**3. Road construction**

Road construction may involve rigid pavement or flexible pavement. Rigid pavement is made up of portland cement concrete base. It could be reinforced with steel bar or could be an ordinary plain concrete slab.Because of its rigidity and high modulus of elasticity, rigid pavement tends to distribute the applied load over a relatively wide area of soil and the major portion of the structural capacity is provided by the slab. Hence the major consideration in the design of rigid pavement is the strength of the concrete. Minor variations in the sub grade and/or base strength have little influence on the structural capacity of the pavement. Provision of drainage assets is therefore not a priority. In flexible pavement the strength is brought about by building up relatively thick layers of sub base, base course and surface coating over a sub grade. These layers are composed of natural aggregates. The manners of load distribution in flexible pavement make it sensitive to moisture variation. Therefore the provision of drainage asset is a priority in the design and construction of flexible pavement.

In flexible pavement design four main features are considered [2]: the pavement structure, pavement materials, cross sectional profile and size of the drain invert.

## **3.1 Pavement structure**

A flexible road pavement structure may comprise the subgrade, subbase, base course and asphalt coating in that order. The subgrade is the in-situ or borrowed earth material spread and compacted in line with the specifications, across the width and lenght of the proposed road to form its foundation. The thickness of the sub grade is semi infinite and the elevation relative to the ground level is dependent on the environmental factors. The subbase is placed over the subgrade and serves as a transition layer to the base course which is the main load spreading layer of the pavement. The uppermost layer is the surfacing or asphalt coating that has a water proofing and abrasion resistant properties.
