Environmental challenges in incorporating recycled plastics in asphalt pavements
Creators: name: Boom, Yeong Jia
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400505
subject: Construction materials

description: Plastic consumption worldwide has increased significantly over the last decades, causing severe environmental pollution with no alternative ways to dispose of and recycle. Recycling plastic waste is a better solution than other means of dealing with it, such as composting, incineration or landfilling. Along with the ever-growing asphalt industry, road pavement technicians and scientists are adopting new techniques and methods to improve construction and maintenance practices. These practices have the purpose of minimising the damage to the current environment. The research on utilising recycled plastics in the asphalt industry has significantly increased in the past decade. The team at RMIT University proposed that using recycled plastics provides mechanical enhancement and a gateway to potentially reduce the environmental impacts of recycled plastics in asphalt compared to conventional methods. This research is a continuation and extensive expansion of the parent research on incorporating recycled plastics in asphalt roads conducted at RMIT. 
The initial stage of the research involved a comprehensive survey from 40 sources in the plastics industry, including the manufacturing industry, material recycling facilities (MRFs), and plastic and recycling associations. The survey aimed to develop techniques for selecting the types of recycled plastics to represent common plastics circulating in Australian and New Zealand households and industries without compromising on the objective of tackling the waste issue. Based on a series of site visits and interviews, 38 samples of plastics varying in categorisation and type were obtained.
 
The second part of the research focused on developing screening tests to narrow down the list of plastic samples received from the industry. The development of the screening methodology involved eight individual tests in determining an overall rank scoring system compared to commercially available products. 2 types of recycled plastics were selected for the dry method. In contrast, five types of recycled plastics were selected for the wet method bitumen-only testing and later narrowed down to two; these two plastics were also employed for the mixed method.

The third part of the research investigated a comprehensive laboratory investigation to develop necessary techniques to incorporate recycled plastics in asphalt and bitumen production. Specifically, experimental setup and methods were developed to analyse the emissions during recycled plastic modified bitumen and asphalt production. The developed methods were subjected to a series of sensitivity and verification tests to determine the optimum parameters employed for the subsequent studies.

The fourth part of the research investigated the fuming and emissions analysis from the wet method of incorporating recycled plastics in bitumen via bitumen modification. Recycled plastic modified bitumen samples were prepared to incorporate 1 %, 2 %, 4 % and 6 % by weight of bitumen at 160 °C, 180 °C and 200 °C. Total VOC and PAH emitted compounds were quantified using gas chromatography/mass spectrometry (GC/MS). The results were compared against conventional methods (regular bitumen and commercially available polymer-modified bitumen). An increase in working temperature, regardless of the presence of plastics, resulted in higher concentrations of VOCs and PAHs. However, using recycled plastics resulted in a reduction in PAHs and VOCs with increasing plastic content leading to higher reduction rates. The thermogravimetric analysis (TGA) test showed that the enhancement in thermal stability from using recycled plastics in bitumen could effectively inhibit portions of VOCs and PAHs from emitting during high working temperatures. The variation in inhibition effect from each recycled plastic indicates that the recycling process of plastics from their source could potentially contribute to the accumulation of organic compounds in the end product.

The fifth part of the investigation involved the fuming and emissions analysis from the dry and wet asphalt incorporation method. PAHs and VOCs from an asphalt mix (bitumen and aggregate) are quantified using GCMS. A novel method for capturing laboratory-generated methods was developed as part of this study. The emissions analysis was conducted according to the previous method involving four industrial standards. Samples from the dry method were prepared incorporating 0 %, 0.5 %, 1 %, 2 % and 4 % by weight of the asphalt mix at 140 °C, 160 °C and 200 °C. The emissions from recycled plastic-modified asphalt were considerably lower than conventional methods. In one instance, using recycled ABS plastic via the dry method resulted in higher concentration levels than conventional methods. A further compound analysis for that particular type of plastic showed that the carcinogenic compound was elevated even at the lowest working temperature of asphalt production. Despite an increase in the concentration of the carcinogenic compound, the results were found to be below the regulatory limits set by the US EPA for benzene exposure.

As the "mixed" method is the most common method of incorporating recycled plastics, a comprehensive analysis of the mixed method's complete mechanical properties and environmental effects compared to conventional methods was conducted in the sixth part of the investigation. This investigation focused on incorporating recycled plastics into asphalt, where low melting point plastics are used as an asphalt modifier instead of aggregate replacement. The results showed that using low melting point recycled plastics as an asphalt modifier either remained similar or worsened the moisture resistance. However, fatigue and rutting resistance were improved compared to conventional methods. From an environmental perspective, emission levels were reduced compared to conventional methods, while microplastic levels generated were lower than asphalt mixes produced with commercially available polymer-modified bitumen.

The final part of the research investigated the carcinogenic compounds arising from asphalt mixes from both compound groups (VOCs and PAHs). The aim was to conduct a non-carcinogenic and carcinogenic risk assessment to determine the risks involved with workers exposed to fumes. Carcinogenic concentration levels from the wet, dry and mixed methods were compared against conventional methods, and asphalt mixes were produced with commercially available polymer-modified bitumen. The non-carcinogenic and carcinogenic risk assessment of VOCs indicated a negligible risk to exposed workers, while PAHs constitute a possible non-carcinogenic and low carcinogenic risk for workers exposed. Overall, incorporating recycled plastics in asphalt reduced workers' risk of having non-carcinogenic and carcinogenic health issues compared to conventional asphalt mixes.