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1、Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking permitted without license from IHS -,-,- Copyright American Concrete Institute Provided by IHS un
2、der license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking permitted without license from IHS -,-,- Autogenous Deformation of Concrete Editors Ole Mejlhede Jensen Dale P. Benz Pietro Lura h international- SP-220
3、Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking permitted without license from IHS -,-,- First printing, March 2004 DISCUSSION of individual pape
4、rs in this symposium may be submitted in accordance with general requirements of the AC1 Publication Policy to AC1 headquarters at the address given below. Closing date for submission of discussion is October 2004. All discussion approved by the Technical Activities Committee along with closing rema
5、rks by the authors will be published in the January-February 2005 issue of either AC1 Structural Journal or AC1 Materials Journal depending on the subject emphasis of the individual paper. The Institute is not responsible for the statements or opinions expressed in its publications. Institute public
6、ations are not able to, nor intended to, supplant individual training, responsibility, or judgment of the user, or the supplier, of the information presented. The papers in this volume have been reviewed under Institute publication procedures by individuals expert in the subject areas of the papers.
7、 Copyright O 2004 AMERICAN CONCRETE INSTITUTE P.O. Box 9094 Farmington Hills, Michigan 48333-9094 All rights reserved, including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or writ
8、ten or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. Printed in the United States of America Editorial production: Lindsay K. Kennedy Library of Congress catalog
9、card number: 2004103246 ISBN: 0-8703 1- 143-3 Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking permitted without license from IHS -,-,- PREFACE Au
10、togenous deformation of concrete is the free deformation of sealed concrete at a constant temperature. A number of observed problems with early-age cracking of high- performance concrete can be attributed to this phenomenon. During the last 10 years, this has led to an increased focus on autogenous
11、deformation within both concrete practice and concrete research. Since 1996, there has been enough interest to hold yearly international conferences entirely devoted to this subject. The papers in this publication were presented at two consecutive half-day sessions at the American Concrete Institute
12、s Fall Convention in Phoenix, Arizona, October 2002. All papers were reviewed according to AC1 procedures. This publication and the sessions were sponsored by AC1 Committee 236, Material Science of Concrete. The 12 presentations from eight different countries indicate the broad, global research effo
13、rts dealing with autogenous deformation, and the well-attended-of up to 90 people-show that this interest is shared by the general concrete community. Attendees left with an understanding of the complexity of the autogenous deformation of concrete. The sessions showed that cooperation and more resea
14、rch are needed. At the AC1 2002 Fall Convention, the international materiais research organization, RILEM, took steps to promote further research and cooperation within this area. Two technical committees held initial meetings with the involvement of many AC1 members. More collaborative research wil
15、l be carried out! Copenhagen, Denmark, and Santa Cruz, Bolivia, October 2003. Ole Mejlhede Jensen Dale P. Bentz Pietro Lura I 0.Mejlhede Jensen I 1 -1200 ! -?SM) I Age Idavsl Figure 8 - Measured and calculated autogenous deformation from 1 day. Copyright American Concrete Institute Provided by IHS u
16、nder license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SP-220-5 Controlling Plastic and Autogenous Shrinkage in High-Performance Concrete Structures by an Early Water
17、 Curing by PA. Atcin, G. Haddad, and R. Morin bleeding; cost of water curing; curing membrane; early cracking; fogging; high-performance concrete (HPC); plastic shrinkage; susta in ab le deve lo pmen t 69 Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=IHS Emplo
18、yees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking permitted without license from IHS -,-,- 70 Atcin et al. Pierre-Claude Atcin (FACI) is Professor Emeritus of Civil Engineering at the Universit de Sherbrooke and member of the Canadian Academy
19、 of Engineering. He was a member of AC1 committees 234 Silica Fume on Concrete and 363 High-Strength Concrete. From 1990 to 1998, he was Scientific Director of Concrete Canada, the Network of Centres of Excellence on High-Performance Concrete. He is the autor of the book “High-Performance Concrete“.
