effect of vehicle axle loads on pavement performance - sharma

8/2/2019 Effect of Vehicle Axle Loads on Pavement Performance - Sharma

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Effect OfVehicle Axle Loads On Pavement Performance

B.M. Sharma, K. Sitaramanjaneyuiu, and P.K. KanchanCentral Road Research Institute of New Delhi. IndiaABSTRACT

The ever increas ing vehiclepopulat ion and heavy axle loads hascaused subs tan t ia l damage to Indianroads. Trucks carry loads much inexcess of l ega l lLmits and are la rgelyresponsib le fo r poor road conditions inaddi t ion to th e inadequate s t ruc tura lcapacity o f pavements and diminishingal loca t ion of funds year af te r year fo rmaintenance and rehabi l i ta t ion . Veryhuge cap i t a l inves tments are now neededto upgrade and r ehab i l i t a te the exis t ingroad network to make it capable towithstand high s t resses and ty repressures caused by heavy wheel loads.In view of very remote poss ib i l i ty ofsuch l a rge magnitude of funds everbecoming avai l ab le in the near fu ture ,one of the bes t course to remedy th es i tua t ion would be to s t r i c t l y enforcethe l ega l axle l imi t s . Pavementperformance da ta base generated andpavement deter iora t ion models developedfrom th e Pavement Performance Study,recen t ly completed in the country, hasbeen used/appl ied for the presentanalyses . An at tempt has been made inth i s paper to evaluate the ef fec t s ofheavy axle loads on pavement performancein terms of inc rease in serv ice l i f e i fover loading i s r e s t r i c ted through s t r i c tenforcement. Further detai led analysesi s planned to be done fo r obtainingr e l i ab l e and accura te r e su l t s .INTRODUCTION

Transporta t ion i s a v i t a l sector tothe economy of a na t ion . India i s a f as tdeveloping country and has got on e of

th e l a rges t r a i l and road t r anspor ta t ionnetwork in the world. Th e share of roadt r anspor t to r a i l t r anspor t hasincreased to 80 % an d 60 % in 1993 from26% an d 11% in 1951 fo r passenger andgoods t r anspor t respect ively . The roadlength has correspondingly increasedfrom about 0.4 mi1liom km. to 2 mil l ionkm. during the l a s t four decades. Due toindus t r ia l i sa t ion an d increased economy,the vehic le populat ion has increasedfrom 0.3 mil l ion in 1951 to about 21mil l ion during the same per iod, thusreg is te r ing a 70 fo ld increase . Thegrowth of passenger and f r i eght t r a f f i cis al so phenominal and it i s about 10%per annum a t present . Th e passengert r a f f i c has r i sen to 1200 b i l l i o npassenger km. (bp km) in 1991 from 31 bpkm in 1951 and the f r e ight t r a f f i c hasgone to 295 bi l l ion tonne km (b t km) in1991 from 5.5 b t km in 1951. It i sexpected t ha t passenger and f re ightt r a f f i c would be about 3 tLmes and 4tLmes respect ively than the presen tt r a f f i c in the next 15 years , ca l l i ngfo r heavy demand fo r add i t iona l veh ic lesand be t te r roads e t c . Not only thet r a f f i c volume has increased butth e tendency of most of theveh ic le d r ive rs i s to overload

alsoheavyt h e i r

veh ic les in order to save opera t ioncos t . Most of the goods veh ic les inInd ia a re two-axle r ig id chass i s andcons t i t u t e about 98.5 % of the t o t a lf l ee t . OVerloading of t rucks i s a commonscene on Indian roads an d it i s notsurpr i s ing to find heavy veh ic les withhigh ty re pressures than normal valuesand carrying as high as 18 to 20 tonnesaxle loads against th e l ega l ly

Road transport technology-4. University of Michigan Transportation Research Institute. Ann Arbor. 1995. 263


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ROAD TRANSPORT TECHNOLOGY--4permissible axle load of 10.2 tonnes.The a r t e r i a l road system comprisingNational Highways (NH) and StateHighways (SH) i s jus t about 2% and 6% ofthe t o t a l road length respect ively buti s responsible of carrying as high astwo-thi rd of the t o t a l road t r a f f i c .The funds al located for road developmentpurposes i s about 30-40% shor t fo r NHsystem and i s shor t by about 50-60% forSH system than the actua l needs.

Several research studies have beencompleted an d some are in progress forthe purpose of understanding pavementbehaviour (performance) under dif fe ren tcondit ions of t r a f f ic volume andloading, cl imate, subgrade, s t ruc tura lcomposition e tc . One such studyrecent ly completed i s the PavementPerformance Study. The data base andmodels avai l ab le through th i s study hasbeen made use of in conducting analysespresented herein , to evaluate thevehic le loading ef fec ts (overloading) onpavement performance.

