Asphalt is a heavy, dark brown to black mineral substance, one of several mixtures of hydrocarbons called bitumens. Asphalt is a strong, versatile weather and chemical-resistant binding material which adapts itself to a variety of uses. Asphalt binds crushed stone and gravel (commonly known as aggregate) into firm, tough surfaces for roads, streets, and airport run-ways. Asphalt, also known as mineral pitch, is obtained from either natural deposits such as native asphalt or brea or as a byproduct of the petroleum industry (petroleum asphalt). Prehistoric animal skeletons have been preserved completely intact in natural asphalt deposits, one of the most famous being the La Brea Tar Pits in Los Angeles, Califomia.
Asphalt is one of the world's oldest engineering materials, having been used since the beginning of civilization. Around 6000 B.C. the Sumerians had a thriving shipbuilding industry that produced and used asphalt for caulking and waterproofing. As early as 2600 B.C. the Egyptians were using asphalt as a waterproofing material and also to impregnate the wrappings of mummies as a preservative. Ancient civilizations widely used asphalt as a mortar for building and paving blocks used in temples, irrigation systems, reservoirs, and highways. The asphalts used by early civilizations occurred naturally and were found in geologic strata as either soft, workable mortars or as hard, brittle black veins of rock formations (also known as asphaltic coal). Natural asphalts formed when crude petroleum oils worked their way up through cracks and fissures to the earth's surface. The action of the sun and wind drove off the lighter oils and gases, leaving a black residue. Natural asphalts were extensively used until the early 1900s. The discovery of refining asphalt from crude petroleum and the increasing popularity of the automobile served to greatly expand the asphalt industry. Modern petroleum asphalt has the same durable qualities as naturally occurring asphalt, with the added advantage of being refined to a uniform condition free from organic and mineral impurities.
Most of the petroleum asphalt produced today is used for highway surfacing. Asphalt paving material is a dull black mixture of asphalt cement, sand, and crushed rock. After being heated, it is dumped out steaming hot onto the roadbed, raked level, and then compacted by a heavy steamroller. Asphalt is also used for expansion joints and patches on concrete roads. Airport runways, tennis courts, playgrounds, and floors in buildings all use asphalt as well. Light forms of petroleum asphalt called road oils are sprayed on roadways to settle dust and bind gravel. Another major use of asphalt is in asphalt shingles and roll roofing, which usually consists of felt saturated with asphalt. The asphalt helps to preserve and waterproof the roofing material. Other applications for asphalt include the following: waterproofing tunnels, bridges, dams and reservoirs; rustproofing and sound-proofing metal pipes and automotive under-bodies; and soundproofing walls and ceilings.
The raw material used in modern asphalt manufacturing is petroleum, which is a naturally
Crude petroleum is separated into its various fractions through a distillation process at the oil refinery. After separation, these fractions are further refined into other products which include asphalt, paraffin, gasoline, naphtha, lubricating oil, kerosene, and diesel oil. Since asphalt is the base or heavy constituent of crude petroleum, it does not evaporate or boil off during the distillation process. Asphalt is essentially the heavy residue of the oil refining process.
Distilling the crude
1 The refining process starts by piping the crude petroleum from a
storage tank into a heat exchanger or tube heater where its temperature
is rapidly raised for initial distillation. It then enters an
atmospheric distillation tower where the lighter and more volatile
components, or fractions, vaporize and are drawn off through a series of
condensers and coolers. It is then separated for further refining into
gasoline (considered a "light" distillate), kerosene
(considered a "medium" distillate), diesel oil (considered
a "heavy" distillate), and many other useful petroleum
The heavy residue from this atmospheric distillation process is commonly called topped crude. This topped crude may be used for fuel oil or further processed into other products such as asphalt. Vacuum distillation may remove enough high boiling fractions to yield what is called a "straight run" asphalt. However, if the topped crude contains enough low volatile components which cannot be economically removed through distillation, solvent extraction—also known as solvent deasphalting—may be required to produce asphalt cement of the desired consistency.
- 2 Asphalt may next be blended or "cut back" with a volatile substance, resulting in a product that is soft and workable at a lower temperature than pure asphalt cement. When the cut-back asphalt is used for paving or construction, the volatile element evaporates when exposed to air or heat, leaving the hard asphalt cement. The relative speed of evaporation or volatility of the cutting agent determines whether a cutback asphalt is classified as slow, medium, or rapid-curing. Heated asphalt cement is mixed with residual asphaltic oil from the earlier distillation process for a slow-curing asphalt, with kerosene for medium-curing, and with gasoline or naphtha for the rapid-curing asphalt.
