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 lime 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 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.

In addition to the asphalt and rock, 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 rocks near the surface have been impregnated with upwelling asphalt.

The term asphalt concrete is not widely used. It is often called just asphalt. 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. Note that this material is chemically unrelated to cured-lime concrete.

See also

Free floating screed


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