Asphalt containing recycled plastic held together better than conventional asphalt and showed a lower level of plastic-like particles after combined traffic and moisture-related laboratory testing, according to a study led by Penn State researchers. The findings suggest that improved pavement durability may help limit particle release, the team said, but field studies are needed to determine whether the same pattern occurs on real roads.
The study, published in the Journal of Cleaner Production, examined recycled plastic-modified asphalt made with low-density polyethylene, or LDPE. While recycled plastics are increasingly being explored to divert waste from landfills and improve pavement performance, the researchers said less is known about whether plastic-containing pavement could release microplastics as roads age, experience traffic and endure weather.
"Our goal was to understand whether asphalt pavement containing recycled plastic could release microplastic particles as it is damaged over time," said Shihui Shen, distinguished professor of rail transportation engineering at Penn State Altoona and corresponding author on the paper. "That environmental question has received much less attention than the engineering performance of the pavement."
Microplastics are particles smaller than 5 millimeters - the width of a pencil eraser - and have been found in water, soil and air. Pavements can contribute particles to the environment through tire wear, pavement deterioration and plastic-containing construction materials. Particles released from road surfaces could potentially be carried into roadside soil and stormwater by wind and rainfall.
To investigate that possibility, the researchers compared conventional asphalt with asphalt containing recycled LDPE. They prepared the plastic-containing asphalt by mixing shredded plastic with crushed stone and reclaimed asphalt pavement, then adding the binder that holds the mixture together.
The researchers then exposed samples to two established laboratory pavement tests designed to simulate stresses caused by traffic and weather. In the Hamburg Wheel Tracking Test, steel wheels repeatedly rolled over asphalt specimens submerged in hot water, simulating the combined effects of traffic loading, heat and moisture. In a second test, called the Moisture-Induced Stress Tester, samples were subjected to elevated heat, moisture and repeated hydrostatic water pressure.
"Together, these tests reproduce important aspects of roadway damage under controlled laboratory conditions," said Zhen Liu, a research assistant professor at the Chinese University of Hong Kong who completed a postdoctoral fellowship at Penn State's Larson Transportation Institute within the Department of Civil and Environmental Engineering and the study's lead author. "We then collected the released particles and analyzed them for possible plastic content."
Working with Penn State's Environmental Contaminants Analytical Laboratory, the researchers analyzed the chemical signatures of the asphalt's ingredients to create a reference library. They then used infrared imaging to measure individual particles and compare their chemical signatures with those reference materials. This helped the team identify particles resembling LDPE. To compare samples, the team measured the combined area of detected LDPE-like particles relative to the weight of the asphalt tested.
"Pavement debris is not a simple environmental sample," said Odette Mina, director of laboratories for Penn State's Institute of Energy and the Environment and a co-author on the paper. "The combination of bulk-material characterization and particle-level infrared imaging allows us to connect the original pavement material with the identity, abundance and physical characteristics of particles released during simulated service conditions."
Asphalt containing recycled plastic generally held together better than conventional asphalt during laboratory testing. After combined traffic and moisture-related stresses, it also showed a lower measured level of particles with chemical signatures resembling LDPE. Together, those findings suggest that improving pavement durability may help limit particle release, according to the team.
"Asphalt binder and LDPE share some infrared features, which can complicate both identifying particles as plastic and determining their source," Shen said. "The findings do not rule out the possibility that either plastic-modified or unmodified asphalt mixtures can release microplastics."
The study tested one asphalt formulation and one source and amount of recycled plastic, so the researchers cautioned that findings cannot yet be generalized to other pavement materials or real-road conditions.
The study also highlighted the importance of contamination control in microplastics research, Mina explained. The team cleaned testing equipment and internal piping, ran background tests without asphalt specimens, used glass containers and apparatus where possible, used pre-filtered water and covered samples during preparation. The imaging system also generated match-quality scores to help researchers evaluate confidence in particle identification.
Those steps are important because microplastics can be introduced inadvertently through laboratory equipment, air, water or previous samples, potentially causing background particles to be mistaken for particles released by asphalt, Mina said.
The researchers said field studies are needed to determine whether their laboratory findings hold on real roads over longer periods. Future field studies will need to account for traffic volume, rainfall, temperature changes, sunlight, pavement aging, runoff chemistry and differences in recycled-plastic and asphalt formulations.
"Field studies, reliable sampling and long-term monitoring will give transportation agencies stronger evidence for weighing pavement-performance benefits against potential environmental impacts," Shen said. "Ultimately, more reliable environmental data can help agencies make more informed decisions about recycled-plastic pavement technologies."
In addition to Liu, Shen and Mina, the research team included Hlengilizwe Nyoni, assistant research professor of the Institute of Energy and the Environment, and was supported by the following industry partners: Eco Plastic Products of Delaware, New Enterprise Stone and Lime Co., Inc. and HRI, Inc.
The research was supported by the U.S. Department of Transportation Region 3 Center for Integrated Asset Management for Multimodal Transportation Infrastructure Systems and the Penn State Commonwealth Campuses Research Collaboration Development Program. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funders.