The paper presented describes anoverview of the present s ta tus an d roaddevelopmental pol ic ies an d pract ices in. India; importance an d impl ica t ion ofvehic le loads ; studies conducted towardsmodernisat ion of vehicle f l ee t an ddevelopment of ef f ic ien t roadin f ras t ruc ture . The paper discusses inde ta i l the analyses/procedures adoptedto evaluate the ef fec ts of vehicle loadson pavement performance an d the resu l tsobta ined therefrom. Further work inth i s di rec t ion i s also suggested towardsachieving re l iab le resu l ts .AN OVERVIEW OF PRESENT STATUS AND ROADDEVELOPMENTAL POLICIES/PRACTICES ININDIA

The unant ic ipated increase in vehiclepopula t ion and heavy axle loads hasbrought the road network to a crumblings tage . The network i s gross ly short ofi t s s t r uc tu r a l capacity, highlydis t ressed and has s ta r ted showingsigns of premature fa i lure . The roadnetwork i s unable and incapable tosustain high s tresses caused due to

264

heavy wheel loads and increased tyrepressures. I t has been indicated tha tthe condit ion of NH system i s not verysat isfactory and encouraging and tha t 3Km. out of every 5 km. of s t a t e highwaysare in bad shape needing immediateat tent ion.

The assets bu i l t a t huge capi ta l costsare ageing due to continuous shor t fa l lin investments on road maintenance inaddition to excessive u t i l i za t ion ofroads. I t has been estimated t ha t thecountry i s loos ing about Rs. 150,000mill ion ($ US 5,000 mil l ion) on accountof excess vehicle operat ion cost due topoor road condi t ions. The publicinvestment on road sec tor need to bestepped up by 2.7 t imes the presentlevel of funds a l loca t ion fo r roadmaintenance and upgradation. Hugeinvestments are needed fo r removal ofdefic iencies which have bu i l t up becauseof the reason t ha t t imely preventivemaintenance t reatments could not be. undertaken due to pauci ty of funds. Abroad estimate about the requirement offunds for the horizon year 2001 forbuilding expressways and upgradation,improvement and rehabi l i ta t ion of the NHsystem i s about Rs. 600,000 mill ion ($US 20,000 mil l ion) .

Pavement design in India i s empir icaland based on subgrade strength CBR (4days soaking) and cumulat ive standardaxles over a design l i f e . The designs t ipu la tes the mater ia l s and spec if i -cat ions alongwith t he i r thicknesses indi f fe ren t pavement layers . veryrecent ly , design guidel ines have beenbrought out based on mechanistic(analyt ical ) design methods. As regardsmaintenance of roads , norms and guidel ines available are t en ta t ive an d notbased on the resu l t s of an y scien t i f icstudy. The decis ions taken forimplementation of various maintenancetasks are subject ive in nature an ddepend largely on Engineer ' s judge-ment an d personal experience.Rehabil i tat ion / strengtheningrequirements are based on the Benkelmanbeam rebound def lec t ion data . PavementManagement s tudies are now f inding grea t


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appl icat ions in India towards ef f ic ien t ,ef fec t ive and coordinated management ofroad network, within the fundsconstraints . The Government of India i snow encouraging pr iva te enterpreneuresto invest in road sec tor so as toprovide good roads for comfortable,speedy and fue l -e f f ic ien t road t ranspor tin the country.VEHICLE LOADING - ITS IMPORTANCE ANDIMPLICATION

As emphasied e a r l i e r in the paper,t rucks in India carry loads in excess oft he i r capaci ty . There are standardsavai lable in India of s ize , weights anddimensions of the t ruck body but theseare , in genera l , not largely followed.The vehic le owners make changes an d havewider and higher bodies so tha t a truckcan carry more goods than permissible,thus producing a considerable reductionin haulage charges. I t i s not verystrange tha t newly constructed f lexiblepavements, par t icu la r ly those withunbound bases, f requent ly show signs ofdis t ress short ly af te r they are openedto t r a f f i c . The immediate effectsof overloading are less obvious onpavements with bound bases but the i rservice l ives are s ign if ican t ly reducedthan normally expected. Heavy t raf f icloading produce rapid dif fe ren t ia lcompaction in the upper layers ofpavement in addi t ion to fracture of theasphal t surfac ing . Implications ofoverloading on overa l l t ranspor t costshave been examined worldwide an d it i sevident tha t vehicle overloadingser iously af fec ts the improvements ofroad network in many developingcountries including India largelybecause of increased demands formaintenance and r ehab i l i t a t ion du e topavement's damage caused by heavy axles.

