ACI 224R-2001 Control of Cracking in Concrete Structures《混凝土建筑的裂化控制》.pdf

1、ACI 224R-01 supersedes ACI 224R-90 and became effective May 16, 2001.Copyright 2001, 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

2、, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission inwriting is obtained from the copyright proprietors.ACI Committee Reports, Guides, Standard Practices,and Commentaries are intended for guidance in plannin

3、g,designing, executing, and inspecting construction. Thisdocument is intended for the use of individuals who arecompetent to evaluate the significance and limitations ofits content and recommendations and who will accept re-sponsibility for the application of the material it contains.The American Co

4、ncrete Institute disclaims any and all re-sponsibility for the stated principles. The Institute shallnot be liable for any loss or damage arising therefrom.Reference to this document shall not be made in con-tract documents. If items found in this document are de-sired by the Architect/Engineer to b

5、e a part of the contractdocuments, they shall be restated in mandatory languagefor incorporation by the Architect/Engineer.224R-1Control of Cracking in Concrete StructuresACI 224R-01The principal causes of cracking and recommended crack-control proce-dures are presented. The current state of knowled

6、ge in microcracking andfracture of concrete is reviewed. The control of cracking due to dryingshrinkage and crack control in flexural members, overlays, and mass con-crete construction are covered in detail. Long-term effects on cracking areconsidered and crack-control procedures used in constructio

7、n are pre-sented. Information is presented to assist in the development of practicaland effective crack-control programs for concrete structures. Extensive ref-erences are provided.Keywords: aggregates; anchorage (structural); bridge decks; cement-aggregate reactions; concrete construction; concrete

8、 pavements; concreteslabs; cooling; corrosion; crack propagation; cracking (fracturing); crackwidth and spacing; drying shrinkage; shrinkage-compensating concrete;heat of hydration; mass concrete; microcracking; polymer-modified concrete;prestressed concrete; reinforced concrete; restraint; shrinkag

9、e; temperature;tensile stresses; thermal expansion; volume change.CONTENTSChapter 1Introduction, p. 224R-2Chapter 2Crack mechanisms in concrete, p. 224R-22.1Introduction2.2Compressive microcracking2.3FractureChapter 3Control of cracking due to drying shrinkage, p. 224R-113.1Introduction3.2Cause of c

10、racking due to drying shrinkage3.3Drying shrinkage3.4Factors controlling drying shrinkage of concrete3.5Control of shrinkage cracking3.6Shrinkage-compensating concreteChapter 4Control of cracking in flexural members, p. 224R-174.1Introduction4.2Crack-control equations for reinforced concrete beams4.

11、3Crack control in two-way slabs and plates4.4Tolerable crack widths versus exposure conditions inreinforced concrete4.5Flexural cracking in prestressed concrete4.6Anchorage-zone cracking in prestressed concrete4.7Crack control in deep beams4.8Tension crackingReported by ACI Committee 224Mohamed Abou

12、-Zeid David W. Fowler*Edward G. Nawy*John H. Allen Grant T. Halvorsen Randall W. Poston*James P. Barlow Will Hansen*Royce J. RhoadsMerle E. Brander*M. Nadim Hassoun Andrew ScanlonKathy Carlson Harvey Haynes*Ernest K. Schrader*David Darwin*Paul Hedli Wimal Suaris*Fouad H. Fouad*Tony C. Liu Zenon A. Z

13、ielinskiFlorian BarthChairmanRobert J. Frosch*Secretary*Members of ACI 224 who assisted in revisions to this report.224R-2 ACI COMMITTEE REPORTChapter 5Long-term effects on cracking, p. 224R-245.1Introduction5.2Effects of long-term loading5.3Environmental effects5.4Aggregate and other effects5.5Use

14、of polymers in improving cracking characteristicsChapter 6Control of cracking in overlays,p. 224R-256.1Introduction6.2Fiber-reinforced concrete (FRC) overlays6.3Latex- and epoxy-modified concrete overlays6.4Polymer-impregnated concrete (PIC) systems6.5Epoxy and other polymer concrete overlaysChapter

15、 7Control of cracking in mass concrete, p. 224R-287.1Introduction7.2Methods of crack control7.3Design7.4Construction7.5OperationChapter 8Control of cracking by proper construction practices, p. 224R-348.1Introduction8.2Restraint8.3Shrinkage8.4Settlement8.5Construction8.6Specifications to minimize dr

16、ying shrinkage8.7ConclusionChapter 9References, p. 224R-399.1Referenced standards and reports9.2Cited references9.3Other referencesCHAPTER 1INTRODUCTIONCracks in concrete structures can indicate major structuralproblems and detract from the appearance of monolithicconstruction. There are many specif

