Journal - The Institution of Engineers, Malaysia (Vol. 69, No.2, June 2008)54
RESILIENT MODULUS OF MALAYSIAN BASE AND SUBBASE PAVEMENT MATERIALS
(Date received: 24.8.2006)
Ahmad Kamil Arshad and Mohd. Yusof Abd. Rahman * Faculty of Civil Engineering, Universiti Teknologi MARA, 40450 UITM Shah Alam, Selangor
E-mail: [email protected]
ABSTRACT Pavement design approach is shifting towards analytical or mechanistic-based procedures. Resilient modulus is a fundamental parameter used in the procedure and a study was carried out to characterise the parameter for base and subbase pavement materials used in Malaysia according to the Public Works Department of Malaysia’s (PWD) Specification for Road Works (JKR/SPJ/1988). This paper details the study and describes the materials tested, the methodology used and the results obtained from the study. In this study, base (Type II) and subbase (Type E) specimens of 100 mm diameter x 200 mm height were tested using the repeated load triaxial test in accordance with AASHTO T307-99. In addition, the test was carried out at different gradation compositions (but within the required gradation envelope) and moisture contents to study their effects on the resilient modulus value. From the test, the k-θ model was used to characterise the base and subbase materials. Finally, recommendations on a set of resilient modulus values for Malaysian base and subbase materials to be used for mechanistic design of flexible pavements were made, taking into account of the Malaysian environment.
Keywords: Base and Subbase Materials, Flexible Pavement Design, Pavement Materials, Resilient Modulus
(Note: As most of the literature reviewed used U.S. Customary Units, this article used similar units for the purpose of comparing the findings. For conversion: 1 kPa = 6.9 psi)
1 INTRODUCTION AtypicalflexiblepavementinMalaysiaconsistsofasphalticconcretewearingandbindercourse,crushedaggregateorwet-mixbaselayerandgranularsubbase(river/miningsandorquarrydust).Incertaincircumstances,particularlywherethetrafficloadingsarehigh,additionalbituminousmacadamroadbaselayerisincludedabovethecrushedaggregate/wet-mixbaselayer. Current Public Works Department of Malaysia’s (PWDMalaysia) Specifications for Road Works (JKR/SPJ/1988) isa “recipe-based” specification that requires flexible pavementmaterialstocomplywiththephysicalpropertiesdescribedinthespecification [1], such as gradation, compaction density, CBRvalueandplasticity indexformixaggregates,baseandsubbasematerials(Table1).Thesepropertiesensurethatthematerialsareofaspecifiedqualitybutdonotmeasurethestructuralpropertiesrequiredasinputformechanisticpavementdesignmethod.
Requirements Subbase Base
PlasticityIndexLiquidLimitAggregateCrushingValueCBRSoundness(SodiumSulphate)FlakinessIndexFracturedFace(4.75mmsieve)
≤6≤25%≤35≥30---
≤6-
≤30≥30
≤12%≤30
≥80%
Resilientmodulus (MR) isa fundamentalparameterused in themechanisticpavementdesignprocedureandisdefinedastheratioofdeviatorstress,σdovertherecoverablestrain,εr[2]: σdMR=––– (1) εr
A laboratory study was carried out to characterise resilientmodulus for base and subbase pavement materials used inMalaysia according to PWDMalaysia’s Specification for RoadWorks(JKR/SPJ/1988).ThematerialstestedincludebasetypeIIusingcrushed rockaggregates and subbase typeEusingquarrydustandminingsand. The objective of the laboratory tests is to determine thevalues or range of values of resilient modulus for typicalMalaysianflexiblematerialssothatthesecanbeusedinroutinemechanisticbasedflexiblepavementdesign.Testingwascarriedout at different gradation compositions (butwithin the requiredgradationenvelope)andmoisturecontentstostudytheireffectsontheresilientmodulusvalue.
