1、ACI 207.2R-07Reported by ACI Committee 207Report on Thermal andVolume Change Effectson Cracking of Mass ConcreteReport on Thermal and Volume Change Effectson Cracking of Mass ConcreteFirst PrintingSeptember 2007ISBN 978-0-87031-258-8American Concrete InstituteAdvancing concrete knowledgeCopyright by
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10、ting ACI.Most ACI standards and committee reports are gathered together in the annually revised ACI Manual ofConcrete Practice (MCP).American Concrete Institute38800 Country Club DriveFarmington Hills, MI 48331U.S.A.Phone: 248-848-3700Fax: 248-848-3701www.concrete.orgACI 207.2R-07 supersedes ACI 207
11、.2R-95 and was adopted and published September2007.Copyright 2007, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by anymeans, including the making of copies by any photo process, or by electronic ormechanical device, printed, written, or oral
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13、 executing, and inspecting construction. Thisdocument is intended for the use of individuals who arecompetent to evaluate the significance and limitations of itscontent and recommendations and who will acceptresponsibility for the application of the material it contains.The American Concrete Institu
14、te disclaims any and allresponsibility for the stated principles. The Institute shall notbe liable for any loss or damage arising therefrom.Reference to this document shall not be made in contractdocuments. If items found in this document are desired by theArchitect/Engineer to be a part of the cont
15、ract documents, theyshall be restated in mandatory language for incorporation bythe Architect/Engineer.Report on Thermal and Volume Change Effectson Cracking of Mass ConcreteReported by ACI Committee 207ACI 207.2R-07This report presents a discussion of the effects of heat generation andvolume change
16、 on the design and behavior of mass concrete elements andstructures. Emphasis is placed on the effects of restraint on cracking andthe effects of controlled placing temperatures, concrete strength requirements,and material properties on volume change.Keywords: adiabatic; cement; concrete cracking; c
17、reep; drying shrinkage;foundation; heat of hydration; mass concrete; modulus of elasticity;placing; portland cement; pozzolan; restraint; stress; temperature; tensilestrength; thermal expansion; volume change.CONTENTSChapter 1Introduction, p. 207.2R-21.1Scope1.2Mass concrete versus structural concre
18、te1.3Approaches for crack controlChapter 2Thermal behavior, p. 207.2R-32.1General2.2Thermal gradientsChapter 3Properties, p. 207.2R-43.1General3.2Strength requirements3.3Tensile strength3.4Creep3.5Thermal properties of concrete3.6Modulus of elasticity3.7Strain capacityChapter 4Heat transfer and volu
19、me change,p. 207.2R-84.1Heat generation4.2Moisture contents and drying shrinkage4.3Ambient temperatures4.4Placement temperature4.5Final temperature in service4.6Heat dissipation4.7Summary and examplesChapter 5Restraint, p. 207.2R-225.1General5.2Continuous external restraint5.3Internal restraintChapt
20、er 6Crack widths, p. 207.2R-256.1General6.2Crack control joints6.3LimitationsChapter 7References, p. 207.2R-267.1Referenced standards and reports7.2Cited referencesAppendix A, p. 207.2R-27A.1NotationA.2Metric conversionsJeffrey C. Allen Anthony A. Bombich Barry D. Fehl Gary R. MassTerrence E. Arnold
21、 Teck L. Chua Rodney E. Holderbaum Tibor J. PatakyRandall P. Bass Eric J. Ditchey Allen J. Hulshizer Ernest K. SchraderFloyd J. Best Timothy P. Dolen David E. Kiefer Gary P. WilsonStephen B. TatroChair207.2R-2 ACI COMMITTEE REPORTCHAPTER 1INTRODUCTION1.1ScopeThis report is primarily concerned with e
22、valuating thethermal behavior of mass concrete structures to control thecracking in members that occurs principally from thermalcontraction with restraint. This report presents a detaileddiscussion of the effects of heat generation and volumechanges on the design and behavior of mass concrete elemen
23、tsand structures, a variety of methods to compute heat dissipationand volume changes, and an approach to determine mass andsurface gradient stresses. It is written primarily to provideguidance for the selection of concrete materials, mixturerequirements, and construction procedures necessary tocontr
24、ol the size and spacing of cracks. The quality of concretefor resistance to weathering is not emphasized in recom-mending reduced cement contents; however, it should beunderstood that the concrete should be sufficiently durable toresist expected service conditions. This report can be appliedto most
25、concrete structures with a potential for unacceptablecracking. Its general application has been to massive concretemembers 18 in. (460 mm) or more in thickness; it is alsorelevant for less massive concrete members.1.2Mass concrete versus structural concreteMass concrete is defined in ACI 116R as: “a
26、ny volume ofconcrete with dimensions large enough to require thatmeasures be taken to cope with generation of heat fromhydration of the cement and attendant volume change, tominimize cracking.” The most important characteristic ofmass concrete that differentiates its behavior from that ofstructural
27、concrete is its thermal behavior. The generallylarge size of mass concrete structures creates the potentialfor large temperature changes in the structure and significanttemperature differentials between the interior and the outsidesurface of the structure. The accompanying volume-changedifferentials
28、 and restraint result in tensile strains and stressesthat may cause cracking detrimental to the structural design,the serviceability, or the appearance.In most structural concrete construction, most of the heatgenerated by the hydrating cement is rapidly dissipated, andonly slight temperature differ
29、ences develop. For example, aconcrete wall 6 in. (150 mm) thick can become thermally stablein approximately 1-1/2 hours. A 5 ft (1.5 m) thick wall wouldrequire a week to reach a comparable condition. A 50 ft (15 m)thick wall, which could represent the thickness of an arch dam,would require 2 years.
