ConcreteConstructionTroubleshootingTips
Problems •AirLossDuringConcretePumping •Blistering •Bugholes(SurfaceAirVoids) •Crazing •CrustingSurfaces(WavySurfaces) •Curling •Discolouration •Dusting •Honeycombing •LowConcreteStrengthResults •PlasticShrinkageCracking •Scaling •UncontrolledShrinkageCracking •MortarFlaking •Popouts
Aids •ConversionTable •ConcreteQuickSpecifications
www.rmcao.org
Problem:
3
Definition:Changesintheaircontentoftheconcreteduetoconcretepumpingoperations.
Concreteairentrainmentrequirementsaretypicallybaseduponconcretesampledandtestedfromconcretesuppliedattheendofthechuteoftheconcretetruck.Whenconcreteisplacedviaaconcretepumpandtheacceptancesamplesareobtainedattheendofthepumpthereareanumberofadditionalfactorsthatthecontractormustconsidersincepumpingoperationscanreducetheaircontentoftheconcrete.
Air Loss During Concrete Pumping is more likely to occur when:n Theconcreteundergoessignificantverticaldrops
n Theconcretehasahighinitialslump
n Thepumpingpressureisincreasedandthereisasuddenpressuredropastheconcreteexitsthepumphose
How to avoid problems with Air Loss During Concrete Pumping:n Operatetheconcretepumpingboominanupwardorhorizontalposition
asmuchaspossible
n Reduceverticalconcretedropstotheshortestpossiblelength
n Insertmultiple90°elbowsintothelinetocreatearesidualamountofconcreteinthelineandtominimizethelongstraightdropoutofthepumpingline
n Utilizeagateorflowrestrictingdeviceattheendofthepumplinetoprovideresistancetoflow
n Runthelast3–5metresofthepumpinghosehorizontallyalongthesubgradeorformworktoavoidverticaldrops
n Usingcable,looptheendsectionoftherubberhoseintoacircletopreventdirectfreefalloutofthehose
n Elevatetheplasticaircontentabovethespecificationlimitsattheconcretetrucklocationtoaccountfortheactualairloss(Caution–Inspectionoftheaircontentatboththetruckandthepumpinglocationbecomesextremelycriticalwheneverthereisachangeintheboomconfiguration–higheraircontentsinthefinalconcretehaveanegativeimpactoncompressivestrength)
Concretepumpingonacommercialprojectusinga flowrestrictiondevicetoprovideresistancetoflow andtopreventdirectfreefall
Photos courtesy of RMCAO
Concretepumpingonaresidentialradiantfloor heatingproject
References:1. ConcreteinPractice#21–LossofAirContentinPumpedConcrete–NationalReady
MixedConcreteAssociation2. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute3. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Air Loss During Concrete PumpingAir Loss During Concrete Pumping
Problem:
4
Definition:Theirregularraisingofathinlayeratthesurfaceofplacedmortarorconcreteduringorsoonaftercompletionofthefinishingoperation.
Blistersformjustunderthesurfaceoftheconcretewheneitherbleedwaterorentrappedairispreventedfromescapingtheconcrete.Thistypicallyoccurswhenthetopsurfaceoftheconcretehasbeensealedprematurelyduringthefinishingoperations.
Blisters are more likely to form when:n Thesubgradeiscoolerthantheconcreteresultinginnon-uniformsetting
oftheconcrete
n Insufficientorexcessivevibrationisemployed
n Theconcretesurfaceisprematurelysealedduetoimproperfinishingproceduresortools
n Siteconditionsareresultinginrapidsurfaceevaporationcausingthefinishertomistakenlyassumethebleedingperiodiscomplete
n Entrainedairisaddedtononair-entrainedconcreteorwhentheairentrainmentisabnormallyhigh
n Theconcreteelementisextremelythickresultinginlongertimesforbleedwaterandentrappedairtorisetothesurface
n Adryshakeisprematurelyappliedtotheconcretesurface
n Theconcreteisplacedoveravapourbarrier
How to avoid problems with Blistering:n Utilizeproperconcreteconsolidationpractices
n Utilizetheproperfinishingtoolsandkeepthetrowelingbladesas flataspossibletoavoidsealingthesurface
n Placeconcreteduringperiodsoflowersurfaceevaporation
n Avoidplacingconcreteoncoldsub-gradeswheneverpossible
n Avoidtheuseofchemicalretarderssincetheywillonlyintensify theproblem
n Keeptrowelbladesflataslongaspossibletoavoidsealingthe surfaceprematurely
n Considertheuseofchemicalacceleratorstoreducethesettime
n Properlyheatthebuildingand/orsubgradepriortoconcreteplacement
n Avoidplacingconcreteontopofvapourbarrierswheneverpossible
References:1. ConcreteinPractice#13–ConcreteBlisters–NationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
BlisteringBlistering
Cross-sectionthroughaconcreteblister
Blisteringconcretesurface
Photos courtesy of CAC and CTL
Problem:
Photos courtesy of CAC and RMCAO
5
Definition:Smallregularorirregularcavities,usuallynotexceeding15mmindiameter,resultingfromtheentrapmentofairbubblesinthesurfaceofformedconcreteduringplacementandconsolidation.
Bugholesappearassmallvoidontheoutsideofformedconcretesurfaces.Thesedefectsaretypicallyduetothepresenceofsmallairpocketsthatwerenotremovedduringconcreteconsolidationoperationsorbytheincompleteapplicationoftheformreleaseagenttoformworkresultinginconcreteadhesionduringtheformremovalprocess.
