The specifications must take into account the traffic patterns. Asphalt pavement structures must transfer and distribute the wheel load of vehicles to the structure's supporting foundation. The plans must take into consideration the quantity of traffic and the wheel loads of the vehicles. Typically, loads, consisting of passenger automobiles to twin trailers, range from 2,000 to 80,000 pounds.
The aggregate base makes up the foundation for the project. Aggregates consist of a mixture of materials, including sand, gravel, crush rocks fillers and other materials. The base functions as a frost barrier. The frost barrier keeps the asphalt surface from rising due to the ice, snow and cold of the winter season. The foundation also serves as a solid surface that provides durable support for bearing the design load for the asphalt structure under a variety of conditions. The load bearing capacities for residential driveways differ from the support standards for highway traffic.
If the site already has a subgrade or a composition comprising material with a gravel-like makeup, one to two inches of processed stone or recycled aggregates may suffice. When working with soil, an eight-inch to 10-inch thick base may be required. Contractors must consult the recommendations for the given area, including the local Department of Transportation specification for asphalt paving. The material's thickness should not exceed a thickness of six inches, which allows the material to compact adequately.
The presence of water, which may seep up from underneath the pavement, weakens the structure. For asphalt surfaces to drain properly, the base has to have the correct pitch. Most design criteria for residential and commercial projects call for a minimum pitch of two percent or two feet for every 100 feet. Other drainage specifications may require the installation of subdrains to drain water away from the pavement structure.
Asphalt projects up to the size of small commercial projects usually require a compaction density in the 90 to 95 percent range. The final compact finish should equate to no more than five times the size of the largest stone material used for the asphalt mix. In Connecticut and New York, the maximum stone size varies between 3/8 inch and ½-inch; therefore, the specifications allow for maximum poured mix of 2-½ inches, so that when compacted, the pavement has a maximum compacted thickness of two inches. If delivery or garbage trucks use the structure a 3-1/2 inch thickness, consisting of two lifts or pourings -- a binder layer of two inches and a top layer of 1-1/2 inches - is usually required. The load bearing capacity of some commercial structures necessitate greater thicknesses.