Performance Properties of Recycled Plastic Modified (RPM) Asphalt Mixtures Added via the Dry Method

Abstract

According to the United States Environmental Protection Agency (EPA), approximately 35.7 million tons of plastic waste were generated in 2018, of which only 3.1 million tons were recycled, with the rest being buried in landfills, incinerated, or discarded as litter. Numerous possible solutions are being investigated to reuse waste plastics with considerable attention being given to using specific categories of plastics in asphalt pavements. The two methods for incorporating recycled plastics in asphalt mixtures are the dry method and the wet method. In the wet method, recycled plastics are added to the asphalt binder as a polymer modifier or an asphalt replacement. In the dry method, recycled plastics are added directly into the mixture as either an aggregate replacement, mixture modifier, binder modifier, or a combination of these. Of the seven categories of waste plastic, the majority of research has focused on high and low-density polyethylene (HDPE and LDPE), and polypropylene (PP) since these categories have melting points generally in the range of mixing temperatures for asphalt paving mixtures. However, there are currently no robust specifications on the source and properties of recycled plastics for use in asphalt. Therefore, evaluating recycled plastics with different properties is essential to understand their effects on the asphalt mixtures. This research explored the potential of integrating recycled plastics into asphalt pavements and was structured on three experiments designed to assess the implications of incorporating post-consumer recycled (PCR) plastics into asphalt mixtures. These experiments focused on evaluating performance-based properties and friction-related characteristics of both plant-produced and laboratory-prepared recycled plastic-modified (RPM) asphalt mixtures. The first experiment delved into characterizing the performance and friction-related characteristics of plant-produced RPM asphalt mixtures from two field projects. By comparing these properties to control mixtures without PCR plastics, this phase aimed to evaluate the implications of using recycled plastics in real-world paving applications. Various performance tests were considered to provide a testing plan for evaluating the RPM mixture's behavior under various conditions. The test results revealed that incorporating PCR plastics via the dry method enhances stiffness and rutting resistance, yet adversely impacts workability, intermediate-temperature cracking resistance, and fatigue damage resistance. However, their effects on low-temperature cracking resistance, moisture susceptibility, surface texture, and friction properties appeared minimal. The second experiment tried to develop a laboratory procedure that simulates the dry method of adding PCR plastics to asphalt mixtures at production plants. This experiment assessed the performance properties of mixtures prepared by four laboratory PCR addition methods and compared them with the plant-produced asphalt mixtures to identify the laboratory method that best replicates the plant-production process. Mix designs from two field projects used in the first experiment were used for this experiment. The second experiment revealed that none of the four PCR plastic addition methods accurately replicated the production of RPM mixtures at asphalt plants due to possible differences in production and storage aging processes between laboratory and plant production. The third experiment expanded the scope to include laboratory-prepared RPM asphalt mixtures incorporating five different types and sources of PCR plastics. This was essential for evaluating the adaptability of various plastic materials in asphalt mixtures. Mix designs from the National Center for Asphalt Technology (NCAT) and Minnesota Road Research Facility (MnROAD) Additive Group (AG) experiments represented southern and northern mix designs, respectively, to assess the effects of different PCR plastics on the performance properties of both control and RPM asphalt mixtures. The test results showed that RPM mixtures maintained comparable workability, intermediate-temperature cracking resistance, surface characteristics, and low-temperature cracking resistance, but with slightly better rutting resistance and higher stiffness than the control mixtures. However, these RPM mixtures displayed reduced fatigue resistance. Results regarding moisture susceptibility were inconsistent for the two mix designs. Additionally, the FlexPAVETM analysis was conducted on both plant-produced and laboratory-prepared control and RPM mixtures to assess the cracking damage performance over a 20-year analysis period. FlexPAVETM analysis was used for mechanistic asphalt mixture and pavement performance prediction based on fundamental properties and using moving vehicle loads and pavement temperature data. Different pavement structures, varying in the thickness of the asphalt concrete (AC) layer, were also simulated using FlexPAVETM to explore the effects of incorporating recycled plastics into various pavement designs. Based on the FlexPAVETM simulations, pavement test sections with RPM mixtures generally exhibited similar or higher final percent damage, except for two PCR plastics, compared to the control sections after 20 years of service life. However, the impact of different types of recycled plastics on the final percentage of damage varied. Finally, a Pearson correlation analysis was performed to assess the relationship between the physical and thermal properties of recycled plastics and the performance test results of laboratory-prepared RPM asphalt mixtures. These mixtures incorporated five different types and sources of PCR plastics using the southern and northern mix designs. The analysis results indicated that among the various properties of PCR plastics, only the melt flow index (MFI) exhibited strong correlations with mixture workability, Glover-Rowe (G-Rm), and rutting resistance.