Evaluation of Asphalt Absorption by Mineral Aggregates

8/9/2019 Evaluation of Asphalt Absorption by Mineral Aggregates

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EVALUATIONOFASPHALTABSORPTIONBYMINERALAGGREGATES

By

PrithviS.Kandhal

MaqboolA.Khatri

PublishedintheJournaloftheAssociationofAsphaltPavingTechnologists,AsphaltPavingTechnology,

Volume60,1991.

277TechnologyParkway Auburn,AL36830


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EVALUATIONOFASPHALTABSORPTIONBYMINERAL AGGREGATES

By

PrithviS.Kandhal

AssistantDirectorNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

MaqboolA.Khatri

GraduateResearchAssistantAuburnUniversity,Alabama

PublishedintheJournaloftheAssociationofAsphaltPaving

Technologists,AsphaltPavingTechnology,

Volume60,1991.


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DISCLAIMER

Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsorpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

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ABSTRACT

Allmineralaggregatesusedinthehotmixasphalt(HMA)mixtureshavesomeporosityandas

suchtendtoabsorbsomeamountofasphalt.Differenttestmethodsareusedbytheagenciestodeterminetheamountofasphaltabsorbedbythemineralaggregates.Thereisaneedtodeveloparealisticandaccuratestandardtestmethodfordeterminingasphaltabsorption,especiallybecauseofitstimedependentnature,sothatthevoidparameters(suchasairvoidcontent,voidsinthemineralaggregate,andvoidsfilledwithasphalt)ofthecompactedmixspecimenscanbedetermineduniformlyandaccuratelybyallagencies.

Threemineralaggregatesofdifferentabsorptivecharacteristicsandtwoasphaltcements(AC-20

andAC-5grades)fromtheStrategicHighwayResearchProgram(SHRP)MaterialsReferenceLibrarywereusedinthisstudy.Althoughthreeprimaryabsorptionmethodswereused,atotalofninedifferentconditionings/procedureswereattempted.

Basedonthetestresults,ithasbeenrecommendedtouseRicemethodfordeterminingasphalt

absorptionaftertheHMAmixhasbeenagedfor4hoursat2900F.Also,theasphaltabsorptionhasbeenfoundtofollowahyperbolicrelationshipwithtimeandpotentiallyreachesanultimate(limiting)value.Itispossible,basedonthisrelationship,topredictasphaltabsorptionatanygiventimeiftheabsorptionvaluesjustaftermixingandatothertwoagingtimesareknown.

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EVALUATIONOFASPHALTABSORPTIONBYMINERALAGGREGATES

PrithviS.Kandhal&MaqboolA.Khatri

INTRODUCTIONAllmineralaggregatesusedinhotmixasphalt(HMA)mixtureshavesomeporosityandassuchtendtoabsorbsomeamountofasphalt.Althoughsomeabsorptionmayleadtoimprovedstrengthinacompactedmixturethroughparticleinterlocking,theportionoftheasphaltthatisabsorbedisnolongeravailableasbinder(.1).Thesituationisfurtheraggravatedbecauseofthetime-dependentnatureoftheabsorptionphenomenon.Moreover,theasphaltavailableinthinfilmsservingasabindermayhavedifferentphysical,chemicalandrheologicalpropertiesifselectiveabsorptiontakesplace(2).Theabsorptionofasphaltinmineralaggregatesmaycausethefollowingwhichmayleadtoprematurefailureoftheasphaltpavingmixtures(1):

1.Incorrectcomputationofpercentairvoids,voidsinmineralaggregateorvoidsfilledwithasphalt.Sinceoneormoreofthesearegenerallyusedascriteriaforasphaltmixturedesign,incorrectcalculationsmayleadtomixtureslackingdurabilityorstability.

2.Nothavingenougheffectivebindermayleadtoraveling,cracking,orstripping.3.Possibleprematureagehardeningandlowtemperaturecrackingasaresultof

changesinasphaltpropertiesduetoselectiveabsorption.4.Constructionproblemssuchassegregationandtendermixes.

Inviewoftheaboveconsequences,theexactevaluationofabsorptionofasphaltbymineral

aggregatesisofparamountimportance.Theproblemalsoneedstobeaddressedsinceoftentimesaggregateshavingrelativelyhigherasphaltabsorptionmayhavetobeutilizedeitherbecauseofhigherqualityaggregatesgettingdepletedataneverincreasingrateorbecausegeographicproximityoftenpredicatestheuseofsuchaggregates.

AlthoughmosthighwayagenciesutilizetheRicemethod(maximumtheoreticalspecificgravityofvoidlesspavingmixtures)incalculatingtheamountofasphaltabsorbed,othermethodssuchasbulkimpregnatedspecificgravityarealsoused.Also,someaggregateshavebeenobservedtocontinuetoabsorbasphaltwithtimeduringconstructionandtheearlylifeofanasphaltpavement.Hotmixasphalt(HMA)mixtureswhichappearveryrichduringproductioncangetleanerandleanerwithtimeinducingprematureravellingand/orcracking(3).Thereisaneedtodevelopandstandardizearealisticmethodfordeterminingasphaltabsorptionespeciallyinviewofitstimedependentnature.

OBJECTIVES

Thisresearchworkwasundertakentoachievethefollowingobjectives:

1.Evaluatethevariousmethodscurrentlyavailabletodeterminetheamountofasphalt

absorbedbythemineralaggregates,andrecommendthemostrealisticandsuitablemethodbasedonthetestdataobtained.

