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ConcreteConstructionTroubleshootingTips
Problems
AirLossDuringConcretePumping
Blistering
Bugholes(SurfaceAirVoids)
Crazing
CrustingSurfaces(WavySurfaces)
Curling
Discolouration
Dusting Honeycombing
LowConcreteStrengthResults
PlasticShrinkageCracking
Scaling
UncontrolledShrinkageCracking
MortarFlaking
Popouts
Aids
ConversionTable
ConcreteQuickSpecications
www.rmcao.org
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Problem:
Defnition:
Changesintheaircontentoftheconcreteduetoconcretepumping
operations.
Concreteairentrainmentrequirementsaretypicallybaseduponconcretesampledandtestedfromconcretesuppliedattheendofthechuteof
theconcretetruck.Whenconcreteisplacedviaaconcretepumpand
theacceptancesamplesareobtainedattheendofthepumpthereare
anumberofadditionalfactorsthatthecontractormustconsidersince
pumpingoperationscanreducetheaircontentoftheconcrete.
Air Loss During Concrete Pumping is
more likely to occur when:
n Theconcreteundergoessignicantverticaldrops
n Theconcretehasahighinitialslump
n Thepumpingpressureisincreasedandthereisasuddenpressuredrop
astheconcreteexitsthepumphose
How to avoid problems with Air Loss
During Concrete Pumping:
n Operatetheconcretepumpingboominanupwardorhorizontalposition
asmuchaspossible
n Reduceverticalconcretedropstotheshortestpossiblelength
n Insertmultiple90elbowsintothelinetocreatearesidualamountof
concreteinthelineandtominimizethelongstraightdropoutofthe
pumpingline
n Utilizeagateorowrestrictingdeviceattheendofthepumplineto
provideresistancetoow
n Runthelast35metresofthepumpinghosehorizontallyalongthe
subgradeorformworktoavoidverticaldrops
n Usingcable,looptheendsectionoftherubberhoseintoacircleto
preventdirectfreefalloutofthehose
n Elevatetheplasticaircontentabovethespecicationlimitsattheconcretetrucklocationtoaccountfortheactualairloss(Caution
Inspectionoftheaircontentatboththetruckandthepumpinglocation
becomesextremelycriticalwheneverthereisachangeintheboom
congurationhigheraircontentsinthenalconcretehaveanegative
impactoncompressivestrength)
Concretepumpingonacommercialprojectusingaowrestrictiondevicetoprovideresistancetoowandtopreventdirectfreefall
Photos courtesy of RMC
Concretepumpingonaresidentialradiantoorheatingproject
References:1. ConcreteinPractice#21LossofAirContentinPumpedConcreteNationalReady
MixedConcreteAssociation
2. ACI116CementandConcreteTerminologyAmericanConcreteInstitute3. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Air Loss During Concrete PumpingAir Loss During Concrete Pumping
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Problem:
4
Defnition:
Theirregularraisingofathinlayeratthesurfaceofplacedmortaror
concreteduringorsoonaftercompletionofthenishingoperation.
Blistersformjustunderthesurfaceoftheconcretewheneitherbleedwaterorentrappedairispreventedfromescapingtheconcrete.Thistypically
occurswhenthetopsurfaceoftheconcretehasbeensealedprematurely
duringthenishingoperations.
Blisters are more likely to orm when:
n Thesubgradeiscoolerthantheconcreteresultinginnon-uniformsetting
oftheconcrete
n Insufcientorexcessivevibrationisemployed
n Theconcretesurfaceisprematurelysealedduetoimpropernishing
proceduresortools
n Siteconditionsareresultinginrapidsurfaceevaporationcausingthe
nishertomistakenlyassumethebleedingperiodiscomplete
n Entrainedairisaddedtononair-entrainedconcreteorwhentheair
entrainmentisabnormallyhigh
n Theconcreteelementisextremelythickresultinginlongertimesfor
bleedwaterandentrappedairtorisetothesurface
n Adryshakeisprematurelyappliedtotheconcretesurface
n Theconcreteisplacedoveravapourbarrier
How to avoid problems with Blistering:
n Utilizeproperconcreteconsolidationpractices
n Utilizethepropernishingtoolsandkeepthetrowelingbladesasataspossibletoavoidsealingthesurface
n Placeconcreteduringperiodsoflowersurfaceevaporation
n Avoidplacingconcreteoncoldsub-gradeswheneverpossible
n Avoidtheuseofchemicalretarderssincetheywillonlyintensifytheproblem
n Keeptrowelbladesataslongaspossibletoavoidsealingthesurfaceprematurely
n Considertheuseofchemicalacceleratorstoreducethesettime
n Properlyheatthebuildingand/orsubgradepriortoconcreteplacement
n Avoidplacingconcreteontopofvapourbarrierswheneverpossible
References:1. ConcreteinPractice#13ConcreteBlistersNationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
BlisteringBlistering
Cross-sectionthroughaconcreteblister
Blisteringconcretesurface
Photos courtesy of CAC and CTL
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Problem:
Photos courtesy of CAC and RMC
Defnition:
Smallregularorirregularcavities,usuallynotexceeding15mmindiameter,
resultingfromtheentrapmentofairbubblesinthesurfaceofformed
concreteduringplacementandconsolidation.
Bugholesappearassmallvoidontheoutsideofformedconcretesurfaces.
