Asphalt concrete

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.
  • Mastic Asphalt Concrete or sheet asphalt is produced by heating hard grade blown bitumen (Oxidation) in a green cooker (mixer) until it has become a viscous liquid after which the aggregate mix is then added.

The bitumen aggregate mixture is cooked (matured) for around 6-8 hours and once it is ready the mastic asphalt mixer is transported to the work site where experienced layers empty the mixer and either machine or hand lay the Mastic asphalt contents on to the road. Mastic asphalt concrete is generally laid to a thickness of around 20-30mm for footpath and road applications and aroumd 10mm when flooring or roof applications are concerned.

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.

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