The top layer of a road surface is called the surface course. This surface is PCC or Portland Cement Concrete. Portland cement is 85 percent lime and silica and contains calcium, aluminum, iron, silicon and trace amounts of gypsum. This concrete may also contain coarse aggregate (crushed stone or gravel), fine aggregate (such as sand), fly ash, and water. It may also include air entraining admixtures, retarders, accelerators, water reducers and trace amounts of chemical admixtures such as pumping acids, expansive agents, and pigments. This layer provides friction, noise control, drainage, smoothness and is a waterproofing for the underlying layers. The thickness of the surface course depends on the type of traffic that will use the surface. Thickness usually runs between 6 inches for light loading to 12 inches for high traffic and heavy loads.
The base course is the layer immediately below the surface course. This layer is a secure support for the construction equipment and gives even support to the surface course. It aids in load distribution, adds to frost resistance and drainage and helps prevent any movement in the soil underneath the road. The base course contains either aggregate (gravel, sand, lime rock, clay and sand mixed), stabilized aggregate or soil, dense-graded HMA (hot mixed asphalt), occasionally permeable HMA or lean concrete (a mixture containing less Portland cement paste than the Surface Course).
Although not always used or needed, the subbase course is below the base course and above the subgrade. Although it functions mainly as structural support, it also improves drainage, minimizes frost action damage, impedes the movement of fine material in the sublayer, stopping it from creeping up into the pavement, and provides a support base for construction of the upper layers. Consisting of lower quality materials than those used in the base course, appropriate materials for the subbase include high quality structural fill (such as coal ash) and aggregate.
Under the three layers of rigid pavement courses is the subgrade. This is the soil under the road. The soil must be able to support the upper layers and the traffic on the road above. To strengthen the soil, heavy rollers or vibratory compactors compresses this layer. The subgrade must meet stabilization requirements. Soil can be stabilized by incorporating chemical admixtures such as fly ash, Portland cement or lime; mechanically by including granular materials or stabilization expedient such has sand-grid (metal mats), geosynthetics (thin, strong polypropylene material) or matting (fiberglass, plastic hexagonal, corduroy/wood or roll-up mesh).