Thesedefectsaretypicallyduetothepresenceofsmallairpocketsthat
werenotremovedduringconcreteconsolidationoperationsorbythe
incompleteapplicationoftheformreleaseagenttoformworkresultingin
concreteadhesionduringtheformremovalprocess.
Bugholes are more likely to orm when:
n Theconcreteisnotadequatelyconsolidatedtoremoveallentrappedair
n Internalconcretevibratorinsertionsarenotsufcientlyoverlappedor
wellspaced
n Formreleaseagentsarenotusedortheformsareremovedbefore
theconcretehasgainedsufcientstrengthtoresisttheformremoval
pressures
n Concretemixturesareverystifforsticky
How to avoid problems with Bugholes:
n Uniformlyconsolidatetheconcreteensuringthatthevibratorinsertionpatternisuniformandwellspaced
n Reducetheheightofeachconcreteplacementlifttomakeairremoval
easer
n Movethevibratorasclosetotheformworkaspossible
n Properlyapplytheformreleaseagentandonlyremovetheformsonce
theconcretehasgainedsufcientstrengthtoresistanysuctionforces
n Considertheuseofspecialtyconcreteproductslikeselfconsolidating
concrete(SCC)
References:1. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
2. ACI116CementandConcreteTerminologyAmericanConcreteInstitute3. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Bugholes (Surace Air Voids)Bugholes (Surace Air Voids)
Bugholesonaplywoodformedsurface
Bugholesonaconcretesurfacecreatedwithaformliner
SelfConsolidaConcrete(SCCsignicantlyreducesbughoandcanbeusinarchitecturaconcreteapplications
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Problem:
6
Defnition:
Intersectingcracksthatextendbelowthesurfaceofhardenedconcrete;
causedbyshrinkageofthedryingsurfaceconcretethatisrestrainedby
concreteatgreaterdepthswhereeitherlittleornoshrinkageoccurs;varyinwidthfromneandbarelyvisibletoopenandwell-dened.
Crazingcracksarenerandomcracksonthesurfaceoftheconcretethat
donotaffectthestructuralintegrityoftheconcretebutwhicharevery
unsightly.Thesecracksrarelyaffectthedurabilityorwearresistanceofthe
concreteandareparticularlyevidentwhentheconcretecontainscalcium
chlorideasanacceleratingadmixture.
Crazing cracks are more likely to orm when:
n Theconcreteisnotproperlycuredandprotected.Thesecracksformdue
tothetopsurfaceoftheconcretedryingoutandshrinkingfasterthantheremainderoftheconcrete
n Theconcreteisexposedtorapidorprolongedsurfacedrying(low
humidity,hightemperature,directsunlight,dryingwinds,etc.)
n Thereisexcessoatingortheuseofajitterbugtodepressthecoarse
aggregateresultingintoomuchcementpasteatthesurface
n Theconcretesurfaceishandtrowelnishedandwhenitdoesntcontain
asurfacehardener
n Broadcastingcementbackontothesurfaceoftheconcretetodryup
bleedwater
n Utilizingcalciumchlorideinhotdryweather
How to avoid problems with Crazing:
n Startcuringassoonaspossible.Wetcuringproceduresthatensurethe
concretesurfaceremainscontinuouslywetduringthecuringperiodare
mosteffective
n Avoidintermittentcuringprocedures
n Donotapplycementtothesurfaceoftheconcretetoeliminate
excessivebleeding.Ordertherequiredslumpfromyourconcrete
producerandutilizechemicaladmixtureslikewaterreducerstoeliminateexcessivebleeding
n Utilizeinitialprotectionmethodstopreventthedryingoutofthe
concretesurfaceduringthenishingperiod.Thiscanincludetheuseofevaporationretardantchemicalsorfogspraying
References:1. ConcreteinPractice#3CrazingConcreteSurfacesNationalReadyMixed
ConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
CrazingCrazing
Crazingonaresidentialgarageoorslab
Crazingonexterioratwork
Photos courtesy of CAC and RMCAO
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Problem:
Defnition:Amouldedlayerofplainorreinforcedconcretewithanon-uniformsurfacedueto
differentialconcretesetting.
Wavyconcretesurfacesorconcretecrustingoccurswhenthetopsurfaceoftheconcretebeginstosetwhiletheunderlyingconcreteisstillinaplasticstate.Surfacecrustingiscausedbydifferentialstiffeningorsettingoftheconcrete.Athinportionofthetopsurfaceoftheconcretebeginstosetwhiletheunderlying
concreteisstillplastic.Surfacenishingoperationsmustthenbecompletedovera
jellylikeconcretebaseresultingindifferentialmovementoftheslabsurfaceduring
nishingoperationswhichresultsinwavyand/orcrackedconcretesurfaces.
