CE 303 – Transportation Transportation Engineering Highway Engineering Lecture 10 - Pavement Design 10.1 10.1
ntr ntro! o!uc ucti tion on to to high highway way eng engin inee eeri ring ng
Transportation planning and traffic planning are the initial stages of transportation engineering pertaining to road transport. Having planned highways, the next stage is the construction of the highways. The roads have to be constructed in different ground conditions and in different environments. The conditions and environments pose complex issues in highway construction. In Sri Lankan context, these issues are, !" !" #ong #onges esti tion on on urba urban n roa roads ds $" %ccidents &" 'a(or 'a(or roads roads runni running ng throu through gh built built up areas areas #it #ities ies and and townsh townships ips"" )" *arrow roads +" +" Stru Structu ctural ral inad inade eua uacy cy of of pave paveme ment ntss -" -" oor oor geom geomet etri rica call desi design gn /" /" Small Small stru struct ctur ures es such such as brid bridge gess 0" 1undin 1unding g for mainte maintenan nance ce and and rehabi rehabilita litatio tion n 2" 2" 1und 1undin ing g for for expa expans nsio ion n and and new fac facil ilit ities ies !3" !3" 4nviro 4nvironme nmenta ntall pollut pollution ion These issues provide the following challenges to the highway engineer. !" #hallenges #hallenges of design, design, constructio construction, n, rehabil rehabilitation itation,, reconstru reconstruction ction and expans expansion ion i." i." 5esig 5esign n and and reco recons nstr truc uctt usin using g mod modern ern tech techno nolo logi gies es ii." ii." 6ede 6edesig sign n olde olderr facil faciliti ities es to meet meet toda today7 y7ss dema demand nds. s. iii iii.." Secu Secure re budge udgett pro provi visi sio ons. iv. iv." %dopt %dopt cost cost effe effectiv ctivee and and envi environ ronmen mentall tally y soun sound d solu solutio tions. ns. $" $" #hal #halle len nges of saf safet ety y an and env envir iro onmen nmentt i." i." Identi Identify fy necessa necessary ry safety safety reuir reuireme ements nts of the the road system system espec especial ially ly,, to protect protect vulne vulnerab rable le road users. ii." ii." Implem Implement ent regu regulat lation ionss contr controll olling ing noise, noise, air and water water poll polluti ution. on. 10." 10." Pave Paveme ment nt Desi Design gn
The main purpose of a pavement is to provide a means of reducing the stress due to the wheel load to a value bearable to ground under the pavement. 1ig. !3.! shows how the high stress that exists at the point of wheel contact is reduced down the pavement structure until the stress is brought down to a level acceptable to the less competent naturally existing ground called the subgrade.
#ig. 10.1 5istribution of wheel load to the ground
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The pavement may be a single layer of one material or multiple layers of different material. There are three types of pavements, which are, !. 1lexible pavements $. 6igid pavements &. #omposite pavements #$e%i&$e pavements are constructed using granular material and bitumen. They can be subdivided into two types, conventional flexible pavements which consist of two or more layers of different material and full depth flexible pavements which have only one layer. 'igi! pavements are constructed of ortland cement concrete ##" Composite pavements have a base layer of ## and a surface layer of hot8mix asphalt. They have strength of rigid pavements and smooth surface of flexible pavements.
There are two factors which lead to the development of layered flexible pavement construction. They are i." the stresses from vehicles travelling on the road are highest near the surface ii." a smooth riding surface is necessary to reduce fatigue due to varying stresses on surface.
#ig. 10." % typical section through a flexible pavement
1ig. !3.$ shows a typical cross section of a flexible pavement. The functions of the different layers of flexible pavement are as follows, !. 9earing course a" 9ithstands direct traffic loading. b" rovides smooth riding c" rovides skid resistant surface d" 9aterproofs the pavement $. :asecourse a" Supports wearing course b" %ssists protecting layers below &. 6oadbase a" 'ain load spreading layer of the pavement structure ). Sub8base a" %ssists load spreading b" %ssists subsoil drainage c" %cts as temporary road for construction traffic $
The design of a flexible pavement is based on, i." The strength of the subgrade. #alifornia :earing 6atio #:6" is one measure of subgrade strength. ii." The number of wheel load applications on the pavement during the design life. iii." %n empirical relationship, layer thicknesses have with #:6 value of subgrade and number of wheel load applications. iv." Locally available materials for construction. 10.".1 (e$ection an! properties o) materia$s use! in pavement $ayers
To design the pavement layers it is necessary to select the materials for the pavement construction. The different layers can be constructed with the materials described below, (u&-&ase !. ;ranular sub8base, Type ! $. ;raded ;ranular sub8base, Type $. #rushed rock, slag or other hard material." Smaller si
!. 9et mix macadam #rushed rock graded and mixed with $8-= water. Laid in $33 mm layers and compacted or rolled. $. 5ry bound macadam &/.+ mm to +3.3 mm single si
&.&+ mm &0=" mixed with bitumen !3 pen to $33 pen, +3 ? +0 o# ect". ;ood load spreading properties ). 6olled asphalt 9ell graded crushed rock &+= fine aggregate and -+= coarse aggregate" plant mixed with +3 ? /3 = pen grade bitumen. +.
