pavement design project notes by denis ssekamatte








PavementDesign



Overview

Ahighwaypavement isastructureconsistingofsuperimposedlayersofprocessed materialsabovethenaturalsoilsub-grade,whoseprimaryfunctionistodistributethe appliedvehicle loadstothesub-grade.Thepavementstructureshouldbeableto providea surfaceof acceptableridingquality,adequateskidresistance,favorablelight reflectingcharacteristics,andlownoisepollution.Theultimateaimistoensurethat thetransmittedstressesduetowheelloadaresufficientlyreduced,sothattheywill
notexceedbearingcapacityofthesub-grade.Twotypesofpavementsaregenerally
recognizedasservingthispurpose,namelyflexiblepavementsandrigidpavements. Thischaptergivesanoverviewofpavementtypes,layers,andtheirfunctions,and pavementfailures.Improperdesignofpavementsleadstoearlyfailureof pavements affectingtheridingquality.



Requirements ofa pavement

Anidealpavementshould meetthefollowingrequirements:

        Sufficientthicknesstodistributethewheelloadstressestoasafe valueonthe sub-gradesoil,
Structurallystrongtowithstandalltypesofstressesimposedupon it,
Adequatecoefficientoffrictiontopreventskiddingofvehicles,
Smoothsurfacetoprovidecomforttoroadusersevenat highspeed,
Produceleastnoisefrommovingvehicles,
Dustproofsurfacesothattrafficsafetyisnotimpairedbyreducingvisibility,
Impervioussurface,sothatsub-gradesoiliswellprotected,and
Longdesignlifewithlowmaintenancecost.



Types ofpavements

Thepavementscanbeclassifiedbasedonthestructuralperformanceintotwo,flexible pavementsandrigidpavements.Inflexiblepavements,wheelloadsaretransferredby grain-to-graincontactoftheaggregatethroughthegranularstructure.Theflexible pavement,havinglessflexuralstrength,actslikea flexiblesheet(e.g.bituminous road).Onthecontrary,inrigidpavements,wheelloadsaretransferredtosub-grade soilbyflexuralstrengthofthepavementandthepavementactslikea rigidplate(e.g. cementconcreteroads).Inadditiontothese,compositepavementsarealsoavailable.
Athin layerofflexiblepavementoverrigidpavementisanidealpavementwithmost desirablecharacteristics.However,suchpavementsarerarelyused innew constructionbecauseof highcostandcomplexanalysisrequired.

Flexible pavements

Flexiblepavementswilltransmitwheelload stressestothelowerlayersbygrain-to- graintransferthroughthepointsofcontactin thegranularstructure(seeFigure1).

































Figure1:Loadtransferingranularstructure


Deflectiononflexiblepavement

Thewheelloadactingonthepavementwillbedistributedtoawiderarea,andthe stressdecreaseswiththedepth.Takingadvantageof thisstressdistribution



characteristic,flexiblepavementsnormallyhas manylayers.Hence,thedesignof flexiblepavementusestheconceptoflayeredsystem.Basedonthis,flexible pavementmaybeconstructedinanumberoflayersandthetop layerhastobeof best qualitytosustain maximumcompressivestress,inadditiontowearandtear.The lowerlayerswillexperiencelessermagnitudeof stressandlowqualitymaterialcan
beused.Flexiblepavementsareconstructedusingbituminousmaterials.Thesecanbe
eitherintheformofsurfacetreatments(suchasbituminoussurfacetreatments generallyfoundonlowvolumeroads)or,asphaltconcretesurfacecourses(generally usedonhighvolumeroadssuchasnationalhighways).Flexiblepavementlayers reflectthedeformationofthelower layersontothesurfacelayer(e.g.,ifthereisany undulationinsub-gradethen itwillbetransferredtothesurfacelayer).Inthecaseof flexiblepavement,thedesign isbasedonoverallperformanceof flexiblepavement, andthestressesproducedshouldbekeptwellbelowtheallowablestressesofeach pavementlayer.

Types ofFlexible Pavements

Thefollowingtypesofconstructionhavebeenusedinflexiblepavement:

Conventionallayeredflexiblepavement,
Full- depthasphaltpavement,and
Containedrockasphaltmat(CRAM).

Conventionalflexiblepavementsarelayeredsystemswithhighqualityexpensive materialsareplacedin thetopwherestressesarehigh,andlowqualitycheap materialsareplacedinlowerlayers.

Full- depthasphaltpavementsareconstructedbyplacingbituminouslayersdirectly onthesoilsub-grade.This ismoresuitablewhenthereishightrafficand local materialsarenotavailable.

