What are Different Types of Asphalt?
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Asphalt is most well known as a road covering more technically termed asphalt concrete, but there are a few different forms in which the substance may appear. It is a naturally occurring material present in crude oil and in natural deposits, notably around certain bodies of water and in oil sands. This substance is found in either liquid or semi-solid form in nature and is characterized by its high viscosity and its sticky, black appearance. It consists almost exclusively of bitumen, a substance composed of polycyclic aromatic hydrocarbons (PAHs). The major types used in construction are rolled and mastic.
Rolled asphalt is the variety with which most people are probably familiar, as it makes up about 80% 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 make roads and other surfaces, such as parking lots, by being applied in layers and compacted. Different types are distinguished according to the process used to bind the aggregate with the asphalt.
Hot mix asphalt concrete (HMAC) is produced at 320°F (160°C). This high temperature serves to decrease viscosity and moisture during the manufacturing process, resulting in a very durable material. HMAC is most commonly used for high-traffic areas, such as busy highways and airports.
Warm mix asphalt concrete (WAM or WMA) reduces the temperature required for manufacture by adding emulsions, waxes, or zeolites. This process benefits both the environment and the workers, as it results in less fossil fuel consumption and reduced emission of fumes. In cold mix 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 nearly as durable as HMAC or WAM, and it is typically used for low traffic areas or to patch damaged HMAC.
Cut-back 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 viscosity while the concrete is layered and compacted. The lighter kerosene later evaporates, leaving a hardened surface.
Mastic asphalt, also called sheet asphalt has a lower bitumen content than the rolled forms. It is used for some roads and footpaths, but also in roofing and flooring. Stone mastic asphalt (SMA), another variety, is becoming increasingly popular as an alternative. Its benefits include an anti-skid property and the absence of air pockets, but if laid improperly, it may cause slippery road conditions.