Aggregate.pdf

Concrete Technology

1. Introduction of Concrete
Mr.KiranR. Patil
AssistantProfessor,
DepartmentofCivilEngineering,
D. Y. PatilCollege of Engineering & Technology,
Kolhapur

2. Aggregates
•Theaggregatesprovideabout70–80%ofconcretevolume.Therefore,theymustbe
strong,durable,cleanandofpropershape.
•Theymustbewellgraded.Theyshouldnotbechemicallyreactive.
•Aggregatesarecheaperthancementandgivegreatervolumestabilityanddurabilityto
concrete.Basically,theaggregatesareusedtoprovidebulktoconcrete.
•Theyalsoreduceshrinkage.Toincreasethedensityofconcrete;aggregatesoftwoor
moresizesareused.
•Fineaggregatehelpsinproducingworkabilityanduniformityinconcretemix.
•Classification of Aggregates
•ClassificationbasedonSource–
(i)NaturalAggregates
(ii)ArtificialAggregates
•NaturalAggregates:Naturalaggregatesareobtainedfromnaturaldepositsofsandor
fromquarries.Theseaggregatesmaybeigneous,sedimentaryormetamorphic.
Aggregatesobtainedfromigneousrocksarehard,troughanddense.Examplesofthese
rocksareGranite,Basalt,andTrap.
•ArtificialAggregates:Brokenbricksandblastfurnaceslagareusedasartificial
aggregates.Theaggregatesobtainedfromgoodqualitybrickshavingcrushingstrength
3.5N/mm2aresuitableformassconcretebutnotsuitableforR.C.C.

3. •ClassificationbasedonSize–
(i)FineAggregate:passesfrom4.75mmISsieve
(ii)CoarseAggregate:Retainon4.75mmISsieve
•ClassificationbasedonShape–
(i)RoundedAggregates:Fullywaterwarn
(ii)AngularAggregates:Havingwelldefinededges
(iii)FlakyandElongatedAggregates:Thicknessissmallrelativetothewidthorlength
•ClassificationbasedonUnitWeight–
(i)Normalweightaggregates:Sp.gravity-2.5-2.7,Density:2300-2600kg/m3
(ii)Heavyweightaggregates:Sp.gravity-2.8-2.9,Density:2800-2900kg/m3
(iii)Lightweightaggregates:Density:1200kg/m3

4. Physical Properties of Aggregates:
1.Size:
•Aggregatesaredividedintotwocategoriesbasedonsize-Coarseaggregate(C.A.)and
Fineaggregate(F.A.).Theaggregatebiggerthan4.75mmsizeiscalledcoarse
aggregateandaggregatesmallerthan4.75mmiscalledfineaggregate.
•CoarseAggregate–
•Thegradedcoarseaggregateisdescribedbyitsnominalsize40mm,20mm,16mm,
12.5mm,10mm,etc.
•Agradedaggregateofnominalsize20mmmeansanaggregatemostofwhichpasses
the20mmI.S.sieve.
•FineAggregate–
•Thefineaggregatemaybenaturalsandorcrushedsandobtainedbycrushingthe
boulders.I.S.383-1970hasdividedthefineaggregateintofourgradingzones
dependingupontheparticlesizedistribution.
•ThegradingzonebecomesprogressivelyfinerfromgradingZoneItoZoneIV.ZoneI
aggregateshouldnotbeusedforR.C.C.

