critical review of vma requirements in superpave

8/9/2019 Critical Review of VMA Requirements in Superpave

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ACRITICALREVIEWOFVMAREQUIREMENTSINSUPERPAVE

By

PrithviS.Kandal

KeeY.FooRajibB.Mallick

PublishedinTransportationResearchBoard,TransportationResearchRecord1609,1998

277TechnologyParkway Auburn,AL36830


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By

PrithviS.KandhalAssociateDirector

NationalCenterforAsphaltTechnologyAuburnUniversity,Alabama

KeeY.Foo

AssistantProfessorBradleyUniversity,Peoria,Illinois

RajibB.Mallick

SeniorResearchAssociate

NationalCenterforAsphaltTechnologyAuburnUniversity,Alabama

PublishedinTransportationResearchBoard,TransportationResearchRecord1609,1998


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DISCLAIMER

Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

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ABSTRACT

Reportsofincreaseddifficultiesinmeetingtheminimumvoidsinmineralaggregate(VMA)

requirementshavesurfacedwiththerecentuseofSuperpavevolumetricmixdesign.ThelowVMAofSuperpavemixescangenerallybecontributedtotheincreasedcompactiveeffortbySuperpavegyratorycompactor.Thishasledtotheincreaseduseofcoarserasphaltmixes(gradationsnearthelowercontrolpoints).However,theminimumVMArequirementsinSuperpavevolumetricmixdesignforthesecoarsemixesarethesameasthosedevelopedforthedensemixesdesignedbytheMarshallmethod.

LiteraturereviewhasindicatedthattherationalebehindtheminimumVMArequirementwasto

incorporateatleastaminimumpermissibleasphaltcontentintothemixinordertoensureitsdurability.Studieshaveshownthatasphaltmixdurabilityisdirectlyrelatedtoasphaltfilmthickness.Therefore,theminimumVMAshouldbebasedontheminimumdesirableasphaltfilmthicknessratherthanaminimumasphaltcontentbecausethelatterwillbedifferentformixeswithdifferentgradations.Mixeswithcoarsegradation(and,therefore,lowsurfacearea)have

difficultymeetingtheminimumVMArequirementbasedonminimumasphaltcontentinspiteofthickasphaltfilms.

Arationalapproachbasedonaminimumasphaltfilmthicknesshasbeenproposedand

validated.Thefilmthicknessapproachrepresentsamoredirect,equitable,andappropriatemethodofensuringasphaltmixdurabilityandencompassesvariousmixgradations.

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Kandhal,Foo,&Mallick


PrithviS.Kandhal,KeeY.FooandRajibB.Mallick

INTRODUCTIONOneoftheproblemsencounteredbyhighwayagenciesimplementingSuperpavevolumetricmixdesignisthedifficultyinmeetingtheminimumvoidsinmineralaggregate(VMA)requirement.ThelowVMAofthesemixturescanoftenbeattributedtotheincreasedcompactiveeffortbytheSuperpavegyratorycompactorandtheincreaseduseofcoarserasphaltmixes(gradationsbelowrestrictedzone).AvastmajorityofconventionalasphaltmixesintheU.S.havegradationsabovethemaximumdensityline(1).ManyhighwayagencieshavemadetheirasphaltmixescoarserthanthoseconventionallyusedinordertomeettheSuperpaveVMArequirements.Superpavealsorecommendstheuseofaggregategradationsbelowthemaximumdensitylineespeciallyforhighvolumeroads.Eventhen,itisnotalwayspossibletomeettheVMArequirement.

LiteraturereviewhasindicatedthattherationalebehindtheminimumVMArequirementfor

conventionalasphaltmixeswastoincorporateaminimumdesirableasphaltcontentintothemixtoensureitsdurability.Studieshaveshownthatasphaltmixdurabilityisdirectlyrelatedtoasphaltfilmthickness.Therefore,theminimumVMAshouldbebasedontheminimumdesirableasphaltfilmthicknessratherthanaminimumasphaltcontentbecausethelatterwillbedifferentformixeswithdifferentgradations.Mixeswithacoarsegradation(and,therefore,lowsurfacearea)havedifficultymeetingtheminimumVMArequirementbasedonminimumasphaltcontentinspiteofthickasphaltfilms.AcriticalreviewoftheminimumVMArequirementis,therefore,needed.

