21-05-2012, 12:45 PM
ENGINEERING MATERIALS
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CEMENT
WHAT IS CEMENT
Material with adhesive and cohesive properties
Any material that binds or unites-essentially like glue.
FUNCTION OF CEMENT
To bind the sand and coarse aggregate togetherto fill voids in between sand and coarse aggregate particleto form a compact mass
PROPERTIES OF CEMENT
Goodqualitycementpossesthefollowingproperties(whichdependuponitschemicalcomposition,thoroughnessofburningandfinenessofgrinding).
1.Providestrengthtomasonry.
2.Stiffensorhardenseasily.
3.Possessesgoodplasticity.
4.Easilyworkable.
5.Goodtomoistureresistant
SETTING AND HARDENING OF CEMENT
thatpropertyoffreshlymixedconcreteormortarwhichdeterminestheeaseandhomogeneitywithwhichitcanbemixed,placed,consolidatedandfinishediscalledtheworkabilityofconcrete(ACI).Workabilitydependsonwatercontent,aggregate(shapeandsizedistribution),cementitiouscontentandage(levelofhydration)andcanbemodifiedbyaddingchemicaladmixtures,likesuperplasticizer. Raisingthewatercontentoraddingchemicaladmixtureswillincreaseconcreteworkability.Excessivewaterwillleadtoincreasedbleeding(surfacewater)and/orsegregationofaggregates(whenthecementandaggregatesstarttoseparate),withtheresultingconcretehavingreducedquality.
MIXING CONCRETE
Separatepastemixinghasshownthatthemixingofcementandwaterintoapastebeforecombiningthesematerialswithaggregatescanincreasethecompessivestrengthoftheresultingconcrete.Thepasteisgenerallymixedinahigh-speed,sheartypemixerataw/cm(watertocementratio)of0.30to0.45bymass.Thecementpastepremixmayincludeadmixturessuchasacceleratorsorretarders,superplasticizers,pigments,orsilicafume.
PLACING AND COMPACTION
Themethodschosenforplacingandcompactingtheconcretewilldependonthetypeofconstruction,thetotalvolumetobeplaced,therequiredrateofplacingandthepreferencesandexpertiseoftheconstructioncompaniesinvolved.Thereare,however,severalbasicruleswhichshouldbefollowedtoensurethattheconcreteisproperlyplacedandcompactedintoauniform,voidfreemassonceithasbeendeliveredtotheformworkinasatisfactorystate:
REGULAR CONCRETE
Regularconcrete-isthelaytermdescribingconcretethatisproducedbyfollowingthemixinginstructionsthatarecommonlypublishedonpacketsofcement,typicallyusingsandorothercommonmaterialastheaggregate,andoftenmixedinimprovisedcontainers.Thisconcretecanbeproducedtoyieldavaryingstrengthfromabout10MPa(1450psi)toabout40MPa(5800psi),dependingonthepurpose,rangingfromblindingtostructuralconcreterespectively