Critical Parameters Affecting the Carbon Footprint of Asphalt Mixes

There is the need for up-to-date environmental assessment of the different types and sources of asphalt mixes, including the new and developing modified mixes with engineered recycled wastes, such as recycled asphalt pavement (RAP), ground tire rubber (GTR), waste plastic pellets, etc. In recent years, the asphalt industry has shifted its focus to accelerating and adopting more sustainable practices to mitigate environmental issues pertaining to asphalt consumption, such as carbon dioxide emissions, resource depletion and human toxicity. Such impacts can be quantified and interpreted using life cycle assessment (LCA) tools, where the total environmental burden of a product or system is estimated. LCA can help us realize the sustainability potential of asphalt mixes reinforced with recycled material. Environmental impacts of a life cycle for a product or system are available in documents known as Environmental Product Declarations (EPDs), which consist of the following components in detail: product description and ingredients, LCA framework and functional unit, life cycle inventory and allocation method, data of environmental impacts indicators and impact category summary. In this study, 41 EPDs from an asphalt mixing plant in the State of Missouri in the U.S., were analyzed to identify the critical parameters contributing to the total carbon footprint per metric tonne of asphalt mix, considering the extraction of raw materials, transportation and production phases of a pavement life cycle. One of the more dominant environmental factors examined was the global warming potential (GWP) that is measured in terms of the effect of kilograms of carbon dioxide. These mixes varied in terms of type of asphalt mix, source of aggregates, aggregate size, binder grade, modifiers and additives, and recycled material content, mainly RAP. All unmodified mixes showed embodied GHG emissions of about 80 kg-CO2e. The mix that had the highest environmental impact, i.e., 231 kg-CO2e, was a PG76–22 SMA mix with 3.5% SBS modified binder. It is evident in all cases, however, that the extraction of raw materials and preparation of ingredient materials (i.e., Phase A1) dominates the carbon footprint of the production of asphalt mixes. Mixes incorporating lime filler had significantly higher GHG emission in A1 compared to those with other types of mineral filler. The transportation phase (A2) was the least impacting component of the material production stage, though most PPA modified mixes exhibited higher GWP in A2, compared to the rest, suggesting that the PPA is transported over large distances. A GWP of around 42 kg-CO2e (with negligible difference) is found for mix production processes (A3) at the asphalt plant in this study. This research can further bridge knowledge gaps and distinguish inconsistencies in asphalt pavement LCA that can make databases and assessments more efficient.