A flexible pavement is also called an asphalt pavement or Tarmac. Flexible pavement is composed of three or four layers. A four-layer flexible pavement includes a surface course, a base course and a sub-base course laid over a compacted subgrade. A three-layer flexible pavement omits the sub-base layer and places the base course directly on the compacted subgrade. The surface layer of a flexible pavement generally consists of hot-mix asphalt, or HMA. For the base course, graded crushed aggregate, lime rock, or stabilized aggregate may be used. The base course is required to be compacted to the maximum possible density but not less than 100 percent of the ASTM D 1557 standard.
For flexible pavements, the greatest amount of stress takes place at the surface of the road. Because of this, the materials strongest under tensile stress need to be used for the surface course of the pavement while lower-quality, less flexible materials can safely be used beneath it. The design of flexible pavement is based on the fact that the intensity of any load is diminished as the load is transmitted downwards into the ground. Therefore, the quality of the subgrade plays a major role in determining the thickness of the three or four layers that make up a flexible pavement.
Flexible pavement is given its name because if its ability to flex and bend when a load is applied to it. The strength of flexible pavement lies in its ability to carry large tensile loads before yielding to plastic deformation, thereby making it a good choice for roads that require a certain amount of resilience. Over time, however, plastic deformation of the asphalt will take place, resulting in wheel path ruts. A flexible pavement can generally last anywhere from 20 to 30 years, depending on the level of traffic it is subjected to, the local weather conditions, the materials used and the quality of the paving process.
Care must be taken when laying down a flexible pavement. Too little bituminous material will cause prevent the aggregate from holding together, thereby greatly reducing the integrity of the road. Too much bituminous material can cause the road to become slick as the oils rise to the surface of the road, making the road unsafe to drive on.