What is Asphalt Concrete?

Asphalt concrete, commonly called asphalt, tarmac, pavement or black top, is a composite material used in the construction of roadways and parking lots. This composite is a mixture of a petroleum byproduct, asphaltic bitumen and aggregate materials. In asphalt concrete, the asphaltic bitumen acts as a sort of glue that binds the aggregate pieces together.

Although the process of paving streets with tar goes backs as far as eighth century Baghdad, no real changes in the process came about until the 20th century brought the first automobiles to public use. In 1901, Edgar Purnell Hooley patented a material called tarmacadam that would become the precursor of modern asphalt concrete. The shortened name of Hooley's material, tarmac, is often used to refer to asphalt concrete despite the fact that modern asphalt concrete does not contain tar.

Historically, asphalt concrete has been used for a variety of purposes, including bullet-proofing British warships in the early 1940s. The asphalt concrete was applied in a thick layer to a steel backing plate that was attached to the exterior hull of the ship. This use of asphalt concrete was called plastic armor and was quite effective at stopping the armor-piercing bullets of the time.

Asphalt concrete is mixed using six main methods. Each of these methods have their own particular pros and cons. These six methods are: hot mix, warm mix, cold mix, cut-back, mastic and natural asphalt.

Hot and warm mix asphalt relies on heat to soften the asphaltic bitumen and cause it to bind with the aggregate. Although both of these processes produce smooth, durable roadways, the temperatures required to work with the hot mixed material can be prohibitive for winter road construction and can require large amounts of fossil fuels to be used to generate the required heat. For this reason, the warm mixed materials are generally preferred because they hold up better during cold weather construction, and the reduced temperatures result in less pollution.

Cold mix asphalt is primarily used as a patch for road surfaces. Mixed with soap and water, the asphaltic bitumen is emulsified and mixed with the aggregate. As the water evaporates from the mixture, the asphalt hardens and takes on the characteristics of hot mix asphalt concrete. Cut-back asphalt uses the same process but, rather than using soap and water, kerosene or light petroleum products are employed to emulsify the asphalt binder.

Mastic asphalt is used for foot paths, roofing, flooring and other light-use paving projects. It is produced by cooking the asphaltic bitumen in a mixer for several hours before adding the aggregate. The aggregate is added after the bitumen has reached a viscous liquid state, and the mixture is allowed to cook for another six to eight hours. It is then transported to the job site for use.

Natural asphalt occurs as a result of upwelling bitumen. This bitumen is a naturally occurring substance in some areas of the Earth, and it exists below the surface. As the bitumen seeps up toward the surface, it travels through the porous sedimentary stone and becomes lodged in a form of naturally occurring asphalt.