Crusting Suraces are more likely
to occur when:n Therearesignicanttemperatureand/ormoisturedifferencesbetweenthe
bottomandtopoftheslab
n Concreteisplacedonacoldsubgrade.Thecoldsubgradeactsasaheatsink
whileportableheatersarebeingusedonthetopsurfaceoftheslabtomaintain
thenecessaryplacementandcuringtemperatures
n Environmentalconditionsareresultinginrapidsurfacemoistureloss.Highwinds,
directsunlightandlowrelativehumidityconditionsresultinconcretesurface
stiffeningduringthedryingprocess
n Concretemixdesignswithlowbleedingcharacteristicsaremoreproneto
concretecrusting
n Chemicalretardersareusedintheconcrete
How to avoid problems with Crusting Suraces:n Minimizetemperaturedifferentialsbetweentheconcretesurfaceandthe
subgradeorformwork.Duringwintermonthsthesubgrademustbeproperly
heatedwellbeforetheconcreteplacementandduringsummermonthswork
shouldbescheduledtoavoidplacingconcreteduringperiodswhentheair
temperatureisrisingsignicantly
n Utilizeproperinitialcuringprotectionstrategiessuchasfogsprayingand
evaporationretardantcompoundtominimizemoisturedifferentialsbetween
thebottomandtopoftheslab
n Considertheuseofnishingtoolsthatopenthesurfaceoftheconcreteand
dontresultinsurfacesealing.Delayoatingaslongaspossiblesincethisistheoperationthatcreatesmostofthehumps
n Ensurethatalloatingandtrowelingequipmentisusedinaatpositionto
minimizethechanceofsealingthesurface
n Considertheuseofchemicaladmixturestoacceleratethesettingrateof
theconcrete.Donotusechemicalretarderstodelaythesettingtimeofthe
concretesincethiswillonlyintensifytheconcretecrustingproblem.Minimize
anyvariationsbetweenloadsofconcreteandavoidunloadingdelays
n Delaypowertrowelingaslongaspossibletominimizethewavinessthatwillbecreatedduringthenishingoperations
n Usethepansonpowertrowelstobreakopentheconcretesurfaceimmediately
afterthebleedwaterperiodendstopromoteuniformconcretestiffening
References:1. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup2. ACI116CementandConcreteTerminologyAmericanConcreteInstitute
3. TroubleshootingCrustedConcreteBruceSuprenantConcreteConstruction4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Crusting Suraces (Wavy Surace)Crusting Suraces (Wavy Surace)
Walkbehindpowertrowel
Rideonpowertrowel
Photos courtesy of CAC and RMC
Anexampleofawavyconcretesurface
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Problem:
8
Photos courtesy of CAC and W. R. Grace
Defnition:
Adeviationofaslabfromitsoriginalshape,usuallycausedbyeither
temperatureormoisturedifferentialsorbothwithintheslab.
Curlingisthedistortionofaconcreteslabintoacurvedshapebyeitherupwardsordownwardsbendingoftheedgesofslab.Thisoccurswhenthe
topandbottomoftheslabchangesize(shrink)atdifferingrates.
Curling is more likely to occur when:
n Thereisasignicantdifferenceineithermoisturecontentortemperaturebetweenthetopandbottomoftheslab
n Theconcreteslabisrelativelythin
n Whenthedistancebetweenjointsislarge
n Theslabisunreinforced
How to avoid problems with Curling:
n Utilizethebestpossiblecuringsystemandconsiderextendingthecuringperiod
n Usethelowestpracticalwaterslumpandavoidretemperingwithwater
n Avoidtheuseofvapourbarriersbeneaththeslab
n Implementmixdesignprincipalsthatminimizeconcreteshrinkage(thelargestpossiblecoarseaggregatesize,minimizethecement
content,etc.)
n Useasmallerjointspacingtoreducethesizeoftheresultingconcrete
panels
n Considerincreasingthethicknessoftheconcreteslaband/orutilizeslab
reinforcementtoreducecurling
n Considertheuseofspecialtychemicaladmixturestoreduceconcrete
shrinkage/curling
References:1. ConcreteinPractice#19CurlingofConcreteSlabsNationalReadyMixed
ConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
CurlingCurling
Illustrationofcurlingofaconcreteslabongrade
Exampleofdifferentlevelsofconcretecurlingbasedupontwodifferentmixdesigns
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Problem:
Defnition:
Thedepartureorvariationofcolourfromthatwhichisnormalordesired.
Discolourationcantakemanyformsinconcrete.Theseformsinclude:gross
colourchangesoverlargeareasofconcretetypicallycausedbychangestotheconcretemix;lightpatchesofdiscolourationcausedbyeforescence;
darkblemishesoramottledsurfaceappearance.
Discolouration is more likely to occur when:
n Therearesignicantchangestothemixdesignrelatedtotheraw
materials(cementitiousmaterials,aggregates,admixtures,etc.)
n Therearesignicantchangesintherateofhydrationovertheconcrete
slabtypicallyduetoinconsistentcuringperiodsoroperations
n TherearesignicantvariationsintheW/CMratiooftheconcrete
n Chemicaladmixturessuchascalciumchlorideareusedintheconcrete
n Theatworknisherimproperlyestimatesthetimingofthenishing
operationsresultinginahard-troweledsurface
How to avoid problems with Discolouration:
n Eliminatetheuseofcalciumchloride.Concreteisfarmorelikelyto
discolourwhencalciumchlorideisused
n Conductnishingoperationsatthecorrecttime.Earlynishingtends
toelevatethewater/cementingmaterialsratioatthetopsurfaceandlightenthecolour.Latenishingtendstolowerthewater/cementing
materialsratioatthetopsurfaceanddarkenthecolour
n Implementauniformcuringsystemforthenecessarytimeperiod.