2 LITERATURE REVIEWA The Importance of Resilient Modulus The development of mechanistic-based pavement designproceduressuchastheShellPavementDesignManual[3]andtheAsphaltInstitute’sThicknessDesignManual(MS-1)9thedition[4]providetheneedforthemeasurementofresilientmodulusasinputformechanisticdesignofflexiblepavements.Inmechanisticdesign, flexible pavement ismodeled as amulti-layered system
Table 1: PWD Malaysia’s requirements for subbase and base materials [1]
RESILIENT MODULUS OF MALAYSIAN BASE AND SUBBASE PAVEMENT MATERIALS
Journal - The Institution of Engineers, Malaysia (Vol. 69, No.2, June 2008) 55
andrequired inputssuchas thematerialproperties, thicknessofeach layerand traffic loadings.Thematerialproperties requiredaretheelasticmodulusandPoisson’sratio(usuallyassumedfromotherstudies).AccordingtoHuang[2],theelasticmodulustobeusedistheresilientmodulus. In addition, although the AASHTO Guide for Design ofPavement Structures [5] is still empirical, the determination oflayercoefficientsforitsdesignprocedurerequiresinputvaluesofresilientmodulusforsubgrade,subbaseandbaselayers.AASHTO[5] recommended direct laboratory measurement using theAASHTOMethodT274-82 for subgrade and unbound granularmaterials(includingbaseandsubbase)andASTMD4123-82forasphaltic concrete andasphalt stabilisedmaterials.Furthermore,the increasing use of performance-based specifications requiresthemeasurement of resilientmodulus of pavementmaterials asoneofthekeypropertiestobeachieved.
B Factors Affecting the Resilient Modulus of Granular Materials Theresilientmodulusofgranularmaterialsareknowntobeafunctionoffactorssuchasstresslevel,density,grading,finescontent, maximum grain size, aggregate type, particle shape,moisturecontent,stresshistoryandnumberof loadapplications[6]. However,most researchers agreed that themost influentialfactorsarethelevelofappliedstressandtheamountofmoisturecontentinthematerial. In a recent study by Khogali and Zeghal [7], fourparameters that affect the resilientmoduluswere investigated,namely:deviatorstress,confiningpressure,moisturecontentandmaterialdrydensity.Itwasfoundthatdeviatorstressisthemostsignificant factor followed by the effect of moisture content.Theremainingfactorsappearedtohavelittleornoeffectontheresilientmodulusvalue.
C Constitutive Models for Material Characterisation Thelaboratorytestingofabaseorsubbasematerialwillprovidedataforconstitutivemodelingofresilientmodulusbehaviouroverarangeofappliedstress.Thepavementresponsesthataretobecalculatedusingmultilayerelasticanalysisaredependentontheconstitutive model selected to represent the materials’ resilientmodulusbehaviour.Variousconstitutivemodelshavebeenusedforpavementdesign. Theconstitutiveequations thathavebeenusedwithvaryingcomplexitiesaregivenbelow:Fine-GrainedSoils–1986/93AASHTODesignGuide[5]: MR=K1(σd)
K3 (2) Coarse-GrainedSoils–1986/93AASHTODesignGuide[5]:
MR=K1(θ)K2 (3)
UniversalEquation(Uzan,1985)[8]:
[θ ]K2 [ θd]
K3 MR=K1Pa
_____ (4) PaPa
ExpandedUniversalConstitutiveEquation(NCHRPProject1-28A)[9]:
θ –3k4 k2 τoct k3
MR=k1pa [______] [ ___ +1] (5)
pa pa
where, θ = σ1 + σ2 + σ3
1 __________________________ τoct = __ √(σ1 – σ2)
2+(σ1 – σ3)2+(σ2 – σ3)
2
3where pa=Atmosphericpressure. θ=Bulkstress: σd =Deviatorstress. σ1=Majorprincipalstress. σ2 =Intermediateprincipalstress. σ3=Minorprincipalstress/confiningpressure. τoct =Octahedralshearstress. k1,k2, k3,k4=Regression constants from repeated load resilient
modulustests.