17、ic causes of cracking.This report presents the principal causes of cracking and adetailed discussion of crack-control procedures. The reportconsists of eight chapters designed to help the engineer andthe contractor in developing crack-control measures.This report is an update of previous committee r

18、eports(ACI Committee 224 1972, 1980, 1990). ACI Bibliogra-phy No. 9 supplemented the original ACI 224R (1971). TheCommittee has also prepared reports on the causes, evaluation,and repair of cracking, ACI 224.1R; cracking of concrete in di-rect tension, ACI 224.2R; and joints in concrete construction

19、,ACI 224.3R.In this revision of the report, Chapter 2 on crack mechanismshas been revised extensively to reflect the interest and attentiongiven to aspects of fracture mechanics of concrete during the1980s. Chapter 3 on drying shrinkage has been rewritten.Chapter 4 has been revised to include update

20、d informationon crack-width predictive equations, cracking in partiallyprestressed members, anchorage zone cracking, and flexuralcracking in deep flexural members. Chapter 6 on concreteoverlays has been reorganized and revised in modest detailto account for updated information on fiber reinforcement

21、and on polymer-modified concrete. Chapter 7 on massconcrete has been revised to consider structural consequencesmore extensively. CHAPTER 2CRACK MECHANISMS IN CONCRETE2.1IntroductionCracking plays an important role in concretes response toload in both tension and compression. The earliest studies of

22、the microscopic behavior of concrete involved the responseof concrete to compressive stress. That early work showedthat the stress-strain response of concrete is closely associatedwith the formation of microcracks, that is, cracks that form atcoarse-aggregate boundaries (bond cracks) and propagateth

23、rough the surrounding mortar (mortar cracks) (Hsu, Slate,Sturman, and Winter 1963; Shah and Winter 1966; Slate andMatheus 1967; Shah and Chandra 1970; Shah and Slate1968; Meyers, Slate, and Winter 1969; Darwin and Slate1970), as shown in Fig. 2.1.During early microcracking studies, concrete was cons

24、ideredto be made up of two linear, elastic brittle materials; cementpaste and aggregate; and microcracks were considered to bethe major cause of concretes nonlinear stress-strain behaviorin compression (Hsu, Slate, Sturman, and Winter 1963; Shahand Winter 1966). This picture began to change in the19

25、70s. Cement paste is a nonlinear softening material, asis the mortar constituent of concrete. The compressive non-linearity of concrete is highly dependent upon the responseof these two materials (Spooner 1972; Spooner and Dougill1975; Spooner, Pomeroy, and Dougill 1976; Maher and Dar-win 1977; Cook

26、 and Chindaprasirt 1980; Maher and Darwin1982) and less dependent upon bond and mortar microcrackingthan originally thought. Research indicates, however, that a sig-nificant portion of the nonlinear deformation of cement pasteand mortar results from the formation of microcracks thatare several order

27、s of magnitude smaller than those observed inthe original studies (Attiogbe and Darwin 1987, 1988). Thesesmaller microcracks have a surface density that is two tothree orders of magnitude higher than the density of bondand mortar microcracks in concrete at the same compres-sive strain, and their dis

28、covery represents a significantstep towards understanding the behavior of concrete andits constituent materials in compression.The effect of macroscopic cracks on the performance andfailure characteristics of concrete has also received considerableattention. For many years, concrete has been conside

29、red a brittlematerial in tension. Many attempts have been made to useprinciples of fracture mechanics to model the fracture ofconcrete containing macroscopic cracks.The field of fracture mechanics was developed by Griffith(1920) to explain the failure of brittle materials. Linear elasticfracture mec

30、hanics (LEFM) predicts the rapid propagation of amicrocrack through a homogeneous, isotropic, linear-elasticmaterial. The theory uses the stress-intensity factor K thatCONTROL OF CRACKING IN CONCRETE STRUCTURES 224R-3represents the stress field ahead of a sharp crack in a struc-tural member which is

31、 a function of the crack geometry andstress. K is further designated with subscripts, I, II, and III,depending upon the nature of the deformation at the cracktip. For a crack at which the deformation is perpendicular tothe crack plane, K is designated as KI, and failure occurswhen KI reaches a criti

32、cal value KIc, known as the criticalstress-intensity factor. KIc is a measure of the fracture tough-ness of the material, which is simply a measure of the resis-tance to crack propagation. Often the region around the cracktip undergoes nonlinear deformation, such as yielding inmetals, as the crack g

33、rows. This region is referred to as theplastic zone in metals, or more generally as the fracture processzone. To properly measure KIc for a material, the test specimenshould be large enough so that the fracture process zone issmall compared with the specimen dimensions. For LEFMto be applicable, the