Unevencuringdirectlyaffectsthedegreeofhydrationofthecementandthecolouroftheconcrete
n Ensurethatplasticsheeting,whenusedforcuringpurposes,doesnot
comeindirectcontactwiththeconcrete.Plasticsheetingtendstoleave
colourstreaksontheconcretesurfacewhereitisindirectcontactwith
theconcrete
n
Utilizeeffectiveformreleaseagentsthatpreventthenon-uniformloss
ofmoisturefromtheformedsurface
n Ensurethattherawmaterialsourcesarenotvariedduringtheconcrete
placement.Architecturalconcreteprojectsmayrequirethestockpilingof
rawmaterialsattheconcreteplant(ataconsiderablecost)fortheentire
project
References:1. ConcreteinPractice#23DiscolorationNationalReadyMixedConcreteAssociation
2. ACI116CementandConcreteTerminologyAmericanConcreteInstitute3. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
DiscolourationDiscolouration
Discolouringofaresidentialdrivewayduetotheuseofcalciumchloride(accelerator)inonlyoneofthetwoloads
Theproperapplicationofacuringcompound
Photos courtesy of C
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Problem:
10
Defnition:
Thedevelopmentofapowderedmaterialatthesurfaceofhardened
concrete.
Dustingistheformationofloosepowdercausedbythedisintegrationofthetopsurfaceoftheconcreteduetoanextremelyweaksurface.
Dusting is more likely to occur when:
n Anynishingoperationisperformedwhilethereisbleedwateronthe
topsurfaceoftheslab
n Theconcreteisnotproperlycuredresultinginanextremelyweaksurface
n Theconcreteisexposedtocarbondioxidewhilestillinitsplasticstate
n Theconcretesurfaceisnotprotectedduringtheinitialconcrete
placement(wind,rain,snow,freezing)
How to avoid problems with Dusting:
n Donotperformanynishingoperationsontheconcretewhilebleed
waterisstillpresentonthesurface.Ifthenishingoperationsworkthe
bleedwaterbackintotheconcretetheyelevatethew/cmratioofthe
surfacelayersignicantlyanddramaticallyreducethesurfacelayers
strength
n Initiatepropercuringpracticesassoonaspossibleandcurethe
concreteforthespeciedperiod
n Avoidplacingconcreteonvapourbarrierssincetheycanextendthe
bleedingperiodintotheinitialset/nishingperiod
n Ensurethatallheatingdevicesandconstructionequipmentisvented
outsidetheworkareatoavoidcarbondioxidebuild-upabovetheslab
whiletheconcreteisstillplastic
n Addingwatertotheconcretesurfacetoaidthenishingoperationswill
signicantlyweakenthesurfaceandincreasetheproblemswithdusting
References:1. ConcreteinPractice#1DustingConcreteSurfacesNationalReadyMixed
ConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
DustingDusting
Anexampleofadustingconcretesurface
Properconcretecuringpracticessignicantlyreducedustingproblems
Photos courtesy of CAC and RMCAO
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Problem:
Photos courtesy of CAC and RMC
Defnition:
Voidsleftinconcreteduetofailureofthemortartoeffectivelyllthe
spacesamongcoarseaggregateparticles.
Honeycombingofconcretetypicallyoccurswhentheconcreteisallowedtosegregateduetoobstructionsintheowofconcreteduringplacementor
duetoinadequateconsolidationtechniquesaftertheconcreteisplaced.
Dependingontheextentandlocationofthehoneycombing,thisdefectcan
beeitheracosmeticorstructuraldefectinthenalproduct.
Honeycombing is more likely to occur when:
n Theconcreteelementcontainsreinforcingsteelthatlimitstheabilityof
theconcretetoowthroughtheformwork
n Thereislimitedaccessorlocationsforplacingconcreteintheformwork
resultinginlargespacingsbetweenconcreteplacementlocations
n Highconcentrationsofreinforcingsteelpreventtheinsertionofinternal
concretevibratorsinkeystructurallocations
n Largesizecoarseaggregateisusedinthemixdesignandreinforcement
ispresentintheformwork
n Concretemixdesignsarenotproperlyproportionedforthenecessary
owabilityandworkability
How to avoid problems with Honeycombing:
n WorkwiththeEngineerandthereinforcingsteelcontactortominimize
restrictionsduetoreinforcingsteel.Thiscanincludetheresizingofthe
rebarandavoidingrebarlapsplicesinkeystructuralareas
n Ensurethatoptimumconcretevibrationpracticesarefollowed.This
includesdecreasingtheheightofthelifts,reducingthedistance
betweenvibratorinsertions,etc.
n Designingorre-engineeringtheconcreteelementtoallowforacceptable
concreteplacementmethodsandlocations
n Allowingthecontractortoselecttheappropriateconcreteslumpforthe
placementtechniquebeingutilized
n Designingthemixtooptimizeowabilityandtominimizethecoarse
aggregatesize
n Consideringtheuseofspecialtyconcreteproductslikeselfconsolidating
concrete(SCC)
References:1. PlacingandVibratingPouredConcreteWallsConcreteConstruction2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
HoneycombingHoneycombing
Honeycombinginaresidentialshearwall
Honeycombinginaconcretecolumn
SelfConsolidatingConcrete(SCC)canbeusedtosuccessfullyaddresschallengingconcreteplace-ments.However,pleasekeepinmindthatthequalitoftheformworkisalsocriticalsincetheformworksurfacewillbeperfectlymirroredintheconcreteelement
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Problem:
12
Defnition:
Whenthetestresultsfromtheconcretecylindersorbeamsusedasthe
basisofmaterialacceptancefailtomeetthestrengthrequirementsofCSAA23.1Clause4.4.6.6.1.