The dimension of the overloadingproblem i s such t ha t speci f icprecautions need to be adopted tominimise i t s af fec ts . One way i s to makesure t ha t pavements are designed using ar ea l i s t i c assessment of the expectedt r a f f ic loading because incorrectestimates of vehicle loading would

P A V E M E N T P E R F O ~ C E ser iously af fec t i t s behaviour. Th eother opt ion i s to s t r i c t ly enforce th elegal axle l imits and thus obtainincreased pavement's service l i fe an dperformance.

It has been establ ished t ha ts t ruc tura l damage to road pavements i scaused largely by commerCial t r a f f i c an dtha t the pavement damage increases verysteeply with the axle loadings. Thedamage which a heavy commercial vehicledoes i s a function of the degree towhich the various axles are loaded. Thedamaging ef fec t of t r a f f ic i s expressedas the number of standard axles per 100.commercial vehic les an d i s termed asVehicle Damage Factor (VDF).

To design pavements, it i s necessaryto know the d is t r ibu t ion of axle loadsto which the pavement wil l be subjectedduring the design l i f e . The dis t r ibu t ioni s general ly refer red to as the axleload spectrum. The axle load surveyshave been conducted in India from t imeto t ime on various roads an d the VDFS'which" i s a mult ipl ier for convert ing ofaxle-load repe t i t ion , have beendetermined. The indicat ive VDF valuesrecommended by Indian Roads Congress(IRe), for di f fe ren t range of t r a f f i cvolume (commercial vehicles per day) andthe t e r r a in , are given in Table-1.These are the VDF values used for designof pavements in India, i f spec i f ic VDFis not avai lable/assessed for a givenroad. The resu l ts of axle load surveysconducted in India in the recent pas thave brought out the fol lowing;i ) Damaging ef fec ts of heavy

commercial vehicles have increasedalarmingly par t icu la r ly on moreheavily t raff icked corr idors andhighways.

i i ) overloading beyond the legal axlel imi ts i s very frequent and i s ofvery large magnitude which, as amatter of fact , has led to thebel ief /conclusion t ha t overloadedvehicles are responsib le fo rpavement damage an d thus poor roadcondit ions .

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ROAD TRANSPORT TECHNOLOGY-4Table . 1 Indica t ive VDF Values

I n i t i a l t r a f f i ci n t ens i ty interms of numberof commercialvehic les /day

Less then 150

150-1500

More then 1500

Terra in

Hil lyRoll ingPla inHil lyRoll ingPla inHil lyRoll ingPla in

Th e l ega l l imi t for axle loads inInd ia has increased from 8.16 tonnes to10.2 tonnes in the l a s t decade or so .The Motor Vehicle Act s t i pu la t e s maximumaxle and vehic le weights as under:s ing le axle with two tyres 6.0 tonnesSingle axle with four ty res 10.2 tonnesTandem axle with eight ty res 19.0 tonnes

Very little attempt i s genera l ly madeto enforce the la w in th i s r espec t . Theincrease in axle l imi ts has beenef fec ted but the economic ju s t i f i c a t ionfo r changes in these l imi ts has not beenexamined very close ly an d accura te ly inIndia because of non-avai l ab i l i ty ofdata which would enable evaluat ion ofvehic le opera t ing costs and thees t ima t ion o f addi t ional pavement damagecaused by th e heavier axle loads . Indeveloped coun t r ie s , seve ra l majors tudies have been conducted which haveiden t i f i ed f ac tor s inf luencing th e costo f opera t ing vehic les and it i s nowposs ib le to conduct incremental analysesto show the economic e f f ec t s ofincreases in axle loads . I t has alsobeen concluded through these s tudiest h a t increases in legal axle load l imi t sa re j u s t i f i ed under a wide var ie ty ofcond i t ions .

