Asphalt concrete
Asphalt concrete is a construction material commonly used for paving roads, highways and parking lots. It consists of
asphalt and mineral aggregate mixed together, laid down in a mat and compacted. Mixing of asphalt and aggregate is accomplished in one of three 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 approximately 160 degrees Celsius, while asphalt paving and compaction are performed at approximately 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.
- 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 mix is easy to work and compact. The emulsion will break after enough water evaporates back out, and the cold mix will ideally take on the properties of cold HMAC. Cold mix is commonly used as a patching material, and on lower level of service roads.
- Cut-back Asphalt Concrete is produced by dissolving the asphalt 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 out. Due to the air pollution this causes, cut-backs have been illegal in the US since the 1970s, however they are still widely used in Europe and the rest of the world.
Natural asphalt concrete can be found in some parts of the world where rocks near the surface have been impregnated with upwelling asphalt.
Asphalt concrete pavements, especially those at airfields are sometimes called tarmac in Europe for historical reasons, although they do not contain tar and are not constructed using the macadam process.