The AASHTO model for coarse-grained soils/granularmaterials isbasedon theworkofHicks [10]whostudiedon thefactors affecting the resilient properties of granular materials.However,VonQuintusandKillingsworth[11]found that theso-called“universalconstitutivemodel”whichisbasedonstudiesmadebyUzan[8]ismoreaccurateinsimulatingtheresponsesmeasuredinthelaboratory.Thisisbecausethemodeltakesintoaccountboththeconfiningstressandthedeviatorstress,comparedtotheHicks’modelwhichonlytakesintoaccountoftheconfiningstress. ThedraftAASHTO2002PavementDesignGuideadopteda modified version of the equation, called the “expandeduniversalconstitutiveequation”whichisapplicabletoalltypesofunboundpavingmaterials,rangingfromplasticclaystocleangranular bases [9]. Von Quintus and Yau[12] found good fitof theequationusingdatafor thepavementmaterialsobtainedfrom the Long Term Pavement Performance (LTPP) programconductedintheUnitedStates. Thispaperfocusedonthek-θmodelbecauseofitssimplicityanditswidespreaduseinmodelinggranularmaterials.Inaddition,the model is also incorporated in multilayer elastic analysissoftwarespresentlyavailablewhichisusedtoanalysepavementstructures. Table 2 shows the k1 and k2 values from previousresearchbyvariousinvestigatorsusingthek-θmodel.
Table 2: Ranges of k1 and k2 for untreated granular materials [2]
Reference Material k1(psi) k2
Hicks(1970)Hicks&Finn(1970)
Allen(1973)Kalcheff&Hicks(1973)
Boyceet.al(1976)Monismith&Wictzak(1980)
Partiallycrushedgavel,crushedrockUntreatedbaseatSanDiegoRoadTest
Gravel,crushedstoneCrushedstone
Well-gradecrushedlimestoneIn-servicebase&subbasematerials
1600-50002100-54001800-80004000-9000
80002900-7750
0.57–0.730.61
0.32-0.700.46-0.640.67
0.46-0.65
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Journal - The Institution of Engineers, Malaysia (Vol. 69, No.2, June 2008)56
3 METHODOLOGYA Equipment UTM-5Pservo-pneumaticallycontrolledtestingmachinewasusedfortheresilientmodulustestingofbaseandsubbasematerials.Realtimecontrolofthemachineandthegenerationoftherequiredwaveformwereprovidedbyadigitalsignalprocessingunit;whileadataacquisitionsystemtookalltransducerreadingsatthesametime.Thesignalprocessingunitandthedataacquisitionsystemwereprovidedinacontrolanddataacquisitionsystem,whichwasintegratedwiththeUTM-5Ptestingmachine.Auniversaltriaxialcellcapableoftesting100mmdiameterx200mmhighspecimenswasusedforrepeatedloadtestofthebaseandsubbasematerials(Figure1).
of the envelope (named Mid+25% thereafter) and (3) betweenmiddleoftheenvelopeandthelowerlimitoftheenvelope(namedMid-25%thereafter).One(1)sampleeachwaspreparedforeachgradationline.
B.2 Subbase Materials Miningsandandquarrydustwereusedforsubbasematerials(typeE).ThegradationofthesubbasematerialswereinaccordancewiththegradationenvelopeasperPWDMalaysia’sSpecificationsforRoadWorks(Table4).Similartobasematerials,three(3)specificgradationlinesweresetforeachgradationenvelope:(1)middleofthe envelope (namedMid thereafter), (2) betweenmiddle of theenvelope and the upper limit of the envelope (namedMid+25%thereafter)and(3)betweenmiddleof theenvelopeandthe lowerlimitoftheenvelope(namedMid-25%thereafter).One(1)sampleeachwaspreparedforeachgradationline.
