Asphalt aggregate consists of the bits of rock and gravel that are typically seen in an asphalt, or "blacktop" surface. Some standard components of asphalt aggregate include sand, gravel, crushed stone and slag. These aggregates not only add strength to the finished asphalt concrete, they also are used without the asphalt mixture to create a stable foundation on which roads, railroad tracks and buildings are constructed.
Asphalt aggregate is usually produced in rock quarries that are located near most heavily populated areas. Depending on the geological makeup of the area, the aggregate is usually composed of granite, marble, limestone or some other bedrock-quality stone. In areas that don't have minable deposits of these types of stone, natural sand and gravel are often mined and used as aggregates.
To the untrained eye, asphalt pavement might look like a chaotic mess of debris, but that is not the case at all. A lot of research and development has gone into creating the right mix of asphalt aggregate for any given application. Aggregate is sorted and graded by size, and the amounts and sizes used in an asphalt mix can vary from one type of job to the next.
A hot mix asphalt (HMA), which is typically used for roads and parking lots, is as much as 95 percent asphalt aggregate by weight. The majority of aggregate in the mix is usually coarse, or large in size, followed by crushed stone and gravel. The remaining aggregate is composed of fillers, such as sand.
Many construction companies and local governments have saved money and helped the environment by using recycled asphalt aggregate. Depending on local codes and regulations, recycled aggregate might come from a number of places. Some sources of recycled aggregate include used asphalt pavement that has been reclaimed during road construction or repairs, concrete that has been saved from demolished buildings and even some types of recycled plastics. The largest source of recycled asphalt aggregate is slag from blast furnaces and steel furnaces.
Recycled aggregate is often less expensive, because it can be made on site with portable crushers and aggregate washers. It also makes efficient use of old aggregate that would otherwise find its way into landfills. In some areas, the use of recycled aggregate has helped local governments reduce the disposal rate by as much as 50 percent. Many areas do not allow recycled aggregates to be used to completely replace traditional asphalt aggregate because it produces a lower quality of asphalt, but even partial use of recycled aggregates can conserve natural resources.