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Unit6ConcreteConstruction
PartⅠIllustratedWordsandConceptsFigure6-1ASlumpTestforConcreteConsistencyFigure6-2TheGrowthofCompressiveStrengthinConcreteoverTimeFigure6-3PretensioningoftheSteelStandsintheConcreteFigure6-4PosttensioningoftheSteelStandsintheConcrete
PartⅡPassagesPassageAMakingandPlacingConcretePassageBHandlingandPlacingConcreteUnit6ConcreteConstructionPartⅠIllustratedWordsandConceptsFigure6-1ASlumpTestforConcrete
ConsistencyThehollowmetalconeisfilledwithconcreteandtampedwithrodaccordingtoastandardprocedure.Theconeiscarefullyliftedoff,allowingthewetconcretetoslumpunderitsownweight.Theslumpininchesismeasuredinthemannershown.Unit6ConcreteConstructionPartⅠIllustratedWordsandConceptsFigure6-2TheGrowthofCompressiveStrengthinConcreteoverTime
Moist-curedconcreteisstillgainingstrengthafter6months,whereasAir-driedconcretevirtuallystopscuringaltogether.Unit6ConcreteConstructionPartⅠIllustratedWordsandConceptsFigure6-3PretensioningoftheSteelStrandsintheConcrete
Pretensioning,thepretensionedsteelstrandsforabeam
Unit6ConcreteConstructionPartⅠIllustratedWordsandConceptsFigure6-4PosttensioningoftheSteelStrandsintheConcrete
Posttensioning,usingdrapedstrandstomorenearlyapproximatetheflowpathoftensileforcesinthebeam.Unit6ConcreteConstructionPartⅡ
Passages
PassageAMakingandPlacingConcreteProportioningConcreteMixesThequalityofcuredconcreteismeasuredbyanyofseveralcriteria,dependingonitsenduse.Forstructuralcolumns,beams,andslabs,compressivestrengthandstiffnessareimportant.Forpavingsandfloorslabs,surfacesmoothnessandabrasionresistancearealsoimportant;forpavingsandexteriorconcretewalls,ahighdegreeofweatherresistanceisrequired.Unit6ConcreteConstructionPartⅡ
Passages
PassageA
Watertightnessisimportantinconcretetanks,dams,andwalls.Regardlessofthecriteriontowhichoneisworking,however,therulesformakinghighqualityconcretearemuchthesame:Useclean,soundingredients;mixtheminthecorrectproportions;handlethewetconcreteproperlytoavoidsegregatingitsingredients;andcuretheconcretecarefullyundercontrolledconditions.Unit6ConcreteConstructionPartⅡ
Passages
PassageAThedesignofconcretemixturesisasciencethatcanbedescribedhereonlyinitsbroadoutlines.Thestartingpointofanymixdesignistoestablishthedesiredworkabilitycharacteristicsofthewetconcrete,thedesiredphysicalpropertiesofthecuredconcrete,andtheacceptablecostoftheconcrete,keepinginmindthatthereisnoneedtospendmoneytomakeconcretebetterthanitneedstobeforagivenapplication.Unit6ConcreteConstructionPartⅡ
Passages
PassageA
Concreteswithultimatecompressivestrengthsaslowas2000poundspersquareinch(13.8MPa)aresatisfactoryforsomefoundationelements.Concreteswithultimatecompressivestrengthsof22000psi(150MPa),producedwiththeaidofsilicafume,flyash,andsuperplasticizeradmixtures,arecurrentlybeingemployedinthecolumnsofsomehighrisebuildings,andhigherstrengthsthanthisarecertaintobedevelopedinthenearfuture.Acceptableworkabilityisachievableatanyofthesestrengthlevels.Unit6ConcreteConstructionPartⅡ
Passages
PassageAGivenapropergradationofsatisfactoryaggregates,thestrengthofcuredconcreteisprimarilydependentontheamountofcementinthemixandonthewater-cementratio.Althoughwaterisrequiredasareactantinthecuringofconcrete,muchmorewatermustbeaddedtoaconcretemixthanisneededforthehydrationofthecement,inordertogivethewetconcretethenecessaryfluidityandplasticityforplacingandfinishing.Unit6ConcreteConstructionPartⅡ