Table 4: Gradation envelope for subbase type E [1]
B.S.Sieve Size
% Passing by weight
Subbase ‘E’
50.0mm25.0mm9.5mm4.75mm2.00mm425um75um
--
10055–10040–10020–506–20
C Testing Procedure Foreachtypeofsample,theoptimummoisturecontentwasfirstdeterminedusingmodifiedproctortest(4.5kgrammer).Theoven-driedsamplewasthenpreparedatthree(3)differentlevelsofmoisturecontentbyaddingwatertotherequiredamountpriortocompaction:optimummoisturecontent(OMC),OMC-2%andOMC+2%. Eachofthe100mmdiameterx200mmhighspecimenwaspreparedusing a split sand former, using a 0.3mm thick rubbermembrane over it. The split sand former was placed above atriaxial base-plate,with a porous stonebeingplacedon the baseplatepedestal.For each specimen, avibratoryhammerwasused
Figure 2: Gradation lines and envelope for base (Type II)
Figure 1: Repeated load triaxial cell and UTM 5-P machine
B MaterialsB.1 Base Materials CrushedrockaggregateswereusedforbasetypeIImaterial.ThegradationinaccordancetoPWDMalaysia’sspecificationsisasfollows:
Table 3 : Gradation envelope for base type II [1]
BS SieveSize (mm)
% Passing by weight
50 100
37.5 85–100
28 70–100
20 60–90
10 40–65
5 30–55
2 20–40
0.425 10–25
0.075 2–10
Three(3)specificgradationlinesweresetforeachgradationenvelope (Figure 2): (1) middle of the envelope (named Midthereafter),(2)betweenmiddleoftheenvelopeandtheupperlimit
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Journal - The Institution of Engineers, Malaysia (Vol. 69, No.2, June 2008) 57
tocompactfiveequallayersofapproximately40mminthickness(Figure3).The split sand formerwas then removedandanotherlayerofrubbermembranewasplacedaroundthespecimentomakeit air-tight.The triaxial top capand theporous stonewasplacedon the specimen; rubberO-ringswere thenplaced around it andthebottompedestal,priortotheplacementofthespecimeninthetriaxialchamber. Figure4shows theset-upof therepeated loadtriaxialtestforbaseandsubbasematerials[13].
Figure 3: Compaction of samples using vibratory hammer
ThespecimensweretestedforresilientmodulususingtheUTM-5PmachineatthedeviatorstressandconfiningpressuresasperAASHTOT307-99 test [13].The repeated loadwas setat adurationof0.1 secondswith a restperiodof0.9 seconds.Pre-conditioningofthespecimenwascarriedoutataconfining
pressureof103.4kPaandamaximumaxialstressof103.4kPafor 1000 repetitions for basematerials and 500 repetitions forsubbbase materials. Resilient modulus tests were then carriedout at the required confining pressures and deviator stress (15cycles)for100repetitions(Table5).Theaverageofthelastfivereadingsforeachcycleistakenastheresilientmodulusfortheparticularcycle.
Table 5 : Testing sequences for base/subbase materials [13]
SequenceNo.ConfiningPressure,
S3
Max.AxialStress,Smax
CyclicStressScyclic
ConstantStress0.1Smax No.ofLoad
ApplicationskPa psi kPa psi KPa psi kPa Psi
0123456789101112131415
103.420.720.720.734.534.534.568.968.968.9103.4103.4103.4137.9137.9137.9
15333555101010151515202020
103.420.741.462.134.568.9103.468.9137.9206.868.9103.4206.8103.4137.9275.8
1536951015102030101530152040
93.118.637.355.931.062.093.162.0124.1186.162.093.1186.193.1124.1248.2
13.52.75.48.14.59.013.59.018.027.09.013.527.013.518.036.0
10.32.14.16.23.56.910.36.913.820.76.910.320.710.313.827.6
1.50.30.60.90.51.01.51.02.03.01.01.53.01.52.04.0
500–1000100100100100100100100100100100100100100100100
Figure 4: Set-up of the repeated load triaxial test [13]
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Journal - The Institution of Engineers, Malaysia (Vol. 69, No.2, June 2008)58
Thedataobtainedforthe15cycleswerethenplotted(Figure5) based on the following relationship for the base/subbasematerials[5]:
MR(psi)=k1(θ)k2 (6)
where θ= StressinvariantorBulkstress(psi)=(σ1 + σ2+ σ3) =(σd+3σ3). σ1= Majorprincipalstress(psi). σ2 = Intermediateprincipalstress(psi). σ3= Minorprincipalstress/confiningpressure(psi). σd= Deviatorstress(psi).k1,k2 = Regression constants from repeated load resilient
modulustests.
