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. This substance is at least 80% carbon, which explains its deep black color. Sulfur is another ingredient found in the tar-like asphalt, as well as some trace minerals. It 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 it 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.
Asphalt is also a popular sealant for roofs. When heated, it can be pumped to the roof of a new building and poured into place. While it is still pliable, roofers can spread an even layer to form a nearly-impenetrable barrier between the building and the elements. Over time, the aggregate may work its way out of the tar, but the overall integrity is comparable to other roofing methods.
Because asphalt bears the weight of cars extremely well, it has become a very popular material for parking lot construction. The material can be applied quickly on a prepared surface, which means a parking lot can be graded, poured and painted with little delay. Patching it is usually a matter of bringing new material to the affected area and pressing it into the cracks or potholes. This makes asphalt preferable to more permanent materials such as concrete. Repair crews can fix most problems without blocking traffic or removing entire sections of the roadway.
Asphalt does have some drawbacks as a construction material, however. The sulfur fumes released during the heating process can be hazardous to workers and very unpleasant for passers-by. Traffic and constant exposure to the elements can cause the surface to wear out faster than anticipated. Because the ground beneath the asphalt can repeatedly freeze and thaw, roads made with it are susceptible to cracks and pothole formation. But for general durability and low production costs, this material is hard to beat for many projects.
