Hawaii is currently investigating the responsible use of recycled plastics in road construction. This initiative aims to address the environmental and economic impacts of plastic waste by reducing the need to transport, incinerate, or landfill plastics within the islands. Polymers, specifically styrene-butadiene-styrene (SBS), are currently used in Hawaii's polymer-modified asphalt (PMA) pavements to enhance durability and strength, qualities essential for the state's tropical climate.
HDOT Initiates Research
The Hawaii Department of Transportation (HDOT) initiated research to explore the incorporation of discarded plastics into asphalt pavements. Key questions included how modified pavements made with recycled plastics would perform and whether they would release microplastics or associated chemicals into the environment. HDOT partnered with environmental chemist Jennifer Lynch and her CMDR team for this investigation.
The CMDR team provided derelict fishing nets, identified as a significant contributor to Hawaii's marine debris problem, for use in the research. The team also utilized its laboratory capabilities to measure potential microplastic shedding from pavements made with plastic waste compared to standard SBS-modified pavement.
Experimental Design and Materials
Waste plastics, including derelict fishing nets and repurposed polyethylene from Honolulu's recycling containers, were converted into products suitable for asphalt incorporation. Experimental sections of a residential road on the island of Oahu were then paved with asphalt containing standard SBS, polyethylene from recycling containers, and polyethylene from fishing nets.
Initial Findings on Microplastic Shedding
After approximately 11 months of regular traffic usage, Lynch's team collected road dust samples from each pavement section to analyze for microplastic shedding.
Initial tests indicated that pavements made with recycled polyethylene did not release a higher amount of polymers than the control pavement made with SBS.
This finding was consistent across mechanical performance tests with pavement samples and analyses of simulated stormwater collected from the experimental road sections. While microplastic-sized particles were detected, very few were identified as polyethylene, regardless of the pavement type tested. This is attributed to polymers being melted into the asphalt binder, meaning shed particles are a mixture of rock, binder, and melted polymer chains rather than plastic alone.
Broader Context and Future Research
Further comparisons by the CMDR team revealed that polymer shedding from tire wear significantly outweighed any polyethylene signals detected in the road dust. Additional research is necessary to fully assess pavement durability. Researchers express optimism that repurposing used plastics into pavement could contribute to reducing landfill waste and marine debris in Hawaii. The Hawaii Department of Transportation funded this research.