground the moistened funnel with trap door closed is filled completely with fresh concrete without tamping or tapping. The concrete is struck off level with the top and the trap door is opened within (10 ±2) seconds from filling the funnel and the time taken by concrete to flow down (emptying the funnel) under gravity is recorded in seconds to an accuracy of 0.1 second. The whole test is performed within five minutes. A V-funnel flow time between 8 and 12 seconds is considered appropriate for self-compacting concrete. The inverted cone shape restricts flow and prolonged flow times may give some indication of the susceptibility of the mix to blocking. High flow time can also be associated with low deformability due to a high paste viscosity, and with high inter-particle friction. When the concrete is allowed to rest for some time, the segregation of concrete will show a less continuous flow with an increase in flow time. The greater the increase in flow time, the greater will be the concrete’s susceptibility to segregation. Further, non-uniform flow of concrete from the funnel suggests a lack of stability or segregation resistance. Thus, the test can be used as a pass/fail test measuring multiple aspects of workability; it does not provide an independent indication of filling ability, passing ability, or segregation resistance; thus is ineffective for troubleshooting in case of problematic mixtures with high V-funnel times.

592

Concrete Technology

Although the apparatus is simple, but the test frame is large, bulky, and must be placed on a level surface; the effect of the angle of the funnel and the wall effect on the flow of concrete are not clear. 490mm

4 425mm

1 150mm 75mm 65mm (a) V-funnel 230mm

465 5mm

150 0mm 75 mm

(c) V-funnel Test T

(b) O-funnel

Fig. 16.9

Shape, dimensions (mm) of funnels: (a) V-funnel. (b) O-funnel and (c) V-funnel test

4. Box-type tests These tests are used for assessing the filling (flowing) and passing ability of self-compacting concrete to flow through tight openings including spaces between reinforcing bars and other obstructions without segregation or blocking. The test is suitable for SCC with coarse aggregates of maximum size of less than 25 mm. The apparatus consists of two chambers A and B with a gate at the intersection of two chambers. The concrete is allowed to flow on the release of gate from the chamber A to chamber B passing through the bar obstructions placed between two chambers as shown in Fig. 16.10(a). The time taken and the height to which the chamber B is filled are noted. The amount of concrete which has passed through the obstacle measures the self-compactability of the mix. There are two commonly used types of box-test methods; the two bar test and the three bar test. The three bar test simulates more congested reinforcement. U-type test The U-type test shown in Fig. 16.10(c) may be considered to be the appropriate, as it uses small amount of concrete compared to others. In

High-Performance Concretes

593

this test, the degree of compactability can be indicated by the height that the concrete reaches after flowing through obstacles. Concrete with the filling height of over 300 mm as shown in Fig. 16.10(c) can be judged as selfcompacting. Generally, the concrete is considered to be self-compacting if the filling height is more than 85 per cent of the maximum height possible.

(a) Box type filling unit

(b) Flow obstacle

680 mm

cpen the center gate obstacle 200

280

Height, H > 300 mm (c) U-test apparatus

Fig. 16.10

Shape of filling units and flow obstacle

L-box test (European Guidelines-2005) This test measures the filling and passing abilities of SCC. The apparatus shown in Fig. 16.11 consists of a rectangular-section L-shaped box of a stiff non-absorbent, non-corrodible material like steel, plastic or plywood with flat and smooth surfaces. The vertical and horizontal portions are separated by a freely moving sliding gate in front of which obstructions in the form of vertical lengths of reinforcement bar are fitted. The horizontal portion of the box is marked at 200 mm and 400 mm from the gate. The vertical portion is provided with a removable hopper. The test procedure consists in placing the L-box with sliding gate closed on a level, firm horizontal base. The concrete is poured from a container in to the filling hopper of L-box and the vertical column is filled in one lift without

594

Concrete Technology

rodding or any other consolidation. The concrete in vertical column is allowed to stand for (60 ±10) seconds. Any segregation is recorded and the gate is raised to allow SCC to flow in to the horizontal trough. The sliding gate is lifted and the stopwatch is started simultaneously. The concrete is allowed to flow out from the vertical column into the horizontal section of the box through the bar obstructions. The times T200 and T400 taken by the concrete to reach the 200 and 400 mm marks in the horizontal trough, respectively, are recorded. When the concrete has stopped flowing, the depth of the concrete fill at beginning and at the end of the horizontal portion of the L-box (H1 and H2) are measured to an accuracy of 0.01 mm as the average of depths at three positions equally spaced across the width of the box. The test is performed within five minutes.

Gate Rebar

100

20 0

600

150

800 Dimensions in mm H1

Measurement Points

H2

(a) General assembly of L-box

Fig. 16.11

(b) L-box test

Dimensions and typical design of L-box and performance of the test

The L-box test measures both passing ability (PA) and filling ability (FA) of SCC because the extent to which concrete flows down the horizontal portion of the box depends on the yield stress (filling ability) of the concrete and the extent of blocking caused by the row of bars. It enables visualization of the flow of the concrete in the test, especially any blocking behind the bars. The stability, i.e., resistance to segregation can be assessed visually. A concrete sample with coarse aggregate particles that reach the far end of the horizontal part of the box exhibits good resistance to segregation. However, there is no evidence of what effect the wall of the apparatus and the consequent ‘wall effect’ might have on the concrete flow, but this arrangement does, to some extent, replicate what happens to concrete on site when it is confined within formwork. The passing ability (PA) typically expressed as H2/H1 generally lies in the range 0.8 to 0.85. However, passing ability of greater than 0.9 is preferable. Passing ability is indicated by visual inspection of the area around the bar obstructions, even distribution of aggregate indicates good passing ability. The major advantages of the L-box test over J-ring test is that the larger amount of mass available to push concrete through the bars is more representative of field conditions than in the J-ring test. The relationship between the test results

High-Performance Concretes

595

and field performance is better established than for the J-ring test. However, the test does not the measure the passing ability sufficiently independent of filling ability, i.e., it does not distinguish between passing ability and filling ability. Therefore, the test is essentially a pass/fail test because it is not clear whether concrete with a low blocking ratio exhibits inadequate filling ability, passing ability, or both. The term blocking ratio generally defined as the ratio of concrete height in the horizontal portion to that in the vertical portion of the box is a misnomer, because higher blocking ratios correspond to less blocking, greater filling ability, or both. A term such as passing ratio is more appropriate. The L-box does reflect field conditions; however, the number of bars through which the concrete must pass is limited. The J-ring has more bars and would likely exhibit less variability from one test to another. The test can be used as a reference test for passing ability. However, if the L-box results are used to qualify an SCC mixture design, then an L-box with the same rebar spacing and dimensions should be used during production QC testing. The test is not as simple as the slump flow test, the test apparatus is relatively bulky, difficult to clean, and thus, not well-suited for use in the field. The L-box test is preferred to the U-box test. 5. Column segregation test (ASTM C 1610/C 1610M-06) The column segregation test provides an independent measurement of stability or segregation resistance by replicating static conditions in formwork and quantifying the segregation of coarse aggregate after a fixed time. However, the test does not measure dynamic stability. The standardized column segregation test apparatus (ASTM C1610) shown in Fig. 16.12 consists of a 200 mm diameter, 650 mm long vertical PVC pipe split into four 165 mm long sections. In an alternative arrangement, two middle sections are replaced with single 330 mm long section. The section are clamped together to form a water-tight seal. The assembled column is filled with SCC in one lift without rodding or any other external compaction effort and allowed to rest undisturbed for 15 minutes after placement as most but not all segregation occurs within the 15 minute duration of the column segregation test. Using collector plate each section is removed individually with the concrete inside. The contents of the top and bottom pipe sections are transferred to separate no. 4 sieves. The contents of the middle section(s) are discarded. The concrete from each section is washed over the ASTM no. 4 sieve to remove all paste and fine aggregate, leaving behind only clean coarse aggregates on each sieve. The coarse aggregates retained on each sieve is collected in a separate container and dried in an oven or microwave to the constant mass and weighed. The segregation is computed as a function of the relative amount of aggregate in the top and bottom quarters of the pipe. The use of only the top and bottom sections is the preferred approach because it requires less work and the relative difference in aggregate mass in the middle two sections is likely to be low in most cases.

596

Concrete Technology

A non-segregating mix will have a consistent aggregate mass distribution in each section. A segregating mix will have higher concentrations of aggregate in the lower sections. For the column segregation test, the maximum segregation should be less than 15 per cent for most cases but may need to be reduced in some applications. The sampling conditions should be well defined.

Fig. 16.12

Column static segregation test apparatus for SCC

The test takes at least 30 minutes to perform including filling the column, allowing the concrete to remain undisturbed for 15 minutes, collecting the concrete from the column, washing and sieving the aggregate, and drying the aggregate to its saturated surface-dry condition with towels. If the aggregate is oven-dried, results are not available for at least several more hours. Determination of mass of the aggregates in saturated-surface dry condition may increase the variability of test results. Static stability or segregation resistance measured with the column segregation test and sieve stability test are closely related. The column segregation test is relatively difficult and time consuming to perform and requires balance; hence the test is not considered appropriate as a rapid field acceptance test. Thus, the test is conducted in a laboratory. 6. Sieve segregation resistance test (European Guidelines 2005) The sieve stability or segregation resistance test, also called wet-sieve stability test, is used to assess the degree to which a self-compacting concrete mix is likely to segregate. The test measures mainly the static segregation.

High-Performance Concretes

597

After sampling, (10 ± 0.5) liter of fresh concrete is placed in the sample container made from plastics or metal having an internal diameter (300 ± 10) mm and the lid fitted on it. The sample is allowed to stand undisturbed in the sample container with lid on for (15 ± 0.5) minutes, any segregation or bleeding that occurs during this period is due to static segregation. Segregation of coarse aggregate and bleeding lead to more mortar and paste at the top of the sample. A quantity of (4.8 ± 0.2) kg of top part of the concrete sample (including any bleed water) is then poured onto the centre of a perforated plate test sieve shown in Fig.16.13 with frame diameter 300 mm, height 40 mm and 5 mm square apertures conforming to ISO 3310-2. The amount of mortar passing the sieve depends to some extent on dynamic segregation resistance because viscous, cohesive mortar is less likely to pass through the sieve. Since this evaluation of dynamic segregation is determined after the concrete has remained undisturbed for 15 minutes, it may not reflect the dynamic segregation resistance of the concrete during placement conditions where the concrete is sheared continuously.

Fig.16.13

Sieve stability test

598

Concrete Technology

After the concrete has rested (120 ± 5) seconds on the test sieve, the sieve is removed vertically without agitation and the weight of material which has passed through the sieve is recorded on a weighing machine to an accuracy of 20 g. The segregation ratio is then calculated as the proportion of the sample passing through the sieve. Higher the amount of mortar passing through the sieve greater will be the probability for segregation. It is generally noticed that if the percentage of mortar which has passed through the test sieve, i.e., the segregation ratio is between 5 and 15 per cent of the weight of the sample, the segregation resistance is considered satisfactory. Below five per cent, the resistance is excessive, and likely to affect the surface finish; there may be possibility of blow holes. For the typical percentages of mortar passing through the sieve, the degree of potential segregation can be interpreted as given in Table 16.4. Table 16.4

Segregation potential with percentage of sample passing the sieve

Sample passing the sieve, per cent 30

Potential of segregation Could be too cohesive/viscous Provides optimum resistance to segregation Likelihood segregation Possibility of severe segregation.

The test requires approximately 20 minutes to perform which includes filling the bucket, waiting for the 15 minute rest period, pouring the concrete on the sieve and allowing it to remain there for two minutes, and measuring the final mass of material passing the sieve. When the sieve stability test is used in the laboratory to qualify mixture proportions, mixtures should be prepared with the range of water contents and slump flows expected during production. If these mixtures exhibit adequate segregation resistance and the slump flow test is used in the field to control concrete rheology indirectly, it is not necessary to use the sieve stability test in the field. The test is simple to perform and provides an independent measurement of segregation resistance. However, the test conditions are neither directly representative of field conditions for static segregation nor fully take in to account dynamic stability. The test is slow and requires an accurate weighing machine, making it unsuitable for use as a rapid acceptance test in the field. Currently, the most commonly used test methods during construction are slump flow, T500 and J-ring test. These three parameters combine to provide information on filling ability (flowability), passing ability and stability of SCC mixtures.

16.4.4

Structural Properties of Hardened SCC

The compressive and tensile strengths, modulus of elasticity and durability of well-designed self-compacting concrete are in the same order of magnitude as the

High-Performance Concretes

599

conventional vibrated concrete. In general, the SCC bond strengths expressed in terms of the compressive strengths are higher than those of conventional concrete.

16.4.5

Mixing and Placing of Self-Compacting Concrete

Generally, the locally available materials and the equipment commonly available at the plants can be used for producing SCC economically. This includes many of the procedures common to local precast plants, such as methods of concrete mixing and transportation of the concrete from the mixing plants to the casting area. A well-designed SCC can flow horizontally a distance of 15 to 20 m without segregation and it may have a free fall of as much as 8 m without segregation. However, it is recommended that the distance of horizontal flow be limited to 10 m and the vertical free fall distance be limited to 5 m. For deck slab of a bridge, it would be difficult for the SCC to flow too far. This could be handled by designing an SCC with a lower slump flow. When pouring a new layer of SCC on an old layer, the bond between the old and new SCC is equal to or better than in the case of conventional vibrated concrete. If the period passed between the layers is longer than 30 minutes, these cold joints or pour surfaces can be eliminated with a limited amount of active compaction (by vibration with hand held pokers for a short period of time), causing the two pours to integrate together. This procedure may also be necessary at the intersection between the release points where the concrete flows meet.

16.4.6

Formwork System

All commonly used formwork materials are suitable for SCC. For better surface quality of SCC, wood is preferable to plywood, and plywood to steel. SCC is more sensitive to temperature during the hardening process than the conventional vibrated concrete. Due to the cohesiveness of properly designed SCC, the formwork need not be tighter than that for conventional vibrated concrete.

16.4.7

Surface Finishing and Curing

As there is little or no bleeding water at the surface, SCC tends to dry faster than conventional vibrated concrete. SCC should be cured as soon as practicable after its placement to prevent surface shrinkage cracking. Moreover, since SCC often has a very low water-cement ratio (generally below 0.40), moist curing is desirable to provide a continuous water source to the concrete as it cures. Moist curing ensures that the capillary pores are filled and the hydration reaction continues to take place. Though increase in coarse aggregate contents can reduce plastic shrinkage, but this may affect the fresh properties of the SCC mix. Sometimes release of air from fresh concrete may cause surface quality problems as shown in Fig. 16.14. Drying shrinkage for SCC is very close to that of conventional concrete. However, self-desiccation could cause plastic shrinkage during the first 24 hours in a very low water-to-cement ratio mix. This self-desiccation is the result of hydrating cement

600

Concrete Technology

(a) Surface bubbles in the concrete

Fig. 16.14

(b) Blowholes in hardened concrete

Surface quality problems due to release of air from fresh concrete

particles consuming the available free water during the hydration process. As the water is consumed, the capillary pores within the concrete remain partially empty, causing the internal relative humidity to drop considerably. This could lead to bulk shrinkage, resulting in internal micro-cracking and affecting the overall strength and durability of the product.

16.4.8

Advantages of Using Self-compacting Concrete

Although at the start of a project the production of SCC is little more expensive than conventional concrete and it is difficult to maintain the desired consistency over a long period of time. However, SCC offers many advantages for the precast, prestressed concrete industry and for cast-in-place construction: 1. Easier and rapid placement in members with dense reinforcement and complicated formwork, as shown in Fig. 16.15(a) results in faster construction and reduction in cost of production. 2. Reduction in on-site manpower for all operations. 3. Relatively low water-to-cement ratio results in rapid strength development, improved quality, strength and durability. 4. Produces good surface finish particularly for slabs as shown in Fig. 16.15(b). 5. Cost efficient and rational solution in thin overlays on prefabricated elements. Thinner concrete sections can be cast easily. 6. Reduced noise levels in the plants and at construction sites due to absence of vibration. 7. Safer and cleaner working environment.

16.4.9

Disadvantages for Self-Compacting Concrete

1. Tight joints in the formwork for deep elements are required for SCC with high fluidity (pressure). But the problem can be solved by pouring concrete in smaller lifts. 2. Increased formwork pressure requires slower casting rate for walls. 3. Presumptive poor surface quality of walls leads to increased costs of finishing works when removable formwork is used.

High-Performance Concretes

601

(a) Casting an SCC slab

(b) Leveling a SCC slab with a skip flot

Fig. 16.15

Casting and leveling of a SCC slab

4. Tight schedule for delivery is to be maintained in order to avoid undesirable cold joints or pour surfaces between newly cast layers. 5. SCC with low water-to-cement ratio results in rapid drying and thus requires intense curing to avoid increased plastic shrinkage cracking shown in Fig. 16.16(a).

(a) Plastic shrinkage on a slab

Fig. 16.16

(b) Settlement cracks above rebars

Examples of heavy plastic shrinkage on slab, and settlement cracks parallel with and above reinforcement bars in construction using SCC

602

Concrete Technology

6. SCC is more prone to settlement cracking shown in Fig. 16. 14(b). 7. On the part of ready-mix producer, SCC could lead to added production costs as it requires trained and experienced plant personnel, and more care than the conventional vibrated concrete to ensure adequate quality control. This may lead to decreased produtivity and increased responsibility for technical performance problems at the site. 8. Though most of the common concrete mixers can be used for producing SCC, but the mixing time may be longer than that for the conventional vibrated concrete. 9. Since SCC is more sensitive to the total water content in the mix, it is utmost necessary to take into account the moisture in the aggregates and water content in the admixtures before adding the remaining water in the mix. The mixer must be clean and moist, and contains no free water. 10. Admixtures for the SCC may be added at the plant or at the construction site. There is cost benefit in adding the admixtures at the site. In transportation extra care must be taken for long delivery distances. EFNARC has prepared specifications and guidelines for self-compacting concrete for European Nations for the use both at site and in precast concrete works. The test methods have been standardized. Some countries like Germany, Norway, Turkey and Sweden have their own national companion codes. There is no British national codes explicitly covering self-compacting concrete, however, it is possible to specify SCC within the BS 8500 system using the proprietary concrete category. Based on the current state-of-the-knowledge, the following performance specifications for SCC are generally stipulated for proper mix design and testing: Workability in terms of slump flow: Mix remain flowable: Pumpable mix remain flowable:

> 700 mm ≥ 90 minutes ≥ 90 minutes through pipes ≥ 100 m long

Mechanical properties 28-day compressive strength: Creep and shrinkage:

Similar to HPC Similar to HPC

Durability parameter:

≥ HPC

16.4.10

Recommended Range of Constituents

The current recommended ranges of constituents for mix proportions for highstrength self-compacting concrete are given in Table 16.5. These guidelines are intended to provide an indication of the typical range of constituents in SCC. These mix proportions are in no way restrictive and many SCC mixes may outside this range for one or more constituents.

High-Performance Concretes Table 16.5

603

Recommended mix proportions for high-strength self-compacting concrete

Recommendations

NFNARC

Recommended range (based on current knowledge)

I. Mix proportions: ranges of ingredients Fines content (2

2 to 5

–

0 to 10

1 to 25

8 to 15

0.8 to 1.0

U-Box type-300 mm

–

4.5-6.0

III. Structural properties of hardened concrete 28-days compressive strength 91-days compressive strength 28-days splitting tensile strength Elastic modulus, (GPa) Shrinkage strain (×10-6)

– – – – –

40 to 80 MPa 55 to 100 MPa 2.4 to 4.8 MPa 30 to 36 600 to 800

604

Concrete Technology

However, if silica fume is used in low water-binder ratio SCC mix from strength considerations it may reduce the workability to an unacceptable level for self-consolidating requirements. To have much higher level of workability required for SCC, a higher dosage of superplasticizer is needed which may have adverse side effects. Therefore, proper selection of additives and superplasticizers is required. A typical mix consisting of different sizes of aggregates is represented in Fig. 16.17.

Fig. 16.17

16.4.11

Typical mix constituents (by weight) for self-compacting concrete

SCC Mix Design Development

The aim of self-compacting concrete mix proportioning is to develop procedures that consistently produce high quality concrete using locally available materials and processes. This mix design development may require numerous trial and modification procedures that iterate on the desired responses from a specific fresh concrete property or a combination of fresh concrete properties. Once satisfactory fresh concrete performance is achieved, the mix is evaluated and refined for hardened concrete performance. As discussed in the foregoing sections, self-compacting concrete mix proportioning requires in general a low coarse aggregate content, increased paste content, high powder content, low water to powder ratio, high doses of superplasticizer and viscosity modifying agent; and thus, there are large number of variables to be considered. 1. Extension of conventional mix proportioning For the case when the amount of cement and the target water to cement ratios are fixed from durability and other considerations, and mineral additives are not used in the production of SCC, the reduction in the number of the variables simplifies the mix design process considerably.

High-Performance Concretes

605

According to the IS 10262-2009 guidelines for concrete mix proportioning, the provisions of IS 456:2000 through reference constitute its own provisions. The most important provision is concerning the durability; as per IS 456:2000 stipulations from durability considerations, for reinforced concrete the amount of cement shall not be less than 300 kg/m3 of concrete. In the conventional concretes generally used in India, the cement content varies between 325 to about 400 kg/m3. In view of the above observations, with fixed cement content of say 325, 350, 375, 400 kg/m3, etc., and using the locally available appropriate materials, it is possible to tailor the properties of self-compacting concrete to meet the demands of any particular application, as a substitute of conventional concrete. Typical flowing mix proportions with fixed cement content of 350 kg/m3, but without mineral additives are given in the following table. Table 16.6 Cement, kg/m3

Water, kg/m3

Sand, kg/m3

350

125

800

Gravel, HRWR, W/C ratio kg/m3 l/m3

750

2.95

0.357

CA/FA ratio

FA/TA ratio

0.937

0.516

In this section, proportioning of self-compacting concrete mix with fixed cement content of 350 kg/m3 and slump-flow value (spread or patty diameter) in the range of 600+ mm has been explored using locally available materials. With this cement content, the 28 days concrete strength expected is of the order of 35 MPa. The SCC mixes can be developed through trial batches and adjustments are made to the mix proportions based on the test results until consistently satisfactory fresh concrete performance is achieved. In a general case, the basic SCC trial mixes can be based on the mix proportions for normally compacted concrete (NCC). The fixed and variable/trial batch parameters given in the following table may be considered for the basic/preliminary mix proportioning of SCC. Table 16.7 1. Fixed parameters Amount of cement

350 kg/m3

Water/powder ratio by mass

0.38

Stabilizer (cohesion agent) content

0.85-0.9 per cent of cement

2. Trial or variable parameters Amount of fly ash content

(30 and 35 per cent of cement)

Fine to coarse aggregate ratio by mass

(1.0 and 1.1)

Superplasticizer content by mass of powder (i.e., cement (0.9 and 1.0 per cent) and fly ash)

606

Concrete Technology

The following procedure for preliminary mix proportioning of SCC may be adopted: 1. Select locally available materials with the best grading and shaped aggregate economically possible. (a) G-43 grade Portland cement. (b) Normal-density coarse aggregate, preferably of maximum nominal size of 10/20 mm. (c) Rounded well-graded fine aggregate. (d) Pulverized fly ash as filler material: use good quality fine flyash. 2. Determine following properties of envisaged ingredients as per relevant code. (a) (b) (c) (d) (e) (f)

Specific gravity of cement. Average values of specific gravity and absorption of the coarse aggregate. Average specific gravity and absorption of the fine aggregate. Specific gravity of fly ash. Specific gravity of the superplasticizers (provided by the supplier). Specific gravity of stabilizer (specified by the supplier).

3. Entrapped air content may be considered as two per cent. 4. Calculate quantities of the ingredients for each of the trial mixes by absolute volume method. 5. Mix the materials as per standardized procedure. (To avoid variations due to mixing, the mixing process of trial mixes is generally standardized). 6. Perform slump flow test; measure the average slump flow in mm. 7. Check the concrete spread/patty for stability of mix which appears in the form of segregation and bleeding. 8. Fill three 150 mm cubes with concrete and test them after 28 days of curing; calculate average compressive strength The above procedure is repeated for other trial mixes. General recommendations The moisture control and consistency of materials and processes are the keys to making quality self-consolidating concrete. Ongoing testing is imperative to either confirm that consistency is being maintained, or to determine when, and what, adjustments need to be made. Following guidelines may prove useful: Mix proportioning 1. Since SCC is less tolerant of changes than conventional concrete. Small changes to the mix, especially the amount of water, can have large effects on its plastic properties. Consequently, tighter control limits on SCC are required than on conventional concrete; particularly on raw materials variability, batching accuracy and moisture measurement and compensation. 2. If the concrete is not required to flow very far from the pouring site through light reinforcement, then a flow of 500–600 mm may be adequate. However, if the concrete must flow large distance through a highly reinforced element,

High-Performance Concretes

3. 4.

5.

6.

607

then a flow of 700–750 mm may be adequate. Generally, better form finishes are achieved with concrete that has a higher slump flow and lower viscosity. Manufactures instructions for suitability of a particular admixture, its dosages and resulting interactions with cement is the best guide. It is advisable to make an additional batch with an additional 5 L/m3 of water to determine the robustness of the mix to moisture changes. If the mix segregates, the mix may be adjusted to obtain greater stability, especially if sand moisture is controlled to ±1 per cent, which works out to about 7.5 L/m3 for many SCC mixes. The performance a mix design should be confirmed in the laboratory and in small production batches before commercial production. A good laboratory mix design is only as good as the ability of the production plant to reproduce the laboratory results. A good SCC mix requires field trials for fine tuning prior to actual production. The differences in mixing energy between a laboratory mixer and a truck mixer or central mix plant can significantly change the performance of the mix design, especially with respect to air content management, admixture dose response, and slump flow retention. It is good practice to establish the water demand of a mix design by measuring the slump without HRWR. This baseline can be used to troubleshoot the mix if performance changes.

Batching and Mixing Recommended practices for batching and mixing are listed below: 1. Determination and adjustment of moisture in aggregates is crucial. For production of SCC moisture meters for sand are strongly recommended. 2. Once the admixture dosage is established, it should not be changed unless water demand of the materials has changed. 3. The gradings of aggregates should be checked a minimum of once per week or whenever the SCC mix appears to have changed. In case of change in gradations of aggregates mix proportions should be adjusted accordingly. 4. Laboratory trial test should be performed once per month, or whenever the mix appears to have changed, to investigate possible water demand changes. 5. Change in amperage reading is an important indicator of changes in materials. The amperage reading should be stable before adding the SCC admixture. In the absence of admixture (HRWR), amperage reading that corresponds to ‘zero-admixture slump’ which produces good SCC upon later addition of the SCC admixture should be identified; it may serve as bench mark. If the amperage reading is too high, it may be adjusted with water. Similarly, target amperage reading for ‘slump after addition of SCC admixture’ should be established. If the reading is too high it should be adjusted with admixture.

608

Concrete Technology

6. In case of truck mixer, moisture control and water demand are occasionally confirmed by measuring the slump before SCC admixture is added. Changes in ‘zero-admixture’ slump are an indication of changes in sand moisture or water demand. The ‘zero-admixture’ slump should be within ±25 mm of the predefined target. 7. Since the polycarboxylates that are designed to produce highly stable SCC develop workability more slowly than typical high range water reducers, fast mixing leads to poor reproducibility and can induce undesirable excess foaming. 8. Generally 5 L/m3 change in water content in SCC can change the slump flow by 50–75 mm. 2. Formal approaches to SCC mix proportioning There are two basic approaches to mix design: Approach 1 Proportioning with moderate water to cement ratios (e.g.,, 0.45), and use of HRWR and VMA to provide fluidity and increase stability, respectively. Approach 2 Mixes without viscosity-enhancing admixture, but with lower water to cement ratios (e.g.,, 0.35) to reduce free water content and provide stability and use of a relatively high content of HRWR to provide high fluidity. This mix is to be obtained by using the NCC mix as a basis. Typically, this requires a decrease of the coarse aggregate content with a corresponding increase of sand, or increase in the sand-to-paste ratio. This approach will require moderate use of HRWRs and VMAs. Evolution process of three alternate SCC mix proportions consisting of two basic types identified earlier as powder-type SCC and VMA-type SCC, and a combination of a powder and a VMA-type SCC mixes are presented in Table 16.6. The mix proportions for the conventional vibrated or normal concrete as obtained by testing trial batches as per ACI method are given in the table. In the design Type-III cements, 2NS-natural sand, 17A-coarse aggregate are used and a target air content of 6 ± 1.5 per cent is envisaged. (a) Powder-type SCC mixes In a powder-type SCC mix, the concrete fluidity is achieved by using a reduced amount of coarse aggregate and a relatively high amount of HRWR, while the segregation resistance comes from an increased amount of fine aggregate. The high amount of HRWR allows the water to cement ratio to be kept to a relatively low level, say 0.35. The mix development process is illustrated in Table 16.8. The first mix is the baseline SCC trial mix and the succeeding mixes are the adjusted mixes to obtain desired or target response. In a trial mix evaluation, any target value which is violated will result in the proposal being modified or rejected. (b) Combination of a powder and VMA-type SCC mix The target water to cement ratio is 0.4. Because of the increased amount of water in this mix a lower amount of HRWR is used than in the powder-type SCC mix. Compared to powder-type mix, the combined mix design shall have a larger coarse aggregate content and a reduced amount of fine aggregate. The first mix is the baseline SCC trial mix. The succeeding mixes are the adjusted mixes to obtain desired responses.

High-Performance Concretes Table 16.8

Development of medium strength self-compacting concrete mixes

Mix proportions, kg/m3 Cement Water

609

F.A.

C. A.

W/c ratio

Admixtures, (ml/m3) Air en- HRWR training

Remark

VMA

I. Normally compacted concrete 415

166

721

937

0.40

180

360

0

335

129

800

1120

0.39

340

2070

650 0

Base trial mix

II. Powder-type SCC Mixes 415

145

901

818

0.35

90

1130

415

147

956

800

0.35

390

2500 (+)

0

x ≥ 600

75 ≥ x ≥ 60/MPa

Shrinkage

Creep

30 MPa ≥ x

x ≥ 95 MPa

4

x is creep in micro-strain. Strain/pressure unit. Creep mesurements to be taken for a creep period of 180 days.

x is shrinkage in micro strain. Shrinkage measurements are to start 28 days after moist curing and be taken for a drying period of 180 days.

x is modulus of elasticity.

x is compressive strength. The 56-day compressive strength is recommended.

x is in coulombs.

x is average depth of wear/abrasion in mm measured at three different locations. At each location 98 N force, for three 2-minute abrasion periods shall be applied for a total of six minutes of abrasion time per location.

x is visual rating of the surface after 50 cycles.

x is acoustically measured relative dynamic modulus of elasticity after 300 cycles.

Notes

*All the performance characteristics are ascertained by standard test procedure stipulated in relevant ASTM standards.

Note

x ≤ 50 GPa

40 ≤ x < 50 GPa

28 ≤ x < 40 GPa

Elasticity 400 ≥ x

70 ≤ x < 95 MPa

55 ≤ x < 70 MPa

40 ≤ x < 55 MPa

800 ≥ x

0.5 > x

x = 0, 1

3

Compressive strength

2000 ≥ x > 800

x = 2, 3

x = 4, 5

Scaling resistance

Chloride-ion permeability 3000 ≥ x > 2000

80 % ≤ x

2

60 % ≤ x < 80 %

1

HPC performance grade

Grades of performance characteristics for high performance structural concrete

Freeze–thaw durability

Performance characteristic*

Table 16.12

High-Performance Concretes

635

been suggested for each definition parameter. Relationships between performance and severity of field conditions have been estimated to assist designers in selecting the grade of HPC that should be used for a particular project. However, depending on the specific application, a given HPC may require different grade of performance for each performance characteristics. For example, a bridge located in an urban area with moderate climate may require Grade-3 performance for strength, elasticity, shrinkage, creep, and abrasion resistance, but only Grade-1 performance for freezing–thawing durability, scaling resistance, and chloride-ion permeability. Each parameter grade must represent a measure of performance when subjected to a field condition. Using grades to represent performance, an engineer can specify a mixture to yield a desired concrete service life. Each parameter can be independently specified by a grade. The grades start at low performance levels and small enough increments are defined to enable to specify higher quality concrete incrementally. The strength grades start at a performance level that is easily attainable and spans to a superior grade. The definition covers all grades of concrete that can be readily used by the concrete industry. Table 16.5 does not represent a comprehensive list of all characteristics that a good concrete should exhibit. It does list characteristics that can quantifiably be divided into different performance groups. Other characteristics should be checked. For example, HPC aggregates should be tested for detrimental alkali–silica reactivity. Due consideration should also be paid to (but not necessarily limited to) acidic-environments and sulfate-attacks. For a given high performance concrete, the mix design is specified by a grade for each desired performance characteristic. For example, a concrete may perform adequately at Grade-4 in strength and elasticity, Grade-3 in shrinkage and scaling resistance and Grade-2 in all other categories. In view of diversity of strength needs and the variation of strengths used in practice, SHRP has suggested a wide range of strength grades starting at 40 MPa for Grade-1 to greater than 95 MPa for Grade-4. Bridge engineers currently specifying strengths less than Grade-1 may begin the transition to a higher durability and strength concrete by specifying minimum HPC performance grades. The highest level is specified to define the state-of-the-art in highway concrete usage. Static modulus of elasticity grades ranges from a low of 28 GPa for Grade-1 to greater than 50 GPa for Grade-3. The general field environment to which the concrete may be exposed during its working life has been classified in IS:456–2000 into five levels of severity, i.e., mild, moderate, severe, very severe and extreme as described in Table 16.13. Compliance to specified stipulations regarding minimum cement content, maximum water–cement ratio and minimum grade of concrete will ensure adequate performance levels in the given exposure conditions. IS:456–2000 stipulations for following exposures are the following: 1. Freezing–thawing Where freezing–thawing actions under wet conditions exist, enhanced durability can be obtained by the use of suitable air-entraining admixtures. When concrete lower than grade M50 is used under these conditions, the mean total air content by volume of the fresh concrete at the time of delivery into the construction should be

Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal area. Concrete surfaces sheltered from severe rain or freezing whilst wet. Concrete exposed to condensation and rain. Concrete continuously under water. Concrete in contact or buried under non-aggressive soil/ground water. Concrete surfaces sheltered from saturated salt air in coastal area. Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immersed in sea water. Concrete exposed to coastal environment. Concrete surfaces exposed to seawater spray, corrosive fumes or severe freezing conditions whilst wet. Concrete in contact with or buried under aggressive subsoil/ground water. Surface of members in tidal zone. Members in direct contact with liquid/solid aggressive chemicals.

Exposure conditions

280

260

250

240

220

Min. cement content, kg/m3

0.40

0.45

0.50

0.60

0.60

M25

M20

M20

M15

—

Max .water– Min. grade cement ratio of concrete

Plain concrete

360

340

320

300

300

Min. cement content, kg/m3

0.40

0.45

0.45

0.50

0.55

Max. watercement ratio

Reinforced concrete

M40

M35

M30

M25

M20

Min. grade of concrete

Minimum cement content, maximum water-cement ratio, and minimum grades of concrete for different environmental exposure conditions with 20 mm maximum size normal weight aggregates

*Cement content is irrespective of the grades of cement. Mineral additives are taken into account into cement content and water-cementing ratio suitably.

