Asphalt concrete

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As shown in this cross-section, many older roadways are smoothed by applying a thin layer of asphalt concrete to the existing portland cement concrete.

Base layer of asphalt concrete in a road under construction

Asphalt concrete, normally known simply as asphalt, is a composite material commonly used for construction of pavement, highways and parking lots. It consists of asphalt binder and mineral aggregate mixed together then laid down in layers and compacted.

Mixing of asphalt and aggregate is accomplished in one of several ways:

Hot Mix Asphalt Concrete (commonly abbreviated as HMAC) is produced by heating the asphalt in order to decrease its viscosity and drying the aggregate to remove moisture from it prior to mixing. Mixing is generally performed at about 160 degrees Celsius, while paving and compaction are performed at about 140 degrees Celsius. HMAC is the form of asphalt concrete most commonly used on highly trafficked pavements such as those on major highways and airfields.
Warm Mix Asphalt Concrete (commonly abbreviated as WMA or WAM) is produced by adding either zeolites, waxes, or asphalt emulsions to the mix. This allows significantly lower mixing and laying temperatures and results in lower consumption of fossil fuels, thus releasing less carbon dioxide, aerosols and vapours. Not only are working conditions clearly improved, but the lower laying-temperature also leads to more rapid availability of the surface for use (in construction sites with critical time schedules).
Cold Mix Asphalt Concrete is produced by emulsifying the asphalt in water with (essentially) soap prior to mixing with the aggregate. While in its emulsified state the asphalt is less viscous and the mixture is easy to work and compact. The emulsion will break after enough water evaporates and the cold mix will, ideally, take on the properties of cold HMAC. Cold mix is commonly used as a patching material and on lesser trafficked service roads.
Cut-back Asphalt Concrete is produced by dissolving the binder in kerosene or another lighter fraction of petroleum prior to mixing with the aggregate. While in its dissolved state the asphalt is less viscous and the mix is easy to work and compact. After the mix is laid down the lighter fraction evaporates. Due to the resulting air pollution cut-backs have been illegal in the US since the 1970s. They are still widely used in Europe and the rest of the world, especially for recycling old asphalt pavement.

In addition to the asphalt and aggregate, additives, such as polymers, and antistripping agents may be added to improve the properties of the final product.

Natural asphalt concrete can be found in some parts of the world where rock near the surface has been impregnated with upwelling asphalt.

The term asphalt concrete is typically only used in engineering jargon. It is often called just asphalt by laypersons who tend to associate the term concrete with portland cement concrete only. The engineering definition of concrete is any composite material composed of mineral aggregate stuck together with a binder, whether that binder is portland cement, asphalt or even epoxy.

Asphalt concrete is often touted as being 100% Recyclable. Several in-place recycling techniques have been developed to rejuvenate oxidized binders and remove cracking, although the recycled material is generally not very water-tight or smooth and should be overlaid with a new layer of asphalt concrete. Asphalt concrete that is removed from a pavement is usually stockpiled for later use as a base course material.

Very little asphalt concrete is actually disposed of in landfills. Sometimes waste materials, such as rubber from old tires, are added to asphalt concrete but there is a concern that the hybrid material may not be recyclable.

Asphalt concrete pavements—especially those at airfields—are sometimes called tarmac for historical reasons, although they do not contain tar and are not constructed using the macadam process.

Asphalt concrete

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