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VDF values (standard ax les o f 8.16 tonnesper commercial vehic le)Unsurfaced

0.501.502.00

ThinBituminousSurfacing

0.751. 752.251.002.002.501.252.252.75

ThickBituminousSurfacing

1.252.252.751.502.503.00

The cos t of opera t ing a road t r anspor tsystem cons is t s o f tw o main c o m p o n e l ) ~ , s , namely the opera t ing cos t of vehic lesf lee t using the f a c i l i t y and the costof c o n ~ t r u c t i n g and maintaining theroads. I t i s now a wel l known f ac t t ha tthe t r anspor t cos t o f a par t icu la rf re ight tonnage decreases qui te rapid lywith increase in amount of f r e ightcarried by vehic les and also tha t thecost of providing an d maintaining theroads increases as the vehic le axle loadincreases . The magnitude of thesecomponent costs var ies with extent ofaxle loads car r i ed by vehic les . Indiaspends a la rge proport ion of her scareresources on road t r anspor ta t ion an d iti s therefore des i rable t ha t the roadt ranspor t system should opera te underconditions which minimises the t o t a lcost .

The most widely used r e la t ionshipbetween vehic le loading and pavementperformance wa s der ived from the AASHORoad Test in ear ly s i x t i e s . An axle loadcarrying a load of 8.16 ton was def inedas a s tandard axle , with a damagingef fec t of uni ty . Th e damaging e f f ec t s ofl igh te r and heavier ax les were expressedas equivalency fac tors . Th e s t ruc tura ldamage to a pavement caused by wheel


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loads i s given by an empir ical equat ionof th e fo l lowing form:

Pavement damage (Axle load)nThe r e su l t s .have revealed t h a t value

of exponent, n , can vary from 2 .4 to 6.6under extreme condi t ions . It wasconcluded th a t fo r heavy wheel loads onroads of medium or high s t r ength , asmeasured by s t ruc t u ra l number, the valueof n i s in the range of 3 .2 to 5.6 .The conclus ion emerged from theAASHO Road Test was t h a t the re la t ivedamage to both f l ex ib le and r ig idpavements varied approximately as thefour th power of the applied wheel loads .It i s t h i s r e la t ionsh ip t h a t providesth e bas i s fo r assess ing th e ef fec t s ofvehic le loading in most cur ren t methodsof pavement design globa l ly . There la t ionsh ip enables conver t ing thees t ima ted spectrum of ax le loadings in toan equiva lent number of repe t i t i ons of as tandard ax le load of 8.16 tonnes . Thef ac t o r s fo r th i s conversion toEquiva len t Standard Axles (ESALs ) werederived based on th i s re l a t ionsh ip . InInd ia , th e value of exponent n i s takenas 4 and the t r a f f i c i s def ined by theEquiva len t Standard Axles (ESALs ) whichi s ca lcu la t ed by mult ip lying th e numberof commercial vehic les and VehicleDamage Factor (VDF) der ived from axleload spectrum using s tandard "FourthPower Law".STUDIES CONDUCTED IN INDIA TOWARDSDEVELOPMENT, IMPROVEMENT ANDMODERNISATION OF ROAD INFRASTRUCTURE ANDVEHICLE FLEETPAVEMENT PERFORMANCE STUDY

This i s a long term research study ofna t iona l importance, sponsored by th eMinis t ry of Surface Transport , Govt. ofIndia , and was commenced in the year1986. The study i s a sequal to thea lready completed Road User Cost studyin Ind ia , which had, e a r l i e r in 1982,success fu l ly brought out road use r . cos tmodels fo r d i f f e ren t roadway and t r a f f i cs i t u a t i o n s . The Pavement PerformanceStudy i s planned to evolve road cos t

PAVEMENT PERFORMANCEmodels, c o m p r ~ s ~ n g i n i t i a l const ruc t ioncos t and subsequent maintenance cos tduring the des ign / se rv ice l i f e , andwhich in conjunct ion with road usercosts , would develop information/datafor t o t a l t r an spo r t a t i on c os t modelsunder Indian condi t ions . The studyObjectives are proposed to be achievedthrough the fo l lowing:i ) Development of pavement performance

data fo r pavement mater ia l snormally used i n t h e country

i i ) Based on th e performance data , toa t tempt development of l aye requivalencies , as feas ib le

i i i ) Limited s tud ies on the e f fec t ofmaintenance l eve l on pavementperformance

iv) Generat ion o f data on theconst ruc t ion and maintenanceinputs of d i f f e r en t pavements

The study cons i s t s of two pa r t s :a) Study on Exis t ing Pavement Sect ionsb) Study on New Pavement Sect ionsstUdy On Exis t ing Pavement Sect ions

This pa r t of s tudy has recent ly beencompleted on i n - se rv i ce road sec t ions .A to ta l of 113 road sec t ions on a r te r i a lroad network, each 500 meter l ength , inthe s t a t e s of Haryana, Uttar Pradesh,Rajasthan and Gujara t were monitered fo rthe i r performance, a t every ha l f yeari n t e rva l , over a per iod of 3 to 5years . The various observat ions/measurements t aken on these t e s tsec t ions included the . fo l lowing:i ) Pavement s t ru c tu r a l de ta i l s(mater i a l s / spec i f i ca t ions and th e i r

th icknesses in d i f f e r en t l ayers)i i ) Surface defec t s c r a c k i n g ,patching, pothole s , r ave l l ing ,

shoving, depress ion e t c . )

