Asphalt

Asphalt is a dark, sticky substance composed of hydrocarbons mixed with aggregates and oils. It is produced mainly from crude oil through refining, though it also occurs naturally in some parts of the world, such as natural asphalt deposits. Its principal property is that it is adhesive and water-resistant.

The largest application is road construction. There, asphalt acts as a binder holding together stone and sand. This mixture is known as asphalt concrete and forms the surface of modern roads. The term 'asphalt' is sometimes used to describe the entire mixture, and sometimes only the binding agent.

Production

In oil refineries, crude oil is heated in furnaces and then sent through a distillation tower. Lighter components such as gas, petrol, and diesel rise to the top. What remains at the bottom, the heaviest fraction with the highest boiling point, is the asphalt base. A separate distillation process at lower pressure extracts other components present in the crude.

Types and preparation

To lay asphalt, it is heated to high temperature and mixed with stone. This is the hot mix method. Other methods use water and chemicals to break down the asphalt into an emulsion, allowing work at lower temperatures. There is also asphalt modified with polymers to make it more resistant to traffic loads and climate changes.

Other applications

Beyond roads, asphalt is used for roof waterproofing, waterproofing of steel pipes to reduce friction, and damp-proofing of building foundations. Historically, natural asphalt was used for sealing boats and construction from ancient times.

Environment and recycling

One noted property is that worn asphalt road can be broken up, mixed, and reused, reducing the need for new material. On the other hand, hot mix production is energy-intensive, and efforts are underway to lower process temperatures. Dark roads also absorb much heat, contributing to urban heat islands.