5. 2.Shape:
•Aggregatesareclassifiedintothreetypesbasedonshape–Roundedaggregates,
Angularaggregates,FlakyandElongatedaggregates.
•RoundedAggregate:Theaggregatewithroundedparticles(rivergravels)has
minimumvoids.Itgivesminimumratioofsurfaceareatothevolume,thusrequiring
minimumcementpastetomakegoodconcrete.Theseaggregatemakesconcretemore
workable.Theonlydisadvantageisthattheinterlockingbetweentheparticlesisless
andhencedevelopmentofthebondispoor,makingitunsuitableforhighstrength
concreteandforroadpavements.
•AngularAggregate:Theaggregatewithsharpedgesandangularparticlehashigher
voids.Itrequiresmorecementpastetomakeworkableconcreteofhighstrengththan
thatrequiredbyroundedaggregate.Theinterlockingbetweentheparticlesisexcellent
anditprovidesagoodbond.Theseaggregatearesuitableforhighstrengthconcreteand
forroadpavements.
•Flaky and Elongated Aggregate: A particle is said to be flaky when its thickness
(least dimension) is less than 0.6 times the mean dimension. The mean dimension of an
aggregate is the average of the sieve sizes through which the particle passes and
retained, respectively.
•e.g. 20 mm sized aggregate is that which passes through 20 mm sieve and is retained
on 16 mm sieve. Its mean dimension is
20+16
2
= 18 mm and0.6??????18=10.8????????????
•Thus, the aggregate particles having the minimum dimension less than 10.8 mm is
considered as flaky.

6. •Aparticleissaidtobeelongatedwhenitslongestdimensionisgreaterthan1.8times
themeansievesizetowhichtheparticlebelongs.
•e.g.Fora20mmsize,anaggregateparticlewiththelongestdimensiongreaterthan
18??????1.8=32.4????????????wouldbeconsideredelongated.
•Flakyandelongatedaggregatesarenotdesirable.
•Theyreducetheworkabilityandtendtobeorientedinaplaneduringcompaction.The
hardenedconcretemayresultinstratifiedstructure.
3.Texture:
•Thesurfacetextureisameasureofsmoothnessorroughnessoftheaggregate.The
surfacetexturemaybeglassy,smooth,granular,rough,crystalline,porousor
honeycombed.
•Thebonddependsuponthesurfacetextureoftheaggregate.Anaggregatewithrough
textureispreferredtoonewithasmoothtexture.
•Assurfacesmoothnessincreases,contactareadecreases,henceahighlypolished
particlewillhavelessbondingareawiththematrixthanaroughparticleofthesame
volume.
•Asmoothparticle,however,willrequireathinnerlayerofpastetolubricateits
movementswithrespecttootheraggregateparticles.Itwill,therefore,permitdenser
packingforequalworkabilityandhence,willrequirelowerpastecontentthanrough
particles.
•Ithasbeenalsoshownbyexperimentsthatroughtexturedaggregatedevelopshigher
bondstrengthintensionthansmoothtexturedaggregate.

7. 4.Strength:
Generallythreetestsareconductedtodeterminethestrengthofaggregate–
(i)AggregateCrushingValue:
Thecrushingstrengthismoreimportant.I.S.383–1970prescribesamaximumcrushing
valueas45%fortheaggregateusedforconcreteotherthanforwearingsurfacesand35%for
concreteforwearingsurfacessuchasroads,pavementsandrunways.
(ii)AggregateImpactValue:
Toughnessistheresistanceoftheaggregatetofailurebyimpact.Toughnessismeasuredby
ImpactValueTest.AccordingtoI.S383–1970,theimpactvalueshouldnotexceed45%for
theaggregateusedforconcreteotherthanforwearingsurfacesand35%forconcretefor
wearingsurfaces.
(iii)AggregateAbrasionValue:
Hardnessoftheaggregateisitsresistancetowear.ItisobtainedintermsofAggregate
AbrasionValuebyusingLosAngelesmachine.Satisfactoryaggregateshouldhavean
abrasionvaluenotmorethan30%fortheaggregateusedforwearingsurfacesand50%for
aggregateusedfornon-wearingsurfaces.
5.SpecificGravity:
Thespecificgravityofanaggregateisdefinedastheratioofmassofsolidsampletothemass
ofanequalvolumeofwateratthesametemperature.
•Themainuseofspecificgravityistodesignaconcretemix,tocalculatetheyieldof
concreteforagivenproportionandtocalculatevoidsratioforagivenaggregate.

