Investigation of Oxygen Diffusion Behavior in Asphalt Mixtures Using Molecular Dynamics Simulations and Laboratory Test

The long-term durability of asphalt pavements is closely related to the oxidative aging of asphalt binders. Oxidative aging depends on the oxygen diffusion in asphalt mixtures. To further investigate oxidation reactions and guide the design of age-resistant asphalt mixtures, we conducted a combined experimental and molecular dynamics (MD) study to investigate the oxygen diffusion in the asphalt mixture and to analyze the effects of volumetric characteristics of the asphalt mixture and environmental factors on the oxygen diffusion. A diffusion apparatus was used to measure oxygen diffusion in compacted asphalt mixtures. MD simulations were performed on the asphalt mixture-oxygen diffusion model to simulate the oxygen transport process. The results showed that oxygen diffusion was influenced by the temperature, oxygen concentration, air void, and height of the asphalt mixture. The intrinsic mechanism was that the temperature accelerated the oxygen transport by increasing the self-diffusion coefficient of the oxygen molecules. Although the oxygen concentration did not affect the oxygen self-diffusion coefficient, it increased the oxygen base, which in turn increased the number of oxygen molecules passing through the asphalt mixture. Moreover, the increased air voids of the asphalt mixture decreased the obstruction in the oxygen diffusion process and increased its transmission speed, whereas the increase in the height of the asphalt mixture increased the oxygen transmission path, thereby reducing the oxygen transmission efficiency. The results of MD simulation were consistent with the experimental results. Therefore, it is feasible to use MD simulations to investigate diffusion behavior of oxygen in asphalt mixtures.