BACKGROUNDONDEVELOPMENTOFVMACRITERIA

InapaperpresentedtotheHighwayResearchboardin1956(2),McLeodpointedoutthatthebasiccriteriaforboththedesignandanalysisofasphaltpavingmixturesshouldbeonavolumetricbasisandnotonthebasisofweight.Mostspecificationsinthosedaystendedtospecifyarangeofasphaltcontentbyweightalongwithgradingbandsorlimitsfortheaggregate,whichineffectrequiredadesignonthebasisofweight.

McLeod(2)illustratedthevolumetricrelationshipbetweenthetotalasphaltbinder,airvoids

betweenthecoatedaggregateparticles,andthetotalaggregateinacompactedpavingmixture.HedevelopedthevolumetriccriteriasuchasVMAbasedonspecimenscompactedwithaMarshallhammerwith75blowsoneachsideofthespecimen.HerecommendedthattheVMA,whichisthevolumeofvoidsbetweentheaggregateparticles,shouldberestrictedtoaminimumvalueof15%,thevolumeoftheairvoids(withintheVMA)shouldliebetween3and5%,which

inturnrestrictedthevolumeofasphaltcementbinderinthecompactedmixturetoapermissibleminimumof10%byvolume.Therefore,hisproposalforaspecificationofaminimum15%VMA,alongwith5%airvoids,automaticallyestablishedaminimumasphaltcontentofabout4.5%byweight(10%byvolume).

McLeod'scalculationswerebaseduponabulkspecificgravityof2.65fortheaggregateand

1.01fortheasphaltcement.Noasphaltabsorptionwasconsideredinthevolumetricanalysis.

AnotherpaperpresentedbyMcLeodin1959(3)totheAmericanSocietyofTestingand

Materials,advocatedtheuseofbulkspecificgravityoftheaggregateforcalculatingboththeVMAandtheairvoids.Absorptionoftheasphaltcementintotheaggregatewasalsotakenintoaccountinthevolumetricanalysis.McLeodrecommendedagainthatthelowestpermissible

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asphaltcontentinahotmixasphalt(HMA)mixshouldbe4.5%byweight,toensuremix

durability.Thisamountstoabout10%asphaltcementbyvolume.NoHMAperformancedatawerepresentedtosupporttheminimumasphaltcontentof4.5%onwhichtheminimumVMArequirementwasbased.Inthispaper,McLeodalsoproposedarelationshipbetweenthe

minimumVMAandthenominalmaximumparticlesizeoftheaggregate,whichwasadoptedbytheAsphaltInstitutein1964(4).Hebasedthisrelationshipuponthebulkspecificgravityoftheaggregateandanairvoidscontentof5%forthecompactedmix.However,thebackgrounddataforrelatingtheminimumVMArequirementstothenominalmaximumsizeoftheaggregatewasnotgiven(3).

Duringthelast30yearsorso,mostasphaltpavingtechnologistsdidnotrealizethatthese

minimumVMArequirementswerebasedon5%airvoidcontent(andnot4%airvoidcontentgenerallyusedformixdesign)and75-blowMarshallcompaction.Obviously,theminimumVMArequirementscorrespondingto4%airvoidcontentwouldbe1%lowerthanthoserecommendedinearliereditionsofAsphaltInstituteMS-2(4).Thiswasrecognizedin1993andtheAsphaltInstituteMS-2wasrevised(5)togiveminimumVMArequirementscorresponding

to3,4,and5%airvoidcontents.TheserevisedminimumVMArequirementshavealsobeenincorporatedintheSuperpavemixdesignprocedures.