2.Studythetime-dependentphenomenonofasphaltabsorptionespeciallyinviewofthestorageofHMAinsilosand/ortransportationoverlongdistancestothepavingsite,anddevelopasphaltabsorptionpredictivetechniques.

REVIEWOFLITERATURE

Manyinvestigatorsinthepasthaveattemptedtoevaluateasphaltabsorptionofaggregatesthroughcorrelationswithabsorptionusingotherliquidsofwhichkerosenehasbeenthemostcommonlyused.Hveem(4)devisedthecentrifugekeroseneequivalenttest(CKE)in1942.TheCKEisdefinedasthequantityofkeroseneabsorbedby100-gofaggregateunderspecified

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conditionsofsoakingandcentrifuging.Ithasbeenshowntobeafunctionofthesurfaceareaand

absorptivecapacityoftheaggregateandhasbeenusedasapartofthedesignofbituminousmixturesbyHveemmethod.Lohn(5)alsousedasimilarapproachandcorrelatedasphaltabsorptionofanaggregatewithkeroseneabsorption.Hestudiedtheeffectsoffactorssuchas

saturationtime,centrifugeforceandcentrifugetimeaswellandfinallyadopted10-minutesofsaturationand8-minutesofcentrifugingataforceof400timesgravity.Donaldsonetal(6)furtherproposedsomemodificationstotheHveemCKEmethodbyincreasingthesoakingtimeto30-minutesandbytestinganon-absorptiveaggregateofthesamegradationforthepurposeofcomparison.Sincekerosenehaswettingpropertiessimilartothatofasphalt,ithasbeenbelievedtogiveabetterrepresentationasanabsorptionagentascomparedtowater(6,7).AmodifiedCKEprocedurewasdevelopedbyCechetini(8),whichcandeterminethesurfaceareasandevaluateabsorptivebehaviorofcrushedaggregatemixes.Togetherwiththisinformationandthedatacharacterizingtheasphalttobeused,theamountofasphaltneededbyanaggregatecanbedeterminedfromanomograph.Someotherinvestigators(9,10)havetriedtouseoilstoevaluatetheabsorptivecapacityofcoarseaggregates.However,becauseofthedifferencesinwettingpropertiesandviscositiesoftheseliquidsandasphalts,onlyapproximateestimationscouldbe

madeoftheasphaltabsorptionoftheaggregates.In1936,aprocedurewasdescribedbyReagel(11)fordeterminationofrelativeabsorptionof

waterandliquidbituminousmaterialbythecoarseaggregateusingawaterdisplacementmethod.Itwasrecommendedthatabsorptionofliquidbituminousmaterialbeestimatedat75percentofthewaterabsorption.

In1942,GoshornandWilliams(12)developedtheimmersionmethodwhichisdescribedlater.

Sincetheaggregateisincontactwithanunlimitedsupplyofasphaltatrelativelylowviscosityforanextendedperiodoftimeinthismethod,theabsorptionismuchhigherthanwouldbeexpectedinactualHMAmixtures(13).However,thevaluescanbetakenastheabsorptivepotentialoftheaggregatesusedforHMAmixtures.Accuracyofthismethodislimitedbythehomogeneityoftheaggregateandaccuracyofthebulkspecificgravity.

Rice(14,15)proposedavacuumproceduretodeterminethemaximumspecificgravityofthe

voidlessHMAmixture.Inthismethod,theabsorptionofasphaltbyaggregatesiscalculatedfromthemaximumspecificgravityofthemixture,theasphaltcontent,andthebulkspecificgravityoftheaggregateusedintheHMAmixture.ThismethodisnowstandardizedasASTMD2041.Ifaggregatesarenotthoroughlycoated,erroneousresultsmaybeobtainedbecauseofpossibleabsorptionofwaterbytheaggregateduringvacuumsaturation.TheASTMtestmethodD2041,however,allowstheuseofasupplemental(dryback)proceduretocorrectforthewaterabsorptionproblem.Themethodcanbeappliedtoactualmixesandisnottootimeconsuming.TheaccuracyofthismethodisalsodependentupontheaccuracyofbulkspecificgravityoftheaggregatesusedintheHMAmixture.

AdifferentapproachwasusedbyLarsen(16)whoconductedahighpressuretestinsteadof

vacuumsaturationtodeterminetheeffectivespecificgravityoftheaggregate.Asphaltabsorptionwascalculatedaspercentofthevolumeofaggregateandrangedfrom26to88percentofthewaterabsorption.

TheU.S.ArmycorpsofEngineers(17,18)developedandhasusedthebulkimpregnated

specificgravityinthedesignandcontrolofbituminousmixtures.Itisafunctionoftheratioofasphalttowaterabsorption,whichvarieswidelybutfollowsadefinitepatternwithdifferenttypesofaggregates.Theasphaltabsorptioncanbefoundknowingthebulkimpregnatedspecificgravityofthemixture,thebulkspecificgravityoftheaggregateandthespecificgravityoftheasphaltusedintheHMAmixture.McLeod(19)hasrecommendedtheuseofthismethodtodeterminethemaximumorupperlimitofasphaltabsorptionforanaggregate.Lee(13)has,however,comparedthedataobtainedthuswithothermethodsandconcludedthatthese

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absorptionvalueswerebynomeanstheabsolutemaximumabsorptionvaluesforaparticular

aggregate,butcanbeconsideredasthe"realistic"maximumabsorptionvaluesaggregateswillhaveinHMAmixtures.Oneseriouslimitationofthismethodistheremovalofairbubblesentrappedinasphaltwhenbothcoarseandfineaggregatesareaddedtotheasphaltcement.

