Study: Recycled-plastic pavement withstands heavy trucks, extreme weather
RENO — Asphalt pavement formulated with recycled plastic has successfully survived heavy truck traffic and the atmospheric rivers of the winter of 2022-23 after a section of California State Route 99 flooded in January, officials announced Tuesday.
“It’s a testament to the new technology,” said Elie Hajj, professor of civil and environmental engineering at the University of Nevada, Reno and associate director of their Superpave Center. “We are conducting extensive performance and environmental testing to get to this asphalt mixture.”
While the research on the use of recycled plastic in the lab is continuing, Granite Construction, which partnered with the University’s Western Regional Superpave Center, approached Caltrans on testing the new mixture of asphalt in the real world.
“We did an overlay for a Caltrans project on Highway 99, replacing their conventional mix with 10% plastic by weight of the binder,” Edgard Hitti, national asphalt manager for Granite Construction said. “That was the first successful low-carbon mix utilized by Caltrans.”
Asphalt pavement is traditionally a mixture of aggregates – crushed rocks, gravel and sand – held together by an asphalt binder, usually a sticky black liquid or semi-solid form of petroleum to create a cohesive mixture.
This innovative asphalt binder blend used plastic from a single source, used printer cartridge toner. It has been named MTP, which stands for modified toner polymer. It is a post-consumer waste plastic that was headed to incineration or landfills in the United States. Many public agencies are requiring the use of recyclable materials in asphalt pavements and this source of plastic meets those requirements.
“Only 9% of all waste plastic is recycled, so we didn’t want to use materials from existing recycling streams,” Hajj said. “And we wanted post-consumer polymers, a low or net-zero carbon product.”
Toner powder and cartridges are typically classified as electronic waste, making them difficult to recycle and contributing an estimated 9,000 to more than 25,000 tons of waste each year in the United States, as there is no application for better utilization.
“We wanted to find a source that wasn’t currently recycled so we can improve the total plastic recycled in the U.S. and not carve from an existing stream that is currently being recycled and used in other industries,” Hitti said. “It is also a great tool to improve the Environmental Product Declaration of the asphalt mix, which is referred to as the ‘Nutrition Value’ of the asphalt mix.”
The approximate volume of waste plastic from toner cartridges recycled on the short section of Highway 99 is equivalent to 450 large-wheeled residential curbside recycling bins.
In addition to its other properties, this plastic has the potential to reduce carbon dioxide emissions from the liquid asphalt by approximately 5% while also diverting 200 pounds of plastic out of the waste stream for every ton of liquid asphalt produced.
The toner from used printer cartridges is added to the asphalt binder as a partial replacement to improve its mechanical properties. The modified toner polymer binder is combined with aggregates to produce the asphalt mix used to construct the asphalt pavement surface. Hajj said that using the toner is better than other plastic products, as they aren’t oxidized like a lot of other recyclables.
“Plastics are not all the same, so with toner cartridges it’s a consistent product – uniform and homogenous,” Hajj said. “Many plastics are very hard material and difficult to work with. Toner cartridges are nicer because it is a mixed softer post-consumer plastic that is better suited to binder modification because of its low melting point and better elasticity than conventional hard plastics.”
The 500-foot section of Highway 99 in Elk Grove, just south of Sacramento, was paved after an initial test of the mixture the year prior proved successful in a Target store parking lot pavement application. In October, the highway section was paved, and subsequent tests show it is doing well, even after the flooding from a levee break during an atmospheric storm.
“The levee breach that flooded Hwy 99, coincidentally happened at the low-carbon plastic section and it held well with no damage,” Hitti said.
Following the winter, a team of pavement experts from the University of Nevada, Reno, Granite, and Caltrans visited the plastic section for a field review to observe and test the pavement for performance. Being a main artery for trucks transporting goods across the state and the country, thousands of heavy trucks traveled the highway for more than three months in sweltering hot as well as damp and freezing weather. The team observed that the pavement has been performing very well, and there is no cracking or rutting.
“Overall, I was very impressed by the high quality of the test section,” Peter Sebaaly, professor in the College of Engineering and Director of the Pavement Engineering & Science Program, said. “At this early stage of pavement life, we typically look for signs of issues that may impact long-term performance such as non-uniformity of the mix throughout the section and openness of the surface. It was great to see that the section does not display any of the issues.”
Julissa Larios, a doctoral candidate at the University of Nevada, Reno, is using state-of-the-art laboratory equipment and techniques to evaluate the full engineering behavior of this innovative technology. The lab results so far indicate that the MTP (modified toner polymer) asphalt binders exhibit increased stiffness and viscosity proportional to the toner content, resulting in improved resistance to permanent deformation.
The plastic binder can be used in any type of application, from heavy traffic applications like the Highway 99 project to parking lots like the Target project, including pavement preservation applications such as slurry seal.
“We had the first ever successful attempt to use emulsified recycled plastic in a project in San Luis Obispo in October 2022,” Hitti said.
The project used a 5% mix of toner plastic in the binder for the Type 2 slurry seal, which is a maintenance treatment of a thin layer of material to prolong the life of asphalt using very fine crushed aggregates.
Hajj said that the team at the University’s Superpave Center will continue researching mixtures and product applications and have expanded experiments while preparing a study for publication.
“Our work to date provides insights on the effective use of modified toner polymers in asphalt binders, providing a promising approach to improve the deformation properties of asphalt binders while diminishing environmental risks associated with landfilling toner,” he said. “We feel there is still more data needed on the impact of MTP on asphalt mixtures and their long-term performance, so we’ll continue additional research on the effects of MTP on mixtures rutting and cracking characteristics.”
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