An evaluation of waste plastic in asphalt pavement towards a circular economy

Incorporating waste plastic into asphalt pavement is an evolving recycling strategy based on a circular economy approach. Dow has cooperated with MU to use optimal recycle plastic to target better pavement performance with less plastic leaking pollution. The goal is to find the optimal method to cooperate waste plastic with asphalt mixture, that is, an economical and environment-friendly way to target better mixture performance. Linear low-density polyethylene (LLDPE) and waste Polyethylene Terephthalate (PET) is the main plastic used in this project. Other additives (GTR, ElvaloyTM, and PPA) are also added to the mixture by dry process or wet process with the goal to improve the compatibility of plastic in the asphalt mixture. Various binder tests were conducted to evaluate the effect of LLDPE and PET modified binder by a wet process. AASHTO Superpave binder performance grading (PG) tests (Viscosity, DSR, BBR) test was utilized to characterize the workability, high-temperature (rutting) performance, and low temperature (cracking) performance. AASHTO Superpave method was performed to explore the LLDPE modified mixture performance with MoDOT criteria. Rutting resistance, cracking resistance at different temperatures were studied with a suite of laboratory tests, such as the Hamburg Wheel Tracking test (HWT), DC(T) test, IDEAL-CT test. Water samples from the rutting test and the permeability test were also further tested for microplastic detection. In terms of performance grade (PG) of chemically treated waste PET modification by wet process, this study is analogous to Leng's study (2018). Waste PET modification, up to 15 percent by weight of the binder, slightly increases the workability, high temperature, and low-temperature performance. Appreciate range for the amount of waste plastic was determined to be 2-3 percent by weight of the bitumen regard to PG performance. Elvaloy (PPA)-only or LLDPE pellet-only modified binder increases binder viscosity, which indicates a harder binder at the same temperature. Thus, the fail temperature from the DSR test at high temperature and intermediate temperature also improve. The increase in viscosity from the additives has a negative effect on the m value and stiffness of BBR test results. The combination of both LLDPE pellets and Elvaloy (PPA) made the binder even stiffer with viscosity increasing from 0.421 pa*s to 1.319 pa*s, which is more than three times. The addition of plastic by dry process affected specimens in mixture volumetric properties and performance test results. Melted plastic remains very viscous and dense and was not coated on the dense-graded aggregates used in this study. Plastic modified asphalt mixtures decreased the theoretical maximum gravity of the mixture for the volumetric property. Performance tests results show that plastic modified mixture greatly improve rutting resistance with rut depth from 17.1mm to 0.9 mm, which satisfied the MODOT criteria of 12.5 mm rut depth and enhance low- temperature cracking resistance to some degree. Even the CT index of LLDPE modified mixtures fails to meet the recommended threshold of MODOT, the CT index improves with the increase of LLDPE amount and the decrease of LLDPE size. It's also worth mentioning that the smaller size of LLDPE also helps to disperse itself in asphalt mixture, which produces a more stable and reliable asphalt mixture in the long term. Even stiffer with viscosity increasing from 0.421 pa*s to 1.319 pa*s, which is more than three times. The addition of plastic by dry process affected specimens in mixture volumetric properties and performance test results. Melted plastic remains very viscous and dense and was not coated on the dense-graded aggregates used in this study. Plastic modified asphalt mixtures decreased the theoretical maximum gravity of the mixture for the volumetric property. Performance tests results show that plastic modified mixture greatly improve rutting resistance with rut depth from 17.1mm to 0.9 mm, which satisfied the MODOT criteria of 12.5 mm rut depth and enhance low- temperature cracking resistance to some degree. Even the CT index of LLDPE modified mixtures fails to meet the recommended threshold of MODOT, the CT index improves with the increase of LLDPE amount and the decrease of LLDPE size. It's also worth mentioning that the smaller size of LLDPE also helps to disperse itself in asphalt mixture, which produces a more stable and reliable asphalt mixture in the long term.