Absorptionofasphaltbyaggregateshasalsobeendeterminedbycalorimetricanalysiswithphotometer(13).Thebasicprincipleisthattheamountoflightabsorbedbyagivensolutionisproportionaltotheintensityoftheincidentlightandtotheconcentrationoftheabsorbingspeciesinthepathofthelightbeam.However,onlysolutionsoflightconcentrationcanbeusedandthemechanismofabsorptionofasphaltinsolutionislikelytobedifferentfromthatinanHMAmixture.

Cross-sectionalmeasurementshavealsobeenemployedtoevaluateasphaltabsorption(13).

CompactedspecimenoftheHMAmixtureiscutinhalvesbyadiamondsawtoexposetheinnersurfaceoftheaggregates.Thesampleisthenplacedinanopaqueprojectorandfromitsimageprojectedonascreen,tracingsaremadeoftheexternalcontoursoftherockparticlesandlinesof

deepestasphaltpenetration.Thetotalareaofaparticleandareapenetratedbytheasphaltaremeasuredbyaplanimeter,andthepercentageofasphaltabsorptioniscalculated.Limitationsofthismethodareselectiveabsorptionofasphaltanddifferentsizesofaggregateinamixture.

MorerecentlytheMethyleneBluetesthasbeenusedinEuropetoindirectlymeasurethe

absorption/adsorption,surfacearea,cationicexchangecapacity,soundness,andoverallclaycharacteristicsofaggregates.Thistest,firstdevelopedinFrance(20),usesMethyleneBluetoquantifytheabsorption/adsorptionofanaggregate.Themethodhasseriouslimitations:a)apowderedaggregateisused,andb)noasphaltcementisused.

FrancoandLee(21)haverecentlyevaluatedtheviabilityofusinganairmeterfordetermining

themaximumspecificgravityofHMAmixtures.TheairmeternormallyhasbeenusedfordeterminingthepercentofairentrainedinPortlandcementconcrete.Theso-calledpressure

methodworksontheprincipleofBoyle'slaw.AweighedsampleofHMAmixisintroducedintothebowloftheairmeter(TypeBairmeterasspecifiedinAASHTOT152)andwaterisintroducedtofillthemetertothecapacity(noattemptismadetoremovetheentrappedair).Thefilledairmeterisweighedandtheweightofwaterobtained.TheaircontentofthemeteristhendeterminedinaccordancewithAASHTOT152.BackcalculationsarethenperformedandthevolumeoftheHMAmixisdetermined.Furtherworkisneededtoimprovethedesignoftheairmetersothatitsconsistencyandsensitivityisacceptable.Thefollowingisthesummaryofconclusionsbasedonthereviewofliteratureconcerningasphaltabsorptionasaffectedbythemethodsofdetermination.

1.Nosinglestandardmethodiscurrentlyinuseforevaluatingandspecifyingtheabsorptivecharacteristicsofaggregateswithrespecttoasphaltcement.

2.Theindirectestimationofasphaltabsorptionusingwater,keroseneandoilisneither

realisticnorreliable.3.Theimmersionmethodmaybeusedtoquantifythepotentialmaximumabsorptive

capacityofacoarseaggregate.However,itcannotbeusedforthewholeHMAmixconsistingofcoarseandfineaggregates.

4.Thebulkimpregnatedspecificgravitymethodcanbeusedtodeterminetherealisticmaximumabsorptivecapacityofanaggregatebutithaslimitationsduetotheproblemofremovalofairbubbles.

5.RicemethodcanbeusedforthewholeHMAmixandisthemostrepresentativeofallmethodssinceitteststheactualHMAmixwhereinasphaltispresentonlyinthinfilmsaroundtheaggregate.

6.Thephenomenonofasphaltabsorptionisverymuchtime-dependentandneedstobeevaluatedassuch.

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7.Theaccuratedeterminationofasphaltabsorptionbymostmethodsisdependenton

theaccuracyofthedeterminationofbulkspecificgravityoftheaggregatesusedintheHMAmixture.

8.Thereisanurgentneedforadoptingarealisticandsuitablestandardmethodfor

determiningasphaltabsorptionsothatreproducibleresultsareobtainedandtheHMAmixescanbedesignedproperly.

MATERIALS

TheMaterialsReferenceLibrary(MRL)forStrategicHighwayResearchProgram(SHRP)has11differentaggregates.Threeaggregateswereselectedforuseinthecurrentstudy,basedonthewaterabsorptiondatafortheMRLaggregates,coveringawiderangeofwaterabsorption.Theseaggregatesare:

1.RC-McAdamLimestone2.RB-WatsonvilleGranite3.RD-FrederickLimestone

Theseaggregateshavehigh,mediumandlowwaterabsorption,respectively.DetailedpropertiesfortheaggregatesusedarereportedinTable1.Itwasdecidedtousecoarseaggregatesonlyofuniformsinglesize,namely,passingl/2-insieveandretainedon3/8-insieve.Thiswasnecessitatedbecauseoneofthemethods(immersionmethod)usedindeterminingasphaltabsorptioncanbeusedforcoarseaggregateonly,and,forbulkimpregnatedmethod,resultsareaffectedbythepresenceoffineaggregate.Useofauniformgradationwouldgiveconsistentresultswithdifferentabsorptionmethods.Althoughfineaggregate,whichhasalargesurfacearea,isbelievedtohaveapronouncedinfluenceonasphaltabsorption,itsasphaltabsorptionisnotlikelytobetime-dependent.Allaggregateswerethoroughlywashedanddriedbeforeuseinordertoexcludetheeffectoffinesandmoisture.Inadditiontotheabove,glassbeadswerealsousedinordertoevaluatetheoperatorandproceduralerrorsassociatedwitheachmethod.Theglassbeadsusedwerealsoofthesamesize.