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ROAD TRANSPORT TECHNOLOGY-4i i i ) Roughness by Fi f th

in tegra toriv ) Rebound def lec t ion bybeam

wheel bump

Benkelman

v) Subgrade moisture contentv i) Traff ic volume surveys (on

representa t ive s i t e s )v i i ) Axle load surveys ( o n representa

t ive s i t e s , annual ly)v i i i )La te ra l placement of vehic les

(during s ix th and t en th ser i es ofobservations)ix ) Transverse pro f i l e

Modified Struc tu ra l Number (MSN) i s anindicator of pavement 's s t ruc tura ls t rength and has been used fo r th i ss tudy. I t i s calcula ted using thefol lowing equation developed duringKenya study

MSN:: SN+ SNSGwhereSM :: Struc tu ra l Number

a 1 , a 2 , a 3 , ..... an a re th e s trengthcoef f i c i en t s of d i f f e r en t mater ia ls invarious pavement courses andt 1 , t 2 , t 3 ' ..... n are theth icknesses of various pavement layersin inches .SNSG::structural suppor t due to subgrade=3.51(10910 CBR)-0.S5(lo910 CBR)2 -1 .43whereCBR : :Cal i forniamoisturedensi ty

Bearing Rat io a t Fieldcontent and Field dry

The voluminous t ime-ser i es pavementperformance an d t r a f f i c - r e la ted datawas analysed and incremental modelsdeveloped for predic t ion of var ious

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modes of d is t r e s s for pavements withsurfac ings of Premix carpet (PC), SemiDense Bituminous Concrete (SDBC) andBituminous Concrete (BC) Th e variousmodels developed (avai lable ) a re :a) In i t i a t ion of Crackingb) Progress ion of Crackingc) In i t i a t ion of Ravelling

( for premix carpet s u r f a ~ e only)d) Progress ion of Ravelling( for premix carpet surface only)

e) In i t i a t ion of Potholesf) Progress ion of Potholesg) Progress ion of Roughness

These models a re capable of predic t ingthe s t a t e of heal th (various defec t s ) ofthe pavements , over a passage o f timeand t r a f f i c app l ica t ions , which wouldas s i s t and. enable highway planners ,p ro fess iona ls an d policy makers indeciding upon optimal and r a t i ona lmaintenances t r a t eg i e smodels mayfollowing

andfo r road

also ber ehab i l i t a t i on

i )

network.used fo r

Thesethe

To evaluate thes t r uc tu r a l s t rengthperformance (pavement

e f f ec t ofon pavement

deter iora t ion)i i ) To evaluate th e ef fec t of t r a f f i cloading on pavement performancei i i ) To evaluate the exist ing pavement

des igniv) To support the analy t i ca l pavementdesign methodsv ) Pavement maintenancesystem development

management

For th e work reported in t h i s paper ,the pavement deter iora t ion modelsdiscussed above have been appl ied toevaluate the ef fec ts of increased axleloads (over loading) on pavementperformance.Study On New Pavement Sect ions

This p a r t of s tudy has recent lylaunched and i s in progress . Th e

beenstudy


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i s to be conducted on special ly designedand construc ted experimental roadsec t ions , for deta i led and comprehensivecoverage of various parameters , in orderto obtain re l iab le an d ref ined pavementdeter iora t ion models for a variety ofappl icat ions .VEHICLE FLEET MODERNISATION AND ROAD USECHARGES STUDY

This study undertaken with the WorldBank ass i s t ance has been completed an dwas taken up to determine the optimalfuture composition of the motor vehiclef l ee t and appropriate levels of road usecharges. The study est imated futuredemand fo r road t ransport an d theanalyses of the changing scenario of thef re ight market. I t involved assesment ofthe appropr ia teness of the present s t a t eof the road in f ras t ruc ture and t ransportindust ry . The study establ i shed the needfor modernising Ind ia ' s t ruck f lee t androad infras tructure , along withiden t i f ica t ion of areas and level ofmodernisation required and the policyframework desi red to achieve them.