CSAA23.1recognizesthefactthatwhenconcreteisproportionedtomeet
therequirementsofthestandardlowstrengthresultswilloccurabout
onceortwiceperevery100tests.Acceptableconcretecomplieswiththe
requirementsofCSAA23.1Clause4.4.6.6.1when:
n Theaveragesofallsetsofthreeconsecutivestrengthtestsforthatclass
ofconcreteequalorexceedthespeciedstrength
n Noindividualtestresultismorethan3.5MPabelowthespecied
strength
Low Concrete Strength Results are morelikely to occur when:
n Theconcretetestingisperformedbyunqualiedpersons
n CSAA23.1/.2concretetestingrequirementsarenotfollowed
n Whentestspecimensareimproperlyhandled,curedandtested
n Thejobsiteadditionofexcessiveamountofwaterisallowed
n Thewrongconcreteisorderedfortheconcreteplacement
How to avoid problems with Low ConcreteStrength Results:
n EnsurethatallconcreteeldtestingisonlyperformedbyCSAorACI
Certiedconcretetechnicians
n Ensurethatallconcretetestingisperformedinstrictaccordancewith
CSAA23.1/.2
n Verifythattheproperfacilitiesexistforthesitestorageoftestsamples
priortothestartofconcreteplacement.Everyjobsiteshouldincludea
curingboxwhichmaintainsthesamplesatatemperaturebetween15C
and25C
n Reviewtheconcretedeliveryticketforeveryloadofconcreteand
comparethattospecicationrequirementsfortheelementbeing
constructed
n UtilizethePre-PourMeetingFormtoclearlydesignatethepersonwith
authorityforjobsitewateradditionandspecicallydenetheconditions
whenwateradditionwillbeallowed
References:1. CIP#9LowConcreteCylinderStrengthNationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Low Concrete Strength ResultsLow Concrete Strength Results
Concretecylindersusedforconcreteacceptancepurposesmustbestoredinatemperaturecontrolledenvironment-notlefttobakeinthesun
Anexampleofpropereldcuringmethodsincludestheuseofatemperaturecontrolledcuringbox
Photos courtesy of RMCAO
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Defnition:
Shrinkagecracksthatdevelopbeforethecementpaste,mortar,grout,or
concretesets.
Plasticshrinkagecracksarecausedbytherapidlossofwaterfromthesurfaceoftheconcretebeforetheconcretehashadtimetoset.These
cracksformafterconcreteplacementandconsolidation,whiletheconcrete
isstillplastic.Theyarerelativelyshallowindepthandtendtoformin
paralleltooneanother.Thecracksareunappealingbutrarelycause
signicantdamagetothestrengthordurabilityoftheoorduetotheir
shallowdepth.
Plastic Shrinkage Cracking is more likely
to occur when:
n Thereisarapidlossofmoisturefromthesurfaceoftheconcrete
n Thereislowrelativehumidity
n Thereishighambientairorconcretetemperatures
n Highwindsareblowingacrosstheslabsurface
n Limitedbleedwaterispresentintheconcreteresultinginrapidsurface
moistureloss
How to avoid problems with Plastic
Shrinkage Cracks:
n Considererectingwindscreensorsunshadestoreducethesurface
evaporationrate
n Utilizefogsprayingequipmentontheup-windsideoftheslabto
maintaintherelativehumidityoftheslabat100%
n Considerpre-dampeningthesub-gradeduringhotdryweathertoreduce
themoisturelossfromtheconcreteintothesubgrade
n Considertheuseofsyntheticbrestoresistthetensileforcescreated
duringtheplasticshrinkageperiod
n Considerplacingtheconcreteduringoff-hourstoavoidhigh
temperature,highwind,highsunexposureperiods
n Considertheuseofevaporationretardantchemicalsappliedbetween
thevariousnishingoperationstopreventtherapidlossofmoisture
fromthesurfaceoftheconcrete
References:1. CIP#5PlasticShrinkageCrackingNationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Plasticshrinkagecracksonexterioratwork
CSAA23.1denesSeveredryingconditionsaswhesurfacemoistureevaporationexceeds1.0kg(m2.hr
Photos courtesy of CAC and RMC
Plastic Shrinkage CrackingPlastic Shrinkage CrackingProblem:
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Problem:
14
Photos courtesy of CAC and RMCAO
Defnition:Localakingorpeelingawayofthenear-surfaceportionofhardenedconcreteor
mortar.
