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Typical layers of a conventional flexible pavement includes seal coat, surface
course, tack coat, binder course, prime coat, base course, sub-base course,
compacted sub-grade, and natural sub-grade (Figure 1).
Seal coat is a thin surface treatment used to water-proof the surface and to
provide skid resistance.
Tack coat is a very light application of asphalt, usually asphalt emulsion
diluted with water.
It provides proper bonding between two layer of binder course and must be thin,
uniformly cover the entire surface, and set very fast.
Prime coat is an application of low viscous cutback bitumen to an absorbent
surface like granular bases on which binder layer is placed.
It provides bonding between two layers.
Unlike tack coat, prime coat penetrates into the layer below, plugs the voids,
and forms a water tight surface.
Figure 1:
Typical cross section of a flexible pavement
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Surface course is the layer directly in contact with traffic loads and
generally contains superior quality materials.
They are usually constructed with dense graded asphalt concrete(AC).
The functions and requirements of this layer are:
- It 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,
- It must be tough to resist the distortion under traffic and provide a
smooth and skid- resistant riding surface,
- It must be water proof to protect the entire base and sub-grade from the
weakening effect of water.
This layer provides the bulk of the asphalt concrete structure.
It's chief purpose is to distribute load to the base course
The binder course generally consists of aggregates having less asphalt
and doesn't 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.
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
It may be composed of crushed stone, crushed slag, and other untreated or
stabilized materials.
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 the intrusion of fines from the sub-grade in the pavement structure
If 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. For example, a pavement
constructed over a high quality, stiff sub-grade may not need the additional
features offered by a sub-base course.
In such situations, sub-base course may not be provided.
The top soil or sub-grade is a layer of natural soil prepared to receive
the stresses from the layers above.
It is essential that at no time soil sub-grade is overstressed.
It should be compacted to the desirable density, near the optimum
moisture content.
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