From Encyclopedia Jr, free information reference for Kids
- The term asphalt is often used as an abbreviation for asphalt concrete.
Base layer of
asphalt concrete in a road under construction.
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.
[edit] 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 shrink on cooling, so large splashes 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.
[edit] Known Uses
[edit] Ancient Times
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 related to the English word mummy.
In the ancient far east, natural asphalt is slowly boiled to get rid of the higher fractions, leaving a material of higher molecular weight which is thermoplastic and when layered on objects, it becomes quite hard upon cooling. This is used to cover scabbards and other objects that needs water-proofing. Statuettes of household deities are also cast with this type of material in Japan and, most probably also in China
Poured bitumen has also been used as a damp-proof course in building.
[edit] Rolled asphalt concrete
The largest use of asphalt is for making Asphalt concrete for road surfaces, accounts for example for approximately 80% of the asphalt consumed in the United States. Roofing shingles account for most of the remaining asphalt consumption. Other uses include cattle sprays, fence post treatments, and waterproofing for fabrics.
Asphalt road surfaces is the most widely recycled material in the US, both by gross tonnage and by percentage. According to a report issued by the Federal Highway Administration and the United States Environmental Protection Agency, 80% of the asphalt from road surfaces' that is removed each year during widening and resurfacing projects is reused as part of new roads, roadbeds, shoulders and embankments.
[edit] Mastic asphalt
Mastic asphalt is a type of asphalt which differs from dense graded asphalt (Asphalt concrete) in that it has more bitumen (Binder) usually around 7-10% of the whole aggregate mix,as where roler asphalt has only around 5% added bitumen. Another asphalt which is fast gaining global popularity is Stone mastic asphalt or (SMA), SMA's advantages over roller asphalt is its high anti skid qualities due to its high aggregate density and the lack of void content (air pockets). Another advantage of SMA is its longer durability over alternative road asphalt surfaces, but its manufacture and application if not controlled closely, can result in slippery road surfaces due to excess bitumen pooling (bleeding) on to the surface.
[edit] Low temperature mixing
Since the mid 1980s a number of technologies have been developed which allow asphalt to be mixed at much lower temperatures. These involve mixing the asphalt with additives which either reduce its melting point or turn it into an emulsion.
[edit] Etymology
The word asphalt is derived from the late Middle English : from French asphalte, based on late Latin asphalton, asphaltum, from Greek asphalton, asphaltos (άσφαλτος).
[edit] References
- Barth, Edwin J., Asphalt: Science and Technology Gordon and Breach (1962). ISBN 0-677-00040-5.
[edit] External links