Note

Extreme

Very severe

Severe

Moderate

Mild

Environment class

Table 16.13

High-Performance Concretes

Nominal maximum size of aggregate, mm Entrained air, per cent

20 5±1

637

40 4±1

Since air-entrainment reduces the strength, suitable adjustments may be made in the mix design for achieving required strength. 2. Exposure to sulfate attack The total water-soluble sulfate content of concrete mix expressed as SO3, should not exceed four per cent by mass of cement in the mixture except for the concrete using super-sulfate cement complying IS: 6909. For the very high sulfate concentration conditions, some form of lining such as polyethylene or polychloroprene sheet; or surface coating based on asphalt, chlorinated rubber, epoxy; or polyurethane materials should also be used to prevent access by the sulfate solution. 3. Chloride-ion penetration The total amount of acid-soluble chloride content (as Cl) in normal concrete at the time of placing shall be limited to 0.4 and 0.6 kg/m3 of concrete containing metal but cured at elevated and normal temperatures, respectively. 4. Alkali-aggregate reaction ASR can be controlled by (i) limiting cement content (ii) using low alkali ordinary Portland cement having total alkali content not more than 0.6 per cent (as Na2O equivalent).

16.9

STANDARD TEST PROCEDURES

SHRP has identified test procedures to ascertain performance for all the eight definition parameters. However, while considering the severity of field conditions for estimating the performance of concrete in the service life of actual structures, the results of laboratory tests on concrete durability should be used with caution because the cracking behavior of concrete is highly dependent on the specimen size, curing history, and environmental conditions. Laboratory specimens are small and usually not restrained against volume changes. Laboratory tests of rich mix containing a fast-hydrating cement may yield low permeability values. The same concrete mix when used in an actual structure may not prove to be durable if exposed to frequent cycles of wetting–drying, heating–cooling, and freezing–thawing. Under similar circumstances, inadequately cured concrete containing a high volume of fly ash or slag will also crack and deteriorate in the field, whereas well-cured specimens may have given excellent performance in a laboratory test on permeability. The tests, performance parameters, and respective grades given in Table 16.14 are described below. 1. Resistance to freezing–thawing Two HPC grades of resistance to freezing– thawing are delineated by the percentage of residual dynamic modulus of elasticity after 300 cycles. Grade-1 is defined as 60 to 80 per cent retention of the original dynamic modulus of elasticity and Grade-2 is defined as greater than 80 per cent retention of the original dynamic modulus of elasticity. 2. Scaling test Scaling performance is evaluated after 50 cycles by visually inspecting specimens. Grade-1 is defined by a visual inspection rating of 4 or 5, Grade-2 by a rating of two or three, and Grade-3 by 0 or 1.

638

Concrete Technology

Table 16.14

Recommendations for the application of HPC grades

Exposure condition

Recommended HPC grade for given exposure condition N/A*

Grade-1

Grade-2

Freeze–thaw durability, exposure (x = F/T cycles per year)

x 4000

Portland cement concrete w/c > 0.6

High

3000 < x < 4000

Composite cement concrete 0.5 < w/cm < 0.6

Moderate

2000 < x < 3000

Composite cement concrete 0.4 < w/cm < 0.5

Low

1000 < x < 2000

High performance concrete w/cm < 0.4

Very low

100 < x < 1000

Latex-modified concrete

Negligible

x < 100

Polymer concrete

The permeability of high-strength lightweight concrete is low but is higher than that of normal-weight concrete at a similar strength level. The permeability depends on the porosity of the mortar matrix rather than on the porosity of the lightweight aggregate. There is optimum cement content for permeability. Mineral additives like fly ash, silica fume, and GGBFS improve the pore structures and thus the durability of concrete. A concrete with mineral additives is much less permeable to chloride ions than the concrete without mineral additives. High performance concretes are produced often with large quantities of these materials. Replacing large volumes of OPC with pozzolanic materials results in a significant drop in the pH of the pore solution, which may increase the risk of depassivation of steel in reinforced concrete. However, resulting refinement of the pore structure due to pozzolanic activity increases the electrolytic resistance of concrete and reduces corrosion to the negligible levels. Ternary OPC–FA–SF concrete has very high resistance to penetration of chloride ions. This resistance to diffusivity of ternarybinder concrete continues to increase with time or aging.

Deterioration Parameters 1. Abrasion, erosion and cavitation resistance Abrasion and erosion are wearing due to repeated rubbing and friction. For pavements, abrasion results from traffic wear and the pavement becomes polished reducing its skid resistance. Abrasion or erosion resistance of concrete is a direct function of its compressive strength, and thus of its water-cement ratio and constituent

662

Concrete Technology

materials. High-quality paste and strong aggregates are essential to produce an abrasion resistant concrete. When compared with high-quality asphalt pavement, the abrasion resistance of the very high-strength concrete represents an increase in the service life of the pavement by a factor of nearly 10. The coarse aggregate is the most important factor, followed by water– cementing material ratio in rank, affecting the abrasion resistance of concrete. The abrasion resistance of concrete is strongly influenced by the relative abrasion resistance of its mortar and coarse aggregate. When the coarse aggregate and mortar have nearly the same abrasion resistance, the surface wear of the concrete would be fairly uniform and the concrete can present serious skidding and slipping problems when wet. When the water-cementing material ratio is very low, it can make the concrete almost as abrasion resistant as high-performance rocks. The abrasion resistance of concrete generally varies inversely with the water-cementing material ratio, the porosity, and the cement paste volume in the concrete. Therefore, in HPC the use of superplasticizer to reduce substantially the water–cementing material ratio, would improve the abrasion resistance of concrete considerably. Introducing mineral additives without using superplasticizer would reduce the abrasion resistance of concrete since more water would be needed to maintain a constant workability. The abrasion resistance for high-volume fly ash systems relative to no-fly ash concrete is lower. HPC with silica fume and with low water-cement ratio has considerably high resistance to abrasion and is often used for repair of cavitation damaged water-retaining structures. If erosion of the concrete surface of hydraulic structures is due to a gradual wearing as a result of small particles of debris rolling over the surface at low velocities, then the quality of aggregate and the hardness of the surface determine the rate of erosion. Hence fibers have no effect in this regard. On the other hand, when erosion is due to abrasion resulting from high velocity flow and impact of large debris, steel fiber concretes provide significant erosion resistance. 2. Wet-dry exposure The addition of polypropylene fibers effectively retard the deterioration process of the surface skin of the concrete cured in hot weather environment but subjected to cyclic wet-dry seawater exposure. 3. Freezing-thawing Reduction in permeability improves resistance to damage caused by cyclic freezing-thawing. An adequate entrained-air-void system, however, is the most important factor affecting resistance to freezing-thawing. For the normal strength concrete, entrained air of four to eight per cent by volume of concrete provides an effective defence against frost damage. The exact amount of entrained air is dependent on the maximum size of the coarse aggregate, provided that the coarse aggregate itself is frost resistant. The optimum spacing factor of the air voids should be no more than the ACI recommended maximum value of 0.2 mm and the air voids should be of small size with their diameter being in the range of 0.05 to 1.25 mm to ensure that the required spacing factor is obtained with low air contents. Moreover, the frost resistance of concrete is related not only to the water-binder ratio, but also to the overall quality of the paste. The most important factor for frost durability

High-Performance Concretes

663

is not the total porosity of the concrete or of the paste, but the size, distribution of the capillary pores, which determines the amount of freezable water and the paste permeability. The damage in the paste is characterized by surface scaling, while in mortar and concrete, distress appears in form of a few large cracks. The resistance of concrete to freezing–thawing, and wetting–drying cycles is superior for high performance concrete than that for normal concrete of similar workability and strength. For the given number of cycles, long freezing–thawing cycles are more severe than the short freezing–thawing cycles. A maximum water-cementing material ratio (w/cm) of 0.40 for concrete exposed to freezing–thawing conditions and deicing salts is recommended since at a w/cm less than 0.40, a properly consolidated cement paste has a low permeability. In the range of 0.40 to 0.55, permeability begins to increase significantly. For an increase in the w/cm above 0.55, permeability increases at a very high rate due to an increase in capillary channels within the cement hydrate phases. Properly air-entrained concretes containing superplasticizers can have adequate freezing-thawing resistance even at spacing factors greater than 0.2 mm. The non-air-entrained high-strength concretes with water–cementing material ratio of less than 0.24 (critical value) is frost resistant. For values of water-binder ratio higher than 0.30, the use of silica fume may be detrimental. The use of air entrainment is necessary if water-binder ratio is higher than 0.30. Some fly ashes with high carbon contents reduce air content. Thus it is important to ensure that concrete containing fly ash also has desired air-void system. However, for properly air-entrained concrete, silica fume does not seem to have any detrimental effect. The very low water-cement ratio, for an adequately cured concrete, can reduce or even eliminate the amount of freezable water in the pores for practical temperature ranges. These mixes will also dramatically reduce the ingress of water, therefore, reducing the amount of damage due to physically freezing water in the concrete. The high-strength lightweight concretes are extremely resistant to frost action. The use of calcium chloride should be avoided, as it contributes to reduced freezing-thawing resistance. A lightweight coarse aggregate with its surface coated by high molecular paraffin produces lightweight concrete with high durability against freezing-thawing, abrasion and fire. The addition of fibers themselves has no significant effect on the freezing– thawing resistance of concrete, i.e., the concretes that are not resistant to freezing-thawing will not have improved resistance by the addition of fibers. Hence, the well-known practices for achieving durable concrete and the standard air entrainment criteria for plain concrete should also be used for fiber reinforced concrete. 4. Deicer scaling Scaling is caused by repeated application of deicing salts. Concrete surface damaged by salt scaling becomes rough and pitted as a result of spalling and flaking of small pieces of mortar near the surface. Even highquality concrete with adequate air entrainment can suffer scaling by deicing chemicals.

664

Concrete Technology

Scaling is most likely to occur where there is a weak layer of paste or mortar at/or near the concrete surface. Scaling distress appears in the form of—lift-offs or pop-offs, checkerboard, and coalesced. The pop-offs are usually a result of improper finishing operations that develop poor bond, and sometimes micro-separations between the surface mortar layer and underlying single near-surface coarse aggregate particles. The mortar popoffs subsequently result due to either/or combination of—impact of traffic, expansion due freezing of water that accumulates at the mortar cover and aggregate interface, and drying shrinkage of mortar (because the stresses developed at the mortar-aggregate interface by shrinkage are accumulative instead of being distributive along the interface due to poor bond). This condition is similar in some respects to a classic wearing surface distress feature known as a pop-out. Pop-outs are associated with non-frost-resistant coarse aggregate particles, such as some shales and chert, which exhibit high porosity, low strength, or both. When these sub-standard aggregate particles become saturated, they expand greatly during freezing. The particles fracture and pop-out the overlying thin mortar layer. Pop-offs differ from pop-outs as coarse aggregate particles involved in pop-off do not fracture. In checkerboard and coalesced scaling distress, a small number of pop-offs join together forming alternating small regions of original unscaled wearing surface and scaled areas. The best prevention of scaling is to eliminate the weak layer of material by proper mix design and good construction practice in placing, finishing, and curing. Over-vibration, too much trowelling, and excessive bleeding should all be avoided. Finishing practices that result in a higher water-cementing ratio in the wearing surface layer relative to bulk concrete at lower depths, and reduce or eliminate the entrained air voids in the wearing surface should be avoided. Well-cured concrete pavement, which has achieved an acceptable strength level and has been allowed to dry for a period before deicing salts are applied, generally will have good scaling resistance. Use of concrete of grade (28-day compressive strength potential) of at least 25 to 30 MPa with an adequate entrained air-void system can eliminate scaling problem to a greater extent. The non-air-entrained high-strength concretes with good deicer salt scaling resistance can be produced with a Portland cement plus silica fume and good quality coarse aggregate by using water–cementing material ratio of 0.30, even after only 24 hours of curing. The concrete with up to 30 per cent fly ash has adequate scaling resistance. However, when higher volume of fly ash (55 to 60 per cent) are used in air-entrained concrete, the scaling performance of the concrete becomes unsatisfactory. 5. Carbonation Carbonation is a process where CO2 present in the atmosphere reacts readily in the presence of moisture with hydrated cement minerals. CO2 mainly reacts with Ca(OH)2 to form calcium carbonate, while other cement compounds are carbonated to hemi-carbo-aluminate (4CaO · Al2O3 · ½ CO2 · 18H2O). Ca(OH)2 + CO2 → CaCO3 + H2O

High-Performance Concretes

665

Limited carbonation of hydrated Portland cement paste results in increased strength and reduced permeability (because of formation of calcium carbonate which is insoluble, plug up the pores and micro-cracks, the calcite layer formed on the surface may therefore protect the concrete from further attack), whereas, excessive carbonation in the presence of aggressive CO2 reduces impermeability (because the formation of calcium bicarbonate, which is soluble and can be leached away, widens the existing pores and microcracks). However, carbonation neutralizes the corrosion protection of steel provided by the alkaline environment of hydrated cement paste. The reaction progressively lowers the pH value of pore solution from its initial saturation level of 12 to 13 and thus depassivates the reinforcement. The destruction of the passive film on steel occurs at pH between 9 and 10. The concrete having a pH value below 9 is usually categorized as carbonated concrete. Once the cementing paste in contact with reinforcement is carbonated, corrosion begins. The rate of reinforcement corrosion depends on the electrical resistivity of the concrete. High-replacement pozzolana concretes have higher accelerated carbonation depths compared with normal concrete. Such an increase may not be of major consequence in HPC having very low permeability. At a given age carbonation levels for HPC are roughly twice as for ordinary concrete with water-cement ratio of 0.55. 6. Sulfate attack Sulfate attack is one of the major causes of concrete failure. Sulfates occur naturally in the environment, sea water, ground water and soils. They react chemically with certain Portland cement constituents causing premature degradation of concrete structure. The mechanism of sulfate attack on concrete apparently involves two chemical reactions: combination of sulfate ions with calcium ions to form gypsum, and combination of sulfate ions and hydrated calcium aluminate to form calcium-sulfo-aluminate (ettringite) (3 CaO · Al2 O3 · 3 CaSO4 · 3H2O). The reaction of sulfate ions with the Ca(OH)2 in the concrete to form gypsum is called acidic type of sulfate attack. It can be symbolically represented as: Ca(OH)2 + Na2SO4 → CaSO4 + 2 NaOH and/or

Ca(OH)2 + MgSO4 → CaSO4 + Mg(OH)2 The CaSO4 precipitates as CaSO4 · 2H2O (secondary gypsum). The second type of attack mainly results from the chemical reaction between the tricalciumaluminate (C3A) present in Portland cement and sulfate ions supplied by the aggressive environment. By this reaction, ettringite is formed. The chemical reaction can be represented as: 3CaSO4 + C3A + 32H2O → C3A · 3CaSO4 · 32H2O The reaction products, i.e., gypsum and ettringite have greater volume than the compounds they replace. The increase in volume due to gypsum has been reported to be more than double the original volume and that due to ettringite

666

Concrete Technology

to be more than triple the original volume. This expansion causes cracking of the matrix, which leads to the loss in strength and disruption. Fly ash and other pozzolanas improve sulfate resistance by reducing permeability, thus limiting the amount of external sulfate that can penetrate the concrete. Because calcium hydroxide (free lime) is involved in the chemical reactions that cause sulfate attack, pozzolanas can also help to control damage by converting the free lime into calcium–silicate–hydrates. Concretes made with low calcium fly ashes (with CaO content less than 15 per cent) resist sulfate attack better than concretes made with high-calcium fly ashes. When three per cent by mass of the cementing material is replaced by silica fume, the expansion of the ternary OPC-high-calcium FA-SF mortars has been found to decrease significantly (expansion may be less than 0.1 per cent). Tricalcium aluminate, a cementing component of hydration product of cement is attacked by sulfates. Replacing some Portland cement with GGBFS lowers the tricalcium aluminate content, thus reducing the damage caused by sulfate attack. By reacting with calcium hydroxide, the GGBFS also reduces the permeability of concrete and thus further increases resistance to sulfate attack. Concrete in which GGBFS comprises more than 50 per cent of the total cementing content has very strong sulfate resistance. Depending upon the concentration of sulfates expressed as SO3 in the soil or in ground water, Portland slag cement or Portland pozzolana cement could be used for low concentrations (10 per cent 5 ~ 8 per cent < 25 per cent < 135 kg/m3

Type-B < 0.40 >10 per cent 5 ~ 8 per cent < 25 per cent < 140 kg/m3

3. High-rise buildings The reasons for using the high-strength concrete in the area of high-rise buildings are to reduce—the deadload, the deflection, the vibration and the noise, and the maintenance cost.

High-Performance Concretes

673

4. Miscellaneous applications Fiber reinforced concrete has been used with and without conventional reinforcement in many field applications. These include bridge deck overlays, floor slabs, pavements and pavement overlays, refractories, hydraulic structures, thin shells, rock slope stabilization, mine tunnel linings and many precast products. The addition of steel fibers is known to improve most of the mechanical properties of concrete, namely, its static and dynamic tensile strengths, energy absorption and toughness, and fatigue resistance. Hence, proper utilization of steel fiber-reinforced concrete depends on the skill of the engineer.

REVIEW QUESTIONS 16.1 What is high performance concrete (HPC)? How is HPC classified according to different levels of performance requirements? 16.2 What is self-compacting concrete, and what are the advantages and disadvantages of using it? 16.3 Describe characteristics of self-compacting concrete in the fresh condition. How is the flowability of SCC measured? 16.4 Enlist the test methods for self-compacting. Describe the slump flow and T500 tests.

16.5 Describe briefly the lightweight foamed or aerated concrete, its types, properties, advantages and disadvantages. 16.6 Write short notes on two following: (a) Applications of lightweight foamed concrete (b) Durability-performance grades (c) Application of high performance concrete

MULTIPLE-CHOICE QUESTIONS 16.1 A properly designed self-consolidating concrete (SCC) should be (a) highly flowable (b) stable (no separation or segregation) (c) spread readily into place (low viscosity) (d) Fill the formwork without any consolidation and without undergoing significant separation (e) All of the above 16.2 Identify the false statement: (a) Self-compacting concrete is characterized by high performance; better and more reliable quality, dense and uniform surface texture, improved durability, high-strength, and faster construction. (b) The conventional concrete is transformed into self-compacting concrete, by carefully selecting and proportioning the aggregate with

suitable shape, size, grading, cement and water contents, and admixture dosage to ensure the selfconsolidating properties. (c) In addition to high range water reducers and viscosity modifying admixtures, traditional mineral additives including silica fume, fly ash, blast furnace slag, and limestone powder help to achieve the balance between fluidity and cohesion. (d) As there is significant bleeding water at the surface, SCC tends to dry at slower rate than conventional vibrated concrete. There is reduced possibility of surface shrinkage cracking. (e) As compared with conventional concrete of comparable strength,

674

Concrete Technology

the materials cost of SCC is approximately 10 to 15 per cent higher and labor cost is 25 per cent lower and the overall construction costs are comparable. 16.3 Assertion A: The yield stress of SCC is reduced by using an advanced synthetic high-range water-reducing admixture (HRWR), while the viscosity of the paste is increased by using a viscosity-modifying admixture (VMA) or by increasing the percentage of fines incorporated into the SCC mix design.

Reason R: SCC mix should have low yield stress and high plastic viscosity so that it requires minimal force to initiate flow; yet have adequate cohesion to resist aggregate segregation and excessive bleeding. (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true (e) Both are false

16.4 Match list-I with list-II and select the correct answer using the codes given below the lists. List-I: (Test) 1. 2. 3. 4.

List-II: (Characteristic)

Flowability and stability Segregation resistance Plastic viscosity Flowability

A. B. C. D.

Slump flow test V-Funnel time test T500 Test Sieve Segregation Resistance

Codes: (a) (b) (c) (d) (e)

A 1 1 3 2 4

B 2 4 4 1 1

C 3 3 2 3 3

D 4 2 1 4 2

16.5 Assertion A: The mixes of high slumpflow and low plastic viscosity are beneficial in helping to achieve excellent surfaces. A mix that is close to seggregation will usually give the best surface. Reason R: SCC with poor filling and passing ability; high viscosity or high yield stress, low slump-flow and/ or long T500 times is prone to cause blowholes on the surface of cast concrete as they make it difficult for the air to leave the concrete. (a) Both (A) and (R) are true and (R) is correct explanation of (A)

(b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true (e) Both are false 16.6 In case of SCC, the ‘zero-admixture’ slump should be within…. of the predefined target (a) ± 12.5 mm (b) ± 25 mm (b) ± 37.5 mm (d) ± 40 mm (e) ± 5.0 mm

High-Performance Concretes

675

16.7 Match list-I containing the test values for a good SCC with list-II of desired SCC property and select the correct answer using the codes given below the lists. List-I: (Test value) A. B. C. D.

List-II: (Property)

3-8 sec 10-15 mm 90 minutes 100 m

1. 2. 3. 4.

Pumpable distance Flowability retention V-funnel time J-ring value

Codes: (a) (b) (c) (d) (e)

A 1 1 3 2 2

B 2 4 4 1 1

C 3 3 2 3 4

D 4 2 1 4 3

16.8 For lightly reinforced member, the concrete flow of ________mm may be sufficient and if the concrete has to flow a large distance through a highly reinforced element, a flow of _________ mm may be necessary. The maximum size of aggregate of uniform

shape and grading is generally limited to ______mm. (a) (400-500), (600-650) and 10 mm (b) (500-600), (700-750) and 10 mm (c) (500-600), (700-750) and 20 mm (d) (600-650), (700-760) and 10 mm (e) (600-650), (700-760) and 20 mm

Answers to MCQs 16.1 (e) 16.7 (c)

16.2 (d) 16.8 (c)

16.3 (a)

16.4 (b)

16.5 (a)

16.6 (b)

17 17.1

REPAIR TECHNOLOGY FOR CONCRETE STRUCTURES

INTRODUCTION

Though concrete is a relatively durable building material, it may suffer damage or distress during its service life due to a number of reasons. Because of the varying conditions under which it is produced at various locations, the quality of concrete suffers occasionally either during production or during service conditions resulting in distress. The structural causes of distress of concrete may include externally applied and environmental loads exceeding the design stipulations, accidents and subsidences. Sometimes distress in a structure is brought about by poor construction practices, error in design and detailing, and construction overloads. The other causes may be drying shrinkage, thermal stress, weathering, chemical reactions and corrosion of reinforcement. In addition to the distress in hardened concrete the plastic concrete may also suffer damage due to plastic shrinkage and settlement cracking as shown in Fig. 17.16. Sometimes on stripping off the forms a number of surface defects such as bulges, ridges, honeycombing shown in Fig. 17.16, bolt-holes, etc., are noticed on the fresh concrete members. Such defects can be avoided to a large extent by providing a watertight and rigid formwork in such a way that stripping can be done without the use of crowbars or other tools. In addition to these defects, blowholes develop during concreting operations due to improper design of formwork. These are formed in the surface of concrete by trapped air and water bubbles against the face of formwork. These can be reduced if the form face is slightly absorbent and adequate compaction is provided. In case the blow-holes are exceptionally large, or if a smooth surface is required, they must be filled with 1 : 1 or 1 : 2 cement–sand mortar. The sand should be sieved through a 300 μm or 600 μm sieve, depending upon the smoothness of the finish required. Crushed limestone dust is preferable. The mortar should be rubbed over the affected area with a rubber-faced float, and finally rubbed down with a smooth stone or mortar block for a smooth finish. The honeycombing consisting of groups of interconnected deep voids normally indicate inadequate compaction or loss of grout through joints in formwork or between formwork and previously cast-concrete. The affected area is delineated with a saw cut to a depth of 5 mm. The unsound material is chipped out to the solid concrete. After the surface has been prepared a bonding coat should be applied to all exposed surfaces, and new concrete should be placed against the prepared surface. The bonding coat may consist of a slurry of cement and water, but it is desirable to incorporate a polymer admixture. The repair of bulges and projections can be carried out by chipping off the concrete from the surface and then rubbing the surface with a grinding stone. Scouring of

677

Repair Technology for Concrete Structures

a vertical surface of hardened concrete making it resemble a map of delta of a river is caused by water moving upward against the face of formwork. This is a sign of excessive wet or harsh concrete. It is a superficial defect so, unless it is unusually deep and the cover to reinforcement is unusually small, the remedial measures consist of early facing up in the same way suggested for filling blowholes. This type of repair (more appropriately called finishing) should be done as soon as possible after the forms are stripped and before the concrete becomes too hard. The repair should preferably be completed within 24 hours of the removal of the forms. This is done to develop a good bond and make the repaired portion as durable and permanent as the original work. If the repairs are not properly done, the newly placed concrete becomes loose and drummy with the passage of time and finally gets detached from the main concrete. The darker color of the repaired patches can be corrected by adding 10 to 20 per cent white Portland cement to the patching mortar to obtain uniform surface color. The various defects occurring during construction are outlined in Table 17.1. Table 17.1 Symptom

Summary of defects occurring during construction Cause

Cracks in horizontal surface, as concrete stiffens or very soon thereafter. Cracks form above ties, reinforcement, etc., or at arrisses. especially in deep lifts.

Plastic shrinkage: rapid drying of surface.

Cracks in thick sections, occurring as concrete cools.

Prevention

Remedy

Shelter during placing. Cover as early as possible. Use air entrainment. Change mix design. Use air entrainment.

Seal by brushing in cement or lowviscosity polymer.

Restrained thermal contraction.

Minimize restraint to contraction. Delay cooling until concrete has gained strength.

Seal cracks.

Blowholes in form faces of concrete.

Air or water trapped against formwork: Inadequate compaction. Unsuitable mix design. Unsuitable release agent.

Improve vibration. Change mix design. Use appropriate release agent. Use absorbent formwork.

Fill with polymermodified fine mortar.

Voids in concrete.

Honeycombing: Inadequate compaction. Grout loss.

Improve compaction. Reduce maximum size of aggregate. Prevent leakage of grout.

Cut out and make good. Inject resin.

Erosion of vertical surfaces, in vertical streaky pattern.

Scouring: Water moving upwards against form face.

Change mix design, Rub in polymerto make more cohesive modified fine or reduce water content. mortar.

Plastic settlement: concrete continues to settle after starting to stiffen.

Recompact upper part of concrete while still plastic. Seal cracks after concrete has hardened.

Contd.

678

Concrete Technology Table 17.1 Symptom

Cause

(continued) Prevention

Remedy

Color variations.

Variations in mix proportions, curing conditions, materials, characteristics of form face, vibration, release agent. Leakage of water from formwork.

Ensure uniformity of all relevant factors. Prevent leakage from formwork.

Apply surface coating.

Powdery formed surface.

Surface retardation, caused by sugars in certain timbers.

Change form material. Seal surface of formwork. Apply limewash to form face before first few uses.

Generally, none required.

Rust strains.

Pyrites in aggregates. Rain streaking from unprotected steel. Rubbish in formwork. Ends of wire ties turned out.

Avoid contaminated aggregates. Protect exposed steel. Clean forms thoroughly. Turn ends of ties inwards.

Clean with dilute acid or sodium citrate/sodium dithionite. Apply surface coating.

Plucked surface.

Insufficient release More care in applicaRub in fine mortar, agent. Careless re- tions of release agent and or patch as for moval of formwork. removal of formwork. spalled concrete.

Lack of cover to reinforcement.

Reinforcement moved during placing of concrete, or badly fixed. Inadequate tolerances in detailing.

17.2

Provide better support for reinforcement. More accurate steel fixing. Greater tolerances in detailing.

Apply polymermodified cement and sand rendering. Apply protective coating.

SYMPTOMS AND DIAGNOSIS OF DISTRESS

In addition to minor structural defects outlined above, the other distresses can be observed in the form of cracks, spaIling and scaling of concrete. Cracking is the most common indication of the distress in a concrete structure. It may affect appearance only, or indicate significant structural distress or lack of durability. Cracks may represent the total extent of the damage, or they may point to problems of greater magnitude. These, in turn, may cause corrosion of reinforcement due to the entry of moisture and oxygen. All the concrete structures crack in some form or the other. Most buildings develop cracks in their fabric which are superficial and occur soon after the construction. Cracks, even if harmless, may have an adverse psychological effect. However, cracking in concrete structures is not necessarily a cause for accusing the designer, builder or

Repair Technology for Concrete Structures

679

supplier. What really matters is the type of structure and the nature of cracking. Cracks that are acceptable for building structures may not be acceptable for water-retaining structures. Cracking of concrete structures can never be totally eliminated, but the practitioner should be aware of the causes, evaluation techniques, and the methods of repair. The approach to diagnosis of the problem of cracking should be identical to that of a doctor to the patient. An engineer should have a sound knowledge of all the facets of concrete technology, i.e., of the behavior of construction materials, construction techniques, types of cracks likely to occur, their causes and respective remedial measures. In short, treatment of cracks involves detection, diagnosis and remedy. Before remedies are sought, correct diagnosis will decide whether satisfactory repair is possible. The development of cracks and their repair is a perpetual problem involving considerable cost and inconvenience to the occupants. The problem should be tackled on two fronts, i.e., by adopting preventive measures and repairing them. However, prevention is better than repair. The designer and builders should attempt to reduce the formation of cracks by using appropriate construction materials, and by adopting appropriate design and construction techniques. The cracks in a structure are broadly classified in two categories: superficial cracks and structural cracks. The structural cracks may be active and dormant. A crack where a movement is observed to continue is termed active, whereas the crack where no movement occurs is termed dormant or static. The following information may help in diagnosing the cracks: 1. 2. 3. 4. 5. 6. 7.

Whether the crack is new or old Type of crack, i.e., whether it is active or dormant Whether it appears on the opposite face of the member also Pattern of the cracks Soil condition, type of foundation used, sign of movement of ground, if any Observations on the similar structures in the same locality Study of specifications, method of construction used and the test results at the site, if any 8. Views of the designer, builder, occupants of the building, if any 9. Weather during which the structure has been constructed From the above discussion, it is evident that the cracking is a complex phenomenon. The various aspects of the problem are discussed as follows. The latent defects in a concrete structure may be caused by inadequacy of design, materials or construction practices which may not become evident until sometime after its completion. The immediate cause of deterioration may be a chemical action or corrosion of reinforcement, but in majority of the cases the basic cause may be traced back to something such as unrealistic detailing or poor workmanship. The incompatible dimensional changes caused by drying shrinkage and thermal movements during and after the hardening period may also cause cracks in concrete members. Before any repair work is taken in hand, the cause of damage must be clearly identified, for which careful investigation is required. Some of common causes are discussed in the following subsections.

680

Concrete Technology

17.2.1

Cracking of Plastic Concrete

When the exposed surfaces of freshly placed concrete are subjected to a very rapid loss of moisture caused by low humidity, wind, and/or high temperature, the surface concrete shrinks. Due to restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks that are usually short, discontinuous running in all directions and very seldom extend to the free edge. In an unreinforced slab they are typically diagonal as shown in Fig. 17.1(a). In the presence of reinforcement their pattern may be modified. Plastic shrinkage usually occurs prior to final finishing before curing starts. The cracks are often fairly wide at the surface. They range from few centimeters to many meters in floors or slabs or other elements with large surface areas. Plastic shrinkage cracks may extend the full depth of elevated thin structural elements. Plastic shrinkage cracks can be controlled by reducing the relative volume change between the surface and the interior concrete by preventing a rapid moisture loss due to hot weather and dry winds. This can be accomplished by using fog nozzles to saturate the air above the surface and use of plastic sheeting to cover the surface between the final finishing operations. Wind breakers reducing the wind velocity, and sunshades reducing the surface temperatures are also helpful, and it is good practice to schedule flatwork after the walls have been erected. The remedial measures after the cracks have formed usually consists of sealing them against entry of water by brushing in cement or low viscosity polymer. After initial placement, vibration and finishing, the concrete has a tendency to continue to consolidate or settle, especially in deep sections after it has started to stiffen. During this period the plastic concrete may be locally restrained by reinforcing bars, previously placed concrete, or formwork tie-bolts. These local restraints may cause cracks and/or voids adjacent to the restraining element as shown in Fig. 17.1(e). When associated with the reinforcing bars, this settlement cracking increases with increasing bar size, increasing slump, and decreasing cover. The degree of settlement cracking may be magnified by insufficient compaction or the use of leaking or highly flexible forms. The use of lowest possible slump, an increase in concrete cover, provision of a sufficient time interval between placement of concrete in various elements, adequate vibration and proper form design will reduce settlement cracking. Air entrainment may help in avoiding these cracks. The remedial measures, after concrete has hardened, consists in sealing the cracks in order to protect the reinforcement.

17.2.2

Cracking of Hardened Concrete

The moisture-induced volume changes are characteristic of concrete. A loss of moisture from cement paste results in a volume shrinkage by as much as one per cent, whereas the internal restraint provided by the aggregate reduces the magnitude of this volume change to about 0.05 per cent. On the other hand, an increase in moisture in the concrete tends to increase its volume. If these volume changes are restrained (usually by another part of structure or by the subgrade), the tensile stresses develop. When the tensile strength of concrete is exceeded, it will crack. The cracks may propagate at much lower stresses than are required to cause crack initiation.

(a) Plastic shrinkage cracking

(b) Crazing (map pattern of fine cracks)

(c) Alkali-aggregate reaction cracking

(d) Reinforcement corrosion cracking Settlement of concrete surface

Void under reinforcing bar (e) Typical plastic settlement crack over reinforcing bar

Fig. 17.1

Typical examples of crack patterns

682

Concrete Technology

In massive concrete elements, tensile stresses are caused by differential shrinkage between the surface and the interior concrete. The larger shrinkage at the surface causes the cracks to develop that may with time penetrate deeper into the concrete. Surface cracking on walls and slabs usually occurs due to drying shrinkage when the surface layer of concrete has a higher water content than the interior concrete. The surface crazing appears in the form of a series of shallow, closely spaced fine cracks. The extent of shrinkage cracking depends upon the amount of shrinkage, degree of restraint, modulus of elasticity, and amount of creep. The amount of drying shrinkage is influenced mainly by the amount and type of aggregate and the water content of the mix. The shrinkage decreases with the increase in the amount of aggregate, and the reduction in water content. The higher the stiffness of the aggregate, the more effective it is in reducing the shrinkage of concrete, e.g. the shrinkage of concrete containing sandstone aggregate may be more than twice that of concrete with basalt or granite. Therefore, the drying shrinkage can be reduced by using the maximum practical amount of aggregate and lowest usable water content in the mix. Shrinkage cracking can be controlled by using properly spaced contraction joints and proper steel detailing. The shrinkage cracking can also be controlled by using shrinkage compensating cement.

