When you hear the word asphalt, you probably imagine the black tar stuff on roads and highways, right? But that's not exactly correct.
"Asphalt is the liquid that is in the road," says J. Richard Willis, Ph.D., vice president for engineering, research and technology at the National Asphalt Pavement Association (NAPA), who has a doctorate in asphalt design and construction. "It's the binding agent that kind of holds the rocks together." Asphalt comes from crude oil, while tar comes from coal.
Asphalt also is found naturally in the earth, and there are lakes of it where oil from underground has risen to the surface, like the La Brea Tar Pits in Los Angeles and Pitch Lake in Trinidad, which is the largest natural deposit of asphalt in the world.
But the most common way the binder is made today is through the oil refining process. Asphalt is the heaviest of materials in a barrel of oil; it's basically the waste product.
"Asphalt is the heavy residue that settles to the bottom," Willis says. 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 mixture we drive on. The road is really an asphalt mixture or better termed "asphalt pavement."
The History of Asphalt
All the talk of oil refining may make asphalt sound like a relatively modern invention, but the first recorded use of asphalt in a road was in Babylon in 615 B.C.E.; asphalt and burned brick were used to pave a procession street during the reign of King Nabopolassar, according to the 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.
"It's been used in other non-road functions throughout history," Willis says. 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.
Enterprising 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 invigorated the asphalt industry. By 1907, natural asphalt production was overtaken by refined petroleum asphalt.
"People started demanding better modes of transportation," Willis explains. "The roads where people started using the asphalt to keep the rocks together held up longer than the conventional dirt road that people were used to." Driving on a gravel road versus one that was paved offered a significantly different experience. Finally, the 1956 Federal-Aid Highway Act helped transform the roads in the United States still made of packed dirt and created the 48,876-mile (78,658-kilometer) Interstate System in the U.S.
How Asphalt Is Used Today
Although it's most often associated with roads, asphalt is used for many purposes, though roads account for its most extensive use. Of the more than 2.7 million miles (4.3 million kilometers) of paved roads in the U.S., 94 percent are surfaced with asphalt, according to NAPA.
Interestingly, though, all of that includes a mixture of about 95 percent stone, sand and gravel, and just 5 percent asphalt cement. Asphalt also is used for parking lots, airport runways and racetracks.
"Asphalt is a really flexible and versatile product," explains Willis. It can be used to line fishponds and water reservoirs or for sporting purposes like tennis courts. A couple of years ago, it was chosen as the base surface for the field at the Minnesota Vikings stadium in Minneapolis.
Since the early days of asphalt production, the industry has continued to innovate new products, becoming more scientific and rigorous, according to Willis.
"We've changed the way we engineer the mixes," he says. "We're at an era today where you are seeing a giant shift in how the industry and how states work." Using advanced testing methods, asphalt researchers have been aiming to improve performance. Incorporating new materials, additives and technologies, they are seeking to learn how various recipes will perform in different temperatures and climates.
One major update has been the creation of warm-mix asphalt (WMA), which reduces the production temperature of asphalt at a plant, thereby reducing energy usage and saving time in both production and road surfacing. WMA also improves working conditions with lowered exposure to fuel emissions, fumes and odors, according to the U.S. Department of Transportation Federal Highway Administration. WMA is technology that did not exist in U.S. in 2002 and now accounts for about 40 percent of the market, says Willis.
Criticisms of Asphalt
Asphalt probably isn't something you think of as ecofriendly; it could be partly guilt by association because asphalt is naturally aligned with major polluters — driving automobiles and oil production. And some of the negativity is warranted: Because asphalt has low reflectivity, it has been determined to be a significant contributor to the urban heat island (UHI) effect, Abbas Mohajerani, Jason Bakaric and Tristan Jeffrey-Bailey wrote in a 2017 article in the "Journal of Environmental Management." Anyone who has sat in a highway traffic jam on a hot summer day can attest to that.
As far as asphalt's contributions to the UHI, the Environmental Protection Agency states that conventional asphalt pavements can be modified with materials or treated after installation to raise reflectance. For decades, this has been sometimes implemented on surfaces like parking lots and highways. The EPA includes porous asphalt and rubberized asphalt as examples of permeable pavements.
Asphalt has also earned bad marks for being impermeable, for the gases it produces when melted and the fumes it exposes workers to during paving and roofing. Occupational Safety and Health Administration (OSHA) says those fumes can lead to headache, skin rash, fatigue and even skin cancer. While OSHA's standards do not specifically address asphalt fumes, the administration recommends that controlling exposure can be done through "engineering controls, administrative actions and personal protective equipment."
And of course, there's still the fact that asphalt is made from petroleum. But asphalt does have positive eco-qualities too.
100 Percent Recycling Efforts
"What a lot of people don't know is all of the environmentally friendly things the asphalt industry is actually doing," Willis says. For starters, asphalt is 100 percent recyclable, and more importantly, it actually does get recycled. In 2018, 82.2 million tons (74.5 million metric tons) of Reclaimed Asphalt Pavement (RAP) was put back into new mixes. That means every asphalt mix put down in the U.S. included about 21 percent RAP. In fact, the combined weight of all items people recycle annually in the U.S. — paper, plastic and aluminum — totaled a fraction of (about 68 percent) of the weight of RAP the asphalt industry recycles annually.
"That's just one material we recycle," says Willis. "We are the most active recycling industry in the country." It is also one of the biggest recyclers of tire rubber, which is used as a modifier for mixtures in some states. Roof shingles also are recycled into new asphalt mixtures, and the industry is looking into how plastic might become part of the discussion. "When people bring those questions to us, we try to find solutions."
A lot of engineering and material science goes into constructing a road. Today, asphalt roads are designed around the concept of "perpetual pavement," or at least to last 40 years or more. Routine maintenance consists of "milling" the surface — taking off the top inch or so — every 12 to 20 years and replacing it with a new overlay. That top inch can be recycled, and the periodic overlays "significantly improve the ride quality and fuel consumption of vehicles traveling on these roads," according to the Asphalt Pavement Alliance.
Asphalt of the Future
Until it's time for hover cars, asphalt roads are likely to stick around. And the industry plans to keep innovating in product and production. Willis describes recent breakthroughs like autonomous rollers and equipment, as well as the increased use of virtual reality for training.
"I see technology as a big part of the industry's future," he says. As asphalt experts get better at handling big data, they can use it for production and placement to improve efficiencies in real time. One day, he could even see intelligent pavements with nano-sensors in the roads providing feedback on how the pavement is behaving and lasting. "Our roads are going to get a lot smarter. We've got the technology to really improve the experience of the riding public."