Bugholes are more likely to form when:n Theconcreteisnotadequatelyconsolidatedtoremoveallentrappedair
n Internalconcretevibratorinsertionsarenotsufficientlyoverlappedorwellspaced
n Formreleaseagentsarenotusedortheformsareremovedbeforetheconcretehasgainedsufficientstrengthtoresisttheformremovalpressures
n Concretemixturesareverystifforsticky
How to avoid problems with Bugholes:n Uniformlyconsolidatetheconcreteensuringthatthevibratorinsertion
patternisuniformandwellspaced
n Reducetheheightofeachconcreteplacementlifttomakeairremovaleaser
n Movethevibratorasclosetotheformworkaspossible
n Properlyapplytheformreleaseagentandonlyremovetheformsoncetheconcretehasgainedsufficientstrengthtoresistanysuctionforces
n Considertheuseofspecialtyconcreteproductslikeselfconsolidatingconcrete(SCC)
References:1. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup2. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute3. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Bugholes (Surface Air Voids)Bugholes (Surface Air Voids)
Bugholesonaplywoodformedsurface
Bugholesonaconcretesurfacecreatedwith aformliner
SelfConsolidating Concrete(SCC) significantly reducesbugholes andcanbeused inarchitectural concrete applications
Problem:
6
Definition:Intersectingcracksthatextendbelowthesurfaceofhardenedconcrete;causedbyshrinkageofthedryingsurfaceconcretethatisrestrainedbyconcreteatgreaterdepthswhereeitherlittleornoshrinkageoccurs; varyinwidthfromfineandbarelyvisibletoopenandwell-defined.
Crazingcracksarefinerandomcracksonthesurfaceoftheconcretethatdonotaffectthestructuralintegrityoftheconcretebutwhichareveryunsightly.Thesecracksrarelyaffectthedurabilityorwearresistanceoftheconcreteandareparticularlyevidentwhentheconcretecontainscalciumchlorideasanacceleratingadmixture.
Crazing cracks are more likely to form when:n Theconcreteisnotproperlycuredandprotected.Thesecracksformdue
tothetopsurfaceoftheconcretedryingoutandshrinkingfasterthantheremainderoftheconcrete
n Theconcreteisexposedtorapidorprolongedsurfacedrying(lowhumidity,hightemperature,directsunlight,dryingwinds,etc.)
n Thereisexcessfloatingortheuseofajitterbugtodepressthecoarseaggregateresultingintoomuchcementpasteatthesurface
n Theconcretesurfaceishandtrowelfinishedandwhenitdoesn’tcontainasurfacehardener
n Broadcastingcementbackontothesurfaceoftheconcretetodryupbleedwater
n Utilizingcalciumchlorideinhotdryweather
How to avoid problems with Crazing:n Startcuringassoonaspossible.Wetcuringproceduresthatensurethe
concretesurfaceremainscontinuouslywetduringthecuringperiodaremosteffective
n Avoidintermittentcuringprocedures
n Donotapplycementtothesurfaceoftheconcretetoeliminateexcessivebleeding.Ordertherequiredslumpfromyourconcreteproducerandutilizechemicaladmixtureslikewaterreducersto eliminateexcessivebleeding
n Utilizeinitialprotectionmethodstopreventthedryingoutoftheconcretesurfaceduringthefinishingperiod.Thiscanincludetheuse ofevaporationretardantchemicalsorfogspraying
References:1. ConcreteinPractice#3–CrazingConcreteSurfaces–NationalReadyMixed
ConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
CrazingCrazing
Crazingonaresidentialgaragefloorslab
Crazingonexteriorflatwork
Photos courtesy of CAC and RMCAO
Problem:
7
Definition:Amouldedlayerofplainorreinforcedconcretewithanon-uniformsurfaceduetodifferentialconcretesetting.
Wavyconcretesurfacesorconcretecrustingoccurswhenthetopsurfaceof theconcretebeginstosetwhiletheunderlyingconcreteisstillinaplasticstate.Surfacecrustingiscausedbydifferentialstiffeningorsettingoftheconcrete. Athinportionofthetopsurfaceoftheconcretebeginstosetwhiletheunderlyingconcreteisstillplastic.Surfacefinishingoperationsmustthenbecompletedoverajellylikeconcretebaseresultingindifferentialmovementoftheslabsurfaceduringfinishingoperationswhichresultsinwavyand/orcrackedconcretesurfaces.
