Pavement Design Seminar

8/12/2019 Pavement Design Seminar

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PAVEMENT DESIGN

1.1Introduction:-

A highway pavement is a structure consisting of superimposed

layers of processed materials above the natural soil sub-grade, whose

primary function is to distribute the applied vehicle loads to the sub-grade.

The pavement structure should be able to provide a surface of acceptable

rid in g quality, adequate skid resistance, f a vo ra b l e light reflecting

characteristics, and low noise pollution. The ultimate aim is to ensure

that the transmitted stresses due to wheel load are sufficiently reduced,

so that they will not exceed bearing capacity of the sub- grade. Two

types of pavements are generally recognized as serving this purpose, namely

flexible pavements and rigid pavements. This chapter gives an overview of

pavement types, layers, and their functions, and pavement failures.

mproper design of pavements leads to early failure of pavements affecting

the riding quality.

1.2 Requirements of a pavement:-

An ideal pavementshould meet the following requirements!-

" #ufficient thickness to distribute the wheel load stresses to a safe value

o n the sub-grade soil,

" #tructurally strong to withstand all types of stresses imposed upon it,

" Adequate coefficientof friction to preventskidding of vehicles,

" #mooth surface to provide comfort to road users even at high speed,

" $roduce least noise from moving vehicles,

" mpervious surface, so that sub-grade soil is wellprotected, and

" %ong design life with low maintenance cost.

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1.3 Types of pavements:-

The pavements ca n be classified based on the structuralperformance i n t o two, flexible pavements and rigidpavements. n flexible

pavements, wheel loads are transferred by grain-to-grain contact of the

aggregate through the granular s t r uc tu re . The flexi ble pavement, havingless flexural strength , acts like a flexible sheet &e.g.bituminous road'.

Wheel LoadP

Granular

Structure

Figure 1:1 Load transfer in granular structure

(n the contrary, in rigid pavements, wheel loads are transferred

to sub-grade soil by flexural strength of the pavement and the pavement acts

like a rigid plate &e.g. cement concrete roads'. n addition to these,

composite pavements are also available. A thin layer of flexible pavementover rigid pavement is an ideal pavement with most desirable

characteristics . )owever, such pavements are rarely used in new

constructionbecause of high cost and complex analysis required.

1.3.1 Flei!le pavements:-

*lexible pavements wil l transmit wheel load stresses to the lowerlayers by grain-to-grain transfer thr ou gh thepoints of contact in the granularstructure &see *igure +!+'. The wheel load acting on the pavement will be

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distributed to a wider area, and the stress decreases with the depth. Taking

advantage of these stress distribution characteristic, flexible pavements

normally has many layers. )ence, the design of flexiblepavement uses theconcept of layered system. ased on this, flexible pavement may be

constructed in a number of layers andthe top layer has to be of best qualityto sustain maximum compressive stress, in addition t o wear and tear.

*lexible pavements are constructed using bituminous materials. These can

be either in the form of surface treatments &such as bituminous surface

treatments generally found on low volume roads' or , asphalt concrete

surface courses &generally used on high volume roads such as national

highways'. *lexible pavement layers reflect the deformation of the lowerlayers on to the surface layer &e.g., if there is any undulation in sub-grade then

it willbe transferred to the surface layer'. n the case of flexible pavement,

the design is based on overall performance of flexible pavement, and thestresses produced should be kept well below the allowable stresses of each

pavementlayer.

#eal coat

Track coat prime coat

#urface course &-/ mm'

inder course &/-

+// mm'

ase course &+//-

0// mm'

#ub base course &+//-

0// mm'

1ompacted sub grade&+/-0// mm'

Figure 1:2 typical cross section of a flei!le

pavement

1.3.1." Types of Flei!le #avements:-

The following types of construction have been used in flexible

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pavement!-

" 1onventional layered flexiblepavement,

" *ull-depth asphaltpavement, and

" 1ontained rock asphalt mat &12A3'.

1onventional flexible pavements are layered

systems with high quality expensive materials are placed in the

top where stresses are high, and low quality cheap materials are

placed in lower layers.

*ull-depth asphalt pavements are constructed byplacing bituminous layers directly on the soil sub- grade. This is

more suitable when there is high traffic and local materials are not

available.1ontained rock asphalt mats are constructed by

placing dense4open graded aggregate layers in between twoasphalt

layers. 3odified dense graded as phalt concrete i s placed above

the sub-grade wi l l significantly reduce the vertical compressivestrain on soil sub-grade and protect from surface water.

1.3.1.$ Typical layers of a flei!le

pavement:-

Typical layers of a conventional flexible pavementincludes seal coat, surface course, tack coat, binder course,prime

coat, base course, sub-base course, compacted sub-grade, and

natural sub-grade &*igure +.'.

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%eal &oat: - #eal coat is a thin surface treatment used to

water-proof the surface and to provide skid resistance.

Tac' &oat: -Tack coat is a very light application of asphalt,

usually a sphalt emulsion diluted with water. t provides properbonding between two layer of binder course and must be thin,

uniformly cover the entire surface, and set very fast. #rime &oat: - $rime coat is an application of low viscous

cutback bitumen to an absorbent surface like granularbases on

which binder layer is placed. t provides bonding between two

layers. 5nlike tack coat, prime coatpenetrates into the layer

below, plugs the voids, and forms a water tight surface.

%urface course: -#urface course is the layer directly in contact

with traffic loads and generally contains superior quality

materials. They are usually constructedwith dense graded asphaltconcrete &A1'. The functions and requirements of this layer are!

