Cracks and Crack Control in Concrete Structures

1 Special ReportCracks and Crack Controlin Concrete StructuresFritz LeonhardtProfessor EngineerStuttgart, FRGThe material presented in this paper isbased on more than 30 years of re-search, observations and experienceconcerning causes, Control , and conse-quences of cracking in Concrete struc-tures. This extensive background washelpful in the preparation of this paperwhich deals with questions of presence of cracking does notnecessarily indicate deficiency instrength or serviceability of concretestructures. While currently available de-sign code provisions lead to reasonablecontrol of cracking, additional controlcan be achieved by understanding thebasic causes and mechanisms of Crack -Note: This paper is a revised and updated version ofan article originally published in the Proceedings ofthe International Association for Bridge and Struc-tural Engineering (1 ABSE), Zurich, Switzerland,1987, p.

2 In Concrete Structures . In this paper,causes of Concrete cracking are dis-cussed, including tensile strength ofconcrete, temperature, shrinkage andcreep effects. Recommended crackwidths are presented along with designmethods for sizing reinforcement tocontrol Crack OF CRACKINGC oncrete can Crack due to a number ofcauses. Some of the most significantcauses are discussed in Strength of ConcreteThe tensile strength of Concrete is awidely scattering quantity. Cracking oc-curs when tensile stresses exceed thetensile strength of Concrete . Therefore,to Control Concrete cracking, the tensilestrength of Concrete is of primary im-124portance. Laboratory test data con-ducted by H. Busch were analyzedstatistically. As presented in Ref. 1, thisanalysis furnished the following re-lationships for the mean direct tensilestrength, f tm, related to the 28-daycompressive cylinder strength f,' of con-crete:fcm= (fc)z/3(psi)fi'm= (ff)2"'(N/mm2)The statistical analysis indicated thatthe coefficient in this equation can bemodified to ( ) and ( ) toobtain the 5 and the 95 percentiles, re-spectively, of the tensile strength, tensile strength of Concrete isslightly higher in flexure.

3 However, it isrecommended that values for direct ten-sion be used in practice. Concretecracks when the tensile strain, t, ex-ceeds to percent. Thislimiting tensile strain is essentially in-dependent of Concrete 5 percentile of the tensilestrength, f, should be used in design tolocate areas in the structure that arelikely to Crack by comparing calculatedstresses with the expected concretestrength. The 95 percentile, f5, shouldbe used to obtain conservative values forrestraint forces that might occur beforethe Concrete Cracks . These restraintforces are used to calculate the amountof reinforcement needed for Crack of Cracking DuringConcrete HardeningConcrete cracking can develop duringthe first days after placing and beforeany loads are applied to the develop due to differentialtemperatures within the occurs when these stresses ex-ceed the developing tensile strength, f;,of the Concrete as indicated in Figs.

4 1and 2. Differential temperatures aremainly due to the heat of hydration ofSynopsisSimple design rules are presentedto Control cracking in Concrete struc-tures. Causes of cracking and its ef-fect on serviceability and durability arediscussed. The paper is primarily ap-plicable to large Structures such asbridges. However, general conceptspresented are applicable to any con-crete structure . Prestressing forcesare considered. A numerical exampleshowing application of the methodand use of simple design charts is during Concrete hardening. Thiseffect is usually neglected except inmassive Structures as indicated in Ref. , depending on cement contentand type of cement, the temperaturewithin Concrete members with dimen-sions of 12 to 36 in. (30 to 91 cm) canincrease approximately 36 F to 108 F(20 C to 60 C) during the first 2 daysafter Concrete members are allowed tocool quickly, tensile stresses may reachvalues higher than the developing ten-sile strength of the Concrete .

5 Even if thisprocess results only in microcracking,the effective tensile strength of thehardened Concrete is reduced. How-ever, very often wide Cracks appeareven when reinforcement is addition, the reinforcement may notbe fully effective since bond strength isalso developing and is yet too low. It isnecessary to minimize such early cracksby keeping temperature differentialswithin the Concrete as low as can be done by one or more of thefollowing measures:1. Choice of cement A cement withlow initial heat of hydration should beselected. Table 1 shows that there is asignificant variation in heat develop-PCI JOURNAUJuly-August 1988 125b concretecompressionGT+ +G = OT aT EctensionInternal stressesin equilibriumFig. 1. Temperature distribution due to heat of hydration andinternal stresses caused by outside cooling in a free standingconcrete among different types of cement content of Concrete shouldbe kept as low as possible by goodgrading of the aggregates.