Crusting Surfaces are more likely to occur when:n Therearesignificanttemperatureand/ormoisturedifferencesbetweenthe
bottomandtopoftheslab
n Concreteisplacedonacoldsubgrade.Thecoldsubgradeactsasaheatsinkwhileportableheatersarebeingusedonthetopsurfaceoftheslabtomaintainthenecessaryplacementandcuringtemperatures
n Environmentalconditionsareresultinginrapidsurfacemoistureloss.Highwinds,directsunlightandlowrelativehumidityconditionsresultinconcretesurfacestiffeningduringthedryingprocess
n Concretemixdesignswithlowbleedingcharacteristicsaremorepronetoconcretecrusting
n Chemicalretardersareusedintheconcrete
How to avoid problems with Crusting Surfaces:n Minimizetemperaturedifferentialsbetweentheconcretesurfaceandthe
subgradeorformwork.Duringwintermonthsthesubgrademustbeproperlyheatedwellbeforetheconcreteplacementandduringsummermonthsworkshouldbescheduledtoavoidplacingconcreteduringperiodswhentheairtemperatureisrisingsignificantly
n Utilizeproperinitialcuringprotectionstrategiessuchas“fogspraying”and“evaporationretardant”compoundtominimizemoisturedifferentialsbetweenthebottomandtopoftheslab
n Considertheuseoffinishingtoolsthatopenthesurfaceoftheconcreteanddon’tresultinsurfacesealing.Delayfloatingaslongaspossiblesincethisis theoperationthatcreatesmostofthehumps
n Ensurethatallfloatingandtrowelingequipmentisusedinaflatpositiontominimizethechanceofsealingthesurface
n Considertheuseofchemicaladmixturestoacceleratethesettingrateoftheconcrete.Donotusechemicalretarderstodelaythesettingtimeoftheconcretesincethiswillonlyintensifytheconcretecrustingproblem.Minimizeanyvariationsbetweenloadsofconcreteandavoidunloadingdelays
n Delaypowertrowelingaslongaspossibletominimizethewavinessthatwill becreatedduringthefinishingoperations
n Usethepansonpowertrowelstobreakopentheconcretesurfaceimmediatelyafterthebleedwaterperiodendstopromoteuniformconcretestiffening
References:1. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup2. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute3. TroubleshootingCrustedConcrete–BruceSuprenant–ConcreteConstruction4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Crusting Surfaces (Wavy Surface)Crusting Surfaces (Wavy Surface)
Walkbehindpowertrowel
Rideonpowertrowel
Photos courtesy of CAC and RMCAO
Anexampleofawavyconcretesurface
Problem:
8
Photos courtesy of CAC and W. R. Grace
Definition:Adeviationofaslabfromitsoriginalshape,usuallycausedbyeithertemperatureormoisturedifferentialsorbothwithintheslab.
Curlingisthedistortionofaconcreteslabintoacurvedshapebyeitherupwardsordownwardsbendingoftheedgesofslab.Thisoccurswhenthetopandbottomoftheslabchangesize(shrink)atdifferingrates.
Curling is more likely to occur when:n Thereisasignificantdifferenceineithermoisturecontentor
temperaturebetweenthetopandbottomoftheslab
n Theconcreteslabisrelativelythin
n Whenthedistancebetweenjointsislarge
n Theslabisunreinforced
How to avoid problems with Curling:n Utilizethebestpossiblecuringsystemandconsiderextendingthe
curingperiod
n Usethelowestpracticalwaterslumpandavoidretemperingwithwater
n Avoidtheuseofvapourbarriersbeneaththeslab
n Implementmixdesignprincipalsthatminimizeconcreteshrinkage (thelargestpossiblecoarseaggregatesize,minimizethecementcontent,etc.)
n Useasmallerjointspacingtoreducethesizeoftheresultingconcretepanels
n Considerincreasingthethicknessoftheconcreteslaband/orutilizeslabreinforcementtoreducecurling
n Considertheuseofspecialtychemicaladmixturestoreduceconcreteshrinkage/curling
References:1. ConcreteinPractice#19–CurlingofConcreteSlabs–NationalReadyMixed
ConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
CurlingCurling
Illustrationofcurlingofaconcreteslabongrade
Exampleofdifferentlevelsofconcretecurlingbasedupontwodifferentmixdesigns
Problem:
9
Definition:Thedepartureorvariationofcolourfromthatwhichisnormalordesired.
Discolourationcantakemanyformsinconcrete.Theseformsinclude:grosscolourchangesoverlargeareasofconcretetypicallycausedbychangestotheconcretemix;lightpatchesofdiscolourationcausedbyefflorescence;darkblemishesoramottledsurfaceappearance.
Discolouration is more likely to occur when:n Therearesignificantchangestothemixdesignrelatedtotheraw
materials(cementitiousmaterials,aggregates,admixtures,etc.)
n Therearesignificantchangesintherateofhydrationovertheconcreteslabtypicallyduetoinconsistentcuringperiodsoroperations
n TherearesignificantvariationsintheW/CMratiooftheconcrete
n Chemicaladmixturessuchascalciumchlorideareusedintheconcrete
n Theflatworkfinisherimproperlyestimatesthetimingofthefinishingoperationsresultinginahard-troweledsurface
How to avoid problems with Discolouration:n Eliminatetheuseofcalciumchloride.Concreteisfarmorelikelyto
discolourwhencalciumchlorideisused
n Conductfinishingoperationsatthecorrecttime.Earlyfinishingtendstoelevatethewater/cementingmaterialsratioatthetopsurfaceandlightenthecolour.Latefinishingtendstolowerthewater/cementingmaterialsratioatthetopsurfaceanddarkenthecolour
n Implementauniformcuringsystemforthenecessarytimeperiod.Unevencuringdirectlyaffectsthedegreeofhydrationofthecement andthecolouroftheconcrete
n Ensurethatplasticsheeting,whenusedforcuringpurposes,doesnotcomeindirectcontactwiththeconcrete.Plasticsheetingtendstoleavecolourstreaksontheconcretesurfacewhereitisindirectcontactwiththeconcrete
n Utilizeeffectiveformreleaseagentsthatpreventthenon-uniformloss ofmoisturefromtheformedsurface
n Ensurethattherawmaterialsourcesarenotvariedduringtheconcreteplacement.Architecturalconcreteprojectsmayrequirethestockpilingofrawmaterialsattheconcreteplant(ataconsiderablecost)fortheentireproject
References:1. ConcreteinPractice#23–Discoloration–NationalReadyMixedConcreteAssociation2. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute3. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
DiscolourationDiscolouration
Discolouringofaresidentialdrivewayduetothe useofcalciumchloride(accelerator)inonlyoneof thetwoloads
Theproperapplicationofacuringcompound
Photos courtesy of CAC
Problem:
10
Definition:Thedevelopmentofapowderedmaterialatthesurfaceofhardenedconcrete.
