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ConcreteConstructionmatterT.Pauly,M.J.N.PriestleyAbstractViewedintermsofacceptedpractices,concreteconstructionoperationsleavemuchtobedesiredwithrespecttothequality,serviceability,andsafetyofcompletedstructures.Theshortcomingsoftheseoperationsbecameabundantlyclearwhenamagnitude7.6earthquakestrucknorthernPaki-stanonOctober8,2005,destroyingthousandsofbuildings,damagingbridges,andkillinganesti-mated79,000people.Theunusuallylowqualityofconstructionoperationsprevalentwasamajorcauseoftheimmensedevastation.Keywords:ConcretePlacingCuringConstructionTechnologyPlacingConcreteIfconcreteisplacedinthesurface,thesur-faceshouldbefilledwithwatersufficientlytopreventitfromabsorbingtheconcreteofitswater.Iffreshconcreteistobeplacedonornearbytoconcretethathassolidified,thesurfaceoftheplacedconcreteshouldbecleanedabsolutely,preferablywithahigh-pressureairorwaterjetorsteel-wirebrushes.Thesurfaceshouldbewet,butthereshouldbenomuchwater.Alittlequantityofcementgroutshouldbebrushedoverthewholearea,andthenfollowedimmediatelywiththeapplicationofa1/2-inLayerofmortar.Thefreshconcreteshouldbeplacedonoragainstthemortar.Inordertodecreasethedisintegrationre-sultingfromcarriageafteritisplaced.Thecon-creteshouldbeplacedasnearlyasprobablyinitsfinalpoint.Itshouldbeplacedinlayerstopermituniformcompaction.Thetimeintervalbetweentheplacingoflayersshouldbelimitedtoassureperfectbondbetweenthefreshandpreviouslyplacedconcrete.Inplacingconcreteindeeperpatters,aves-selshouldbeusedtolimitthefreefalltonotover3or4ft,inordertopreventconcretedisintegra-tion.Thevesselisapipemadeoflightweightmetal,havingadjustablelengthsandattachedtothebottomofahopperintowhichtheconcreteisdeposited.Asthepattersarefilled,sectionsofthepipemayberemoved.Immediatelyaftertheconcreteisplaced,itshouldbecompactedbyhandpuddingorame-chanicalvibratortoeliminatevoids.Thevibratorshouldbeleftinonepositiononlylongenoughtoreducetheconcretearoundittoaplasticmass;thenthevibratorshouldbemoved,ordisintegra-tionoftheaggregatewilloccur.Ingeneral,thevibratorshouldnotbepermittedtopenetrateconcreteinthepriorlift.Themainlyadvantageofvibratingisthatitpermitstheuseofadrierconcrete,whichhasahigherstrengthbecauseofthereducedwatercontent.Amongtheadvantagesofvibratingcon-cretearethefollowing:1.Thedecreasedwaterpermitsareductioninthecementandfineaggregatebecauselesscementpasteisneeded.2.Thelowerwatercontentdecreasesshrinkageandvoids.3.Thedrierconcretedecreasesthecostoffinishingthesurface.4.Mechanicalvibrationmayreplacethreetoeighthandpuddles.5.Thelowerwatercontentincreasesthestrengthoftheconcrete.6.Thedriermixturepermitstheremovalofsomepattersmorequickly,whichmayreducethecostofpatters.CuringConcreteIfconcreteistogainitsmaximumstrengthandotherdesirableproperties,itshouldbecuredwithadequatemoistureandatafavorabletem-perature.Failuretoprovidetheseconditionsmayresultinaninferiorconcrete.Theinitialmoistureinconcreteisadequatetohydrateallthecement,provideditisnotshouldreplacethemoisturethatdoesevaporate.Thismaybeaccomplishedbymanymethods,suchasleavingthepattersinplace,keepingthesurfacewet,orcoveringthesurfacewithaliquidcuringcompound,whichcomesbeingtoawater-tightmembranethatpreventstheescapeoftheinitialwater.Curingcompoundsmaybeappliedbybrushesorpressuresprayers.Agallonwillcover200to300sqft.Concreteshouldbeplacedatatemperaturenotlessthan40ormorethan80°F.Alowertem-peraturewilldecreasetherateofsetting,whileahighertemperaturewilldecreasetheultimatestrength.PlacingConcreteinColdWeatherWhentheconcreteisplacedduringcoldweather,itisusuallynecessarytopreheatthewater,theaggregate,orbothinorderthattheini-tialtemperaturewillassureaninitialsetandgaininstrength.Preheatingthewateristhemostef-fectivemethodofprovidingthenecessarytem-perature.Forthispurposeawaterreservoirshouldbeequippedwithpipecoilsthroughwhichsteamcanbepassed,orsteammaybedischargeddirectlyintothewater,severaloutletsbeingusedtogivenbetterdistributionoftheheat.Whenthetemperaturesofthemixturesareknown,somespecificchartsmaybeusedtocal-culatethetemperatureofconcrete.Astraightlinepassallthreescales,passingthrougheverytwoknowntemperatures,willassurethedetermina-tionofthethirdtemperature.Ifthesurfaceofsandisdry,thefactlinesofthescalesgivingthetemperatureofconcreteshouldbeused.However,ifthesandcontainsabout3percentmoisture,thedottedlinesshouldbeused.Specificationsusuallydemandthatfreshlyplacedconcreteshallbekeptatatemperatureofnotlessthan70°Ffor3daysor50°Ffor5daysafteritisplaced.Somepropermethodmustbeprovidedtokeepthedemandedtemperaturewhenthecoldweatherisestimated.ReinforcingsteelsforconcreteComparedwithconcrete,steelisahighstrengthmaterial.Theusefulstrengthofordinaryreinforcingsteelsintensionaswellascompres-sion,i.e.,theyieldstrength,isabout15timesthecompressivestrengthofcommonstructuralcon-crete,andwellover100timesitstensilestrength.Ontheotherhand,steelisahigh-costmaterialcomparedwithconcrete.Itfollowthatthetwomaterialsarethebestusedincombinationiftheconcreteismadetoresistthecompressivestressesandthecompressiveforce,longitudinalsteelreinforcingbarsarelocatedclosetotheten-s