Containedrockasphaltmatsareconstructedbyplacingdense/opengraded aggregatelayersinbetweentwoasphaltlayers.Modifieddensegradedasphalt concreteisplacedabovethesub-gradewillsignificantlyreducethevertical compressivestrainonsoilsub-gradeandprotectfromsurfacewater.

Typicallayers ofaflexible pavement

Typicallayersofaconventionalflexiblepavementincludessealcoat,surfacecourse, tackcoat,bindercourse,primecoat,basecourse,sub-basecourse,compactedsub- grade,andnaturalsub-grade(Figure2).



SealCoat:

Sealcoatisathinsurfacetreatmentusedto water-proofthesurfaceandtoprovide skidresistance.

TackCoat:

Tackcoatisaverylightapplicationofasphalt,usuallyasphaltemulsiondiluted with water.It providesproperbondingbetweentwolayerofbindercourseand mustbe thin,uniformlycovertheentiresurface,andsetveryfast.

PrimeCoat:

Primecoatisanapplicationoflowviscouscutbackbitumentoanabsorbentsurface likegranularbasesonwhichbinderlayer isplaced.It providesbondingbetweentwo layers.Unliketackcoat,primecoatpenetratesintothelayerbelow,plugsthevoids, andformsawatertightsurface.























Figure2:Typicalcross sectionofaflexiblepavement


Surfacecourse

Surfacecourseisthelayerdirectlyincontactwithtrafficloadsand generallycontains superiorqualitymaterials.Theyareusuallyconstructedwithdensegradedasphalt concrete(AC).Thefunctionsandrequirementsofthislayerare:

        It providescharacteristicssuchasfriction,smoothness,drainage,etc.Alsoit willpreventtheentranceof excessivequantitiesofsurfacewaterintothe underlyingbase,sub-baseandsub-grade,
        It mustbetoughtoresistthedistortionundertrafficandprovidea smoothand skid-resistantridingsurface,



        It mustbewaterprooftoprotecttheentirebaseandsub-gradefromthe weakeningeffectofwater.

Bindercourse

Thislayerprovidesthebulkoftheasphaltconcretestructure.It'schiefpurposeisto distributeloadtothebasecourseThebindercoursegenerallyconsistsofaggregates havinglessasphaltanddoesn'trequirequalityashighasthesurfacecourse,so replacingapartofthesurfacecoursebythebindercourseresultsinmoreeconomical design.

Basecourse

Thebasecourseisthelayerofmaterialimmediatelybeneaththesurfaceof binder courseandit providesadditionalloaddistributionandcontributestothesub-surface drainageIt maybecomposedof crushedstone,crushedslag,andotheruntreatedor stabilizedmaterials.

Sub-Basecourse

Thesub-basecourseisthelayerofmaterialbeneaththebasecourseandtheprimary functionsaretoprovidestructuralsupport,improvedrainage,andreducetheintrusion offinesfromthesub-gradeinthepavementstructureIfthebasecourseisopen
graded,thenthesub-basecourse withmorefinescanserveasafillerbetweensub- gradeandthebasecourseAsub-basecourseisnotalwaysneededorused.For example,a pavementconstructedoverahighquality,stiffsub-grademaynotneedthe additionalfeaturesofferedbyasub-basecourse.Insuchsituations,sub-basecourse maynotbeprovided.

Sub-grade

Thetopsoilorsub-gradeisalayerofnaturalsoilpreparedtoreceivethestressesfrom thelayersabove.It isessentialthatat notimesoilsub-gradeisoverstressed.It should becompactedtothedesirabledensity,neartheoptimummoisturecontent.

Failure offlexible pavements

Themajorflexiblepavementfailuresarefatiguecracking,rutting,andthermal cracking.Thefatiguecrackingofflexiblepavementisduetohorizontaltensilestrain at thebottomoftheasphalticconcrete.Thefailurecriterionrelatesallowablenumber ofloadrepetitionstotensilestrainandthisrelationcanbedeterminedinthe laboratoryfatiguetestonasphalticconcretespecimens.Ruttingoccursonlyon



flexiblepavementsasindicatedbypermanentdeformationorrutdepthalongwheel loadpath.Twodesign methodshavebeenusedtocontrolrutting:onetolimitthe verticalcompressivestrainonthetopofsubgradeandothertolimitruttingtoa tolerableamount(12mmnormally).Thermalcrackingincludesbothlow-temperature crackingandthermalfatiguecracking.

Rigidpavements

Rigidpavementshavesufficientflexuralstrengthtotransmitthewheelloadstresses toa widerareabelow.Atypicalcrosssectionoftherigidpavementisshown in Figure3.Comparedtoflexiblepavement,rigidpavementsareplacedeitherdirectly
onthepreparedsub-gradeoronasinglelayerofgranularorstabilizedmaterial.Since thereisonlyonelayerofmaterialbetweentheconcreteandthesub-grade,thislayer canbecalledasbaseor sub-basecourse.




