Table1.PropertiesofAggregatesUsed

Property

BulkSpecificGravity

a)ASTMC127

b)ASTMC127+Vacuum*

c)HgPorosimetry**

WaterAbsorption(%Wt.ofAggregate)

a)ASTMC127b)ASTMC127+Vacuum*

MedianPoreDiameter,micron

IndexofParticleShape&Texture(ASTMD3398)

RC-McAdamLimestone

2.485

2.494

2.467

2.882.95

0.178

13.2

RB-WatsonvilleGranite

2.692

2.709

2.759

1.681.58

0.054

14.6

RD-FrederickLimestone

2.713

2.722

2.899

0.380.28

0.013

15.2

*Aggregatewasvacuumsaturatedwithwater(vacuumlevel=30mmHg,vacuumsaturationtime=10min)and thenkeptimmersedinwaterfor24hours **Obtainedat14.7psi

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TheSHRPMaterialsReferenceLibraryhasseveralasphaltsavailableofwhichthefollowing

wereselectedforuseinthecurrentphaseofresearch:1.AAM-l-WestTexasAC-202.AAB-2-WyomingSourAC-5

TheAC-20asphaltwasselectedbecauseitisthemostcommonlyusedviscositygradeintheU.S.LowviscosityAC-5waschosentostudytheeffectofviscosityofasphaltontheamountofabsorption.ThisisalsothelowestviscositygradedasphaltavailableintheMRL,andisnormallyusedforpavinginCanadaandAlaska.VariouspropertiesoftheasphaltcementsusedinthisstudyaregiveninTable2.

Table2.PropertiesofAsphaltUsed

Asphalt

Grade

OriginalAsphaltViscosityat

140F,poise

275F,cSt

Property AAM-1

AC-20

1992

569

AAB-2

AC-5

403

193

Penetration,0.1mm(77F,100g,5sec)

Ductility,cm(39.2F,1cm/min)

SofteningPoint(R&B),F

ComponentAnalysis

Asphaltenes(n-heptane)

PolarAromatics

NaphtheneAromatics

Saturates

ElementAnalysis

Nitrogen,%

Sulfur,%

Vanadium,ppm

Nickel,ppm

ThinFilmOvenTest

MassChange,%

ViscosityofTFOTResidueat

140F,poise

275F,cSt

ViscosityRatio(140F)

64

4.6

125

3.9

50.3

41.9

1.9

0.50

2.40

6029

0.00516

3947

744

1.98

166

81.0

115

16.7

35.7

36.5

10.8

0.50

5.40

16336

0.00149

1073

263

2.66

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TESTPROCEDURES

Threetestmethodswereprimarilyusedfordeterminingtheasphaltabsorption.However,proceduralvariationsweremadewithintwotestmethodsresultinginthefollowingninetesting

techniques:1-2)Immersionmethodwithimmersiontimesof1and3hours.3) Bulkimpregnatedspecificgravitymethod4-9)Ricemethodwithmixagingtimesof0,1,2,4,6,and8hours.

Theimmersionmethodwasfirstusedin1942byWilliams(12)andgivesthemaximumor

potentialGoshornandabsorptivecapacityofanaggregate.Theoriginalprocedureinvolvedheatingtheaggregatetoabout300Fandthensuspendingitbymeansofawirebasketin85-100penetrationasphaltat275Ffor2hours.Thesamplewasthencooledtoroomtemperaturewhileimmersedandagainreheatedtoandmaintainedat275Ffor1hour.Thebasketcontainingcoatedaggregatewasthenremovedandsuspendedinanairbathat275Funtilallexcessasphalthaddrainedoff.Thecoatedaggregatewasremovedfromthebasket,cooledtoroomtemperature

andweighedinbothairandwatertodetermineasphaltabsorption.Forthisstudyauniformtemperatureof290FwasselectedforallphasesofthistestsinceitapproximatestheaveragemixingtemperatureusedbytheHMAindustry.Additionally,theaggregatewasimmersedonlyonceforimmersiontimesofeitheroneorthreehourstoavoidanyvariationsthatmightoccurduringtheprocessofreheating.Thebasketsusedwereweighedbothinairandunderwatertoavoidremovingthecoatedaggregatefromthebasketforweightdeterminations.Thetimeusedfordrainingexcessasphaltwasselectedas10minutestoattainuniformdrainingforallsamples.

TheU.S.ArmyCorpsofEngineers(17,18)developedandusesthebulkimpregnatedspecific

gravityinthedesignandcontrolofbituminouspavingmixtures.TheCorps'procedureinvolvesheatingabout1500gofaggregatetoconstantweightat230-290Fandweighingit.Theasphaltisthenseparatelyheatedto2805Fandpouredintoagallonpailone-thirdfull.Ametalstirrerisinsertedandasphaltisthenallowedtocooltoroomtemperatureforaminimumof8hours.