Scalingbeginsassmalllocalizedpatchesoftheconcretesurfacethatdebonds
fromtheconcretetypicallyduetofreeze-thawexposure.Withtimetheselocalized
patchescanexpandandmergetogethertoproducelargescaledareas.Thevarious
levelsofscalingaredenedbytheAmericanConcreteInstitute(ACI)as:
n Light ScalingScalingoftheconcretethatdoesnotexposethecoarseaggregate
n Medium ScalingScalingoftheconcretethatinvolveslossofsurfacemortarto5to10mmindepthandexposureofthecoarseaggregate
n Severe ScalingScalingthatinvolveslossofsurfacemortarto5to10mmindepthwithsomelossofmortarsurroundingtheaggregateparticlesof10to
20mmindepth
n Very Severe ScalingScalingthatinvolveslossofcoarseaggregateparticlesaswellasmortargenerallytoadepthgreaterthan20mm
Scaling is more likely to occur when:n Theconcretecontainsinsufcientairentrainmentorisnon-airentrained
n Theconcretehasnotbeenproperlycured
n Theconcreteisexposedtodeicingchemicalsofanytype
n Improperconcretehasbeenutilizedforexteriorapplications.TheOntarioBuildingCodeandCSAA23.1bothspecifyaminimumof32MPa,0.45W/CM,
airentrainedconcreteforexteriorapplicationsexposedtofreeze-thawcyclesanddeicingchemicals
n Whennishingoperationsarecompletedwhilethebleedwaterisstillonthe
concretesurface
n Theconcreteisplacedduringadverseweatherconditions(extremeheat,extremecold,duringtherain,etc.)
How to avoid problems with Scaling:n Orderandplacetheproperconcretefortheapplication(32MPa,0.45W/CM,
airentrainedconcrete)
n Properlycuretheconcrete.Exteriorconcreterequiresaminimumof7dayscuringinordertoensurethattheconcretesurfacedevelopsthenecessarydurabilitypropertiestoresistscaling
n Avoidtheuseofdeicingchemicalsontheconcretesurface.Deicingchemicals
shouldnotbeusedatallduringtherstwinterandonlysparinglyafterthat
n
Avoidtheuseofdeicingchemicalsthatchemicallyattackconcreten Allowatleast30daysofairdryingoftheconcreteslabafterthecuringperiod
beforeexposingtheslabtofreeze-thawcycles.Theelevatedmoisturecontentof
newconcreteinitiallyreducestheeffectivenessoftheairentrainment
n Considertheproperuseofconcretesealerstoreducetheabsorptionofmoisture
andchloridesintotheconcrete
n Utilizewetcuringmethodsduringhotweatherandseveredryingconditionsand
fogspraytheconcretesurfacebetweennishingoperations
References:1. CIP#5PlasticShrinkageCrackingNationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
5. OntarioBuildingCode2000,OntarioMinistryofMunicipalAffairsandHousingHousingDevelopmentandBuildingsBranch
Lightscalingofaconcretesidewalk
Mediumscalingofaconcretesidewalk
Properconcretecuringpracticessignicantlyreduceinstancesofconcretescaling
ScalingScaling
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Uncontrolled Shrinkage CrackingUncontrolled Shrinkage CrackingProblem:
Defnition:
Crackingofastructureormemberduetofailureintensioncausedbyexternalor
internalrestraintsasreductioninmoisturecontentdevelops,carbonationoccurs,orboth.
Concretebothexpandsandcontractswithchangesinmoistureandtemperature
anddeectsdependingontheelementsize,reinforcing,loadingforceandsupport
conditions.Allthesefactorscanleadtouncontrolledcrackingiftheproperdesign
andjointdetailsarenotaddressedpriortothetimeofconstruction.Inorderto
preventuncontrolledshrinkagecrackingthedesignerandthecontractorshould
effectivelyutilizethefollowingconcretejointingsystems:
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 Impropernishingproceduresareimplementedortheinstallationofcontraction
jointsisnotcompletedinatimelyfashion
n Thereisinadequateconcretecuring
How to avoid problems with Uncontrolled
Shrinkage Cracks:n Ensurethatthesubgradeisadequatelypreparedbasedupontheexposureand
loadingconditionsthattheslabwillbeexposedtoduringitsservicelife.This
includesusingtheappropriatethicknessofgranularsub-basefortheapplication
andproperlycompactingthesub-base
n Ensuringpositivedrainageofboththesub-gradeandtheconcretesurfaceto
avoidthebuild-upofhighmoisturelevels
n Utilizingtheproperconcretefortheapplication(SeeCSAA23.1minimum
durabilityrequirements)
n Thedesignerandthecontractorshouldreviewtheconcretejointingdetails
priortothestartoftheproject.Itemstoconsiderincludeconcretethickness,
maximumjointspacing(4.5mmax.),brereinforcement,structural
reinforcementandearlyinstallationofcontractionjoints
n Implementcuringproceduresassoonaspossibletoensurethatthe
performancepropertiesoftheconcretefullydevelop
References:1. CIP#4CrackingConcreteSurfacesNationalReadyMixedConcreteAssociation2. ConcreteConstructionTroubleshootingTipsTheAberdeenGroup
3. ACI116CementandConcreteTerminologyAmericanConcreteInstitute4. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Photos courtesy of CAC and RMCAO
Designandconstructionofaproperisolationjoint
Designaconstructofpropercontractijoints
Designandconstructionofproperconstructionjoints
Ifyoufaiproperlydesignfoshrinkageuncontrorandomcrackswoccur
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Problem:
16
Defnition:
Thedislodgingofsmallsectionsofconcretemortardirectlyovertopofthe
coarseaggregateparticlesinexteriorconcreteexposedtofreeze/thaw
conditions.
