Functions of various components of road pavements

 Functions of various components of road pavements 

The following are the components parts of road pavements :

1. Sub-grade or formation

2. Sub-base course

3. Base course

4. Surface course or wearing course

1. SUB-GRADE 

    The finished and compacted surface of earthwork on which a road pavement rests is called sub-grade or formation.

    The soil sub-grade of a pavement may be provided either on embankment or in cutting or directly over the nature ground level depending upon the topography of the area concerned and the final formation level of the road. 

Functions of sub-grade

1. To provide an adequate and uniform support to the road pavement.

2. To bear ultimately the  entire load of pavement including the  load of traffic transmitted through the pavement and to distribute this load on a larger area below it.

3. To behave elastically and adjust itself to the excessive stress condition.

2. SUB-BASE COURSE

A layer of granular material provided in between the sub-grade and base course in a road pavement is called sub-base.

    It is provided as an additional layer when sub-grade is of poor quality. It consists of a layer of comparatively cheaper material like natural gravel, quarry waste, cinders, sand, stabilized soil.

    For cement concrete roads, sub-base layer is not provided. It is usually needed on flexible pavements built on fine grained soils.

Functions of sub-base course

(1) To improve the bearing capacity of the sub-grade.

(2) To improve the drainage conditions and to check capillary rise of sub-soil water.

(3) To eliminate frost heave in frost affected area by preventing capillary rise of water.

(4) To act as filler material and increase the thickness of pavement at a low cost.

3. BASE COURSE

    A layer of boulders or bricks provided below the wearing course is termed as base course.

    This layer is the major component of road pavement and is provided over the sub-base course or immediately above the good quality sub-grade layer in the absence of  sub-base course.

    This course is consider as the most important and a major component of road pavement structure. This course is to bear the impact of traffic transferred through the wearing course. In case of rocky sub-grade, this layer is not provided.

Functions of base course 

   (1) It acts as foundation for road pavement and transfers the load to the sub-base course.

   (2) It withstands the higher shearing stress imposed upon it due to impact of traffic on the wearing course.

4. WEARING COURSE

    The topmost layer of the road pavement which is directly in contact with the traffic is called wearing course.

    It may consist of one or more number of layers in case of flexible pavement. A good wearing course should be impervious from weather resisting.

Functions of  wearing course 

(1) To distribute the traffic load safely to the base course.

(2) To provided a smooth surface for the safe and comfortable movement of traffic.

(3) To provided a water tight layer against infiltration.

(4) To resist abrasion or destructive pressure exterted by wheel loads.

(5) To prevent dust nuisance.