Thermal Cracking The temperature difference within a concrete structure result in differential volume change. When the tensile strain due to differential volume change exceeds the tensile strain capacity of concrete, it will crack. The temperature differentials associated with the hydration of cement affect the mass concrete such as in large columns, piers, footings, dams, etc., whereas the temperature differentials due to changes in the ambient temperature can affect any structure. The liberation of the heat of hydration of cement causes the internal temperature of concrete to rise during the initial curing period, so that it is usually slightly warmer than its surroundings. In thick sections and with rich mixes, the temperature differential may be considerable. As the concrete cools it will try to contract. Any restraint on the free contraction during cooling will result in tensile stresses which are proportional to the temperature change, coefficient of thermal expansion, effective modulus of elasticity (which is reduced by creep) and degree of restraint. The more massive the structure, the greater is the potential for temperature differential and degree of restraint. Thermally induced cracking can be reduced by controlling the maximum internal temperature, delaying the onset of cooling by insulating the formwork and exposed surfaces, controlling the rate of cooling, and increasing the tensile strain capacity of the concrete. Special precautions need to be taken in the design of structures in which some portions are exposed to temperature changes while the other portions of structures are either partially or completely protected. A drop in temperature may result in the cracking of the exposed element while increase in temperature may cause cracking in the protected portion of structure. Temperature gradients cause deflection and rotation in structural members; if these are restrained serious stresses can result. Allowing for movement by using properly designed contraction joints and correct detailing will help alleviate these problems. If the cracks do form. remedial measures are similar to those for cracks that form after a structure in service.

Repair Technology for Concrete Structures

683

Cracking due to Chemical Reactions The most important constituent of concrete namely cement is alkaline; so it will react with acids or acidic compounds in presence of moisture, and in consequence the matrix will become weak and its constituents may be leached out. The concrete may crack, as a result of expansive reactions between aggregate containing active silica and alkalies derived from cement hydration, admixture or external sources (e.g., curing water, ground water, alkaline solutions stored). The alkali-silica reaction results in the formation of a swelling gel, which tends to draw water from other portions of concrete. This causes local expansion and accompanying tensile stresses which if large may eventually result in the complete deterioration of the structure. Control measures include proper selection of aggregate, use of low-alkali cement, and use of pozzolana. Typical symptoms in unreinforced and highly reinforced concrete are map cracking, as shown in Fig. 17.1(c), and gel excluding from cracks. The alkali-carbonate reaction occurs with certain limestone aggregates and usually results in the formation of alkali-silica product between aggregate particles and the surrounding cement paste. Here also the affected concrete is characterized by a network pattern of cracks as shown in Fig. 17.1(c). The problem may be minimized by avoiding reactive aggregate, use of a smaller size aggregate and use of low alkali cement. When the sulfate bearing waters come in contact with the concrete, the sulfate penetrates the hydrated paste and reacts with hydrated calcium aluminate to form calcium sulfoaluminate with a subsequent large increase in volume, resulting in high local tensile stresses causing the deterioration of concrete. The blended or pozzolana cements impart additional resistance to sulfate attacks. The calcium hydroxide in hydrated cement paste will combine with carbon dioxide in the air to form calcium carbonate which occupies smaller volume than the calcium hydroxide resulting in the so called carbonation shrinkage. This situation may result in significant surface grazing and may be especially serious on freshly placed concrete surface kept warm during winter by improperly vented combustion heaters.

Cracking due to Weathering The enviromnental factors that can cause cracking include (i) freezing and thawing, (ii) wetting and drying, and (iii) heating and cooling. Except in tropical regions, the damage from freezing and thawing is the most common weather related physical deterioration. In the aggregate particles saturated above the critical degree of saturation, the expansion of absorbed water during freezing may crack the surrounding cement paste and/or damage the aggregate itself. The control measures include the use of the lowest practical water-cement ratio and total water content, durable aggregate and adequate airentrainment. Adequate curing prior to exposure to freezing conditions is also important. Other weathering processes that may cause cracking in concrete are alternate wetting and drying, and heating and cooling. If the volume changes due to these processes are excessive, cracks may develop and give the impression that the concrete is on the verge of disintegration. The fire and frost actions may also damage the structure. The damage due to these factors may appear in the form of general flacking and spalling of concrete from the surface. Concrete gradually loses strength with increase in temperature above about 300 °C, damage being greater with aggregate having high coefficient of thermal expansion.

684

Concrete Technology

Cracking due to Corrosion of Reinforcement It is the most frequent cause of damage to reinforced concrete structures. This aspect of the cracking problem has been discussed in detail in Chapter 15. However, the salient features are outlined for ready reference. The corrosion of steel produces iron oxides and hydroxides, which have a volume much greater than the volume of the original metallic iron. This increase in volume causes high radial bursting stresses around reinforcing bars and results in local radial cracks. These splitting cracks may propagate along the bar, resulting in the formation of longitudinal cracks parallel to the bar or spalling of concrete as illustrated in Fig. 17.1(d). Cracks provide easy access to oxygen, moisture, and chloride, and thus even a minor split can create a condition in which corrosion continues and causes further cracking. Reinforcing steel usually does not corrode in concrete because a tightly adhering protective oxide coating forms in a highly alkaline environment. This is known as passive protection. However, if the alkalinity of the concrete is reduced through carbonation or the passivity of the steel is destroyed by aggressive chloride ions, the reinforcing bars may corrode. Cracks transverse to reinforcement usually do not cause continuing corrosion of reinforcement if the concrete has low permeability. If the combination of density and cover thickness is sufficient to restrict the flow of oxygen and moisture, corrosion slows down or ceases. For general concrete construction, the best control measure against corrosion-induced splitting is the use of concrete with low permeability. Increased concrete cover over the reinforcing bar is effective in delaying the corrosion process and also in resisting the splitting and spalling caused by corrosion or transverse tension. In very severe exposure conditions, additional protective measures, such as coated reinforcement, sealers or overlays on concrete, and corrosion-inhibiting admixtures can be adopted. Cracking due to Poor Construction Practices Poor construction practices, such as adding water to concrete to improve workability, lack of curing, inadequate form support, inadequate compaction, and arbitrary placement of construction joints, can result in cracking in concrete structures. Adding water to improve workability has the effect of reducing strength, increasing settlement and ultimate drying shrinkage. The early termination of curing will allow for increased shrinkage at the time when the concrete has low strength. Incomplete hydration due to drying will reduce not only the long term strength but also the durability of the structure. Lack of support for forms or inadequate compaction can result in the settlement cracking of concrete before it has developed sufficient strength to support its own weight, while improper location of construction joints can result in cracking at the planes of weakness. Some of the defects occurring during construction are summarized in Table 17.1. Cracking due to Construction Overloads The loads induced during construction can be far more severe than those experienced in service. Unfortunately, these conditions may occur at the early ages when the concrete is most susceptible to damage and often result in permanent cracks. A common error occurs when the precast members are not properly supported during transportation and erection. The use of arbitrary or convenient lifting points

Repair Technology for Concrete Structures

685

may cause severe damage. A big element lowered too fast, and stopped suddenly carries significant momentum which is translated into an impact load that may be several times the dead weight of the element. Storage of materials and equipment can easily result in loading conditions during construction far more severe than any load for which the structure is designed. Damage from unintentional construction overloads can be prevented only if the designers provide information on load limitations for the structure and if the construction personnel heed to these limitations.

Cracking due to Errors in Design and Detailing The design and detailing errors that may result in unacceptable cracking include use of poorly detailed re-entrant corners in walls, precast members and slabs; improper selection and/or detailing of reinforcement; restraint of members subjected to volume changes caused by variations in temperature and moisture; lack of adequate contraction joints, and improper design of foundations resulting in differential settlement within the structure. Re-entrant corners provide a location for stress concentration and, therefore, are prime locations for initial cracks, as in the case of window and door openings in concrete walls and dapped beams. Additional properly anchored diagonal reinforcement is required to keep inevitable cracks narrow and prevent them from propagating further. An inadequate amount of reinforcement may result in excessive cracking. A common mistake is to lightly reinforce an element because it is a non-structural element and tying it to the rest of the structure in such a manner that it is required to carry a major portion of the load once the structure begins to deform. The non-structural element will carry a load in proportion to its stiffness. Since this element is not detailed to act structurally, unsighty cracking may result even though the safety of the structure is not threatened. The restrained members subjected to volume changes frequently develop cracks. A slab, wall or a beam restrained against shortening, even if prestressed, can easily develop tensile stress sufficient to cause cracking. Beams should be allowed to move. Improper foundation design may result in excessive differential movement within a structure. If the differential movement is relatively small, the cracking problem may be only visual in nature. However if there is a major differential settlement, the structure may not be able to redistribute the loads rapidly enough, and a failure may occur. One of the advantages of the reinforced concrete is that, if the movement takes place over a sufficiently long period of time, creep will allow some redistribution of load. Special care need to be taken in the design and detailing of structures in which cracking may cause a major serviceability problem. These structures also require continuous inspection during all phases of construction to supplement the careful design and detailing.

Cracks due to Externally Applied Loads Load induced tensile stresses may result in cracks in concrete elements. A design procedure specifying the use of reinforcing steel, not only to carry tensile forces, but also be obtain both an adequate crack distribution and a reasonable limit on crack width is recommended. Flexural and tensile crack widths can be expected to increase with time for members subjected

686

Concrete Technology

to either sustained or repetitive loading. A well-distributed reinforcing arrangement offers the best protection against undesirable cracking.

17.3

EVALUATION OF CRACKS

As in the case of a medical practitioner prescribing medicine without thoroughly examining the patient, it is difficult for a repair engineer to advocate any repair technology without making a thorough investigation. Before proceeding with repair, the investigations should be made to determine the location and extent of cracking, the causes of damage, and the objectives of repair. Calculation can be made to determine stresses due to applied loads. For detailed information, the history of the structure, structural drawings and specifications, and construction and maintenance records should be reviewed. The objectives of repair include restoration and enhancement of durability, structural strength, functional requirements and aesthetics. The evaluation of cracks is necessary for the following purposes: 1. 2. 3. 4. 5. 6.

To identify the cause of cracking. To assess the structure for its safety and serviceability. To establish the extent of the cracking. To establish the likely extent of further deterioration. To study the suitability of various remedial measures. To make a final assessment for serviceability after repairs.

Apart from visual inspection, tapping the surface and listening to the sound for hollow areas may be one of the simplest methods of identifying the weak spots. The suspected areas are then opened up by chipping the weak concrete for further assessment. The comparative strength of concrete in the structure may be assessed to a reasonable accuracy by non-destructive testing and by the tests on the cores extracted from the concrete. The commonly used non-destructive tests are the rebound hammer test and ultrasonic pulse velocity test.

17.3.1

Visual Examination

The appearance of concrete surface may suggest the possibility of chemical attack by a general softening and leaching of matrix, or in case of sulfate attack by whitening of concrete. Rust stains often indicate corrosion of reinforcement but they may also be caused due to the contamination of aggregate with iron pyrites. If the cracked concrete is broken out, the appearance of the crack surface gives useful information; dirt or discoloration show that the crack has been there for some time. General flaking of an exposed concrete surface suggests frost damage. In fire damaged structure, the color of concrete gives an indication of the maximum temperature reached. The crack pattern may be informative, a mesh pattern suggests drying shrinkage, and surface crazing may indicate frost attack or in rare cases alkali-aggregate reaction. Typical crack patterns are illustrated in Fig. 17.1. The cracks caused by

Repair Technology for Concrete Structures

687

unidirectional bending will be the widest in the zone of maximum tensile stress and will taper along their length, while cracks caused by direct tension will be of roughly uniform width. Pop-outs in concrete are usually associated with particles of coarse aggregate just below the surface. The location and width of cracks should be noted on a sketch of the structure. A grid marked on the surface of the structure can be used to accurately locate cracks on the sketch. Crack widths can be measured to an accuracy of about 0.025 mm using a crack comparator, which is a small hand held microscope with a scale on the lens closest to the surface being viewed. Location of observed spalling, exposed reinforcement, surface deterioration, and rust staining should be noted on the sketch. The use of brittle liquid coatings on the suspected structure can also help detect the crack or growth of cracks over a period of time. The movement of the cracks can be monitored with the help of mechanical movement indicators or crack monitors using electrical resistance thin filaments. which amplify crack movement and indicate the maximum range of movement occurring during the measurement period, i.e., the extent of progressive growth of crack. Linear variable differential transformers (LVDTs) and data acquisition systems (ranging from strip chart recorder to a computer based system) are available. The cracks in concrete may be evaluated at macro, micro, submicro, and atomic levels (Angstroms Å). In the present discussion the macrostructure cracks having a size (i.e. width/depth) in the range of 0.1 to 0.3 mm are of interest.

17.3.2

Non-destructive Testing

Non-destructive tests may be performed to determine the presence of internal cracks and voids, and the depth of penetration of cracks that are visible at the surface. Useful information can often be obtained by tapping the surface of concrete with an ordinary hammer. The difference in sound when concrete is struck may identify areas of delamination of concrete that has been damaged, say by fire. A hollow sound indicates a separation or crack below the surface. In order to assess the strength of a structure, the position and size of reinforcement must be known. A knowledge of reinforcement may also be helpful in interpretation of crack pattern. In the absence of records, or as a check on their accuracy, the depth of cover may be measured by electromagnetic cover meters, and they may also indicate the position of individual bars if they are not too close together. The presence of reinforcement can be determined using a pachometer. Some pachometers are calibrated to give either the depth or the size of the bar. If corrosion is the suspected cause of cracking, the easiest approach entails the removal of a portion of the concrete to directly observe the steel. There are number of electrical techniques for detecting corrosion of reinforcement. In one of the commonly used techniques corrosion potential is detected by electrical potential measurement using a suitable reference half-cell. The most common technique to detect cracking using ultrasonic non-destructive test equipment is the through transmission testing using soniscope. The arrangement is shown in Fig. 17.2. The method consists of transmitting a mechanical pulse to

688

Concrete Technology TT

TT

TT

d

RT Partial depth crack

RT

TT = Transmitting Transducer RT = Receiving Transducer

Direct transmission Increased path length due to discontinuity RT

AMPLITUDE

(a) Pulse transmission through member

Input Signal

Time Full-strength Signal Attenuated Signal

Output Signal

Time

t

t = Time delay between transmitting and receiving signals Ultrasonic Pulse Velocity (UPV) = d /t (b) Oscilloscope signal

Fig. 17.2

Through-transmission ultrasonic testing

one face of the concrete member and receiving it at the opposite face, and calculating the pulse velocity from the time taken by the pulse to pass through the member, and the distance between the transmitting and receiving transducers. A significant change in measured pulse velocity may occur if an internal discontinuity results in an increase in the path length of the signal as it passes around the end of crack. Generally, the higher the pulse velocity, the better the quality and durability of the concrete. Internal discontinuities can also be detected by attenuation of signal strength if the signal is displayed on an oscilloscope. If no signal arrives at the receiving transducer, a significant internal discontinuity, such as a crack or void, is indicated. An indication of the extent of the discontinuity can be obtained by taking readings at a series of positions on the member. The results of ultrasonic testing should be interpreted cautiously, e.g., with fully saturated cracks the ultrasonic testing will generally be ineffective. Another method called the pulse echo method for loation of cracks is shown in Fig. 17.3.

Repair Technology for Concrete Structures

Impactor

689

Impactor

2

1 1 Receiving transducer

Receiving transducer

2

3

(a) Pulse Echo

1

1 2 3

(i) Solid member

(ii) Internal crack (b) Oscilloscope Signals

Fig. 17.3

17.3.3

Pulse echo method for crack location

Tests on Concrete Cores

The basic objective of testing the hardened concrete cores is to check its compliance with specifications. In order to resolve the disputes arising out of the testing of concrete by destructive tests using conventional procedures, concrete cores are drilled and tested to estimate the strength of concrete in the actual structures. However, significant infomation can be obtained from cores taken from selected locations within the structure. Cores and core holes afford the opportunity to accurately measure the width and depth of cracks. Core material and crack surface can be examined petrographically to determine the presence of alkali-silica reaction products or other deleterious substances. The cores can also be used to detect segregation or honeycombing or to check the bond at construction joints. Usually a core is cut by means of rotary cutting tool with diamond bits. Thus a cylindrical specimen is obtained, sometimes containing embedded fragments of reinforcement. The cores are soaked in water, capped and tested in compressions in a moist condition. The height/diameter ratio of the core is kept nearly two to compare its strength with standard cylindrical specimens. In some cases, beam (prism) specimens can be sawn from road or airfield pavement slabs using a diamond or carborundum saw. Such specimens are tested in flexure. The cutting of beams is, of course, a very cumbersome and expensive process and not much used. While comparing the strength of cores with that of standard cylinder specimens, the factors regarding site, curing and the position of the cut-out concrete in the structure should be considered. Cores usually have the lowest strength near the top surface of the structure, be it a column, wall, beam or even a slab. With an increase in depth below the top surface, the strength of cores increases up to about a depth of 300 mm.

690

Concrete Technology

17.3.4

Review of Drawings for Reinforcement Details

Proper detailing of reinforcement, including adequate cover is essential to ensure the required performance characteristics of concrete. The detailing of reinforcement should be based on a proper appreciation of the placing and compaction techniques to be used. Some of the factors contributing to the poor design detailing are: 1. 2. 3. 4. 5.

Re-entrant corners Abrupt changes in section Inadequate joint detailing Poor detailing of expansion and contraction joints Improper or inadequate drainage

The reinforcement drawings and specifications from architects and engineers are examined to determine if and where observed cracking can be attributed to inadequate reinforcement. A comparison should be made between the design loads and the actual loads acting on the structure.

17.4

SELECTION OF REPAIR PROCEDURE

The repair of concrete structures may vary between just giving a cosmetic treatment and a total replacement. By a proper investigation and by using well-designed equipment, tools and materials, a number of structures which may appear to have been damaged beyond repair can be reinstated economically. An appropriate repair method can be selected depending upon the cause and extent of damage, importance of the structural element, and its location. The choice of the method will determine its success. A procedure may be selected to accomplish one or more of the following objectives: 1. 2. 3. 4. 5. 6. 7.

To increase strength or restore load carrying capacity To restore or increase stiffness To improve functional performance To provide water tightness To improve appearance of concrete surface To improve durability To prevent access of corrosive materials to reinforcement

Depending upon the nature and extent of the damage, one or more repair methods may be selected, e.g., tensile strength can be restored across a crack by injecting it with an epoxy. However, it may be necessary to provide additional strength by adding reinforcement. Epoxy injection alone can be used to restore flexural stiffness if further cracking is not anticipated. Cracks causing leaks in water retaining structures should be repaired unless the leakage is considered minor. They can be repaired when cracks result in an unacceptable appearance. However, if the crack location is still visible, some form of coating over the entire surface may be required. To minimize further deterioration due to corrosion of reinforcement, cracks exposed to a moist environment should be sealed. Success of the long term repair procedures chiefly depends on the nature of cracks as well as their cause. For example, if the cracking is primarily due to drying shrinkage,

Repair Technology for Concrete Structures

691

it is likely that after a period of time the cracks will stabilize. On the other hand, if the cracks are due to continuing foundation settlement, repair will not be effective until the settlement problem is corrected. The repair procedure also depends on the capabilities and facilities available with the builder, and on the availability of the repair materials.

17.5

REPAIR OF CRACKS

Once the cracked structure has been evaluated and the causes of cracking established, a suitable repair procedure may be selected which takes these causes into account. The methods of crack repair including the characteristics of cracks that may be repaired with each procedure and the types of structure that have been repaired are described in the following sections. The repair of concrete structures is carried out in the following stages: 1. Pretreatment of surface and reinforcement, i.e., removal of damaged concrete. This process is termed as the preparation of surface for repairs. 2. Application of repair material.

17.5.1 Preparation of Surface Prior to the execution of any repair, one of the essential requirements common to all repair techniques is that all deteriorated or damaged concrete should be removed. This can be accomplished by using tools and equipment, the type of which depends to a large degree on the size, depth, and extent of repair. For smaller jobs, the removal of concrete can be accomplished by hand tools whereas for larger repairs, the surface can be prepared by using light and medium weight air hammers fitted with spadeshaped bits. Care should be taken to avoid any damage to the unaffected portions. For cracks and other narrow defects, sawtoothed bits can be used to obtain sharp edges and suitable under cuts as shown in Fig. 17.4(c). The preparation of a surface for repair involves the following steps: 1. 2. 3. 4.

Complete removal of unsound material Undercutting along with the formation of smooth edges Removal of the cracks from the surface Formation of a well-defined cavity geometry with rounded inside corners. For the damaged area shown in Fig. 17.4(a), the correct cavity geometry is illustrated in Fig. 17.4(c) 5. Providing rough but uniform surfaces for repair The surface so prepared should be clean, dry, free of laitance and strong. A clean surface means that there should be no foreign matter (contaminants) such as dirt, loose particles, grease or oil, paints, resins, etc., on the surface. Dry surface in most cases means that no free water shall be present. Free of laitance means that the skin of high water-cement gel, which appears on the surface during concrete placement, should be removed. This skin has poor integrity and adhesion to the parent concrete. Strong concrete below the surface refers to the ability of concrete to resist fractures due to the stresses exerted on it by the repair material and techniques.

692

Concrete Technology

DAMAGED AREA

(a) Plan of damaged area

SECTION

(b) Incorrect method of cutting out

SECTION

(c) Correct method of cutting out

Fig. 17.4

Preparation of surface by cutting out the damaged area

Dust is any fine visible foreign matter present on the concrete surface and can be detected by wiping across the surface of concrete with a dark cloth, the presence of white powder on the cloth indicates the presence of dust, and such a surface is not suitable for the application of most of the repair materials. Dust and other loose foreign materials present on the surface of the concrete can be removed by wire brushing. If this method is not adequate, and other method may

Repair Technology for Concrete Structures

693

be used which will actually remove some concrete along with contaminants. Grinding, scarifying and sand-blasting are the commonly used methods. Grinding or paring is slow but suitable for smaller areas. Scarifying chips away a thin layer of concrete. Scabblers can be used to scarify up to 6 mm from the concrete surface. These are air-operated units having tungsten carbide bits that pound the concrete surface. The subsequent scarifying may be performed using an abrasive blasting machine. Sand or steel shot blasting is perhaps the most effective method of cleaning a concrete surface. The water blasting with low pressure jets may be effective in some cases. Oil can be detected by sprinkling water on the surface. If the water stands in droplets without spreading out immediately, it indicates that the surface is contaminated with an oily substance which will interfere with the bonding of most repair materials. Oil, grease and animal fat may be removed by chemical cleaning, i.e., scrubbing the surface with detergents, caustic soda solution or trisodium phosphate. A vigorous scrubbing action with a stiff broom should be carried out during the washing procedure. The surface should be washed off thoroughly with a pressure hose to remove all traces of loosened oil and as well as the cleaning solution. The laitance which can be detected by the presence of fine powder on surface, when it is scrapped with the knife blade, may be removed by acid etching. However, if the acid etching is to be used the surface should be precleaned by removing any build-up of dirt, oil, grease or any other foreign matter. In case the presence of chlorides in the concrete is not objectionable, a 10 per cent solution of hydrochloric acid in water is applied in the ratio of 1.15 litre/m2 of the surface. A stiff bristle broom or brush should be used to spread and vigorously scrub the acid solution uniformly on the surface. After the foaming action has subsided. the surface is thoroughly washed off with water still scrubbing it with stiff brooms. This is necessary to remove the salts which may have been formed by the reaction of the acid with the cement. The acid at the surface after washing should be checked with a litmus paper. When the presence of chlorides is objectionable, a 15 per cent solution of phosphoric acid can be substituted. The presence of acids from other sources may also be ascertained by placing a litmus paper in a thin film of water on the surface of concrete. A pH value below 4 indicates that the acidity of concrete is too high for the successful application of repair or barrier systems. Form release agents and curing compounds should be avoided when it is known that a barrier system is to be applied on the concrete surface. If used, they should be such that they can be completely removed before the application of barrier system, It is recommended to avoid chemical release agents altogether. Proprietary paint system which can be applied to the forms to prevent contamination of concrete and provide good release are available. In case of application of overlays, waterproofing or protective barriers, the surface should be uniform. The size of the defects on the concrete surface which can be tolerated depends on the nature of the barrier system itself. The recommendations of the manufacturer of the system is the best guide. However, for most of the decorative systems all protrusions should be removed and visible holes should be filled. For other types of barriers, protrusion higher than 1.6 mm, the spalls and holes greater than 3.2 mm diameter should be repaired. The size of the holes and the surface conditions

694

Concrete Technology

determine the material that may be used. Small surface voids are filled with cement grout whereas larger voids can be filled with drypack or shotcrete. A visual inspection may provide an adequate control of these conditions. In case of any doubt regarding the adequacy of the cleaning process or the presence of undetected contaminants, a patch test should be performed. In this test, the repair material or barrier should be applied to a typical surface whose adequacy of preparation is to be tested. After curing the adhesion of the material should be checked for its effectiveness.

17.5.2 Repair Techniques The repair of cracked or damaged structures is discussed under two distinct categories, namely, ordinary or conventional procedures; and special procedures using the latest techniques and newer materials such as polymers, epoxy resins, etc.

Ordinary Procedures Superficial or fine cracks are generally removed by treating the surface with whitewash, soft distempers, silicate cement paints (Snowcem or Aquacem), etc., The methods and materials used for the repair of patches of deteriorated concrete in the structures are described below. The repair is carried out in four steps. 1. Preparation of surface The cracked and deteriorated areas are cut or chipped out to the solid concrete. Application of a sound patch to an unsound surface is meaningless because patch will eventually come out. Any attempt to take short cuts over surface preparation is false economy. The area to be chipped out should be delineated with a saw cut to a depth of about 5 mm in order to provide neat edge. The edges should be cut out as straight as possible and right angled to the surface with corners rounded within the hole. The edges are slightly undercut as illustrated in Fig. 17.4(c) to provide keys at the edges of the patch. The thickness of edges should not be less than 25 mm to prevent them from breaking under load. The unsound concrete is removed with percussive tools. All the loose material should be cleaned and the surface should preferably be washed off before actual patching work is started. Care should be taken to remove excess water from the cavity. To obtain a good bond, it is generally recommended that the surface of concrete be coated with a thin layer of cement grout before placing the patching material. The grout mix and the quantity of mixing water should be same as that of mortar in the replacement material. The patching material should be filled in before the layer of grout dries. In case of corroded reinforcement the concrete should be removed far enough to ensure that all corroded areas are exposed so that they can be cleaned. Carbonated or chloride contaminated concrete in contact with reinforcement should also be removed and replaced with fresh concrete or an impermeable resin compound. Application of corrosion inhibiting chemicals such as phosphates to exposed reinforcement after cleaning may be recommended. A slurry coating of polymer latex and cement can be used in case of cement-based repair. Resin-based coatings are suitable for the use with both cement-based and resin-based repair materials.

Repair Technology for Concrete Structures

695

2. Selection of materials The repair system should be so selected that the mechanical properties of the repair material are similar to those of the structure being repaired. Cement-based repairs can provide fire resistance while resins soften at relatively low temperature. However, the properties of resins may be adjusted within fairly wide margins by suitable formulation so that they can, to some extent, be tailored to fit the job in hand. But the rate of reaction is both exothermic and temperature dependent, the thermal stresses that develop as the materials cool should be kept in mind while planning the repairwork. For conventional repairs, the cement-based materials to be used for patchwork may either be mortar or concrete depending upon the extent of repair. 3. Application of material The methods generally used for filling the material are: (a) drypacking, (b) concrete replacement, (c) mortar replacement, (d) grouting, (e) large volume prepacking of concrete, and (f ) shotcreting or guniting. After the concrete surface has been prepared, a bonding coat should be applied to all the cleaned exposed surfaces. It should be done with minimum delay. The bonding coat may consist of cement slurry or an equal amount of cement and fine sand mixed with water to a fluid paste consistency. Adequate preparation of surface and good workmanship are the ingredients of efficient and economical repairs. (a) Drypacking The method consists of hand placing of low water content mortar on the prepared surface followed by tamping or ramming of mortar into place, producing an intimate contact between the mortar and the existing concrete. Because of the low water cement ratio of the paste, there is little shrinkage, and the mortar provides a durable, strong and watertight patch. The repair material usually consists of cement and sand mortar in proportions of about 1 : 2.5 or 1 : 3 using medium (passing 1.18 mm IS sieve) or coarse concreating sand. However, for a smooth surface finish, a finer sand for final layer may be used. The first layer of repair material should be applied immediately after the application of bonding coat while latter is still wet. Provision of some mechanical anchorage for the patch by means of dowels drilled and grouted into the surrounding concrete is a wise precaution. The water content for the mix should be carefully chosen because excess water will increase shrinkage which may loosen the replaced material, whereas less water will not make a sound solid pack. The water content should be such as to produce a mortar which will stick together on being moulded into a ball, at the same time not exclude water but leave the hand damp. To minimize the shrinkage in place, the mortar should stand for 30 minutes after mixing and then remixed prior to use. The repair material should be filled properly in compacted layers of about 10 mm thick and each layer should normally be applied as soon as the preceding one is strong enough to support it. The preceding layer should be scratched before placing the succeeding layer to secure a good bond or key. Each layer is compacted over its entire surface by using a hardwood stick of

696

Concrete Technology

about 200 to 300 mm length and up to 25 mm diameter with a hammer. The last overflowing layer is struck flush with the surface. There need be no time delays between layers. In case of delay between layers a fresh bonding coat should be applied when work is resumed. The mortar may be finished by 1aying the flat side of hardwood piece against it and striking it several times with a hammer. Curing is done by covering the patch with absorbent material that is kept damp, preferably covered in turn by polythene sheets sealed at edges. Shading from sun may be necessary. Sprayed-on curing membranes may be used after complete patch has been applied. For dormant (in-active) crack repair the portion of crack adjacent to the surface should be widened (routed or chased) to a slot about 25 mm wide and 25 mm deep with a power driven sawtooth bit. The slot should be undercut so that base width is slightly greater than the surface width. After the slot is thoroughly cleaned and dried, the bond coat followed by repair material should be applied. Moist curing can be done by supporting a strip of folded wet burlap along the length of the crack. (b) Concrete replacement In general, this method is used for large and deep patches like those encountered in the repair of old and deteriorated portions of concrete structures where concrete is to be placed to a minimum depth of about 150 mm. The general applications of this method are in the repair of walls, piers, parapets and kerbs, and for resurfacing walls and relining channels. The method is particularly suitable where the holes extend throughout the concrete section or where the surface area of hole is at least 0.09 m2 with a depth of 100 mm for plain concrete, or 0.045 m2 with a depth a little more than reinforcing steel in case of reinforced concrete. As in case of other types of repairs defective concrete is removed so that the sound surfaces are exposed and reinforcement cleaned. In case the repair material cannot be placed immediately, it may be necessary to apply a protective (corrosion-inhibiting paint) coating to the reinforcement. To ensure good bond, to reinforcement, a further coating with a long open time may sometimes be applied. In plain concrete, the defective area is prepared as explained earlier, but if the repairs are to be made in the reinforced concrete, the reinforcing bars should not be left partially embedded, but at least 25 mm clearance should be provided around each exposed bar. In the case of wall repairs, the top of the hole should be cut to a fairly horizontal line with a 1 to 3 upward slope from back towards face to prevent the formation of air pockets at the top during vibration. The bottom and the sides of the hole should be cut sharp and approximately square as shown in Fig. 17.5. All interior corners should be rounded to a minimum radius of 25 mm. For repairing the wall for a height more than 500 mm, the back form is built in one piece, the front formwork is constructed in horizontal sections to place concrete conveniently in about 300 mm deep lifts. All formwork joints should be mortar tight. Before placing the front sections of formwork for each lift the surface of the old concrete should be coated

Repair Technology for Concrete Structures

697

with cement grout having the same water–cement ratio as that for the mortar in the replacement concrete. To reduce shrinkage, a minimum period of 30 minutes is allowed to elapse between the lifts. The mix proportion and water–cement ratio of the replacement concrete should be the same as that used in the structure. The water content must be as low as possible. Compaction is best achieved by internal vibration if there is access for vibrator. If an external vibrator clamped on to the formwork is used, care must be taken not to damage the seals between formwork and existing concrete. The stripping time may vary from 20 to 48 hours depending upon the location and extent of repair. Round corner A

Slope

Slope 1:3

A

SECTION A–A

Square edge

Fig. 17.5

Repair by concrete replacement

(c) Mortar replacement The method is suitable for the cavities which are too wide for drypack or too shallow for concrete replacement. Generally it is used for shallow depressions no deeper than that for the side of the reinforcing bars nearest to the surface. For replacement of deteriorated concrete, this method is suitable for minor restorations. The mortar replacement can be done by hand or can be applied pneumatically by using a small pressure gun. It is preferable to preshrink the repair mortar by mixing it to a plastic consistency as long in advance of its use as cement permits (the preshrinking time ranges from 60 to 120 minutes). For hand placing, the mortar should have the same proportions as the mortar used in the mix of which the structure is made. In the case of a pressure gun, the ratio recommended is 1 part of cement to 4 parts of sand.

698

Concrete Technology

In case the hole being repaired is deeper than 25 mm, the mortar should be applied in layers not exceeding 15 mm in thickness to avoid sagging loss in the bond. The subsequent layers are laid at an interval of 30 minutes or more. The final layer placed slightly overflowing the hole is struck off level with the surface. (d) Grouting The wide and deep cracks may be repaired by filling them with Portland cement grout. The grout mixtures may contain cement and water or cement, sand and water, depending upon the width of the crack. However, the water-cement ratio should be kept as low as practicable to maximize strength and minimize shrinkage. Water-reducing admixtures may also be used to improve the properties of the grout. The procedure consists of cleaning the concrete along the crack; providing built-up grout ports (nipples or seats) at intervals, sealing the crack between the ports with a cement paint or sealant, etc., testing the seal and then grouting the whole area. After the crack is filled, pressure should be maintained for several minutes to ensure good penetration. The method is particularly useful for repairing wide cracks in gravity dams, concrete walls, etc. For narrow cracks in concrete, chemical grouts consisting of solutions of two or more chemicals that combine to form gel, a solid precipitate, can be advantageously used. The chemical grouts are also applicable in moist environment and provide wide limits of control of gel time. (e) Large volume prepacked concrete Prepacked concrete is used to repair old works and is generally adopted when the conventional placing of concrete is difficult. It is advantageously used for large repair jobs, underwater placement, resurfacing of dams, repair of tunnel linings, piers, retaining wall and spillways. It consists in injecting grout into the voids of compacted mass of clean and well-graded coarse aggregate in the forms. The aggregate is wetted after compacting it and then grout is pumped into the forms. The pumped grout usually contains fine sand, Portland cement, a pozzolanic material of low mixing water requirement, a fluidifier designed to increase the fluidity and to inhibit early stiffening of the grout, and mixing water. The forms must constitute a closed system, vented at the top only to avoid trapping of air pockets. The formwork may be of conventional rigid type that either encloses the member to be repaired or is sealed to it at its edges. In grouting aggregate work transparent panels are sometimes provided so that the progress of grouting can be monitored. The aggregate is then placed in the form, and grout lines are attached to inlet or injection plugs fixed to the forms. Pumping of grout should begin at the lowest point as shown in Fig. 17.6 and proceed upwards in order to prevent the formation of air pockets. More than one injection points built into the formwork at different levels may be required if complete filling from bottom requires very high injection pressure. The grout lines should be spaced no more than 1.5 m center to center, and the grout level in the mass of aggregate should be brought up uniformly as determined by the observations of grout levels in the grout pipes. A positive head, of at least 1 to 2 m should be maintained in the grout pipes above the level of the outlets.

