英语翻译Concreteischaracterizedbystrongcompressivestrength,butisrelativelyweakintensionandshear.Itcansupportlargeaxialloads,andisusedextensivelyinthefoundationsoflargestructuressuchasbridgesandtallbuildings.butit
英语翻译
Concreteischaracterizedbystrongcompressivestrength,butisrelativelyweakintensionandshear.Itcansupportlargeaxialloads,andisusedextensivelyinthefoundationsoflargestructuressuchasbridgesandtallbuildings.butitbreakseasilyunderbendingwithoutreinforcingsteel.Concretewithcompressivestrengthof3,0006,000poundspersquareinch(psi)iscommon.
TheAmericanConcreteInstitutedefinesanyconcreteover6,000psiashigh-strengthconcrete.
Forstructuresthatrequireflexuralstrengthforresistancetobendingmoments,reinforcingsteel--rebar--isembeddedintheconcrete.Insomecases,tensionedcablesareplacedintheconcretetomakeprestressedorpost-tensionedconcrete.Thetensiontoresistthebendingmomentcomesfromthereinforcingsteelorthestressedcables.
Foraconcretemixtoreachitsultimatestrength,alongperiodoftimeisneeded.Theultimatestrengthofconcreteandits28-daystrengtharenotthesame.Fordesignpurposes,engineersusethe28-daystrength,andtheconcreteshouldbesubjectedtoonlyminimumloadsuntilthe28dayshavepassed.Thatruleisveryrigidlyenforcedinheavyconstruction.Itisnotatalluncommonforworkinresidentialandmedium-risecommercialbuildingconstructiontobeprocessedona3-dayfloorcycle.Placeconcreteoneday,formandshorethenextflooroverthenexttwodays,thisputsminimumloadonthenewslab,andthenplaceconcreteonthethirdday.Thisplacementincreasestheloadonthepreviouslyplacedfloor,buttheloadiscarriedprimarilybytheshoring,whichpassescontinuallydownforseveralfloors,therebyshiftingtheloadtoconcretethathashadtimetoreachitsultimatestrength.NotetheshoringandreshoringinFig.6.5isinplacetoadepthoffourfloorsbelowtheslabthatiscurrentlybeingplaced.
(ultimatestrengthofconcrete
Thecompressivestrengthofconcretecontinuestoincreaseslowlyformanymonths.Theultimatestrengthmaybehigherthanthe28-daystrength,whichisusedfordesigncalculations.)