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.

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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.

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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

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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

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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

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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

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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.

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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

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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

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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

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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.

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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

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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

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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

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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

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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.

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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).

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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

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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

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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.

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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.

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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

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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.

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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

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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.

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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

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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

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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

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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

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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.

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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.

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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

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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.

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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

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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

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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

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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

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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

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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

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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.

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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

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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.

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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.

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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

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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.

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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

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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

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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

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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.

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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

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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.

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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

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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.

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(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

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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

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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.

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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

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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

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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

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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

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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.

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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

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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.

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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

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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.

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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.

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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

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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

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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.

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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.

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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

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(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)

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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

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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

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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

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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

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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

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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.

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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=

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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)

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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.

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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

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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.

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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.

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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:

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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,

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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.

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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

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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

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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)

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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.

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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.

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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.

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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

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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

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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

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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

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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

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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.).

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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

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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:

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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

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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

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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.

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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.

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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

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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.

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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

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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

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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

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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.

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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.

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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)

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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

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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.

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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

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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.

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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.

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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):

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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.

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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) _____ _____ _____ _____ _____ _____ _____ _____ _____

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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,

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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

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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).

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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

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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.

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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.

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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.

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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.

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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.

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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

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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

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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.

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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.

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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:

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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.

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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

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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

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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

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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

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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

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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.

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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.

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(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

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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

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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

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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.

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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

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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

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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

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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

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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

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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.

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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-

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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.

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(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.

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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

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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

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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.

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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

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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:

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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

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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.

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(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.

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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

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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

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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

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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

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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

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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

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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.

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(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.

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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

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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

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(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

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(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

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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

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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.

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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

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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

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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

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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.

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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

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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)

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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

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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

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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

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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.

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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

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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.

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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

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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.

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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

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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)

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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)

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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)

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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

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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

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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

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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

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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.

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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.

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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

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(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.

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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

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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

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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.

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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

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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

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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

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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.

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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.

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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