USE OF POST-CONSUMER RECYCLED (PCR) PLASTICS IN ASPHALT MIXES: A LABORATORY STUDY

Abstract

In recent years, the use of recycled materials in asphalt mixes has gained popularity among asphalt producers and users to reduce environmental pollution, enhance sustainability, and conserve natural resources. Among different recycled materials, the use of Post-Consumer Recycled (PCR) plastic poses several challenges. These challenges stem from the processes used to incorporate PCR plastic, mixing protocol, sample preparation, testing, and field implementation. Asphalt mixes containing PCR plastic pose several challenges both from the volumetric properties and performance standpoints. Specifically, this study investigates the changes in the volumetric properties and performance of PCR plastic-modified asphalt mixes designed using the Balanced Mix Design (BMD) approach. For this purpose, a control mix (0% plastic) was designed using the BMD approach. This mix was modified using different percentages of two PCR plastics, namely Low-Density Polyethylene (LDPE) and Linear Low-Density Polyethylene (LLDPE). The PCR plastics were added using the dry process. A suitable mixing protocol was developed for incorporating PCR plastics by simulating the production process used for incorporation of Reclaimed Asphalt Pavement (RAP) in an asphalt plant. Volumetric properties, namely maximum theoretical specific gravity of the mix, bulk specific gravity, voids in mineral aggregate (VMA), voids filled with asphalt (VFA), air void contents, and densities were determined for both control (0% plastic) and plastic-modified mixes. Also, Indirect Tensile Strength (ITS), Indirect Tensile Asphalt Cracking Test (IDEAL-CT), and Hamburg Wheel Track (HWT) tests were performed to assess performance of these mixes. Changes in the volumetric and performance properties of asphalt mixes due to plastic modification were studied and are reported in this dissertation. To study the cracking behavior of plastic-modified mixes, effect of loading rate and notch depths on fracture properties was evaluated using Illinois Flexibility Index Test (IFIT), Louisiana Semicircular Bend (L-SCB) and IDEAL-CT. Fracture properties, specifically, fracture energy, Flexibility Index (FI), Cracking Tolerance Index (CTIndex), Strain Energy at Failure (SEF), and Critical Strain Energy Release Rate (J-integral) were evaluated. In addition, the effect of aging conditions on the performance of plastic-modified mixes was evaluated in this study. For this purpose, asphalt mixes without plastic (control mix) and with (LDPE and LLDPE) modification were subjected to three aging conditions, namely short-, medium- and long-term aging. Mechanical tests, including IDEAL-CT and Dynamic Modulus (DM), were conducted to evaluate the effect of aging on cracking resistance and stiffness. Additionally, binders were extracted and recovered from all aged mixes and the extracted aggregates were visually inspected. The effect of aging on the rheological properties of the recovered binder was assessed using Rotational Viscosity (RV), Dynamic Shear Rheometer (DSR), and Multiple Stress Creep Recovery (MSCR) tests. The impact of aging was further examined by identifying changes in chemical functional groups using Fourier Transform Infrared Spectroscopy (FTIR) tests. Finally, AASHTOWare Pavement ME Design (PMED) simulations were used to evaluate the effect of aging on the field performance. Over stiffening of plastic-modified mixes was observed due to the higher amounts of plastics, which resulted in reduced cracking resistance. Consequently, this study aimed at increasing the percentage of plastic in asphalt mixes by incorporating a bio-rejuvenator. The plastic-modified mixes were modified by adding bio-rejuvenator modified binder. The volumetric properties were determined and the mechanical performance, namely rutting, cracking and moisture induced damage resistance, of the asphalt mixes were evaluated using the HWT and IDEAL-CT tests. The optimum dosage of plastics was determined using the BMD criteria. Moreover, the environmental impact analysis was performed on plastic-modified mixes. A significant reduction in Greenhouse Gas (GHG) emissions was observed from the use of plastic in asphalt mixes. Knowledge gained from this study on the effect of addition of PCR plastic on the performance of asphalt mixes designed using the BMD approach is expected to be helpful in incorporating waste plastic in future asphalt mix designs.