Asphalt Roads Are Actually Not Paved With Asphalt

After winter subsides and snow melts, the first days of spring are punctuated by gaping holes in the black asphalt roads we travel every day. But those roads are not made of asphalt. According to J. Richard Willis, Ph.D., vice president for engineering, research and technology at the National Asphalt Pavement Association (NAPA), asphalt is the binding agent that glues the rocks together.

Asphalt comes from crude oil. It consists of compounds of hydrogen and carbon, with minor proportions of nitrogen, sulfur, and oxygen. It is the heaviest of materials in a barrel of oil. It’s basically the waste product—the heavy residue that settles to the bottom. It cannot be used for energy, so it takes on new life as the sticky stuff that holds materials together. Combined with various amounts and types of rocks and other substances, it eventually becomes the black surface on which we drive. The road is really an asphalt mixture, or, better termed, “asphalt pavement.”

Asphalt softens when heated and is elastic under certain conditions. The mechanical properties of asphalt are of little significance except when it is used as a binder or adhesive. Its principal application is, of course, in road surfacing, which may be accomplished in a variety of ways. Light oil “dust layer” treatments may be built up by repetition to form a hard surface, or a granular aggregate may be added to an asphalt coat, or earth materials from the road surface itself may be mixed with the asphalt.

Types of Asphalt

Natural asphalt (also called brea), is believed to be formed during an early stage in the breakdown of organic marine deposits into petroleum. It characteristically contains minerals whereas residual petroleum does not.

Gilsonite, wurzilite, and similar vein asphalts have special uses in heat-resistant enamels; they are hard and are mined like coal. Gilsonite occurs chiefly along the Colorado-Utah border.

Petroleum asphalt is produced in all consistencies from light road oils to heavy, high-viscosity industrial types.

Bitumen is dense, highly viscous, petroleum-based hydrocarbon that is found in deposits such as oil sands and pitch lakes (natural bitumen) or is obtained as a residue of the distillation of crude oil (refined bitumen). In some areas, particularly in the United States, bitumen is often called asphalt, though that name is almost universally used for the road-paving material made from a mixture of gravel, sand, and other fillers in a bituminous binder.

Bitumen is defined by the U.S. Geological Survey as an extra-heavy oil with an API gravity less than 10° and a viscosity greater than 10,000 centipoise (cP). At the temperatures normally encountered in natural deposits, bitumen will not flow. In order to be moved through a pipe, it must be heated, and, in some cases, diluted with a lighter oil. It owes its density and viscosity to its chemical composition—mainly large hydrocarbon molecules known as asphaltenes and resins, which are present in lighter oils but are highly concentrated in bitumen.

In addition, bitumen frequently has a high content of metals, such as nickel and vanadium, and nonmetallic inorganic elements, such as nitrogen, oxygen, and sulfur. Depending on the particular use of bitumen, these elements may be contaminants that must be removed from the finished product. By far, most refined bitumen is used in paving asphalt and roofing tiles, as is a large amount of natural bitumen.

The History of Asphalt

Asphalt may sound like a relatively modern invention, but the first recorded use of asphalt in a road was actually in Babylon in 615 B.C.E., where asphalt and burned brick were used to pave a procession street during the reign of King Nabopolassar, according to NAPA. The Romans used it to seal structures like baths and aqueducts. When English explorer Sir Walter Raleigh turned up at Pitch Lake in Trinidad in 1595, he used the asphalt for caulking his ships.

The use of asphalt also dates back to its use as a water stop between brick walls of a reservoir at Mohenjo-Daro (about the third Millennium BC) in Pakistan. In the Middle East, it was extensively used for paving roads and sealing waterworks—important applications, even today. Pitch Lake on the island of Trinidad was the first large commercial source, but natural sources have since declined in importance as petroleum has instead become the major source.

According to Willis, it’s been used in other non-road functions throughout history. Using it as a binder in roads became more common in the 1800s. John Loudon McAdam, who built the Scottish turnpike, added hot tar to reduce dust and maintenance on roads. This method also improved driving conditions.

In the United States, bituminous mixtures (asphalt concrete) first appeared in the 1860s, and the first “true asphalt pavement” was laid in Newark in 1870 by Edmund J. DeSmedt, a Belgian, according to NAPA. It was modeled after a natural pavement highway in France. DeSmedt then paved Washington, D.C.’s Pennsylvania Avenue with asphalt from Trinidad, further proving its durability.

Chemists and inventors soon filed patents for different blends of asphalt mixtures, which appeared under a variety of names. As the industry grew, cities began requiring warranties on workmanship and materials. Until the early 1900s, nearly all asphalt came from natural sources, but with the launch of the first modern asphalt facility in East Cambridge, Massachusetts, in 1901, and the increase in automobiles, requests for better roads sparked new discoveries, and by 1907, natural asphalt production was overtaken by refined petroleum asphalt.

As automobiles became more commonplace, people started demanding better modes of transportation. The roads where asphalt was used to keep the rocks together held up longer than conventional dirt roads. And driving on a gravel road versus one that was paved offered a significantly different experience. Today, of the more than 2.7 million miles (4.3 million kilometers) of paved roads in the U.S., 94% are surfaced with asphalt, according to NAPA.

Other important applications of asphalt include canal and reservoir linings, dam facings, and other harbor and sea works. Asphalt so used may be a thin, sprayed membrane, covered with earth for protection against weathering and mechanical damage, or thicker surfaces, often including riprap (crushed rock). Asphalt is also used for roofs, coatings, floor tilings, soundproofing, waterproofing and for other building-construction applications.