1、ACI 231R-10Reported by ACI Committee 231Report on Early-Age Cracking:Causes, Measurement, and MitigationReport on Early-Age Cracking:Causes, Measurement, and MitigationFirst PrintingJanuary 2010ISBN 978-0-87031-359-2American Concrete InstituteAdvancing concrete knowledgeCopyright by the American Con
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10、 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 231R-10 was adopted and published January 201
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15、 for incorporation bythe Architect/Engineer.Report on Early-Age Cracking:Causes, Measurement, and MitigationReported by ACI Committee 231ACI 231R-10Early-age cracking is a challenge for the concrete industry. Materialsselection, environmental conditions, and field practices all have considerableinfl
16、uence on the propensity for early-age cracking to occur. This documentfocuses on thermal- and moisture-related deformations; both are materials-related and contribute to early-age cracking. The document providesdetailed reviews on the causes of deformation and cracking, test methodsfor assessing shr
17、inkage and thermal deformation properties, and mitigationstrategies for reducing early-age cracking.Keywords: autogenous shrinkage; cracking; early-age; heat of hydration;measurement; microstructure; mitigation methods; shrinkage; shrinkagecracking; sustainability; thermal cracking; thermal properti
18、es.CONTENTSChapter 1Introduction and scope, p. 231R-21.1Introduction1.2ScopeChapter 2Notation and definitions, p. 231R-22.1Notation2.2DefinitionsChapter 3Causes of early-age deformation and cracking, p. 231R-33.1Thermal deformation3.2Autogenous shrinkage3.3Drying shrinkage3.4Creep and stress relaxat
19、ion from deformation restraint3.5Mitigation of shrinkageChapter 4Test methods and assessment,p. 231R-104.1Introduction4.2Shrinkage measurements4.3Ring testAkthem A. Al-Manaseer Marwan A. Daye Mohamed Lachemi Jussara TanesiEmmanuel K. Attiogbe Noel J. Gardner Benjamin J. Mohr Carlos C. VidelaDale P.
20、Bentz Zachary C. Grasley Kamran M. Nemati Thomas VoightJoseph J. Biernacki Allen J. Hulshizer Jan Olek W. Jason WeissDaniel Cusson Elin A. Jensen Farro F. Radjy Wayne M. WilsonMatthew D. DAmbrosiaWill HansenChairAnton K. SchindlerSecretary231R-2 ACI COMMITTEE REPORT4.4Rigid cracking frames4.5Coeffic
21、ient of thermal expansion (T) measurement4.6Analysis tools assessing stresses and crackingChapter 5Shrinkage control, p. 231R-275.1Introduction5.2Expansive additives5.3Shrinkage-reducing admixtures5.4Internal curingChapter 6References, p. 231R-396.1Referenced standards and reports6.2Cited references
22、CHAPTER 1INTRODUCTION AND SCOPE1.1IntroductionACI Committee 231 defines “early age” as the period afterfinal setting, during which properties are changing rapidly.For a typical Type I portland-cement concrete moist cured atroom temperature, this period is approximately 7 days. Thisdocument, however,
23、 includes discussions of early-ageeffects beyond 7 days. It is important to understand howconcrete properties change with time during early ages andhow different properties are interrelated, which may not bethe same as for mature concrete. It is also important tounderstand how these early-age change
24、s influence theproperties of concrete at later ages. The temperature historyat early ages has a strong effect on whether concrete maydevelop its potential strength. Poor early-age curing has beendemonstrated to detrimentally affect the strength, service-ability, and durability.Concrete structures ch
25、ange volume due to the thermal- andmoisture-related changes. This may be detrimental becausesubstantial stresses may develop when the concrete isrestrained from moving freely. This is particularly importantat early ages while the concrete has a low tensile strength.Therefore, the assessment and cont
26、rol of early-age crackingshould be based on several factors, such as age-dependentmaterial properties, thermal- and moisture-related stressesand strains, material viscoelastic behavior, restraints, andenvironmental exposure.Temperature control in concrete during the early stages ofhydration is essen
27、tial for achieving early strength as well asultimate strength and to eliminate or minimize uncontrolledcracking due to excessive mean peak temperature rise andthermal gradients (ACI 207.1R and 207.2R). Of particularimportance in determining the risk of early-age cracking ofany concrete member is an
28、assessment of the magnitude ofthe stresses generated in the concrete as a result of restraintto thermally induced movement. In general, there are twotypes of restraint: external and internal. External restraints arecaused by support conditions, contact with adjacent sections,applied load, reinforcem
29、ent, and base friction in the case ofconcrete slabs-on-ground. Internal restraint is a manifestation ofthe residual stresses that develop as a result of nonlinear thermaland moisture gradients within a cross section.New methods were developed and older methods rediscoveredfor evaluating stress and s
30、train development and assessingcracking risk in concrete mixtures under realistic exposureconditions. Categories of evaluation methods discussed inthis document include restrained and unrestrained volumechange tests, coefficient of thermal expansion tests, and toolsfor assessing stress development a
31、nd cracking potential. Someof these evaluation methods have been standardized.Mitigation methods have focused mainly on reducing theautogenous (moisture-related) component of the early-agestresses or compensating for the early-age shrinkage byemploying expansive cement. In the former case, bothshrin