20、 Gilbert Haddad, FACI, is Technical Director, Concrete and Materials, for Terratech, a division of SNC-Lavalin, Inc. He is a member of AC1 Committees 214, Evaluation of Results of Tests Used to Determine the Strength of Concrete; 301, Specifications for Concrete; 306, Cold Weather Concreting; and 30
21、8, Curing Concrete. Richard Morin is Head of the Civil Section for the Laboratories of the City of Montreal, in charge of quality control for all civil construction works. He actively promotes the use of high-performance concrete, self-compacting concrete, and roller-compacted concrete. INTRODUCTION
22、 Some recently built high performance concrete structures are badly cracked because high performance concrete was treated like any other concrete: at the best it received a traditional water curing starting 24 hours after casting, very often none at all. High performance concrete is particularly vul
23、nerable to plastic shrinkage because it does not bleed, is subject to autogenous shrinkage, has low waterhinder ratio, and is subject to thermal shrinkage. High performance concrete does not bleed because the reduction of the watedbinder ratio is essentially obtained by drastically decreasing the am
24、ount of mixing water through the use of a superplasticizer rather than by increasing significantly the amount of cementitious materials and, to a lesser extent, water. Like any other concrete, high performance concrete develops autogenous shrinkage, but as its waterbinder ratio is low, autogenous sh
25、rinkage starts to develop very rapidly and intensely during the first 24 hours, as soon as the hydrated cement paste becomes “structured“ (more or less setting time). At that moment, the capillary network is very fine, because cementitious particles are close to each other due to the low waterhinder
26、 ratio (Figure i), and as the volume of capillary water is not very large, very rapidly menisci created by self-desiccation are developed in fine capillaries. These menisci develop significant tensile stresses at a time when the cement paste is not yet very strong. The lower the waterhinder ratio th
27、e earlier and the greater the autogenous shrinkage developed. As hydration proceeds, high performance concrete generates heat like any other concrete (Figure 2), but no more than usual structural concrete having a 300 to 350 kg of cement per m3 (500 to 600 Ib/cu.yd.), because in high performance con
28、crete hydration is limited by the amount of available water. In high-performance concrete strength is coming from the closeness of cement particles, very few “glue“ is needed to bridge the gap between cement particles to generate strength. Therefore, after plastic shrinkage and autogenous shrinkage,
29、 high performance concrete faces thermal shrinkage before being submitted to drying shrinkage. Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking pe
30、rmitted without license from IHS -,-,- Autogenous Deformation of Concrete 7 1 What makes the situation particularly severe is that these different forms of shrinkage are developed simultaneously at a very early age. Moreover, some times, the effects of a thermal shock hurt concrete during the deform
31、ing of structural elements at an age when high performance concrete is not very strong in tension and the effect of this thermal shock has to be added to the effects of the previous forms of shrinkage. Consequently, the main difference between usual concrete and high performance concrete, as far as
32、their shrinkage behaviors are concerned, is that it is very important to prevent the very rapid development of plastic and autogenous shrinkage in high performance concrete as soon as they have been placed. HOW TO CONTROL PLASTIC SHRINKAGE? In the case of normal concrete, plastic shrinkage can be er
33、adicated either by placing with an evaporation retarder or a curing membrane on the surface of the concrete as soon as it is finished. There is plenty of water within concrete to avoid plastic shrinkage so that it is only necessary to prevent its evaporation in a dry environment. AC1 308.1 Standard,
34、 Specification for Curing Concrete and concrete manuals are treating this subject in details. It should be pointed out that bleed water prevents the development of plastic shrinkage and provides the constructor some time to apply the specific treatment that has been selected to prevent the developme
35、nt of plastic shrinkage. On the contrary the surface of high performance concrete is not protected at all by any bleed water so that it is imperative to start fogging or the application of an evaporation retarder as soon as the surface of the concrete is finished, otherwise, plastic shrinkage cracks
36、 can develop within the very first minutes after its placing, specially when the watedcement ratio is quite low (W/C = 0.30) and the external environment hot andor windy. Curing membranes can or cannot be used depending on the type of the control of autogenous shrinkage that will be implemented late
37、r on: when an early water curing will be used to control autogenous shrinkage as in the case of this paper, the use of a water curing membrane is not appropriate because it will prevent later on the penetration of the water within the concrete. AUTOGENOUS SHRINKAGE As demonstrated by Le Chatelier (2
38、), more than 100 years ago, any hydrated cement paste hardening in a close system is subjected to self-desiccation. Self-desiccation is a consequence of the chemical contraction occurring when Portland cement hydrates (Figure 3). The creation of a nanoporosity when hydration proceeds in a closed sys
39、tem results in the drainage of the water contained in the coarse capillaries toward that nanoporosity created by chemical contraction. So as a consequence of this water migration within the concrete, menisci are formed in the capillary network and eventually in the nanoporosity. Menisci create tensi
40、le stresses that make the hydrated cement paste to shrink. The finer the capillaries and pores in which these menisci are developed the greater the autogenous shrinkage of the cement paste. Copyright American Concrete Institute Provided by IHS under license with ACI Licensee=IHS Employees/1111111001
41、, User=listmgr, listmgr Not for Resale, 03/05/2007 03:13:04 MSTNo reproduction or networking permitted without license from IHS -,-,- 72 Atcin et al. In the case of uncured usual concrete having a waterkinder ratio greater than 0.42 the menisci are developed in large capillaries and the correspondin
42、g autogenous shrinkage is not very significant (3). However, when the watedcement ratio decreases well below 0.42, which according to Powers (4) represents the minimum watedcement ratio that is necessary to reach full hydration in a closed system, the lower the watedcement ratio the more exposed to
43、a severe autogenous shrinkage the paste is. But, when a cement paste is cured under water, not only the cement paste does not shrink because self-desiccation does not occur, it rather swells as demonstrated in the Le Chatelier experiment. The same behavior is found when concrete is cured under water
44、 as soon as it is cast (5) as shown in Figure 4. HOW TO CONTROL AUTOGENOUS SHRINKAGE? Different ways to control autogenous shrinkage have been already proposed in the literature. Some of them are already used in the field. All these methods have in common one .thing: they tend to decrease tensile fo
45、rces in the menisci created by self-desiccation that is occurring in any type of concrete that is cured in a closed system. These different methods are of two types: in the first type, external water is supplied in different ways to the hydrating paste, in the second type a chemical admixture is pro
46、vided within the concrete to reduce the tensile stresses developed in the menisci (6). External water, i.e. water external to the cement paste, can be provided in different ways: it can be included within the concrete during its batching or external curing water can be provided as soon as hydration
47、starts. Weber and Reinhardt (7) are suggesting to soak lightweight aggregate to create an internal source (internal to concrete) of extemal water (external to cement paste). On their side, Jensen and Hansen (8) are suggesting to introduce superabsorptive polymers, like the ones used in diapers, that
48、 start to absorb some extra water during batching time and provide a well dispersed source of external water within the concrete when self-desiccation develops. In both cases, due to the presence of a source of water within concrete, capillary water will not be pumped within the paste, and consequently menisci will not be developed within the capillary system. In such cases the use of a curing membrane is appropriate to prevent plastic shrinkage because the external water is contained within the concrete. In this paper only practical means to impleme