Figure3:Typical Cross sectionofRigidpavement


Inrigidpavement,loadisdistributedbytheslabaction,andthepavementbehaveslike anelasticplaterestingonaviscousmedium(Figure4).Rigidpavementsare constructedbyPortlandcementconcrete(PCC)andshouldbeanalyzedbyplate theoryinsteadoflayertheory,assuminganelasticplaterestingonviscousfoundation. Platetheoryisasimplifiedversionoflayertheorythatassumestheconcreteslabasa mediumthickplate whichisplanebeforeloadingandtoremainplaneafter loading. Bendingoftheslabduetowheelloadand temperaturevariationandtheresulting tensileandflexuralstress.

Elasticplaterestingonviscousfoundation

Deflectiononconcretepavement



Types ofRigidPavements

Rigidpavementscanbeclassifiedintofourtypes:

Jointedplainconcretepavement(JPCP),
Jointedreinforcedconcretepavement(JRCP),
Continuousreinforcedconcretepavement(CRCP),and
Pre-stressedconcretepavement(PCP).

JointedPlainConcretePavement:

areplaincementconcretepavementsconstructedwithcloselyspacedcontraction joints.Dowelbarsoraggregateinterlocksarenormallyusedforloadtransferacross joints.Theynormallyhasajointspacingof5to10m.

JointedReinforcedConcretePavement:

Althoughreinforcementsdonotimprovethestructuralcapacitysignificantly,theycan drasticallyincreasethejointspacingto10to30m.Dowelbarsarerequiredforload transfer.Reinforcementshelptokeeptheslabtogetherevenaftercracks.

Continuous ReinforcedConcretePavement:

Completeeliminationofjointsareachievedbyreinforcement.





Failure criteriaofrigidpavements

Traditionallyfatiguecrackinghasbeenconsideredasthemajor,oronlycriterionfor rigidpavementdesign.Theallowablenumberofloadrepetitionstocausefatigue crackingdependsonthestressratiobetweenflexuraltensilestressandconcrete modulusofrupture.Oflate,pumpingisidentifiedasan importantfailurecriterion. Pumpingistheejectionofsoilslurrythroughthejointsandcracksofcementconcrete pavement,causedduringthedownwardmovementofslabundertheheavywheel loads.Other majortypesofdistressinrigidpavementsincludefaulting,spalling,and deterioration.



Theadvantagesof flexiblepavements include-

»Adaptabilitytostageconstruction

»Availabilityoflow-costtypes thatcanbeeasilybuilt

»Abilitytobeeasilyopenedandpatched

»Easytorepairfrostheaveandsettlement

»Resistancetotheformationoficeglaze

Thedisadvantagesinclude-

»Highermaintenancecosts

»Shorterlifespanunderheavyuse

»Damagebyoils andcertainchemicals

»Weakedges thatmayrequirecurbs oredgedevices

(i)Curbs canbeusedofconcreteis probablythemostcommonmaterialusedforbothbarrier andmountablecurbs.Concretecurbs aredurableandrelativelyeasytoconstruct.

(ii)Asphaltisfrequentlyusedforcurbing,especiallywherecurvedparkingislandsmustbe constructed. Theyareeconomicalandeasytoconstruct.

(iii) Granitecurbs arenotas frequentlyusedas asphaltorconcreteunless graniteis quarriedin thearea.Granitecurbs arefarmoredurablethanconcrete.



Problems

1. Thethinlayerofbitumencoatingbetweenanexistingbituminouslayeranda newbituminouslayer is:
1. Sealcoat
2. Intermediatecoat
3. Tackcoat
4. Primecoat
2. Rigidpavementsaredesignedby
1. Rigidplatetheory
2. Elasticplatetheory
3. Infinitelayertheory
4. Interlockingofaggregates






Solutions

1. Thethinlayerofbitumencoatingbetweenanexistingbituminouslayeranda newbituminouslayer is:
1. Sealcoat
2. Intermediatecoat


3. Tackcoat
4. Primecoat
2. Rigidpavementsaredesignedby
1. Rigidplatetheory

2. Elasticplatetheory
3. Infinitelayertheory
4. Interlockingofaggregates

Summary

Pavementsformthebasicsupportingstructureinhighwaytransportation.Eachlayer ofpavementhasamultitudeof functionstoperform whichhastobedulyconsidered duringthedesignprocess.Differenttypesofpavementscanbeadopteddepending uponthetrafficrequirements.Improperdesignofpavementsleadstoearlyfailureof pavementsaffectingtheridingqualityalso.



Bibliography

www.google.com
 www.SeminarsIIt.blogspot.com
www.wikipedia.com






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