Road Pavement Layers – Components and Functions

Road pavement structure: Road is the most important structure that decides the economic growth of a country. The road structure to be constructed based on the traffic requirements, climatic conditions of the area, terrain, etc. The road structure has to withstand abrasion loads and climatic and terrain conditions without failure. This article is about the components of a flexible pavement structure.

Road pavement structure

The road pavement structure layers are as follows.

  • Subgrade or road bed
  • Sub base course
  • Base Course
  • Surface coarse or wearing course

Also Read : Bitumen for road – Grade & Properties of Bitumen

Also Watch : Flexible pavement vs Rigid Pavement

Road pavement – Subgrade

The subgrade is the compacted natural soil below the pavement layers and it is the finished or compacted surface on which the pavement rests. They are also known as the formation and serves as the foundation of pavement layers.
Depending on the nature of the terrain the sub-grade can be an embankment or a cutting or it will be in line with natural ground level. The load-bearing strength of the sub-grade structure is determined by the California bearing ratio test (CBR).
The material used for subgrade shall be locally available, strong, and cheap.


Function of subgrade

Serves as the foundation and acts as a uniform support to pavements.

Sub grades bear the  entire load of the payments along with the service load of traffic.

Also read : Road Margins -6 types of road Margins in highway

Sub base course

The sub base course comes between the base coarse and sub grade. The material used for this layer shall satisfy the specifications in terms of gradation, strength, and plastic characteristics.
This layer is necessary if the sub grade is of poor quality. In that case, an additional layer of burnt clinkers, gravel, slag, etc is provided to strengthen the sub grade.

Functions of subbase coarse

  • The sub-base layer enhances the bearing capacity of the sub grade and improves the load distribution capacity of the base course and wearing course.
  • The sub base course prevents the entry of finely graded sub grade soil to the base course layer.
  • They prevents the capillary rise of water and enables free drainage of water entering the pavement. The sub base material must be free draining for this application with suitable systems must be included in the pavement design for collecting and removing any accumulated water from the sub base.
  • This layer provides insulation to sub-grades against frost
  • The sub base course is used for raising the heights of pavements to be in line with the natural water table.
  • The Sub base course provides a perfect hard stratum for remaining construction activities.

Base course

The base course is a layer or layers of specified or select material of designed thickness placed on a subbase or subgrade (if a subbase is not used) to provide a uniform and stable support for binder and surface courses.
The base course is the most important layer of a road structure which transfers the stresses developed due to traffic impacts through the wearing course. The base course layer provides the required foundation stiffness and structural strength.
Good quality crushed aggregates in line with technical specifications have to be used. If the crushed aggregate does no meet the required criteria it can be stabilised with Portland cement, lime, or asphalt. In the case of high-quality pavements, the crushed aggregates are treated with asphalt or Portland cement.


Function of base course

  • To act as the foundation of the road pavement and to transfer the traffic load safely to the sub base and sub grade.
  • To withstand high shearing stresses due to the impact of traffic.
  • To prevent the undesirable entry of sub grade soil in the pavement when the base course is constructed directly over the sub grade.

Surface course or wearing course

Surface course is multiple layers of pavement structure where the top layer is directly exposed to traffic. The surface course is also called the wearing course. For flexible pavements, the bituminous surface is the wearing course whereas in rigid pavement the concrete surface act as the base course cum wearing course. The wearing course reducing the percolation of water and provide an anti skid and abrasion-resistant riding surface.


Function of Surface or Wearing Course

The Function of surface or wearing course are as follows:

  • To provide a smooth and uniform rigid surface.
  • To resist the abrasive forces of traffic.
  • To prevent dust nuisance.
  • To act as a structural part of the pavement.

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