ASPHALTBINDERFILMTHICKNESSINDURABILITYCONSIDERATIONS

Itisgenerallyagreedthathighpermeability,highairvoids,andthinasphaltcoatingsontheaggregateparticlesaretheprimarycausesofexcessiveagingoftheasphaltbinderwhichcontributestothelackofdurabilityoftheHMAmixesoftenencounteredinthefield.However,theconceptofan"averagefilmthickness"fordense-gradedasphaltmixturesisnoteasilyunderstood.Howmuchvaliditycanbeassignedtoafilmthickness,calculatedsimplybydividingthetotalsurfaceareaoftheaggregate(obtainedfromitsgradation)bytheeffectiveasphaltcontent?Itishighlyunlikelythatalltheparticlesinamixhavethesamefilmthicknessofasphaltcoating.Fineaggregateparticlesmayhaveamuchthickercoatingascomparedtothe

coarseaggregateparticles,andinfact,forallpracticalpurposes,someveryfineparticlesmightsimplybeembeddedintheasphaltcement/fillermortarsystem.Therefore,theterm"filmthickness"iselusiveanddifficulttodefine.However,forthepurposeofcalculationlaterinthispaper,weshallassumethattheconceptofthe"averagefilmthickness"isindeedvalid,andproceedwiththecalculations.SurfaceareawillbecalculatedusingtheprocedureoutlinedintheAsphaltInstitute'sMS-2(5).

Campen,Smith,EricksonandMertz(6)presentedtherelationshipbetweenvoids,surfacearea,

filmthicknessandstabilityfordensegradedHMA.Theauthorsrecognizedthatthickerasphaltbinderfilmsproducedmixeswhichwereflexibleanddurable,whilethinfilmsproducedmixeswhichwerebrittle,tendedtocrackandravelexcessively,retardedpavementperformance,andreduceditsusefulservicelife.Onthebasisofthedatatheyanalyzed,averagefilmthicknessesrangingfrom6to8micronswerefoundtohaveprovidedthemostdesirablepavementmixtures.

Theyalsoconcludedthatthefilmthicknessdecreasesasthesurfaceareaoftheaggregateisincreased.However,theasphaltbinderrequirementofamixisnotdirectlyproportionaltoitssurfacearea.Theasphaltbinderrequirementwasfoundtoincreaseasthesurfaceareawasincreased,butataratemuchlowerthanthatguidedbyarelationshipofdirectproportionality(6).

GoodeandLufsey(7)alsodidsomesignificantworkinrelatingasphalthardeningtovoids,

permeability,andfilmthickness.Theyrecognizedthatthehardeningoftheasphaltbinderinamixwasafunctionofairvoids,filmthickness,temperature,andtime.Onthebasisoftheirworktheyconcludedthataminimumvalueof0.00123for'bitumenindex'(whichcorrespondstoavalueof6micronsfortheaveragefilmthickness)couldbeincludedasacriterioninallmixdesignprocedures.The'bitumenindex'wasdefinedaspoundsofasphaltcementpersquarefoot

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ofsurfacearea.Theyusedtheconceptofbitumenindextoavoidtheimplicationthatallparticles

werecoatedwiththesameuniformthicknessofasphaltcement.TheirstudyindicatedthatacombinedfactoroftheratiooftheairvoidtothebitumenindexcouldbesatisfactorilyrelatedtotheasphaltbinderhardeningcharacteristicsintheHMAmixture.Theysuggestedthatthe

Marshallmethodofmixdesigncouldbeimprovedbyincorporatingamaximumvalueofvoids-bitumenindexratioinplaceofamaximumvalueofairvoidsalone,andsuggestedavalueof4asthemaximumforthisratio,toensurereasonableresistancetoaging.

KumarandGoetz(8)studiedtheasphaltbinderhardeningasrelatedtoHMApermeabilityand

asphaltfilmthickness.Theystatedthatthebestprocedureforpredictingtheresistanceofhardeningofasphaltbinderinasingle-sizedHMAmixwastocalculatetheratioofthefilmthicknessfactortopermeability.Thefilmthicknessfactorwasdefinedastheratioofthepercentasphaltcontentavailableforcoatingtheaggregatetothesurfaceareaoftheaggregate.

