The road for recycled plastic in asphalt paving materials

Throughout his career, Bill Buttlar actively has found ways to use different types of recycled materials in asphalt.

“Asphalt by tonnage is one of the most recycled materials on earth,” Buttlar says, a professor and Glen Barton chair in Flexible Pavement Technology in the Department of Civil and Environmental Engineering at the University of Missouri in Columbia, Missouri.

Buttlar says he has studied the use of recycled aggregate, roofing shingles and ground tire rubber in asphalt pavement and, in more recent years, has researched the use of recycled linear low-density polyethylene (LLDPE) in asphalt. The University of Missouri partnered with the Missouri Department of Transportation (MoDOT) about three years ago to test mixing LLDPE into asphalt pavement for possible use on roads and bridges.

“What started me down the road of waste plastic recycling and asphalt was that one of our alumni at the University of Missouri is Jim Fitterling—he’s the CEO of Dow. They were looking to find ways to help with waste plastics, turning them more into a circular economy,” Buttlar says. “So, he came to my lab one day and brought me some samples of plastic and asked me if I thought that would be something viable to research in the United States—the use of waste plastic in pavements.

“I saw that in India, China and some other places in Europe had attempted to make waste plastic recycling and asphalt with varied success,” he continues. “And so, our challenge was to figure out how to make it work better and pass the tough standards that we have in the United States.”

Buttlar says the University of Missouri and MoDOT began design on this study in early 2021 and spent about four months performing lab designs for the asphalt mixture. In August 2021, a four-lane road cutting through the university was paved using an asphalt mix that contains LLDPE.

“It really worked,” Buttlar says of the pilot project. “Whenever you use a new recycled material and asphalt, it’s a real delicate, complicated recipe, if you would, it’s a good way to think about how we put together asphalt concrete mixtures. And so, it took us a little while to get the recipes just right. And we did show that they were economical and also easy to construct. So, the takeaways were that a contractor with standard equipment at the production plant and in the field can make these mixtures. And we also saw that the economics look pretty positive. Once the supply chain for this picks up, it looks like it will be very cost competitive with the alternatives.”

With the success of the first pilot project on university roads, Buttlar says university researchers now are looking into a follow-up demonstration project that will be performed on an interstate highway, I-155, in southeast Missouri sometime later this year. He adds that the university also is considering researching the use of high-density polyethylene in this demonstration project in addition to LLDPE.

The University of Missouri and MoDOT aren’t alone in researching the use of LLDPE and other plastic scrap in asphalt pavement. A map shared in a 2022 report from the National Center for Asphalt Technology (NCAT) at Auburn University shows there are at least 19 field projects that have used recycled plastic in asphalt mixtures across the U.S. since 2018.

Recycling Today connected with Buttlar to learn more about the University of Missouri’s research related to the use of plastic scrap in paving materials. He shares some of the university’s findings in the interview that follows.

Recycling Today (RT): What prompted discussions about the pilot project with the university, MoDOT and Dow?
Bill Buttlar (BB): It was over three years ago, and one of the very first things that Dow wanted to look at was the environmental impact of recycling plastic into asphalt. So, they knew that there would be potentially a very positive benefit for diverting plastics away from landfills or from other uses maybe like how ground tire rubber or scrap tires can be used for energy, just burn them, but it’s not a very good reuse. But in a circular economy, you would get that material in and it would just sort of go around and around. And so, you could put it in the road and then a little bit later, you could recycle the road and all the plastic or tire rubber that’s in the road would be recycled back into the mix. And that’s the kind of solution we were looking for.

But there’s environmental consequences to that. Anytime you put anything into asphalt as a new ingredient, you have to make sure that there’s no leaching of chemicals. You have to make sure that there’s no washing out of microparticles (in this case they would be considered microplastics). The Dow engineers we worked with wanted to make sure there was no negative environmental impact of using waste plastic and asphalt, and they were mainly concerned with will the plastic stay in the asphalt or will any of it come out as microplastics or microparticles? And so, we did some work in the lab, some accelerated torture testing of the asphalt to make sure that that plastic-modified asphalt would be stable in the environment and not release any significant microplastics. So, we were able to show that, and then we started thinking about doing a demonstration project.

