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Concrete pavement is a rigid structure and asphalt is a flexible structure. Historically, pavements have been divided into two broad categories: rigid and flexible. These traditional definitions, in some cases, are an over-simplification. However, the terms rigid and flexible provide a good description of how the pavements react to traffic loads and the environment.
Flexible asphalt pavement generally consists of a surface of asphalt built over a base course and sub-base course (usually gravel or stone). These layers rest upon a compacted sub-grade (compacted soil). In contrast, rigid pavements are made up of Portland cement concrete and have only a base course between the pavement and sub-grade using approximately half as much granular material.
By virtue of its rigidity and stiffness, concrete pavements tend to distribute the load over a relatively wide area of sub-grade. The concrete slab itself supplies a major portion of a rigid pavement's structural capacity. Flexible pavement, inherently built with weaker and less stiff material, does not spread loads as well as concrete. Therefore flexible pavements usually require more layers and greater thickness for optimally transmitting load to the subgrade.
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