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1、1Theprinciplesgoverningproperplacementofconcreteare:2Segregationmustbeavoidedduringalloperationsbetweenthemixerandthepointofplacement,includingfinalconsolidationandfinishing.3Theconcretemustbethoroughlyconsolidated,andshouldfillallanglesandcornersoftheforms.4Wherefreshconcreteisp
2、lacedonhardenedconcrete,agoodbondmustbedeveloped.5Thetemperatureoffreshconcretemustbecontrolledfromthetimeofmixingthroughfinalplacement,andprotectedafterplacement.。6Concretemaybeconveyedfromamixertopointofplacementbyanyofavarietyofmethodsandequipment,ifproperlytransportedtoavoids
3、egregation.Selectionofthemostappropriatetechniqueforeconomydependsonjobsiteconditions,especiallyprojectsize,equipment,andthecontractor’sexperience.Inbuildingconstruction，concreteusuallyisplacedwithhand-orpower-operatedbuggies;dropbottombucketswithacrane;inclinedchutes;flexibleand
4、rigidpipebypumping;shot-crete,inwhicheitherdrymaterialsandwateraresprayedseparatelyormixedconcreteisshotagainsttheforms;andforunderwaterplacing,tremiechutes(closedflexibletubes).Formassconcreteconstruction,side-dumpcarsonnarrow-gagetrackorbeltconveyersmaybeusedForpavement,concret
5、emaybeplacedbybucketfromtheswingingboomofapavingmixer,directlybydumptruckormixertruck,orindirectlybytrucksintoaspreader.7Evenwithinthespecifiedlimitsonslumpandwater-cementitiousmaterialsratio,excesswatermustbeavoided.Inthiscontext,excesswaterispresentedfortheconditionsofplacingif
6、evidenceofwaterrise(verticalsegregation)orwaterflow(horizontalsegregation)occurs.Excesswateralsotendstoaggravatesurfacedefectsbyincreasedleakagethroughformopenings.Theresultmaybehoneycomb,variationsincolor,orsoftspotsatthesurface.8Inverticalformwork,waterrisecausesweakplanesbetwe
7、eneachlayerdeposited.Inadditiontothedeleteriousstructuraleffect,suchplanes,whenhardened,containvoidswhichwatermaypassthrough.9Inhorizontalelements,suchasfloorslabs,excesswaterrisesandcausesaweaklaitancelayeratthetop.Thislayersuffersfromlowstrength,lowabrasionresistance,highshrink
8、age,andgenerallypoorquality.10Thepurpose