8. •Specificgravityofnormalweightaggregatevariesfrom2.5to2.9.Itismorethan2.9for
heavyweightaggregate.
6. Bulk Density:
•Thebulkdensityofanaggregateisdefinedasthemassofmaterialinagivenvolume.Itis
expressedinkg/litre.Thebulkdensityofanaggregatedependsuponparticlesize,shape,
gradingandmoisturecontent.
•Higherbulkdensityforacoarseaggregateindicatesthattherearelesservoidstobefilled
bysandandcement.Bulkdensityisusedforconvertingproportionsbyweightintothe
proportionsbyvolume.
7. Voids Ratio:
Theemptyspacesbetweentheaggregateparticlesarecalledvoids.Itisthedifference
betweenthegrossvolumeofaggregatemassandthevolumeoccupiedbytheparticlealone.
•Voids Ratio =1 -
������??????�??????�??????
�??????��??????�??????�����??????�??????
•Percentage voids =
specificgravity−bulkdensity
specificgravity
x 100
8. Soundness:
•Soundnessistheabilityofaggregatetoresistvolumechangesduetochangesinphysical
conditions.Thephysicalconditionsthataffectthesoundnessofaggregatearefreezing,
thawing,variationintemperature,alternatewettinganddrying,etc.
•Aggregatewhichundergomorethanthespecifiedamountofvolumechangeissaidtobe
unsoundaggregate.

9. •Thesoundnesstestconsistsofalternateimmersionofaggregatesampleinasaturated
solutionofsodiumsulphateormagnesiumsulphateandoven-dryingitunderspecified
conditions.Lossinweightismeasuredforspecifiednumberofcycles.
•Theaveragelossofweightafter10cyclesshouldnotexceed12%whentestedwith
sodiumsulphateand18%whentestedwithmagnesiumsulphate.
9.Cleanliness:
•Theaggregateshouldbefreefromimpuritieswhicharelikelytointerferewithhydration
process,preventionofeffectivebondbetweentheaggregateandpaste.
•Excessivesiltandclaymayresultinexcessiveshrinkageandincreasedpermeability.It
alsoreducesbondbetweenconcreteandsteel.
•AsperIS:383–1970,thetotalpercentageofsuchimpuritiesshouldnotbemorethan5%.
10.BulkingofFineAggregate:

10. •Theincreaseinthevolumeofagivenmassoffineaggregatecausedbythepresenceof
wateriscalledbulking.
•Freemoistureformsafilmaroundeachsandparticle.Thisfilmexertssurfacetension
whichkeepstheneighboringparticlesawayfromit.Thus,therewillnotbepointcontact
betweentheparticles.Thiscausesincreaseinthevolume.
•Thebulkingincreasesgraduallywithmoisturecontentuptoacertainpointandthenbegin
todecreasewithfurtheradditionofwaterasthefilmsarebroken.Whenthesandisfully
saturatedwithwater,thebulkingispracticallynil.
•Forordinarysands,thebulkingvariesfrom15to30%.Duetothebulking,sandshows
unrealisticvolume.Ifthesandismeasuredbyvolumeandnoallowanceismadefor
bulking,themixwillbericherthanthespecifiedbecauseforgivenmass,mostsand
occupiesaconsiderablylargervolumethanthesamemassofthedrysand.Thisresultsin
amixundersuppliedinsand(undersandedmix)increasingthechancesofsegregationand
honeycombingofconcrete.Thus,itisnecessarytoincreasethemeasuredvolumeofsand
bythepercentageofbulking.
•e.g.Forabulkingof20%,ifnoallowanceismadeforbulking,anominalmix1:2:4will
become1:1.6:4
11. Maximum Size of Aggregate
•Useofthelargestpossiblesizeaggregatereducesthecementandwaterrequirements.
Thisisduetothefactthatworkabilityofconcretedecreaseswiththemaximumsizeof
theaggregate.Useoflargersizeaggregateisbeneficialinmassconcreteduetothelesser
cementconsumption.