Repair Technology for Concrete Structures

699

The injection of grout should be a smooth, uninterrupted operation and positive head should be maintained in the grout lines after the forms have been filled and until the grout has set. (f ) Shotcreting or guniting As explained in Section 14.8 shotcrete or gunite is mortar or concrete conveyed through pressure hose and applied pneumatically at high velocity onto a surface. This material has found wide applications in several major repair works because of ease with which it can be applied on vertical, horizontal or overhead surfaces. The objective of this type of repair may be to replace concrete that has been lost or removed, and to increase effective cover to the steel reinforcement or to protect the structure against future damage by adding additional concrete. In the preparation of surface to be repaired, all the affected concrete must be cut back to unaffected material.

Fig. 17.6

Prepacked concrete repair of retaining wall

When a large portion of the structure is defective, it is recommended that repair should encase the whole of the assessible external surfaces with a specified minimum thickness of sprayed concrete, which shall be brought out to uniform profile, and eyeable lines by additional infilling of the areas from where the damaged concrete has been removed. This can be achieved by applying sprayed concrete over areas of undamaged concrete. The undamaged surface must also be prepared by thoroughly roughening it to remove all original cement laitance, surface deposits and impurities. For the development

700

Concrete Technology

of good bond, the prepared surface must be sound, rough and homogeneous (i.e., free from shattered or laminated concrete). Generally, a welded steel fabric is used, or steel fibers are incorporated in case of sprayed concrete without polymer admixtures to minimize the risk of cracks of sufficient size developing (which allows penetration of air and water) by encouraging the development of a large number of very fine, insignificant cracks. A typical fabric for this purpose would be of 3 to 4 mm diameter high tensile steel bars in the form of 75 to 100 mm mesh. With such a fabric the sprayed concrete thickness should be 50 mm minimum which will provide adequate cover to the fabric itself. The fabric reinforcement should be securely fixed by the nails driven into plugs set in parent concrete and bent over to grip it, with spacers to hold it at least 12 mm from the surface. Fixing points should be spaced at sufficiently close intervals such that the mesh does not belly out during the concrete spraying. The sprayed concrete thickness may be reduced in case of non-rusting steel fabric. The minimum thickness may be 30 to 50 mm. Chases at least 20 mm wide × 20 mm deep should be cut with pneumatic tools at the perimeter of the area to be sprayed, into which the edge will tuck to provide a sound finish at that point. Light timber profiles fixed securely in correct position should always be used as a guide to thickness and to provide eyeable lines on all main arrises. The aggregate to be used in sprayed concrete should be clean, well graded from 10 mm to fines, but without an excess of fines, clay, silt or dust. Typical mix proportions (aggregate-cement ratios) are 3.5 : 1 or 4 : 1, and typical strength of 30 MPa at 28 days. Construction joints are formed at a slope wherever possible, not at right angles as in cast concrete. The face of sprayed concrete is normally carried forward at a slope so that a construction joint of this kind is formed naturally. The interface of the joint must be cleaned of rebound, overshoot and laitance so that a good bond is achieved at the joint. Where defects are in isolated areas and patchwork repairs are justified the repairs should extend at least 300 mm on to the sound concrete at the perimeter of the defective area and terminate in chases. Rectangular patches are preferable to irregular shapes. Prior to commencing spraying, the interface must be prepared and wetted so that it does not absorb water from the sprayed concrete, but at the same time is not so wet that there is excess free water on it. The adequacy of plant and of the properties of the material to be sprayed are confirmed by a preconstruction test panel. It consists in placing a test box 600 or 750 mm square and 100 mm thick on the surface simulating the construction conditions of proposed work and then spraying the concrete. Concrete cores of suitable age are taken from the test panels for testing for the specified standards of crushing strength, porosity, water penetration resistance, etc., However, previous experience with similar plant and materials is often accepted in lieu of such preconstruction tests except on very big jobs. Test panels also help to judge the competence of nozzlemen.

Repair Technology for Concrete Structures

701

Fabric mesh or bar reinforcement should be checked for secure fixing, correct positioning and overlaps, spacers and cleanliness. Profile guides. whether timber formers or stretched wires must be firm, tight and correctly positioned. The spraying should be carried out with the nozzle held approximately at right angles to the interface, and at such a distance that the concrete compacts effectively. The nozzle should be fanned from side to side, and up and down so that layer is evenly built up over the area in front of the nozzleman. If there is any unevenness in the spray of material, the nozzleman must turn the nozzle away from the work until the spray becomes even again. Where the full thickness has to be built up in more than one layer, it is not advisable to apply more than 50 mm or so at a time on vertical surfaces, or 25 mm or so overhead. The surface of each layer may be lightly trimmed with the edge of a steel float, and must be wetted again once set before applying the next layer. Curing of the sprayed concrete is even more important than that of conventional cast concrete because thinner section may make water loss easier and more serious. The curing method may include a fine water spray, wetted hessian and curing compounds. 4. Curing of repair work The curing of patch material requires much more care than that required for a complete structure. There is a tendency of old concrete absorbing moisture from the replacement material. The curing methods have been discussed earlier in detail but the methods commonly used in conventional repair work are summarized below: (a) Horizontal repaired surface can be cured by ponding or by placing wet gunny bags. (b) The vertical or inclined repaired surface may be cured using damp hessian or wet burlap pads. (c) Where the above two methods are not applicable, membrane curing can be used. Initial curing with water followed by membrane curing is very effective. (d) Deliquescent salts which hasten curing by keeping the patch moist may also be used.

Special Procedures The polymers have been recently introduced in concrete technology for multi-purpose applications in the repair and maintenance of concrete buildings and other structures. The polymers used in concrete repair principally consist of two different types of materials: 1. Polymers used as modifiers for cementing systems 2. Reactive-thermosetting resins For the repair of both active as well as dormant cracks, epoxy mortars consisting of an epoxy, hardener and sand, have been used effectively to seal the cracks. The epoxy resins and polymers possess excellent adhesive and sealing properties, although the cost of this repair is quite high. Some materials and techniques for repair of structures are described in the following pages. The physical properties of typical systems used in concrete repair are given in Table 17.2.

702

Concrete Technology Table 17.2

Physical properties of typical products used in concrete repairs

Property

Cementing grouts, mortars and concretes

Polymer modified cementing systems

Polyester resin grouts, mortars and concretes

Epoxy resin grouts mortars and concretes

Compressive strength, MPa Compressive modulus

20–70

10–80

55–110

55–110

E-value, GPa Flexural strength, MPa Tensile strength, MPa

20–30 2–5 1.5–3.5

1–30 6–15 2–8

2–10 25–30 8–17

0.5–20 25–50 9–20

0–5

0–2

0–15

Elongation at breaking 0 point, per cent Linear coefficient of (7–12) × 10–6 thermal expansion per °C Water absorption, 7 days 5–12 at 25 °C, per cent

(8–20) × 10–6 (25–35) × 10–6 (25–30) × 10–6

0.1–0.5

0.2–0.5

0–1

Maximum service temperature under load, °C

In excess 300 dependent upon mix design

100–300

50–80

40–80

Rate of development of strength at 20 °C

1–4 weeks

1–7 days

2–6 hours

6–48 hours

1. Polymer modified cementing system The polymers used as admixtures for cementing systems are normally available as latex (milky white dispersion in water) and are used to gage the cementing mortar as a whole or as a partial replacement of mixing water. The polymer latex forms a network of polymer strands interpenetrating the cement matrix and improves the structural properties and reduces permeability of mortar. Such mortars provide the same alkaline passivation protection to steel as do the conventional materials, and can be readily placed in a single application of 12 to 16 mm thickness which gives adequate protective cover. The functions of polymer latex are: (a) It acts as a water reducing plasticizer. (b) Provides a good bond between repair mortar and concrete surface being repaired. (c) Improves tensile and flexural strengths of the mortar. (d) Reduces permeability of repair mortar to water. There are different types of polymer latexes which have been used as modifiers for cementing systems. They include polyvinyl acetate (PVAc), styrene butadiene rubber (SBR), polyvinyldiene dichloride (PVDC), acrylics and modified acrylics (generally styrene acrylics). PVDC latexes are not recommended for repair mortars for reinforcement concrete. PVAc latexes are widely used as general-purpose bonding aids and admixtures for building industry for interior applications. SBR, acrylic and modified acrylic latexes are most commonly used as admixtures in concrete repair mortars.

Repair Technology for Concrete Structures

703

2. Resin-based materials Resin mortars are extensively used in the locations where the cover is less than 12 mm and areas to be repaired are relatively small. In case of resin mortars the protection of reinforcement depends upon total permeability of envelope. This requires that the reinforcement surface be prepared to a very high standard. Resin repair mortars use reactive resins filled with carefully graded aggregates. Epoxy resin mortars are most commonly used in concrete repairs. Polyester and acrylic resin-based mortars are generally used for small area repairs where very rapid development of strength is required. Since in most repair situations, the polymer-based repair material is bonded directly to concrete or other cementing material, it is, therefore, important to match the mechanical and physical properties of polymer repair composition and concrete. Polymer bonding aids can assist in achieving a reliable bond between green uncured concrete and cured concrete.

17.5.3

Polymer-based Repairs

As explained earlier in Chapter 14 the polymer concrete includes composite prepared by one of the following methods.

Polymer Impregnated Concrete (PIC) This is a portland cement concrete impregnated by a monomer system which is subsequently polymerized by radiation or heat, and the use of a catalyst. Polymer Cement Concrete (PCC) This is a concrete in which the monomer is added during the mixing of Portland cement, water and aggregate, followed by polymerization or curing of the replaced material after its placement. Polymer Concrete (PC) This is a composite material obtained by adding a polymer or its precursor to the aggregate and polymerizing or curing the material after its placement. The concrete polymer materials provide high strength and improved durability under aggressive conditions as compared to conventional concretes. PIC has proved to be the most successful concrete polymer material for construction. Liquid and gaseous monomers can fully penetrate concrete by external pressure and can be polymerized. To obtain maximum polymer loading in the concretes and hence maximum improvement in the desired properties, it is necessary to dry the concrete to constant mass, remove air, soak in a low viscosity liquid monomer, pressurize with nitrogen and wrap the specimen in polyethylene sheet to reduce evaporation prior to polymerization. The polymer systems commonly used are the following: 1. 2. 3. 4. 5.

Methyl-methacrylate (MMA) MMA + 10 per cent trimetholpropane trimethacrylate (TMPTA) Styrene and polyester styrene Methanol Vinyl monomer

704

Concrete Technology

The sulfur impregnated concrete using commercially available (99.9 per cent) pure sulfur may provide a practical and inexpensive substitute of PIC. The polymer cement concretes may be obtained by substituting at least 30 per cent of epoxy resin for cement in ordinary Portland cement concrete mixes. The addition of fly ash to the epoxy cement mixes is favorable for strength. The polymer-based crack repair can be affected as described in the following sections. 1. Polymer impregnation The technique consists of flooding the cleaned, dried cracked concrete surface with a monomer which is then polymerized in place, thus filling and structurally repairing the crack. A monomer system is a liquid that consists of small organic molecules capable of combining to form a solid plastic. The monomer systems used for impregnation contain a catalyst or an initiator and the basic monomer. They also contain a cross-linking agent. When heated the monomers join together or polymerize becoming a tough, strong and durable plastic that greatly enhances a number of concrete properties. Monomers have varying degrees of volatility, toxicity and flammability and do not mix with water. These are low viscosity fluids which may soak into dry concrete, filling the cracks in much the same manner as water does. However, if the cracks contain moisture, the monomer will not soak into the concrete at each crack face, and consequently the repair will be unsatisfactory. If a volatile monomer evaporates before polymerization, the repair will be ineffective. Polymer impregnation can be used for repairing the fractured elements, by drying the fracture, temporarily encasing it in a watertight sheet metal, soaking the fracture with monomer and polymerizing the monomer. Large voids in compression zones may be filled with fine and coarse aggregates before flooding them with the monomer, thus providing a polymer concrete repair. For treating the concrete surfaces that contain a large number of cracks, vacuum impregnation may be used. The process essentially consists of enclosing the part of structure to be repaired within an air-tight plastic cover and applying vacuum to exhaust air from all cracks within the cover. Resin grout or monomer is then admitted which is forced into cracks and pores in the concrete surface by atmospheric pressure. On completion of impregnation, the cover is removed before the impregnant hardens. The selection of appropriate impregnants and degree of vacuum may be based on experience. The process is extensively used as a means of reducing permeability of weak concrete or masonry. It can also be used to improve the abrasive resistance of industrial concrete floor slabs. Polymer impregnation has not been used successfully to repair fine cracks. 2. Drilling and plugging The method is only applicable for the cracks running in reasonably straight lines and accessible at one end. It consists in drilling a hole down the length of crack and grouting it to form a key. A hole 50 to

Repair Technology for Concrete Structures

705

60 mm in diameter should be drilled, centerd on and following the crack as shown in Fig. 17.7. The hole must be large enough to intersect the crack along its full length and provide sufficient repair material to structurally take the loads exerted on the key. The drilled hole is then cleaned and filled with grout. For problems concerning watertightness, the drilled hole should be filled with a resilient material of low modulus in lieu of grout. In case where both load transfer and watertightness are desired, two holes are drilled, one filled with resilient material and the second being grouted.

17.5.4

Resin-Based Repairs

Cracks in reinforced concrete wider than approximately 0.3 mm may require sealing to prevent the entry of moisture, oxygen and other materials, or for other reasons. The choice of the method and materials will depend upon the cause of cracking and, whether a permanent structural filling of crack is needed to carry out any other required strengthening. For restoring the structure to its original strength, the lowviscosity epoxy resin may be injected. Using pressure injection techniques it is possible to completely fill cracks finer than 5 mm with epoxy resin system. However, the work should be carried out skilfully to avoid blowing of surface seals due to back pressure that may develop in case of very fine cracks. Sustained pressure for several minutes may be required to completely fill a fine crack.

Fig. 17.7

Repair by drilling and plugging

706

Concrete Technology

The resin and the hardener are generally in a liquid form and each by itself is stable for an indefinite period. When these are mixed a chemical reaction takes place which converts the system from liquid to a tough plastic solid at ambient temperatures. They develop excellent strength and adhesive properties, and are resistant to many chemicals. They have good chemical and physical stability; they harden rapidly and resist water penetration. In all, they provide a toughness that couples durability with crack resistance. The resin mortar may be obtained by adding fillers such as coarse sand. The epoxy-based compounds are invariably formulated with plasticizers, extenders, diluents and fillers to produce a large number of products which have a wide range of properties. A specific formulation, may thus be made available for each application. The fast setting properties, excellent adhesion characteristics, high strength and chemical stability have led to their extensive use in the concrete construction.

General Applications Expoxies in concrete construction have been used in various ways, e.g., in providing skid resisting overlays and wearing surfaces on concrete floors, as waterproofing membrane, to bond new concrete to old, to bond precast units, to anchor dowel bars, etc. However, they have been most extensively used in the repair of potholes and other defects on concrete floors and to seal cracks in the structural members. The cleaned and dry surface is painted with epoxy compounds before placing the repair materials. The cracks may be sealed with epoxy compound, an epoxy mortar or a Portland cement mortar after priming the surface with epoxy compound. The polymer or resin overlays can be put back into use quickly due to faster curing. Being seamless they are more hygienic, and are chemical resistant.

Materials Epoxy, polyester and acrylic resins are as a class designated as thermosetting materials because when cured the molecular chains are locked permanently together. Unlike thermoplastics they do not melt when heated but lose strength with an increase in temperature. They are generally supplied as two or three component systems: resin, hardener and fillers. The resins are broadly classified as: 1. Epoxy resins 2. Unsaturated reactive polyester resins 3. Unsaturated acrylic resins Acrylic resin systems form high strength materials and are based on monomers of very low viscosity or blends of monomers with methyl-methacrylate monomer. Polyester and acrylic resins contain volatile constituents which are inflammable. Most acrylic resins are highly inflammable with a flash point below 10°C, and vapors also cause toxic reaction. The properties of commonly used resins are: 1. Epoxy resins These have high strength, good bonding characteristics, high impact resistance, high chemical resistance and may be made to provide a non-slip finish. 2. Polymer resins These differ from epoxy resins in that these can be laid over wider temperature range and have a better resistance to heat. They are mixed with cement and fine hard aggregate, and laid in thickness up to 15 mm.

Repair Technology for Concrete Structures

707

3. Polyvinyl acetate (PVAc) It is used as a bonding aid when thin mortar overlays are applied to existing concrete. The liquid can be applied straightaway onto a clean, sound surface and allowed to dry. The slight re-emulsification of the film on being rewetted by application of fresh mortar topping provides a good bond. 4. Natural rubber latex It is an admixture with excellent adhesive properties and is difficult to mix with ordinary Portland cement. It is often used with less alkaline high alumina cement for patching or for underlayments on floors which are to receive vinyl tiles. 5. Styrene butadiene rubber (SBR) It is an effective alternative to PVAc with high water resistance. Unlike PVAc, the dried film does not develop grab on rewetting so it will act as a bonding layer if allowed to dry out. Therefore, mortar mix should be applied while the tack coat of SBR is still wet. 6. Acrylic resins These admixtures have excellent water resistance and improve bond strength when mixed with mortars. Seamless, non-dusting thin floor overlays can readily be produced with acrylic resins. 7. Styrene-acrylic resins A mixture of tough styrene with acrylic resin using 1:3 cement-sand mortar can be used to produce hard-wearing floor overlays at a reasonable cost.

Repair Procedure 1. Resin-mortar The surface preparation requirements are similar to those for cement-based repairs. The constituents of resin-based material must be mixed together thoroughly by use of mechanical mixers or stirrers. Most of the failures of resin-based repairs have been traced to improper proportioning or inadequate mixing. For smaller repair jobs, to obtain correct proportions the constituents are normally available in pre-batched packs. After the preparation of surface a primer or tack coat of unfilled resin is applied to the freshly exposed surfaces of concrete and reinforcement. In general, one coat will be enough, but two may be needed if the substrata is porous. If two coats are used, the second must be applied while the first is still tacky. The patching material must be applied while the primer is still tacky, and each successive layer of patching material must be applied before the previous one has cured too much. The resin-based materials cure by chemical reaction which starts as soon as the constituents are mixed, so they have a limited pot life. The quantity of materials to be mixed in any one batch is precalculated such that it can be used before it becomes too stiff. The resin-based patches should be well compacted and impermeable. Normal safety precautions should be observed while using the resins and hardeners, i.e., gloves should be worn, splashes should be washed off the skin but solvents should not be used for this purpose; adequate ventilation should be provided; and smoking, eating or drinking should be prohibited. 2. Resin-injection The injection of polymer under pressure will ensure that the sealant penetrates to the full depth of the crack. The technique in general consists of drilling holes at close intervals along the length of cracks and injecting the epoxy under pressure in each hole in turn until it starts to flow out of the

708

Concrete Technology

next one. The hole in use is then sealed off and injection is started at the next hole and so on until full length of the crack has been treated. Before injecting the sealant it is necessary to seal the crack at surface between the holes with rapid curing resin. For repair of cracks in massive structures, a series of holes (usually 20 mm in diameter and 20 mm deep spaced at 150 to 300 mm interval) intercepting the crack at a number of location are drilled. Epoxy injection can be used to bond the cracks as narrow as 0.05 mm. It has been successfully used in the repair of cracks in buildings, bridges, dams and other similar structures. However, unless the cause of cracking is removed, cracks will probably recur possibly somewhere else in the structure. Moreover in general this technique is not very effective if the cracks are actively leaking and cannot be dried out. Epoxy injection is a highly specialized job requiring a high degree of skill for satisfactory execution. The general steps involved are as follows. (a) Preparation of the surface The contaminated cracks are cleaned by removing all oil, grease, dirt and fine particles of concrete which prevent the epoxy penetration and bonding. The contaminants should preferably be removed by flushing the surface with water or a solvent. The solvent is then blown out using compressed air, or by air drying. The surface cracks should be sealed to keep the epoxy from leaking out before it has cured or gelled. A surface can be sealed by brushing an epoxy along the surface of cracks and allowing it to harden. If extremely high injection pressures are needed, the crack should be routed to a depth of about 12 mm and width of about 20 mm in a V-shape, filled with an epoxy, and struck off flush with the surface. (b) Installation of entry ports The entry port or nipple is an opening to allow the injection of adhesive directly into the crack without leaking. The spacing of injection ports depends upon a number of factors such as depth of crack, width of crack and its variation with depth, viscosity of epoxy, injection pressure, etc., and choice must be based on experience. In case of V-grooving of the cracks, a hole of 20 mm diameter and 12 to 25 mm below the apex of V-grooved section, is drilled into the crack. A tire valve stern is bonded with an epoxy adhesive in the hole. In case the cracks are not V-grooved, the entry port is provided by bonding a fitting, having a hat like cross-section with an opening at the top for adhesive to enter, flush with the concrete face over the crack. (c) Mixing of the epoxy The mixing can be done either by batch or continuous methods. In batch mixing, the adhesive components are premixed in specified proportions with a mechanical stirrer, in amounts that can be used prior to the commencement of curing of the material. With the curing of material, pressure injection becomes more and more difficult. In the continuous mixing system, the two liquid adhesive components pass through metering and driving pumps prior to passing through an automatic mixing head. The continuous mixing system allows the use of fast-setting adhesives that have short working life. (d) Injection of epoxy In its simplest form, the injection equipment consists of a small reservoir or funnel attached to a length of flexible tubing, so as to

Repair Technology for Concrete Structures

709

provide a gravity head. For small quantities of repair material small hand held guns are usually the most economical. They can maintain a steady pressure which reduces chances of damage to the surface seal. For big jobs power driven pumps are often used for injection. The pressure used for injection must be carefully selected, as the use of excessive pressure can propagate the existing cracks, causing additional damage. The injection pressures are governed by the width and depth of cracks, and the viscosity of resin and seldom exceed 0.10 MPa. It is preferable to inject fine cracks under low pressure in order to allow the material to be drawn into the concrete by capillary action and it is a common practice to increase the injection pressure during the course of work to overcome the increase in resistance against flow as crack is filled with material. For relatively wide cracks gravity head of a few hundred millimeters may be enough. In the case of vertical and inclined surfaces, the injection process should begin by injecting epoxy into the entry port at the lowest level until the epoxy level reaches the entry port above. The injection tube is then removed and the lower entry port is capped. Using an inert gas, a pressure up to 0.7 MPa is applied for a period of 1 to 10 minutes on the port from which the injection tube has just been removed. This forces the epoxy into hairline cracks. The process is repeated at successively higher ports until the cracks have been completely filled and all ports capped. On the other hand, for horizontal cracks injection should proceed from one end of the crack to the other in the same manner. If the pressure can be maintained, it indicates that the crack is full. If the pressure cannot be maintained, it indicates that the epoxy is still flowing into unfilled portions or leaking out of the crack. (e) Removal of surface seal After the injected epoxy has cured, the surface seal may be removed by grinding or other means as appropriate. Fittings and holes at the entry ports should be painted with an epoxy patching compound. ( f) Health and safety precautions Epoxy materials are toxic and skinirritant. Contact with skin, inhalation of vapours and ingestion must always be avoided. The following precautions may be helpful: (i) Full-face shield and goggles should be used during all the mixing and blending operations. (ii) Protective overalls and polyethylene or rubber gloves should be used. (iii) Protective cream for the skin can be used. (iv) Adequate fire protection should be provided.

17.6 17.6.1

COMMON TYPES OF REPAIRS Sealing of Cracks

The crack or joint sealers are very important in concrete structures as every concrete structure has cracks or joints. The crack sealers should ensure the structural integrity and serviceability. In addition they provide protection from the ingress of harmful liquids and gases.

710

Concrete Technology

The method consists of enlarging the crack along its length on the exposed surface (called chasing or routing) and sealing it with a suitable joint sealant as illustrated in Fig. 17.8. Omission of routing may effect the permanency of repair. The routing operation consists of cutting a groove at the surface that is sufficiently large to receive the sealant, using a concrete saw or hand tools. A minimum surface width of routing of 6 mm is desirable, as repairing the narrower grooves is difficult. The surfaces of the routed joints should be cleaned with an air jet and allowed to dry before placing the sealant. 6m

CRACK Joint sealer

Fig. 17.8

Crack repair by routing and sealing

The function of the sealant is to prevent water from reaching the reinforcement, hydrostatic pressure from developing within the joint, staining the concrete surface, or causing moisture problems on the far side of the member. The epoxy compounds are often used as sealant material. Hot-poured joint sealants are used when thorough watertightness of joints is not required and the appearance is not important. Urethanes, which remain flexible through large temperature variations, have been used successfully in sealing the cracks up to 20 mm in width and of considerable depth.

17.6.2 Flexible Sealing For repairing an active crack, it is necessary to provide for its continuing movement. One way to achieve this is to rout or chase the crack along its length. The prepared

Repair Technology for Concrete Structures

711

routed crack is filled with a suitable field molded flexible sealant with strain capacity being at least as large as the one to be accommodated. A wide crack spreads movement over a greater width so that the resulting strain is compatible with sealant to be used. The sealant must adhere to the sides of the chase but debonded from bottom so that the movement in the crack spreads over the full width of the chase. This can be achieved by providing a bond breaker or debonding strip of a material such as polyethylene or pressure sensitive tape at the bottom of the chase before sealant is applied. This debonding strip does not bond to the sealant before or during cure and allows the sealant to change shape without stress concentration at the bottom. The dimensions of the seal are an integral part of its performance, Figure 17.9 shows a sectional view of a typical flexible sealing or movement joint. With an increase in chase width, the crack movement which induces shear or tension in sealant will exert considerably reduced stress on the adhesive interface with concrete, and thus enabling the face seal to cope with extensive movement. Sealant

nd breaker h

W1

h = Depth of sealant W1 Width of joint/crack W idt of sea ant

Crack

Fig. 17.9

17.6.3

Repair of an active crack by flexible sealing

Providing Additional Steel

The cracked reinforced elements (usually bridge decks) can be successfully repaired using epoxy injection and reinforcing bars. The technique consists of sealing the crack, drilling holes of 20 mm diameter at 45° to the element surface and crossing the crack plane at approximately 90° as shown in Fig. 17.10. Reinforcing bars are placed into the drilled holes, and the holes and the crack plane is filled with an epoxy pumped under low pressure varying from 0.35 to 0.55 MPa. Typically, 12 or 16 mm diameter bars extending at least 500 mm on each side of the crack are used. The reinforcing bars can be spaced to suit the needs of the repair and design criteria. The epoxy bonds the bar to the sides of the hole, fills the crack plane and brings the cracked concrete surface back to the monolithic form.

712

Concrete Technology Cracks Reinforcement in holes

Fig. 17.10

Repair by providing additional steel

An elastic exterior crack sealant is required for a successful repair. Gel-type epoxy crack sealants are useful. The sealant should be applied in a uniform layer approximately 1.5 to 2.5 mm thick and shall span the crack by at least 20 mm on each side. Resin bonding of flat steel plates to the external surface of critical structural member of bridges or buildings may prove to be the most practical and economical way of achieving local strengthening.

17.6.4 Stitching of Cracks The stitching procedure consists of drilling holes on both sides of the crack, cleaning the holes, and anchoring the legs of the stitching dogs (U-shaped metal units with short legs as shown in Fig. 17.11) that span the crack, with either a non-shrink grout or an epoxy resin-based bonding system. The stitching dogs should be variable in length and orientation or both, and should be so located that the tension transmitted across the crack is not applied to a single plane within the section but spread ove an area. The spacing of stitching dogs should be reduced at the ends of cracks.

Holes drilled in concrete Stitching dogs

Fig. 17.11

Repair by stitching the cracks

Stitching may be used when tensile strength of the member is to be re-established across major cracks. Stitching does not close a crack but can prevent it from propagating further. Stitching tends to stiffen the structure which may accentuate the overall structural restraint, causing the concrete to crack elsewhere. It may, therefore, be necessary to strengthen the adjacent sections using external reinforcement embedded in a suitable overlay. In the case of bending members, the stitching is done on the tension face where the movement is occurring. If the member is in a state of axial tension, the dogs must be placed symmetrically even if excavation or demolition is required to gain access to opposite sides of the section. The dogs are relatively

Repair Technology for Concrete Structures

713

thin and long and cannot take much compressive force. Accordingly, if there is a tendency for the cracks to close as well as to open, the dogs must be strengthened by encasement in an overlay. In water-retaining structures, the crack must be first made watertight before the stitching begins. The remedial measures for repairing the structural cracking of a slab, and a beam are shown in Figs. 17.12(a) and (b), respectively.

Fig. 17.12

17.6.5

Repair of flexural cracks in slab and beam

Repair by Jacketing

Jacketing is a process of fastening a durable material, e.g., fiber glass over the existing concrete and filling the gap with a grout that provides the needed performance characteristics. The jacketing thus restores or increases the section of an existing member by encasement in a new concrete. The technique is applicable for protecting the member against further deterioration as well as for strengthening. In either case, the concrete section may be increased beyond the value required for the design loads to allow for some deterioration in future. The method is particularly useful for the compression members like columns and piles. The column jacket can also be used for increasing the punching shear strength of column-slab connections by using it as

714

Concrete Technology

a column capital. When the jacket is provided around the periphery of the column, it is termed a collar. In most of the applications, the main function of the collar is to transfer vertical load to the column. Circular reinforcement can be used for load transfer. The practice of transferring load through dowel bars embedded into columns or shear keys has a disadvantage in that they require drilling of holes for dowels or cutting shear keys which are costly and time consuming, and can damage the existing column. Reinforcement encircling the column can be used to transfer the load through shear friction as shown in Fig. 17.13. The expansion of collar as it slides along the roughened surface causes the tensioning of circular reinforcement resulting in radial compression which provides normal force needed for load transfer. The shear transfer strength is provided by both frictional resistance to sliding and dowel action of reinforcement crossing the crack.

(i) Column to be repaired

(iv) Finished column

Fig. 17.13

(ii) Fixing of two-piece jackets

(v) View of finished bridge columns (rectangular)

(iii) Filling gap with concrete

(vi) View of finished bridge columns (circular)

Repair and strengthening of columns by jacketing

The collar can also be used as mid-column bearing surface, acting as circumferential beam to distribute the concentrated load around the column. The collar is subjected to shear and bending along the collar circumference as well as direct bearing stress under concentrated load. Thus in addition to shear transfer reinforcement, the

Repair Technology for Concrete Structures

715

collar should be provided with reinforcement for shear and moment within the collar. The repair can be used as an alternate load path from the column to the collar and then to the connecting structural component. Column collars can be provided below the slab to act as column capital to improve punching shear strength of the slab column connection as shown in Fig. 17.13. The collars are reinforced with circular ties and with dowel bars embedded 150 mm into columns. For repairing the bridge piers, a repair scheme using steel encased column collars can be used. In this design, the steel shell provides the circular reinforcement. Unless the shear transfer strength is verified by load tests, the collar design and construction should meet the following criteria: 1. The collar height should be at least 0.8 times of the original column crosssectional dimension. 2. The collar reinforcement is located at or near the outside face. The steel is lapped or welded for full development of strength. 3. The concrete strength should be at least 22.5 MPa. 4. The column surface should be roughened in the shear transfer zone by bushhammer. The surface of the column is then cleaned by a wire brush and high-pressure air to remove any loose concrete before placing the concrete in the formwork. Fiber glass reinforcement plastics, ferrocement, and other hard materials like polypropylene have been recently used for jacketing.

17.6.6

Autogenous Heeling

The natural process of crack repair known as autogenous heeling has a practical application in closing dormant cracks in a moist environment. Such a case may be found in mass concrete structures. Heeling occurs through the carbonation of calcium hydroxide in cement paste by carbon dioxide, which is present in surrounding air and water. Calcium carbonate and calcium hydroxide crystals precipitate, accumulate, and grow within the cracks. The crystals interlace and twine, producing a mechanical bonding effect which is supplemented by a chemical bonding between adjacent crystals, and between the crystals and the surfaces of the paste and aggregate. As a result, some of the tensile strength of the concrete is restored across the cracked section, and the crack may become sealed. During the process of heeling it is essential for development of any substantial strength, that the crack and adjacent concrete be saturated with water. The saturation must be continuous for the entire period of heeling. A single cycle of drying and reimmersion will result in a drastic reduction in the heeling strength. Heeling should commence as soon as possible after the crack appears.

17.7

TYPICAL EXAMPLES OF CONCRETE REPAIR

The slabs, beams and columns are the most important components of the concrete structures. Due to large number of cracks or damages observed in these components in the structures built in the past, the repair and rehabilitation of such members assume a greater importance. The materials and techniques described in this chapter can be

716

Concrete Technology

used to reinstate the structure by repairing these members. Typical repair techniques used for columns, beams and slabs will be discussed in the following sections.

17.7.1

Repair and Strengthening of Columns

The column jackets and collars discussed in Section 17.6.5 can be advantageously used in the repair of deteriorated/damaged concrete columns. The material used for the jacket may be metal, plastic or concrete. The arrangement will restore the integrity, and protect the reinforcement from the aggressive environments, and may improve the appearance of the original concrete. The jacketing material is secured to the concrete by means of bolts, screws, dowels, etc., as shown in Fig. 17.14.

GE Saw cut

elineated damaged area

Fig. 17.14

Repair of damaged area on a concrete floor slab

Repair Technology for Concrete Structures

717

In case of badly damaged columns additional vertical steel and binding medium is required. Before jacketing the column the concrete surface is suitably prepared, additional reinforcement is fixed as shown in Fig. 17.13(ii) and column is then built out to the required profile with gunite. The column surface is roughened and collar reinforcement in the form of rings is placed at the outer face before placing the concrete in the formwork. The bonding is ensured by drilling a number of holes about 50 mm deep in the old concrete and placing dowels bonded by epoxy.