Thepailwithasphaltandstirreristhenweighedbothinairandwater.Asphaltpailandaggregatearethenseparatelyheatedto2805Funtilthetemperatureisstabilized.Atthisstagepailisremovedfromtheovenandaggregateisslowlyaddedtoitwhilestirringwiththestirrer.Stirringiscontinuedtilltheelapsedtimefromstartofmixingis2minutes.Thecontentsarethencooledtoroomtemperatureandairbubbles,ifany,removedusingaflame.Thepailwithasphalt,aggregateandstirreristhenweighedbothinairandwater.Theabovefivemeasurementsallowthecomputationofthebulkimpregnatedspecificgravityandhencetheasphaltabsorption.Forthepurposeofpresentstudy,somemodificationstothetestweredone.Theweightofaggregateusedwas500ginsteadof1500gbecausetheaggregatehadoneuniformsinglesizeandthuswaslessvariable.Asaconsequencetheasphaltquantitywasalsoreducedfromone-thirdgallontoone-halfquart.Additionally,auniformtemperatureof290Fwasusedatallstagesofthetestwhereuseofovenwasrequired.

TheRicemethodwasoriginallyproposedin1956(14,15)todeterminethemaximumspecificgravityofabituminousmixturebyusingvolumetricflasks.Forthisstudytrialmixeswerepreparedandheatedintheovenat290Fupto8hourstoobservetheavailableeffectivebinderafteragingatvariousasphaltcontents.Anasphaltcontentof2.5percentwasselectedforallthreeaggregates.Sufficientavailableeffectiveasphaltbinderwasobservedvisuallyatthisasphaltcontentevenafter8hoursofagingintheoven.Athigherasphaltcontentsthefilmsweretoothickanddifficultieswereexperiencedinhandlingthemixes.MixeswerepreparedusingthestandardpracticeforpreparingmixturesfortheMarshallmixdesignmethodexceptthattheywerenotcompacted.Bothasphaltcementandaggregatewereheatedto290Ftomaintainuniformityoftemperaturethroughoutthewholetestingscheme.Themixeswereagedintheovenat290Fincoveredcontainersfor0,1,2,4,6,and8hourstostudytheeffectoftimeontheabsorptionofasphalt.Thetheoreticalmaximumspecificgravityofthemixtureswas

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determinedusingASTMstandardprocedureD2041(Ricemethod)usinga4000mlthick-walled

glasspycnometer.KandhalandWenger(22)developedthislargePennsylvaniapycnometertodeterminetheasphaltcontentofHMAmixes.ItisstillusedbytheCentralAsphaltLaboratoryofthePennsylvaniaDepartmentofTransportationtodeterminetheRicespecificgravity.

Accuratevolumetricmeasurementsarepossibleinthispycnometerbecauseofaspecialcapillarystopperandanoverflowcap.

EXPERIMENTDESIGN

Theexperimentwasdividedintotwophases.PhaseIoftheexperimentwasa9x3x2factorialdesignwith3replicatesforeachcombinationgivingatotalof9x3x2x3=162tests.Thethreereplicatesweretreatedasblocksandtestswithineachblockwerecompletelyrandomizedgivingablockedrandomizeddesign.

PhaseIIinvolvedtheuseofglassbeads(absorption=0)asacontrolmaterialtoascertainand

quantifytheerrorsassociatedwiththetestequipment,testprocedure,andoperatorforeach

method.Sinceviscosityoftheasphaltcementandimmersionoragingtimeswillnotaffecttheresults,onlyoneasphaltAAM-l(AC-20)wasusedduringthisphase.Obviously,methods2and5-9werenotconsidered.

TESTDATAANDDISCUSSION

TheresultsofthephaseIexperimentsarepresentedinTable3andFigures1-4.ThevaluesreportedinTable3areaveragesofthreetestswhichwererunatdifferenttimes.TherangeoftheresultsobtainedfordifferentmethodsisalsoindicatedinTable3.BecauseofverylowasphaltabsorptionvaluesobtainedwithRDaggregate,plotteddataforthisaggregateisnotincludedinthispaper.AnANOVA(analysisofvariance)oftheresultsobtainedisshowninTable4.TheANOVAindicatesthatallthefactors,namely,methods(A),aggregates(B),andasphalts(C)aresignificantat"=0.01.Inotherwords,thevaluesofasphaltabsorptionobtainedgreatlydepend

ontheaggregateandasphaltpropertiesaswellasthemethodusedfordeterminingasphaltabsorption.Moreover,theinteractions:AxB(aggregatesxmethods)andBxC(aggregatesxasphalts)arealsofoundsignificantat"=0.01.Allthissuggeststhatonlyastandardmethodofdeterminingasphaltabsorptionforanyparticularsetofaggregateandasphaltmustbeusedbyallagenciestoreasonablyreproducethatmixdesign.

AbsorptionbyImmersionMethod

Asexpected,theresultsreportedinTable3andFigures1-4indicatethatimmersionmethodyieldsthehighestvaluesofasphaltabsorptionforanyaggregate-asphaltcombination.Moreover,thethree-hourimmersionabsorptionvaluesaresignificantlyhigherthanone-hourimmersionvalues.Thisagainindicatesthetime-dependentnatureoftheabsorptionphenomenon.AsseeninFigures1-4andalsoinTable3,thelowerviscosityasphaltAAB-2giveshigherabsorption

valuesasexpected.Thatmeansthehighertheviscosityofasphaltused,theloweristheabsorptionforaparticularaggregate.Obviously,thelowestabsorptionaggregateRDdoesnotshowanysensitivityeithertotheviscosityofasphaltortotheimmersiontime.Theasphaltabsorptionvaluesforl-hourimmersionarefoundtobe28to48percentofthewaterabsorptionvaluesbasedonthetestdatareportedinTables1and3.Forthree-hourimmersionthispercentagevariesfrom27to61percent.

