Asphalt and concrete are two commonly used materials for roadway pavements. The term "flexible" for asphalt pavement and "rigid" for concrete pavement refers to the stiffness of the material and the way each material transmits vehicle loads to the underlying road base.
Flexible pavements are composed of a layer of asphalt placed on top of compacted soil, or if needed, a layer of crushed rock or gravel. A pavement designer determines the needed thickness of the asphalt pavement by assessing the soil conditions and the volume of traffic and the mix of vehicles -- autos, trucks, buses and more -- that the roadway will carry.
Concrete is composed of crushed rock and sand, hydraulic cement, water and additives that adjust its engineering properties. Concrete also is placed on a soil or crushed rock base. In both asphalt and concrete pavement designs, adding lime or cement to the underlying soil may improve its stability.
The selection of a flexible or rigid pavement for a given application depends on a number of factors. Frequently, an analyst conducts a life-cycle cost analysis to evaluate pavement alternatives based on initial cost as well as the rehabilitation and maintenance costs required over the life of the pavement. The Texas Department of Transportation notes that other pavement selection considerations include ease of construction, safety, such as skid resistance, traffic noise and climate or weather issues, among others.