RATIONALAPPROACHBASEDONASPHALTFILMTHICKNESS

RatherthanspecifyingaminimumVMArequirementbasedonminimumasphaltcontentasrecommendedbyMcLeodandadoptedbySuperpave,amorerationalapproachistodirectlyspecifyaminimum,averageasphaltfilmthicknesstoensurethedurabilityofasphaltmixtures.Fordensegradedmixtures,Campen,Smith,EricksonandMertz(6)recommendanaveragefilmthicknessrangingfrom6to8microns.KandhalandChakraborty(9)quantifiedtherelationshipbetweenvariousasphaltfilmthicknesses(rangingfrom4to13microns)andtheagingcharacteristicsofadense-gradedHMAmixsothatanoptimumaverageasphaltfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.TheyusedtheStrategicHighwayResearchProgram(SHRP)agingprocedurestosimulatebothshortandlongtermagingofHMAmixtures.Anoptimumfilmthicknessof9-10micronswasindicatedinthisstudy,belowwhichtheHMAmix(compactedto8%airvoidscontenttofacilitateaging)agedatanacceleratedrate.Obviously,theoptimumfilmthicknessforHMAcompactedto4-5%airvoidscontentinserviceshouldbesomewhatlowerthan9-10micronsbecausetherateofagingwouldbeconsiderably

lowerat4-5%airvoidscomparedto8%airvoids.Basedonthepastresearchexperience,anaverageasphaltfilmthicknessof8micronsisrecommendedandhasbeenusedinthispaper.

EffectofAggregateGradationonVMA

TheminimumdesirableVMAfordifferentaggregategradationswerecalculatedforanaveragefilmthicknessof8micronsat4%airvoids.Anexampleofsuchcalculationfollows.Thesurfaceareacalculation(5)isgiveninTable1.

Table1.CalculatingSurfaceAreaFromGivenAggregateGradation

SieveSize,mm PercentPassing SurfaceAreaFactor(m2/kg)SurfaceArea(m2/kg)

25

1912.5

9.5

4.75

2.36

1.18

0.6

0.3

0.15

0.075

100

9578

66

51

35

25

19

14

95

0.41

0.41

0.82

1.64

2.87

6.14

12.29

32.77

0.41

0.2091

0.28372

0.41492

0.53669

0.84118

1.1061

1.6385

3=5.44021

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Example1

Assumingspecificgravityofasphaltis1.02andbulkspecificgravityofaggregate=2.70

TwoHMAmixtureswithnominalmaximumsizesof19mmand12.5mmhavebeenconsidered.Theaggregategradations:abovetheSuperpaverestrictedzone(ARZ),throughtherestrictedzone(TRZ),andbelowtherestrictedzone(BRZ)havebeenconsideredforeachmix(Table2

andFigures2and3).TheVMAforthevariousmixaggregategradationswerecalculatedandtheresultstabulatedinTable3forfilmthicknessof6,7,and8microns.Table3showsthatforaconstantfilmthickness,theaggregategradationdoeschangetheminimumdesirableVMA.Thegradationsbelowtherestrictedzone(BRZ)whichrepresentacoarsemixhavethelowestVMAinbothmixeswith12.5mmand19mmnominalmaximumsize.ThisillustratestheshortcomingofthecurrentSuperpaveminimumVMArequirementbasedonMcLeod'swork.ByspecifyingaminimumVMA,Superpavehasgenerallypenalizedcoarsemixes(thosebelowtherestrictedzone)whichrepresentamajorityofSuperpavemixesthatarecurrentlybeingplaced.