+. t provides characteristics such as friction, smoothness,

drainage, etc. Also it will prevent the entrance of excessive

quantities of surface water into the underlying base, sub-base

and sub-grade.

. t must be tough to resist the distortion under traffic and

provide a smooth and skid- resistant riding surface.

0. t must be water proof to protect the entire base and sub-

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grade from the weakening effect of water.

$inder course: - This layer provides the bulk of the asphalt

concrete structure. ts chief purpose is to distribute load to the

base course The binder course generally consists of aggregateshaving less asphalt and doesn6t require quality as high as the

surface course, so replacing a part of the surface course by the

binder course results in more economical design.

$ase course: - The base course is the layer of material

immediately beneath the surface of binder course and it

provides additional load distribution and contributes to the

sub-surface drainage t may be composed of crushed stone,

crushed slag, and other untreated or stabilized materials.

%u!-$ase course: - The sub-base course is the layer of

material beneath the base course and the primary functions are to

provide structural support, improve drainage, and reduce theintrusion of fines from the sub-grade in thepavement structuref the base course is open graded, then the sub-base course with

more fines can serve as a filler between sub-grade and the base

course A sub-base course is not always needed or used. *or

example, apavementconstructed over a high quality, stiff sub-

grade ma y not need the additional features offered by a sub-base course. n such situations, sub-base course may not be

provided.

%u!-grade: - The top soil or sub-grade is a layer of natural

soil prepared to receive the stresses from the layers above. t is

essential thatat no time soil sub-grade is overstressed. t should

be compacted to the desirable density, near the optimum

moisture content.

1.3.1.& Failure of flei!le pavements:-

The ma7or flexible pavement failures are fatigue

cracking, rutting, and thermal cracking. The fatigue crackingofflexiblepavement is due to horizontal tens i le strain at the bottom

of the asphaltic concrete. The failure criterion relates allowable

number of load repetitions to tensile strain and this relation can be

determined in the laboratory fatigue test on asphaltic concretespecimens. 2utting occurs only on flexible pavements as indicated

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by permanent deformation or rut depth along wheel load path.

Two design methods have been used to control rutting one to limitthe vertical compressive strain on the top of sub grade and other to

limit rutting to a tolerable amount &+ mm normally'. Thermal

cracking includes both low-temperature cracking and thermalfatigue cracking.

1.3.2 Rigid #avements:-

2igid pavements have sufficient flexural strength to

transmit the wheel load stresses to a wider area below. A typicalcross section of the rigid pavement is shown in *igure +!0.

1ompared to flexible pavement, r i g id pavements are placed either

directly on the preparedsub-grade or on a single layer of granularor stabilized material. #ince there is only one layer of material

between the concrete and the sub-grade, this layer can be called as

base or sub-base course.

$ortland rigid pavement+/-0//

mm

ase of sub base course +//-0//

mm

Figure 1:3 Typical &ross section of rigid pavement

8lastic plate

9iscous foundation contact pressure

Figure 1:( Typical &ross section of rigid pavement

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n rigid pavement, load is distributed by the slab action, and the

pavement behaves like an elasticplate resting on a viscous medium

&*igure +!:'. 2igid pavements are constructed by $ortland cementconcrete &$11' and should be analyzed by plate theory instead of

layer theory, assuming an elastic plate resting on viscous foundation.$late theory is a simplified version of layer theory that assumes

the concrete slab as a medium thick plate which is plane beforeloading and to remain plane after loading. ending of the slab due

to wheel load and temperature variation and the resulting tensile

and flexural stress.

1.3.2." Types of Rigid #avements:-

2igid pavements can be classified into four types!

" ;ointed plain concretepavement&;$1$',

" ;ointed reinforced concrete pavement &;21$',

" 1ontinuous reinforced concrete pavement &121$', and

" $re-stressed concrete pavement &$1$'.

+. )ointed #lain &oncrete #avement: - are plain cementconcrete pavements constructedwith closely spaced contraction

7oints.


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1.3.2.$ Failure criteria of rigid pavements:-

Traditionally fatigue cracking has been considered as the

ma7or or only criterion for rigid pavement design. The allowable

number of load repetitions to cause fatigue cracking depends on the

stress ratio between flexural tensile stress and concrete modulus of

rupture. (f late, pumping is identified as an important failure

criterion. $umping is the e7ection of soil slurry through the 7oints

and cracks of cement concrete pavement, caused during the

downward movement of slab under the heavy wheel loads. (ther

ma7or types(f distress in rigid pavements include faulting, spilling, and

deterioration

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1.3.3 %emi rigid pavement:-

1oncrete pavement with good wear4tear and chemical resistances

has great difficulties in maintenances and repairing, while asphalt concrete can

easily be repaired but is not durable if under heavy loading and chemical impact.

The semi-rigid pavement system has been developed by combining the

advantages of the both pavements to perform like concrete but to be easily

maintained like asphalt concrete. This system is formed by porous asphalt

concrete and polymer modified grouting material which is poured into the

porous asphalt concrete. t has gradually become popular for years

internationally as a surface layer of those pavements under serious conditions

such as in road 7unctions, airport aprons and heavy loading yards. A commonly

used semi-rigid pavement system in #ingapore for both airport parking aprons

and road 7unctions etc.

*"+ *$+

*"+: =routing of the mortar figure &'! #emi-rigid pavement surface

These semi rigid pavement materials have low resistance to impact and abrasion

and therefore are usually provided with flexible pavement surface course.


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1., Reference:-

)ighway engineering by .>.>)A??A @ 1.8.=. ;5#T('

http!44www.faadooengineers.com4threads4/B0-$avement-

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Pavement Design Seminar