AASHTOT307-99 require the samples to be tested at thein-situmoisturecontentorOMCifthein-situmoisturecontentisnotknown/unavailable.Table5liststhevaluesofk1andk2forthebasematerialsobtainedatOMC:
Table 5: Values of k1 and k2 for different gradations of base type II at OMC
Base Type
Moisture Content
(%)
Max. Dry Density (Mg/m3)
k1 k2
Mid-25% 5.40 2.342 2,831.7 0.6968
Mid 5.70 2.350 2,8114.0 0.6917
Mid+25% 6.30 2.363 2,729.7 0.6576
Fromtheabovetable,itcouldbeseenthatbasematerialwithMid-25% grading line has the highest k1 value,which is likelyduetothelowerfinescontent(particlespassingthe75umsieve)andlowermoisturecontentwhichproducesamuchstiffermaterialthan the base materials with higher fines content and highermoisturecontent.Thevaluesobtainedabovecanbecomparedtothefollowingk1andk2valuesthatweresuggestedbyAASHTOforbasematerials:
Table 6: Typical values of k1 and k2 for Base Materials [5]
Moisture Condition K1 K2
DryDampWet
6,000 – 10,0004,000 – 6,0002,000 – 4,000
0.5 – 0.70.5 – 0.70.5 – 0.7
Table6abovesuggeststhatk1valuesareaffectedbymoisturecontentwhereask2valuesarewithinarangebetween0.5to0.7.ReferringtoTable5, itcouldbeseenthatmostofthek1valuesobtained in the laboratoryareat the lowerendof thesuggestedrange(wetmoisturecondition),whilefork2,mostofthevaluesarewithintherangeofthesuggestedvalues. It is likely that the base moisture condition is below theoptimummoisturecontentas thecurrentpractice inMalaysia istocompactthebaseindryconditionsandachieveminimum95%ofthemaximumdrydensityobtainedinthelaboratorymodifiedproctor test. In addition, Bulman and Smith [14] measuredsubgrademoisturecontentunderthepavement’sbaseandsubbaselayersat73differentlocationsinMalaysiaandfoundthatmorethanhalfofthesemoisturecontentswereequalordrierthantheoptimummoisturecontent(OMC)ofthesubgradesoilgivenbytheBritishStandard2.5kgrammercompactiontest.Furthermore,CroneyandBulman[15]foundthatthereisnoevidenceoflong-term moisture exchange with the subgrade sufficient to affectthestrengthofthesub-base/baselayers.Therefore,itispossiblethat theresilientmodulusatOMC-2%ismorerepresentativeoftheconditionsachieveatsite.Figure7belowshowstheresilientmodulusplotforallgradationstestedatOMC–2%.ReferringtoTables6and7,itcouldbeseenthatthek1valuesareinthedryconditionwhilefork2,mostofthevaluesarewithintherangeofthesuggestedvalues.This,however,havetobeconfirmedatsitebymeasuringthein-situmoisturecontentofthebasematerialintheactualpavementconstructed.
Figure 5: Plot of resilient modulus-bulk stress for base and subbase materials
Figure 6: Resilient modulus plot for base type II (Mid-Gradation)
4 RESULTS AND DISCUSSION Thissectionpresents theresults,analysisanddiscussionofthelaboratoryresilientmodulustestingcarriedoutonbaseTypeIIandsubbaseTypeE(quarrydustandminingsand).