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Figure1.WeightandVolumePhaseDiagramforCompactedHMA

Table2.AggregateGradationsConsidered

PercentPassingSieveSize,mm 19mmNominalMaximumSize 12.5mmNominalMaximumSize

ARZ TRZ BRZ ARZ TRZ BRZ

25 100 100 100

19 100 95 90 100 100 100

12.5 88 78 67 100 96 90

9.5 79 66 52 89 83 76

4.75 65 51 36 73 60 47

2.36 49 35 23 55 39 30

1.18 38 25 17 35 28 23

0.6 28 19 14 25 21 170.3 19 14 10 20 16 10

0.15 11 9 7 11 9 7

0.075 5 5 5 5 5 5

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Figure2.AggregateGradationfor19mmNominalSize

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Figure3.AggregateGradationfor12.5mmNominalMaximumSize

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Table3.CalculatedVMABasedonFilmThickness

Film 19mmNominalMaximumSize 12.5mmNominalMaximumSizeThickness

(Micron) ARZ TRZ BRZ ARZ TRZ BRZ6 13.35 11.78 10.58 13.36 12.20 10.96

7 14.73 12.95 11.59 14.75 13.43 12.03

8 16.07 14.09 12.57 16.09 14.63 13.06

Avg 14.72 12.94 11.58 14.73 13.42 12.02

ThecurrentSuperpaveminimumVMArequirementfor19mmnominalmaximumsizemixis13%.IfthatVMArequirementisassumedtobebasedongradationthroughtherestrictedzone,thentheminimumVMArequirementforacoarsergradation(belowtherestrictedzoneandnearthelowercontrolpoints)shouldbeloweredbya"gradationfactor"ofapproximately1.2to1.5

percentdependingonthefilmthicknessused(Table3).Itshouldbenoted,however,thatthegradationonwhichthecurrentVMArequirementsarebasedisnotavailableintheliterature.

TheadjustmentofthecurrentSuperpaveVMArequirementwitha"gradationfactor"for

gradationbelowtherestrictedzoneisatbestatemporary,quickfixsolution.Table4showsthe"gradationfactors"fordifferentnominalmaximumaggregatesizescorrespondingtoafilmthicknessof8microns.

Table4.GradationFactors

NominalMaximumSize

37.5mm 25mm 19mm 12.5mm 9.5mm

0.5 1.0 1.5 1.5 2.0Abetterandequitablesolutiontoensuremixdurabilityistospecifytheminimumaverageasphaltfilmthickness.Anaveragefilmthicknessof8micronsisrecommendedatthistimebasedonpastresearch.Therefore,thefinalstepintheSuperpavevolumetricmixdesignshouldbetocalculatetheaverageasphaltfilmthickness,andiftheaverageasphaltfilmthicknessisequaltoorgreaterthan8micronsthenthemixdesigncanbeapproved.

VALIDATIONOFASPHALTFILMTHICKNESSAPPROACH

Superpavevolumetricmixdesignswereconductedonsixdifferentmixes.Ofthese,threemixesconsistedof100%crushedgraniteaggregate,andtheotherthreeconsistedof80%crushed

graniteand20%naturalsand.ThepropertiesoftheaggregateusedinthemixesareshowninTable5.A3-10millionESALstrafficlevelwasselectedforcompaction.ThedesignparametersareshowninTable6.TheN initial,NdesignandNmaximumvalueswere8,96,and152,respectively.APG64-22gradeasphaltcementwasusedinallmixtures.

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Table5.PropertiesofAggregatesUsedintheLaboratoryValidationStudy

Property GraniteCoarse Granite Natural BlendofGranite,Aggregate Fine Sand FineAggregate,

Aggregate andNaturalSandBulkSpecificGravity 2.688 2.712 2.618 2.693

Uncompactedvoids --- 49.4 40 47.9(AASHTOT304,methodA)

Table6.DesignParametersandSuperpaveCriteria

Traffic,ESALs

3-10million

Ninitial

8

Ndesign

96

Nmaximum

152

VMA,%(minimum)

14

VFA

65-75

Themixwith100%crushedaggregate(CR)andthemixcontaining20%naturalsand(NS)weremadewiththethreefollowingdifferentaggregategradations:(1)abovetherestrictedzone(ARZ),(2)throughtherestrictedzone(TRZ),and(3)belowtherestrictedzone(BRZ).ThegradationsareshowninTable7andFigure4.Thegradationsaredifferentonlyaroundtherestrictedzoneforthesemixesof12.5mmnominalmaximumsize.Inotherregionsthegradationswerekeptthesame.MixdesignswereconductedforeachofthemixeswiththeSuperpavegyratorycompactor.