Althoughthismethodyieldsthehighestvaluesofabsorption,itcannotbetermedasthemost

realisticmethodofdeterminingasphaltabsorptionbecauseasphaltispresentinbulkquantityaroundtheaggregate,whichisunrealisticforaHMAmixture.Moreover,ithasthelimitationofusingthecoarseaggregateonlywhereasmostcommonlyuseddensegradedHMAmixturescontainbothcoarseandfineaggregates.

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Table3.AsphaltAbsorptionDataforPhaseI(PercentbyWeightofAggregate)

RC-McAdamLimestone RB-WatsonvilleGranite RD-FrederickLimestoneMethod/Conditioning (WaterAbsorption=2.88%) (WaterAbsorption=1.68%) (WaterAbsorption=0.45%)

AAM-1 AAB-2 AAM-1 AB-2 AAM-1 AAB-2(AC-20) (AC-5) (AC-20) (AC-5) (AC-20) (AC-5)

1.ImmersionMethod(1hr) 0.80 1.17 0.71 0.81 0.18 0.17

2.ImmersionMethod(3hr) 1.23 1.63 0.91 1.02 0.12 0.21

3.Bulk-ImpregnatedMethod 0.73 0.97 0.60 0.73 0.18 0.15

4.RicewithNoAging 0.17 0.26 0.35 0.36 0.10 0.04

5.Ricewith1hourAging 0.42 0.60 0.44 0.59 0.07 0.02





Notes:1.Eachreportedvalueisanaverageof3testrestults.2.Ontheaverage,therangeofvaluesforthe3replicateswas:

Method

Immersionmethod

BulkimpregnatemethodRicemethod

Overall

0.17

0.17*

Range

0.13

0.16

*TheleastaveragerangewasobservedforRicemethodwith4hoursofagingtobe0.09.3.

Nooutlierswereobserved.

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Figure1.AsphaltAbsorptionvs.AgingTimeforRCandAAM-1

Figure2.AsphaltAbsorptionvs.AgingTimeforRCandAAB-2

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Figure3.AsphaltAbsorptionvs.AgingTimeforRBandAAM-1

Figure4.AsphaltAbsorptionvs.AgingTimeforRBandAAB-2

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Table4.ANOVAforPhaseIExperiments

Source df SS MS Fo

Total 161 23.5166 - -

Methods(A) 8 4.6830 0.5854 70.0*

Aggregates(B) 2 14.5063 7.2531 867.2*

AxB 16 2.3494 0.1468 17.6*

Asphalts(C) 1 0.3746 0.3746 44.8*

AxC 8 0.1452 0.0181 242

BxC 2 0.3244 0.1622 19.4

*

AxBxC 16 0.1162 0.0073 0.9

Blocks(Replicates) 2 0.1311 -- --

Error 106 0.8865 0.0084 --*Significantat"=0.01

AbsorptionbyBulkImpregnatedSpecificGravityMethod

Ingeneral,theasphaltabsorptionobtainedbybulkimpregnatedspecificgravitymethodisexpectedtobelowerthanthatobtainedusingimmersionmethodforthesameaggregate-asphaltcombination.Thisisduetothefactthatalthoughasphaltisstillavailableinbulkaroundaggregateinthebulkimpregnatedspecificgravitymethod,theexposuretimewithhotasphaltisreducedbecauseinthismethodasphalt-aggregatemixtureisallowedtocoolimmediatelyaftermixing.ThisisconfirmedbythedatareportedinTable3andFigures1-4.LowabsorptionaggregateRD,again,seemstohavenosensitivitytowardstheaggregate-asphaltcontacttime

whilehot.Theasphaltabsorptionbythismethodasapercentageofwaterabsorptionisfoundtorangefrom25to43percentbasedondatainTables1and3.

Theasphaltabsorptionobtainedusingthebulkimpregnatedspecificgravitymethodcanbe

consideredtorepresenttherealisticmaximumvalueofasphaltabsorption.Althoughthismethodcantestthewholemixforabsorption,oneseriouslimitationofthismethodisthepotentialproblemofremovingairbubbleswhenbothcoarseandfineaggregatesareimmersedinhotasphalt.

AbsorptionbyRiceMethod

MostHMAmixescontainbothcoarseandfineaggregates,andasphaltispresentintheformofthinfilmsaroundtheaggregate.Thismethod,therefore,canbeappliedtoactualHMAmixes.

ThedataonasphaltabsorptionobtainedusingRicemethodisreportedinTable3andplottedinFigures1-4.ThedatafortheleastabsorptiveaggregateRDwasnotplottedbecauseofverylowabsorptionvalues.Thedataforallaggregatesindicatesthatasphaltabsorptionincreaseswithtime.Viscosityoftheasphaltalsohasaneffectinthatthelowerviscosityasphaltinvariablygiveshigherasphaltabsorption.

Theasphaltabsorptionvaluesfor8hoursofagingintheovenat290Faregenerallyquiteclose

tothevaluesobtainedbytheimmersionmethodwithone-hourimmersion,ascanbeseeninFigures1-3.ForaggregateRB-asphaltAAB-2combination,the8-hourRiceabsorptionissomewhatlessthantheone-hourimmersionmethodvalue.Thisindicatesthatone-hourimmersionmethodindeedgivesconsiderablyhighabsorptionvaluesofasphaltabsorption.LookingatFigures1-4,theabsorptionvaluesobtainedbybulkimpregnatedspecificgravitymethod,whichisthesecondmostcommonlyusedmethodforasphaltabsorptionafterRice

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Kandhal&Khatri

method,appeartointersectthecorrespondingRiceabsorptioncurvesataboutfourhours(three-

fivehoursrange)ofagingtime.Moreover,thecurveforasphaltabsorptionappearstoleveloffataboutfourhoursagingtime.Basedontheseobservationsfromthisstudy,determinationofasphaltabsorptionvaluesobtainedafterfour-houragingofthemixat290Fisrecommendedfor

useasastandardgeneralpracticewhendealingwithabsorptiveaggregates.Thiswillinturnassurecomputationofrealisticandconsistentairvoidscontentinthemix,whichisoneofthecriteriaintheMarshallmixdesignmethod.Agingtimecanbeincreasedinexceptionalcasesifitisdemonstratedthatabsorptioncontinuessubstantiallybeyondfourhours.

