lecture 10 pavement design

8/9/2019 Lecture 10 Pavement Design

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CE 303 – Transportation EngineeringHighway Engineering

Lecture 10 - Pavement Design

10.1 ntro!uction to highway engineering

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 roadshave 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,

!" #ongestion on urban roads

$" %ccidents

&" 'a(or roads running through built up areas #ities and townships"

)" *arrow roads

+" Structural inadeuacy of pavements

-" oor geometrical design

/" Small structures such as bridges

0" 1unding for maintenance and rehabilitation

2" 1unding for expansion and new facilities!3" 4nvironmental pollution

These issues provide the following challenges to the highway engineer.

!" #hallenges of design, construction, rehabilitation, reconstruction and expansion

i." 5esign and reconstruct using modern technologies

ii." 6edesign older facilities to meet today7s demands.

iii." Secure budget provisions.

iv." %dopt cost effective and environmentally sound solutions.

$" #hallenges of safety and environment

i." Identify necessary safety reuirements of the road system especially, to protect vulnerable

road users.

ii." Implement regulations controlling noise, air and water pollution.

10." Pavement Design

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 andfull 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

). Sub8basea" %ssists load spreading

b" %ssists subsoil drainage

c" %cts as temporary road for construction traffic

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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


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(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 mmcoated 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,

. ( )+.)

0!-3

= AxleLoad Ef Factor Equvalent !3.!"

10.".3.1 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*" !!.+ !/.- $$.$ $-.& &3.& &&.+

enetration mm" $.3 ).3 -.3 0.3 !3.3 !$.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! 103 2g/ %$e Loa! 103 2g/ Euiva$ent #actor E)/

!.+ &.3 3.3!

$.3 ).3 3.3)

$.+ +.3 3.!!

&.3 -.3 3.$+

&.+ /.3 3.+3

).3 0.3 3.2!).+ 2.3 !.++

+.3 !3.3 $.+3

+.+ !!.3 &.0&

-.3 !$.3 +.-/

-.+ !&.3 0.!&

/.3 !).3 !!.&3

/.+ !+.3 !+.+3

0.3 !-.3 $3./3

0.+ !/.3 $/.$3

2.3 !0.3 &+.$3

2.+ !2.3 )).23

!3.3 $3.3 +-.+3

Example:

:ase Eear 4uivalent Standard %xles esa"

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%$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.

( )r

r esaesa

n

basecum

!! −+×= !3.&"

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

Tra))ic C$ass 104 esa 'ange (u&gra!e (trength C$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

%$e $oa! o) vehic$e c$ass +ase year esa6rowth #actor

5/

Design $i)e

n years/

Cumu$ative esa

esacum/

&.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&)&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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#hart ! 6'79L' ':D+(E ; (9'#CE D'E((76

*otes ! Fp to !33mm of sub8base may be substituted with selected fill

provided the sub8base is not reduced to les than the roadbase thickness

or $33mm whichever is the greater. The substitution ratio of sub8base

to selected fill is $+mm8&$mm.

$ % cement or lime stabili


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#hart $ C:


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#hart & 6'79L' ':D+(E ; (E


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#hart ) C:


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#hart + 6'79L' ':D+(E ; (T'9CT9'L (9'#CE

*otes ! Fp to !33mm of sub8base may be substituted with selected fill provided the

sub8base is not reduced to les than the roadbase thickness or $33mm whicheveris the greater. The substitution ratio of sub8base to selected fill is $+mm8&$mm.

$ % cement or lime stabili


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#hart - C:


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#hart / +T9


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#hart 0 CE

lecture 10 pavement design

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