17.7.2 Repair of Concrete Floor Slab System Repair, renovation and upgradation of concrete floor slabs is generally required for increasing their life span, or for change of occupancy. These may require a complete structural upgrading necessitating changes in the surface levels, the repair of visual damage only, and provision of a thin surface topping to existing concrete to produce a wearing surface requiring particularly good resistance to abrasion and wear. Provision of services below the floor surface may also require the replacement of floor surface. Before repairing the concrete floor slabs, it is necessary as usual to investigate the causes of damage and to draw the specifications for the repair keeping in mind the future use of the floor. The floor should be surveyed for the defects, the existing levels should be recorded with respect to a known datum, the existing services in floors should be plotted and tested to ensure their satisfactory condition. Many situations may require a provision of a bonding coat or topping over the whole floor area, but before this is carried out damaged areas and the joints should be repaired, otherwise these defects may reflect up through the new topping. The slabs in concrete structures under certain circumstances may deteriorate in selective locations exposing the reinforcement. The deterioration in the form of scaling may occur as a direct result of an inadequate internal air void system in the concrete. In cold regions in the presence of moisture, freeze-thaw action can cause considerable scaling deterioration. The surface delaminations of the concrete in the slab may also be caused by the corrosion of the reinforcement. The delaminations above and around the reinforcement is a condition of concern for long term structural integrity. The repair schemes to reduce further corrosion of embedded reinforcement by preventing moisture from being absorbed and infiltrating the concrete are recommended. An evaluation of the live load capacity of the slab should be made to determine the feasibility of adding a thick overlay system to the slab. A thick overlay system generally adds 550 to 700 Pa to dead load usually at the expense of live load capacity. The repair schemes consisting of overlays, isolated patches, membrane type systems, sealers, and cathodic protection should be evaluated and estimated cost of each scheme be compared to find a competitive and structurally acceptable solution. For extensively damaged large slab systems, a repair scheme using a thin methyl-methacrylate polymer concrete overlay may provide definite advantage not offered by the other schemes. In order to assess its application feasibility, sample tests of the polymer concrete of different thicknesses may be carried out for water absorption and chloride penetration characteristics. Usually, polymer concrete does not absorb moisture.

718

Concrete Technology

It is seen that the delaminated areas are usually concentrated in the negative bending moment regions of slab and near the reinforcement terminations, and hence welldefined areas may be identified as the repair areas.

Preparation of Surface Each repair area should be delineated by saw cut 3 mm wide by 6 mm deep around its perimeter lines at least 100 mm outside the damaged area as shown in Fig. 17.14. The entire surface within this area should be scarified by a scabbling machine to remove the concrete to a level below the wear or damage so as to obtain a sound clean concrete surface suitable for repair. Subsequent to this scarification of the repair area, the sounding technique can be used to locate surface delaminations by using small chipping guns. The reinforcement in the delamination area should be exposed and chipping continued until all concrete within 12 mm of the entire exposed portion of the reinforcing bar is removed. The prepared surface should be resounded to ensure that all delaminated unsound concrete has been removed. The exposed reinforcement should be sand blasted to remove all corrosion by-products. Finally, the entire repair area is blown off with compressed air to remove any loose corrosion particles, concrete, blasting sand and dust. In case of cement mortar repair the prepared reinforcement surface is coated with a cement paste layer which will provide additional protection to the reinforcement. After the coated surface has cured, the repair area is thoroughly kept wet for 24 hours if possible. All surface water must be removed from the area before filling in the cementing material consisting of 1 : 3 cement sand mix with sufficient water for firm pressing by hand. When the quantities required for repair are small, the proprietary materials may be used which are carefully batched and their quality controlled. Because of higher workability they require only hand tamping. For other cement sand mixes, vibrating hammers with a square plate on foot are often used, but for a large area a short beam fitted with form vibrator may be used to press the material into the repair area. The repair is finished off with a hand trowel and kept covered with polythene for 7 days. Dry cementing mortar materials along with a polymer, such as styrene butadiene rubber latex, can be used for high quality local repairs. The following procedure may be adopted. The exposed concrete and reinforcement surfaces are coated with a primer compatible with the repair system. The primer can be applied by rolling it on to the surface with a paint roller and allowed to cure. The coat of primer on the reinforcement provides additional protection against corrosion. After the primed surface is adequately cured, the surface becomes impermeable to moisture and could remain unprotected from environmental effects. An initial bedding layer of polymer concrete is placed in deep areas around the exposed reinforcement. The purpose of this initial bedding layer is to assure that the exposed reinforcement be encapsulated by the polymer concrete. In deeply removed areas beneath the reinforcing bars, the polymer concrete is spaded to remove the air pockets. The chipped areas are then back filled with sand loaded polymer concrete. A skin coat of neat polymer concrete followed by a 6 mm layer is applied over the exposed reinforcement.

Repair Technology for Concrete Structures

719

The material is allowed to cure before proceeding to the application of the second layer of polymer concrete to build-up the cover thickness in the area over the reinforcement. The final lift of polymer concrete is applied over the entire repair area to provide a minimum thickness of 6 mm. A minimum of 12 mm polymer cover is desired over the exposed reinforcement. After the final layer of polymer concrete has cured, the joint between the repair material and unrepaired concrete is sealed with a coat of the same primer as placed on original prepared concrete surface. In deeper patches the material has to be applied in lifts to control shrinkage. At higher temperatures (above 21°C) the rapid evaporation of polymer liquid reduces the working time available before the material gains an initial set. Placing, screeding and trowelling must be done quickly. Working at night or early in the morning for reduced ambient temperature gives a better control of the situation. In the floor slab of Fig. 17.15, the concrete movement (expansion) is found to exceed capabilities beyond that of a control joint. Hence to allow for additional movement, an expansion joint is provided. The preparation of the surface and the view of the surface after completion of the job are shown in Fig. 17.15.

(a) Expansion joint of floor slab (before)

Fig. 17.15

17.7.3

(b) Expansion joint of floor slab (after)

Construction of expansion joint to allow additional movement

Overlays and Surface Treatments

Dormant cracks in both structural and pavement slabs may be repaired using bonded overlays or toppings. However, most cracks in slab are subjected to movement caused by the variation in loading, temperature, and moisture. These cracks will reflect through any bonded overlays negating the overlay so far as crack repair is concerned, but the one-time occurrence or drying shrinkage cracks can be effectively repaired by the use of overlays. The slabs and decks containing fine dormant cracks can be repaired by applying an overlay of polymer modified Portland cement concrete or mortar. In highway bridge applications, a minimum overlay thickness of 40 mm may be used. Polymers suitable for such applications are latexes of styrene butadi-

720

Concrete Technology

ene acrylic, non-reemulsifiable polyvinyl acetate, and certain water-compatible epoxy-resin systems. The minimum resin solids should be 15 per cent by mass of the Portland cement. Before applying an overlay or topping the old floor slab should be thoroughly shot blasted or scabbled, cleaned and saturated with clean water. The localized damages or depressions are repaired as explained earlier, before bonding the cementing topping. There are two types of commonly used toppings, namely the bonded toppings and unbonded toppings. The thickness af topping will be governed by the strength and thickness of old floor slab.

Bonded Toppings They require bonding aids which may include polymers resins and cementing grout. A cementing grout mixed to creamy consistency can be brushed on to the floor slab immediately before placing the topping mix. Usually, a concrete mix having proportions of 1:1:2 by mass of cement and sand, 10 mm aggregate, respectively, may be adequate for topping. The sand should be medium grade (belonging to Zone II) and coarse aggregate should be hard and clean. Granite aggregates are commonly used; flint or quartzite gravel, ballast and hard limestone can also be used. The amount of water to be added should be minimum to attain full compaction. The topping mix should be laid in 20 to 40 mm thick layers in bays such that the construction joints in old floor must reflect up through the topping. The mix should be compacted on the old floor and troweled level at the intervals while topping is hardening, and after final troweling the topping should be cured by covering it with polythene for at least 7 days. In case of polymer modified Portland cement toppings a bond coat consisting of broomed latex mortar or an epoxy adhesive should be applied immediately before placing the overlay. The polymer-modified overlay should be mixed, placed and finished rapidly (within 15 minutes in warm weather). Such overlays should be cured for 24 hours.

Unbonded Toppings As the unbonded topping is an additional slab laid over the old floor slab no further surface preparation is required but construction joints in the old floor must be reflected up through the new slab. Properly lapped polythene sheets are laid as damp-proof membrane over the base slab and an M30 grade concrete having a minimum cement content of 350 kg/m3 should be placed and compacted on it. The concrete mix laid is 100 mm thick in bay sizes of up to 15 m2. Before this concrete hardens a high strength 10 to 15 mm thick topping mix described above may be placed and compacted on to the surface. The troweling at intervals during hardening and curing is accomplished as in bonded toppings.

17.7.4

Reconstruction of Slabs

In case of broken slabs, it is preferable to remove the affected slab and reconstruct it. In case of ground floor slabs, the subgrade should be inspected, compacted and brought to correct level by using 150 mm thick well-graded crushed rock material or lean concrete. Before concreting a polythene sheet should be placed over the top of sub-base to act as dampproof layer. The concrete should be fully compacted with

Repair Technology for Concrete Structures

721

vibrating beam when placed, and finished. It should be cured by covering it with polythene for at least seven days and should not be loaded for 28 days.

17.7.5

Repairs of Beams

In the case of extensively damaged or deteriorated beams, additional reinforcement at the bottom of beam together with the new stirrups are provided. The stirrups can be anchored by expanding bolts set in the sides of the beam below the slab soffit or may be taken right round the beam through the holes drilled in the slab. The roughened or irregular surface of the prepared concrete usually ensures a good bond between the old and new concretes placed by guniting. However, if required, shear connectors can also be provided by expanding bolts or other means.

17.7.6

Surface Coatings

A layer of material applied to a surface, which forms a continuous membrane is termed coating. The coatings generally adhere to the concrete and form films after application. During curing of concrete approximately 25 per cent of water is retained as water of crystallization and 15 per cent as gel. Capillary pores are formed during the evaporation of the remaining 60 per cent and eventually 15 per cent gel water. The capillary pores allow carbon dioxide and other gases to diffuse into concrete which may dissolve in pore water to form acidic solutions. The porosity of concrete may also lead to the absorption of water which may carry harmful reagents in solution. Concrete is strongly alkaline, and as such it is susceptible to attack from acidic reagents. Application of suitable coatings which afford the necessary protection while still allowing free passage of water vapor, is required for the protection of the structure. These preventive measures are described as follows.

Anti-carbonation Coatings The coatings applied to concrete surface to arrest the carbonation process are known as anti-carbonation coatings. These are normally based on chlorinated rubber, polyurethane resin or acrylic emulsions. Anti-carbonation coatings may be effectively used to resist carbonation and general atmospheric deterioration of reinforced concrete. In situations where corrosion and spalling are more wide spread, use of anti-carbonation coating will not be satisfactory. Coatings to Resist the Effects of Acid Environments The concrete structures are occasionally subjected to abnormally acidic environments. This may be due to the release of sulfur dioxide from power stations, steel plants and oil refineries into the atmosphere which readily dissolves in rainwater and forms sulfurous acid resulting in direct etching of the concrete. The concrete, in prolonged contact with water due to poor drainage, may disintegrate. The coatings are required to arrest the process of acid attack and to provide high degree of chemical resistance to the concrete. Under most circumstances, two part polyurethane coating will be suitable. Coatings to Protect Cracked Concrete The cracks which are very fine and not considered to have structural significance may be protected by applying the coating

722

Concrete Technology

locally over cracks. These are termed conventional coatings. The coatings should have flexibility as well as ability to bridge cracks. High-build polyurethane and epoxy-polyurethane formulations have been successful.

17.8

LEAK SEALING

Leakage in the concrete structures causes inevitable damage to the reinforcement. Construction joints, shrinkage and restraint cracks may form leak paths. The amounts of water involved vary from damp patches which tend to evaporate as they are formed, to running leaks which may eventually form pools on undrained surfaces. Damp patches may also be formed when water passes through the voids along reinforcing bars formed due to plastic settlement. The other common routes for larger volume leaks are honeycombed concrete, movement joints like expansion and contraction joints. In case of water-retaining structures, the extent of leakage may be mearured by monitoring loss of liquid from the structure. As per BS:5337 the structure can be considered watertight if the total drop in surface level does not exceed 10 mm in seven days. For an effective leak sealing, it is essential to identify the routes and sources of leakage and due consideration must be given to their likely cause, and their behavior once the structure is in service.

17.8.1

Leak Sealing Techniques

Leak sealing is expensive, so the operations must be necessary and worthwhile. The leak-sealing methods can be classified as: 1. Conventional leak sealing methods 2. Leak sealing by injection techniques 1. Conventional methods Some sources of minor leakage may dry up by autogenous healing which is an accumulation of calcium salts along the leak path. This will obstruct the passage of water over a period of time and reduce the leakage to negligible proportions. Once leak spots have been identified, the remedial action may involve the application of local or complete surface seal in the form of a coating system. The following format is recommended: (a) (b) (c) (d)

Surface preparations Filling of surface imperfections with resin-based grouts Application of primer Application of two coats of high-build paint

The procedure may require quite extensive preparatory work including the injection of suspect joints and random shrinkage cracks with a low viscosity resin. Honeycombed concrete if not particularly extensive may be filled out using a resin-based mortar or putty. Laitance and surface contaminants may be removed by sand blasting and power wire brush. The movement joints can be sealed by filling a resin into the joint which will cure to form flexible sealant as explained in Section 17.6.2. The concrete joint

Repair Technology for Concrete Structures

723

must be prepared and thoroughly cleaned prior to the application of sealant, and an appropriate bonding coat or primer should be used, if recommended. 2. Injection sealing From liquid flow and pressure considerations the simplest and most cost-effective way is to seal the leakage from the water retaining side of the structure. However, when the wet side is inaccessible, the leakage must be tackled from the dry side which is considerably more difficult. Successful leak sealing requires injection of sealant (grout) to fill water passages completely, and it is necessary to attain a relatively high flow velocity to achieve this, because of the short pot-life or working time of the typical repair material. The first basic step is to restrict or confine the water flow to a tube through which the sealant may be introduced. Once the flow of water has been controlled, the connection between the tube and concrete must be made strong enough to withstand the injection pressure. However, due to possibility of concrete being stressed during injection, it is preferable to maintain lower pressures. A typical direct method entails the injection of material up the pressure gradient from the down stream side. On the other hand, an indirect method involves the introduction of the sealant on the pressure side, so that the pathways are filled under the acting hydrostatic head. The direct methods are very slow due to sealant being pumped slowly through very narrow passages against pressure, and the pressure cannot be maintained for long enough to achieve complete penetration. In many cases water may find another finer pathway leading from the same source. In contrast the indirect methods enable the work to be completed quickly because surface seals are not required and mechanical anchorages can be used.

17.9

UNDERWATER REPAIRS

Many of the methods discussed in the preceding sections for abovewater (dry) repairs may also be used underwater with only minor modifications. However, the materials specified for use in air are often unsuitable for underwater application. The special features of underwater repair are: 1. Due to high cost and complexity of underwater working, the repair operations need be made as simple as possible. The choice of repair technique is influenced by the available method of access. 2. Adequate preparation of damaged area may require specially adapted techniques. 3. The repair materials must be compatible with underwater application both during placing and curing. Cementing systems have been found to be better suited for underwater use. 4. Formwork and placement method adopted must minimize mixing between repair material and water. 5. Underwater supervision of repair operations is difficult and costly. Generally, laboratory trials on both materials and repair methods are used to identify possible problem areas and ensure smooth site operations. Before a repair is undertaken it is necessary to clean the damaged area of marine encrustation

724

Concrete Technology

(contaminants) to allow detailed inspection to assess the extent of damage. In case of smaller areas, this can be accomplished by using mechanical wire brushes, needle guns or scabbling tools. However, for larger areas a high pressure jet may provide a solution. Once the area has been cleaned, the extent of cracked and spalling concrete may be defined with the help of divers or remote operated vehicles to photograph the area. The steps involved in underwater repair are: 1. Preparation of damaged area The preparation of surface may require removal of cracked or badly damaged concrete and cutting of distorted reinforcement. If only the damaged concrete is to be removed, high pressure water jetting (with pressure typically between 200 and 1000 atmospheres) directed onto the concrete surface can be used to remove hardened cement paste mortar from the spaces between the aggregate. In this process, the reinforcement itself is cleaned but not cut by the water jet, enabling it to be utilized in the repair. If the reinforcement is also to be cut, then an abrasive slurry injected into the cutting jet. Alternatively, the damaged concrete can be cut by splitting techniques. Wherein hydraulic expanding cylinders are inserted into pre-drilled holes and pressurized until splitting of concrete occurs. The splitting can also be achieved by mixing expansive cement with water to form a paste which is poured into plastic bags. The filled plastic bags are in turn deposited in the pre-drilled holes in the structure. The expansion of the cement over the next 12 to 24 hour period generates stresses approximately 30 MPa which may be adequate to split concrete. The splitting of concrete may also be achieved by using soft explosions. The procedure consists of placing and caulking-in firmly pressurized carbon dioxide cartridges in predrilled holes. The pressure is then released by electrically detonating a small initiating charge in each cartridge which produces a comparatively gentle explosion resulting in the controlled splitting of concrete by cracks running between the prepared holes. The mechanical underwater cutting using hydraulically powered diamondtipped saws and drills is suitable only for minor works, e.g., core cutting, etc. The reaction force to the tools can be provided by strap or strut arrangement bolted to the structure. After the removal of damaged concrete, all broken or distorted reinforcement will have to be removed and replaced before reinstating the cover. The commonly used methods for cutting steel underwater are; oxygen fuel gas cutting, oxy-arc cutting, and mechanical cutting. The first two methods rely upon the burning process which consists of oxidizing the metal (i.e., carbon steel) and removing the oxidized products from working surface. Once all damaged concrete and distorted steel have been removed, the reinforcing bars are replaced with new lengths joined either by couplers or lapped with existing bars. Immediately prior to reinstating the damaged concrete, the surface must be flushed with clean water to remove any bacterial or micro-biological growth which may otherwise significantly reduce the bond between the repair material and structure.

Repair Technology for Concrete Structures

725

2. Application of materials (a) Mortar placement In case of minor damage or to prevent future deterioration, the defects may be filled with cementing or resin-based materials. This patchwork is suited to application in small volumes. To reduce washout of cement from conventional cementing mortars and grouts, adhesive admixtures are used. High-performance mortars mixed above water can be poured by free fall through water to fill the formwork. The mixes are normally formulated to be self levelling to ensure good compaction without vibration, and laid in thickness of 20 to 150 mm. To avoid damage due to wave action on vertical surfaces, the repair should be carried out using either formwork and a free flowing grout or a hard epoxy putty. The normal epoxy or polyester resin mortars are totally unsuited for underwater applications. However, for underwater use, special formulations have been developed. These are normally free flowing and hence can be poured through water directly into formwork. For vertical work special types of underwater grade epoxy putty have been developed. (b) Injection into cracks The general principles for injecting grouts in the cracks underwater are the same as for above water injecting. However, owing to the risk of washout of cement, non-conventional epoxy resin injection are normally preferred. Epoxy putty is used to seal the crack between injection points as shown in Fig. 17.16. Epoxy resins must be low viscosity solvent free underwater grades in order that the water in the crack is replaced by a structural material.

Water-jetted crack

poxy putty

Injection port with one-way nipple injection point

Fig. 17.16

An underwater crack injection arrangement

726

Concrete Technology

For small repairs the use of hand held cartridge injection guns is a satisfactory procedure. (c) Large-scale repairs For bulk underwater, placement of repair material an easy-to-erect formwork complete with inlet pipes and external vibrators and tolerant of variations in the existing structure, is required. Flexible seals are preferred as they ensure a leak tight fit. For vertical repairs positive attachment using steel straps or rock bolts drilled into the concrete to secure the form as illustrated in Fig. 17.17, with a thick layer of compressible gasket (e.g., neoprene rubber) to form final seal, may be employed. Finally, the gaps due to unexpected variations in line or level are sealed. Width suitable for tremie or bucket

Filling height Opening for concrete

Concrete interface Wood or steel form Existing concrete Neoprene seal

Rock bolt

Fig. 17.17

Typical formwork details for underwater vertical repair

The mix design for underwater repair may require certain modifications depending upon the nature of the work. Using available information, concrete mix proportions are selected to give the required strength using slightly oversanded mix. The cement content is then raised by approximately 25 per cent. Lean mixes of less than 350 kg/m3 are not likely to be suitable due to washout of cement. Well-washed marine dredged aggregates and round river gravels are suitable for the tremie and pump placing methods. For small repairs in heavily reinforced areas, superplasticizing admixtures will be essential to give necessary flow characteristics. The placement method for underwater repair must be so selected as to minimize the area of contact between concrete and water, and to prevent

Repair Technology for Concrete Structures

727

turbulence. The concrete can be placed by a bottom opening bucket (skip) or a tremie pipe or by pumping as explained in Section 12.4.1. The prepacked aggregate concrete method of placement is ideally suited to underwater or tidal works, especially where access conditions are limited or fast flowing water could wash away conventional concrete. This method has been explained in the Sections 12.4.1 and 17.5.2.

17.10

DISTRESS IN FIRE DAMAGED STRUCTURES

A large number of reinforced concrete structures salvaged from destruction in fires by timely fire-fighting operations can be put to further service after strengthening and providing some cosmetic repairs since the cost of restoration of such structures less than that for dismantling and construction of new ones. The fire may cause different degrees of damage to the structure: the structure may be completely burnt or destroyed; its surface may be slighly damaged or a slight deformation may occur. In the first case, the whole of the damaged portion has to be replaced during restoration of structure while in the latter, only repair and finishing may be required. The extent of damage caused to the structure during a fire depends on the duration of fire, and the temperature to which the structure was subjected during the fire. High temperature during a fire reduces the strength of reinforced concrete structures due to change in the strength and deformability of materials, reduction in crosssectional dimensions, weakening of bond between the reinforcement and concrete, which determines structural action under the load. The maximum temperature reached during a fire is normally estimated indirectly, e.g., from the melting of metallic or other non-combustible articles. A temperature of 1000–1100°C has been observed during fire in residential and administrative buildings. The duration of these fires was mostly between one and two hours. It has been observed that during fires in theatres and departmental stores, temperature rises up to 1100–1200°C and the fire duration exceeded two–three hours in some cases. Still higher temperatures have been observed during fires in industrial buildings and warehouses in which considerable quantities of solid and liquid combustible materials were processed or stored. During a fire in a store of combustible liquid and lubricants which lasted more than two hours, temperature of 1300°C has been reported. Thus duration of a fire and maximum temperature reached can vary over a wide range. Temperatures of 1000–1100°C in fires which lasted for one–two hours have been observed more frequently than 1300°C. An accurate estimate of the performance characteristics of structures which have been damaged in a fire helps in developing effective restoration/rehabilitation measures. The performance characteristics take into account the physico-chemical and mechanical properties of the materials burnt, and that of heated concrete. There is an accumulation of irreversible damages of mechanical (cracking, creep, shrinkage and plastic deformations) and physico-chemical (corrosion, absorption and degradation) origin. This information improves the reliability of estimation of residual load carrying capacities of structural elements thereby resulting in a considerable saving in the cost of restoration of the structure. However, this prediction of loss in the carrying capacity of fire-ravaged reinforced concrete elements taking into consideration

728

Concrete Technology

the physico-chemical characteristics of materials and geometric dimensions of the structure is difficult. The strength and stiffness of concrete and steel decrease as the temperature of the member increases and dimensional changes occur. The changes in strength, stiffness of concrete are influenced by the type of cement and aggregate, and water content. The stresses due to thermal strain cause the beam, column or slab to crack or spall, reducing concrete area available to resist the applied forces. The following observations will bring out the causes of the damage in fire-ravaged structures and help in obtaining complete performance characteristics of the structure.

Axially Loaded Columns The column under fire normally fail at mid-height due to brittleness. The failure is accompanied by disintegration of concrete in the whole section and buckling of longitudinal bars. It must be realized that due to considerable drop of temperature (800°C or more) between the periphery and center of the section, the strength of concrete varies along the cross section from its initial value at the central portion to zero at the surface. The temperature at which crushing strength of concrete is reduced to half its initial value is termed as critical temperature. The critical temperature depends upon the type of aggregate used in the concrete. This temperature is 550°C for concrete with granite or sandstone aggregates, and 700°C for concrete with limestone. Due to non-uniformity of temperature in the cross section, the hottest layers of concrete and main reinforcement bars near the surface of column are relieved due to thermal creep and loss of strength, and also due to contraction in the case of concrete. This causes increased stresses in the center of the section where moderately hot concrete retains its strength and elasticity. Complete failure of column starts when stresses in the central portion of the cross section becomes equal to the initial prism strength of concrete with deformation approaching its limiting value (0.0025–0.0030).

17.10.1

Restoration of Fire Damaged Elements

The eccentrically loaded columns fail when reinforcement bars in tension heat up. The fire resistance of such elements can be increased by increasing the thickness of protective layer. Heat transmission and temperature of bottom reinforcement are keys to the behavior of reinforced concrete slab exposed to fire. The reinforcing bars are assumed to retain one half of their original strength. Carrying capacity of slabs can be enhanced by increasing their thickness. For beams, depth and width can be increased. However, it should be kept in mind that in case of beams, weakening of bond between transverse reinforcement and concrete on account of heating reduces the residual shear load carrying capacity considerably. The required increase in the dimensions of the beam, longitudinal and transverse reinforcements should be computed by taking into account the change in compressive strength of concrete and moduli of elasticity of concrete and reinforcement. The carrying capacity of axially loaded column depends upon the cross section of the column, coefficient of change in strength of concrete under high temperature and corresponding critical temperature. The carrying capacity can be restored by increasing the cross section with suitable increase in the longitudinal steel.

Repair Technology for Concrete Structures

729

In retrofitting of the structure, a convenient method of anchoring reinforcing bars into existing concrete walls and foundations is to drill a hole in the concrete somewhat larger in diameter than that of the bar and set the bar in an epoxy gel. In slabs epoxy coated bars may be used but epoxy coating thicker than 0.25 mm are not recommended.

17.11

STRENGTHENING WITH COMPOSITE LAMINATES

Strengthening and retrofitting is a growing need for infrastructure renewal, as highways, bridges, and other exposed structures are becoming functionally obsolete or deteriorating with the passage of time under environmental loads, and an increase in service loads. Apart from the need to increase load capacity, upgrading of structure may be necessitated due to a change in the structural system during the modernization/renovation of buildings, where the removal of individual supports and walls may lead to a redistribution of forces or to rectify initial structural deficiency due to design and construction faults. These infrastructure rehabilitation challenges demand new technologies and new materials. The strengthening or retrofit of deficient or damaged reinforced concrete members by adhesively bonding of prefabricated strips of thin composite material laminates, also known as fiber-reinforced-polymer (FRP) plates represents the stateof-the-art in rehabilitation techniques and is becoming increasingly popular in the construction industry. FRP offers excellent properties not available from traditional materials. It is lightweight, non-metallic, corrosion resistant and possesses higher strength and stiffness compared to steel. An ease of handling and application gives FRP an edge over traditional materials for certain applications such as strengthening of bridge beams and slabs. Advanced fibrous polymer composite materials such as thin carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) strips or plates provide an economical and efficient solution for extending the service life of deficient structures in general, and RC structures in particular. FRP strips also offer other high performance advantages such as high specific strength and stiffness, high durability, low creep, and high fatigue resistance in comparison to conventional cementing materials. The performance of these strips, however, depends on the strength of the adhesive used to bond them with the concrete surface, the state of stress at the interface of the concrete and FRP strips, and the failure modes in the concrete. However, RCC beams strengthened with CFRP and GFRP laminates experience brittle failure, with reduced energy absorption and ductility. Reinforced concrete beams or slabs strengthened with externally bonded or attached FRP plates to the tension face (soffit) increase the ultimate flexural strength several times their original strength. The technique of strengthening concrete beams with composite strips is as simple and convenient as the earlier method that has been used to repair concrete beams with steel plates. The advantages of this technique are that the work can be carried out while the structure is still in use and it is economical compared to other methods. In one of the systems, the composite strip is bonded to the concrete surface with a room temperature curing, two-part epoxy adhesive. The procedure for this method is time consuming because it can take days to sandblast,

730

Concrete Technology

clean, and smoothen the concrete to render it suitable for bonding. The two-part epoxy system must be mixed in a precisely controlled fashion and applied in a careful manner to produce a good bond surface. As an alternative to the above method of bonding composite strips to concrete structures, readily available inexpensive commercial off-the-shelf powder-actuated fastening system may be used to attach composite strips to concrete. Such a technique could meet requirements for rapid strengthening in situations where time is critical, as there is no need to prepare the surface or clamp the strip while the epoxy cures. Other systems utilize the preformed fiber fabrics and apply the epoxy resin system to the fabric and to the concrete substrate simultaneously. These systems also require the same time consuming, and careful preparation and curing as in the case of bonding a prefabricated composite strip to the concrete. In many cases, the high ultimate capacity provided to the repaired structure cannot be attained, due to either debonding of the FRP plate from the concrete or development of horizontal cracks and subsequent delamination of the covercrete (anchorage failure) along the longitudinal reinforcement. Both events are initiated by high-stress concentrations developed at the plate curtailment. Thus, generally a provision of continuous mechanical anchorage to the bonded strip at its ends is required to prevent catastrophic brittle failure of the strengthened beam due to strip detachment. The strip end anchorages are more effective in beams that are shorter (with a high ratio of shear force to bending moment) than in longer beams. This anchorage is usually provided in the form of anchor bolts or cover plates. The arrangement is similar to the mechanical anchorages normally recommended for use with epoxy-bonded steel plates. The use of multiple small fasteners, as opposed to large diameter bolts, distributes the load evenly over the composite strip and does not cause premature failure due to excessive stress concentrations at the holes in the composite strip. These mechanical anchorages are used to prevent delamination failures, and are not intended to be the primary load transfer mechanism between the concrete and the composite strip. For effective strengthening, the beam to be strengthened should be ductile (under reinforced), the laminate thickness for bonding should be such that the addition of laminate should not alter under reinforced condition and the shear capacity should also be improved by diagonal plating on both sides of the beam.

Surface Preparation Bond of the FRP plate to the concrete substrate is of critical importance for the effectiveness of the technique, because it is the means to develop composite action by the stress transfer between concrete and FRP. The concrete surface should be mechanically abraded or sand blasted, and a primer should be applied to achieve the best possible bond. The surfaces of the composite plates should be roughened through the use of bead blasting and then cleaned with an appropriate solvent such as acetone. The performance of the surface roughened by chiselling is much better than that obtained by sand blasting.

17.11.1

Flexural Members

Flexural Mechanisms Reinforced concrete beams strengthened with externally bonded FRP laminates or plates may exhibit one of the following modes of failure:

Repair Technology for Concrete Structures

731

1. FRP plate failure The tension reinforcement yields followed by rupture of FRP plate as stress in the composite exceeds its tensile strength in the maximum moment zone. 2. Compression failure The strain in concrete in compression exceeds the ultimate strain (0.003) in the maximum moment zone and the strain in tension reinforcement is below the yield stress. 3. Tension failure The tension reinforcement yields and the stress in FRP plate is below its tensile strength; the beam eventually fails due to crushing of concrete in the compression zone. 4. Shear failure This type of failure occurs when the nominal shear capacity of the beam in the vicinity of the support is exceeded due to lack of proper shear reinforcement. The beams may fail by diagonal cracking with local delamination or debonding. 5. Delamination failure in flexure Enhanced failure loads and desirable failure behavior in laminated concrete beams are both strongly reliant on the successful transmission of bond stresses between the plate and the beam via the adjacent connecting layers of the FRP plate, adhesive and cover concrete or covercrete (the concrete layer outside of the region confined by the stirrups). High bond stresses eventually trigger failure of one of these layers. When this occurs, the plate-to-beam connection is lost and the plate debonds from the beam. Of the materials in the three connecting layers, the covercrete usually possesses the lowest strength, so debonding commonly occurs through delamination of the covercrete layer. Concrete is a brittle material, so any mode of debonding can propagate rapidly, with little advance warning. This brittle form of failure, defined by delamination of the cover concrete attached to the adhesive causes the plates to debond from the beam. When the tension reinforcement yields and the beams fail by the onset of delamination at the interface of the strip and the concrete surface, both with and without covercrete failure (shear/tension delamination). The strips may not be stressed to their maximum capacity which leads to ductile failures. There are three possible modes of delamination failure: (a) Mode-I In this delamination mode, the interface shear stress between concrete and adhesive exceeds the shear strength of the concrete–adhesive interface and the laminate debonds from the concrete face. This mode called end peel mode, starts at the ends of the plates and propagates inwards along the beam. Inclined and horizontal cracks form in the covercrete causing it to break away from the beam while remaining firmly attached to the plate. (b) Mode-II The interface shear stress between the FRP and the adhesive exceeds the shear strength of the FRP-adhesive interface and the laminate debonds from the concrete face. (c) Mode-III The concrete between FRP laminate and tension reinforcement fails in shear and debonds from the steel reinforcement. The debonding of FRP laminate is thus initiated by the development of a flexural crack in the maximum bending moment zone. The debonding begins at toes of

732

Concrete Technology

the flexural cracks in the mid span region of the beam and propagates out to the ends of plates. Mid span debond action is triggered by high shear stresses transmitted from the plates through the adhesive to the covercrete. The delaminated concrete, adhesive, and plate remain a single unit after debonding, with the remaining covercrete staying an integral part of the original beam. 6. Local shear-tension failure The crack initiates in the vicinity of one of the strip ends at the level of the tension steel reinforcement and propagates horizontally either toward mid span, or upward to the location of the concentrated load resulting in delamination of the covercrete (anchorage failure).

17.11.2 Compression Members Fiber reinforced polymer (FRP) plate jackets and steel hoops have been successfully used in the past to enhance the shear strength and curvature ductility capacity in reinforced concrete compression members. The enhancement is due to the dual confinement effects provided by an external FRP jacket and by internal steel hoops. The jackets are also very effective in preventing longitudinal bar buckling. FRP jackets are relatively easy to instal and are cost competitive when compared with conventional options such as concrete or steel jacketing, particularly when access to the columns is limited. FRP jacketing of columns in deficient buildings is an increasingly attractive retrofit option in seismically active areas. The details are given in Section 17.6.5.

17.12

STRENGTHENING OF DEFICIENT STRUCTURES

In case of deficient or under-designed structures the distress may appear in the form of deflections exceeding the serviceability limits, i.e., excessive sagging, accompanied by micro-cracking along the length of beams in their top fibers. The structural behavior of members is dependent on their stiffness. In order to assess the magnitude of deficiency in design, the first step is to compute flexural stiffness of the elements with their existing sizes. Before embarking on strengthening of flexural members like beams and slabs, it is to be ensured that columns and foundations are adequate. To illustrate the procedure, consider the case of a framed structure where slabs and beams have been found to be under designed in thickness, however, columns and their foundations are adequate. In order to strengthen or enhance the flexural stiffness of beams and slabs, it is proposed to increase the cross section of beams and thickness of slab.