Toverifytheconclusionsdrawnabove,Duncan'smultiplerangetestwasrunontheaverage

valuesoftheabsorptionobtainedbyvariousmethods.Sinceinteraction:AxC(methodsxasphalts)wassignificant,resultsforeachasphaltwerehandledseparately.Thisanalysisresultedinthefollowinggroupingsofmethods:

1.AsphaltAAM-l(AC-20):

Methodb,c R0 R1 R2 R4 R6 BI R8 I1 I3

Average 0.210 0.310 0.440 0.463 0.497 0.503 0.554 0.561 0.751Absorption

Groupings -----------------

---------------------------

-------------------------------------

-------------------------------------

-------------------------------------b I1andI3refertoimmersionmethodwith1and3hoursimmersion,respectively;BIreferstobulkimpregnatedmethod;andR0throughR8refertoRicemethodwith0,1,2,4,6,and8hoursofagingintheoven,respectively. cMethodsarearrangedinascendingorderofthemagnitudeofaverageasphaltabsorption.

ItcanbeseenthatasphaltabsorptionvaluesobtainedusingbulkimpregnatedmethodandtheRicemethodwith2,4,and6hoursofagingarenotsignificantlydifferentfromeachotherat "=0.05.Moreover,thereisnosignificantdifferenceintheasphaltabsorptionvaluesobtainedbytheRicemethodwith4to8hoursofaging.

2.AsphaltAAB-2(AC-5):

Methodb,c R0 R1 R2 R4 R6 BI R8 I1 I3

Average 0.219 0.402 0.438 0.560 0.600 0.619 0.650 0.714 0.953Absorption

Groupings ----------------- ----------------- --------------------------------------------

b

-------------------------------------

I1andI3refertoimmersionmethodwith1and3hoursimmersion,respectively;BIreferstobulkimpregnatedmethod;andR0throughR8refertoRicemethodwith0,1,2,4,6,and8hoursofagingintheoven,respectively. cMethodsarearrangedinascendingorderofthemagnitudeofaverageasphaltabsorption.Again,itcanbeseenthattheabsorptionvaluesobtainedusingtheRicemethodwith4,6,and8hoursofagingintheovenarenotsignificantlydifferentfromeachotheraswellasthose

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Kandhal&Khatri

obtainedusingthebulkimpregnatedmethod.CombiningtheresultsoftheDuncan'smultiple

rangetestforboththeasphalts,wecanstatethattheRicemethodwith4-hoursofagingintheovenresultsinasphaltabsorptionvaluesthatarenotsignificantlydifferentfromthoseobtainedusingthebulkimpregnatedmethod.

Thispointisfurtherillustratedbyconsideringanexample.AssumethatthebulkspecificgravityofthecompactedmixusingRCaggregateandAAM-lasphaltis2.325.TheRicespecificgravityvalueat0hour(justaftermixing)of2.422(0.17percentasphaltabsorption)willgiveanairvoidcontentof4.0percentinthecompactedspecimen.Whereas,the4-houragedRicespecificgravityvalueof2.458(0.72percentasphaltabsorption)willsubstantiallyincreasetheairvoidcontentto5.4percent.

AbsorptionUsingGlassBeads

Asphaltabsorptionwasdeterminedusingglassbeads(absorption=0)duringphaseIIofthisstudyandtheresultsthusobtainedarereportedinTable5.Asmentionedearlier,glassbeads

wereusedtoevaluatetheerrorsassociatedwiththetestequipmentandtestproceduresusedinthethreeabsorptiontestmethods.At-testwasconductedtocheckwhethertheasphaltabsorptionobtainedwassignificantlydifferentfromzero.Themethodswererankedaccordinglyanditwasfoundthatthebulkimpregnatedspecificgravitymethodgavethebestresults,followedbytheimmersionmethodandthenbytheRicemethod,asreportedinTable5.TheresultsofthisphasesuggestaneedforrefiningtheRicemethodwhichiscurrentlybeingdoneattheNationalCenterforAsphaltTechnology(NCAT)undertheSHRPA-003Bcontract.

Table5.AsphaltAbsorptionDataforPhaseII(AbsorptionbyPercentWeightofBeads)

Method Individual Average Standard #t#Ranking RemarksValues AbsorptionDeviation oft

ImmersionMethod

Bulk-ImpregnatedMethod

RiceMethod

(percent)

0.02

0.03

0.06

0.01

0.08

-0.02

-0.08

-0.03

(percent)

0.04

0.02

(percent)

0.021.76

0.050.45

2

1

NotSignificantat"=0.05

NotSignificantat"=0.05

NotSignificantat

Note:t0.025.2=4.303

-0.04 -0.05 0.03 1.89 3 "=0.05

RelationshipofAsphaltAbsorptionwithTimeAstheabsorptionisatime-dependentphenomenon,dataforabsorptionusingtheRicemethodwasregressedagainstagingtimeintheoven.AsshowninFigures1-4,theaverageasphaltabsorptionvaluesfollowalmostahyperboliccurvewhenplottedagainstagingtime.Thecoefficientofdetermination(R2)forthefitrangesfrom0.81to0.97.Thiswasfurtherconfirmed