32、kage-reducing admixtures (SRAs) and internal curinghave been demonstrated to reduce the magnitude of theearly-age shrinkage of specimens cured under sealed,isothermal conditions.The prevention or mitigation of early-age cracking willimprove the long-term durability of concrete structures and,therefo
33、re, enhance their sustainability by increasing theservice life.1.2ScopeThis document reviews the causes of early-age deformationand cracking. The test methods for quantifying the early-agestress development and hence the risk of cracking due tothermal and moisture conditions are described. Mitigatio
34、nmethods for stress reduction are also discussed.CHAPTER 2NOTATION AND DEFINITIONS2.1NotationC = cement factor (content) for concrete mixture,lb/yd3(kg/m3)Cf= correction factor accounting for the change inlength of the measurement apparatus withtemperature, 0.56 106/F (1 106/C)CS = chemical shrinkag
35、e of cement (mass of water/mass of cement)D = moisture diffusion coefficient of concretedT/dt = temperature changedhygral/dt = rate of nonthermal deformation due toautogenous shrinkage, drying shrinkage, or bothE(t) = Youngs modulus at time t, psi (MPa)Ec= creep-adjusted modulus of elasticity ofconc
36、rete, psi (MPa)Ec(t) = age-dependent elastic modulus of concreteECON= elastic modulus of concreteEsteel= modulus of elasticity of steel ring, psi (MPa)erfc = complementary error functionf = geometry function (Moon and Weiss 2006)h = humidity (0 to 1)G = coefficient relating stress to steel ring stra
37、in10.44 106psi (72.2 GPa) for the ASTM ring)= stress amplification factorKr= degree of restraint factorL =lengthLo= measured length of specimen at roomtemperature, in. (mm)MLWA= mass of (dry) fine lightweight aggregate neededper unit volume of concrete, lb/yd3(kg/m3)R = degree of restraintKEARLY-AGE
38、 CRACKING: CAUSES, MEASUREMENT, AND MITIGATION 231R-3R = ideal gas constant 8.314 J/(molK)RIC= inner radius of concrete ringRIS= inner radius of steel ringROC= outer radius of concrete ringROS= outer radius of steel ringr =radiusS = degree of saturation of aggregate (0 to 1)Tc= average concrete temp
39、erature F (C)Tmin= minimum concrete temperature on a coldnight, F (C)Tzero-stress= concrete zero-stress temperature, F (C)tCR= time to crackingVW= molar volume of pore solutionmax= maximum expected degree of hydration ofcement (0 to 1)T= coefficient of thermal expansion, strain/F(strain/C) = coeffic
40、ient relating shrinkage rate to shrinkage(0.056 day1) = strain increment from autogenous and dryingshrinkageLa= actual length change of specimen duringtemperature change, in. (mm)Lm = measured length change of specimen duringtemperature change, in. (mm) (increase =positive; decrease = negative)Lf= l
41、ength change of the measuring apparatusduring temperature change, in. (mm)T=measured temperature change (average of thefour sensors) F (C) (increase = positive;decrease = negative) = stress increment, psi (MPa)c(t) = creep straine(t) = elastic strainsh(t) = free shrinkage strainhygral= nonthermal de
42、formation due to autogenous ordrying shrinkageSH= free shrinkage at time tSH-CONST= shrinkage coefficientST= deformation of steelLWA= desorption of lightweight aggregate from satu-ration down to 93% RH (mass water/mass drylightweight aggregate)(t) = creep coefficient at time t = surface tension = co
43、ntact angle = Poissons ratiocand s= Poissons ratio of concrete and steel = stress, psi (MPa)Elastic-Max= theoretical maximum elastic stress(t) = relaxation coefficient at time t(t) = aging coefficient to account for the reducedcreep coefficient due to a gradually increasingload in restrained specime
44、n; 0.6 to 0.9 forordinary hardened concrete, and 0.9 to 1.0 foryoung concrete2.2DefinitionsACI provides a comprehensive list of definitions throughan online resource, “ACI Concrete Terminology,” http:/terminology.concrete.org. Definitions provided hereincomplement that resource.early agethe period a
45、fter final setting, during whichproperties are changing rapidly. For a typical Type I portland-cement concrete moist cured at room temperature, thisperiod is approximately 7 days.CHAPTER 3CAUSES OF EARLY-AGE DEFORMATION AND CRACKINGThe two major driving forces for early-age volumechange are the ther
46、mal deformation due to cement hydrationand shrinkage deformation (autogenous shrinkage anddrying shrinkage). The hydration reaction leads to a netreduction in the total volume of the hardened paste, whichcauses self-desiccation of pores and associated shrinkage.The incremental stress development in
47、a hardeningconcrete structure with thermal and shrinkage deforma-tions being considered is written as = (T T+ ) E R (3-1)3.1Thermal deformationThermal deformation is a dimensional change resultingfrom a temperature change in concrete. Thermal deformationdepends on the coefficient of thermal expansio
48、n (T), whichis a function of the state of internal moisture. This was firstexperimentally identified in 1950 by Meyers (1950), laterfound by Zoldners (1971), and more recently validated byBjontegaard (1999). Both Meyers and Zoldners found that,while the Tat fully dried or saturated conditions wasapp
49、roximately the same, there was a dramatic increase in theTat intermediate relative humidities (RHs). Meyers hypothe-sized that in the intermediate RH range, there is additionaldilation due to changes in the pore water pressure exerted onthe solid skeleton. Powers and Brownyard (1947) noted thatwith changes in temperature, water expands and undergoes areduction in surface tension. As the surface tension of watergoes down, the negative pressure exerted on the pore systemgoes do
