Asphalt is a dark brown-to-black cement-like material obtained by petroleum refining and containing bitumens as the predominant component. Bitumen is a generic term for natural or manufactured black or dark-colored solid, semisolid, or viscous cementitious materials that are composed mainly of high-molecular weight hydrocarbons. The term includes tars and pitches derived from coal. Asphalt is used primarily for road construction and roofing materials due to its remarkable waterproofing and binding properties. The hard surfaces of roads, for example, depend on the ability of asphalt to cement together aggregates of stone and sand.
The first recorded use of asphalt as a road building material was in Babylon around 625 B.C., in the reign of King Naboppolassar. In "A Century of Progress: The History of Hot Mix Asphalt", author Hugh Gillespie notes that “an inscription on a brick records the paving of Procession Street in Babylon, which led from his palace to the north wall of the city, ‘with asphalt and burned brick.’”
Properties of asphalt
Asphalts are highly complex and not well-characterized materials containing saturated and unsaturated aliphatic and aromatic compounds with up to 150 carbon atoms. Their composition varies depending on the source of crude oil. Many of the compounds contain oxygen, nitrogen, sulfur, and other heteroatoms. Asphalt typically contains about 80% by weight of carbon; around 10% hydrogen; up to 6% sulfur; small amounts of oxygen and nitrogen; and trace amounts of metals such as iron, nickel, and vanadium. The molecular weights of the constituent compounds range from several hundred to many thousands.
The compounds are classified as asphaltenes or maltenes according to their solubility in hexane or heptane. Asphaltenes are high-molecular weight species that are insoluble in these solvents, whereas maltenes have lower molecular weights and are soluble. Asphalts normally contain between 5 and 25% by weight of asphaltenes and may be regarded as colloids of asphaltene micelles dispersed in maltenes.
Many of the compounds in asphalt are polar since they contain alcohol, carboxyl, phenolic, amine, thiol, and other functional groups. As a result of this polarity, the molecules self-assemble to form multimolecular clusters with molecular weights up to 100,000. The adhesion of asphalt to aggregate is also thought to depend on the polar attraction between molecules in asphalt and the polar surfaces of aggregates.
Production of asphalt
Asphalt is a portion of the residual fraction that remains after primary distillation operations. It is further processed to impart characteristics required by its final use. In vacuum distillation, generally used to produce road-tar asphalt, the residual is heated to about 750° F and charged to a column where vacuum is applied to prevent cracking.
Asphalt for roofing materials is produced by air blowing. Residual is heated in a pipe still almost to its flash point and charged to a blowing tower where hot air is injected for a predetermined time. The dehydrogenization of the asphalt forms hydrogen sulfide, and the oxidation creates sulfur dioxide. Steam, used to blanket the top of the tower to entrain the various contaminants, is then passed through a scrubber to condense the hydrocarbons.
A third process used to produce asphalt is solvent deasphalting. In this extraction process, which uses propane (or hexane) as a solvent, heavy oil fractions are separated to produce heavy lubricating oil, catalytic cracking feedstock, and asphalt. Feedstock and liquid propane are pumped to an extraction tower at precisely controlled mixtures, temperatures (150°-250° F), and pressures of 350-600 psi. Separation occurs in a rotating disc contactor, based on differences in solubility. The products are then evaporated and steam stripped to recover the propane, which is recycled. Deasphalting also removes some sulfur and nitrogen compounds, metals, carbon residues, and paraffins from the feedstock.
- The National Asphalt Pavement Association. History of Asphalt.
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