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Ciria Report C660 Earlyage Thermal Crack Control In Concrete


CIRIA report C660 (Early-age thermal crack control in concrete) was published in 2007 and included significant changes to CIRIA R91 (1992) of the same title. The application of C660 has identified some concerns with the apparently more onerous requirements of BS EN 1992-3 in some circumstances and less robust design in others, and these are addressed in this third edition. Further guidance has been provided in some areas that previously lacked clarity.Click here to download errata note that accompanies this titleExcerpt from product (PDF)Trouble downloading this PDF?




Ciria Report C660 Earlyage Thermal Crack Control In Concrete



Abstract: In the UK, BS8007 has provided the basis for the design for early-age thermal cracking. This is to be replaced by EN1992-3 and in conjunction with the replacement of the general design code, BS8110 by EN1992-1-1 this has led to significant changes in some aspects of design. BS8007 was supported by CIRIA 91 which provided background to the design method and data for use in the design process. This has been updated and replaced by CIRIA C660 which brings the design into line with the Euro-codes and provides current information required to support the design process. The significant changes are described and their implications are discussed. AS3600 and AS3735 crack control provisions are based on the principles of BS 8007 and may need revision to take account of the changes outlined herein.


The new Eurocode for early-age concrete crack control generally follows and builds on BS 8007 for liquid-retainingand containment structures and the fundamental theory for early-age thermal cracking in concrete. In conjunction withCIRIA Report C660 summarising recent research and experimental data evenmore effective and economical designshould be possible. Unfortunately however there can be serious conflict in one major respect where large andunnecessary increases in reinforcement can be required for controlled cracking of edge-restrainedmembers. Thus efficient use of rebar and fibre reinforcement combinations to achieve well distributed fine cracks withminimumtotal reinforcement at the critical steel ratio could be seriously undermined for other types of concrete structures as well as water-retaining structures. The paper therefore examines themajor changes in the new code for early-age concrete crack control and compares these both with BS 8007 for convenience and with the fundamental theory for early-age cracking to resolve any conflicts or errors.


The new Eurocode for early-age concrete crack control generally follows and builds on BS 8007 for liquid-retainingand containment structures and the fundamental theory for early-age thermal cracking in concrete. In conjunction withCIRIA Report C660 summarising recent research and experimental data evenmore effective and economical designshould be possible. Unfortunately however there can be serious conflict in one major respect where large andunnecessary increases in reinforcement can be required for controlled cracking of edge-restrainedmembers. Thus efficient use of rebar and fibre reinforcement combinations to achieve well distributed fine cracks withminimumtotal reinforcement at the critical steel ratio could be seriously undermined for other types of concrete structures as well as water-retaining structures. The paper therefore examines themajor changes in the new code for early-age concrete crack control and compares these both with BS 8007 for convenience and with the fundamental theory for early-age cracking to resolve any conflicts or errors.


This third edition guidance document continues to look at the causes, effects, and risks of cracking in concrete. It occurs when tensile strain is greater than the tensile strain capacity of concrete. Considers design aspects such as estimating the magnitude of crack-inducing strain, how to control the size of cracks using reinforcement and ways to minimise risk. First published in 1992 as CIRIA R91 and updated in 2007 as C660 to take into account design to Eurocodes, this 3rd edition provides further clarity for design relating to BS EN 1992-3.


The accompanying spreadsheets are available from CIRIA customer services: enquiries@ciria.org. Supersedes C660 Early-age thermal crack control in concrete. 2014 reprint (CIRIA, 2007). First published as CIRIA Report R 91, Early-age thermal crack control in concrete (revised edition), 1992. Based on RP722. Errata issued February 2019 for Page 42, .4.3 reference should be Figure 4.10. Errata issued 25 March 2019, a table with replacement information for pages 5, 38, 41, and 42. Also details corrections for the spreadsheets.