Passages
PassageATheextrawatereventuallyevaporatesfromtheconcrete,leavingmicroscopic
voidsthatimpairthestrengthandsurfacequalitiesoftheconcrete.Absolutewater-cementratiosbyweightshouldbekeptbelow0.60formostapplications,meaningthattheweightofthewaterinthemixshouldnotbemorethan60percentoftheweightofthePortlandcement.Higherwater-cementratiosthanthisareoftenfavoredbyconcreteworkersbecausetheyproduceafluidmixturethatiseasytoplaceintheforms,Unit6ConcreteConstructionPartⅡ
PassagesPassageA
buttheresultingconcreteislikelytobedeficientinstrengthandsurfacequalities.Lowwater-cementratiosmakeconcretethatisdenseandstrong,butunlessair-entrainingorwater-reducingadmixturesareincludedinthemixtoimproveitsworkability,theconcretewillnotfloweasilyintotheformsandwillhavelargevoids.Itisimportantthatconcretebeformulatedwiththerightquantityofwaterforeachsituation,enoughtoassureworkabilitybutnotenoughtoadverselyaffectthefinalpropertiesofthematerial.Unit6ConcreteConstructionPartⅡ
Passages
PassageAMostconcreteinNorthAmericaisproportionedatcentralbatchplants,usingup-to-datelaboratoryequipmentandengineeringknowledgetoproduceconcreteoftheproperqualityforeachproject.Theconcreteistransitmixedenrouteinarotatingdrumonthebackofatrucksothatitisreadytopourbythetimeitreachesthejobsite.Unit6ConcreteConstructionPartⅡ
Passages
PassageAEachloadoftransit-mixedconcreteisdeliveredwithacertificatefromthebatchplantthatlistsitsingredientsandtheirproportions.Asafurthercheckonquality,aslumptest(seeFigure6-1)maybeperformedatthetimeofpouringtodetermineifthedesireddegreeofworkabilityhasbeenachievedwithoutmakingtheconcretetoowet.Unit6ConcreteConstructionPartⅡ
Passages
PassageAForstructuralconcrete,standardtestcylindersarealsopouredfromeachtruckload.Within48hoursofpouring,thecylindersaretakentoatestinglaboratory,curedforaspecifiedperiodunderstandardconditions,andtestedforcompressivestrength.Ifthelaboratoryresultsarenotuptotherequiredstandard,testcoresaredrilledfromtheactualmembersmadefromthequestionablebatchofconcrete.Unit6ConcreteConstructionPartⅡ
Passages
PassageAIfthestrengthofthesecoresamplesisalsodeficient,thecontractorwillberequiredtocutoutthedefectiveconcreteandreplaceit.Frequently,testcylindersarealsocastandcuredontheconstructionsiteunderthesameconditionsastheconcreteintheforms;thesemaythenbetestedasawayofdeterminingwhentheconcreteisstrongenoughtoallowremovalofformsandtemporarysupports.Unit6ConcreteConstructionPartⅡ
Passages
PassageBHandlingandPlacingConcreteFreshlymixedconcreteisnotaliquid,butaslurry,anunstablemixtureofsolidsandliquids.Ifitisvibratedexcessively,droppedfromverymuchofaheight,ormovedhorizontallyforanysubstantialdistanceinformwork,itislikelytosegregate,whichmeansthatthecoarseaggregateworksitswaytothebottomoftheformandthewaterandcementpasterisetothetop.Unit6ConcreteConstructionPartⅡ
Passages
PassageBTheresultisconcreteofnonuniformandgenerallyunsatisfactoryproperties.Segregationispreventedbydepositingtheconcrete,freshfromthemixer,asclosetoitsfinalpositionaspossible.Ifconcretemustbedroppedadistanceofmorethan3or4feet(ameterorso),