11. •Thiswillalsoreducetheheatofhydrationandshrinkagecracks.Duetosmallersurfacearea
ofthelargesizeaggregate,thewater-cementratiocanbedecreasedwhichincreasesthe
strength.
•In practice, the size of aggregate is limited by –
1. Thickness of section
2. Spacing of reinforcement
3. Clear cover
4. Mixing, handling and placing methods
•For strengths up to 20 N/mm², aggregates up to 20 mm may be used.
•According to IS: 456 –2000, the maximum nominal size of coarse aggregate:
1.should not be greater than ¼ ththe minimum thickness of the member
2.should be 5 mm less than the minimum clear distance between the main bars
3.5 mm less than the minimum cover to the reinforcement whichever is smaller.
12.FinenessModulus(F.M.):
•F.M.isanumericalindexofcoarsenessorfinenessofthematerial.Itgivessomeideaofthe
meansizeoftheparticlespresentintheentirebodyoftheaggregate.
•F.M.isobtainedbyaddingthecumulativepercentagesofaggregateretainedoneachofthe
standardsievesrangingfrom80mmto150micronanddividingthissumbyanarbitrary
number100.ThelargertheF.M.,thecoarseristhematerial.
•ThesievesthatareusedforthesieveanalysisoftheaggregateforconcreteasperIS:2386
(PartI)–1963,are80mm,40mm,20mm,10mm,4.75mm,2.36mm.1.18mm,600
micron,300micronand150micron.

12. •TheF.M.canberegardedasanaveragesizeofasieveonwhichthematerialisretained
andthesievesarecountedfromthefinest.
•e.g.AF.M.of5indicatesthatthe5
th
sieve(2.36mm)istheaveragesize
•F.M.forfineaggregatevariesbetween2and3.5.
•F.M.forcoarseaggregatevariesbetween5.5and8
•TheobjectoffindingF.M.istogradethegivenaggregateforthemosteconomicalmixfor
therequiredstrengthandworkabilitywithminimumquantityofcement.
•ThehigherF.M.willgiveaharshmixandlowerF.M.willgiveanuneconomicalmix.The
F.M.isalsoimportantformeasuringtheslightvariationsintheaggregatefromthesame
source.
•Example: Sieve analysis of Fine Aggregate
•Weight of sample taken = 500 gm

14. •Grading of Aggregates
•Theparticlesizedistributionofanaggregateasdeterminedbysieveanalysisis
calledgradingofaggregates.Ifallparticlesofanaggregateareofuniformsize,the
compactedmasswillcontainmorevoids;whereasanaggregatecomprisingof
particlesofvarioussizeswillgiveamasscontaininglesservoids.Theparticlesize
distributionofanaggregateshouldbesuchthatsmallerparticlesfillthevoids
betweenthelargerparticles.
•Thepropergradingofanaggregateproducesdenseconcreteandneedlessquantity
offineaggregateandcementpaste.Thus,itisessentialthatcoarseandfine
aggregateshouldbewellgradedtoproducequalityconcrete.
•Grading Curve:
•Thecurveshowingcumulativepercentageofmaterialpassingthesievesrepresenting
Y–axiswiththesievesizesonX–axisiscalledtheGradingCurve.Thegrading
curveindicateswhetherthegradingofagivensampleconformstothespecifiedoris
toocoarseortoofineordeficientinaparticularsize.

16. •Alkali-Aggregate Reaction (AAR)
•Thealkali-aggregatereactionisthereactionbetweenthereactivesilicaintheaggregates
andalkaliesincement.
•Thereactivesilicaoccursintrap,chert,siliciouslimestone,rhyolite,andesite,etc.The
reactionstartswithattackonthereactivesilicaintheaggregatebyalkalinehydroxidesin
cement.
•Thehydroxidesarederivedfromthealkalies(Na2OandK2O)inthecement.Asaresult
ofthisreaction,analkalisilicategelisformed.Swellingofthisgelmaycausecracking
anddisruptionofcementpaste.
❖ThefactorspromotingAARare,
➢Reactivetypeofaggregates
➢Highalkalicontentincement
➢Availabilityofwater
➢Alternatewettinganddrying
➢Temperatureintherangeof10°Cto38°C
❖ControlofAAR:TherateofAARcanbecontrolledby,
➢Selectingnon-reactiveaggregate
➢Usinglow-alkalicement
➢Controllingmoistureandtemperature
➢AddingadmixtureslikePozzolana