Early-age thermal effects in concrete structures have been the subject of extensive research since the thirties of the twentieth century, when the construction of large dams raised problems with hydration heat. Some studies are devoted to hydration heat [10,11,12,13], temperature and stress variation in concrete members [14,15,16,17,18], the analysis of cracking risk [19,20,21,22], as well as measures preventing this risk [23,24,25,26]. Thus, the behavior of early-age concrete concerning these issues is fairly well recognized. Not many articles have been devoted to the role of reinforcement and the methods for its calculation in mass concrete members under thermal loading. Admittedly, reinforcement does not limit the level of the load and thermal stresses. Consequently, reinforcement does not prevent cracking but is rather for limiting the width and spacing of the cracks to acceptable values. The available works discuss mainly early-age cracking in reinforced concrete walls on slab [27,28] or general requirements for shrinkage and temperature reinforcement in concrete structures [29,30]. Issues concerning early thermal cracks and minimum reinforcement in mass foundation slabs are outlined in [31,32], but not all aspects of the discussed complex issue are covered.


In Europe, the determination of reinforcement for cracking control is based on the design recommendations provided by Eurocode 2 [33], which is actually applicable for typical mechanical loads and does not accurately specify the early-age thermal behavior of concrete members. The topic is more widely discussed in the CIRIA C660 [34] and CIRIA C766 [35] guidelines, described as the British commentary on the Eurocode standards. Nevertheless, the recommendations and worked examples focus on ground slabs without external restraints. Furthermore, the distribution of stresses throughout the concrete hardening period is not precisely analyzed, and especially the cooling phase is neglected. Other national standards can be characterized in a similar way because they usually provide only very general recommendations for reinforcement limiting cracks of thermal origin [36,37].


In detail, the Eurocode 2 standard [33] does not provide detailed recommendations for the application of Equation (1) for mass foundation slabs subjected to early-age thermal effects. First, no clarification regarding the area of the concrete in tension Act and the distribution of induced thermal stresses is given. Next, the mean value of the concrete tensile strength, fct,eff = fctm(t) is recommended to be assumed for the concrete age, t, when cracks are expected. Simultaneously, the concrete age, t, is not specified. These inaccuracies have been discussed in [42,43], and using the German standard DIN EN 1992-1-1/NA [44] is recommended. In this standard, fct,eff is equal to 0.5fctm. The same assumption has been made in [5].


I have extensive experience of the assessment of early thermal behaviour (and sit on the Steering Committe of C660), and am able to provide comprehensive services for the analysis, interpretation and recommendation of measures to mitigate the risk of early thermal cracking in concrete structures.


Early-age thermal cracking occurs when the tensile strain, arising from either restrained thermal contraction or a temperature differential, exceeds the tensile strain capacity of the concrete. In high strength concretes autogenous shrinkage may also contribute to early contraction. Numerous factors influence the risk of early-age cracking including the temperature rise; the coefficient of thermal expansion of the concrete; the restraint to movement offered either by adjacent elements or by differential strain within an element; and the ability of the concrete to resist tensile strain.


2 Early-age thermal crack control in concrete Bamforth, P CIRIA CIRIA C660 CIRIA 2007 RP722 ISBN British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Keywords Reinforced concrete, early-age behaviour, thermal cracking, shrinkage, design, reinforcement, crack control Reader interest Reinforced concrete designers and specifiers, construction planners and engineers, concrete technologists Classification AVAILABILITY CONTENT STATUS USER Unrestricted Design and construction guidance Committee-guided Structural and civil engineers in design and construction Published by CIRIA, Classic House, Old Street, London EC1V 9BP, UK. This publication is designed to provide accurate and authoritative information on the subject matter covered. It is sold and/or distributed with the understanding that neither the authors nor the publisher is thereby engaged in rendering a specific legal or any other professional service. While every effort has been made to ensure the accuracy and completeness of the publication, no warranty or fitness is provided or implied, and the authors and publisher shall have neither liability nor responsibility to any person or entity with respect to any loss or damage arising from its use. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright-holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature. If you would like to reproduce any of the figures, text or technical information from this or any other CIRIA publication for use in other documents or publications, please contact the Publishing Department for more details on copyright terms and charges at: publishing@ciria.org Tel: +44 (0) CIRIA C660


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