A Base Materials The resilient modulus values obtained at various confiningpressuresanddeviatorstresseswereplottedonalog-loggraphandthevaluesofk1andk2wereobtainedfromthegraph.Atotalof3specimens(Mid,Mid-25%andMid+25%)weretestedat three(3)differentmoisturecontents(OMC,OMC-2%andOMC+2%).Figure6showsatypicalplotofresilientmodulus-stressinvariant(bulk stress) relationship for base type II (mid-gradation) at thethreedifferentmoisturecontents. Itcouldbeseen that the lowerthemoisturecontentthehigheristhek1value,howeverk2valueincreaseslightlywiththeincreaseinmoisturecontent.
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Table 7: Values of k1 and k2 for different gradations of base type II at OMC-2%
Base Type k1 k2
Mid-25% 8,841.1 0.4819
Mid 7,357.0 0.5324
Mid+25% 6,518.5 0.5028
4.1 SUBBASE MATERIALS Figures8and9showtheresilientmodulusplotofsubbasematerials(TypeE)usingquarrydustandminingsandforthree(3)differentgradationstestedatOMC.Thek1andk2valuesobtainedaresummarizedinTable9andcanbecomparedtothefollowingk1 and k2values thatwere suggestedbyAASHTOfor subbasematerials:
Table 8: Typical values of k1 and k2 for subbase materials [5]
Moisture Condition k1 k2
DryDampWet
6,000 – 8,0004,000 – 6,0001,500 – 4,000
0.4 – 0.60.4 – 0.60.4 – 0.6
Figure 8: Resilient modulus plot for subbase type E (quarry dust) at OMC
ItcouldbeseenthatbycomparingTables7and9thatsubbasematerialshavemuchlowerk1valuesthanbasematerialsandonlyslightdifferencesink2values.Thisispossiblyduetothefactthatsubbasematerialshaveasmallermaximumaggregatesize thanbasematerialsandalsobecauseofthehigheroptimummoisturecontent(OMC)thanbasematerials.Gray[16]reportedthattheresilient modulus increased with increasing maximum particlesizeforaggregateswithsameamountoffinesandsimilarshapeof size distribution. Also, Hicks andMonismith [10], Dawsonet al. [17] andHeydinger et al. [18] reported that the resilientmodulus of granular materials decreases with the increase inmoisturecontent.
MID-25% : y = 8.8411x0.4819
R2 = 0.8242
MID : y = 7.357x0.5324
R2 = 0.6721
MID+25%: y = 6.5185x0.5029
R2= 0.8597
1
10
100
1 10 100Stress Invariant (psi)
Res
ilien
t Mod
ulus
(x10
3 psi
)
MID - 25% : y = 2.948x0.4826
R2 = 0.9031
MID + 25% : y = 1.6246x0.5251
R2 = 0.7435
MID : y = 1.9291x0.5172
R2 = 0.7621
1.0
10.0
100.0
1.0 10.0 100.0Stress Invariant (psi)
Res
ilien
t Mod
ulus
(x 1
000
psi)
MID + 25% : y = 4.0748x0.5451
R2 = 0.939
MID - 25% : y = 6.5863x0.5682
R2 = 0.6756MID : y = 4.7077x0.5305
R2 = 0.6715
1.0
10.0
100.0
1.0 10.0 100.0Stress Invariant (psi)
Res
ilien
t Mod
ulus
(x 1
000
psi)
Figure 7: Resilient modulus plot for all gradations at OMC
Figure 9 : Resilient Modulus plot for subbase type E (Mining Sand) at OMC
ReferringtotheTables8and9,itcouldalsobeseenthatforquarrydustatOMC,mostofthek1valuesobtainedareatthelowerendofthesuggestedrange(wetmoisturecondition),whilefork2,thevaluesarewithintherangeofthesuggestedvalues.MID-25%gradationhasthehighestk1valuesforbothquarrydustandminingsand,whileMID+25%gradationhas the lowest k1 values.Thissuggestthatthegradationwiththelowerfinescontent(%passing75µm)hasahigherk1value.
