Molecular-Dynamics Simulation Approach to Identifying PAH Release from Asphalt Concrete Pavements due to Mechanical and Environmental Factors

Abstract

Asphalt is a primary source of polycyclic aromatic hydrocarbons (PAHs) in urban runoff originating mainly from asphalt concrete (AC) pavements and parking lots. High levels of PAHs have been reported in soil and water samples collected at close proximities to AC pavements. No major study has investigated the release mechanism of PAHs from asphalt due to mechanical (e.g., tire pressure) or environmental (e.g., temperature and moisture content) factors. In this study, molecular-dynamics (MD) simulation has been utilized to investigate the behavior and interaction of the asphaltene component (e.g., asphaltene phenol) present in asphalt with three different PAH molecules containing two (e.g., naphthalene), four (e.g., fluoranthene) and five (e.g., benzo[a]pyrene) aromatic rings under varying mechanical and environmental conditions. The simulation has been conducted to predict the potential and extent of PAHs released from asphalt at different temperatures varying from −18°C to 38°C, at different moisture contents ranging from 1% to 5%, and also at different vehicular tire pressure varying from 0.55 to 0.68 MPa. The potential and extent of the PAHs release have been predicted by the total energy loss of PAHs in asphaltene phenol by comparing energy loss at 5% moisture with no moisture present. Results show that there is higher energy loss in asphaltene phenol at higher moisture contents and lower temperatures during no-tire-pressure conditions. At 5% moisture and −18°C temperature with no tire pressure, the three PAHs are losing 90%, 88%, and 89% of total energy when compared with 0% moisture at the same −18°C temperature and no tire pressure. The total energy loss decreased with the increase in tire pressure. The study indicates that mechanical (e.g., tire pressure), and environmental (e.g., temperature and moisture) factors have significant effects on PAH release from AC pavements.