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.
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 rejuvenate oxidized binders and
remove cracking, although the recycled material is generally not very
water-tight or smooth and should be overlaid 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. Sometimes
waste materials, such as rubber from old tires, are added to asphalt concrete
but there is a concern that the hybrid material may not be recyclable.
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.
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, however 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, however 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.
The largest use of asphalt is for making asphalt concrete for pavements, 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 pavement. According to a report
issued by the Federal Highway Administration and the United States Environmental
Protection Agency, 80 percent of the asphalt pavement 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 for the English word mummy.
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