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Components of Road Structure

By: Haseeb Jamal / On: Apr 15, 2017 / Definition
Components of Road Structure

A road structure consists of the following components;

1. Sub Grade:

Subgrade is the foundation of the road, thus its the lowest and most important component of road structure.


  • If natural surface is above the formation level then the surface is cut down to proposed sub grade surface
  • If natural surface is below the formation level then the sub grade will be above the ground level
  • It should be constructed at least 60 cm (2ft) high from highest flood level of the area

Function of Sub grade:

  • Bears all the load thus acts as a foundation of road

  • Transfer load through grain to grain contact


Material should be;

  • Strong enough
  • Easily accessible in the surrounding vicinity
  • Cheap


Consists of:

  1. Upper Base Course
  2. Sub or Lower Base Course


  • Constructed above the sub grade
  • Not needed if the sub grade is of very high strength
  • In case of flexible pavement upper and lower base courses are separated having different materials
  • In case of rigid pavement only upper base course is provided
  • Thickness varies from 7.5(3in) to 15cm (6in)

Functions of sub base:

  • Prevent rise of water or capillary action


  • Should be better than the material of Sub Grade
  • The Upper Base Course is made up of sand, gravel, and stone
  • The Lower Base Course is made up of cheaply available material i-e rock and stone fragments


Due to quality of material used in the road base it is divided into

  1. Upper Road Base
  2. Lower Road Base


  • Constructed above the Sub Base

Functions of Raod Base:

  • To avoid the distortion of wearing course due to its sufficient density
  • Supports the wearing course


  • In case of Upper Road Base the material is of high quality as the load intensity is high
  • In case of Lower Road Base the material is of high quality as the load intensity decreases


It is the upper most layer of road cross section. It can be provided in one or two layers:


Constructed usually in two layers

  • Binder Course
  • Wearing Course (It is the layer which is in direct contact with the tyres of the vehicle)

Functions of Surfacing of Road:

  • Prevent penetration of water in to the pavement
  • Binder Course binds the Wearing Course with the Road Base
  • Wearing Course provide a smooth riding
  • Saves the lower layers from abrasion and weathering effects of the moving vehicles


  • Made up of bituminous material
  • For Flexible Pavement asphalt concrete is used
  • For Rigid Pavements Reinforced Cement Concrete (RCC) is used

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