Asphalt
Authors: Speight James G. 
Published: July 2004

Asphalt is defined as a dark brown to black cementitious material in which the predominating constituents are bitumens that occur in nature or are obtained in petroleum processing. Asphalts or bituminous materials are further classified as solids, semisolids, or liquids. Asphalt characteristically contains very high molecular weight molecular polar species, called asphaltenes, which are soluble in carbon disulfide, pyridine, aromatic hydrocarbons, chlorinated hydrocarbons, and tetrahydrofuran. Naturally occurring materials include native asphalt (bitumen), lake asphalt, and rock asphalt. Asphalt (bitumen) also occurs in various oil sand (also called tar sand) deposits which occur widely scattered through the world, and the bitumen is available by means of various extraction technologies. Petroleum asphalts derive their characteristics from the nature of their crude origins with some variation possible by choice of manufacturing process. In crude‐oil refining, the crude oil at 340°C is injected into a fractionating column. The residuum is straight‐run (straight‐reduced) asphalt. Asphalt is also a product of the propane deasphalting and fractionation process, air‐blowing (an exothermic process) a cracking (thermal decomposition) process, grades by blending (proportioning) the extreme, and emulsion processes. Determination of the components of asphalts has always presented a challenge because of the complexity and high molecular weights of the constituents. The methods employed include fractionation by precipitation and distillation, separation by chromatographic techniques, chemical analysis using spectrophotometric techniques, and molecular weight analysis. The principal outcome of many of the composition studies has been the delineation of the asphalt system as a colloidal system at ambient or normal service conditions. Asphalt is not composed of a single chemical species, but is rather a complex mixture of organic molecules that vary widely in composition from nonpolar, saturated hydrocarbons to highly polar and condensed aromatic ring systems. Most molecules contain one or more of the heteroatoms nitrogen, sulfur, and oxygen. Because the heteroatoms often impart functionality and polarity to the molecules, their presence may make a disproportionately large contribution to the differences in physical properties among asphalts from different sources. Generally, most asphalts are 79–88 wt % C, 7–13 wt % H, trace–8 wt % S, 2–8 wt % O, and trace–3 wt % N. The molecular weights of the individual fractions of asphalt have been considered to be of importance. Molecular weight data are used as an indication of the intermolecular interactions in the asphalt, which can confer property differences in asphalts. Asphalt is a viscoelastic material whose rheological properties reflect crude type and, to a lesser extent, processing. The ability of the asphalt to perform under many conditions depends on flow behavior. The term “durable” is used to describe an asphalt that possesses the necessary chemical and physical properties required for the specified pavement performance, being resistant to change during the in‐service conditions that are prevalent during the life of the pavement. One particularly valuable property is the ability of the asphalt to undergo movement without fracture. Failure of an asphalt film is reflected in the appearance of typical crack patterns because of a decrease in the plasticity of the asphalt through insufficient lower molecular weight constituents remaining in the continuous phase. Mineral fillers are often added to asphalts to influence their flow properties, reduce costs, and are commonly used as stabilizers in roofing coatings. Mineral‐filled films show improved resistance to flow at elevated temperatures, improved impact resistance, and better flame‐spread resistance. Fillers commonly used are ground limestone, slate flours, finely divided silicas, trap rocks, and mica; they are often produced as by‐products in rock‐crushing operations. Most tests applied to petroleum asphalts are empirical in nature. The ASTM tests are not the only ones applied to asphalt testing. Private tests of a proprietary nature have been used within companies. Some of these tests are now being made public and may become a part of the ASTM standards. There are a multitude of uses for asphalt, including hydraulics (dam facings, canal linings, pond linings), recreation (substrate for artificial surfaces, tennis courts, running tracks), agriculture (mulches, underground water barriers, stockyard paving), transportation (railroad ballast treatment and roadbeds), and metals (ore leaching pads, ore and coal briquetting, etc). The largest use of straight‐run asphalts is in the paving industry where they serve primarily as binders in paving mixes and as bases in liquid asphalts. Air‐blown asphalts can be used in prepared roofings. Thermal asphalt products are especially useful as binders and as saturants for fiberboards, and as sizings for fiberboard. Liquid asphalt products comprise cutback asphalts and emulsions. The heavier grades are used for mix‐in‐place road asphalts. A variety of materials have been proposed to modify the properties of asphaltic binders to enhance the properties of the mix, including fillers and fibers to reinforce the asphalt–aggregate mixture. The properties of the aggregate also play a significant role in determining the ultimate properties of the asphalt–aggregate mix, factors to consider include the functionalities in the asphalt, absorption of the asphalt into the pores of the aggregate, the aging properties of the asphalt when it is on the aggregate, as well as methods for coating the asphalt on to the aggregate. The word asphalt has been carelessly used. It is essential to differentiate asphalt from materials, especially coal tar and derivatives, which contain known carcinogens and health hazards. For this reason, the use of cracked asphalts must be treated with caution.