Chapter 19, Problem 1P

To determine

The structural components of a flexible pavement.

Explanation of Solution

The structural components of a flexible pavement are as follows:

1. The subgrade or prepared roadbed,
2. The subbase,
3. The base and
4. The surface course.

It is necessary to check for the evaluation of every component of the flexible pavement and

Subgrade (Prepared Roadbed)

It is first in the line of materials that are to be laid and serves as the foundation of the flexible pavement structure. It is being spread along the horizontal alignment of the pavement. The materials used in this layer are very well compacted to meet the prescribed specifications of strength. In this layer, borrow materials may also be used according to the pavement being constructed.

Subbase Course

This layer is kept above the subgrade and the materials used in this layer are of better quality as compared to that of the materials used in subgrade. The materials used in this flexible pavement structure must meet the specifications in terms of gradation, strength and plastic characteristics.

And in case, if the materials for the subbase course aren't readily available then the subgrade materials are treated to meet the specifications that of the subbase course material properties.

Base Course

This course comes at the third place in the line of pavement structures. It isplaced just above the subbase course and if subbase is not used then it lies immediately above the subgrade of the pavement structure. The materials that are used in this pavement section are of much better quality then those used in the previous layers in terms of gradation, strength and plastic requirements. Granular materials likestone,slag, gravel, and sand are used in this course of flexible pavement structure.Wherein, slag and gravel can be used in both forms i.e., (crushed or uncrushed) stones should be crushed. In case if some of the materials are not meeting the specifications for this line of course then they are used in the base if appropriately stabilized with Portland cement, lime or asphalt.For high quality pavements the material is treated with Portland cement or asphalt to improve the stiffness characteristics of such pavements.

Surface Course

This is the topmost layer of the flexible pavement and is placed above the base course of the road pavement. The materials in this pavement include a mixture of aggregates and asphalt. The properties of this material must be able to hold out against the high tire pressure, withstand abrasive forces due to the traffic flow, prevention against the infiltration of surface water to avoid rupturing of surface course and weakening of the underlying structural bed of the pavement. Considering the expected traffic flow the thickness of this layer varies from 3 inches to 6 inches.