itshouldbedepositedthroughdropchutesthatbreakthefalloftheconcrete.Ifconcretemustbemovedalargehorizontaldistancetoreachinaccessibleareasoftheformwork,itshouldbepumpedthroughhosesorconveyedinbucketsorbuggies,ratherthanpushedacrossorthroughtheformwork.Unit6ConcreteConstructionPartⅡ
Passages
PassageBConcretemustbeconsolidatedintheformstoeliminatetrappedairandtofillcompletelyaroundthereinforcingbarsandintoallthecornersoftheformwork.Thismaybedonebyrepeatedlythrustingarod,spade,orimmersion-typevibratorintotheconcreteatcloselyspacedintervalsthroughouttheformwork.Excessiveagitationoftheconcretemustbeavoided,however,orsegregationwilloccur.Unit6ConcreteConstructionPartⅡ
Passages
PassageBSelf-consolidatingconcrete,whichfillsformscompletelywithoutrequiringvibration,hasrecentlybeendeveloped.Itisformulatedwithmorefineaggregatesthancoarseones,whichisareversaloftheusualpropotions.Itincludessuperplasticizingadmixturesthatarebasedonpolycarboxylateethers.Theresultisaconcretethatflowsveryfreely,yetdoesnotallowitscoarseaggregatetosinktothebottomoftheforms.Unit6ConcreteConstructionPartⅡ
Passages
PassageBCuringConcreteBecauseconcretecuresbyhydrationandnotbydrying,itisessentialthatitbekeptmoistuntilitsrequiredstrengthisachieved.Thecuringreactiontakesplaceoveraverylongperiodoftime,butconcreteiscommonlydesignedonthebasisofthestrengththatitreachesafter28days(4weeks)ofcuring.Ifitisallowedtodryatanypointduringthistimeperiod,Unit6ConcreteConstructionPartⅡ
Passages
PassageBthestrengthofthecuredconcretewillbereduced,anditssurfacehardnessanddurabilityarelikelytobeadverselyaffected(seeFigure6-2).Concreteelementscastinformworkareprotectedfromdehydrationonmostoftheirsurfacesbytheformwork,butthetopsurfacesmustbekeptmoistbyrepeatedlysprayingorfloodingwithwater,bycoveringwithmoisture-resistantsheetsofpaperorfilmorbysprayingonacuringcompoundthatsealsthesurfaceoftheconcreteagainstlossofmoisture.Unit6ConcreteConstructionPartⅡ
Passages
PassageBThesemeasuresareparticularlyimportantforconcreteslabs,whoselargesurfaceareasmakethemespeciallysusceptibletodrying.Prematuredryingisaparticulardangerwhenslabsarepouredinhotorwindyweather,whichcancauseaslabtocrackevenbeforeitbeginstocure.Temporarywindbreaksmayhavetobeerected,shademayhavetobeprovided,andfrequentfoggingofthesurfaceoftheslabwithafinesprayofwatermayberequireduntiltheslabishardenoughtobecoveredorsprayedwithcuringcompound.Unit6ConcreteConstructionPartⅡ
Passages
PassageBFormworkBecauseconcreteisputinplaceasashapelessslurrywithnophysicalstrength,itmustbeshapedandsupportedbyformworkuntilithascuredsufficientlytosupportitself.Formworkisusuallymadeofwood,metal,orplastic.Itisconstructedasanegativeoftheshapeintendedfortheconcrete.Unit6ConcreteConstructionPartⅡ
Passages
PassageBFormworkforabeamorslabservesasatemporaryworkingsurfaceduringtheconstructionprocessandasthetemporarymeansofsupportforreinforcingbars.Formworkmustbestrongenoughtosupporttheconsiderableweightandfluidpressureofwetconcretewithoutexcessivedeflection,whichoftenrequirestemporary
supportsthataremajorstructuresinthemselves.Unit6ConcreteConstructionPartⅡ
Passages
PassageB
Duringcuring,theformworkhelpstoretainthenecessary
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