The following major rcommendationsconcerning to the par t of vehic le f leetmodernisat ion study emerged from theresu l t s of th i s study are:i ) In order to meet the future

t ranspor t demand an d to reduce thet o t a l t ransportat ion cost , it i snecessary to improve the exis t ingroad network an d modernise thevehicle f l ee t .

i i ) Old and obsolete vehic les should bephased out and economically moreef f i c i en t multi-axle vehiclesshould be introduced.i i i ) There i s an urgentundertake var ious measurespromotion of technologica lin vehicle design tosafe ty , fuel eff ic iencyreduce road damage.

need tosuch asprogressimprovean d to

iv) Two- axle r ig id vehicles arepaying the i r fa i r shareproport ion to the roadnot

incost

in f l ic ted by them by way of road useand road damage.EVALUATIOH OF EFFECT OF VEHICLE AXLELOADS OH PAVEMEHT PERFORMAHCE

The pavement performance data basegenerated under the Pavement PerformanceStudy-Study on Exist ing Pavementsect ions (PPS-EPS), wa s ut i l i s ed forevalua t ing the ef fec ts of increased axleloads (overloading) on pavementperformance. The road sect ions (s i tes)selected for the present analyses wereselected in a manner t ha t they representthe most prevai l ing prac t ices ofconstruct ing an d maintaining theNational highways and would largelycover the varying operat ing conditionsof t r a f f ic volume and loading,pavement's s t ruc tura l s t rength ,di f fe ren t materials an d specif icat ionsin const i tuent layers and surface typese tc . The t e s t sect ions included in theanalyses can be regarded as windowrepresentat ion of Nat ional highwaysystem of the country. The analyse.s i scar r ied out in the fol lowing tw o .. steps.DETERMINATIONOVERLOADING

OF DAMAGE DUE TO

As stated ear l i e r , th e majori ty ofcommercial vehicles ( t rucks) in Indiaare two-axle r igid body. From the la rgeaxle load data avai lable for thesevehicles , it i s observed t ha ti ) The Gross Vehicle Weight (GVW) i s

normally about 16 tonnesi i ) The unladen weight (empty weight)of such t rucks i s normally about5.5 tonnes

Based on the ear l i e r r esu l t s avai lablefrom di f fe ren t s tudies and as perthe general loading norms on the twoaxles of a t ruck in the r a t io of 1:2(front axle versus rear axle ) , it isassumed tha t front axle weight should bel imited to 5.5 tonnes and the rear axleweight to 10.5 tonnes which would leadto the presumption tha t a vehic le havingGVW in excess of 16 tonnes (5.5 tonnes

269


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ROAD TRANSPORT TECHNOLOGY-4front axle and 10.5 tonnes rear axle) i sa vehic le carrying loads beyond legalaxle l imi t s and i s referred to asoverloaded vehic le for the purpose ofpresent analyses . The axle load data fo rdif fe ren t s i t e s i s analysed to obtainVDF a t the ac tua l loads and the same isdenoted by VDFA. The same data i sreanalysed to obtain the res tr ic ted VDFby l imi t ing the individual axle loads tothe ones discussed above. The VDF soderived i s expressed as VDFR. ESAL sbased on VDFA i s represented by ESALAand are the ones for actual t r a f f icrepe t i t ions . ESALS based on VDFRrepresent the load repe t i t ions i f s t r i c tenforcement of axle l imi t s is employedand no overloading i s allowed onpavements. The r e su l t s obtained from theanalyses car r ied out indicate tha t thereis a subs tan t ia l reduction in vehicledamage fac tor when the load in excess oflegal axle l imi ts i s removed.EVALUATION OF DAMAGING (OVERLOADING)EFFECTS ON PAVEMENT PERFORMANCE

Having es tab l i shed the reduction invehicle damaging ef fec ts due toexclusion 'of excess loads carr ied by thevehicles in comparis ion to legal axleload l imi t s , the subsequent analyses wasconducted to quant i fy/evalua te theef fec t of vehicle overloading onpavement performance. Variousdeter ioration relat ionships developedfor pavements with surfac ing of PC, SDBCan d BC towards predict ion of in i t ia t ionan d progression of various dis t ressmodes were appl ied to the different roadsect ions included in the presentanalyses. In view of an y threshold Iintervent ion l eve ls not being avai lablefor Indian condi t ions, it i s consideredtha t resurfacing should be needed in thefollowing tw o cases .a)

b)

