C. J. DuBois

Using recycled plastics in asphalt could be challenging because of possible composition variations and contaminants. Post-consumer recycled (PCR) and post-industrial recycled (PIR) plastics can differ in composition, consistency over time, amount of contaminates, and degradation potential. Additionally, PCR and PIR plastics may contain chemical additives from their manufacturing process and residues of other materials after recycling. Thus, significant health and safety uncertainties surround the occupational exposure of asphalt workers to hazardous air pollutants (HAPs) emitted during the production and construction of recycled-plastic-modified (RPM) asphalt mixtures. When subjected to elevated temperatures, certain recycled plastics may release HAPs, including polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs). This study examined fumes emitted during four laboratory methods of adding PCR plastics in asphalt mixtures via the dry process that simulated the production of these RPM mixtures at asphalt plants. The fume analysis followed the US National Institute for Occupational Safety and Health (NIOSH), the US Environmental Protection Agency (EPA), and the International Organization for Standardization (ISO) methods for collecting, characterizing, and quantifying PAHs. The chemical characterization of the utilized PCR plastics was performed using gas chromatography/mass spectrometry (GC/MS) analysis. Results indicated that the evaluated PCR plastics did not introduce new compounds to the fumes but may have changed the observed concentration of the compounds in the fumes. Nonetheless, the concentrations of the detected HAP VOCs were significantly below the US Occupational Safety and Health Administration (OSHA) threshold values.

This study evaluated the short-term performance properties and predicted cracking performance of asphalt mixtures containing polymer-modified asphalt (PMA) binders and recycled polyethylene (RPE). To that end, binder rheological testing, mixture performance testing, and FlexPAVE simulations were conducted. Furthermore, three PMA mixtures were selected for field evaluation on the National Center for Asphalt Technology Test Track and rapid performance testing during production. The PMA binders modified with styrene-butadiene-styrene (SBS), reactive elastomeric terpolymer (RET), and RET-compatibilized RPE had significantly improved elasticity and rutting resistance compared with the unmodified binder. The PMA binders also exhibited good storage stability with minimal separation tendency. Both polymer modification of the asphalt binder and adding RPE via the dry process improved the rutting resistance of the mixture. Although the unmodified and PMA mixtures exhibited notably different load-versus-displacement curves from the Indirect Tensile Asphalt Cracking Test (IDEAL-CT), they had similar cracking tolerance index ( CT _index ) results. The G _f -versus- l ₇₅ /m ₇₅ interaction diagram allowed a more robust understanding of the IDEAL-CT results by considering mixture toughness and brittleness, and their interactions with CT _index . In all cases, polymer modification increased the stiffness and fatigue damage resistance of asphalt mixtures, while adding RPE via the dry process embrittled the mixtures, making them more susceptible to fatigue damage. The PMA mixtures had significantly better predicted cracking performance in FlexPAVE simulations than the unmodified mixture. This improvement was mainly caused by polymer modification of the asphalt binder. Finally, adding dry RPE pellets improved the rutting resistance but reduced the intermediate-temperature cracking resistance of the plant-produced PMA mixture containing an SBS-modified binder.

Research has demonstrated that asphalt pavements can be a strategic destination for some of the major streams of waste materials around the globe, such as scrap tires and plastics. Ground tire rubber (GTR) from scrap tires has been used in asphalt pavements since the 1960s but has yet to approach its full potential in relation to market adoption. The heightened restrictions imposed by China in relation to waste stream contamination in 2018 have catalyzed research on incorporating post-consumer recycled (PCR) plastics into asphalt pavements. A field demonstration project was carried out in Columbia, MO to evaluate modern recycled plastic and GTR additives in an asphalt pavement overlay. The demonstration focused on dry-process modification, which requires minimal alterations to an existing asphalt plant and allows a higher amount of recyclates to be utilized. The project was also designed to assist in the Missouri Department of Transportation’s early roll-out of balanced mix design asphalt design and quality assurance specifications. The results of the study indicated the viability of dry-process GTR and PCR plastic additives used with neat (unmodified) asphalt binder as an alternative to binders modified with virgin polymers, chemical treatments, or both, in a wet process. The results of the study also suggest the viability of hybrid wet polymer-modified binder with dry-process PCR plastic as a greener alternative to binders modified with virgin polymers, chemical treatments, or both. Future studies will be needed to examine additional PCR plastic streams, especially mixed polymer streams.