Dustingistheformationofloosepowdercausedbythedisintegrationofthetopsurfaceoftheconcreteduetoanextremelyweaksurface.
Dusting is more likely to occur when:n Anyfinishingoperationisperformedwhilethereisbleedwateronthe
topsurfaceoftheslab
n Theconcreteisnotproperlycuredresultinginanextremelyweaksurface
n Theconcreteisexposedtocarbondioxidewhilestillinitsplasticstate
n Theconcretesurfaceisnotprotectedduringtheinitialconcreteplacement(wind,rain,snow,freezing)
How to avoid problems with Dusting:n Donotperformanyfinishingoperationsontheconcretewhilebleed
waterisstillpresentonthesurface.Ifthefinishingoperationsworkthebleedwaterbackintotheconcretetheyelevatethew/cmratioofthesurfacelayersignificantlyanddramaticallyreducethesurfacelayersstrength
n Initiatepropercuringpracticesassoonaspossibleandcuretheconcreteforthespecifiedperiod
n Avoidplacingconcreteonvapourbarrierssincetheycanextendthebleedingperiodintotheinitialset/finishingperiod
n Ensurethatallheatingdevicesandconstructionequipmentisventedoutsidetheworkareatoavoidcarbondioxidebuild-upabovetheslabwhiletheconcreteisstillplastic
n Addingwatertotheconcretesurfacetoaidthefinishingoperationswillsignificantlyweakenthesurfaceandincreasetheproblemswithdusting
References:1. ConcreteinPractice#1–DustingConcreteSurfaces–NationalReadyMixed
ConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
DustingDusting
Anexampleofadustingconcretesurface
Properconcretecuringpracticessignificantly reducedustingproblems
Photos courtesy of CAC and RMCAO
Problem:
11
Photos courtesy of CAC and RMCAO
Definition:Voidsleftinconcreteduetofailureofthemortartoeffectivelyfillthespacesamongcoarseaggregateparticles.
Honeycombingofconcretetypicallyoccurswhentheconcreteisallowedtosegregateduetoobstructionsintheflowofconcreteduringplacementorduetoinadequateconsolidationtechniquesaftertheconcreteisplaced.Dependingontheextentandlocationofthehoneycombing,thisdefectcanbeeitheracosmeticorstructuraldefectinthefinalproduct.
Honeycombing is more likely to occur when:n Theconcreteelementcontainsreinforcingsteelthatlimitstheabilityof
theconcretetoflowthroughtheformwork
n Thereislimitedaccessorlocationsforplacingconcreteintheformworkresultinginlargespacingsbetweenconcreteplacementlocations
n Highconcentrationsofreinforcingsteelpreventtheinsertionofinternalconcretevibratorsinkeystructurallocations
n Largesizecoarseaggregateisusedinthemixdesignandreinforcementispresentintheformwork
n Concretemixdesignsarenotproperlyproportionedforthenecessaryflowabilityandworkability
How to avoid problems with Honeycombing:n WorkwiththeEngineerandthereinforcingsteelcontactortominimize
restrictionsduetoreinforcingsteel.Thiscanincludetheresizingoftherebarandavoidingrebarlapsplicesinkeystructuralareas
n Ensurethatoptimumconcretevibrationpracticesarefollowed.Thisincludesdecreasingtheheightofthelifts,reducingthedistancebetweenvibratorinsertions,etc.
n Designingorre-engineeringtheconcreteelementtoallowforacceptableconcreteplacementmethodsandlocations
n Allowingthecontractortoselecttheappropriateconcreteslumpfortheplacementtechniquebeingutilized
n Designingthemixtooptimizeflowabilityandtominimizethecoarseaggregatesize
n Consideringtheuseofspecialtyconcreteproductslikeselfconsolidatingconcrete(SCC)
References:1. PlacingandVibratingPouredConcreteWalls–ConcreteConstruction2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
HoneycombingHoneycombing
Honeycombinginaresidentialshearwall
Honeycombinginaconcretecolumn
SelfConsolidatingConcrete(SCC)canbeused tosuccessfullyaddresschallengingconcreteplace-ments.However,pleasekeepinmindthatthequalityoftheformworkisalsocriticalsincetheformworksurfacewillbeperfectlymirroredintheconcrete element
Problem:
12
Definition:Whenthetestresultsfromtheconcretecylindersorbeamsusedasthebasisofmaterialacceptancefailtomeetthestrengthrequirementsof CSAA23.1Clause4.4.6.6.1.