We started our designs early in 2021, so about two years ago. And then we did a number of test sections with different formulations. It took about four months to do all the lab designs for the asphalt. We also did one comparison section of ground tire rubber. Those were constructed in August 2021.

RT: Based on the research you have done, what percentage of LLDPE works best in asphalt mixtures?

BB: The sweet spot that we’re seeing for higher traffic routes is around half a percent by weight of the mix, which might not seem like a lot, but as far as recycled materials go, it’s a pretty good dosage. It interacts with the liquid asphalt binder, which is around 5 percent of the weight of the mix. So, by adding half a percent of waste plastic by weight to the mix, it’s about 10 percent as compared [with] the weight of the asphalt binder in the mixture, and that’s a pretty heavy modification rate. Oftentimes, for an interstate pavement, our modification rate might be between 3 and 5 percent of virgin or new polymer that’s added to the binder. So, we’re already looking at maybe twice the dosage of polymers that we would use for new polymers so it helps use up more of the waste plastic stream.

RT: When you say half a percent of the mix is recycled plastic, how many pounds of plastic would be used to pave a road at the end of the day?

BB: One interesting way to look at it is with [our project], we paved somewhere around four lane miles of plastic-modified asphalt at an inch and a half of thickness. That used 15 tons, or about 30,000 pounds, of waste plastic--the equivalent of about 3 million shopping bags.

RT: For that project, where did you source the plastic scrap?

BB: [It] can be sourced locally—that’s the direction we’re moving with for our future demonstrations. The early demonstration, there was a very well-known LLDPE source Dow had used in a prior demonstration. That gave us some confidence. … So, the pellets we used came from Houston in the first demonstration. We’re trying to use locally sourced plastics in the future demonstrations.

RT: Does the cleanliness of the recycled material matter in this application?

BB: We are more tolerant to that and that’s why we’re trying to stay away from using some of the more … highly clean materials you would need for other recycling applications. In the construction industry, we can use again ground-up pavement in our new pavement. We can use ground-up roofing shingles. They just have to have a certain purity level, but it’s nowhere near what you would need for reuse in food or health applications. So, we can tolerate some variability in the material and some mixing of waste plastic types.

RT: Why does LLDPE work well in these percentages in asphalt mixtures?

BB: Asphalt is a byproduct of the refining of crude petroleum. So, it’s just a heavy liquid, a very dense liquid. That’s how asphalt gets its strength. Also, it’s considered a flexible pavement system; it’s really moldable [and] very agile and flexible in its use. What the polymers do is they’re engineered to have high toughness and strength—they’re really hard and tough materials. When you add even just a little bit of engineered polymer along with this naturally occurring, refined asphalt, it acts like a composite material where you can elicit the best properties of both materials, which is a hallmark of a good composite. So, you’re really building a composite binder system that holds together aggregates. It’s more durable.

RT: Other universities and state departments of transportation also are looking into the use of recycled plastics in paving materials. When do you think recycled plastic might be used in paving projects across the U.S. more broadly?

BB: Our project on I-155 is part of three different teams—one in the North, we’re in the Midwest and one in the Southeast—looking at three different climate zones for waste plastic recycling. So, we are already part of a larger pooled fund type of study where we’ll be able to compare notes and best practices. The scientific community is forming committees around this and working groups to push [this technology] forward.

I would think for Missouri, that might just be a few years. In the case of ground tire recycling, it took a few years of demonstrations and then we had a specification in the books in Missouri, so, I would expect that will take a few years [for recycled plastics in roadways], but that’s a pretty rapid time frame for departments of transportation. [Those departments are] building so many millions of tons of infrastructure, you really have to make sure you have it right before you start building things in the millions of tons range.

RT: What are your thoughts on the future use of recycled plastics in roadways?

BB: I’ve worked in this field for almost 30 years now. Recycling has kind of been hot and cold in communities—you’ll see it pick up and then it will die down or pick up again. But I think it’s here to stay. There are a lot of signs for that. One is federal funding and federal regulations. There’s a lot happening on the legislative side. The other is just that the engineering community has caught up to the point where we’ve figured out how to make plastic recycling in asphalt work not only from a durability standpoint but also from an economic standpoint. I think that’s really going to drive the use. I think we can see a bright future for recycling in pavements.