Roadmap for repurposing waste plastics in asphalt mixes

Authors: AL-QADI, IMAD - University Of Illinois; BENNERT, THOMAS - Rutgers University; BHASIN, AMIT - University Of Texas At Austin; HABBOUCHE, JHONY - Virginia Department Of Transportation; YIN, FAN - Auburn University; HAJJ, ELIE - University Of Nevada; MOGAWER, WALAA - University Of Massachusetts; Sharma, Brajendra - Bk; UNDERWOOD, SHANE - North Carolina State University

Submitted to: American Society of Civil Engineers
Publication Type: Popular Publication
Publication Acceptance Date: 4/16/2024
Publication Date: 5/17/2024
Citation: Al-Qadi, I.L., Bennert, T., Bhasin, A., Habbouche, J., Yin, F., Hajj, E.Y., Mogawer, W.S., Sharma, B.K., Underwood, S. 2024. Roadmap for repurposing waste plastics in asphalt mixes. American Society of Civil Engineers. 1:1-13.

Interpretive Summary: The environmental impact of waste plastics is widely recognized. Efforts are ongoing worldwide to mitigate their proliferation into the ecosystem by public, private, and non-profit sectors. One area that has captured the imagination of policymakers, industry, and researchers is the incorporation of waste plastics in the construction of asphalt pavements (flexible pavements). However, repurposing waste plastics in flexible pavements must be evaluated in the context of other waste mitigation efforts and their impact on long-term flexible pavement performance. A team of experts met in the spring of 2023 and discussed the context to pursue the use of waste plastic streams in asphalt binder as pavement construction material, evaluated this approach in the broader context of its overall environmental impact, and summarized findings from their respective laboratories and field studies using treated and untreated waste plastics in asphalt mixtures and pavements. The team outlined the known benefits and limitations of using waste plastic and identified gaps that need to be addressed before advocating for its widespread adoption in flexible pavements. The outcome can help identify suitable waste agricultural plastics for incorporation into asphalt pavement thereby reducing the amount of agricultural plastic sent to landfills.

Technical Abstract: The global environmental impact from waste plastics is widely recognized, and efforts by public, private, and non-profit sectors are underway worldwide to mitigate their proliferation into the ecosystem. One area that has captured the imagination of policy makers, industry, and researchers is the incorporation of waste plastics in the construction of asphalt pavements (referred to herein as simply flexible pavements). However, repurposing waste plastics in flexible pavements must be evaluated in the context of other waste mitigation efforts and their impact on long-term flexible pavement performance. When considering the use of waste plastics in the construction of flexible pavements, it’s important to understand that term “plastic” encompasses a broad range of solid polymeric materials with varying chemical compositions and physical properties that are derived from petro-chemical manufacturing and processing. These polymers are typically produced by chemically linking blocks of similar or different types of molecules with each other to form long chains or networks. From the perspective of recycling, the Society of the Plastics Industry (SPI) classifies these polymers into seven different categories: (#1) Polyethylene terephthalate (PETE/PET); (#2) High-density polyethylene (HDPE); (#3) Polyvinyl chloride (PVC); (#4) Low-density polyethylene (LDPE); (#5) Polypropylene (PP); (#6) Polystyrene (PS); and (#7) polycarbonate, polyamides, and others. The polymers within each family can also be very different (e.g., two different plastics may be classified as HDPE, but exhibit different chemical and physical properties). In the context of consuming waste plastics in the construction of flexible pavements, it must be recognized that there are only certain types of waste plastics that could potentially be used without risking worker safety, additional harm to the environment, and/or pavement performance. Waste management and environmental agencies employ the five R’s (refuse, reduce, reuse, recycle, repurpose) in mitigating waste impact. These five R’s also represent a hierarchy of impact. The most effective method for environmental protection is to not use a potentially harmful waste product (Refuse). However, since outright refusal is often challenging, the focus shifts to reducing consumption. After reduction, the next priorities are reusing and recycling the material. While repurposing is less effective in reducing waste and its environmental impact, it is important to consider competing methods for repurposing waste plastic streams. The waste plastics streams that could be used in flexible pavement construction may also show potential with other processes that have a higher or lower net carbon impact. For example, LDPE can potentially be broken down into its constituent building blocks to produce fuel. In the context of using waste plastic for the construction of flexible pavements, it's important to evaluate alternative repurposing routes and compare the net carbon impacts of each option. Pilot studies show that selected types of waste plastics can be consumed in the construction of asphalt pavements. The remainder of this white paper presents more technical details on this topic. These studies suggest that waste plastic can be consumed in the order of approximately 2 to 3% by weight of the asphalt binder used in the production of the asphalt mixture. In the context of the magnitude of the asphalt industry, if consumption of waste asphalt was used for every flexible pavement construction project throughout the US, this strategy would alleviate only about 7% of the LDPE waste stream and less than 2% of the total waste plastic stream. Finally, the consumption of waste plastic for the construction of flexible pavements must carefully consider the impact on durability and life of the pavement, as well as the need to pre-process the was