Hawaii has been testing whether recycled plastic can be safely incorporated into road pavement with a pilot project on Fort Weaver Road in Ewa Beach using material equivalent to about 195,000 plastic bottles.
The Hawaii Department of Transportation (HDOT) began the experiment in 2022 using about 1,950 tons of plastic modified asphalt. The project was designed to determine whether adding recycled plastic polymer to conventional hot mix asphalt could improve pavement performance while keeping plastic waste out of landfills.
The road was not made entirely of plastic. The recycled material accounted for only about 0.1% of the asphalt mixture. Three sections were constructed using different mixes, including a conventional section that researchers could use as a control for comparison.
For Hawaii, which faces challenges associated with limited land and waste management capacity, the approach could provide another potential use for discarded plastic. However, researchers said the long term performance and environmental consequences needed to be examined before the method could be adopted more widely.
Researchers investigate what happens to plastic
The project raised an important question beyond whether the pavement could withstand traffic what happens to the recycled plastic once it is embedded in a road?
Researchers from the University of Hawaiʻi at Mānoa and Hawaiʻi Pacific University examined the pavement and surrounding environment. By 2023 scientists at Hawaiʻi Pacific University’s Center for Marine Debris Research were collecting simulated stormwater runoff from the road.
The samples were analysed for microplastics and synthetic chemicals that could potentially be released from the asphalt. Researchers also examined tyre wear particles because not all plastic particles found around a road necessarily originate from the pavement.
Vehicles themselves generate rubber and polymer particles as tyres gradually wear down, making it important to distinguish between pavement derived material and traffic related pollution.
The environmental monitoring was intended to determine whether using recycled plastic in roads could simply shift the waste problem from landfills to surrounding soil and water.
The Fort Weaver project was also linked to Hawaii’s efforts to increase the use of reclaimed asphalt pavement, or RAP in future road construction.
According to HDOT some asphalt mixtures used in the programme contained about 50% reclaimed asphalt. Researchers planned to assess whether the recycled material could maintain adequate pavement performance and potentially support greater use of recycled asphalt in future projects.
The evaluation was expected to continue for about 18 months, allowing researchers to observe cracking, rutting surface wear and other changes that may emerge over time.
The extended monitoring was considered important because a newly constructed road can initially appear successful while developing performance problems later.
The Fort Weaver project was part of a broader effort to determine whether recycled materials could reduce reliance on virgin resources without compromising road quality or creating new environmental risks.
Hawaii had previously noted that plastic containing asphalt had been used elsewhere in the United States but the department wanted to determine how such mixtures would perform under local conditions.
The pilot therefore served as a real world laboratory, with normal traffic continuing while researchers studied pavement performance and environmental runoff.
The roughly 195,000 bottles’ worth of plastic used in the project did not remain in recognisable bottle form. It was processed and incorporated into the asphalt mixture, representing plastic waste that could otherwise have been sent to landfill.
The experiment’s success ultimately depends on two factors whether the modified asphalt remains durable over time and whether the recycled plastic can remain safely contained without releasing harmful particles or chemicals into the environment.
Hawaii later moved into a second phase of its plastic-road research, including materials recovered from consumer recycling and discarded fishing nets. The developments highlight a broader challenge in plastic recycling finding useful applications for waste is only part of the solution its long term environmental behaviour must also be understood.