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1、Concrete,ReinforcedConcrete,andPrestressedConcreteConcreteisastonelikematerialobtainedbypermittingacarefullyproportionedmixtureofcement,sandandgravelorotheraggregate,andwatertohardeninformsoftheshapeanddimensionsofthedesiredstructure.Thebulkofthematerialc
2、onsistsoffineandcoarseaggregate.Cementandwaterinteractchemicallytobindtheaggregateparticlesintoasolidmass.Additionalwater,overandabovethatneededforthischemicalreaction,isnecessarytogivethemixtureworkabilitythatenablesittofilltheformsandsurroundtheembedded
3、reinforcingsteelpriortohardening.Concreteswithawiderangeofpropertiescanbeobtainedbyappropriatesadjustmentoftheproportionsoftheconstituentmaterials.Specialcements,specialaggregates,andspecialcuringmethodspermitanevenwidervarietyofpropertiestobeobtained.The
4、sepropertiesdependtoaverysubstantialdegreeontheproportionsofthemix,onthethoroughnesswithwhichthevariousconstituentsareintermixed,andontheconditionsofhumidityandtemperatureinwhichthemixismaintainedfromthemomentitisplacedintheformsofhumidityandhardened.Thep
5、rocessofcontrollingconditionsafterplacementisknownascuring.Toprotectagainsttheunintentionalproductionofsubstandardconcrete,ahighdegreeofskillfulcontrolandsupervisionisnecessarythroughouttheprocess,fromtheproportioningbyweightoftheindividualcomponents,trou
6、ghmixingandplacing,untilthecompletionofcuring.Thefactorsthatmakeconcreteauniversalbuildingmaterialaresopronouncedthatithasbeenused,inmoreprimitivekindsandwaysthanatpresent,forthousandsofyears,startingwithlimemortarsfrom12,000to600B.C.inCrete,Cyprus,Greece
7、,andtheMiddleEast.Thefacilitywithwhich,whileplastic,itcanbedepositedandmadetofillformsormoldsofalmostanypracticalshapeisoneofthesefactors.Itshighfireandweatherresistanceareevidentadvantages.Mostoftheconstituentmaterials,withtheexceptionofcementandadditive
8、s,areusuallyavailableatlowcostlocallyoratsmalldistancesfromtheconstructionsite.Itscompressivestrength,likethatofnaturalstones,ishigh,whichmakesitsuitableformembersprimarilysubjecttocompression,suchascolumnsandarches