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Th ehas cracking level of areached 25 % of thesur face area an d I orThe roughness level has5000, 4500 and 4000 mm/kmsurface type i s PC, SDBCrespec t ive ly

pavementt o t a l

reachedwhen

and BC

The above levels have been f ixed basedon pas t experience and exis t ingmaintenance standards for providingresurfacing / renewal coa t etc . Theselevels are regarded as c r i t i c a l /terminal level and it i s a t t h i s timetha t the resurfacing i s assumed to beapplied on a pavement if otherwise theroad i s s t ruc tura l ly adequate. Extensivemeasurements of roughness conducted inIndia on newly constructedrehabi l i ta ted pavements have indicatedtha t i n i t i a l roughness fo r pavementswith surfacing of PC, SDBC an d BC arenormally of the order of 2500, 2000 and1500 mm/Km respect ively when the const ruction qual i ty i s good. These values ofi n i t i a l roughness have been used fo r thepresent analyses. With these assumptionsan d considerations in mind, the t ime toreach the above defined c r i t i c a l levels(based on cracking and roughnessseparately) i s determined throughappl icat ion of various pavementdeter iorat ion models, fo r the twosi tuat ions viz . i ) when the actualloads are plying an d i i ) when theoverloading has been excluded and thevehic les are carrying l ega l axleloads. The time so obta ined i s theservice life for t ha t par t icu la rsurfacing because another resurfacingwould be needed af t e r the pavement hasreached pre-defined intervent ion levelsas above. Such t reatments are appliedprimari ly for the purpose of providingbet te r r ideabi l i ty and retarding fur therdeter iorat ion of pavement s t ruc ture .Based on the comparision of servicel ives obtained for ac tua l loads andres t r ic ted loads, the percent increasein service l i fe i s calculated fo r thetwo cr i t e r i as separately viz . crackingan d roughness l eve ls . The resu l tsobtained from the analyses discussedabove are presented in Table-2. I t i sseen tha t there is an increase of about30 - 40 % in service l i f e fo r surfacingsof PC an d BC an d upto about 20 % forSDBC when cracking l eve l i s consideredas the cr i t i ca l c r i t e r i a . The increasein service l i f e i s about 10 - 20 % forPC, upto about 20% for BC and upto about10 % fo r SOBC when roughness l eve l i sregarded as c r i t i c a l c r i t e r i a . The low


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Table. 2 Increase in service l i f e du e to removal of excess axle loads (overloading)on d i f f e r en t pavements subjected to varying degree of t r a f f ic volume an dloadings

Si te Surf- Modified Comme- VDF a t VDF a t ESALs ESALs Percent increaseNo. ace Struc t - r c i a l actua l r e s t r i (x 106 ) (x 106 ) in service l i f eType ura l vehic les loads -c ted per year per year due to removal ofNumber per day (VDFA) loads a t actua l a t r e s t r excess axle loads(MSN) (CVPD) (VDFR) loads - ic ted ------------------(ESALA) loads Cracking Roughness(ESALR) level l eve l--------------------------------------------------------------------------------------1 PC 3.72 920 6.18 2.752 PC 3.06 587 5.61 3.003 PC 3.81 4449 4.49 3.004 PC 4.35 889 5.53 2.755 PC 3.67 2484 5.53 3.006 BC 4.08 3623 6.01 3.007 BC 4.20 4886 6.18 3.008 BC 4.14 6101 4.92 3.009 BC 5.02 5832 6.29 3.0010 SDBC 4.48 1552 4.17 2.7511 SDBC . 5.01 2316 5.13 3.00

increase in service l i f e a t roughnessl eve l as compared to high increase inservice l i f e a t cracking l eve l can besaid to be probably due to differencesin pavement 's surface tex ture ,o the r (associa ted) surface defectsinf luencing roughness, va r iab i l i ty inconstruc t ion qual i ty and seasonal/cl imat ic changes etc .The increase inservice l i f e for d i f f e r en t pavementswould depend on the t r a f f i c repet i t ions(ESALs / year) , s t ruc tu ra l s t rength ofthe pavement (MSN) , mater ia ls /speci f i ca t ions in pavement st ructure,construc t ion and maintenance standardsand environmental condi t ions e tc . In theu l t ima te analyses , it can r igh t ly beconcluded tha t there would beconsiderable improvement in service l i f ewhen s t r i c t compliance of legal axlel imi t s are enforced.CONCLUDING REMARKS

Based on the analysesthe r e su l t s obta ined, conductedit can andbe

2.08 0.92 40 101.20 0.64 30 97.29 4.87 33 101. 79 0.89 31 65.01 2.72 38 207.94 3.97 40 21

