Surface course:
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.
 It must be water proof to protect the entire base and sub-grade from the weakening effect of water.
 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.
 

Binder course:
This layer provides the bulk of the asphalt concrete structure.
Its 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. 
   

Base 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.
It may be composed of crushed stone, crushed slag, and other untreated or stabilized materials.
This layer of pavement should withstand high level of deformation as it plays major role in load distribution. 

 

Sub-base 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 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. 

 

Sub-grade course: 
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.