Table7.GradationofAggregatesUsedintheLaboratoryValidationStudy

PercentPassing

SieveSize(mm)19

12.5

9.5

4.75

2.36

1.18

0.60.3

0.15

0.075

ARZ100

96

83

60

55

35

25

20

75

TRZ100

96

83

60

39

28

21

16

75

BRZa

100

9683

6030

2317

10

75

aARZ-AboveRestrictedZoneTRZ-ThroughRestrictedZoneBRZ-BelowRestrictedZone

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Figure4.GradationsofLaboratoryValidationMixes

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TheSuperpavevolumetricmixdesignresultsforthesixmixesareshowninTable8.Atthe

optimumasphaltcontent,allofthemixesexcept(TRZ-NS)and(BRZ-NS)meettheVMAcriteria(>14.0%).TheBRZ-NSmixbarelyfailstheVMAcriteria(VMA=13.9).Theaverageasphaltfilmthicknessforeachmixwascalculatedfromthemixdesignresultsbasedonthe

optimumasphaltcontentandsurfacearea.FollowingisanexampleofhowtheaverageasphaltfilmthicknessforARZ-CRmixwascalculated.

Table8.MixDesignResults

Properties

Mixa ACContent,% VTM,% VMA,%b Avg.FilmThickness,:m

ARZ-CR 5.1 4.0 15.2 8.6

TRZ-CR 4.7 4.0 14.4 8.8

BRZ-CR 4.9 4.0 14.9 10.4

ARZ-NS 4.6 4.0 14.0 7.7

TRZ-NS 4.1 4.0 13.0 7.6

BRZ-NS 4.5 4.0 13.9 9.5a

b

ARZ-CR-AboveRestrictedZone,AllCrushedTRZ-CR-ThroughRestrictedZone,AllCrushedBRZ-CR-BelowRestrictedZone,AllCrushedARZ-NS-AboveRestrictedZone,20PercentNaturalSandTRZ-CR-ThroughRestrictedZone,20PercentNaturalSand BRZ-NS-BelowRestrictedZone,20PercentNaturalSandMinimumVMARequirement=14.0

Example2

Theaggregatesurfaceareawascalculatedfromtheaggregategradation.Thiscalculationis

showninTable9.

Specificgravityofasphalt=1.02

FromTable8,thefollowingisgivenforARZ-CRmix:VMA=15.2% VTM=4.0% Optimumasphaltcontent=5.1%

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Table9.CalculatingSurfaceAreaFromAggregateGradationofARZ-CRMix

SieveSize,mm

19

12.5

9.5

4.75

2.36

1.18

0.60.3

0.15

0.075

PercentPassing

100

9683

6055

3525

2075

SurfaceAreaFactor(m2/kg)

0.41

0.41

0.82

1.64

2.87

6.14

12.29

32.77

SurfaceArea(m2/kg)

0.41

0.2460

0.4510

0.5740

0.7175

1.2280

0.8603

1.6385

3=6.1253

AllmixesexceptARZ-NSandBRZ-NShavemorethan8micronsaverageasphaltfilmthickness.Asexpected,mixescontainingnaturalsandgenerallyhavelowerVMAcomparedtomixescontaining100%crushedaggregate.ThemajorobservationisthatmixBRZ-NSfailedtomeettheminimumVMArequirementof14.0%butdoeshaveanaverageasphaltfilmthicknessgreaterthan8microns(9.5microns).TheBRZ-NSmixshouldbesignificantlymoredurablewithafilmthicknessof9.5micronsthantheARZ-NSmixwithafilmthicknessof7.7microns,althoughbothhavecomparableVMAvalues.Ifaminimumasphaltfilmthicknessof8.0micronsisspecifiedthentwomixes:ARZ-NSandTRZ-NSwillbeunacceptable,whichislogical.Asexpected,theVMArequirementunfairlypenalizescoarsermixes(thosewithgradationbelowtherestrictedzone).