CSAA23.1recognizesthefactthatwhenconcreteisproportionedtomeettherequirementsofthestandardlowstrengthresultswilloccuraboutonceortwiceperevery100tests.AcceptableconcretecomplieswiththerequirementsofCSAA23.1Clause4.4.6.6.1when:
n Theaveragesofallsetsofthreeconsecutivestrengthtestsforthatclassofconcreteequalorexceedthespecifiedstrength
n Noindividualtestresultismorethan3.5MPabelowthespecifiedstrength
Low Concrete Strength Results are more likely to occur when:n Theconcretetestingisperformedbyunqualifiedpersons
n CSAA23.1/.2concretetestingrequirementsarenotfollowed
n Whentestspecimensareimproperlyhandled,curedandtested
n Thejobsiteadditionofexcessiveamountofwaterisallowed
n Thewrongconcreteisorderedfortheconcreteplacement
How to avoid problems with Low Concrete Strength Results:n EnsurethatallconcretefieldtestingisonlyperformedbyCSAorACI
Certifiedconcretetechnicians
n EnsurethatallconcretetestingisperformedinstrictaccordancewithCSAA23.1/.2
n Verifythattheproperfacilitiesexistforthesitestorageoftestsamplespriortothestartofconcreteplacement.Everyjobsiteshouldincludeacuringboxwhichmaintainsthesamplesatatemperaturebetween15°Cand25°C
n Reviewtheconcretedeliveryticketforeveryloadofconcreteandcomparethattospecificationrequirementsfortheelementbeingconstructed
n Utilizethe“Pre-PourMeetingForm”toclearlydesignatethepersonwithauthorityforjobsitewateradditionandspecificallydefinetheconditionswhenwateradditionwillbeallowed
References:1. CIP#9–LowConcreteCylinderStrength–NationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Low Concrete Strength ResultsLow Concrete Strength Results
Concretecylindersusedforconcreteacceptancepurposesmustbestoredinatemperaturecontrolledenvironment-notlefttobakeinthesun
Anexampleofproperfieldcuringmethodsincludes theuseofatemperaturecontrolledcuringbox
Photos courtesy of RMCAO
13
Definition:Shrinkagecracksthatdevelopbeforethecementpaste,mortar,grout,orconcretesets.
Plasticshrinkagecracksarecausedbytherapidlossofwaterfromthesurfaceoftheconcretebeforetheconcretehashadtimetoset.Thesecracksformafterconcreteplacementandconsolidation,whiletheconcreteisstillplastic.Theyarerelativelyshallowindepthandtendtoforminparalleltooneanother.Thecracksareunappealingbutrarelycausesignificantdamagetothestrengthordurabilityofthefloorduetotheirshallowdepth.
Plastic Shrinkage Cracking is more likely to occur when:n Thereisarapidlossofmoisturefromthesurfaceoftheconcrete
n Thereislowrelativehumidity
n Thereishighambientairorconcretetemperatures
n Highwindsareblowingacrosstheslabsurface
n Limitedbleedwaterispresentintheconcreteresultinginrapidsurfacemoistureloss
How to avoid problems with Plastic Shrinkage Cracks:n Considererectingwindscreensorsunshadestoreducethesurface
evaporationrate
n Utilizefogsprayingequipmentontheup-windsideoftheslabtomaintaintherelativehumidityoftheslabat100%
n Considerpre-dampeningthesub-gradeduringhotdryweathertoreducethemoisturelossfromtheconcreteintothesubgrade
n Considertheuseofsyntheticfibrestoresistthetensileforcescreatedduringtheplasticshrinkageperiod
n Considerplacingtheconcreteduringoff-hourstoavoidhightemperature,highwind,highsunexposureperiods
n Considertheuseofevaporationretardantchemicalsappliedbetweenthevariousfinishingoperationstopreventtherapidlossofmoisturefromthesurfaceoftheconcrete
References:1. CIP#5–PlasticShrinkageCracking–NationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Plasticshrinkagecracksonexteriorflatwork
CSAA23.1definesSeveredryingconditionsaswhensurfacemoistureevaporationexceeds1.0kg(m2.hr)
Photos courtesy of CAC and RMCAO
Plastic Shrinkage CrackingPlastic Shrinkage CrackingProblem:
Problem:
14
Photos courtesy of CAC and RMCAO
Definition:Localflakingorpeelingawayofthenear-surfaceportionofhardenedconcreteormortar.
Scalingbeginsassmalllocalizedpatchesoftheconcretesurfacethatdebondsfromtheconcretetypicallyduetofreeze-thawexposure.Withtimetheselocalizedpatchescanexpandandmergetogethertoproducelargescaledareas.ThevariouslevelsofscalingaredefinedbytheAmericanConcreteInstitute(ACI)as:
n Light Scaling–Scalingoftheconcretethatdoesnotexposethecoarse aggregate
n Medium Scaling–Scalingoftheconcretethatinvolveslossofsurfacemortarto5to10mmindepthandexposureofthecoarseaggregate
n Severe Scaling–Scalingthatinvolveslossofsurfacemortarto5to10mm indepthwithsomelossofmortarsurroundingtheaggregateparticlesof10to20mmindepth
n Very Severe Scaling–Scalingthatinvolveslossofcoarseaggregateparticlesaswellasmortargenerallytoadepthgreaterthan20mm
Scaling is more likely to occur when:n Theconcretecontainsinsufficientairentrainmentorisnon-airentrained
n Theconcretehasnotbeenproperlycured
n Theconcreteisexposedtodeicingchemicalsofanytype
n Improperconcretehasbeenutilizedforexteriorapplications.TheOntario BuildingCodeandCSAA23.1bothspecifyaminimumof32MPa,0.45W/CM,airentrainedconcreteforexteriorapplicationsexposedtofreeze-thawcycles anddeicingchemicals
n Whenfinishingoperationsarecompletedwhilethebleedwaterisstillontheconcretesurface
n Theconcreteisplacedduringadverseweatherconditions(extremeheat, extremecold,duringtherain,etc.)