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Kandhal&KhatribyplottingtheasphaltabsorptiondatareportedbyKandhalandKoehler(23)forPennsylvaniamixes.Figure5showstwocurvesobtainedonHMAmixescontainingabsorptivegravelaggregates.Cross&Co.'smixhad7.5percentasphaltcontentandthewaterabsorptionofthecombinedaggregatewas1.92percent.InterstateAmiesiteCorp.'smixalsohad7.5percentasphaltcontent,andthecombinedaggregatehad1.79percentwaterabsorption.Figure6showssimilarcurvesfortwoadditionalmixesusedbytheseproducersusingabsorptivegravelaggregates.Inthiscase,Cross&Co.'smixhad5.1percentasphaltcontent,andthecombinedaggregatehad2.0percentwaterabsorption.TheasphaltcontentforInterstateAmiesiteCorp.'smixwas7.5percent.Informationonwaterabsorptionoftheaggregateusedwasnotavailable.Again,theR2valuesrangebetween0.97and0.99.Thismeansthattheasphaltabsorptionatanytimecanbepredictediftheabsorptionisdeterminedat0timeandtwoothertimesinitially.

Figure5.AsphaltAbsorptionvs.AgingTime(PennsylvaniaData)

Theequationsforobtaininganestimateofasphaltabsorptionatanygivenagingtime,basedonthishyperbolicrelationship,whenasphaltabsorptionvaluesat0agingtimeandanytwoadditionaltimesareknown,isgivenbelow.

where,A=asphaltabsorptionatanygiventimetA0=asphaltabsorptionat0agingtime

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Kandhal&Khatri

Figure6.AsphaltAbsorptionvs.AgingTime(PennsylvaniaData)

andtheconstantsaandbareobtainedusing

where,

)A1=Al-A0and)A2=A2-A0aredifferentialabsorptionatanyotheragingtimest landt2.

Onemoreadvantageofthehyperbolicrelationshipaboveisthattheultimate(orlimiting)value

ofabsorptionatinfiniteagingtime,A1,mayalsobeestimatedusing

ThevaluesofthelimitingabsorptionforthepresentstudyaswellasforthedataforPennsylvaniamixes(23)areindicatedinFigures1-6.

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Kandhal&KhatriItisworthwhilementioningthatalthoughonlythreeobservationsofasphaltabsorption(atagingtime0andanytwoadditionaltimes)aresufficienttodeterminethecourseofthewholehyperbolicrelationship,itwouldbemoreappropriatetoconductmorethanonetestateachofthesepointsandusetheaveragevaluesintheaboveequations.Anotherapproachwouldbetocarryoutseveralobservationsatdifferentagingtimesandobtainaleastsquaresestimateoftheconstantsaandbtobeusedforpredictionpurposes.

Itisbelievedthatthispredictivetechniquewillespeciallybeusefulwhendealingwithsome

problemaggregateswhichhaveahistoryofcontinualasphaltabsorptionduringconstructionandsubsequentlyinservice(3).

CONCLUSIONSANDRECOMMENDATIONS

Basedontheasphaltabsorptiondataobtainedby9differentmethods/techniquesusingthreeaggregatesandtwoasphaltcementsthefollowingconclusionsaredrawnandrecommendationsmade.

1.TheRicemethodafter4hoursofagingintheovenat290Fisbelievedtobethemostrealisticandsuitablemethodfordeterminingasphaltabsorption.ThisrecommendedmethodwillalsoensurearealisticvalueoftheRicespecificgravityfordeterminationofthevoidpropertiesofthemixformixdesignpurposes.

2.Theasphaltabsorptionisfoundtofollowahyperbolicrelationshipwithagingtime.Basedonthisrelationshipthewholecourseofasphaltabsorptionwithtimecanbedeterminedifasphaltabsorptionvaluesatagingtime0andanyothertwoagingtimesareknown.Therelationshipcanalsobeusedtopredicttheultimate(orlimiting)valueofasphaltabsorptionatinfiniteagingtime.Thiswillespeciallybeusefulwhendealingwithproblemaggregateswhichhaveahistoryofcontinualasphaltabsorptionduringconstructionandsubsequentlyinservice.

ThereisaneedtoimprovetheRicemethodforbetterreproducibilityoftestresults.Thisworkis

currentlybeingconductedatNCATaspartoftheSHRPA-003Bproject.

Themethodsfordeterminingthebulkspecificgravityofcoarseandfineaggregateswhichare

usedincomputingtheamountofasphaltabsorbed,alsoneedtobeimproved.Establishingthesaturatedsurfacedryconditionofthefineaggregateshasbeenaseriousproblemindeterminingthebulkspecificgravity.ImprovedtechniquessuchascalorimetricproceduresattemptedbyKandhalandLee(24)needtobeexplored.

ACKNOWLEDGMENTS

ThisstudywasconductedbytheNationalCenterforAsphaltTechnology(NCAT)atAuburnUniversityundertheStrategicHighwayResearchProgram(SHRP)contractA-003BonFundamentalStudiesofAsphaltAggregateInteractionsIncludingAdhesionandAbsorption.Theopinions,findingsandconclusionsexpressedherearethoseoftheauthorsandnotnecessarilythoseofSHRP,NCAT,orAuburnUniversity.

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Evaluation of Asphalt Absorption by Mineral Aggregates

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