What Is Asphalt Composed Of?

When you think of asphalt, you're likely envisioning the dark tar-like substance on streets and highways, right? However, that's not entirely accurate.

"Asphalt is the liquid that makes up part of the road," explains J. Richard Willis, Ph.D., Vice President of Engineering, Research, and Technology at the National Asphalt Pavement Association (NAPA). With a doctorate in asphalt design and construction, he states, "It acts as the adhesive that holds the stones in place."

So, what is asphalt made of? Asphalt is derived from crude oil, while tar originates from coal.

The Road Is Asphalt Pavement

Asphalt is also found naturally in the earth, with places like the La Brea Tar Pits in Los Angeles and Pitch Lake in Trinidad, the world's largest natural asphalt deposit, where oil from beneath the ground rises to the surface.

However, the most typical method of creating asphalt today involves the oil refining process. Asphalt is the heaviest component in a barrel of oil and is essentially the waste left over.

"Asphalt is the dense residue that settles at the bottom," says Willis. Unable to be used as an energy source, it transforms into the sticky substance that binds materials. Combined with various types of rocks and other elements, it forms the mixture we use for our roadways. The result is what we call an asphalt mixture, or more accurately, "asphalt pavement."

The History of Asphalt

While the concept of oil refining may suggest that asphalt is a modern invention, its first known use in road construction dates back to Babylon in 615 B.C.E. According to the NAPA, asphalt and burnt brick were used to create a ceremonial street during the reign of King Nabopolassar. The Romans later employed it to seal structures such as baths and aqueducts. When English explorer Sir Walter Raleigh arrived at Pitch Lake in Trinidad in 1595, he used asphalt for ship caulking.

"Asphalt has served many purposes throughout history," says Willis. It became more commonly used as a road binder in the 1800s. John Loudon McAdam, who built the Scottish turnpike, added hot tar to reduce road dust and maintenance while also improving 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 Belgian Edmund J. DeSmedt, according to NAPA. It was based on a natural pavement highway in France. DeSmedt then paved Washington, D.C.'s Pennsylvania Avenue using asphalt from Trinidad, demonstrating its long-lasting quality.

Inventive chemists and engineers quickly patented various asphalt pavement mixture formulas, each with a different name. As the industry expanded, cities began mandating warranties on both materials and workmanship. Until the early 1900s, nearly all asphalt was sourced naturally. However, with the first modern asphalt plant opening in East Cambridge, Massachusetts, in 1901 and the rise of automobiles, the demand for improved roads fueled the asphalt industry's growth. By 1907, refined petroleum asphalt had surpassed natural asphalt production.

"People started demanding better transportation options," explains Willis. "The roads that used asphalt to bind the rocks together outlasted the traditional dirt roads." Driving on a gravel road versus one with a smooth asphalt surface created a distinctly different experience. Ultimately, the 1956 Federal-Aid Highway Act helped modernize U.S. roads that were still made of packed dirt, leading to the creation of the 48,876-mile (78,658-kilometer) Interstate System.

How Asphalt Cement Is Used Today

Although asphalt is most commonly linked to roadways, it has numerous other applications, with roads being its largest use. Of the more than 2.7 million miles (4.3 million kilometers) of paved roads in the U.S., 94 percent are asphalt-covered, as reported by NAPA.

Interestingly, this mixture consists of approximately 95 percent stone, sand, and gravel, with only 5 percent asphalt cement. Asphalt is also utilized for parking lots, airport runways, and racetracks.

"Asphalt is an incredibly adaptable and versatile material," says Willis. It can be used to line fish ponds and water reservoirs, or even for recreational spaces like tennis courts. Just a few years ago, it was chosen as the base surface for the field at the Minnesota Vikings stadium in Minneapolis.

Since the inception of asphalt production, the industry has constantly innovated, becoming more methodical and scientific, according to Willis.