17.12.1

Section Enlargement

This method of strengthening involves placing additional bonded reinforced concrete to an existing structural member in the form of an overlay or a jacket. The load carrying capacity or stiffness of columns, beams, slabs, and walls can be enhanced with section enlargement. Typically enlargements in the range of 40 to 70 mm for slabs and 75 to 125 mm for beams and columns are provided. Figure 17.18 depicts

Repair Technology for Concrete Structures

733

details of a section enlargement generally used to increase the capacity of a beam required for increase in the service loading. The enhancement required is achieved by providing additional flexural and shear steel. Longitudinal steel bar

Section enlargement

Fig. 17.18

PT strands

Steel stirrups Original beam section

Beam strengthening using section enlargement

17.12.2 Strengthening of Beams Before taking up the strengthening of a beam, the load acting on it should be reduced by removing the tiles, bed mortar, etc., from the slab. In addition props may be erected at mid-span of each slab and tightened in such a manner that slab is not damaged. After chipping off of the existing plaster on the beam, additional reinforcement at the bottom of beam together with new stirrups (if required) are provided. The bars are passed through or inserted in the supporting columns through holes of appropriate diameter drilled in the columns. The spaces between bars and surrounding holes are filled with epoxy grout to ensure a good bond. Expanded wire mesh is fixed and anchored on three sides of the beam as shown in Fig. 17.19. To ensure a good bond between old concrete and polymer modified mortar, an epoxy bond coat is applied to the concrete surface. While the bond coat is still fresh, a layer of polymer modified mortar is applied. The required thickness on all the three sides is achieved by application of two to three layers of mortar. While applying mortar at the bottom of beam, the thickness of mortar layers should be so adjusted that sagging is completely covered and beam looks undeflected. The mortar is cured for appropriate period in water and thereafter it is allowed to cure in air. Epoxy resin should also be injected in the cracks along top of beams. If new stirrups are required for shear strength enhancements the procedure outlined in Section 17.7.5 should be followed.

17.12.3 Strengthening of Slabs The strengthening of a slab is taken up only after the strengthening of beams is completed. A reinforced structural concrete topping over the existing slab can be used which provides a composite construction of old and new slabs, with additional depths to slab and beam. To ensure a good bond between new and old concretes, mechanical anchorages consisting of steel bolts (shear connectors) inserted in holes

734

Concrete Technology

drilled into the slab at a suitable intervals may be provided. The spaces surrounding the holes are filled with epoxy grout. A shear connector is embedded for half of its length in old concrete and the remaining half which is projected will subsequently be embedded in new concrete.

(a) xpanded wire mesh in position

Bond coat Expanded wire mesh poxy mortar Additional reinforcement (b Repair scheme of the beam 50–75 mm thick new concrete

Old reinforcement xpanded wire (16 guage) Epoxy mortar Additional reinforcement (c) Strengthened section of the beam 10 mm diameter,100 mm long (shear connector)

lt

slab, M 20 grade with extra einforcement Bon coat Old slab Gap filled with epoxy (d) Strengthening schem

f slab

\ Fig. 17.19

Strengthening of a slab-beam system

Before applying topping, the surface of old floor slab should be thoroughly scabbled and cleaned. Additional reinforcement may be required over the supports, because the

Repair Technology for Concrete Structures

735

old reinforcement at supports acquires a position which is near to the neutral axis of composite section. After the preparation of old concrete surface, epoxy bond coat is applied on it and while this coat is still touch-dry, 25 to 50 mm thick M20 grade concrete topping is laid. The thickness of topping is governed by the strength and thickness of old floor slab. However, application of topping increases the dead weight on the slab. With suitable treatment the top layer of topping may be utilized as floor finish, etc. After curing the beam and slab for 14 to 21 days props can be removed. In case of slabs deficient both in thickness and reinforcement a strengthening layer of 25 mm thickness consisting of hardened steel wire fabric of 8 mm Φ @ 150 mm c/c (5.26 kg/m2) and 15 mm thick layer of concrete of grade M20 can be provided at the bottom. In slabs requiring thickness increase of 25 mm at both top and bottom, the strengthening layer will consists of fabric of 10 mm Φ @ 150 mm c/c (8.22 kg/m2) and 15 mm thick layer of concrete on either side. The wire fabric is to be properly connected by welding to the beam reinforcement at the ends. As an example for typical epoxy mortar mix, Araldite of grade GY 25; hardeners of grades GY 825, GY 830 and GY 850; and fine silica in the ratio of 5:1:1:1:1 can be used for an application at the interface between old and new concrete layers @ 350 gm of mortar/m2 of surface.

REVIEW QUESTIONS 17.1 Enlist common causes of cracking or damage concrete. Describe phenomena of cracking due to weathering and due to corrosion of reinforcement. 17.2 Explain briefly the purpose and procedure for the damage assessment (i.e., evaluation of cracks or damage) in concrete element 17.3 Write a short note on selection of repair material and its placement, 17.4 Explain crack repair by routing and sealing. 17.5 Explain the criteria for selection of repair procedure for damaged concrete structures. What are the four steps

17.6

17.7

17.8 17.9

carrying out the repair? Describe briefly the preparation of surface. What is meant by jacketing? Discuss repair and strengthening of columns by jacketing. Write short notes on three of the following: (a) Autogenous heeling, (b) Polymer-based repairs and (c) Overlays and surface treatment, (d) Crack repair by grouting and sealing and (e) Underwater repairs Write a short note on strengthening and retrofitting with composite laminates Explain briefly the strengthening of deficient structures

MULTIPLE-CHOICE QUESTIONS 17.1 The distress in concrete structures may be due to (a) poor construction practices (b) errors in design and detailing (c) environmental loads exceeding design stipulations (d) shrinkage, thermal stresses and corrosion of reinforcement (e) All of the above

17.2 Blow-holes are caused by (a) improper design of formwork (b) lack of compaction (c) inadequate workability (d) excess of water content (e) None of the above 17.3 The repair of bulges, projections, boltholes and blow-holes should be done within_____of stripping off the forms.

736

17.4

17.5

17.6

17.7

Concrete Technology (a) 24 hours (b) 3 days (c) 7 days (d) 21 days (e) 28 days The most common symptom of distress in a concrete structure is (a) spalling of concrete (b) cracking of concrete (c) scaling of concrete (d) surface crazing (e) None of the above Identify the incorrect statement(s). (a) Cracks may represent the total extent of damage or they may point to problems of greater magnitude (b) The cracking in concrete structure is not necessarily a cause for blaming the designer, builder or supplier. (c) A crack where movement is observed to continue is termed dormant. (d) Cracks in concrete members usually occur due to incompatible dimensional (volume) changes (e) None of the above The cracking in plastic concrete is caused by (a) relative volume changes between surface and interior concrete by very rapid loss of moisture due to low humidity, high wind or high temperature (b) local restraint to the continuing consolidation or settlement of the concrete (c) leaking or highly flexible forms (d) Any of the above (e) None of the above The cracking of plastic concrete can be controlled by (a) using plastic sheeting to cover the surface between final finishing operations (b) using lowest possible slump (c) adequate vibration and proper form design (d) providing sufficient time interval between placement of concrete in various elements (e) All of the above

17.8 The cracking due to weathering in structural concrete can be controlled by the following measures except (a) use lowest practical water–cement ratio (b) use lowest practical water content (c) use non air-entrained concrete (d) reduce the temperature differences within a concrete structure (e) None of the above 17.9 The cracking due to corrosion of reinforcement is characterized by (a) exposed reinforcement (b) splitting and spalling of concrete in definite patterns (c) longitudinal cracks parallel to the bar or spalling of concrete (d) rust staining (e) All of the above 17.10 Before proceeding with the repair of the cracks, diagnosis is made to determine the (a) location and extent of cracking (b) causes of cracking (c) likely extent of further deterioration (d) suitability of various remedial measures (e) All of the above 17.11 The weak spots in a concrete member can be identified by (a) tapping the surface and observing the sound for hollow areas (b) opening up (by chipping) the suspected weak concrete (c) observing spalled areas, exposed reinforcement, surface deterioration and rust staining (d) by non-destructive tests (e) All of the above 17.12 Microstructure cracks in concrete have a size, i.e., width/depth ranging from (a) 0.01 to 0.05 mm (b) 0.05 to 0.1 mm (c) 0.1 to 0.3 mm (d) 0.3 to 0.5 mm (e) 0.5 to 1.0 mm 17.13 The selection of repair technique is based on the following objective(s): (a) to restore load carrying capacity

Repair Technology for Concrete Structures

17.14

17.15

17.16

17.17

17.18

17.19

(b) to improve functional requirements (c) to improve durability (d) to prevent access of corrosive agents to the reinforcement (e) One or more of the above The preparation of surface for repair consists of (a) complete removal of unsound material (b) undercutting with formation of smooth edges (c) removing crack from the surface (d) providing rough but uniform surface (e) All of the above The laitance at the surface of the concrete can be best removed by (a) grinding, scarifying and sand blasting (b) sand blasting only (c) acid etching (d) cleaning with detergents or caustic soda solution (e) Any of the above For repair of large and deep patches of deteriorated concrete in a structure, the material filling is done by (a) dry packing (b) concrete replacement method (c) mortar replacement method (d) grouting (e) prepacked concrete Wide and deep cracks in concrete members may be repaired by (a) grouting (b) shotcreting or guniting (c) mortar replacement (d) epoxy injection (e) Any of the above The technique of epoxy injection is used for (a) sealing actively leaking cracks (b) sealing narrow cracks in structural members (c) repairing water tanks/hydraulic structures (d) Any of the above (e) None of the above For sealing the cracks in concrete structures by using epoxy, the minimum width of routing required is

17.20

17.21

17.22

17.23

17.24

737

(a) 3 mm (b) 6 mm (c) 9 mm (d) 15 mm (e) 20 mm The stitching of cracks is done to accomplish all of the following except (a) to re-establish tensile strength across major cracks (b) to prevent the crack from further propagation (c) to stiffen the structure (d) to strengthen the cracks which have a tendency to close as well as to open (e) None of the above The bonded overlays of polymermodified Portland cement concrete or mortar (a) can be used for repairing slab and decks containing fine dormant cracks (b) have minimum overlay thickness of 75 mm (c) the mix should be mixed, placed and finished within 30 minutes (d) should be cured for 72 hours (e) None of the above Polymer impregnation of concrete is not effective in the following cases except (a) when cracks are dry (b) when cracks contain moisture (c) when volatile monomer evaporates (d) when cracks to be repaired are fine (e) None of the above Sulfur impregnated concrete can be used (a) as a practical and inexpensive substitute of polymer impregnated concrete (PIC) (b) for repairing fractured elements (c) for repairing slab and decks containing fine dormant cracks (d) as a practical substitute of polymermodified cement concrete overlay (e) None of the above The polymer system(s) commonly used for polymer concrete is (a) latexes of styrene butadiene acrylic (b) methyl methacrylate (c) 99.9 per cent pure sulfur

738

Concrete Technology

(d) All of the above (e) None of the above 17.25 Jacketing (a) is a process of fastening a durable material over concrete and fi lling the gap with grout (b) increases the section of an existing member by encasement in a new concrete (c) is used for compression members like columns and piles (d) along with collars can be advantageously used for repairing deteriorated concrete columns (e) All of the above

17.26 Autogenous healing (a) is a natural process of crack repair occurring in the presence of moisture (b) cannot take place in continuous saturation (c) is particularly effective in cycles of drying and re-immersion (d) All of the above (e) None of the above

Answers to MCQs 17.1 (e) 17.7 (e) 17.13 (e) 17.19 (b) 17.25 (e)

17.2 (a) 17.8 (c) 17.14 (e) 17.20 (c) 17.26 (a)

17.3 (a) 17.9 (e) 17.15 (c) 17.21 (a)

17.4 (b) 17.10 (e) 17.16 (b) 17.22 (a)

17.5 (c) 17.11 (e) 17.17 (a) 17.23 (a)

17.6 (d) 17.12 (c) 17.18 (b) 17.24 (b)

APPENDIX

1. Which of the following is not a cement in the real sense? (a) blast-furnace-slag cement (b) low heat cement (c) high alumina cement (d) none of the above 2. Percentage loss on ignition in Portland cement shall not be more than (a) 4 (b) 5 (c) 3 (d) 6 3. Bulking of sand is due to (i) viscosity (ii) air voids (iii) surface moisture The correct answer is (a) only (i) (b) only (iii) (c) both (ii) and (iii) (d) both (i) and (iii) 4. Steam curing is not recommended for concrete using (i) high alumina cement (ii) ordinary Portland cement (iii) rapid hardening cement The correct answer is (a) only (i) (b) both (i) and (iii) (c) both (ii) and (iii) (d) (i), (ii) and (iii) 5. Early gain of strength is due to (a) C3S (b) C2S (c) C3A (d) both (a) and (c) 6. The specific surface of cement is expressed in (a) g/mm3 (b) g/mm2 3 (c) mm /g (d) mm2/g

7. The mix for underwater concreting uses should have (a) very low slump (b) low slump (c) high slump (d) none of the above 8. Initial setting of cement is due to reaction of (a) C3S (b) C2S (c) C3A (d) both (a) and (c) 9. Assertion A: Pozzolana is added to cement to increase early strength Reason R: It reduces the heat of hydration (a) both (A) and (R) are true and (R) is correct explanation of (A) (b) both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true 10. A cement having lesser tricalcium aluminate shall have (a) lesser initial strength but higher ultimate strength (b) lesser initial and ultimate strengths (c) no effect on initial and ultimate strengths (d) higher initial and final strengths 11. Creep coefficient of concrete at 1 year age is approximately (a) 3.3 (b) 2.2 (c) 4.4 (d) 1.1 12. Air-entraining agent (a) increases workability (b) decreases strength

740

13.

14.

15.

16.

17.

18.

19.

Concrete Technology

(c) increases resistance to freezing and thawing (d) all of the above Which of the following is responsible for most of the undesirable properties of concrete (a) C3S (b) C2S (c) C3A (d) C4AF The water–cement ratio is expressed by (a) volume (b) weight (c) density (d) none of the above The approximate ratio of concrete strength at 7 days to its strength at 28 days is (a) 3/4 (b) 2/3 (c) 1/2 (d) 1/3 Aerated cement is produced by the addition of (i) zinc sulfate (ii) magnesium sulfate (iii) powdered aluminum (iv) sodium nitrate The correct answer is (a) only (iii) (b) both (i) and (iii) (c) both (ii) and (iii) (d) (i), (ii) and (iv) Which of the following is correct if they are arranged in decreasing order of heat of hydration (a) C3A > C4AF > C3S > C2S (b) C3A > C4AF > C2S > C3S (c) C3A > C3S > C2S > C4SAF (d) C3A > C3S > C4AF > C2S The purpose of concrete compaction is to (a) increase the density (b) increase the weight (c) increase the voids (d) all of the above The increased rate of strength gain of rapid hardening cement is achieved by (a) higher content of C3S (b) higher content of C3A (c) finer grinding of cement (d) (a) and (c)

20. Which of the following characteristics is same for both ordinary Portland cement and rapid hardening cement (a) initial setting time (b) final setting time (c) both initial and final setting times (d) none of the above 21. The freshly prepared concrete mix should be consumed normally within (a) 24 hours (b) 8 hours (c) 1½ hours (d) 15 minutes 22. High alumina cement should not be used in the location where temperatures exceed (a) 15 ºC (b) 18 ºC (c) 20 ºC (d) 25 ºC 23. Ultimate strength of cement is provided by (i) tricalcium aluminate (ii) tricalcium silicate (iii) dicalcium silicate The correct answer is (a) only (iii) (b) only (i) (c) (i), (ii) and (iii) (d) none of the above 24. Air permeability method is used to determine (a) specific surface of cement (b) soundness of cement (c) setting time (d) none of the above 25. If the fineness modulus of fine aggregate is 2.51, it can be graded as (a) fine sand (b) medium sand (c) very fine sand (d) coarse sand 26. Specific surface of rapid hardening cement (cm2/g) shall not be less than (a) 2250 (b) 2250 (c) 3250 (d) 3750 27. Humidity causes (a) hardening of cement in a bag (b) setting of cement in a bag (c) strengthening of cement in a bag (d) none of the above 28. Which of the following cements has maximum percentage of C3S

Appendix

29.

30.

31.

32.

33.

34.

35.

(a) ordinary Portland cement (b) low heat cement (c) sulfate resisting cement (d) rapid hardening cement The resin-impregnated concrete (a) reduces the porosity (b) increases the durability (c) both of the above (d) none of the above Workability of concrete can be increased by the (a) increase in maximum size of aggregate (b) decrease in temperature (c) use of round aggregate which has smooth surface texture (d) all the above Higher percentage of C3S makes cement (i) hydrate more quickly (ii) generate heat more rapidly (iii) develop early strength The correct answer is (a) both (i) and (iii) (b) both (ii) and (iii) (c) (i), (ii) and (iii) (d) None of the above Which of the following statements is a correct statement (a) rich mixes are less prone to bleeding than lean ones (b) lean mixes are less prone to bleeding than rich ones (c) bleeding is decreased by decreasing fineness of cement (d) Both (a) and (c) White cement should have minimum percentage of (a) silica (b) magnesium oxide (c) calcium oxide (d) iron oxide Bleeding can be reduced by (a) addition of pozzolanas (b) addition of aluminum powder (c) increasing the fineness of cement (d) All of the above The increase in the strength of concrete with time is (a) linear (b) non-linear

36.

37.

38.

39.

40.

41.

741

(c) asymptotic (d) All of the above Calcium sulfo-aluminate is produced due to reaction of hydrated tricalcium aluminate with (a) gypsum (b) water (c) Both (a) and (b) (d) None of the above The strength of concrete is decreased by (i) vibration (ii) application of sudden load (iii) fatigue The correct answer is (a) Only (i) (b) Both (ii) and (iii) (c) (i), (ii) and (iii) (d) None of the above The strength of concrete can be approximately expressed as (a) fck = K [C/( W + C + A)]2 (b) fck = K [ (C + A) / (W + C + A)]2 (c) fck = K [(C + W) / (W + C + A)]2 (d) fck = K [(C + W ) / (W + A)]2 where, fck is strength of concrete; A, C and W are the absolute volumes of aggregates, cement and water, respectively, and K is a constant. The residual stress developed in RCC due to thermal gradient is of the order of (a) negligible (b) 10% (c) 25% (d) 40% ∑ (curing period × temperature) is known as (a) curing (b) maturity (c) shrinkage (d) None of the above Low percentage of C3S and high percentage of C2S in cement will result in (i) rapid hardening (ii) higher ultimate strength with less heat generation (iii) better resistance to chemical attack The correct answer is (a) Only (i) (b) Only (ii) (c) Both (i) and (iii) (d) Both (ii) and (iii)

742

Concrete Technology

42. Assertion A: Finer grinding of cement results in early development of strength Reason R: Rate of hydration of cement is increased (a) both (A) and (R) are true and (R) is correct explanation of (A) (b) both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true 43. The permanent deformation of concrete with time, under sustained load is called (a) creep (b) relaxation (c) viscosity (d) visco-elasticity 44. Stripping time in case of rapid hardening cement is x times that in ordinary cement, where x is (a) 1/2 (b) 3/7 (c) 3/4 (d) 4/7 45. The shrinkage of concrete is due to change in (a) length (b) cross-sectional area (c) volume (d) surface area 46. Concrete shrinkage is more pronounced in (a) rich mix (b) lean mix (c) very lean mix (d) normal mix 47. Regarding blast-furnace-slag cement, which of the statements is correct (i) its color is blackish grey (ii) it has lower evolution of heat (iii) can be used in sea water construction The correct answer is (a) Only (i) (b) Only (ii) (c) Both (i) and (ii) (d) (i), (ii) and (iii) 48. High percentage of C3S and low percentage of C2S in a cement will result in (i) rapid hardening (ii) high early strength with high heat generation (iii) more resistance to chemical attack

49.

50.

51.

52.

53.

54.

55.

The correct answer is (a) Only (i) (b) Only (iii) (c) Both (i) and (ii) (d) Both (ii) and (iii) Air-entrainment in the concrete decreases (a) bleeding (b) workability (c) Both (a) and (b) (d) None of the above The water, having acidic/alkaline nature, should be neutralized before use in concrete by (a) NaOH/HCl (b) B2O3/HNO3 (c) H2SO4/KCl (d) KOH/NaCl Finer the cement (i) more is the surface area (ii) higher is the rate of hydration (iii) lesser is the surface area (iv) lower is the rate of hydration The correct answer is (a) Only (i) (b) Both (i) and (ii) (c) Both (i) and (iv) (d) Both (ii) and (iii) The performance of concrete can be enhanced by (a) restricting accessibility of cement grain surfaces (b) making it a well-dispersed system (c) adding a minimum amount of fly ash (d) All of the above Concrete with aggregate of higher modulus of elasticity will shrink (a) by same amount (b) less (c) more (d) more or less the same The split tensile strength of M15 grade concrete when expressed as a fraction of its compressive strength is (a) 0.10 to 0.15 (b) 0.15 to 0.20 (c) 0.20 to 0.25 (d) 0.25 to 0.30 Increase in the fineness modulus of aggregates indicates (a) gap grading

Appendix

56.

57.

58.

59.

60.

61.

62.

(b) finer grading (c) coarser grading (d) Both (i) and (iii) The time dependent phenomenon in concrete is (a) gain of strength (b) shrinkage (c) creep (d) All of the above Vicat’s apparatus is used to determine the following properties of cement (i) normal consistency (ii) initial setting time (iii) final setting time (iv) fineness The correct answer is (a) Both (i) and (iii) (b) Both (ii) and (iv) (c) Both (i) and (iv) (d) (i), (ii) and (iii) Quantity of cement in fresh concrete can be estimated by (i) HCl heat of solution method (ii) EDTA titration method (iii) H2SO4 heat of solution method The correct answer is (a) Only (iii) (b) Both (ii) and (iii) (c) Both (i) and (ii) (d) None of the above Minimum water–cement ratio required for a workable concrete is (a) 0.30 (b) 0.40 (c) 0.50 (d) 0.60 Grading of aggregate in a concrete mix is necessary to achieve (a) adequate workability (b) higher density (c) reduction of voids (d) better durability Bulking of sand is maximum if the percentage of moisture content is of the order of (a) 5 (b) 8 (c) 10 (d) 15 Creep of the concrete is influenced by (a) strength of concrete (b) age of concrete (c) water-cement ratio (d) All of the above

743

63. The compound of Portland cement which contributes to the later strength (for two-three years) is (a) tri-calcium silicate (b) di-calcium silicate (c) tri-calcium aluminate (d) None of the above 64. Presence of air-entraining admixture in concrete (i) decreases the workability (ii) decreases the strength (iii) increases the durability The correct answer is (a) Only (ii) (b) Only (iii) (c) Both (i) and (ii) (d) Both (ii) and (iii) 65. Workability of concrete is influenced most by its (a) cement content (b) aggregate-cement ratio (c) water-cement ratio (d) water-content 66. The ratio of ultimate creep strain to elastic strain is known as (a) creep modulus (b) creep coefficient (c) creep-strain ratio (d) tertiary creep 67. Consider the following statements: Sand in mortar is needed for (i) decreasing the quantity of cement (ii) reducing shrinkage (iii) decreasing the surface area of binding material (iv) increasing the strength The correct answer is (a) (ii), (iii) and (iv) (b) (i), (ii) and (iii) (c) (i), (iii) and (iv) (d) (i), (ii) and (iv) 68. The lower water-cement ratio in concrete causes (a) smaller creep and shrinkage (b) improved frost resistance (c) greater density and smaller permeability (d) All of the above 69. Use of accelerators in concrete (i) shortens the setting time

744

70.

71.

72.

73.

74.

Concrete Technology

(ii) increases the early strength of concrete (iii) increases the period of curing The correct answer is (a) Only (i) (b) Only (iii) (c) Both (i) and (ii) (d) Both (ii) and (iii) Reduction in aggregate–cement ratio while keeping w/c ratio constant causes (a) decrease in workability (b) workability is not affected (c) increase in workability (d) None of the above Lightweight concrete can be used in (a) air-conditioned buildings (b) non-load bearing walls (c) providing reduced thickness of structure (d) All of the above Consider the following statements The effect of sea water on hardened concrete is to (i) increase its strength (ii) reduce its strength (iii) retard setting (iv) decrease its durability The correct answer is (a) (i) and (iii) (b) (ii) and (iii) (c) (ii) and (iv) (d) (i) and (iv) The addition of CaCl2 in concrete results in (i) increased shrinkage (ii) decreased setting time (iii) decreased shrinkage (iv) increased setting time The correct answer is (a) Only (i) and (iii) (b) Only (iv) (c) Only (i) (d) Only (i) and (ii) If x, y and z are the fineness moduli of coarse, fine and combined aggregates, respectively, the ratio of fine aggregates to combined aggregates, is (a) [z – x]/[z + x] (b) [x – y]/[z – y] (c) [x – z]/[z + y]

(d) [z – x]/[y – z] 75. For making lightweight concrete, the gas bubbles are produced by adding (a) aluminum powder to the slurry (b) air bubbles to the slurry (c) water bubbles to the slurry (d) zinc powder to the slurry 76. Aggregates for use in concrete should be in (a) wet condition (b) surface dry condition (c) moist surface condition (d) All of the above 77. The process of mixing some mortar in the mixer at the beginning of the first batch concrete mixing is called (a) buttering (b) borrowing (c) intiating (d) accelerating 78. Wide and deep cracks in concrete can be repaired by (a) guniting (b) grouting (c) epoxy injection (d) None of the above 79. Shrinkage of concrete (i) depends upon water–cement ratio (ii) is inversely proportional to water content (iii) increases with the increase in percentage of cement content The correct answer is (a) Only (i) (b) Only (iii) (c) Both (ii) and (iii) (d) None of the above 80. Hardened concrete is (a) linearly elastic material till the fracture (b) non-linearly elastic material till the fracture (c) linearly elastic up to the level where stress is less than 0.5 times the maximum stress in compression (d) None of the above 81. Concreting in hot weather will result in (a) increased strength (b) increased cracking (c) retarded setting (d) unevenness

Appendix 82. Compaction factor of 0.855 indicates a mix of (a) low workability (b) high workability (c) very low workability (d) medium workability 83. The reaction during cement hydration is (a) exothermic (b) indothermic (c) either of (a) and (b) depending upon the stage of hydration (d) None of the above 84. The reasonable slump for mass concrete should be (a) 90–180 mm (b) 10–30 mm (c) 50–75 mm (d) 25–50 mm 85. Match list-I with list-II and select the correct answer using the codes given below the lists List-I: (type of concrete) A. Concrete for RCC beams and slabs B. Concrete for plain footing C. Vibrated concrete D. Mass concrete

Codes: (a) (b) (c) (d)

A 4 3 4 3

B 3 4 3 4

C 2 2 1 1

88. Assertion A: In slurry infiltrated fiber concrete (SIFCON), there is substantial enhancement of tensile load carrying capacity Reason R: Fibers suppress the localization of micro-cracks into macro-cracks (a) both (A) and (R) are true and (R) is correct explanation of (A) (b) both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true 89. Match list-I with list-II and select the correct answer using the codes given below the lists List-I: (test) A. B. C. D.

Vicat-apparatus Le-chatelier apparatus Slump test Fineness modulus

List-II: (property) 1. 2. 3. 4.

Soundness of cement Initial setting time of cement Workability of concrete Relative size of aggregates

Codes: A

List-II: (slump, mm) 1. 75 2. 50 3. 150 4. 100 D 1 1 2 2

86. The nature of pigments used in colored cement is chemically (a) inactive (b) active (c) alkaline (d) acidic 87. Efflorescence in cement is caused due to an excess of (a) alkalis (b) iron oxide (c) silica (d) alumina

745

B

C

D

(a)

1

2

3

4

(b)

1

4

3

2

(c)

3

4

2

1

(d)

2

1

3

4

90. The product formed on hydration of C2S, in the abbreviated symbols is (b) C3SH (a) C2SH (d) C3S2H3 (c) C2S3H2 91. The commercial name of white and colored cements in India is (a) colocrete (b) silvicrete (c) snocem (d) All of the above 92. Workability tests most suitable for concrete of very low workability are (i) Vee-Bee test (ii) slump test (iii) compaction factor test The correct answer is

746

Concrete Technology below the lists List-I: (type of job)

(a) Only (i) (b) Only (iii) (c) Both (i) and (iii) (d) None of the above 93. Match list-I with list-II related to OPC and select the correct answer using the codes given below the lists

A. B. C. D.

List-II: (slump, mm)

List-I: (compounds) A. C3S C. C3A

(1) 90 (3) 50

B. C2S D. C4AF

25–50 8–14

2. 5–12 4. 20–45

Codes: (a) (b) (c) (d)

A 1 1 1 2

B 2 4 2 1

C 3 2 4 3

D 4 3 3 4

94. The product formed on hydration of C3S, in the abbreviated symbols is (a) C2SH (b) C3SH (c) C2S3H2 (d) C3S2H3 95. Abnormal and premature hardening within few minutes of water mixing is a phenomenon known as (a) flocs (b) false set (c) flocculation (d) permanent setting 96. Which law governs the flow equation of fresh concrete (a) Le-chatelier’s law (b) Bingham law (c) Newton’s law (d) None of the above 97. Segregation in concrete results in (i) creation of porous layers (ii) honeycombing (iii) scaling of surface The correct answer is (a) Only (i) (b) Only (iii) (c) Both (i) and (iii) (d) (i), (ii) and (iii) 98. Match list-I with list-II and select the correct answer using the codes given

(2) 75 (4) 25

Codes:

List-II: (percentage mass) 1. 3.

Precast work Footings Columns Beams

99.

100.

101.

102.

103.

104.

A B C D (a) 4 3 2 1 (b) 1 2 3 4 (c) 2 3 1 4 (d) 3 4 1 2 The flakiness index (in per cent) of coarse aggregate is normally (a) above 75 (b) between 50 and 75 (c) up to 25 (d) between 25 and 50 For the concrete of grades M20 or above the individual test strength (in MPa) compliance requirement is (a) with in fck ± 4 (b) > fck – 4 (c) > fck – 3 (d) within fck ± 3 The shrinkage strain in concrete is approximately (a) 0.0030 (b) 0.0300 (c) 0.0003 (d) 0.3000 The measure of variation in statistical quality control of concrete is generally determined by (a) coefficient of variation (b) curtosis (c) skewness (d) moments The membrane curing of the concrete may be done by (a) solid membranes (b) liquid membranes (c) gaseous membranes (d) Both (a) and (b) Fly ash can be used as (i) partial replacement of fine aggregate (ii) partial replacement of cement (iii) as an admixture

Appendix

105.

106.

107.

108.

109.

110.

The correct answer is (a) Only (i) (b) Only (ii) (c) Both (ii) and (iii) (d) Both (i) and (ii) Portland cement is heavier than water by about (a) 1.15 times (b) 2.30 times (c) 3.85 times (d) 3.15 times The approximate value of the thermal coefficient of expansion of concrete is (a) 9 × 10–5 per °C (b) 10 × 10–6 per °C (c) 9 × 10–7 per °F (d) 8 × 10–6 per °F Underwater concreting is done by (a) dripping method (b) tremie method (c) cofferdam method (d) All of the above Which of the following is/are responsible for the inelastic behavior of concrete (i) shrinkage in concrete (ii) propagation of cracks (iii) improper curing of concrete The correct answer is (a) Only (iii) (b) Both (i) and (iii) (c) Both (ii) and (iii) (d) Both (i) and (ii) Specific surface (cm2/g) of 43-Grade Portland cement should not be less than (a) 2500 (b) 2000 (c) 2250 (d) 3250 The root cause of durability problems in concrete is lack of emphasis on (i) strength-durability relationship (ii) cracking-durability relationship (iii) closer working relationship between designer, material engineer and construction engineer The correct answer is (a) Only (i) (b) Only (iii)

111.

112.

113.

114.

115.

116.

117.

118.

747

(c) Both (ii) and (iii) (d) (i), (ii) and (iii) For mass concrete foundation works, the degree of workability should be (a) low (b) average (c) high (d) very high In the field the commonly used vibrator for compaction of concrete is (a) form vibrator (b) external vibrator (c) needle vibrator (d) None of the above ‘Tremie’ is a (a) bucket (b) water-tight pipe (c) bag (d) prepack concrete Low water-cement ratio in concrete results in (i) greater density and lesser permeability (ii) smaller creep and shrinkage (iii) improved frost resistance The correct answer is (a) Both (i) and (iii) (b) Both (ii) and (iii) (c) (i), (ii) and (iii) (d) None of the above The presence of common salt in sand results in (a) corrosion of reinforcement (b) scaling (c) pitting (d) All of the above Modulus of elasticity of concrete increases with (a) the age (b) the increase in w/c ratio (c) the decrease is curing period (d) All of the above The water-reducing admixture is/are (a) lignosulfonic salt (b) hydroxylated carboxylic acid (c) Both (a) and (b) (d) None of the above Specific gravity of fly ash falls in the range (a) 1.0 to 1.6

748

119.

120.

121.

122.

123.

124.

125.

126.

Concrete Technology (b) 1.6 to 1.9 (c) 1.9 to 2.4 (d) 2.4 to 2.8 The air-entraining agents are used to make (a) air-tight concrete (b) cellular concrete (c) air-extraction concrete (d) All of the above With the increase in the angularity number of aggregates, the workability of concrete will (a) increase (b) not change (c) decrease (d) None of the above Addition of pozzolanas into concrete mixes improves (a) workability (b) resistance to chemical attack (c) Both (a) and (b) (d) None of the above Verback’s dilatometer measures aggregate’s (a) specific heat (b) thermal conductivity (c) coefficient of thermal expansion (d) All of the above In concrete mix design, allowance for bulking of sand is necessary in case of (a) weigh batching (b) volume batching (c) Both (a) and (b) (d) None of the above ‘Shotcrete’ is used in the application of (a) soil stabilization (b) waterproofing (c) stabilization of rock slopes (d) None of the above The permeability of moist cured concrete in comparison to that of steam cured concrete is (a) smaller (b) higher (c) equal (d) None of the above The Tributyl-phosphate used as an admixture serves the purpose of (a) set-controlling (b) water-reducing (c) air-detraining

(d) grouting 127. With the increase in the rate of loading during testing of concrete specimens, the compressive strength of concrete (a) does not change (b) increases (c) decreases (d) None of the above 128. The unit weight (kN/m3) of fly ash concrete is of the order of (a) 10.0 (b) 12.5 (c) 18.0 (d) 22.0 129. Assertion A: Ready-mixed concrete is popular building material. Reason R: Ready mixed concrete embodies the concept of treating concrete in its entity as a building material rather than ingredients (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true 130. “Ultramarine” pigment is used to produce a concrete of (a) black color (b) blue color (c) white color (d) yellow color 131. Concreting in cold weather (i) results in cracking due to the temperature differential within concrete mass (ii) increases the rate of strength development (iii) delays the removal of formwork The correct answer is (a) Only (iii) (b) Only (ii) (c) Both (ii) and (iii) (d) Both (i) and (iii) 132. The settlement of coarse aggregate towards bottom with scum rising towards the surface is known as (a) bleeding (b) capillarity (c) laitance (d) permeability 133. The value of pulse velocity of good quality concrete should be

Appendix

134.

135.

136.

137.

138.

139.

140.

