In 2018, UC San Diego tested a campus road made with recycled plastic in its asphalt mix, highlighting the promise and uncertainty of plastic-modified pavement

Road surfaces rely on bitumen, a petroleum-derived binder that holds the mineral aggregates in asphalt together. In 2018, the University of California San Diego tested a different approach on a section of campus roadway, using recycled plastic materials in place of petroleum-based bitumen. The project simultaneously addressed the growing volume of discarded plastic and the ongoing need to maintain roads. MacRebur, instead of treating plastic as just a material to be disposed of or recycled, developed a process for blending it into asphalt mixtures. As per the university’s October 2018 news report, Gary Oshima, a construction commodity manager at UC San Diego, discovered the technology through a Facebook post featuring a TED Talk by MacRebur co-founder and chief executive Toby McCartney. This led Oshima to contact the company, which found a receptive response in their desire to make their debut in America.The methods for blending recycled plastic into asphalt Asphalt pavement typically combines mineral aggregates with a petroleum-derived binder called bitumen, which glues the particles together. Recycled plastic can be added using two primary methods: the dry process and the wet process, with each method differing in how the plastic interacts with the other materials. In the dry process, shredded or ground plastic is added directly to the mixture, and it serves either as a partial replacement for aggregate or as a modifier, whereas in the wet process, before it is combined with the aggregate, the plastic is blended into the asphalt binder. Ultimately, both methods strive to yield a modified binder with properties apt for the pavement application.According to the study, Use of Recycled Plastic in Asphalt and Concrete Pavement Applications, there are different technical challenges when these two methods are used. The National Centre for Asphalt Technology cautioned that the dry process could result in inconsistent mixture quality. Conversely, the wet process could separate the plastic from the binder because of chemical incompatibility and differences in viscosity and density.Understanding these distinctions matters because plastic waste is not a homogeneous material; different polymers such as polypropylene and polyethene terephthalate have distinct characteristics. Simply put, any one formulation that performs well with one type of polymer cannot be assumed to work the same with another, while other factors, such as particle size, dosage, mixing conditions, and the existing asphalt mixture, can also affect the result.What later research found Studies conducted in several countries have reported improvements, particularly in asphalt properties after recycled plastic was introduced. A 2020 Australian laboratory study, published two years after the campus test, found that recycled-plastic products led to an increase in binder resistance to flow and elasticity. The performance of the binder increased by three to four levels, and the resulting mixtures became stiffer and more deformation-resistant. However, the study did not find any notable improvements in terms of crack resistance or resistance to moisture damage.A review by Mashaan and colleagues in 2019, a year after the campus test, discussed studies and affirmed that the amount of plastic used also mattered, recommending different proportions of waste high-density polyethene and polyethene terephthalate for particular asphalt applications. However, those recommendations were in relation to the formulations examined; they do not establish a universal optimum for plastic-modified roads. The California Department of Transportation’s review also exhibited a shortage of domestic research and long-term field evidence. Although laboratory experiments and pilot installations opened up avenues for further investigation, at the time they could not determine the behaviour of plastic-modified pavement over many years of traffic, weather exposure and maintenance. The UC San Diego road has now carried traffic for eight years, yet no public condition data has surfaced, so its long-term performance remains unverified.Furthermore, environmental questions remain pertinent to that assessment, as researchers and officials have raised concerns about the potential for leaching, worker exposure during production and the difficulty of recycling plastic-containing asphalt at the end of its service life. Incorporating waste into pavement does not, by itself, demonstrate a net environmental benefit.You use AI every day. Now get your AI Quotient. Take the AIQ test.