11.02 5.35 44 2110.95 6.68 29 2013.38 6.39 42 16

2.36 1.56 13 64.23 2.54 18 10

concluded tha t there i s a s igni f icantimprovement in service l i f e when s t r i c tenforcement of legal axle l imi t s i sdone. Th e var ious benef i t s ar i s ing froms t r i c t enforcement of legal l imi t s a reas given below:

i ) Reduction in maintenance andr ehab i l i t a t i on cos t of roads

i i ) More funds would be avai lable forupkeep of pavements

i i i ) The road pavementsmaintained a t desired/se r v ic iab i l i ty levels

Based on the l imited workr e su l t s avai l ab le , it wil l beand premature to quant i fy andthe saving in maintenanceaccount of load r e s t r i c t ions .

can beminimum

done andtoo ear ly

es t imatecos tI t

however, expected an d believed tha ton

i s ,thesavings in maintenance cos t on rough

basis should a t l e a s t be of the order of20-25% of what i s present ly spent i f

271


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ROAD TRANSPORT TECHNOLOGY--4l ega l axle l imi t s are enforced and noveh ic le i s allowed to car ry loads beyondth e permiss ib le one. Savings o f suchmagnitude would cer ta in ly help roadorganisa t ions in providing be t te r roadsto its use rs .

The work presented here in t h i s paperi s prel iminary an d i s based on asmall sampling of t r a f f i c volume an dloading charac ter i s t i cs on few roadpavements . I t i s , therefore , recommendedt h a t l a rge sampling of da ta would needto be included before ar r iv ing a tde f in i t e , r ea l i s t i c , r e l i ab le andaccura te r esu l t s . The work repor ted i splanned to be continued/ extended byinvolving varying operat ing s i tua t ionso f pavements and t r a f f ic loading,d i f f e r e n t category of roads and o the rinf luencing parameters af fec t ingpavement performance.ACKNOWLEDGEMENT

The authors are thankful to theDirec to r , Central Road ResearchIn s t i t u t e , New Delhi , fo r h is constantencouragement and kind permiss ion topubl ish t h i s paper . Thanks a re a l so dueto Mr. Suni l Ja in , Sc ien t i s t , for h isass i s t ance in the f i na l s tage of theprepara t ion of th i s paper .REFERENCES

1 .

2.

3.

4.

J . R o l t , " Optimum axle loads ofcommercial vehic les in developingcountr ies" , Laboratory Report 1002,Transporta t ion and Road ResearchLaboratory, Crowthorne, England,1981 .IRC: 37-1984, .. Guidel ines for thedes ign of f lexible pavements" ,Indian Roads Congress, New Delhi ,1984.R.P. Sikka, "Pavement managementsystem" , Journal of Indian RoadsCongress , Vol. 51-3, New Delhi ,Nov. 1990.D.P.Gupta, M.K.Bhalla, S.S.Chakrabor ty ,"Strategy fo r road and

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road t r anspor t : EmergingJournal of Indian RoadsVol. 51-3, Paper No. 402,, Nov. 1990.

i ssues" ,Congress,New Delh i

R.P. Sikka, "Growing cr i s i s o fmanaging the t runk route system inIndia" ,Journal of Indian RoadsCongress, Vol.53-2, Paper No. 414,New Delhi , Sept . 1992.V.K. Sood, B.M.Sitaramanjaneyulu,Performance Studytowards development

Sharma, K.Pavement

An inpu to f pavement

management system in India" ,Conference Proceedings , SeventhRoad Engineer ing Associat ion o fAsia and Aust ra l i a , Singapor , June1992.R.S. Milland, .. Road building inthe t r op i c s - Sta te of the A rtreview" , Transpor ta t ion ResearchLaboratory, Crowthorne, England,1993."Pavement Performance Study- Studyon Exis t ing Pavement Sect ions" ,Final Repor t , Vol. 1 Studyob jec t ives , scope an d methodologyfo r data co l l ec t i on , Cent ra l RoadResearch In s t i t u t e , New Delhi ,Sept . 1993.

9. D.P.Gupta," Road in f ras t ruc ture - Apol icy framework" , Journal ofIndian Roads Congress, Vol. 54-3,Paper No. 424, New Delhi , Nov.1993.

10. V.K. Sood, B.M. Sharma, P.K.Kanchan, K. Sitaramanjaneyulu,Pavement de t e r i o r a t i on modell ing inIndia" , Proceedings , ThirdIn terna t ional Conference onManaging Pavements, Texas, U.S.A.,May 1994.

11. "Pavement Performance Study - Studyon Exis t ing pavement Sect ions" ,Final Repor t , Vol. 2 Datamanagement and model development,Central Road Research In s t i t u t e ,New Delhi , Sept . 1994.

effect of vehicle axle loads on pavement performance - sharma

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