(a) more than 3.5 km/s (b) less than 3.5 km/s (c) less than 2.0 km/s (d) None of the above Concreting in hot weather (i) increases tendency to cracking (ii) makes air-content control difficult (iii) increases strength of hardened concrete The correct answer is (a) Only (i) (b) Both (i) and (ii) (c) Both (ii) and (iii) (d) (i), (ii) and (iii) The phenomenon of bleeding occurs in concrete mix which is (a) lean and wet (b) coarse and wet (c) coarse and dry (d) lean and dry Assertion A: In the quick setting cement, the setting action starts within five minutes of addition of water and it becomes hard like stone within 30 minutes. Reason R: The cement contains small amount of aluminum sulfate, reduced amount of gypsum and is finely ground. (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true The subject of ‘Rheology’ is more closely related to (a) strength of materials (b) fluid mechanics (c) engineering mechanics (d) None of the above The ratio of cement to sand in ferrocement should not be less than (a) 1:7 (b) 1:6 (c) 1:5 (d) 1:3 Rheology of concrete deals with its (a) deformation (b) flow properties (c) compactability (d) All of the above The ratio of shear stress to shear rate in flow of concrete is a

749

(a) constant (b) variable (c) may be both of the above (d) cannot be predicted 141. The water-cement ratio for ferrocement should be (a) less than 0.35 (b) between 0.50 and 0.60 (c) greater than 0.60 (d) between 0.35 and 0.40 142. Vibration test of hardened concrete specimen provides (a) chord modulus (b) secant modulus (c) tangent modulus (d) dynamic modulus 143. Match list-I with list-II and select the correct answer using the codes given below the lists List-I: (phenomenon) A. Deflocculation B. Widening of range of grain size C. Balling effect D. Alkali–aggregate reaction List-II: (material) 1. Steel fibers 2. Siliceous materials 3. Superplasticizer 4. Microfillers Codes: A (a) 2 (b) 4 (c) 3 (d) 3

B 4 3 4 2

C 1 2 1 1

D 3 1 2 4

144. The flow of fresh concrete is of (a) non-Newtonian type (b) Newtonian type (c) Lagrangian type (d) Eulerian type 145. Percentage humidity level at which concrete is cured should be of the order of (a) 40 (b) 55 (c) 80 (d) 90 146. The corrosion of steel in concrete is faster when the element is immersed in (a) alkaline solution (b) acidic solution (c) sea water (d) None of the above

750

Concrete Technology

147. Carbonation of concrete results into (a) increased shrinkage (b) increased strength (c) Both (a) and (b) (d) increased permeability 148. Study the following statements: (i) presence of five per cent voids in concrete may reduce the strength by 35 per cent (ii) pumpable concrete is high slump concrete (iii) ready mixed concrete is weigh batched, mixed in a centrally located plant and is transported in a truck mixer and delivered in a ready to use condition The correct statement(s) is/are (a) Only (i) (b) Only (ii) (c) Both (ii) and (iii) (d) (i), (ii) and (iii) 149. As per IS: 456–2000, the modulus of elasticity of concrete is taken as (a) 5700

f ck

(b) 5000

f ck

(c) 570

f ck

(d) 50 f ck 150. The use of relatively weak mortar (i) will accommodate movements due to loads, and cracking, if any, will be distributed as thin hair cracks which are less noticeable or harmful. (ii) will result in reduction of stresses due to differential expansion of masonry units The correct answer is (a) Only (i) (b) Only (ii) (c) Both (i) and (ii) (d) Neither (i) nor (ii) 151. Match list-I with list-II and select the correct answer using the codes given below the lists. List-I: (property) A. Reinforcing index B. pH Value C. Cementing efficiency factor

D. Toughness List-II: (related to) 1. Mineral additives 2. Corrosion of reinforcement 3. Stress–strain curve 4. Fiber-reinforced concrete Codes: A (a) 3 (b) 4 (c) 4

B 2 2 2

C 1 3 1

D 4 1 3

(d) 3

4

1

2

152. As per IS: 456–2000, the relationship between modulus of rupture (fcr) and characteristics of strength of concrete (fck) is (a) 0.70

f ck

(b) 0.12

f ck

(c) 0.70

f ck

(d) 1.0

f ck

153. Assertion A: The specific surface of aggregates decreases with increase in size of the aggregates. Reason R: The workability of a mix is influenced more by the finer fraction than the coarse particles. (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true 154. Match list-I with list-II and select the correct answer using the codes given below the lists. List-I: (parameter) A. B. C. D.

Specific surface Surface tension Water-binder ratio Porosity-permeability

List-II: (material) 1. 2. 3. 4.

Aggregate Mineral additives Cementitious materials Superplasticizer

Appendix Codes: A (a) 1 (b) 3 (c) 1 (d) 1

B 2 4 4 4

C 3 1 2 3

D 4 2 3 2

155. Match list-I with list-II and select the correct answer using the codes given below the lists List-I: (property) A. Self-desiccation B. Expansive reaction C. Volume change D. Cracking behavior List-II: (parameter) 1. Corrosion 2. Shrinkage 3. Autogenous-shrinkage 4. Curing-history Codes: A (a) 3 (b) 4 (c) 4 (d) 2

B 1 3 1 4

C 2 1 3 3

D 4 2 2 1

156. Aerated concrete is suitable for applications such as (a) wall insulation (b) fire protection (c) filler wall (d) All of the above 157. Good quality concrete should have fineness modulus of fine aggregate in the range of (a) 2.0 to 3.5 (b) 1.5 to 2.3 (c) 3.0 to 5.5 (d) None of the above 158. The compressive strength of a standard 1:3 Portland cement–sand mortar in MPa after three days of curing should not be less than (a) 7.0 (b) 11.5 (c) 17.5 (d) 21.0 159. The diameter in mm of tremie pipe for underwater concreting shall be not less than (a) 100 (b) 150

751

(c) 200 (d) 300 160. Match list-I with list-II and select the correct answer using the codes given below the lists. List-I: (stripping of forms) A. Slabs (props not removed) B. Removal of props to slab spanning over 4.5 m C. Beam soffits (props not removed) D. Removal of props to beam spanning over 6 m List-II: (time) 1. 14 days 2. 21 days 3. 3 days 4. 7 days Codes: A (a) 1 (b) 3 (c) 3 (d) 4

B 2 4 4 3

C 3 2 1 1

D 4 1 2 2

161. Slump (mm) of concrete in case of underwater concreting shall be (a) 50–100 (b) 100–150 (c) 100–180 (d) 150–200 162. Good quality concrete should have fineness modulus of coarse aggregate in the range of (a) 5 to 6 (b) 6 to 9 (c) 3 to 4 (d) 4 to 5 163. The sand used in cement test should be in accordance with (a) IS: 1343–1980 (b) IS: 456–2000 (c) IS: 650–1991 (d) IS: 1365–1980 164. Grade of concrete for RCC for sea-water application shall not be less than (a) 15 (b) 20 (c) 30 (d) 10 165. Match list-I with list-II and select the correct answer using the codes given below the lists.

752

Concrete Technology List-I: (admixture) A. Water-reducing admixture B. Air-entraining agent C. Superplasticizer D. Accelerator List-II: (chemicals) 1. Sulfonated melamine formaldehyde 2. Calcium chloride 3. Lignosulfonate 4. Neutralized vinsol resin Codes: A

B

C

D

(a) 3 4 1 2 (b) 2 4 1 3 (c) 1 3 4 2 (d) 3 4 2 1 166. Low heat cement contains lower percentage of (a) C3A (b) C3S (c) C2S (d) none of above 167. Match list-I with list-II and select the correct answer using the codes given below the lists List-I: (nominal mix) A. 1 : 1 : 2 B. 1 : 1½ : 3 C. 1 : 2 : 4 D. 1 : 4 : 8

B 2 3 4 4

C 3 2 1 2

D 4 1 2 1

168. Match list-I with list-II and select the correct answer using the codes given below the lists List-I: (test) A. Crushing test B. Impact test C. Shape test D. Solubility test List-II: (property)

Codes: A (a) 1 (b) 3 (c) 4 (d) 2

B 2 1 2 3

C 3 2 3 1

D 4 4 1 4

169. Excess alkalies in cement cause (a) alkali–aggregate reaction (b) efflorescence (c) staining (d) All of the above 170. Match list-I with list-II and select the correct answer using the codes given below the lists. List-I: (type of cement) A. B. C. D. 1. 2. 3. 4.

Footing Foundation base Heavily loaded columns Beams

Codes: A (a) 1 (b) 4 (c) 3 (d) 3

Resistance against sudden jerks Share of flak particles Compressive strength Soluble matter

Air-entraining Portland cement Low heat Portland cement Hydrophobic Portland cement Rapid hardening cement

List-II: (characteristics)

List-II: (applications) 1. 2. 3. 4.

1. 2. 3. 4.

Suitable for large dams Unsuitable for large mass concrete Greater resistance to frost attack Safe storage under unfavorable condition of humidity.

Codes: A B C D (a) 3 4 1 2 (b) 4 2 1 3 (c) 3 1 4 2 (d) 4 1 2 3 171. Assertion A: The alumina should not be present in excess amount in the raw materials used for manufacture of cement. Reason R: Alumina acts as a flex and lowers the clinkering temperature. (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true 172. Roller-compacted concrete (RCC) is a (a) stiff (dry and lean) concrete mix

Appendix

173.

174.

175.

176.

177.

178.

(b) zero slump concrete (c) mix having consistency of damp gravel (d) All of the above Addition of fibers in concrete results in (a) modest increase in compressive strength (b) increased ductility (c) enhanced toughness (d) All of the above Assertion A: Ready-mixed concrete (RMC) is one of the most versatile and popular building materials. Reason R: The RMC producer is an expert in selecting the proportions for the specific application based on experience for the required per-formance. (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true Blow holes are the result of (a) excess water–cement ratio (b) insufficient workability (c) improper design of shuttering (d) None of the above Study the following statements: (i) for constant w/c ratio, finer sand decreases the workability (ii) creep is the deformation of concrete under sustained loading (iii) creep is directly proportional to cement paste The correct statement(s) is are (a) Only (i) (b) Both (i) and (ii) (c) Both (ii) and (iii) (d) (i), (ii) and (iii) Most suitable cement for concrete for under-sea applications is (a) low heat cement (b) high-alumina cement (c) super sulfate cement (d) All of the above For complete hydration of cement, the water–cement ratio needed is of the order of (a) less than 0.25

179.

180.

181.

182.

183.

184.

753

(b) more than 0.25 but less that 0.35 (c) more than 0.35 but less than 0.45 (d) more than 0.45 but less than 0.60 Assertion A: The availability of high-early strength OPCs has resulted in decreasing cement content and increasing water–cement ratio for the given consistency. Reason R: The concrete mixtures are proportioned on the basis of 28-day compressive strength. (a) Both (A) and (R) are true and (R) is correct explanation of (A) (b) Both (A) and (R) are true and (R) is an incorrect explanation of (A) (c) (A) is true and (R) is false (d) (A) is false and (R) is true Epoxy injection technique is used for (a) repairing the water retaining structures (b) sealing of large cracks (c) sealing the narrow cracks (d) All of the above Ready-mixed concrete (RMC) is (i) specified in terms of performance parameters (ii) produced under factory conditions (iii) produced and supplied by weight The correct answer is (a) Both (i) and (ii) (b) Both (i) and (iii) (c) Both (ii) and (iii) (d) (i), (ii) and (iii) For compaction of concrete, surface vibrator can be used for (i) columns (ii) slabs (iii) raft foundation The correct answer is (a) Only (i) (b) Only (ii) (c) Both (i) and (ii) (d) Both (ii) and (iii) Number of samples for 31–50 m3 of concrete work shall be (a) 2 (b) 3 (c) 4 (d) 5 Match list-I with list-II and select the correct answer using the codes given below the lists.

754

Concrete Technology List-I: (property enhancement) A. Self-compactability B. Freeze-thaw durability C. Paste-refinement D. Concrete-concrete bond List-II: (material) 1. High-range water reducer 2. Epoxy resin 3. Mineral additive 4. Air-entraining admixture Codes: A (a) 4 (b) 1 (c) 2 (d) 3

B 3 4 3 2

C 2 3 4 1

D 1 2 1 4

(iii) process improvement to tailor mix to the needs of specific job The correct answer is (a) Only (i) (b) Only (ii) (c) Both (ii) and (iii) (d) (i), (ii) and (iii) 187. The heat liberated during hydration of cement is about (a) 360 cal/g (b) 240 cal/g (c) 60 cal/g (d) 120 cal/g 188. Match list-I with list-II and select the correct answer using the codes given below the lists. List-I: (phenomenon)

185. The RMC producer (i) guarantees the desired performance (ii) receives instructions through job specifications (iii) receives instructions in terms of prescriptive specifications The correct answer is (a) Only (i) (b) Both (i) and (ii) (c) Both (i) and (iii) (d) None of the above 186. In concrete industry for economy the emphasis should be placed on (i) prescriptive specifications (ii) performance-based specifications

A. B. C. D.

Pop-off Rapid slump loss Steel corrosion Low bleeding

List-II: (reason) 1. 2. 3. 4.

Non-retarding HRWR Improper finish High volume of fly ash Chloride-ion penetration

Codes: (a) (b) (c) (d)

A 1 2 3 4

B 2 1 2 3

C 3 4 1 2

D 4 3 4 1

Answers to MCQs 1.(d)

2.(b)

3.(b)

4.(a)

5.(a)

6.(d)

7.(c)

8.(c)

9.(d)

10.(a)

11.(d)

12.(d)

13.(c)

14.(b)

15.(b)

16.(a)

17.(d)

18.(a)

19.(d)

20.(c)

21.(c)

22.(b)

23.(a)

24.(a)

25.(a)

26.(c)

27.(b)

28.(d)

31.(c)

32.(a)

33.(d)

34.(d)

35.(b)

36.(a)

37.(c)

38.(a)

39.(a)

40.(b)

41.(d)

42.(a)

43.(a)

44.(b)

45.(c)

46.(a)

47.(d)

48.(c)

49.(a)

50.(a)

51.(b)

52.(b)

53.(b)

54.(a)

55.(c)

56.(d)

Appendix

755

57.(d)

58.(c)

59.(b)

60.(c)

61.(a)

62.(d)

63.(b)

64.(d)

65.(d)

66.(b)

67.(d)

68.(d)

69.(c)

70.(c)

71.(b)

72.(c)

73.(d)

74.(b)

75.(a)

76.(b)

77.(a)

78.(b)

79.(b)

80.(c)

81.(b)

82.(d)

83.(a)

84.(d)

85.(d)

86.(a)

87.(a)

88.(a)

93.(b)

94.(d)

95.(b)

96.(b)

97.(d)

98.(a)

99.(c)

100.(b)

101.(a)

102.(a)

103.(d)

104.(d)

105.(d)

106.(b)

107.(b)

108.(d)

109.(c)

110.(c)

111.(a)

112.(c)

113.(b)

114.(c)

115.(a)

116.(a)

117.(c)

118.(c)

119.(b)

120.(c)

121.(c)

122.(c)

123.(b)

124.(c)

125.(a)

126.(c)

127.(b)

128.(d)

129.(a)

130.(b)

131.(d)

132.(c)

133.(a)

134.(b)

135.(a)

136.(a)

137.(b)

138.(d)

139.(d)

140.(b)

141.(d)

142.(d)

143.(c)

144.(a)

145.(d)

146.(b)

147.(c)

148.(c)

149.(a)

150.(a)

151.(c)

152.(c)

153.(b)

154.(d)

155.(a)

156.(d)

157.(a)

158.(b)

159.(c)

160.(c)

161.(d)

162.(a)

163.(c)

164.(c)

165.(a)

166.(a)

167.(c)

168.(b)

161.(d)

162.(a)

163.(c)

164.(c)

165.(a)

166.(a)

167.(c)

168.(b)

169.(d)

170.(c)

171.(a)

172.(d)

173.(d)

174.(a)

175.(c)

176.(b)

177.(b)

178.(c)

179.(a)

180.(c)

181.(a)

182.(a)

183.(c)

184.(b)

185.(b)

186.(c)

187.(d)

188.(b)

BIBLIOGRAPHY BIBLIOGRAPHY

1. ACI Committee 211, 1991(re-approved in 2002), Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete, ACI211.1-91, American Concrete Institute, Farmington Hills, Michigan, USA, 2002. 2. ACI Committee 211, Guide for Selecting Proportions for High Strength Concrete with Portland Cement and Fly Ash, ACI-211.4R-93, American Concrete Institute, Farmington Hills, Michigan, USA, 1993. 3. ACI Committee 211, Guide for Selecting Proportions for No-Slump Concrete, ACI-211.3R-97, American Concrete Institute, Farmington Hills, Michigan, USA, 1997. 4. ACI Committee 211, Standard Practice for Selecting Proportions for Structural Lightweight Concrete, ACI-211.2-98, American Concrete Institute, Farmington Hills, Michigan, USA, 1998. 5. ACI Committee 222, Protection of Metals in Concrete Against Corrosion, ACI- 222.R-01, American Concrete Institute, Farmington Hills, Michigan, 2001, 41 pages. 6. ACI Committee 232, Use of Fly Ash in Concrete, ACI-232.2R-96, American Concrete Institute, Farmington Hills, Michigan, 1996, p. 34. 7. ACI Committee 234, Guide for the Use of Silica Fume in Concrete, ACI-234.R96, American Concrete Institute, Farmington Hills, Michigan, 1996, p. 51. 8. ACI Committee 301, Specifications for Structural Concrete, ACI-301-99, American Concrete Institute, Farmington Hills, Michigan, USA, 1999. 9. ACI Committee 305, Hot Water Concreting, Journal of American Concrete Institute, Vol. 74, August 1977, pp. 317–332. 10. ACI Committee 313, Guide for Structural Lightweight Aggregate Concrete, ACI- 213.R-79, Concrete International, Vol. 1, No. 2, February 1979, pp. 33-62. 11. ACI Committee 318, Building Code Requirements for Structural Concrete, ACI- 318-05, American Concrete Institute, Farmington Hills, Michigan, 2005, p. 443. 12. ACI Committee 506, Specifications for Materials, Proportioning and Application of Shotcrete, ACI-506, 2-77, Journal of American Concrete Institute, Vol. 73, December, 1976, pp. 679-685; Vol. 74, November, 1977, pp. 563.

Bibliography

757

13. ACI Committee 548, Polymers in Concrete, Journal of American Concrete Institute, Farmington Hills, Michigan, 1977. 14. Allen, R.T.L. and S.C. Edward (ed.), Repair of Concrete Structure, Blackie, London, 1987. 15. BS:1881-Part 201, Guide to Use of Non-destructive Methods of Test for Hardened Concrete, British Standards Institution, London. 16. BS EN:206-1, Concrete-Part 1 Specification, Performance, Production and Conformity, British Standards Institution, 2002, 74 Pages. 17. BS EN:480-1, Admixtures for Concrete, Mortar and Grout, Test Methods, Reference Concrete and Reference Mortar for Testing, British Standards Institution, 2006. 18. BS EN:1992-1-1, Eurocode 2- Design of Concrete Structure, General Rules and Rules for Buildings, British Standards Institution, 2004. 19. BS:8500-1, Concrete - Complementary British Standard to BS EN 206-1 - Method of Specifying and Guidance for the Specifier, British Standards Institution, 2006. 20. BS:8500-2, Concrete - Complementary British Standard to BS EN 206-1 - Specification for constituent materials and concrete, British Standards Institution, 2006. 21. BS EN:12350, Testing Fresh Concrete; BS EN: 12390: Testing Hardened concrete, BS EN:12504: Testing Concrete in Structures, British Standards Institution, 2000. 22. Burgueño, R. and Bendert, D. A., Experimental Evaluation and Field Monitoring of Prestressed Box Beams for SCC Demonstration Bridge, Report No. CEE-RR – 2007/01, Department of Civil and Environmental Engineering, Michigan State University, East Lansing, Michigan, July 2007. 23. Buttler, W.B., Durable Concrete Containing Three or Four Cementitious Materials, Fourth CANMENT/AC1 International Conference on Durability of Concrete, Sydney, Australia, SP-170, Vol. 1, V.M. Malhotra, (ed.), American Concrete Institute, Framington Hills, Mich., 1997, pp. 309–330. 24. Cooke, T. H., Concrete Pumping and Spraying - A Padical Guide, Van Nostrand Reinhold, London, 1990. 25. CSA Standard A23.1-00/A23.2-00, Concrete Materials and Methods of Concrete Construction/Methods of Test for Concrete, Canadian, Standards Associations, Toronto, 2000. 26. Department of the environment (DoE), Design of Normal Concrete Mixes, British Research Establishment, Walford, U.K., 1988. 27. ERMCO BIBM, CEMBUREAU, EFCA AND EFNARC, The European Guidelines for Self Compacting Concrete Specification, Production and Use, Warrington: SCC European Project Group, May 2005. 28. EFNARC: Specifications and Guidelines for Self-Compacting Concrete, EFNARC, Association House, 99 West street, Farnham, Surrey, UK, 2002. 29. Ferraris, C. F. and Larrard, F. de, Testing and Modelling of Fresh Concrete Rheology, Building and Fire Research Laboratory, National Institute of Standards and Technology, NISTIR 6094, February 1998.

758

Concrete Technology

30. Gambhir, M. L., Design of Reinforced Concrete Structures, Prentice-Hall of India Pvt. Ltd, New Delhi, 2008. 31. Gambhir, M. L., Fundamentals of Reinforced Concrete Design, Prentice-Hall of India Pvt. Ltd, New Delhi, 2006. 32. Gambhir, M. L., Concrete Technology, McGraw-Hill Education (India), New Delhi, 3rd edition, 2004. 33. Gambhir, M. L., Concrete Manual: Laboratory Testing for Quality Concrete, Dhanpat Rai & Sons, New Delhi, 4th edition, 1992. 34. IS: 269-1989(fourth revision), Specification for 33 Grade Ordinary Portland Cement (with Amendment No. 3), Bureau of Indian Standards, New Delhi. 35. IS: 383-1970 (re-approved in 1997), Specifications for Coarse and Fine aggregates from Natural sources for Concrete, Bureau of Indian Standards, New Delhi, India. 36. IS: 455-1989 (fourth revision), Specification for Portland Slag Cement (with Amendment No. 3), Bureau of Indian Standards, New Delhi, India. 37. IS: 456-2000 (fourth revision), Code of Practice for Plain and Reinforced Concrete (with Amendment No. 2), Bureau of Indian Standards, New Delhi, India. 38. IS: 460 (Part-I)-1978 (second revision), Test Sieves: Part-I Wire Cloth Test Sieves, Bureau of Indian Standards, New Delhi, India. 39. IS: 460 (Part-II)-1978, Test Sieves: Part-II Perforated Plate Test Sieves, Bureau of Indian Standards, New Delhi, India. 40. IS: 516-1959, Method of Test for Strength of Concrete (with Amendment No. 2), Bureau of Indian Standards, New Delhi, India. 41. IS: 650-1991 (second revision), Specification for Standard Sand for Testing of Cement, Bureau of Indian Standards, New Delhi, India. 42. IS: 1199-1959, Methods of Sampling and Analysis of Concrete, Bureau of Indian Standards, New Delhi, India. 43. IS: 1343-1980 (first revision), Code of Practice for Prestressed Concrete (with Amendment No. 1), Bureau of Indian Standards, New Delhi, India. 44. IS: 1489 (Part-I)-1991 (third revision), Specification for Portland Pozzolana Cement: Part-I Fly ash Based (with Amendment No. 2), Bureau of Indian Standards, New Delhi, India. 45. IS: 1489 (Part-II)-1991 (third revision), Specification for Portland Pozzolana Cement: Part-II Calcined Clay Based (with Amendment No. 2), Bureau of Indian Standards, New Delhi, India. 46. IS: 2386 (Part-I)-1963, Methods of Test for Aggregates for Concrete: Part-I Particle Size and Shape (with Amendment No. 2), Bureau of Indian Standards, New Delhi, India. 47. IS: 2386 (Part-Ill)-1963, Methods of Test for Aggregates for Concrete: PartIll Specific Gravity, Density, Voids, Absorption and Bulking, Bureau of Indian Standards, New Delhi, India. 48. IS: 2386 (Part-TV)-1963, Methods of Test for Aggregates for Concrete: Part-IV Mechanical Properties (with Amendment No. 3), Bureau of Indian Standards, New Delhi, India.

Bibliography

759

49. IS: 2386 (Part-VII)-1963, Methods of Test for Aggregates for Concrete: PartVII Alkali Aggregate Reactivity, Bureau of Indian Standards, New Delhi, India. 50. IS: 2430-1986 (first revision), Methods for Sampling of Aggregates for Concrete, Bureau of Indian Standards, New Delhi, India. 51. IS: 3466-1988 (second revision), Specification for Masonry Cement (with Amendment No. 1), Bureau of Indian Standards, New Delhi, India. 52. IS: 3535 (first revision)-1986, Methods of Sampling Hydraulic Cement, Bureau of Indian Standards, New Delhi, India. 53. IS: 3558-1983 (first revision), Code of Practice for Use of Immersion Vibrators for Consolidating Concrete, Bureau of Indian Standards, New Delhi, India. 54. IS: 3812-2003, Specifications for Pulverized Fuel Ash, Bureau of Indian Standards, New Delhi, India. 55. IS: 4031-1988(Part-II) (first revision), Methods of Physical Tests for Hydraulic Cement: Part-II Determination of Fineness by Specific Surface by Blaine Air Permeability Method, Bureau of Indian Standards, New Delhi, India. 56. IS: 4031-1988 (Part-VI) (first revision), Methods of Physical Tests for Hydraulic Cement: Part-VI Determination of Compressive Strength of Hydraulic Cement (other than Masonry Cement) (with Amendment No. 1), Bureau of Indian Standards, New Delhi, India. 57. IS: 4031-1988 (part-IX) (first revision), Methods of Physical Tests for Hydraulic Cement: Part-IX Determination of Heat of Hydration, Bureau of Indian Standards, New Delhi, India. 58. IS: 4031-1988 (Part-X) (first revision), Methods of Physical Tests for Hydraulic Cement: Part-X Determination of Drying Shrinkage, Bureau of Indian Standards, New Delhi, India. 59. IS: 4305-1967, Glossary of Terms Relating to Pozzolana, Bureau of Indian Standards, New Delhi, India. 60. IS: 4634-1968, Method for Testing Performance of Batch Type Concrete Mixer, Bureau of Indian Standards, New Delhi, India. 61. IS: 4845-1968, Definitions and Terminology Relating to Hydraulic Cement, Bureau of Indian Standards, New Delhi, India. 62. IS: 4926-1976 (first revision), Specification for Ready Mixed Concrete, Bureau of Indian Standards, New Delhi, India. 63. IS: 5640-1970, Method of Test for Determining Aggregate Impact Value of Soft Coarse Aggregate, Bureau of Indian Standards, New Delhi, India. 64. IS: 5816-1999 (first revision), Method of Test for Splitting Tensile Strength of Concrete Cylinders, Bureau of Indian Standards, New Delhi, India. 65. IS: 6452-1989, Specification for High Alumina Cement for Structural Use (with Amendment No. l), Bureau of Indian Standards, New Delhi, India. 66. IS: 6461-1972 (Part-I), Glossary of Terms Relating to Cement Concrete: Part- I Concrete Aggregates, Bureau of Indian Standards, New Delhi, India. 67. IS: 6461-1972 (Part-II), Glossary of Terms Relating to Cement Concrete: Part- II Materials, Bureau of Indian Standards, New Delhi, India. 68.15:6461-1972 (Part-IV), Glossary of Terms Relating to Cement Concrete: PartIV Types of Concrete, Bureau of Indian Standards, New Delhi, India.

760

Concrete Technology

69. IS: 6461-1973 (Part-VII), Glossary of Terms Relating to Cement Concrete: Part-VIl Mixing, Laying, Compaction, Curing and Other Construction Aspects, Bureau of Indian Standards, New Delhi, India. 70. IS: 6461-1973 (Part-VIII), Glossary of Terms Relating to Cement Concrete: Part-VII Properties of Concrete, Bureau of Indian Standards, New Delhi, India. 71. IS: 6491-1972, Methods for Sampling of Fly Ash, Bureau of Indian Standards, New Delhi, India. 72. IS:6909-1990, Specification for Supersulfated Cement (with Amendment No. 3), Bureau of Indian Standards, New Delhi, India. 73. IS:7861-1975 (Part-I), Code of Practice for Extreme Weather Concreting: Part-I Recommended Practice for Hot Weather Concreting (with Amendment No. 1), Bureau of Indian Standards, New Delhi, India. 74. IS: 7861-1981 (Part-II), Code of Practice for Extreme Weather Concreting: Part-VII. Recommended Practice for Cold Weather Concreting (with Amendment No. 1), Bureau of Indian Standards, New Delhi, India. 75. IS: 8041-1990 (second revision), Specification for Rapid Hardening Portland Cement (with Amendment No. 2), Bureau of Indian Standards, New Delhi, India. 76. IS: 8042-1989 (second revision), Specification for White Portland Cement (with Amendment No. 4), Bureau of Indian Standards, New Delhi, India. 77. IS:8043-1991 (second revision), Specification for Hydrophobic Portland Cement (with Amendment No. 1), Bureau of Indian Standards, New Delhi, India. 78. IS: 8112-1989 (re-affirmed in 2000), Specifications for 43 grade Portland cement, Bureau of Indian Standards, New Delhi, India. 79. IS: 8142-1976, Method of Test for Determining Setting Time of Concrete by Penetration Resistance, Bureau of Indian Standards, New Delhi, India. 80.1S: 9012-1978 (re-affirmed in 1997), Recommended Practice for Shotcreting, Bureau of Indian Standards, New Delhi, India. 81. IS: 9013-1978 (re-affirmed in 1997), Method of Making, Curing and Determining Compressive Strength of Accelerated Cured Concrete, Test Specimens, Bureau of Indian Standards, New Delhi, India. 82. IS: 9103-l999 (first revision), Specification for Admixtures for Concrete, Bureau of Indian Standards, New Delhi, India. 83. IS: 10262-1982 (re-affirmed in 1999), Recommended Guidelines for Concrete Mix Design, Bureau of Indian Standards, New Delhi, India. 84. IS: 12089-1987, Specification for Granulated Slag for Manufacture of Portland Slag Cement, Bureau of Indian Standards, New Delhi, India. 85. IS: 12269-1987, Specification for 53 Grade Ordinary Portland Cement (with Amendment No. 3), Bureau of Indian Standards, New Delhi, India. 86. IS: 12330-1988 (re-affirmed in 2000), Specification for Sulfate Resisting Portland Cement (with Amendment No. 4), Bureau of Indian Standards, New Delhi, India. 87. IS: 12600-1989 (re-affirmed in 2000), Specification for Low Heat Portland cement (with Amendment No. 3), Bureau of Indian Standards, New Delhi, India.

Bibliography

761

88. IS: 12803-1989, Methods of Analysis of Hydraulic Cement by X-Ray Fluorescence Spectrometer, Bureau of Indian Standards, New Delhi, India. 89. IS: 13311 (Part-I)-1992, Methods of Non-Destructive Testing of Concrete: Part-I Ultrasonic Pulse Velocity, Bureau of Indian Standards, New Delhi, India. 90. Japan Society of Civil Engineers, High-fluidity Concrete Construction Guideline, Concrete Library 93, 1999. 91. Khayat, K., Workability, Testing and Performance of Self-Consolidating Concrete, ACI Materials Journal No 3, American Concrete Institute, Farmington Hills, Michigan, 1999, pp 346–354. 92. Kosmatka, S. H., Kerkhoff, B., Panarese, W. C., MacLeod, N. F. and McGrath, R, J., Design and Control of Concrete Mixtures, 7th Canadian edition, Engineering Bulletin 101, Cement Association of Canada, Ottawa, Canada, 2002. 93. Malhotra, V.M., Making Concrete Greener with Fly Ash, Concrete International, Vol. 21, No. 5, May 1999, pp. 61–66. 94. Nagamoto, N., Ozawa K., Mixture Properties of Self-Compacting, HighPerformance Concrete, Proceedings, Third CANMET/ACI International Conferences on Design and Materials and Recent Advances in Concrete Technology, SP-172, V. M. Malhotra, American Concrete Institute, Farmington Hills, Michigan, 1997, p. 623–637. 95. Nagataki, S., Fujiwara H., Self-Compacting Property of Highly-Flowable Concrete, Second Conference on Advances in Concrete Technology, ACI SP154,V.M. Malhotra, American Concrete Institute, June 1995, p. 301-304. 96. Nagataki, S. S., Migazato, and T. Saitoh, Effects of Fly Ash and Silica Fume in High Performance Concrete, Fly Ash, Slag, Silica Fume, and Natural Pozzolan-Proceedings, CANMET/ACI Sixth International Conference, Bangkok, Thailand, SP-178, Vol. 1, V.M. Malhotra, (ed.), American Concrete Institute, Farmington Hills, Mich., 1995, pp, 307–330. 97. Nelson, P.K., Handbook of Non-destructive and Innovative Testing Equipment for Concrete, Federal Highway Administration, Washington, D.C., 2003. 98. Neville, A.M., Properties of Concrete, 4th edition, Pitman Publishing Company, 1996. 99. Nielsen, C., SCC and the Working Environment, Presentation at the Nordic SCC Network Workshop, Copenhagen: DTI concrete Centre, 2006. 100. Okamura H. and Ouchi, M., Self-Compacting Concrete, Journal of advanced Concrete Technology, Japan Concrete Institute, Tokyo, 2003. 101. Okamura, H. and Ouchi, M., Self-Compacting Concrete: Development, present use and future, Proceedings of first International RILEM Symposium on Self-Compacting Concrete, Stockholm: RILEM Publications S.A.R.L., 1999, pp. 3–14. 102. Okamura, H. and Ozawa, K., Mix Design for Self-Compacting Concrete, Concrete Library of Japanese Society of Civil Engineers, June 25, 1995, p. 107–120. 103. PCI:TR-6-03, Interim Guidelines for the Use of Self-Consolidating Concrete in PCI Member Plants, Precast/Prestressed Concrete Institute, Chicago, Illinois, April, 2003, 88 Pages.

762

Concrete Technology

104. Peterson, M., Ph.D. Thesis entitled: High-Performance and Self-Compacting Concrete in House Building: Field Tests and Theoretical Studies of Possibilities and Difficulties, submitted to Lund Institute of Technology, Division of Building Materials, Lund University, 2008. 105. SP: 23-1982, Handbook on Concrete Mixes, Bureau of Indian Standards, New Delhi, 1982. 106. Swamy, R.N. and A.A., Darwish, Engineering Properties of Concretes with Combinations of Cementitious Materials: Fly Ash, Slag, Silica Fume, and Natural Pozzolanas-Proceedings, CANMET/ACI Sixth International Conference, Bangkok, Thailand, SP-178, Vol. 1, V.M. Malhotra, (ed.), American Concrete Institute, Framington Hills, Mich., 1998, pp. 331–359. 107. Tattersall G.H. and P.F.G. Banfill, Rheology of Fresh Concrete, Pitman Advanced Publishing Program, 1983. 108. Testing SCC, Measurement of Properties of Self-Compacting Concrete, Final Report European Union Growth Contract No. G6RD-CT-2001-00580, Paisly: ACM Centre, University of Paisly, 2007. 109. Teychenne, D.C., R.E. Franklin and H. Erntroy, Design of Normal Concrete Mixes, Department of Environment, HMSO, London, 1975, p. 31. 110. Tomsett, H.N. Ultrasonic Pulse Velocity Measurement in the Assessment of Concrete Quality, Magazine of Concrete Research, Vol. 32, No. 110, 1980, pp. 7–16.

