Asphalt covers more than 90 percent of the 2.7 million miles of paved roads in the United States.
It’s a durable yet flexible material, creating smooth pavement for our traveling convenience. But the steady pounding of semi trucks, the yearly squeeze and release of the freezing and thawing cycles, the heat of the sun’s rays, and the effects of constant oxidation eventually win out.
Asphalt will break down—and when it does, we rip up our roads and start over.
But the asphalt rubble we see piled alongside the highway during road construction season isn’t simply junk to be discarded. With a little rehabilitation, it can regain its flexibility and thus, its usefulness—and thanks to environmental, logistical and financial motivations, there’s growing interest in putting more recycled asphalt back on the road.
“This is the new frontier,” says Hussain Bahia, a Vilas Distinguished Professor of civil and environmental engineering at the University of Wisconsin-Madison and an internationally sought-after expert on asphalt.
Reusable, in theory
Humans have used asphalt since ancient times, hence the rug on the floor of Bahia’s Engineering Hall office depicting the Babylonian king Hammurabi, who oversaw expansive building projects and included construction laws in his famous Code of Hammurabi. The French began using it on roads in the first half of the 19th century, and the method made its way to America in the late 1800s.
Liquid asphalt—a bottom-of-the-barrel byproduct of crude oil refining—binds together rocks or sand (referred to as “aggregate”), providing road builders with a quicker-drying and cheaper alternative to concrete. It’s billed as a reusable material.
“The asphalt industry remains the country’s most diligent recycler, with more than 99 percent of reclaimed asphalt pavement being put back to use,” the National Asphalt Pavement Association states in the 2017 report on its annual recycled materials and warm-mix asphalt usage industry survey.
Yet the results of that same survey put the average percentage of reclaimed asphalt pavement in new asphalt mixtures nationwide at 20.1—meaning the overwhelming majority of the material that’s laid down on roads is still newly refined liquid asphalt and mined aggregate. Meanwhile, there were more than 102 million tons of reclaimed asphalt pavement stockpiled across the country at the end of 2017, according to the report.
“It’s not only the asphalt; we cannot continue to mine these aggregates from our mountains. It’s just not sustainable,” says Bahia. “Asphalt is very expensive, relatively speaking, so we can reuse it, and more importantly, these rocks are very good rocks. Recycle the good rocks—don’t throw them away. Recycling is becoming almost a necessity because we want to reduce cost.”
With that in mind, Bahia’s lab, the Modified Asphalt Research Center, recently began an 18-month project with the Recycled Materials Resource Center in the College of Engineering to performance-test asphalt mixes using 30- and 50-percent recycled content. By defining the properties that affect performance and establishing more effective testing methods, his group hopes to pave the way for wider adoption of asphalt containing higher levels of recycled content.
Green and growing
One of Bahia’s former PhD students, Nima Roohi Sefidmazgi (MSCE ’11, PhDCE ’13), wants to go even further. He’s vice president of Green Asphalt, a New York-based company that’s one of the only in the country producing 100-percent recycled asphalt.
“On the scale we’re doing it in New York City, no other plant is doing it in the whole world,” Roohi Sefidmazgi says of the company’s Long Island City plant, which was featured in The Wall Street Journal in October 2018.
To produce a 100-percent recycled mix, Green Asphalt tweaked the typical process, heating the material indirectly, using a different type of filtering system for fumes from the heated asphalt, and treating the melted blend with a soy-based oil to recover its flexibility. The company tested an array of rejuvenating oils, including one derived from waste food oil that workers loved because it smelled like McDonald’s French fries.
The hardest part, though, was securing regulatory approval, which was Roohi Sefidmazgi’s main charge when he joined Green Asphalt in 2014. He worked with the New York City and New York departments of transportation to develop specifications for the production process and final product, backed by performance data from on-the-road testing.
New York City approved Green Asphalt’s mix for use in 2015, thus becoming the first major city in the country to green-light 100-percent recycled asphalt. In doing so, the city also found a convenient outlet for some of its milled asphalt.
Now, Green Asphalt hopes to make use of its hard-won technical and bureaucratic knowledge by partnering with companies around the world. Roohi Sefidmazgi says the company’s vision is that every road around the world will use 100-percent recycled asphalt by 2038.
The potential environmental and financial impacts are tantalizing. According to Roohi Sefidmazgi, using 1 million tons of 100-percent recycled asphalt instead of a 30-percent recycled product would result in a carbon footprint savings equivalent to roughly 16,000 houses’ worth of electricity per year. (The National Asphalt Pavement Association estimates the United States used 379.4 million tons of asphalt in 2017.)
And since Green Asphalt sells its mix for about 30-percent cheaper than an average 30-percent recycled mix, Roohi Sefidmazgi says, states could save millions of dollars.
“I think we’re very close to a tipping point,” he says.
The path forward
Still, significant roadblocks remain for fully recycled asphalt. State departments of transportation aren’t eager to risk pavement performance without more proof; without agency interest, companies have little motivation to spend money overhauling their production processes and testing new mixes.
“There’s kind of a constant negotiation between state agencies and contractors,” says Amir Golalipour (MSCE ’11, PhDCE ’13), a project manager consultant with the Federal Highway Administration.
He says at this point the agency is promoting different sustainable practices, such as using recycled tires in asphalt and warm-mix asphalt, which is produced at lower temperatures. Golalipour believes pavement with 100-percent recycled asphalt needs more performance evaluation. He says the Federal Highway Administration has focused on performance-driven specifications to ensure pavement longevity and to provide flexibility in materials selection.
Projects like Bahia’s could supply the data to help nudge state agencies toward approving mixes with higher levels of recycled content. He and his students are testing different rejuvenating oils, as well as methods of adding them to aged asphalt.
The end goal: Provide agencies and industry players with tests and standards that ensure recycled mixes perform as well as or better than conventional mixes.
As for acceptance? That will require movement on both sides, Bahia says. Agencies will need to adapt their thinking from controlling the components—something that’s harder to do for recycled mixes—to focusing on performance properties. Companies will need to buy equipment for new tests and train their workers.
“These machines cost more,” says Bahia. “But in the bigger picture, the costs of this equipment and the training are really very, very small compared to the cost of not recycling.”
Author: Tom Ziemer