Table 9: Values of k1 and k2 for subbase materials at OMC
Subase Type k1 k2
QuarryDustMid-25%Mid
Mid+25%
2,948.01,929.11,624.6
0.48260.51720.5251
MiningSandMid-25%Mid
Mid+25%
6,586.34,707.74,074.8
0.56820.53050.5451
Also,mining sandwas found to have higher k1 value thanquarrydust,whilek2valuedoesnotshowanyspecifictrendwithchangesinmoisturecontent.Asthek1valueforquarrydustislowatOMC(andthushasalowerresilientmodulusvalue),itislikelythatquarrydustisunsuitableasasubbasematerialasitisadverselyaffectedbyanincreaseinmoisturecontent.Forminingsand,theOMCvaluegivesthehighestk1valueandthusisappropriatetobe
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testedatOMC.Thereasonsforthisdifferencesarepossiblyduetofirstly,thelowermoisturecontentanddegreeofsaturationatOMCforminingsandandsecondly,itislikelythatthefinescontentofminingsand(below0.075mm)iscoarserthanthefinescontentofquarrydust,thusislessplasticandislessinfluencedbytheincreaseinmoisturecontentatOMC.
5 RESlLIENT MODULUS OF BASE AND SUBBASE MATERIALS TO BE USED IN PAVEMENT DESIGN Base and subbase materials are granular in nature andthereforetheresilientmodulusforgranularmaterialsisnon-linearand depends on particularly the confining pressure that exist intheparticularlayer.Todeterminetheresilientmodulus,iterativecalculationsmustbecarriedoutandthisisnormallydoneusingcomputer software that calculate the stresses using elastic layerassumptionsorfiniteelementprocedures.However,AASHTO[5]providessomesuggestionsregardingthevaluestobeused.
A Base Materials Thebasemodulusisnotonlyafunctionofmoisturebutalsothestressstatewhichinturn,varywiththesubgrademodulusandthicknessoftheasphalticconcretesurfacinglayer.Thefollowingis recommended values by AASHTO [5] for use in design ofpavements:
Table 10: Values of stress state of base recommended by AASHTO [5]
Asphalt Concrete Thickness (inches)
Subgrade Soil Resilient Modulus (psi)
3,000 7,500 15,000Less than 2
2 - 44 - 6
Greater than 6
201055
2515105
3020155
In Malaysia, most pavements are designed based on anassumed CBR value of 5 and asphaltic concrete thickness ofbetween4-6inches(100-150mm).Subgraderesilientmodulusareusuallycalculatedbasedonthefollowingrelationship:
MR(psi)=1500×CBR(7)
AsCBR=5,MR=7,500psi(51.8MPa)Thestressstatetobeusedis10psi.(0.069MPa)AssumingthebasematerialtobeusedisfromtheMIDgradationline (moisture content of OMC-2%), the following equationderivedfromthelaboratorytestisused:
Mr(psi)=7357θ 0.5324
Usingθ=10psi, Mr(psi)=7357(10)0.5324
Mr(psi)=25,067psi.
Ifafactorof1.2isusedtocompensateforthereductionofresilientmodulusvalueduetothescalpingofmaterialslargerthan25mminthelaboratorytesting(Barksdaleetal.1997),then:
MR(psi)=25,067psi×1.2=30,080psi(207.6Mpa)
ForMalaysianBaseTypeII,MIDgradationline(moisturecontent of OMC-2%), the recommended values of resilientmodulusisshowninTable11.Itshouldbenotedthattheresilientmodulus is dependant onboth asphaltic concrete thickness andtheresilientmodulusofthesubgradesoil.Forthesameasphalticconcretethickness,thehigherthesubgraderesilientmodulus,thehigher is the resilientmodulus value of the base.On the otherhand, for the same subgrade modulus value, the increase inthe thicknessof theasphalticconcretewill result in lowerbaseresilientmodulusvalue.