30、A 500 ft (152 m) thick dam, such asHoover, Shasta, or Grand Coulee, would take approximately200 years to achieve the same degree of thermal stability.Temperature differentials never become large in typicalstructural building elements and, therefore, typical structuralbuilding elements are relatively
31、 free from thermal cracking.In contrast, as thickness increases, the uncontrolled interiortemperature rise in mass concrete becomes almost adiabatic,and this creates the potential for large temperature differentialsthat, if not accommodated, can impair structural integrity.There are many concrete pl
32、acements considered to bestructural concrete that could be significantly improved ifsome of the mass concrete measures presented in this reportwere implemented. Measures include consideration of issuessuch as required concrete strengths, age when strength isrequired, cement contents, supplemental ce
33、mentitiousmaterials, temperature controls, and jointing.1.3Approaches for crack controlIf cementitious materials did not generate heat as theconcrete hardens, if the concrete did not undergo volumechanges with changes in temperature, and if the concrete didnot develop stiffness (high modulus of elas
34、ticity), therewould be little need for temperature control. In the majorityof instances, this heat generation and accompanyingtemperature rise will occur rapidly before the developmentof elastic properties and, consequently, little or no stress devel-opment during this phase. A continuing rise in te
35、mperature formany more days is concurrent with the increase in elasticmodulus (rigidity). Even these circumstances would be oflittle concern if the entire mass of the placement could:1. Be limited in maximum temperature to a value close toits final, cooled, stable temperature;2. Be maintained at the
36、 same temperature throughout itsvolume, including exposed surfaces;3. Be supported without restraint (or supported on foundationsexpanding and contracting in the same manner as theconcrete);4. Relieve its stress through creep; and5. Have no stiffness or rigidity.None of these conditions, of course,
37、can be achievedcompletely. The first and second conditions (such astemperature controls) can be realized to some extent in mostconstruction. The third condition (such as limited restraint)is the most difficult to obtain, but has been accomplished ona limited scale for extremely critical structures b
38、y preheatingthe previously placed concrete to limit the differentialbetween older concrete and the maximum temperatureexpected in the covering concrete. The fourth and fifthconditions can be somewhat influenced if there is an optionto use lower-strength concrete and aggregates with lowercoefficients
39、 of thermal expansion and lower modulus. Thisreport provides discussion and explanation about theseissues and other issues related to controlling thermal volumechanges and subsequent cracking.All concrete elements and structures are subject to volumechange in varying degrees dependent upon the makeu
40、p,configuration, and environment of the concrete. Uniformvolume change will not produce cracking if the element orstructure is relatively free to change volume in all directions.This is rarely the case for massive concrete membersbecause size alone usually causes nonuniform change, andthere is often
41、 sufficient restraint either internally or externally toproduce cracking.The measures used to control cracking depend, to a largeextent, on the economics of the situation and the seriousnessof cracking if not controlled. Cracks are objectionable wheretheir size and spacing compromise the strength, s
42、tability,serviceability, function, or appearance of the structure.While cracks should be controlled to the minimum practicablewidth in all structures, the economics of achieving this goalTHERMAL AND VOLUME CHANGE EFFECTS ON CRACKING OF MASS CONCRETE 207.2R-3should be considered. The change in volume
43、 can be minimizedor controlled by such measures as reducing cement content,replacing part of the cement with pozzolans, precooling, post-cooling, insulating to control the rate of heat absorbed or lost,and by other temperature control measures outlined in ACI207.4R. Restraint is modified by installi
44、ng joints to permitcontrolled contraction or expansion and also by controlling therate at which volume change takes place. Construction jointsmay also be used to reduce the number of uncontrolled cracksthat may otherwise be expected. By appropriate considerationof the preceding measures, it is usual
45、ly possible to controlcracking or at least to minimize the crack widths.In the design of reinforced concrete structures, cracking ispresumed mitigated through the effective placement ofreinforcement. For this reason, the designer does notnormally distinguish between tension cracks due to volumechang
46、e and those due to flexure. Instead of employing manyof the previously recommended measures to control volumechange, the designer may choose to add sufficient reinforcementto distribute the cracking so that one large crack is replacedby many smaller cracks of acceptably smaller widths. Theselection
47、of the necessary amount and spacing of reinforcementto accomplish this depends on the extent of the expectedvolume change, the spacing or number of cracks that wouldoccur without the reinforcement, and the ability of reinforcementto distribute such cracks.The degree to which the designer will either
48、 reducevolume changes or use reinforcement for control of cracks ina given structure depends largely on the massiveness of thestructure itself and on the magnitude of forces restrainingvolume change. No clear-cut line can be drawn to establishthe extent to which measures should be taken to control t
49、hechange in volume. Design strength requirements, placingrestrictions, and the environment itself are sometimes sosevere that it is impractical to mitigate cracking solely bymeasures to minimize volume change. On the other hand,fortunately, the designer normally has a wide range of choiceswhen selecting design strengths and structural dimensions.In many cases, the cost of increased structural dimensionsrequired by the selection of lower-strength concrete (withinthe limits of durability requirements) is more than repaid bythe savings in reinforcing ste