- 3 The asphalt cement may also be emulsified to produce a liquid that can be easily pumped through pipes, mixed with aggregate, or sprayed through nozzles. To emulsify, the asphalt cement is ground into globules 5 to 10 microns and smaller (one micron is equal to one millionth of a meter). This is mixed with water. An emulsifying agent is added, which reduces the tendency of the asphalt and water to separate. The emulsifying agent may be colloidal clay, soluble or insoluble silicates, soap, or sulphonated vegetable oils.
4 Asphalt may also be pulverized to produce a powdered asphalt. The
asphalt is crushed and passed through a series of fine mesh sieves to
ensure uniform size of the granules. Powered asphalt can be mixed with
road oil and aggregate for pavement construction. The heat and pressure
in the road slowly amalgamates the powder with the aggregate and binding
oil, and the
- 5 If the asphalt is to be used for a purpose other than paving, such as roofing, pipe coating, or as an undersealant or water-proofing material, the asphalt may be oxidized, or air blown. This process produces a material that softens at a higher temperature than paving asphalts. It may be air blown at the refinery, at an asphalt processing plant, or at a roofing material plant. The asphalt is heated to 500°F (260°C). Then air is bubbled through it for one to 4.5 hours. When cooled, the asphalt remains liquid.
Asphalt Paving Mixtures
Since asphalt cement is a major constituent used in road paving, the following is a brief
- 6 Asphalt cement and aggregate are combined in a mixing facility where they are heated, proportioned, and mixed to produce the desired paving mixture. Hot-mix facilities may be permanently located (also called "stationary" facilities), or it may be portable and moved from job to job. Hot-mix facilities may be classified as either a batch facility or a drum-mix facility, both can be either stationary or portable. Batch-type hot-mixing facilities use different size fractions of hot aggregate which are drawn in proportional amounts from storage bins to make up one batch for mixing. The combination of aggregates is dumped into a mixing chamber called a pugmill. The asphalt, which has also been weighed, is then thoroughly mixed with the aggregate in the pugmill. After mixing, the material is then emptied from the pugmill into trucks, storage silos, or surge bins. The drum-mixing process heats and blends the aggregate with asphalt all at the same time in the drum mixer.
- 7 When the mixing is complete, the hot-mix is then transported to the paving site and spread in a partially compacted layer to a uniform, even surface with a paving machine. While still hot, the paving mixture is further compacted by heavy rolling machines to produce a smooth pavement surface.
The quality of asphalt cement is affected by the inherent properties of the petroleum crude oil from which it was produced. Different oil fields and areas produce crude oils with very different characteristics. The refining method also affects the quality of the asphalt cement. For engineering and construction purposes, there are three important factors to consider: consistency, also called the viscosity or the degree of fluidity of asphalt at a particular temperature, purity, and safety.
The consistency or viscosity of asphalt cement varies with temperature, and asphalt is graded based on ranges of consistency at a standard temperature. Careless temperature and mixing control can cause more hardening damage to asphalt cement than many years of service on a roadway. A standardized viscosity or penetration test is commonly specified to measure paving asphalt consistency. Air-blown asphalts typically use a softening point test.
Purity of asphalt cement can be easily tested since it is composed almost entirely of bitumen, which is soluble in carbon disulfide. Refined asphalts are usually more than 99.5% soluble in carbon disulfide and any impurities that remain are inert. Because of the hazardous flammable nature of carbon disulfide, trichloroethylene (TCE), which is also an excellent solvent for asphalt cement, is used in the solubility purity tests.
Asphalt cement must be free of water or moisture as it leaves the refinery. However, transports loading the asphalt may have moisture present in their tanks. This can cause the asphalt to foam when it is heated above 212°F (100°C), which is a safety hazard. Specifications usually require that asphalts not foam at temperatures up to 347°F (175°C). Asphalt cement, if heated to a high enough temperature, will release fumes which will flash in the presence of a spark or open flame. The temperature at which this occurs is called the flashpoint, and is well above temperatures normally used in paving operations. Because of the possibility of asphalt foaming and to ensure an adequate margin of safety, the flashpoint of the asphalt is measured and controlled.
