Control of Cracking in Reinforced Concrete Structures: Research Project CEOS.fr

By Francis Barre, Philippe Bisch, Daniele Chauvel and 11 more

This book presents new guidelines for the control of cracking in massive reinforced and prestressed concrete structures. Understanding this behavior during construction allows engineers to ensure properties such as durability, reliability, and water- and air-tightness throughout a structure’s lifetime. Based on the findings of the French national CEOS.fr project, the authors extend existing engineering standards and codes to advance the measurement and prediction of cracking patterns.

Various behaviors of concrete under load are explored within the chapters of the book.  These include cracking of ties, beams and in walls, and the simulation and evaluation of cracking, shrinkage and creep. The authors propose new engineering rules for crack width and space assessment of cracking patterns, and provide recommendations for measurement devices and protocols.

Intended as a reference for design and civil engineers working on construction projects, as well as to aid further work in the research community, applied examples are provided at the end of each chapter in the form of expanded measurement methods, calculations and commentary on models.

• Language: English
• Publisher: Wiley
• Release date: Aug 16, 2016
• ISBN: 9781119347392

Book preview

Series Editor

Jacky Mazars

Control of Cracking in Reinforced Concrete Structures

Research Project CEOS.fr

Francis Barre, Philippe Bisch, Danièle Chauvel, Jacques Cortade, Jean-François Coste, Jean-Philippe Dubois, Silvano Erlicher, Etienne Gallitre, Pierre Labbé, Jacky Mazars, Claude Rospars, Alain Sellier, Jean-Michel Torrenti, François Toutlemonde

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First published 2016 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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The rights of Research Project CEOS.Fr to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2016941705

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN 978-1-78630-052-2

Foreword

The control of cracking in reinforced and prestressed concrete is an essential factor in ensuring the reliability and durability of structures, together with many other important properties including water-tightness and air-tightness.

Eurocode 2 (EC2) and, more recently, fib Model Code 2010 (MC2010) address the durability of structures and contain guidelines and rules for estimating and limiting cracking as a function of the characteristics of concrete and its reinforcement, and the exposure classifications of the works. However, these rules are normally only intended to be applied to the most common design situations. As a result, they do not take sufficient account of the behaviour of works containing massive reinforced and prestressed concrete structures, nor works which are subject to special service requirements in terms of water-tightness and air-tightness or service life and so forth. These rules are also inadequate for works requiring enhanced load protection against natural hazards or external attack. In these works, thermo-hydro-mechanical (THM) effects, scale effects and structural effects can all result in specific cracking behavior. In the case of thick rafts and walls, shrinkage and creep shall be taken into account, both in early-age concrete and in the long-term.

The purpose of this book is to provide further guidelines which can extend the existing standards and codes to cover these types of special works, especially those which are massive in nature, taking account of their specific behavior in terms of cracking and shrinkage together with other important properties such as water- and air-tightness.

The proposed rules and guidelines given in this book are based on the results of the French CEOS.fr project (Comportement et Evaluation des Ouvrages Spéciaux – fissuration, retrait) covering the behavior and evaluation of special reinforced concrete (RC) works with regard to cracking and shrinkage. The CEOS.fr project took place between 2008 and 2015, involving 41 French Ministère de l’Environnement de l’Énergie et de la Mer (MEEM), clients and project managers. The project was funded jointly by the partners of the MEEM.

The CEOS.fr project consisted partly of tests, some using full-scale solid concrete blocks and others performed on a smaller scale using laboratory models, together with the development of simulation models in collaboration with the MEFISTO project1 under the auspices of the French Agence Nationale pour la Recherche (ANR). The experimental results were presented to the international scientific community and a panel of experts in these complex and rapidly changing fields assessed the simulation models. The CEOS.fr project also took account of experimental results and actual experience feedback of concrete works from the various partners.

These guidelines are addressed primarily to designers and civil engineers responsible for construction projects. Engineering rules and recommendations are illustrated at the end of each chapter using examples of design calculations, commentary on the use of models, or applicable measurement methods. Further supporting details of the basis for these guidelines may be found in the CEOS.fr test report, titled Results obtained in the understanding of cracking phenomena [PN 13b], which describes the results of the associated tests, the interpretation of these results and the justifications for each proposed modification to EC2 and MC2010.

The guidelines given herein reflect the latest state of the art understanding at the time of going to press. They are therefore subject to expansion and modification as new experimental data becomes available, further experience is gained and new technologies are used in future projects.

We would like to express our sincere thanks to all those who have contributed to the publication of this document, its English version and to the, Institut pour la recherche appliquée et l’expérimentation en génie civil (IREX) for their administrative and logistical support.

Pierre LABBÉ, EDF

December 2015

1 Maîtrise durablE de la Fissuration des InfraSTructures en bétOn.

Notations

Symbols are mentioned only when they are specific and not used currently by Eurocode 2 (EC2) [NF 04, NF 06a, NF 06b] and fib model code 2010 (MC2010) [CEB 12]. However some symbols used less in these codes are also quoted. The units refer to the International System (IS).

