Environmental assessment of asphalt mixtures modified with polymer coated rubber from scrap tires

Abstract

Polymer coated rubber (PCR) is an engineered crumb rubber developed to address the drawbacks of regular crumb rubber in asphalt mixtures, such as the absorption of asphalt binder and premature cracking. Assessing the use of PCR using life cycle assessment (LCA) is important to evaluate whether its use lowers the environmental impact of asphalt mixtures. In LCA, the choice of allocation procedure significantly impacts the results. The cut-off method is often used in LCA studies of products containing recycled materials. The objectives of this work were to evaluate: i) if asphalt mixtures modified with PCR had lower environmental impact than mixtures modified with synthetic polymers, ii) if using more PCR decreased the environmental impact of the asphalt mixtures, iii) if the results changed by using different allocations methods, and finally, iv) if the environmental impact of asphalt mixtures changed when considering the long-term mechanical performance of the blends. In this work, the global warming potential (GWP), fossil depletion, and cumulative energy demand of one ton of asphalt mixture were calculated using the cut-off method, economic allocation, and system expansion. For the system expansion, the recovery energy in cement kilns from scrap tires was compared to burning petroleum coke or natural gas. The LCA of asphalt mixtures modified with two amounts of PCR (0.5% and 1% by weight of total mixture) compared to the synthetic polymer styrene-butadiene-styrene (SBS) was performed by including results of the experimental investigation (asphalt mixture design and mechanical testing conducted in the asphalt laboratory at Michigan State University). Results were different based on the allocation method considered. Overall, the GWP of PCR 0.5% and PCR 1% mixtures was lower than the SBS mixture, except in one case. The GWP of PCR 1% was 2% higher than the SBS mix when petroleum coke was used in the system expansion. Adding more PCR did not necessarily increase the environmental benefits. PCR 1% had a higher GWP than PCR 0.5% (2.6%, 2.7%, and 6.3% by using cut-off, economic allocation, and system expansion, respectively), except for the system expansion using natural gas as a replacement, which was 3.7% lower than the PCR 0.5%. However, the system expansion with natural gas had the lowest GWP compared to the other methods. On the contrary, the system expansion considering petroleum coke had the highest CO2eq emissions among all the methods used. The GWP of PCR 0.5% and PCR 1% was similar by using cut-off and economic allocation. As the cement industry transition to low-carbon energy, the carbon footprint associated with using scrap tires in pavements will decrease when using system expansion, while it will remain the same with the cut-off method. Considering the mechanical performance of the mixtures, the GWP for the pavement with 0.5% PCR was 8.04% lower over its service life than the synthetic polymer blend.