concrete technology properties of fresh concrete

8/10/2019 Concrete Technology Properties of Fresh Concrete

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PropertiesofFreshandHardenedConcrete


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PropertiesofFreshConcrete


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Workability SlumpLoss

Segregation/Bleeding

PlasticShrinkage

TimeofSet

Temperature

Outline

[email protected] 04/01/2013DepartmentofCivilandEnvironmentalEngineering,UniversityofLagos

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Workability

Slumtest

CompactingFactor

VBConsistometer

Tests

[email protected] 04/01/2013DepartmentofCivilandEnvironmentalEngineering,UniversityofLagos

5


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SlumpTest

Invertedcone

fillitupwiththreelayersofequalvolume

rodeachlayer25timesscrapeoffthesurface

200

100

300


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SlumpTest

slumpcone

rod

concrete


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Slumptest

Slump

Ruler


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TypesofSlump

Collapse Shear True

Inacollapseslumptheconcrete

collapses

completely.

Inashearslumpthetopportionoftheconcrete

shearsoffandslipssideways.

Inatrueslumptheconcretesimplysubsides,keeping

moreorlesstoshape


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Europeanclassesofslump

According

to

European

Standard

EN

206

1:2000

five

classesofslumphavebeendesignated,astabulatedbelow

Slump

class Slump

in

mm

S1 10 40

S2 50 90

S3 100 150

S4 160210

S5 220


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Factoraffectingworkability

Cementwater

Admixtures

Aggregate

AmbientConditions

Time


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Cementwater

Theworkabilityoftheconcretedecreasesasthefinenessofthecementincreases.


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Admixtures

Theprincipaladmixtureseffectingimprovementintheworkabilityofconcretearewaterreducingandair

entrainingagents. Workabilityadmixturesareusedtoincrease

workabilitywhilethemixproportionsarekeptconstantsortoreducethewatercontentwhilemaintainingconstantworkability.


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Aggregate

Theworkabilityofconcreteismainlyinfluencedbythetotalsurfaceareaoftheaggregate.

Thesurfaceareaisgovernedbythemaximumsize,gradingandshapeoftheaggregate.

Workabilitydecreasesasthespecificsurface

increases..


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AmbientConditions

Environmentalfactorsthatmaycauseareductioninworkabilityaretemperature,humidityandwind

velocity

.


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Time

Timeofmixingofconcreteanditsfinalcompactiondependsonthegeneralconditionsofworksuchas

thedistancebetweenthemixerandthepointofplacing,siteproceduresandgeneralmanagement.

.


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Segregation

Whenthereisasignificanttendencyforthelargeandfineparticlesinamixtobecomeseparated,

segregationissaidtohaveoccurred.i.e.coarseaggregatetendstosegregateduetogravity.Typicalofdrymixes.

.


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Bleeding

Wheretheconsistencyofamixissuchthatitisunabletoholdallitswater,someofitisgradually

displacedandrisestothesurface. Typicalofwetmixes.


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Bleeding


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Causesofbleedingandsegregation

Improperslump

Excessiveamountofcoarseaggregate

Lackoffines

Inappropriateplacingandcompacting

.


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ControlofBleeding

Reductionofwater

Introductionoffinesandair

ProperCompaction(toomuchcompaction

causesbleeding).


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PropertiesofHardenedConcrete


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Outline

Compressivestrength

Tensilestrength

Factoraffectingstrengthofconcrete

Deformation


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Compressivestrength

Thecompressivestrengthofconcreteistakenasmaximumcompressiveloaditcancarryperunitarea.

ConcretestructuresarenormallydesignedonthebasisthatconcreteiscapableofresistingONLYcompression,thetensionbeingcarriedbysteelreinforcement.


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Tensilestrength

Thetensilestrengthisoneofthebasicandimportantpropertiesoftheconcrete.Theconcreteisnotusually

expectedtoresistthedirecttensionbecauseofitslowtensilestrengthandbrittlenature.However,thedeterminationoftensilestrengthofconcreteisnecessarytodeterminetheloadatwhichthe

concretemembersmaycrack.Thecrackingisaformoftensionfailure.


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Tensilestrength

Apartfromtheflexuretesttheothermethodstodeterminethetensilestrengthofconcretecanbebroadlyclassifiedas

(a)directmethods,and

(b)indirectmethods.Thedirectmethodsuffersfromanumberofdifficultiesrelatedtoholdingthespecimenproperlyinthetestingmachinewithoutintroducingstressconcentration,andtothe

applicationofuniaxialtensileloadwhichisfreefromeccentricitytothespecimen


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Tensilestrength

Inducedtensilestressatfailureisgivenas

2

=Maximumappliedload

=cylinderlengthanddiameterrespectively


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Tensilestrength

Flexuralstrengthofconcreteisanotherindirecttensile.Flexuralstrength(modulusofrupture)isgivenby

=Maximumappliedload

=distancebetweenthesupports

=beambreadthanddepthrespectively


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FactorsAffectingStrength

1. Influenceofconstituentmaterials

Cement:finenessandchemicalcomposition

Water:a minimumamountofwaterforaproper

hydrationAggregate:thebond(aggregatematrixinterface)


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FactorsAffectingStrength

2. Admixtures

Calciumchlorideisanacceleratoradmixtures,itincreasestherateofdevelopmentofstrengthatearlydays.

3. Methodsofpreparation

4. Influenceofcuring

5. Influenceoftestcondition(Cube,Cylinderandsquareprism)

6. Specimen

moisture

content

and

temperature7. Methodofloading


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StrengthandCuring

time

inairentiretime

moistcuredentiretime

inairafter3days

inairafter7days

Strength

28

100%


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Deformation

Concretedeformsunderload,thedeformationincreaseswiththeappliedloadelasticdeformation.

Whenitcontinuestodeformwithtimeunderconstantloadtimedependentdeformationorcreep


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Elasticdeformation

Deformationrelationshipforconcretesubjectedtoacontinuousincreasingloadisnonlinearincharacter

Whentheappliedloadisreleasedtheconcretedoesnot

fullyrecovereditsoriginalshape.


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Creepdeformation

Concretesubjectedtosustainedloadwillundergoelasticandcontinuestodeformwithtime.

Theincreaseinstrainwithtimeistermedcreep


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Shrinkage

Settlementofsolidsandthelossoffreewaterfromtheplasticconcrete,bythechemicalcombinationofcementwithwater(autogenous shrinkage)andbythedryingofconcrete(dryingshrinkage)

PlasticShrinkage:takesplacebeforeconcreteset

Autogenous Shrinkage:ashydrationcontinuesinanenvironmentwherewatercontentisconstant,thereis

a

decrease

in

volume

of

the

cement

paste

which

results

inshrinkageoftheconcrete.


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Shrinkage

Dryingshrinkage:waterlosesfromthevoidsandcapillaryporesafterconcretehardened,furthershrinkagewherewaterisdrawnoutofcementgel.

concrete technology properties of fresh concrete

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