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.
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.
Asphalt is rather hard to transport in bulk (it hardens unless kept very hot) so it is sometimes mixed with
diesel oil or
kerosene before shipping. 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 largest use of asphalt is for making
asphalt concrete for road surfaces, which accounts 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.
While Americans are accustomed to thinking about
recycling their newspapers and beverage containers, the most widely recycled product in terms of both percentage and tonnage is actually asphalt road surfaces. According to a report issued by the
Federal Highway Administration and the
United States Environmental Protection Agency, 80 percent 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.
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.
The word
asphalt is derived from the late Middle English : from French
asphalte, based on late Latin
asphalton,
asphaltum, from Greek
asphalton,
asphaltos (άσφαλτος).
Barth, Edwin J.,
Asphalt: Science and Technology Gordon and Breach (1962). ISBN 0677000405.
*
Black Stuff*
Hawaii Asphalt Pavement Guide*
Asphalt Recycling
