Asphalt is a naturally occurring material found in crude oil and natural deposits. It can appear in different forms, with rolled asphalt being the most common. Other types include hot-form asphalt concrete, hot mixed concrete, cold-worked asphalt concrete, and mastic asphalt. Cut asphalt concrete is illegal in the US. Stone mastic asphalt is becoming increasingly popular but can lead to slippery road conditions if improperly laid.
Asphalt is best known as a road covering more technically asphalt concrete, but there are a few different forms the substance can appear in. It is a naturally occurring material found in crude oil and in natural deposits, particularly around certain bodies of water and in the oil sands. This substance is found in nature in liquid or semi-solid form and is characterized by its high viscosity and its sticky and black appearance. It consists almost exclusively of bitumen, a substance composed of polycyclic aromatic hydrocarbons (PAH). The main types used in construction are laminates and mastic.
Rolled asphalt is the variety most people are probably familiar with, as it makes up about 80 percent of that used in the United States. It is made of aggregate or solid materials such as sand, gravel or recycled concrete, with an asphalt binder. This type is used to build roads and other surfaces, such as parking lots, by layering and compaction. Different types are distinguished according to the process used to bind the aggregate with the asphalt.
Hot-form asphalt concrete (HMAC) is produced at 320°F (160°C). This elevated temperature serves to reduce viscosity and moisture during the manufacturing process, resulting in a very strong material. HMAC is most commonly used for high traffic areas, such as busy highways and airports.
Hot mixed concrete (WAM or WMA) reduces the temperature required for production by adding emulsions, waxes or zeolites. This process benefits both the environment and the workers, as it results in lower consumption of fossil fuels and reduced fume emissions. In cold-worked asphalt concrete, the asphalt is emulsified in soapy water before mixing it with the aggregate, eliminating the need for high temperatures altogether. However, the resulting material is not as durable as HMAC or WAM, and is typically used for low traffic areas or to repair damaged HMAC.
Cut asphalt concrete has been illegal in the United States since the 1970s, but many other countries around the world still use it. This type of concrete is the least environmentally friendly option, resulting in significantly more air pollution than the other forms. It is made by dissolving the binder in kerosene before mixing it with the aggregate, reducing the viscosity as the concrete is poured and compacted. The lighter kerosene then evaporates, leaving a hardened surface.
Mastic asphalt, also called sheet asphalt, has a lower bitumen content than rolled forms. It is used for some roads and sidewalks, but also for roofing and paving. Stone mastic asphalt (SMA), another variety, is becoming increasingly popular as an alternative. Its advantages include a non-slip property and the absence of air pockets, but if improperly laid, it can lead to slippery road conditions.
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