Bitumen
- 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% 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.
Mastic asphalt is a type of asphalt which differs from Dense graded asphalt (asphaltic concrete) in that it has a lower quantity of bitumen 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.
The world has become increasingly concerned over the global climate change problem in recent years and to the pollution that is emitted into the atmosphere from burning fossil fuels. This has lead to the introduction of asphalt alternatives that are more environmentally friendly, safer to use for road construction workers and can withstand increasing road surface temperatures better than asphalt made with conventional residue bitumen.
Mastic roller hybrid or (MRH) asphalt is the latest breakthrough technology being developed which is similar to the SMA Stone Mastic Asphalt with the exception that it only has about 4-5% bitumen loading, which means that the asphalt surface remains non-slip even under wet weather conditions. MRH asphalt is compacted at low temperatures around 90 °C (Warm mix asphalt) the aggregate mix in the (MRH) can also be made from alternative rock sources eg, recycled crushed brick and concrete to help preserve our natural rock reserves. The inherently lighter asphalt surface of the MRH Asphalt helps to reduce the Urban heat island effect by providing cooler road surfaces.
Approximately 2000 million tonnes of Asphalt Bitumen is used annually around the world and its use will grow dramatically especially in Asia during the next decade. Bitumen has been around for millions of years in crude oil, it won't last forever if not for its negative environmental impact but because its source is slowly being depleted (Peak oil theory). A number of factors have an effect on the performance of asphalt, these include its composition and crude oil source, the type and amount of aggregate used, the presence of moisture, the method of road construction, temperature and of course the volume of traffic.
Around 30% of the world crude oil supply is heavy grade which is required to manufacture bitumen,it is a costly exercise for the oil industry having to spend roughly two barrels of fuel to extract one barrel of heavy grade crude oil from the ground, most of the cost goes into heating the wells, pipelines and into transportation which all at the end of the day are ultimately carried at the expense to people's health and the environment.
Conventionally the asphalt manufacturing process has to be monitored very closely to achieve a positive paving outcome and is generally un-forgiving to minor changes to the bitumen or aggregate loading and processing temperatures. The chance of asphalt mix failure is therefore increased due to human or mechanical error, normal road grade bitumen is made from the by-product waste (bottoms) of the petrochemical distillation process this by-product is processed further by adding oils and cutters volatiles (fumes) to achieve a desired grade. The fumes that are emitted into the atmosphere during the bitumen manufacturing process, hot storage and at the asphalt manufacturing stage not only have a negative impact on health but are also environmentally damaging and therefore undesirable.
The surfaces of asphalt roads made from normal bitumen have a tendency of becoming greasy (slippery)under wet weather conditions, this is due ironically to the high oil content of residue bitumen. Moving traffic tends to pick up (binder shift) this oily residue and distribute it from one area to another and consequently contribute to the slippery road surfaces. Considering all the shortcomings of residue bitumen and its impact on the environment such as leaching ,fuming, heat sink absorption Urban heat island, and the occupational health and safety issues for the asphalt workforce having to handle a product at elevated temperatures, and also taking into consideration the global move towards renewable resources and sustainability, there is a pressing need to revitalize research, development and engineering especially into alternative asphalt bitumen compositions and asphalt materials.
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).
References
- Barth, Edwin J., Asphalt: Science and Technology Gordon and Breach (1962). ISBN 0-677-00040-5.
- The Asphalt Bitumen Story