IftheminimumVMArequirementwasbasedonaminimumaverageasphaltfilmthicknessof8

microns,itwouldvarydependingonthegradationasshowninTable10.AllmixesexceptARZ-NSandTRZ-NSmeettheminimumVMArequirementsbasedonanaverageasphaltfilmthicknessof8microns.

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Table10.VMARequirementsandActualValues

VMAMix

MinimumRequired MinimumBasedon8 ActualObtainedinbySuperpave MicronFilm MixDesignThickness

ARZ-CR

TRZ-CR

BRZ-CR

ARZ-NS

TRZ-NS

BRZ-NS

14.0

14.0

14.0

14.0

14.0

14.0

15.2

14.2

13.2

15.2

14.2

13.2

15.2

14.4

14.9

14.0

13.0

13.9

CONCLUSIONSANDRECOMMENDATIONTheminimumVMArequirementcurrentlyadoptedbySuperpavetoensuremixdurabilityisinadequate.Itisnotequitabletomixeswithdifferentgradations.Therequirementpenalizesmixeswithcoarsegradations(thosebelowtheSuperpaverestrictedzone)whichmayhavelowVMAbuthaveincreasedasphaltfilmthickness.Variousstudieshaveshownthattheasphaltmixdurabilityisdirectlyrelatedtofilmthickness.

Itisrecommendedthatminimumaverageasphaltfilmthicknessbeusedtoensuremixdurability

insteadofminimumVMA.Aminimumaveragethicknessof8micronsisrecommendedatthistime.Thefilmthicknesscanbecalculatedfromtheasphaltcontentandsurfaceareaoftheaggregateasshowninanexampleinthispaper.

Thecurrentmethodofcalculatingaggregatesurfaceareausessurfaceareafactorsasgiveninthe

AsphaltInstituteManualSeries2(5).Thebackgroundresearchdataforthesesurfaceareafactorscouldnotbefoundintheliterature.Furtherresearchisneededtoverifythesesurfaceareafactors.However,thecurrentsurfaceareafactorscanbeusedatthistimetocalculatethe"average"asphaltfilmthicknessbecause(a)theoptimumfilmthicknesses(suchas8microns)recommendedintheliteraturearebasedonthesefactors,and(b)theyareconsideredadequateforcomparisonpurposes.

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REFERENCES

1.Kandhal,P.S,K.Y.Foo,andJ.D'Angelo.ControlofVolumetricPropertiesofHotMix

AsphaltbyFieldManagement.InTransportationResearchRecord1543,TRB,National

ResearchCouncil,Washington,D.C.,1996.2.McLeod,N.W.RelationshipsBetweenDensity,BitumenContent,andVoidsPropertiesof

CompactedBituminousPavingMixtures.Proceedings,HighwayResearchBoard,Volume35,1956.

3.McLeod,N.W.VoidsRequirementsforDense-GradedBituminousPavingMixtures.SpecialTechnicalPublication252,ASTM,1959.

4.MixDesignMethodsforAsphaltConcreteandOtherHotMixTypes,TheAsphaltInstitute,MS-2,May1984.

5.MixDesignMethodsforAsphaltConcreteandOtherHotMixTypes,TheAsphaltInstitute,MS-2,SixthEdition,1993.

6.Campen,J.F.,J.R.Smith,L.G.Erickson,andL.R.Mertz.TheRelationshipsBetweenVoids,SurfaceArea,FilmThicknessandStabilityinBituminousPavingMixtures.InProceedings,AAPT,Vol.28,1959.

7.Goode,J.F.andL.A.Lufsey.Voids,Permeability,FilmThicknessvs.AsphaltHardening.InProceedings,AAPT,Vol.34,1965.

8.Kumar,A.andW.H.Goetz.AsphaltHardeningasAffectedbyFilmThickness,VoidsandPermeabilityinAsphalticMixtures.InProceedings,AAPT,Vol.46,1977.

9.Kandhal,P.S.andS.Chakraborty.EffectofAsphaltFilmThicknessonShort-andLong-TermAgingofAsphaltPavingMixtures.InTransportationResearchRecord1535,TRB,NationalResearchCouncil,Washington,D.C.,1996.

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critical review of vma requirements in superpave

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