Table 11: Recommended values of resilient modulus (psi) for Malaysian Base Type II material (MID gradation, OMC-2%)
using stress state values suggested by AASHTO [5]
Asphalt Concrete Thickness (inches)
Subgrade Soil Resilient Modulus (psi)
3,000(CBR = 2)
7,500(CBR = 5)
15,000(CBR = 10)
Less than 22 - 44 - 6
Greater than 6
43,50030,10020,80020,800
49,00037,30030,10020,800
54,00043,50037,30020,800
B Subbase Materials Themodulus of the subbase is dependant on the asphalticconcrete thickness and for subbase thickness between 6 and 12inches (150 mm to 300 mm), AASHTO [5] recommended thefollowingstressstates(inpsi):
Asphalt ConcreteThickness (inches)
Stress State (psi)
less than 22 – 4
greater than 4
107.55
Using subbase Type E (mining sand-mid gradation) as anexample,andassumingthatthethicknessofasphalticconcreteisgreaterthan4inches(100mm)theresilientmodulusisdeterminedasfollows:
MR=4,707.7(θ)0.5305
=4,707.7(5)0.5305 =11,056psi(76.3MPa)
TherecommendedvaluesofresilientmodulusforMalaysiansubbase Type E, using mid-gradation line (moisture content atOMC),isshowninTable13.Itshouldbenotedthattheresilientmodulus of the subbase is dependant on the asphaltic concretethickness.Theincreaseinthethicknessoftheasphalticconcretewillresultinlowersubbaseresilientmodulusvalue.
Table 13: Recommended values of resilient modulus (psi) for Malaysian Subbase Type E material (mid-gradation, OMC)
using stress state values suggested by AASHTO [5]
Asphalt ConcreteThickness (inches)
Mining Sand (psi)
Quarry Dust (psi)
less than 22 – 4
greater than 4
16,00013,70011,100
6,3005,5004,400
Table 12: Values of stress state of subbase recommended by AASHTO[5]
RESILIENT MODULUS OF MALAYSIAN BASE AND SUBBASE PAVEMENT MATERIALS
Journal - The Institution of Engineers, Malaysia (Vol. 69, No.2, June 2008) 61
ENGR. DR. MOHD. YUSOF BIN ABD. RAHMAN CurrentlyanAssociateProfessorintheFacultyofCivilEngineering,UniversityTeknologiMARA,ShahAlam.HespecialisesintheareaofHighwayandTransportationEngineering.
PROFILES
CONCLUSION Themeasurement of resilientmodulus of base type II andsubbase type E using quarry dust andmining sandwas carriedout by laboratory testing according to the procedures set out inAASHTOT307-99.For thebasematerials,MID-25%gradationhas the highest resilient modulus value at OMC. However,comparingwithvaluesobtainedbyresearchelsewhere,itislikelythatthevalueshouldbebasedonlowermoisturecontent,possiblyatOMC-2%.Forsubbasematerials,miningsandgavethehigherresilientmodulus values than quarry dust atOMC.As subbaselayer is located adjacent to the subgrade, itsmoisture conditionislikelytobehigherandpossiblyattheOMC.Resilientmodulusvaluesofbase(TypeII)andsubbase(TypeE)forthemid-gradationlinewasrecommendedbasedonthestressstatevaluessuggestedAASHTO. Field verification using non-destructive techniquessuchasfallingweightdeflectometer(FWD)andthemeasurementofin-situmoisturecontentofbaseandsubbaseshouldbecarriedout to compare the values obtained in the laboratory and thosemeasuredin-situ.n
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ENGR. AHMAD KAMIL BIN ARSHAD CurrentlyaPhDstudentintheFacultyofCivilEngineering,UniversitiTeknologiMARA,ShahAlam.HereceivedhisB.E.(Civil)fromCurtinUniversityofTechnology,WesternAustraliain1987andhisM.S.(HighwayandTransportationEngineering)fromUniversitiPutraMalaysiain2003.