"We've revolutionized how we engineer the mixes," he states. "We're in a time where there’s a massive shift in the way the industry and states operate." With the use of advanced testing methods, asphalt researchers are focused on enhancing performance. By incorporating new materials, additives, and technologies, they're exploring how different asphalt formulas will perform across various temperatures and climates.

A significant development has been the creation of warm-mix asphalt (WMA), which lowers the production temperature of asphalt at plants, thereby saving energy and reducing production and road surfacing time. WMA also improves working conditions by decreasing exposure to fuel emissions, fumes, and odors, as reported by the U.S. Department of Transportation Federal Highway Administration.

Criticisms of Asphalt and Modifying Asphalt Pavements

Asphalt may not immediately strike you as an eco-friendly material. Its association with major polluting industries like automobiles and oil production certainly contributes to its negative image. Some of the criticism is justified: asphalt's low reflectivity has been identified as a key factor in the urban heat island (UHI) effect, as discussed in a 2017 study published in the "Journal of Environmental Management" by Abbas Mohajerani, Jason Bakaric, and Tristan Jeffrey-Bailey. Anyone who has spent time in a traffic jam on a hot summer day can attest to this phenomenon.

Regarding asphalt's impact on the UHI, the Environmental Protection Agency highlights that conventional asphalt pavements can be enhanced by adding materials or treated after installation to increase their reflectivity. This method has been applied for years, particularly on surfaces such as highways and parking lots. The EPA also cites porous and rubberized asphalt as examples of permeable pavement solutions.

Asphalt has faced criticism for being impermeable, as well as for releasing harmful gases when heated and exposing workers to toxic fumes during road and roof paving. According to the Occupational Safety and Health Administration (OSHA), these fumes can cause issues like headaches, skin rashes, fatigue, and even skin cancer. Although OSHA does not have specific regulations on asphalt fumes, it recommends controlling exposure through "engineering controls, administrative actions, and personal protective equipment."

Of course, it's important to remember that asphalt is derived from petroleum. Nonetheless, there are also environmentally beneficial aspects of asphalt production.

100 Percent Recycling Efforts

"What many people don't realize is how environmentally responsible the asphalt industry actually is," says Willis. To begin with, asphalt is 100 percent recyclable, and it is regularly recycled. In 2018, 82.2 million tons (74.5 million metric tons) of Reclaimed Asphalt Pavement (RAP) were incorporated into new mixes. This means that roughly 21 percent of every asphalt mix laid in the U.S. contained RAP. In fact, the total weight of all recyclable items in the U.S. — including paper, plastic, and aluminum — was just about 68 percent of the RAP the asphalt industry recycles annually.

"That's just one material we recycle," Willis explains. "We are the country's leading recycling industry." Asphalt is also one of the top recyclers of tire rubber, which is incorporated as a modifier in some state mixtures. Additionally, roof shingles are recycled into new asphalt mixes, and the industry is exploring how plastic could be included in this process. "When these questions are raised, we work to find solutions."

Building a road involves a significant amount of engineering and material science. Today's asphalt roads are often designed with the idea of creating a "perpetual pavement" — aiming for a lifespan of 40 years or more. Regular maintenance involves "milling" the asphalt surface, removing the top layer of about an inch every 12 to 20 years, and replacing it with a fresh overlay. This top layer can be recycled, and these periodic overlays "greatly enhance ride quality and fuel efficiency for vehicles driving on these roads," according to the Asphalt Pavement Alliance.

Asphalt of the Future

As we wait for the advent of hover cars, asphalt roads are expected to remain in place. The industry is continuing to innovate in both products and processes. Willis highlights recent advancements such as autonomous rollers and equipment, alongside the growing use of virtual reality for training purposes.

"I believe technology will play a significant role in the future of the industry," he says. As asphalt professionals become more adept at utilizing big data, they can apply it to production and placement in real time, improving efficiencies. In the future, he envisions intelligent pavements with nano-sensors embedded in the roads, providing live data on the pavement's performance and durability. "Our roads will become much smarter. We have the technology to greatly enhance the driving experience."