Asphalt is a thick brownish or black substance derived from the same crude oil which produces kerosene, gasoline and vinyl. It is literally scraped from the bottom of the barrel after all other petroleum-based products have been refined or processed. Asphalt is at least 80% carbon, which explains its deep black color. Sulphur is another ingredient found in the tar-like asphalt, as well as some trace minerals. Asphalt is primarily used as a sealant for rooftops and a durable surface for roads, airport runways, playgrounds and parking lots.
The tar from the crude oil is usually mixed with sand or gravel (often called aggregate) to form the finished product we call asphalt. The black tar forms a strong adhesive bond with the aggregate, which makes it durable. When used in road construction, asphalt is usually poured over a bed of heavier aggregate in a heated state, then pressed into place by an extremely heavy steam roller. Once the fresh asphalt cools to ambient temperature, it becomes sturdy enough for automobile traffic. Asphalt may harden even more over the years, but it still retains enough flexibility to accommodate natural variations in the roadbed.