Mortarakingtypicallyoccursinexterioratworkthatwasnotproperly
curedduringtherst7to28days.Mortarakingistypicallyveryshallow
indepthandconsistsofdistinctdelaminations(akes)thatoccurdirectly
overtopofthecoarseaggregateparticlesintheconcrete.Thisistypically
duetothefactthattheconcretemortardirectlyovertopofthecoarse
aggregatewasallowedtodryoutandwasnotproperlycured.Anyexcess
bleedwaterthatispresentintheconcreteisforcedtotravelaroundthe
coarseaggregateparticlesasitrisestothesurfaceandthemortardirectly
abovefailstodevelopthenecessarystrengthrequiredtoachieveproper
durabilityandbond.
Mortar Flaking is more likely to occur when:
n Theconcreteisplacedduringperiodsofrapidsurfacedrying
n Whentheconcreteisnotcuredatall
n Whencuringcompoundisappliedtoolateafterconcreteplacementor
notuntilthefollowingday
How to avoid problems with Mortar Flaking:
n
Ensurethepropercuringproceduresarefollowedatalltimes!Properconcretecuringshouldeliminatethisproblem
n Duringperiodsofrapidsurfacedryingconsiderusinginitialcuring
procedureslikeevaporationretardantsorplacingplasticsheetingover
theconcretesurfacebetweennishingoperations
n Considertheuseofwetcuringmethodsduringseveredryingandhot
weatherconditions
n Considertheuseoffogsprayingsystemstoprotectthesurfaceofthe
concretefromdryingoutuntilwetcuringcanbeinitiated
References:
1. ACI116CementandConcreteTerminologyAmericanConcreteInstitute2. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Mortar FlakingMortar Flaking
MortarakingonaconcretesidewalkNotethattheaggregateissoundandonlythemortarabovetheaggregatehasfailed
Cross-sectionthroughamortarakenotethatonlythemortarabovethecoarseaggregatehasfailed
Photos courtesy of RMCAO and CTL
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Photos courtesy of RMC
Defnition:
Conicalfracturesoftheconcretesurfacecausedbytheexpansionofice
insideporouscoarseaggregateslocatedjustbelowtheconcretesurface.
Popoutsresultwhensoftporousaggregates,foundbothinlimestoneandgravelsources,undergofreeze/thawconditionsinasaturatedstate.While
airentrainmentprovidesprotectionforthecementpastefromtheexpansive
forcesgeneratedduringwinterfreeze/thawcycles,itdoesnotprovideany
protectionforthecoarseaggregateparticles.Ifthelocalaggregatesources
containevenasmallpercentageofsoft,porousparticles(likechert),the
coarseaggregatecanabsorbsignicantquantitiesofmoistureandfractures
duringfreeze/thawcycles.Theresultingexpansionfracturesboththe
aggregateandthesurroundingconcrete.Popoutsaredistinguishedfrom
mortarakingbythepresenceoffracturedaggregateinthepopout(as
showninthesamplephoto).
Popouts are more likely to orm when:
n Thelocalaggregatesourcescontainsoft,porousmaterial
n Theconcreteisinafullysaturatedcondition
n Theconcretehasntachieveditsultimatedurabilitypotentialdueto
inadequatecuring
How to avoid problems with Popouts:
n
Considertheuseofalternativeaggregatesources.Thismayrequiretheimportofaggregatesatasignicantadditionalexpense
n Considerchangestotheaggregateprocessingprocesstofurtherreduce
thequantityofsoftporousmaterial(thecompleteeliminationofallsoft
materialisnotnormallypossible)
n Ensurethattheoveralldesignminimizesthebuild-upofexcessmoisture
intheconcrete.Thiscanbeachievedthroughpropersurfacegrading
anddrainingofthesurroundingsubgrade
n Ensurethatproperconcretecuringandprotectionpracticesarefollowed
toachievethehighestpossibleconcretedurability
n Considertheuseofsealerstolimittheinowmoistureintotheconcrete
References:
1. ACI116CementandConcreteTerminologyAmericanConcreteInstitute2. DesignandControlofConcreteMixtures7thEditionCementAssociationofCanada
Popoutsurfacedamagecasedbythefractureofasoftaggregateparticleduringwinterfreeze/thawcycles
Across-sectionalviewofaconcretecoretakenfromapopoutareanotethelargechertparticleinthislimestoneaggregatesourcethatisresponsiblefortpopout
PopoutsPopoutsProblem:
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To Obtain Multiply By To Obtain Multiply By
millimetres inches 25.40
centimetres inches 2.54
centimetres feet 30.45metres feet 0.3048
metres yards 0.9144
kilometres miles 1.6093
inches millimetres 0.03937
inches centimetres 0.3937
inches metres 39.37
feet metres 3.281
feet kilometres 3280.90
yards metres 1.094
yards kilometres 1094.60
miles kilometres 0.61
grams ounces 28.35
kilograms pounds 0.4536
kilograms shorttons 907.185
kilograms longtons 1016.05
tonnes pounds 0.0004536
tonnes shorttons 0.9072
ounces grams 0.035
pounds kilograms 0.2205