Lean concrete
-. #ement bound roadbase /. Soil cement and cement bound granular road base. 'ixtures of soil or granular material and cement, laid full depth in one layer and rolled. &
(ur)acing has either the wearing course only or wearing course with a base course.
!. 9earing course a" :ituminous surface dressing and a layer of chippings >!3 mm. 6olled and excess chippings removed. b" 5ouble bituminous surface treatment. Tack coat, aggregate layer, rolled. :itumen layer, aggregate later rolled followed by bituminous surface dressing. c" Hot rolled asphalt. The Strongest and durable. 'ade of high fines. Laid )3 mm thick with $3 mm coated chippings rolled into the surface for better skid resistance. $. :asecourse a" @pen textured macadam. #oarse graded, no fines >&.&+ mm. Thickness -3 ? 03 mm for )3mm. Thickness &+ ? +3 mm for $3mm. b" 5ense basecourse 9ell graded crushed rock &+= fine aggregate and -+= coarse aggregate", Thickness -3 ? 03 mm for )3mm. Thickness +3 ? -3 mm for $0 mm. Thickness &+ ? +3 mm for $3mm. c" 6olled asphalt basecourse. +3 ? /+ mm layer of rolled asphalt 10."." C + ' Test
#:6 test is an indirect test for the determination of the strength of a soil. The test is carried out by sub(ecting a sample of the soil held in a mould to the load of a standard plunger. The plunger penetrates into the soil. 1ig.!3.& a" and b" show schematic arrangement of test euipment for a #:6 test. The test compares the loads on the plunger to penetrate $.+ mm into the soil sample and a standard sample of crushed rock. The surcharge load applied using a steel disc represents the loading condition above the subgrade after laying of pavement. #:6 value is an undefined index of strength which depends on the soil condition at the time of testing. It is given by the ratio expressed as percentage of load for $.+mm of penetration in soil sample to load for same penetration in standard crushed rock sample. 1ig. !3.) shows details of mould, plunger and test procedure. Load is applied at the penetration rate of !.$/ mm per min. 1ig. !3.+ shows load vs penetration graphs. %n upward convex graph is expected as shown for sample %. Sometimes the graph behaves as shown for sample :. In this case a correction is reuired to the graph and the graph is shifted by the amount x shown. x is the point at which the tangent to the curve of sample : meets the x axis. The tangent is drawn to get an approximate correction to the curve.
a" enetration in sample
b" enetration in standard crushed rock
#ig. 10.3 Schematic arrangement of test euipment for #:6 test.
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10.".3 ,hee$ $oa! app$ications
The other reuired data for pavement design is the number of times wheel loads are applied to the pavement. This is based on the design life, the anticipated number of different types of vehicles using the pavement during the design life and the conversion euivalent" factors for each vehicle type which converts an axle loading to a standard axle loading. Table !3.! gives these euivalent factors which are based on the empirical relationship, ).+
AxleLoad . Equvalent Factor ( Ef ) = 0!-3 10.".3.1
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Estimation o) the amount o) tra))ic an! the cumu$ative num&er o) euiva$ent stan!ar! a%$es esa/
:ase year traffic flow is the %nnual %verage 5aily Traffic %%5T" of the base year. The no of vehicles is converted into euivalent standard axles esa" using the euivalent factors given in Table !3.!. :ase year esa esabase" A %%5T x &-+ x 4f !3.$"
!. 6ate of penetration is !.$/mmBmin. $. 6ecord load at penetration intervals of 3.$+ mm up to /.+ mm. ). 5etermine moisture content &. lot Load Cs enetration.