INDEX

A Abram’s water–cement law 28, 179, 252 Abrasion 658, 661 Abrasion resistance 63, 112, 260, 362, 662, 671 Absestos–cement 528 Absolute volume concept 307, 611 — method 231 Absorption 70 Absorption of aggregate 70, 73, 428 Accelerated strength tests 429 Accelerating admixtures 104 Accelerated curing 298, 389, 429 — setting 371 — strength 197, 211, 272, 288 Accelerators 104, 131, 176, 177, 411 Acceptance criteria 129, 221, 226, 230, 335, 367, 454 — testing 231, 392 Acceptance tests 221 Acids and Alkalies 98 ACI method for mix proportioning 275, 303, 309 Acoustic insulation 619, 621 Acrylic resins 706 Active 679 Active crack 709, 710 Admixtures 48, 49, 102, 157, 238, 313, 411, 425, 526, 609 Advantages of concretes 8, 15, 504 — heat resistant concretes 548 — quality control 219 — ferrocement 463 Aerated concrete 125, 472, 555, 617 Aggregate 48, 58, 63, 93, 107, 155, 441, 506, 510, 524, 530, 563, 574, 647

— abrasion value 70 — cement ratio 74, 143, 166, 213, 302, 407 — characteristics 212, 276, 283 — grading 72, 81, 156, 175, 219, 228, 254 — impact value 70 — moisture 254, 265, 272, 276 — segregation 530, 537, 541 — shape 155, 519 — cement ratio 74, 143, 166, 213 — cracks 185 — matrix bond 80, 441, 461, 468, 574 Aggregates 4, 7, 9, 13, 17, 32 Aggregate–matrix bond 60, 404, 510, 574 Aggregate-matrix Interface 462 Aggregate-paste bond 656 Aggregate properties 155 Air content 37, 48, 51, 108, 109, 123 — test 150 Air-detraining admixtures 48, 104, 107, 124 Air-entrained concrete 107, 110, 220, 246, 252, 528, 663, 664 Air-entraining agent 36, 37, 51, 490, 645 Air-entraining cement 37 Air-entrainment 69, 116, 127 Air-entrainment in Shotcrete 492 Air voids 107 Air-void analysis 423, 425, 426 Algae 99 Alkali-aggregate-reaction 78, 136, 686, 560, 659 Alkali–Silica reaction 46, 47, 213, 561, 657, 658, 683 Alkali-aggregate expansion 103 Alkali–aggregate reactivity 658 Alkali-carbonate reaction 683 Alkali-silica reactivity 77

764

Index

Alternate wetting and drying 36 All-in-aggregate 65, 66, 79, 86 Analysis of fresh concrete 423, 428 Angular aggregate 66, 71, 112, 175, 187 Angularity number 67 Anti-carbonation coatings 72 Apparent specific gravity 72 Application of — jet cement 541 — steel-fibre reinforced concrete 463 — foamed concrete 617, 620 — refractory concretes 524 Artificial 392 Artificial aggregates 347 Asbestos fibres 506, 530 Aspect ratios 463, 508, 579, 668 Assessment of in-situ quality 434, 444 Autogenous heeling 715, 735

B Balling 579 Ball-penetration test 423 Basement waterproofing 104 Batching 15, 218, 220 — plant 281, 292, 312, 335, 361 Beam strengthening 733 Behaviour of fibre reinforced concrete 463 Binary cements 46, 49, 643 Bingham model 167 Blaine’s air permeability 34 Blast-furnace cement 141 Blast-furnace-slag 60, 64, 65, 68, 134, 141, 487 Bleeding 6, 37, 46, 82, 107, 110, 140, 330, 646 Blended cements 43, 203, 205, 578, 630, 640, 660 Bloated clay 68 Bloated clay aggregates 69, 464 Blowholes 600, 615, 674 Blow-holes 677, 676 Boiling water method 298, 429 Bond 71 Bond strength 127, 166, 212, 379, 393, 462, 639 Bonded overlays 523, 719 Bonding admixtures 104, 127 Boyle’s law 160 Brick aggregate 64, 486 British DoE method 261, 284, 302 Broken-brick Aggregate Concrete 486 Bulk density of an aggregate 72

Bulk volume of coarse aggregate 277, 345 Bulking of fine aggregate 74

C Calcium chloride 98, 101, 105, 125, 413, 570, 653 Capillary action 96 Capillary flow of water 103 Capillary porosity 640 Capillary pores 26, 199, 385, 599, 600, 659 Carbon fibres 483, 487, 530 Carbon-fibre-reinforced polymer 729 Carbonation 46, 78, 140, 263, 486, 523, 641, 654, 657, 658, 664 Carbonation shrinkage 197, 654, 683 Causes of distress of concrete 676 Cellular concrete 104, 110, 125, 466, 617, 626 Cement 2, 17, 98, 201, 206, 427, 558, 653 — factor 208, 577 Cement concrete 17 Cement mortar 17 Cementing efficiency factor 317, 319, 642 Central-mixed concrete 364 Central-mixed plant 366 Centrifugation 154, 374, 375, 380 Ceramic bond 548, 552 Characteristic compressive strength 240, 262, 284, 344, 470, 518 Characteristic strength 225 Characteristic value 231 Chasing 710 Chemical — admixtures 2, 4, 27, 37, 102, 146, 281, 526 — attack 198, 263, 395, 461, 487, 559, 631 — curing 350 — properties of cements 33 Chloride — concentration 434, 512 — penetration 41, 46, 126, 198, 419, 500, 560, 565, 637 Chemical analysis 456 Chemical Requirements 39 Chloride-ion 638 Chloride-ion diffusion 632 Chloride-ion penetration 47 Chloride-ion permeability 660 Chloride penetration 50 Chloride-permeability 660

Index Choice of mixer 356 Choice of vibrators 376 Classification according to geological origin 63 — shape 63 — size 57 — unit weight 63 — of aggregates 63 — of HPC 574 — of ready mixed concrete 361 Clinker 18, 20, 49, 202, 463, 541 Coal ash 13, 137, 487 Coarse aggregate 5, 17, 80, 214, 228, 323 — content 239, 303, 469, 529 Coefficient of expansion 78 Coefficient of thermal expansion 78, 209, 211 — variation 224, 419 Colcrete 419 Cold joints 106, 324, 401, 482, 599 Cold weather concreting 35, 140, 411, 413 Collar 714, 716 Coloured portland cement 35, 37 Colouring admixtures 102, 113, 128, 425 Column capital 715 Column jacket 713, 716 Compacting factor 148, 167, 259, 423, 459 — test 148, 149, 219, 378, 422 Compaction of concrete 108, 159, 249, 373, 468 Compliance testing 256, 260, 422 Composite cements 44 Composite laminates 729 Composition of portland cement 22, 27, 32 Compound composition 19 Compressive strength class 256, 258 Concentration of chloride ions 570 Concrete 2, 100, 104, 113, 228, 371, 477, 551, 653 — surface-hardening admixture 128 — core tests 435, 449, 453, 459 — corrosion 497 — deterioration 425, 523, 539 — durability 198, 523, 557, 581 Concrete in marine environment 206, 213 — mix design 35, 154, 239, 306, 426, 543 — repair 232, 629, 618, 630, 662 Concrete compression tests 32 Concrete cover 444 Concrete replacement 696 Concreting Methods 414

765

Concreting in hot weather 409 Conformity testing 260, 309 Consistency 146 Consistency of the concrete 21, 146, 149, 242 Construction in ferrocement 501 Continuous grading 85, 219, 209, 381, 497, 543 Contraction joints 682 Controlled concrete 18 Control-ratio 227 Conversion of mix proportions 333 Core Tests 449 Core strength 450, 451, 452, 454 Corrosion 26, 199, 252, 413, 444, 508, 638 — inhibiting admixtures 104, 125 — of concrete 504, 506 — of steel 46, 125, 252, 445, 472, 501, 506, 560, 661 Corrosion 523 Corrosion analyzer 445 Corrosion inhibitors 566, 572 Corrosion of reinforcement 76, 566 Corrosion of the reinforcement 569 Corrosion of the reinforcing steel 97 Corrosive reaction 632 — resistance 483, 581 Cover over steel 571 Covercrete 204, 444, 436, 660, 730 Crack comparator 687 Crack pattern 687 Cracks 678 Cracking 31, 187, 362, 435, 574, 650 — durability relationship 497, 508, 574 Crazing of the surface 106 Creep 9, 68, 145, 314, 435, 574, 647, 655 Creep and shrinkage 68, 521, 545, 602 Creep of concrete 92, 198, 655 Criterion sieve 244, 302 Critical fibre content 513 Crystallization 20, 548, 562, 658, 722 Curing 220, 345, 350, 421, 430, 542, 648 — cycle 300, 301, 352, 365 — membrane 391, 494, 697 Curing by infrared radiation 351 Curing — of foamed concrete 618 — periods 377, 387, 390 — temperature 347, 357, 386, 494, 569 — water 81, 86, 89, 379, 603

766

Index

D Dampness 97 Darcy’s equation 200 Debonding strip 711 Deformation meter 444 Degree of control 102, 213, 226, 228 Delamination of the covercrete 729 Delamination failures 730 Delayed curing 396 Depth of carbonation 204 Design of concrete mix as a system 311 Design of high strength concrete mixes 313 — workability concrete mixes 293 Designed mix concrete 5, 9, 299 Deterioration of concrete 25, 76, 102, 205, 207, 561, 560, 683 Development of strength 24, 103, 136, 387, 389, 487, 539, 541, 702 Diagnosis of distress 678 Differential settlement 685 Discrete Fibre Reinforced Concrete 506 Displacement method 74 Drilled-hole method 446 Drilling and plugging 704, 705 Dormant 696, 679 Dosage level of admixture 120 Dry process 19, 20, 54, 494 Drying shrinkages 9, 43, 574, 640, 676 Drying method 74 Drypack 694 Drypacking 695 Dry process 19 Dual drum mixer 354 Dump buckets 357, 405 Dumpers 357 Ductility 670 Durability 577, 46, 45, 684, 236, 220, 218 Durability of concrete 44, 198, 307, 403, 560, 561, 565, 630 Dynamic modulus of elasticity 193, 637

E Effect of Impurities 96 Effect of temperature 649 Efficiency factor 317, 327, 339, 487, 488, 642, 672 Efflorescence 9, 208, 213 Elastic modulus of concrete 639 Elastic modulus 43, 197, 441, 506, 574, 647, 651

Electrical curing of concrete 395 Electro-chemical cells 569 Electro-chemical corrosion 567 Elongated Aggregates 66 Elongation index 67, 307 Entrained air 48, 646, 662 Entrained-air-void system 664 Entrained air content 48, 109, 281, 342, 632 Endurance limit 523, 656, 671 Entrapped air 41, 107, 186, 262, 369 Epoxy 450, 502, 534, 538, 572, 637, 672 Ettringite 26, 27, 205, 665 European standards 255 Evaluation of cracks 686 Exfoliated-vermiculite 465 Expanded — shale 53, 61, 464, 466, 493 — metal lath 499 Expansion-producing admixtures 125 Expansive cement 41, 100, 125, 133, 724 External or shutter vibrators 377

F False set 31 Farris effect 645 Fast track concrete paving 672 Fatigue 522 Fatigue behaviour 503 Fatigue resistance Fatigue strength 656, 671 Ferrocement 463, 433, 495, 497, 498, 715 Fibre — reinforced concrete 3, 445, 463, 506, 513, 578, 667 — reinforced plastics 528, 574, 575, 596, 680 — reinforced polymers 526, 575 — shotcrete 488 Fibre pull-out 523 Fibrous ferrocement 463, 505 Field control 220 Final setting 49 Final setting times 31, 34, 55 Fine aggregates 65, 81, 87, 111, 283 Fineness — modulus 68, 79, 86, 276, 281, 289 — of cement 30, 33 Finishing of concrete 349, 383 Fire damaged structures 648, 727 Fire resistance 68, 208, 463, 549, 555 Flakiness index 67, 71, 89

Index Flash set 24 Flash setting 27 Flexible sealing 710 Flexural strength 67, 130, 180, 183, 479, 486, 621 Flexural tensile strength 180 Flexural test 650 Flocculation 27 Flogging 391 Flowability 103 Flow test 148, 150, 583, 586, 588 Flowing concrete 114 Fly ash 41, 46, 50, 134, 137, 316, 433, 458, 526, 573, 605, 666 Fly ash mineralogy 138 Fly ash particle size 138 Foamed — concrete 111, 539 — blast-furnace slag 463 Formwork 7, 96, 140, 143, 214, 226, 358, 392, 420, 432, 480, 489, 546, 580, 594, 615, 632, 676 Freezing and thawing 63, 69, 78, 109, 123, 140, 252, 277, 362, 395, 480, 491, 659, 683 Fungicidal, germicidal and insecticidal admixtures 104, 129 Freeze–thaw action 717 Frequency of sampling 226 Furnace slag 18, 36, 44, 46, 134, 141, 202, 256, 329, 389, 463, 464, 555, 581

G Gap-graded — aggregate 438, 510 — concrete 470, 486 Gap-grading 85 Gas-forming admixtures 104, 124 Gaussian distribution curve 222 Gel pores 26, 199, 337, 385 Gel/space ratio 640 General field environment 635 Germicidal 129 Glass-fibre-reinforced polymer 729 Glass fibres 484, 529 Grade of concrete 96, 207, 228, 256, 346, 453, 635 Graded aggregates 85, 307, 478, 551 Grades of ordinary portland cement 34 Grading curve 81

767

Grading limits 86, 310 Grading of — aggregate 81, 94, 120, 198, 211, 266, 302, 487 — coarse aggregate 72, 75, 109, 141 — fine aggregate 83, 228, 239 Granulated blast-furnace-slag 141, 207, 640, 646 Ground granulated blast furnace slag 46, 48, 207 Grouting 41, 104, 124, 418, 543, 629, 694 Guniting 487, 493, 695, 699, Gypsum 141

H Highway pavements 671 Honeycombed 96 Honeycombing 75, 689, 676 Hot weather concreting 409 Hydration 45 Hydration of cement 35 Hand mixing 121, 352 Hand rodding 356 Hardness 70 Hardness of the aggregate 70 Hard intrusion 505 Heat of hydration 23, 32, 49, 80, 103, 136, 139, 140, 200, 268, 313, 314, 324, 413, 555, 576, 630, 642, 653 Heat resistant concrete 548 Heavy weight concrete 64, 231, 255, 507 High-alumina cement 42 High calcium fly ash 50, 138, 666 High density concrete 544 High early strength 42, 51, 116, 388, 395, 541, 574, 577 High early strength concrete 43 High frequency vibration 375 High performance concretes 2, 11, 34, 43, 50, 115, 123, 155, 169, 201, 313, 574, 575 High-performance 12 High strength concrete 34, 66, 70, 224, 226, 313, 462, 521, 572, 576 High volume fly ash (HVFA) concretes 48, 139, 641, 662 High-alumina cement 42, 495 High-density aggregates 68, 545 High-density concrete 545 High-grade concretes 34, 512, 572 High-pressure steam curing 394

768

Index

High-range water reducers 112, 114, 115, 581, 608, 641 High range water reducing 577 Highly reactive pozzolana 45 High strength concrete 34 Honeycombed concrete 279, 136, 373, 374, 722 Horizontal shaft mixers 365 Horizontal slip forming technique 402 Hot pressing technique 461 Hot weather concreting 118, 409 Hydrate–space ratio 28 Hydration of cement 24, 25, 57, 100, 143, 157, 179, 194, 385, 395, 410, 542, 578, 641, 644, 666, 682 Hydraulic cement 17, 52, 100, 103, 493, 548 Hydrophobic 37, 126, 164, 474, 528 Hydrosilicates 564

I Igneous rocks 64 Ignition loss 137 Impact-echo testing 394, 398 Impact load 685 Impermeability 45, 220 Impact resistance 503, 522 Indian standard specifications for admixtures 131 Indian standard recommended guidelines 341, 345 Induction period 27 Inert matrix 17 Influence of cement composition 121 Inhibitors of iron corrosion 571 Initial 31 Initial drying shrinkage 395 Initial setting time 31, 34, 49, 97, 107, 315, 541 Injection of epoxy 708 Inorganic Salts 98 Insecticidal admixtures 104, 129 In-situ testing 422 Inspection and testing 220, 367, 422 Insulating formwork 412 Interfacial bond 668, 659 Interfacial shear 650 Interfacial zone 660 Inverted slump flow test 584

J Jacketing 713

Jet cement 541 Jet-crete 488 Joint sealers 709 J-ring test 586, 589, 595, 598

K Kelly ball test 151, 153 Kilns 19, 20, 489

L Laitance 96, 101, 111, 159, 371, 373, 414, 491, 691, 693, 699 Latex-modified cement concrete 538 L-box test 586 Leaching 46, 70, 125, 136, 203, 390, 561, 563, 659 Leaching action 561 Leak sealing 722 Level of confidence 232 Light-weight aggregate 68, 463, 465, 470, 656 — light-weight aggregate concrete 110, 470 — light-weight concrete 60, 136, 170, 184, 186, 225, 408, 423, 430, 495, 541, 572, 583 — light-weight foamed concrete 617, 621 — light-weight Masonry blocks 625 — light-weight precast panels 620 Limestone aggregates 315, 464, 550, 683 Lime reactivity 48 Load tests 454, 715 Loading speed 648 Loss on ignition test 33 Loss of workability 157 Low calcium fly ash 48, 138, 319, 666 Low heat cement 34, 203, 231, 476 Low pressure steam curing 394 Low-alkali cement 77, 78, 683 Low-heat portland cement 13, 32, 35, 242 Low-heat cement 231

M Macro-cracks 667 Macrocracks 200 Management of Uncertainties 232 Magnesium phosphate cement 43 Manual finishing of concrete 383 manufacture of cement 19 Manufactured sand 71, 88, 307, 581 Map cracking 683 Marine environment 36

Index Marine structures 100 Marsh cone test 123 Masonry cement 36 Mass concrete 13, 32, 47, 48, 64, 77, 112, 139, 153, 242, 250, 275, 312, 347, 371, 419, 476, 554, 682, 715 Mass concreting 49 Mat concrete 578 Maturity of concrete 387, 407 Maturity test 432 Maximum nominal size 81, 83, 241, 243, 250, 280, 290, 468, 478, 498, 526, 577, 606, 613 Maximum size of aggregate 175 Maximum size of the aggregate 80 Mean 223 Maximum water-cement ratio 250, 258, 260, 262, 266, 560, 635 Mean strength 222, 225, 227, 237, 255, 290, 297, 469 Mean target strength 253, 284 Measure of variability 223 Measurement of workability 148, 513 Mechanical properties of aggregate 70, 560 Medium strength concrete 226, 253, 340, 345, 467 Membrane curing 392, 701 Metamorphic rocks 64 Metakaolin 50 Methods of curing concrete 389, 390 Micro silica 138, 142, 207, 641 Micro cracks 640, 667 Microcracks 561 Microcracking 441 Micro cracking 639, 656 Microstructure 657 Microstructure of the gel 409 Microwave water-content test 426 Mid-range water reducer 114 Mid-range water-reducing admixtures 114 Mineral 134 Mineral additives 13, 36, 44, 47, 49, 102, 115, 127, 139, 186, 201, 204, 207, 239, 308, 313, 338, 433, 577, 581, 604, 630, 633, 654 Mineral micro fillers 574 Mineralogy of clinker 20 Minimum cement content 250, 252, 295, 411, 420, 566, 635, 666, 720 Mix design 35, 73, 89, 122, 154, 172, 240, 252, 254, 340, 386, 426, 466, 498, 580, 602, 623, 664

769

Mix proportioning for high performance concrete 283 Mix Proportions 154 Mix proportions of RCC 480 Mix proportions with silica fume 327 Mixing Procedures 121 Mixing of concrete 350, 353, 541 Mixing of the epoxy 708 Mixing plants 352, 599 Mixing time 102, 133, 198, 353, 500, 602 Mixing water 28, 52, 77, 82, 110 Mobile plant 352, 365 Mode of addition 121 Modes of delamination 731 Modification in the microstructure 461 Modified slump test 169 Modulus of elasticity 68, 70, 141, 192, 193, 314, 398, 441, 460, 462, 466, 518, 527, 576, 598 Modulus of elasticity of concrete 192 Modulus of rupture 180 Moisture content 73 Moisture content of aggregate 73, 220, 366 Moisture movement 9, 195, 395, 483, 529 Mortar — cube 29, 32, 47, 137 — impregnation 463 — replacement 641, 654 Multiple cracking of matrix 509

N Natural aggregate 63, 72, 463, 485, 487 Natural rubber latex 539, 707 Needle vibrators 376 Newtonian liquids 167 No-fines concrete 544 Nominal maximum size of aggregate 566 Nominal mix 5, 34, 48, 75, 213, 343 Non-air-entrained concretes 247 Non-destructive methods 435 Non-destructive testing 236, 423, 430, 436, 686 Non-OPC cements 35, 42 Non-shrinking cement 125 Non-steel fibres 526 Non-tilting-type mixer 354 Normal — distribution curve 222 — weight aggregate 67, 68, 545 Nuclear concrete 547

770

Index

O Oil Contamination 99 Oil-well cement 41 Optimum amount of fly ash 578 Optimum cement content 314, 469, 661 Optimum or economical dosage 123 — concrete mix design 275 — moisture content 428 — percentage 94, 174, 224, 414 Ordinary portland cement 18, 22, 38, 118, 136 Organic fibres 530 Overlay system 717 Oxide composition of ordinary Portland cement 18 Oxide compositions of cementitious materials 44

P Pachometer 687 Pan mixer 122, 354, 531 Particle-size distribution 50, 641, 643, 645 Partly rounded aggregate 71 Part replacement of cement 139 Part replacement of fine aggregate 139 Pattern cracking 78, 136 Pavements 60, 154, 250, 378, 389, 407, 463, 507, 552, 627 Performance-based environment 575 Penetration resistance test 446 Performance of fresh concrete 644 Performance of hardened concrete 102, 396, 517, 647 Performance oriented specifications 5, 15, 422, 444 Performance requirements 2, 3, 6, 10, 12, 15, 35, 50, 145, 313, 479, 576, 610 Period of curing 104, 254, 387, 392 Permanent set 191 Permeability 28, 45, 58, 92, 103, 111, 159, 179, 444, 461, 479, , 570, 631, 684 Permeability reducing admixtures 104, 126 Permeability of concrete 126 Permeability tester 445 Physical requirements 137 Physical requirements of admixtures 129 Pigments 128 Placing by pump 368 Placing in bags 415, 420

Placing of self-compacting concrete 599 Plant-mixing 363 Plasticizing admixtures 112 Plastic shrinkage 194, 653, 676 Plastic shrinkage cracks 680 Plastic shrinkage cracking 410, 601, 644, Plastic viscosity 148 Plasticizer 102, 107, 114, 117, 157, 201, 231 Pneumatic tire rollers 477 Poisson’s ratio 652 Polyester 512, 521, 533, 536, 669, 672 Polymer based repairs 703 — bonding admixtures 127 — concrete 475, 553, 621 — concrete composites 532 — concrete repair 699 — Impregnated concrete 532, 555, 703 Polymer Cement Concrete 703 Polymer concrete 536, 703 Polymer concrete composites 532 Polymer-impregnated concrete 462, 703 Polymer latex 702 Polymer modified concrete 538 Polymer overlays 670 Polymer resins 533, 706 Polypropylene fibre 506, 526, 664 Polypropylene Fibre-reinforced 526 Pop-offs 653 Pores 73 Pore structure 46 Porositester 444 Porosity 28, 42, 45, 61, 443, 462, 548 Porosity of concrete 73 Portland cement 34 Portland slag cement 47, 48 Portland–pozzolana cement 47, 118 Powder-type SCC 582, 608 Power barrows 357 Pozzolanic — action 44, 113, 120, 124 — cement 42, 178, 184, 186, 579 Pozzolana additives 42 Pozzolana mineralogy 45 Pozzolanas 44, 134, 231 Pozzolanic reactions 45 Prepacked concrete 415, 419, 698, 737 Preplaced aggregate concrete 545 Prepolymer cement concrete 539 Prescribed concrete 258 Prescriptive 650

Index Prescriptive specifications 15 Preparation of Surface 691 Prestressed cement concrete 648 Pre-steaming period 353 Pre-shrink 697 Probabilistic design approach 232 Probability factor 224 Process improvements 12, 574 Processed aggregates 69 Production of foamed concrete 617 Prolonged vibration 379 Proportioning of concrete mixes 238, 241 Proportioning of ready-mixed concrete 362 Proprietary concrete 259, 329, 602 Protective coating 502, 569 Puddling 547 Pull-out — resistance 461 — testing 392, 446, 584 Pulse velocity 648, 686 Pulverized fuel ash 136, 257, 464, 487 Pumped concrete 250, 329, 359, 403

Q Quality — assurance 231, 234 — audit 232, 235 — control 5, 9, 12, 198, 200, 212, 218, , 219, 232, 234, 366, 422 — management 231 — management system 232, 235, 236 — of concrete 89, 132, 159, 191, 198, 326, 511, 585, 599 Quantities of materials 273, 334 Quickset 105

R Radiation shielding 545 Range 223 Random sampling 222, 226 Rapid analysis machine 428 Rapid-hardening cement 34, 156, 175, 197 Rapid-hardening cement 156 Rapid slump loss 645 Rate of hardening 49 Rate of hydration 26, 33, 102, 385, 389 Rate of strength development 27, 393, 542 Reaction 657 Reactive aggregate 683

771

Reactive silica 45, 77, 483, 561, 658 Ready mix plant 117 Ready mix truck 116 Ready-mixed concrete 12, 122, 132, 335, 355 Rebound developed 436 Rebound hammer 686 Recycled aggregate 13, 92, 485 Recycled concrete 485 Recommended Practices and Precautions 410 Red mud aggregate 487 Refractory — concrete 41, 465, 524 — shotcrete 488, 491, 493 Reinforced concrete 47, 64, 199, 231, 377, 395, 402, 444, 463, 472, 560, 605, 656 — corrosion 413, 435, 565, 631, 642 Reinforcing index 668 Repair by jacketing 713 repair of concrete 690 Repair of Concrete Floor Slab System 717 Repair materials 11, 128, 691, 693, 706 Repair Techniques 694 Repair of — pot-holes 676 — structures 47, 476, 625 — beams 652 Requirements of workability 155, 328 Resin systems 533, 706, 720 Resins 112, 127, 392, 533, 691, 695, 703 Resistance to freezing–thawing 70, 637 — wear 64, 70, 438, 474 Resin-mortar 707 Resistance to sulphate attack 487 Retarders 106, 122, 176, 379 Retarding admixtures 104, 106 — plasticizer 102, 104 Retrofit of deficient 729 Revibration 377 Rheology 121 Rheology of fresh concrete 146, 165, 177 Rheological properties 50 Rice husk ash 44, 483, 577 Rigid floor screeds 627 Road paver 477 Roller compacted concrete 140, 476

S Sands 65, 88 Saturation point 120

772

Index

Saturated surface dry aggregate 72, 273, 287, 303 Sealant 710 Seawater 98 Scaling 76, 425, 552, 578, 615, 631, 632, 635 Sedimentary rocks 64 Segregation 6, 46, 689 Segregation 123, 110 Segregation and bleeding 6, 37, 110, 117, 123, 143, 146, 148, 155, 157, 159, 330, 333, 360 Self-compacting concrete 580, 581, 584, 591, 597 Self-desiccation 653 Self-leveling mix 121 Self-leveling concrete 115, 172, 176, 201 Self-stressing cement 41, 125 Semiautomatic batching 344 Serviceability 218 Serviceability conditions 517, 652 Set-retarding admixtures 106, 114 Setting time 34, 42, 48, 55, 96, 127 Settlement cracking 684, 680, 676 Settlement shrinkage 125 Shape of aggregates 9 Shear slump 149 Shotcrete 488, 643, 694 Shrinkage 68, 70, 599, 650 — coefficients 433 — compensating cement 41, 654 — reducing admixtures 112 — strain 179, 184, 480, 535, 603 Shrinkage compensating cement 41, 682 Shrinkage-compensating concrete 126 Shrinkage Reducing Admixtures 125 Shrink-mixed concrete 364 Shuttering 377, 392, 396, 408, 544, 626 Sieve segregation resistance test 597 Silica fume 41, 44, 50, 126, 142, 487, 521, 577 Silicate hydrate 44 Silicate phase 26 Simultaneous replacement of cement and fine aggreg 140 Single-point tests 167 Sintered fly ash 68 Sintered-pulverized fuel ash 465 Single-size-aggregates 66, 543 Sintered fly ash aggregate 69, 465 Site problems 121

Skeleton steel 498 Skip 358, 368, 727 Sleeper cement 34 Slip forming 401 Slump 140, 146, 149 — class 256, 258, 260, 264, 302 — cone 148, 513, 587, 591 — flow by abrams cone 603 — ranges 257, 275 — test 151, 174, 378, 423 — time 148 — flow test 583 Slump loss 122, 123 Slurry infiltrated 667 — concrete 578, 591 — fibre concrete 451, 507, 514, 591 Slurry infiltrated mat concrete 507 Slurry mixer 365 Soft intrusion 506 Soniscope 687 Sound absorption 69 Sound insulation 472, 555 Soundness of aggregate 76 Spalling of concrete 684 Specialty category admixtures 104 Specific 71 Specifications 221 Special-purpose cements 35 Specified grading 302 Specific gravity 73 Specific gravity of an aggregate 71, 72 — of portland cement 33 Specific gravity of portland cement 33 Specific heat 78, 79, 210 Specific heat of the aggregate 79 Specific surface 35 — of cement 30 — of the aggregate 67, 75 Specification compliance 54, 436, 446, 449 — cracks 615, 684 — tensile strength of concrete 180, 183 Splitting test 183 Spraycrete 488 — deviation 222, 224, 226 — mixes 219 — sand 32 — mix design method 611 — quality control 218, 221 Static (secant) modulus 193 Steam curing 393, 553

Index Steel Corrosion 662 — mats 668 — reinforced concrete 463, 472 Steel fibre concretes 668 Steel fibrous shotcrete 492 Steel formwork 397 Steel wire — fabric 678 — meshes 463 Stitching of cracks 712 Stitching dogs 712 Storage of cement 53 Strain capacity 650, 654 Strength of aggregate 64 Strengthening of — beams 652, 733 — columns 682, 716 — slabs 654, 733 Strengths of concrete 179, 298 Stress 148 Stress concentrations 652 Stress–strain 639 Stress–strain curve 190, 191, 639 Strip detachment 730 Stripping of form 35, 140, 400 Stripping time 34, 697 Structural concrete 48, 68, 69, 191, 334, 565, 660 — Light-weight concrete 465, 471 Styrene-acrylic resins 707 Sugar 99 Sulphate — attack 198, 560, 591 — resistance 89, 98 — resisting cement 34 Sulphur — concrete 480, 488 — efflorescence 206 — impregnated concretes 462 — infiltrated concrete 518, 579 Super–plasticizers 46, 176 Superplasticizer 88, 102, 114, 130, 146, 157, 530, 574, 578, 604, 633, 637 Super-workable 663 Supplementary additives 134 Surface — coatings 75, 315, 498, 528, 687 — cracking 538 — crazing 681, 686 — defects 615

773

— finishing 531 — treatments 719 — vibrators 359 Synthetic latex bonding admixtures 127

T T500 Test 586 Target consistence 262 — mean strength 222, 225, 230, 237, 297 Target mean strength 225 Temperature reinforcement 198 Temperature control of concrete 410 Temperature control of ingredients 410 Tensile strength 9 Tensile strength of concrete 180, 181, 183, 207, 649 Ternary cements 49 Test for self-consolidating concrete 580 Testing of durability 444 The compressive strength 668 Thermal — conductivity 78, 79, 208, 210, 214, 413, 463, 550, 570 — cracking 126 — insulation 68, 413, 432 — movement 9, 382, 630 — properties of aggregates 78 — properties of concrete 209, 578, 655 Thermal contraction 35 Thermal cracking 139 Thermal diffusivity 210 Thermal insulation 68 Thermal movement 9 Thermal properties of the aggregates 78 Thermal stress 676 Thermoplastics 533, 706 Thermosetting resins 533, 701 Tilting type mixer 353 Timber formwork 397 Time of mixing 37, 145, 355 Tip skip hoists 358 Tippers 357, 358 Toppings 719 Toughness 70, 463, 492, 504 Transgranular 660 Transgranular fractures 639, 651 Transition zone 643 Transit-mixed concrete 364 Transportation of concrete 356, 405 Tremie method 416

774

Index

Trial batches 254, 272, 275 Trowel vibrators 378 Truck mixer 355, 357 Type of superplasticizer 119 Types of cements 33 Types of Fibres 511

U Ultra-light-weight concrete 436, 473 Ultrasonic pulse velocity method 460 Ultrasonic pulse velocity 441 Ultrasonic-pulse-velocity test 686 Unbonded — toppings 719, 720 Underwater concreting 414 — repairs 671 Unsoundness of cement 31 Using Indian standard recommended guidelines 261 U-type test 592

V Vacuum — concrete 416 — dewatering the concrete 474 Values of workability 153, 250 Variance 224 Vee-Bee 167 Vee-Bee test 150, 163, 423 Vegetable fibres 506, 530 Vermiculite 69, 464 Vertical slip forming 402 Vibrating screed 378 Vibration 109, 113, 128, 368, 374 — technique 313, 344 Vibrators 359, 372, 419, 726 Vibropressing 381, 385 Viscosity-enhancing admixture 608, 646 Visual stability index 584, 585, 589 VMA-type SCC 582, 608 Void content 71, 155, 419, 479, 566 Void ratio 73 Void ratio of an aggregate 73 Volume — batching 218, 220, 344 — equivalency 316 — fraction of fibres 506, 518 — yield of concrete 140

W Warm water method 300, 429 Waste material based concrete 482 Water 2, 83 — absorption 68, 273, 289, 544 — reducing admixtures 104, 125, 644 — cement ratio 28, 34, 42, 49, 71, 74, 85, 95, 161, 178, 200, 213, 227, 231, 236, 419, 500 — cementing material ratio 313, 578 — emulsions 126 Water–cementitious material 653 Water–cementitious material ratio 633 Water–cement ratios 34 Water method 300 Waterproof Portland cement 37 Water-reducing admixtures 107 Water Reducing Admixtures 112 Waterproof — membranes 572 — portland cement 37 Weight equivalency 317 Wearing surfaces 70 Wet process 19 Wheel barrows 352, 357 White cement 15, 33, 124, 536 — death of concrete 564 White death of concrete 564 White Portland cement 37 Windsor probe test 446 Wire mesh 464 Workability 27, 36, 46, 63, 67, 83, 105, 133. 140, 459, 486 — measurements 424 — of fibre reinforced concrete 463, 506 — parameters 522 — tests 148, 422, 432, 458, 474 Workability 46 Workability of concrete 423 Workability retention 117 Working stress method of design 191 Workability tests 160

Y Yield of concrete 72, 75, 89, 140 Yield stress 167, 171, 580, 674

Z Zero-slump concrete 476