pounds tonnes 2204.59
shorttons kilograms 0.0011
shorttons tonnes 1.1023
longtons kilograms 0.0010
longtons tonnes 0.9842
squareinches centimetres2 0.1550
squarefeet centimetres2 0.00108
squarefeet metres2 10.764
squareyards squarefeet 0.1111
squareyards metres2 0.3987
acres metres2 0.000247
acres hectares 2.471
squaremiles hectares 0.003861
centimetres2 squareinches 6.4529
metres2 squarefeet 0.0929
metres2 squareyards 0.8361
kilometres2 squaremiles 2.59
kilometres2 acres 0.00404
cubicinches cm3 0.06104
cubicinches litres 61.02
cubicfeet m3 35.3143
cubicfeet litres 0.3532
cubicyards m3 1.3079
Imp.gallons cubicfeet 6.236
Imp.gallons litres 0.22
Imp.gallons m3 219.97
cm3 cubicinches 16.387
cm3 cubicfeet 28.32
m3 Imp.gallons 3.154
m3
cubicyards 0.764
C F 5/9(F-32)
F C (9/5C)+32
MPa psi 0.006895
psi MPa 145
LENGTH
AREA
TEMPERATURE
WEIGHT/MASS
VOLUME/CAPACITY
PRESSURE
Conversion TableConversion Table
18
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Concrete ApplicationClass oConcrete
MaximumW/CM
Minimum28d Strength
(MPa)
Chloride IonPermeabilityat 56 days
ReerenceStandard
Agricultural
Reinforcedbeams,slabsandcolumnsovermanurepitsexposedtoseveremanuregases
A1 0.40 35 1500Coulombs CSAA23.1-09
Reinforcedwallsinexteriormanuretanks,silos,
andfeedbunkersexposedtoseveretomoderate
manuregases
A2 0.45 32 N/A CSAA23.1-09
Structurallyreinforcedinteriorwalls,beams,
slabsandcolumnsinacontinuouslysubmerged
conditionexposedtomoderatetoseveremanure
gases
A3 0.50 30 CSAA23.1-09
Non-structurallyreinforcedslabsongradeexposedmoderatemanuregasesorliquidswithoutfreeze-thawexposure
44 0.55 25 CSAA23.1-09
Industrial/Commercial/Institutio
nal
HighPerformanceConcrete CXL 0.40 50within56days 1000Coulombs CSAA23.1-09
Structurallyreinforcedconcreteexposedtochlorides(Bridgedecks,parkingdecks,etc.)
C1 0.40 35 1,500Coulombs CSAA23.1-09
Non-structurallyreinforcedconcreteexposedto
chloridesandfreezingandthawing(Garageoors,
pavements,sidewalks,curbs,etc.)
C2 0.45 32 CSAA23.1-09
Continuouslysubmergedconcreteexposedto
chloridesbutnotfreezingandthawing(Under-waterportionsofmarinestructures)
C3 0.50 30 CSAA23.1-09
Non-structurallyreinforcedconcreteexposedto
chloridesbutnotfreezingandthawing(Under-
groundparkingslabsongrade)
C4 0.55 25 CSAA23.1-09
Concreteexposedtofreezingandthawingina
saturatedconditionbutnottochlorides(Pool
decks,patios,tenniscourts,etc.)
F1 0.50 30 CSAA23.1-09
Concreteinanunsaturatedconditionexposedtofreezingandthawingbutnottochlorides(Exterior
wallsandcolumns)
F2 0.55 25 CSAA23.1-09
Concretenotexposedtochloridesorfreezingand
thawing(Footingsandinteriorslabs,wallsand
columns)
N SpeciedbytheDesigner CSAA23.1-09
Residential
Footingsforwalls,columnsandchimneys R1 0.7015
(36%air)CSAA23.1-09
Foundatio ns,walls,gr adebeams,piers,etc. R2 0.7015
(47%air)CSAA23.1-09
Interiorslabsongroundnotexposedtofreeze-
thawwithoutavapourbarrierbeneaththeslabR3 0.65 20 CSAA23.1-09
Garageoorsandallconcreteexposedtofreezing
andthawinganddeicingchemicals(Driveways,sidewalks,patios,steps,etc.)
C2 0.45 32 CSAA23.1-09
OwnermayorderconcretetoPerormance Specifcations(concretesupplierisresponsibleforconcreteasdelivered)orPrescriptive Specifcations(Ownerisresponsibleforconcreteasdelivered).
DetailedConcreteMixProportionsshallnotbedisclosedbytheconcretesupplier.
CONCRETE PLANT CERTIFICATION SPECIFICATION:The concrete supplier shall submit to the concrete purchaser a currently valid Certifcate o Ready Mixed Concrete Production Facilities or a currently Certifcate o Mobile Mix Concrete Production Facilities as issued by the Ready Mixed Concrete Association o Ontario to the plant being used.
References:
1.OntarioBuildingCode-2006,OntarioMinistryofMunicipalAffairsandHousing-HousingDevelopmentandBuildingsBranch
2.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.Cautionshouldbeexercisedinextractingindividualclausesortab
andusingtheminprojectspecications,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
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www rmcao org
THISINFORMATIONTOBEUSEDASAGUIDELINEONLY.FORMOREDETAILEDINFORMATIONORSPECIFICATIONS,PLEASECONSULT REFERENCES.
Photoscourtesyof:
CAC-CementAssociationofCanada
CTL-ConstructionTechnologyLaboratories,
Inc.
RMCAO-ReadyMixedConcreteAssociation
ofOntario
Preparedby:
Ready Mixed Concrete
Association of Ontario
365BrunelRoad,Unit#3
Mississauga,OntarioL4Z1Z5
Tel:905.507.1122
Fax:905.890.8122
Email:[email protected]