Asphalt concrete
From Freepedia
Asphalt concrete, normally known simply as asphalt, is a composite material commonly used for construction of paved roads, highways and parking lots. It consists of asphalt binder and mineral aggregate mixed together, laid down in layers 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 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 out. Due to the air pollution this causes, cut-backs have been illegal in the US since the 1970s. It is 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 rejuvinate oxidized binders and remove cracking, although the recycled material is generally not very water tight or smooth and should be overlayed 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.
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.