How to avoid problems with Scaling:n Orderandplacetheproperconcretefortheapplication(32MPa,0.45W/CM,
airentrainedconcrete)
n Properlycuretheconcrete.Exteriorconcreterequiresaminimumof7days curinginordertoensurethattheconcretesurfacedevelopsthenecessary durabilitypropertiestoresistscaling
n Avoidtheuseofdeicingchemicalsontheconcretesurface.Deicingchemicalsshouldnotbeusedatallduringthefirstwinterandonlysparinglyafterthat
n Avoidtheuseofdeicingchemicalsthatchemicallyattackconcrete
n Allowatleast30daysofairdryingoftheconcreteslabafterthecuringperiodbeforeexposingtheslabtofreeze-thawcycles.Theelevatedmoisturecontentofnewconcreteinitiallyreducestheeffectivenessoftheairentrainment
n Considertheproperuseofconcretesealerstoreducetheabsorptionofmoistureandchloridesintotheconcrete
n Utilizewetcuringmethodsduringhotweatherandseveredryingconditionsandfogspraytheconcretesurfacebetweenfinishingoperations
References:1. CIP#5–PlasticShrinkageCracking–NationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada5. OntarioBuildingCode–2000,OntarioMinistryofMunicipalAffairsandHousing–Housing
DevelopmentandBuildingsBranch
Lightscalingofaconcretesidewalk
Mediumscalingofaconcretesidewalk
Properconcretecuringpracticessignificantlyreduceinstancesofconcretescaling
ScalingScaling
15
Uncontrolled Shrinkage CrackingUncontrolled Shrinkage CrackingProblem:
Definition:Crackingofastructureormemberduetofailureintensioncausedbyexternalorinternalrestraintsasreductioninmoisturecontentdevelops,carbonationoccurs, orboth.
Concretebothexpandsandcontractswithchangesinmoistureandtemperatureanddeflectsdependingontheelementsize,reinforcing,loadingforceandsupportconditions.Allthesefactorscanleadtouncontrolledcrackingiftheproperdesignandjointdetailsarenotaddressedpriortothetimeofconstruction.Inordertopreventuncontrolledshrinkagecrackingthedesignerandthecontractorshouldeffectivelyutilizethefollowingconcretejointingsystems:
n Isolation Joints –Jointsthatpermitbothhorizontalandverticalmovementbetweentheslabandtheadjacentconcrete
n Contraction Joints –Jointsthatpermithorizontalmovementoftheslabandinducecontrolledcrackingatpre-selectedlocations
n Construction Joints –Jointsthatarestoppingplacesintheprocessofconstruction
Uncontrolled Shrinkage Cracking is more likely to occur when:n Thedesignerorcontractorhasnotdevelopedaproperjointingsystemandlayout
priortothestartofconcreteplacement
n Thesub-gradeisnotproperlypreparedandcompacted
n Thereisjobsiteadditionofexcessiveamountsofwater
n Improperfinishingproceduresareimplementedortheinstallationofcontractionjointsisnotcompletedinatimelyfashion
n Thereisinadequateconcretecuring
How to avoid problems with Uncontrolled Shrinkage Cracks:n Ensurethatthesubgradeisadequatelypreparedbasedupontheexposureand
loadingconditionsthattheslabwillbeexposedtoduringitsservicelife.Thisincludesusingtheappropriatethicknessofgranularsub-basefortheapplicationandproperlycompactingthesub-base
n Ensuringpositivedrainageofboththesub-gradeandtheconcretesurfacetoavoidthebuild-upofhighmoisturelevels
n Utilizingtheproperconcretefortheapplication(SeeCSAA23.1minimumdurabilityrequirements)
n Thedesignerandthecontractorshouldreviewtheconcretejointingdetailspriortothestartoftheproject.Itemstoconsiderincludeconcretethickness,maximumjointspacing(4.5mmax.),fibrereinforcement,structuralreinforcementandearlyinstallationofcontractionjoints
n Implementcuringproceduresassoonaspossibletoensurethattheperformancepropertiesoftheconcretefullydevelop
References:1. CIP#4–CrackingConcreteSurfaces–NationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTips–TheAberdeenGroup3. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute4. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Photos courtesy of CAC and RMCAO
Designandconstructionofaproperisolationjoint
Designandconstructionofpropercontractionjoints
Designandconstructionofproperconstructionjoints
Ifyoufailto properlydesignforshrinkageuncontrolledrandomcrackswilloccur
Problem:
16
Definition:Thedislodgingofsmallsectionsofconcretemortardirectlyovertopofthecoarseaggregateparticlesinexteriorconcreteexposedtofreeze/thawconditions.
Mortarflakingtypicallyoccursinexteriorflatworkthatwasnotproperlycuredduringthefirst7to28days.Mortarflakingistypicallyveryshallowindepthandconsistsofdistinctdelaminations(flakes)thatoccurdirectlyovertopofthecoarseaggregateparticlesintheconcrete.Thisistypicallyduetothefactthattheconcretemortardirectlyovertopofthecoarseaggregatewasallowedtodryoutandwasnotproperlycured.Anyexcessbleedwaterthatispresentintheconcreteisforcedtotravelaroundthecoarseaggregateparticlesasitrisestothesurfaceandthemortardirectlyabovefailstodevelopthenecessarystrengthrequiredtoachieveproperdurabilityandbond.
