Concrete Construction Troubleshooting Tips - Rainbow Concrete - Main

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


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


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


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


DustingDusting

Anexampleofadustingconcretesurface

Properconcretecuringpracticessignificantly reducedustingproblems


Problem:

11


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


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)


Plastic Shrinkage CrackingPlastic Shrinkage CrackingProblem:

Problem:

14


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


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


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

Concrete Construction Troubleshooting Tips - Rainbow Concrete - Main

Documents