Asphalt

Where Found

Before the growth of the industry in the early twentieth century, when asphalt began to be mass-produced, naturally occurring asphalt was found in pools of petroleum-heavy deposits, most notably Trinidad’s 46-hectare Pitch Lake (off the coast of Venezuela), the 4,000-hectare Bermudez Lake in Venezuela, and areas around the Dead Sea basin in Israel and Jordan.

Primary Uses

Asphalt is most associated with road-surface paving. Apart from highway construction and repairs, asphalt is used for airport runways, running tracks, and driveways. In addition, asphalt is a waterproofing agent in fabrics, irrigation systems, roofing shingles, jetties and sea walls designed to combat beach erosion, and insulation. More than 60 billion metric tons of manufactured asphalt are used annually in the United States alone.

Technical Definition

Asphalt is a by-product of petroleum refining, either occurring naturally over geologic eras or processed industrially in the controlled refining process called fractional distillation that occurs once naphtha, gasoline, and kerosene have all been extracted from the crude. It is difficult to define a single chemical profile because asphalt varies depending on the grade of crude petroleum and the type of refining production used. Generally, asphalts contain saturated and unsaturated aliphatic compounds (organic compounds whose carbon atoms configure in open branches rather than closed rings) and aromatic compounds (organic compounds whose carbon atoms configure in closed rings) with up to 150 carbon atoms. Asphalt contains about 80 percent carbon, 10 percent hydrogen, and 6 percent sulfur with trace amounts of oxygen and nitrogen. Molecular weight varies. These compounds are further classified by their solubility—more insoluble compounds are called asphaltenes and less soluble compounds maltenes.

Description, Distribution, and Forms

Asphalt is a dark (brown or black), cementitious (highly adhesive) material. Its predominant elements are bitumens, solid and/or semisolid high-molecular-weight hydrocarbons. Because more than 80 percent of commercially produced asphalt is used for paving, the properties of asphalt are graded according to its performance as a pavement adhesive, particularly how well it holds up over time, specifically in the areas of aging, stiffening, and cracking.

Only 5 percent of the asphalt used today is naturally occurring. Its most prominent naturally occurring site is Pitch Lake in the petroleum-rich southwest corner of the island of Trinidad, near the town of La Brea. Although difficult to determine dimensions with any accuracy because it grows and shrinks, this “lake”—it is more accurately a sludgy 40-plus-hectare deposit created by a slow underground seepage of asphalt, or pitch—is estimated to be more than 35 meters across and 107 meters deep at its deepest point (the edges of the lake are crusty and support weight, but toward the center of the lake, the pitch becomes characteristically sticky and far more hazardous, easily swallowing entire objects). Given its dimensions, the lake has provided an apparently inexhaustible supply of asphalt for centuries—large holes left by removing asphalt are quickly filled in as the lake maintains a kind of fluid dynamic. Western explorers were fascinated by the phenomena of this vast tar lake, not merely for the usefulness of the asphalt (they immediately used it to upgrade the waterproofing on their ships) but because the lake also preserved the remains of prehistoric mammals and birds.

Commercially produced asphalt is primarily a taffylike binder known as asphaltic concrete. Asphaltic concrete, a tacky, ductile resin suspended in an oily medium, maintains the integrity of particles. Because asphaltic concrete is highly sticky, it must be heated into liquid to be used for pavement construction. It is most often sprayed on a graded roadbed. It is then compacted into the proper density, usually 25 to 30 centimeters. Because it solidifies into a tough and flexible surface able to maintain its integrity against both the weight and frequency of traffic and the deformations from conditions (notably ice and extreme heat), asphalt remains the primary roadway surface. Given the diminishing supply of and concerns over the depletion of that resource, asphaltic concrete is attractive because more than 80 percent of removed asphalt can be recycled for new road projects.