Mortar Flaking is more likely to occur when:n Theconcreteisplacedduringperiodsofrapidsurfacedrying
n Whentheconcreteisnotcuredatall
n Whencuringcompoundisappliedtoolateafterconcreteplacementornotuntilthefollowingday
How to avoid problems with Mortar Flaking:n Ensurethepropercuringproceduresarefollowedatalltimes!Proper
concretecuringshouldeliminatethisproblem
n Duringperiodsofrapidsurfacedryingconsiderusinginitialcuringprocedureslikeevaporationretardantsorplacingplasticsheetingovertheconcretesurfacebetweenfinishingoperations
n Considertheuseofwetcuringmethodsduringseveredryingandhotweatherconditions
n Considertheuseoffogsprayingsystemstoprotectthesurfaceoftheconcretefromdryingoutuntilwetcuringcanbeinitiated
References:1. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute2. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Mortar FlakingMortar Flaking
Mortarflakingonaconcretesidewalk–Notethattheaggregateissoundandonlythemortarabovetheaggregatehasfailed
Cross-sectionthroughamortarflake–notethatonlythemortarabovethecoarseaggregatehasfailed
Photos courtesy of RMCAO and CTL
17
Photos courtesy of RMCAO
Definition:Conicalfracturesoftheconcretesurfacecausedbytheexpansionoficeinsideporouscoarseaggregateslocatedjustbelowtheconcretesurface.
Popoutsresultwhensoftporousaggregates,foundbothinlimestoneandgravelsources,undergofreeze/thawconditionsinasaturatedstate.Whileairentrainmentprovidesprotectionforthecementpastefromtheexpansiveforcesgeneratedduringwinterfreeze/thawcycles,itdoesnotprovideanyprotectionforthecoarseaggregateparticles.Ifthelocalaggregatesourcescontainevenasmallpercentageofsoft,porousparticles(likechert),thecoarseaggregatecanabsorbsignificantquantitiesofmoistureandfracturesduringfreeze/thawcycles.Theresultingexpansionfracturesboththeaggregateandthesurroundingconcrete.Popoutsaredistinguishedfrommortarflakingbythepresenceoffracturedaggregateinthepopout(asshowninthesamplephoto).
Popouts are more likely to form when:n Thelocalaggregatesourcescontainsoft,porousmaterial
n Theconcreteisinafullysaturatedcondition
n Theconcretehasn’tachieveditsultimatedurabilitypotentialduetoinadequatecuring
How to avoid problems with Popouts:n Considertheuseofalternativeaggregatesources.Thismayrequirethe
importofaggregatesatasignificantadditionalexpense
n Considerchangestotheaggregateprocessingprocesstofurtherreducethequantityofsoftporousmaterial(thecompleteeliminationofallsoftmaterialisnotnormallypossible)
n Ensurethattheoveralldesignminimizesthebuild-upofexcessmoistureintheconcrete.Thiscanbeachievedthroughpropersurfacegradinganddrainingofthesurroundingsubgrade
n Ensurethatproperconcretecuringandprotectionpracticesarefollowedtoachievethehighestpossibleconcretedurability
n Considertheuseofsealerstolimittheinflowmoistureintotheconcrete
References:1. ACI116–CementandConcreteTerminology–AmericanConcreteInstitute2. DesignandControlofConcreteMixtures–7thEdition–CementAssociationofCanada
Popoutsurfacedamagecasedbythefractureofasoftaggregateparticleduringwinterfreeze/thawcycles
Across-sectionalviewofaconcretecoretakenfromapopoutarea–notethelargechertparticleinthislimestoneaggregatesourcethatisresponsibleforthepopout
PopoutsPopoutsProblem:
To Obtain Multiply By To Obtain Multiply By
millimetres inches 25.40centimetres inches 2.54centimetres feet 30.45metres feet 0.3048metres yards 0.9144kilometres miles 1.6093inches millimetres 0.03937inches centimetres 0.3937inches metres 39.37feet metres 3.281feet kilometres 3280.90yards metres 1.094yards kilometres 1094.60miles kilometres 0.61
grams ounces 28.35kilograms pounds 0.4536kilograms shorttons 907.185kilograms longtons 1016.05tonnes pounds 0.0004536tonnes shorttons 0.9072ounces grams 0.035pounds kilograms 0.2205pounds tonnes 2204.59shorttons kilograms 0.0011shorttons tonnes 1.1023longtons kilograms 0.0010longtons tonnes 0.9842
squareinches centimetres2 0.1550squarefeet centimetres2 0.00108squarefeet metres2 10.764squareyards squarefeet 0.1111squareyards metres2 0.3987acres metres2 0.000247acres hectares 2.471squaremiles hectares 0.003861centimetres2 squareinches 6.4529metres2 squarefeet 0.0929metres2 squareyards 0.8361kilometres2 squaremiles 2.59kilometres2 acres 0.00404
cubicinches cm3 0.06104cubicinches litres 61.02cubicfeet m3 35.3143cubicfeet litres 0.3532cubicyards m3 1.3079Imp.gallons cubicfeet 6.236Imp.gallons litres 0.22Imp.gallons m3 219.97cm3 cubicinches 16.387cm3 cubicfeet 28.32m3 Imp.gallons 3.154m3 cubicyards 0.764
°C °F 5/9(°F-32)°F °C (9/5°C)+32
MPa psi 0.006895psi MPa 145
LEN
GTH
AR
EA
TEM
PE
RA
TUR
E
WE
IGH
T /
MA
SS
VO
LUM
E /
CA
PA
CIT
YP
RE
SS
UR
E
Conversion TableConversion Table
18
19
Concrete Application Class ofConcrete
MaximumW/CM
Minimum28d Strength
(MPa)
Chloride IonPermeabilityat 56 days
ReferenceStandard
Agr
icul
tura
l
Reinforcedbeams,slabsandcolumnsover manurepitsexposedtoseveremanuregases
A–1 0.40 35 1500Coulombs CSAA23.1-09
Reinforcedwallsinexteriormanuretanks,silos,andfeedbunkersexposedtoseveretomoderatemanuregases
A–2 0.45 32 N/A CSAA23.1-09
Structurallyreinforcedinteriorwalls,beams,slabsandcolumnsinacontinuouslysubmergedconditionexposedtomoderatetoseveremanuregases
A–3 0.50 30 CSAA23.1-09
Non-structurallyreinforcedslabsongrade exposedmoderatemanuregasesorliquids withoutfreeze-thawexposure
4–4 0.55 25 CSAA23.1-09
Indu
stri
al /
Com
merc
ial /
Ins
titu
tion
al
HighPerformanceConcrete C–XL 0.40 50within56days 1000Coulombs CSAA23.1-09
Structurallyreinforcedconcreteexposedto chlorides(Bridgedecks,parkingdecks,etc.)