34、 value of KIc must be a material property,independent of the specimen geometry (as are other materialproperties, such as yield strength or compressive strength).Initial attempts to measure KIc in concrete were unsuccessfulbecause KIc depended on the size and geometry of the testspecimens (Wittmann 1

35、986). As a result of the heterogeneityinherent in cement paste, mortar, and concrete, these materialsexhibit a significant fracture-process zone and the criticalload is preceded by a substantial amount of slow crack growth.This precritical crack growth has been studied experimentallyby several resea

36、rchers (John and Shah 1986; Swartz and Go1984; Bascoul, Kharchi, and Maso 1987; Maji and Shah1987; Castro-Montero, Shah, and Miller 1990). This researchhas provided an improved understanding of the fracture processzone and has led to the development of more rational fracturecriteria for concrete.Thi

37、s chapter is divided into two sections. The first sectionon compressive microcracking presents the current knowledgeof the response of concrete and its constituent materials undercompressive loading and the role played by the various typesof microcracks in this process. The second section discussest

38、he applicability of both linear and nonlinear fracture mechanicsmodels to concrete. A more comprehensive treatment of thefracture of concrete can be found in ACI 446.1R.2.2Compressive microcrackingDuring early microcracking research, a picture devel-oped that closely linked the formation and propaga

39、tion ofmicrocracks to the load-deformation behavior of concrete.Before loading, volume changes in cement paste cause inter-facial cracks to form at the mortar-coarse aggregate bound-ary (Hsu 1963; Slate and Matheus 1967). Under short-termcompressive loads, no additional cracks form until the loadrea

40、ches about 30% of the compressive strength of the con-crete (Hsu, Slate, Sturman, and Winter 1963). Above thisvalue, additional bond cracks are initiated throughout thematrix. Bond cracking increases until the load reaches about70% of the compressive strength, at which time the microc-racks begin to

41、 propagate through the mortar. Mortar crack-ing continues at an accelerated rate, forming continuouscracks parallel to the direction of compressive load, until theconcrete is no longer able to sustain the load. The onset ofmortar cracking is related to the sustained, or long-term,compressive strengt

42、h. Derucher (1978) obtained a somewhatdifferent picture of the microscopic behavior of concreteusing the scanning electron microscope (SEM). He subjecteddried concrete specimens to eccentric compressive loadingwithin the SEM. He observed that microcracks that existFig. 2.1Cracking maps and stress-st

43、rain curves for concrete loaded in uniaxial compression(Shah and Slate 1968).224R-4 ACI COMMITTEE REPORTbefore loading are in the form of bond cracks, with exten-sions into the surrounding mortar perpendicular to the bondcracks. Under increasing compression, these bond crackswiden but do not propaga

44、te at loads as low as 15% of thestrength. At about 20% of ultimate, the bond cracks begin topropagate, and at about 30%, they begin to bridge betweenone another. The bridging is almost complete at 45% of thecompressive strength. At 75% of ultimate, mortar cracksstart to join one another and continue

45、 to do so until failure.In general, microcracking that occurs before loading has littleeffect on the strength of compressive strength of the concrete.In studies of high-strength concrete, Carrasquillo, Slate,and Nilson (1981) concluded that it was more appropriate toclassify cracks as simple (bond o

46、r mortar) and combined(bond and mortar) and that the formation of combinedcracks consisting of more than one mortar crack signaledunstable crack growth. They observed that the higher theconcrete strength, the higher the strain (relative to the strain atpeak stress) at which this unstable crack growt

47、h is observed.They observed less total cracking in high-strength concretethan normal-strength concrete at all stages of loading.Work by Meyers, Slate, and Winter (1969), Shah andChandra (1970), and Ngab, Slate, and Nilson (1981) demon-strated that microcracks increase under sustained and cyclicloadi

48、ng. Their work indicated that the total amount of micro-cracking is a function of the total compressive strain in theconcrete and is independent of the method in which the strainis applied. Suaris and Fernando (1987) also showed that thefailure of concrete under constant amplitude cyclic loadingis c

49、losely connected with microcrack growth. Sturman, Shah,and Winter (1965) found that the total degree of microcrackingis decreased and the total strain capacity in compression isincreased when concrete is subjected to a strain gradient.Since the early work established the existence of bond andmortar microcracks, it has been popular to attribute most, ifnot all, of the nonlinearity of concrete to the formation ofthese microscopic cracks (Hsu, Slate, Sturman, and Winter1963; Shah and Winter 1966; Testa and Stubbs 1977; Car-rasquillo, Sla