History

Asphalt was used as early at 3600 BCE by the Sumerians as a mortar adhesive for paving and building and as a waterproofing agent in canal construction and public pools (records indicate it also was used for medical treatments, statuary enhancement, and mummification throughout the Middle East). Pitch is mentioned early in the Bible: It binds Noah’s ark and later makes waterproof the bulrush basket in which the baby Moses is set adrift on the Nile.

Following the discovery of huge natural deposits of pitch in South America in the fifteenth and sixteenth centuries (notably by Sir Walter Ralegh), asphalt became a commercially viable waterproofing agent for the burgeoning shipbuilding industry. Not until 1870, however, was asphalt first tested for paving. The idea came from accounts discovered among Incan archives dating back to the twelfth century. A stretch of streets in Newark, New Jersey, was made of Trinidad asphalt. Its appeal was evident—it was cheap to import, easy to apply, flexible, and once hardened made for a smooth ride. Within six years, asphalt was selected for a most ambitious—and prestigious—paving project: Pennsylvania Avenue in Washington, DC. By the turn of the century, more than 35 million square meters of streets in America were asphalt. With the rise of the oil industry, however, commercially produced asphalt became more profitable.

Obtaining Asphalt

Asphalt may be obtained directly from sludgy beds near petroleum fields. However, far more asphalt is obtained as a small fraction of residue distilled during the refining process of crude oil. Heavy crude oil is heated to nearly 370° Celsius in large furnaces. As the crude is processed, lighter components vaporize and are released into the through refinery towers (in the early 1980s, facing pressure from environmental groups, the petroleum industry overhauled the distillation towers to minimize pollution). To render the remaining asphalt into asphaltic cement, the residue is distilled further through a vacuum process designed to prevent the residue from cracking, and then it is mixed with the appropriate aggregate materials, most commonly crushed rock, slag, sand, and/or cement stone. In turn, the asphalt must be stored (and transported) at a constant 65.55° Celsius to ensure liquidity. If the asphalt must be transported a considerable way, kerosene or diesel oil can be temporarily added and then separated before the asphalt is applied to the roadbed.

Uses of Asphalt

Long before the automobile made highway construction a pressing concern, as thousands of kilometers of roadway needed to be laid quickly and economically, asphalt was valued because it was waterproof. It revolutionized the shipping industry because of its caulking ability. Even in modern society, asphalt helps solidify sea walls to protect against the effects of tides, waves, and harsh weather. In addition, asphalt lines the retaining tanks in industrial fish hatcheries, preserves the integrity of irrigation systems (before asphalt, up to one-third of water transported would be lost in transit), and maintains holdings by providing underlining for reservoirs. Furthermore, asphalt has become a primary liner for the disposal of hazardous waste and a liner at landfill sites.

In addition, because it is waterproof and fire retardant, asphalt can be used, when combined with felt and mineral granules, to produce the familiar rectangular roofing shingles. This asphalt is slightly harder than paving asphalt (it is typically heated at a much higher temperature to make it less flexible). Asphalt shingles are remarkably adaptable to a variety of roofing needs and styles; shingles can be adjusted for the slope of the roof, conditions, and even house design.

However, asphalt is best known as a paving agent. It is durable, tough, and flexible enough to provide a comfortable riding surface. Industry standards measure durability of standard asphalt roadways at fifteen to twenty years. That longevity is most often compromised by cracks from water seeping into the surface during the winter, freezing, and then cracking the upper layers or by ruts that appear during hot summers when the asphalt softens. However, asphalt provides for relatively easy repairs. The durability of asphalt, particularly its capacity to hold up under constant traffic and enormous weights, has made it the primary coating for airplane runways. For much the same reason, asphalt is also used for railroad beds and even subway system track beds. Finally, asphalt is used for smaller, high-volume traffic surfaces such as walking trails, tennis courts, biking paths, running tracks, basketball courts, golf-cart paths, and playgrounds. Users of these recreational surfaces appreciate the give in asphalt, as compared to concrete surfaces. In 1986, the National Center for Asphalt Technology (NCAT) was established at Auburn University to develop asphalt production quality.

Bibliography

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