C–1 0.40 35 1,500Coulombs CSAA23.1-09
Non-structurallyreinforcedconcreteexposedtochloridesandfreezingandthawing(Garagefloors,pavements,sidewalks,curbs,etc.)
C–2 0.45 32 CSAA23.1-09
Continuouslysubmergedconcreteexposedtochloridesbutnotfreezingandthawing(Under- waterportionsofmarinestructures)
C–3 0.50 30 CSAA23.1-09
Non-structurallyreinforcedconcreteexposedtochloridesbutnotfreezingandthawing(Under-groundparkingslabsongrade)
C–4 0.55 25 CSAA23.1-09
Concreteexposedtofreezingandthawinginasaturatedconditionbutnottochlorides(Pooldecks,patios,tenniscourts,etc.)
F–1 0.50 30 CSAA23.1-09
Concreteinanunsaturatedconditionexposedtofreezingandthawingbutnottochlorides(Exteriorwallsandcolumns)
F–2 0.55 25 CSAA23.1-09
Concretenotexposedtochloridesorfreezingandthawing(Footingsandinteriorslabs,wallsandcolumns)
N SpecifiedbytheDesigner CSAA23.1-09
Resi
dent
ial
Footingsforwalls,columnsandchimneys R–1 0.7015
(3–6%air)CSAA23.1-09
Foundations,walls,gradebeams,piers,etc. R–2 0.7015
(4–7%air)CSAA23.1-09
Interiorslabsongroundnotexposedtofreeze-thawwithoutavapourbarrierbeneaththeslab
R–3 0.65 20 CSAA23.1-09
Garagefloorsandallconcreteexposedtofreezingandthawinganddeicingchemicals(Driveways,sidewalks,patios,steps,etc.)
C–2 0.45 32 CSAA23.1-09
OwnermayorderconcretetoPerformance Specifications(concretesupplierisresponsibleforconcreteasdelivered)orPrescriptive Specifications(Ownerisresponsibleforconcreteasdelivered).
DetailedConcreteMixProportionsshallnotbedisclosedbytheconcretesupplier.
CONCRETE PLANT CERTIFICATION SPECIFICATION: “The concrete supplier shall submit to the concrete purchaser a currently valid Certificate of Ready Mixed Concrete Production Facilities or a currently valid Certificate of Mobile Mix Concrete Production Facilities as issued by the Ready Mixed Concrete Association of Ontario to the plant being used.”
References:1. OntarioBuildingCode-2006,OntarioMinistryofMunicipalAffairsandHousing-HousingDevelopmentandBuildingsBranch2. CSAA23.1-09ConcreteMaterialsandMethodsofConcreteConstruction,CanadianStandardsAssociationInternational
WiththepermissionofCanadianStandardsAssociation,materialisreproducedfromCSAStandardA23.1-09/A23.2-09, Concrete Materials and Methods of Concrete Construction/Methods of Test and Standard Practices for ConcretewhichiscopyrightedbyCanadianStandardsAssociation,178RexdaleBlvd.,Toronto,Ontario,M9W1R3.Whileuseofthismaterialhasbeenauthorized,CSAshallnotberesponsibleforthemannerinwhichtheinformationispresented,norforanyinterpretationsthereof.A23.1/23.2-04isintendedtobeusedinitsentirety.Cautionshouldbeexercisedinextractingindividualclausesortablesandusingtheminprojectspecifications,sincetakingthemoutofcontextcanchangetheirmeaning.For more information on CSA or to purchase standards, please visit their website at www.shopcsa.ca or call 1-800-463-6727.
Concrete Quick SpecConcrete Quick Spec
www.rmcao.org
THISINFORMATIONTOBEUSED ASAGUIDELINEONLY.FORMORE DETAILEDINFORMATIONOR SPECIFICATIONS,PLEASECONSULT REFERENCES.
Photo’scourtesyof:CAC-CementAssociationofCanadaCTL-ConstructionTechnologyLaboratories,Inc.RMCAO-ReadyMixedConcreteAssociationofOntario
Preparedby:
Ready Mixed Concrete Association of Ontario365BrunelRoad,Unit#3Mississauga,OntarioL4Z1Z5Tel:905.507.1122Fax:905.890.8122Email:[email protected]
©2010RMCAO.Allrightsreserved.07/10
Top Related