Introduction

Most concrete structures in Europe are currently designed according to Eurocode 2 (EC2). However, feedback has shown that EC2 rules do not fully reflect the complete behavior of massive concrete structures such as thick slabs or thick walls throughout time. These structures are subjected to THM effects, scale effects and structural effects that induce specific cracking patterns related to crack spacing and crack width.

To address concerns of the sustainability and durability of structures, in 2008 the French Civil Engineering Community decided to launch a joint national research project, CEOS.fr, with the aim of taking a step forward in engineering capabilities for predicting the crack pattern of special structures, mainly massive structures.

The aims of CEOS.fr project were threefold:

– to provide experimental data representative of massive test specimens;

– to develop numerical nonlinear models and damage models, to simulate concrete behavior under load and imposed deformation in accordance with the test results;

– to propose engineering rules for crack width and space assessment of possible cracking patterns in massive structures, in addition to the EC2 and fib Model Code (MC2010) standards.

The CEOS.fr project was carried out from 2008 up to mid-2015 around three axes: modeling and simulation in parallel with MEFISTO research project, testing on large-scale models and engineering rules.

The guidelines for the control of cracking phenomena in reinforced concrete structures are mainly dedicated to the proposed rules based on the outcomes of the CEOS.fr project and feedbacks from operated structures. These rules aim to supplement those presented in EC2 and MC2010. These guidelines were presented to a panel of experts within the framework of the Concrack4 seminar held at Ispra (Italy) in March 2014, following meetings of the EC2 Committee in charge of reviewing this standard.

The structure of this book is as follows:

Chapter 1 gives a general overview of various tests and modeling approaches, which were performed in the framework of CEOS.fr project to address the difficult topic of concrete cracking control.

Chapter 2 examines two significant effects that were identified during the massive structure tests:

– section 2.1: hydration effects of concrete at an early age (increase of temperature due to cement hydration followed by temperature decrease) and over time (high level of moisture retention) which lead to non-uniform concrete strains in the structural cross section of the considered element;

– section 2.2: scale effect, which results in the decrease of the tensile strength at an early age as observed in massive elements compared to the strength measured on laboratory test specimens, the probability to meet extreme values in the massive element volume being higher than in small size specimens.

Chapter 3 deals in particular with the 3D effect, which is characterized mainly by the non-uniform concrete stresses close to concrete cracks in the cross-section of the element. This effect is taken into account by using the γ0 ≤ 1 coefficient.

Chapter 4 proposes two methods for concrete crack width assessment in the case of massive beams or elements assimilated to massive beams.

Chapter 5 proposes an operational method for applying the rod-tie model as described in MC2010 and the calculation of the crack width derived from the reinforcing bar deformations.

Chapter 6 applies to each type of concrete elements, tie, beam and shear walls, according to whether the functioning of the concrete element is assimilated to a tie or to a shear wall.

Chapter 7 outlines the related equations presented in MC2010 and proposes some adaptations to massive elements.

Chapter 8 analyzes the relations used for the crack width calculation as given by MC2010 and proposes a method to assess the external restraints then the stiffness under the internal strains due to thermal efforts, shrinkage and creep and under external imposed deformations such as settlement.

Chapter 9 proposes an approach for calculating the concrete cracking by distinguishing the structures with waterproofing requirements from structures with sustainability requirements.

Chapter 10 describes the methodology based on the project MEFISTO results supported by the French Agence Nationale de la Recherches (ANR) and describes how to simulate the thermal and hydration effects and how to take into account those effects during the drying phase of concrete.

Chapter 11 provides recommendations on parameters measurement, measurement devices, test protocols in order to facilitate the use of measurements performed on structures, mainly on massive structures under THM effects, and the use of the feedback of experience related to this domain.

Worked examples are presented at the end of each chapter.

At the end of the book, a Bibliography gives the references of all articles referred to in the chapters.

1

CEOS.fr Project Presentation

The overall objective of the CEOS.fr project is to take a significant step towards improving the engineering capabilities for assessing concrete structure crack patterns and predicting the patterns expected under anticipated design conditions. Crack control is crucial to ensure serviceability (durability and sustainability) throughout the working life of concrete structures. Current engineering practice provides some recommendations for limiting concrete cracking, with crack width and spacing control based on formulae supported by empirical data from test beams (small-sized test specimens) submitted to bending moments or tensile force. While the current codes are considered to be reasonably representative for these load cases, previous results indicate that these formulae are not fully consistent when applied to shear walls or massive structures. Hence, within the CEOS.fr national research project, several experiments on massive concrete beams were conducted to improve the knowledge of cracking phenomena, by coupling numerical modeling and experimental approaches.

1.1. CEOS.fr work program

The CEOS.fr program includes three work areas that are relevant for the control of cracking:

– monotonic loads: the purpose of which is to