Another important engineering property of asphalt cement is its ductility, which is a measure of a material's ability to be pulled, drawn, or deformed. In asphalt cements, the presence or absence of ductility is usually more important than the actual degree of ductility because some asphalt cements with a high degree of ductility are also more temperature sensitive. Ductility is measured by an "extension" test, whereby a standard asphalt cement briquette molded under standard conditions and dimensions is pulled at a standard temperature (normally 77°F [25°C]) until it breaks under tension. The elongation at which the asphalt cement sample breaks is a measure of the ductility of the sample.
Environmental protection laws have developed stringent codes limiting water flows and particulate and smoke emissions from oil refineries and asphalt processing plants. Not only dust but sulfur dioxides, smoke, and many other emissions must be rigorously controlled. Electrostatic precipitators, primary dust collectors using single or multiple cone cyclones, and secondary collection units consisting of fabric filter collectors commonly called "baghouses" are all required equipment to control emissions. Hydrocarbons formed in asphalt production, if unchecked, create odoriferous fumes and pollutants which will stain and darken the air. Pollutants emitted from asphalt production are controlled by enclosures which capture the exhaust and then recirculate it through the heating process. This not only eliminates the pollution but also increases the heating efficiency of the process.
Higher costs of asphalt cement, stone, and sand have forced the industry to increase efficiencies and recycle old asphalt pavements. In asphalt pavement recycling, materials reclaimed from old pavements are reprocessed along with new materials. The three major categories of asphalt recycling are 1) hot-mix recycling, where reclaimed materials are combined with new materials in a central plant to produce hot-mix paving mixtures, 2) cold-mix recycling, where reclaimed materials are combined with new materials either onsite or at a central plant to produce cold-mix base materials, and 3) surface recycling, a process in which the old asphalt surface pavement is heated in place, scraped down or "scarified," remixed, relaid, and rolled. Organic asphalt recycling agents may also be added to help restore the aged asphalt to desired specifications.
Because of solvent evaporation and volatility, use of cutback asphalts, especially rapidcure cutback asphalts which use gasoline or naphtha, is becoming more restricted or prohibited while emulsified asphalts (in which only the water evaporates) are becoming more popular because of cost and environmental regulations.
Increasing economic and environmental needs will bring many new technical frefinements to recycling old asphalt pavements, such as using microwaves to completely break down the pavement. Microwaves heat the crushed rock in asphalt pavement faster than the surrounding cement, which is then warmed by the radiant heat from the rock. This method prevents the asphalt cement from burning.
Alternative sources of raw material are being researched, such as the production of synthetic asphalt from the liquefaction of sewage sludge. To ensure consistent product quality, new methods are being developed for manufacturing modified asphalts and emulsions. Many new tests are being developed to help characterize asphalts, such as high-performance gel-permeation chromatography (HP-GPC), which allows many properties to be studied and the results compiled in only a few minutes. New processes, more efficient mixing and milling units, in-line liquid mass flow meters, on-line monitoring systems, and new safety equipment are some other areas being investigated for improvement.
Polymer-modified asphalt crack sealers are gaining in popularity, and many other asphalt modifiers are being developed. Modifiers are added to control pavement rutting, cracking, asphalt oxidation, and water damage. Some commercially available asphalt modifiers are polymers, including elastomers, metal complexes, elemental sulfur, fibers, hydrated lime, Portland cement, silicones, various fillers, and organic anti-strip agents. Many of these modifiers have not been extensively used and are being researched for further development. It might even be possible one day to have "smart asphalt cements" by mixing in certain asphalt friction modifiers which would allow it to change characteristics depending on whether moisture was present. In conjunction with antilock brakes, automatic traction controls, and airbags, this could serve to save many lives on our nation's highways.
Where To Learn More
Asphalt Construction Handbook. 6th ed. Barber-Greene, A Division of Caterpillar Paving Products Inc., 1992.
Bitumens, Asphalts, and Tar Sands. Elsevier North-Holland, 1978.
"The Long and Recyclable Road." High Technology Business, September-October 1989, p. 4.
Planche, Jean-Pascal, Helen W. King, and Gayle N. King. "A Refiner's Approach to Asphalt Characterization." Nature, April 18, 1991, pp. 24-25.
"Physical and Chemical Characteristics of Synthetic Asphalt Produced from the Liquefaction of Sewage Sludge." U.S. Environmental Protection Agency, Municipal Environment Research Laboratory: Center for Environmental Research Information, 1982.
Asphalt Institute, Executive Office and Research Center. Research Park Drive, P.O. Box 14052, Lexington, KY 40512-4052. (606) 288-4960.
— Glenn G. Whiteside