Asphalt
The term asphalt is often used as an abbreviation for asphalt concrete.
Asphalt is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits. Asphalt is composed almost entirely of bitumen. There is some disagreement amongst chemists regarding the structure of asphalt, but it is most commonly modeled as a colloid, with asphaltenes as the dispersed phase and maltenes as the continuous phase.
There are two forms commonly used in construction: rolled asphalt and mastic asphalt. Rolled asphalt (usually hot rolled or HRA) is one of the forms of road surfacing material known collectively as blacktop; another form is the (distinct) macadam, including both tar and bituminous macadams. The terms asphalt and tarmac tend to be used interchangeably in common usage, although they are distinct products.
Background
Asphalt is sometimes confused with tar, which is an artificial material produced by the destructive distillation of organic matter. Tar is also predominantly composed of bitumen, but the bitumen content of tar is typically lower than that of asphalt. Tar and asphalt have very different engineering properties.
Asphalt can be separated from the other components in crude oil (such as naphtha, gasoline and diesel) by the process of fractional distillation, usually under vacuum conditions. A better separation can be achieved by further processing of the heavier fractions of the crude oil in a de-asphalting unit, which uses either propane or butane in a supercritical phase to dissolve the lighter molecules which are then separated. Further processing is possible by "blowing" the product: namely reacting it with oxygen. This makes the product harder and more viscous.
Natural deposits of asphalt include Lake Asphalts (primarily from the Pitch Lake in Trinidad and Tobago and Bermudez Lake in Venezuela), Gilsonite, the Dead Sea in Israel, and Tar Sands.
Both asphalt are typically stored and transported at temperatures around 150 Celsius. They shrinkl on cooling, so large spashes on the skin are especially hazardous. Sometimes diesel oil or kerosene are mixed in before shipping to retain liquidity; upon delivery, these lighter materials are separated out of the mixture. This mixture is often called bitumen feedstock, or BFS. Some dump trucks route the hot engine exhaust through pipes in the dump body to keep the material warm. The backs of tippers carrying asphalt, as well as some handling equipment, are also comonly sprayed with diesel oil before filling to aid release.
In the ancient middle east, natural asphalt deposits were used for mortar between bricks and stones, ship caulking, and waterproofing. The Persian word for asphalt is mumiya, which may be the source of the English word mummy.
Poured bitumen has also been used as a damp proof course in building.
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