*oteD !. 'aximum particle si
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Load on plunger Cs enetration for Standard crushed stone Load on plunger k*" !!.+ !/.$$.$ $-.& enetration mm" $.3 ).3 -.3 0.3
&3.& !3.3
&&.+ !$.3
#ig. 10. The Load on plunger Cs enetration graphs
Ta&$e 10.1 4uivalent factors for different axle loads (ing$e an! !ua$ ,hee$ Loa! 10 3 2g/ %$e Loa! 103 2g/ !.+ &.3 $.3 ).3 $.+ +.3 &.3 -.3 &.+ /.3 ).3 0.3 ).+ 2.3 +.3 !3.3 +.+ !!.3 -.3 !$.3 -.+ !&.3 /.3 !).3 /.+ !+.3 0.3 !-.3 0.+ !/.3 2.3 !0.3 2.+ !2.3 !3.3 $3.3 Example: :ase Eear 4uivalent Standard %xles esa" -
Euiva$ent #actor E)/ 3.3! 3.3) 3.!! 3.$+ 3.+3 3.2! !.++ $.+3 &.0& +.-/ 0.!& !!.&3 !+.+3 $3./3 $/.$3 &+.$3 )).23 +-.+3
%$e $oa! o) vehic$e c$ass DT o) vehic$e c$ass Euiva$ent #actorE)/ +ase year esa &.3 )+3 3.3! !-)& ).3 &03 3.3) ++)0 +.3 $+3 3.!! !33&0 -.3 !33 3.$+ 2!$+ /.3 0+ 3.+3 !++!& 0.3 /+ 3.2! $)2!! 2.3 )3 !.++ $$-&3 !3.3 &+ $.+3 &!2&0 !!.3 $+ &.0& &)2)2 !$.3 !+ +.-/ &!3)& Fse growth factorr" for each vehicle class and the assigned design life n years" to calculate cumulative esa. esacum
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Thus, the wheel load applications during design life are calculated as follows, • %ssess base year traffic flow by classes of commercial vehicles • 5etermine the axle loads and growth rate of each vehicle class • %pply the euivalent axle load factors and growth rates to base year traffic flow to determine the pavement damaging effect Geuivalent standard axles, esa" during the design life. The esacum can be directly used to find the thicknesses of pavement layers if the design charts of 6oad *ote $2 are employed. In order to use the design charts of 6oad *ote &! Ta&$e 10." Traffic and subgrade strength classes. Tra))ic C$asses (u&gra!e (trength C$asses 4 Tra))ic C$ass 10 esa 'ange (u&gra!e (trength C$ass 'ange o) C+' 5 T! >3.& S! $ T$ 3.& ? 3./ S$ &8) T& 3./ ? !.+ S& +8/ T) !.+ ? &.3 S) 08!) T+ &.3 ? -.3 S+ !+8$2 T-.3 ? !3.3 S&3 T/ !3.3 ? !/.3 T0 !/.3 ? &3.3 It is necessary to convert the esacum into Traffic #lasses and #:6 values into subgrade strength classes as given in Table !3.$. Example. #alculation of cumulative esa 6rowth #actor Design $i)e Cumu$ative esa %$e $oa! o) vehic$e c$ass +ase year esa 5/ n years/ esa / &.3 !-)& ) !3 !2/$).3 ++)0 & !3 -&-3$ +.3 !33&0 & !3 !!+3/) -.3 2!$+ ) !3 !32++/.3 !++!& + !3 !2+!$! 0.3 $)2!! + !3 &!&&$0 2.3 $$-&3 & !3 $+2)$0 !3.3 &!2&0 ) !3 &0&)+! !!.3 &)2)2 ) !3 )!2-3! !$.3 &!3)& + !3 &23)+Total $$-2&)& cum
The traffic class is T+ 10.". 'oa! 7ote 31
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@verseas 6oad *ote &!8 % guide to the structural design of bitumen8surfaced roads in tropical and sub tropical countriesJ published by Transport 6esearch Laboratory T6L", Fnited Kingdom gives a simple but adeuate design procedure for most Sri Lankan roads. The charts give directly the pavement layer thicknesses for different combinations of Traffic and Subgrade strength classes. lease refer to #harts ! to 0. 1ig. !3.- gives a flow chart leading to the design of flexible using steps discussed in this note. 1ig. !3./ gives the legend of description of pavement materials necessary to refer #harts ! to 0.
#ig. 10.4 1low chart for the construction of a highway pavement
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#ig. 10.8 Legend of definitions of pavement materials for use with #harts ! to 0
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% granular sub8base may also be used.
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