6 Heat de-velopment can also be reduced by ad-ding fly ash or using slag furnace Evaporation of watermust be prevented by using curingcompounds or by covering the concretewith a membrane. Rapid evaporationcan lead to plastic shrinkage by thermal insulation -Rapid cooling of the surface must beprevented. The degree of thermal insu-lation depends not only on the climate,but also on the thickness of the concretemember and on the type of cement cold water on warm youngconcrete, as it was done years ago, is This is a necessity forlarge massive Concrete Structures suchas dams. For more usual Structures , inwhich shortening after cooling can takeplace without creating significant re-straint forces, precooling is expensiveand unnecessary. In this case, thermalinsulation is preferable and it also hasthe benefit of accelerating concretestrength development.

7 An exceptionmay be made in very hot climates sinceprecooling can keep Concrete workablefor a longer period of shrinkage is considered as acause of early cracking. However, this isnot true under normal climatic condi-tions. Shrinkage needs time to produce a126 Table 1: Heat of hydration of various types ofcements.*Type ofcementtHeat of hydration (Btu/1b)1 day3 days7 days28 daysI92144157167II76115135148 III139184194205IV508194117V5888101124*Da ta obtained from Concrete Manual, Bureau ofReclamation, 1975, pp. Federal Specifications SS-C-192G, including InterimAmendment 2, classified the live types according to usage asfollows: Type I for use in general Concrete construction whenTypes 11, 111, 1V, and V are not required; Type 11 for use inconstruction exposed to moderate sulfate attack; Type III for usewhen high early strength is required; Type IV for use when lowheat of hydration is required; and Type V for use when high sulfateresistance is : Btu/Ib = as high as the tensile rupturestrain.

8 Only in very hot and dry airshrinkage can cause early Cracks inyoung Concrete , if measures againstevaporation are not of Cracking AfterConcrete HardeningTensile stresses due to dead and liveloads cause cracking. Normal rein-cracking due to restrainttensile strength fitInternal Stress5 10 15 20 hConcrete hardening time, hoursFig. 2. Development of the tensile strength and stresses due to nonlineartemperature distribution within the JOURNAL/July-August 1988 127 Deformed Shape considering Upper Faceof Beam Warmer Than Bottom Face andassuming beam freed from interior Supports^I I II MDTM oment DiagramVATS hear DiagramFig. 3. Forces in a Concrete beam due to a temperature rise OT atthe upper face of the beam and external restraint provided byinterior or prestressing should be de-signed to provide required strength andkeep Crack widths within permissiblelimits.

9 Tensile stresses due to serviceloads can be controlled by degree of prestressing can be cho-sen based on structural or economicconsiderations. Normally, partial pre-stressing leads to better serviceabilitythan full can also be initiated by ten-sile stresses due to restrained defor-mations from temperature variations orfrom shrinkage and creep of deformations such as differ-ential settlement between foundationscan also cause are two types of restraint whichcause stress in Concrete members,namely, internal restraint as shown inFig. 1, and external restraint in indeter-minate Structures , as shown in Fig. deformations caused Crack -ing in Concrete bridges and it wasprimarily due to temperature differ-ences produced by heating under thesun and cooling during the night. Ex-treme temperatures that occur at 20 to50-year intervals must be considered.

10 Asindicated in Refs. 3, 4, 5 and 6, temper-atures in bridge Structures were mea-sured in several countries. Recently, Transportation Research Boardpublished in Ref. 7 temperature data forbridge differentials should beconsidered along with recommendedmean temperatures, Tm, used for cal-culating maximum and minimumchanges in the lengths of structuralmembers. In Central Europe values forT. are specified for Concrete bridges asvarying from +68 F to 22 (+20 C to 30 C).The temperature distribution over abeam cross section can be subdividedinto three parts as shown in Fig. 4. Theconstant part, 0 T,, causes axial forces ifoverall length changes are linear part, AT,, causes restraintforces, MAT and VAT, in indeterminatestructures as shown in Fig. 3 for a threespan continuous beam. The nonlinearpart, AT3i causes stresses, which are in128 Table 2.