Study on inherent characteristics of asphalt regenerating agents: Insights from density functional theory calculations and molecular dynamics simulations

The asphalt mixtures containing reclaimed asphalt pavement (RAP) are gradually used in pavement. When the RAP content exceeds 25%, the influence of RAP properties on the performance of the asphalt mixture becomes more pronounced. To address this barrier, the incorporation of regenerating agents is adopted to soften RAP asphalt and ensure the performance of the asphalt mixture. Currently, research on the effect of regenerating agents is rapidly advancing, but there remains limited exploration of their inherent characteristics. This study employs advanced tools such as molecular dynamics (MD) simulations and density functional theory (DFT) to investigate the chemical properties and material attributes of regenerating agents. Fourteen common regenerating agents are selected, which can be classified into eight categories (A similar to H) based on their structural features. The results reveal that linear alkanes and cycloalkanes are nonpolar surfaces due to their inherent structural characteristics, resulting in larger fraction free volume (FFV) values within the molecular structure. Considering the molecular weight and intermolecular interactions, these components exhibit a higher diffusion coefficient (D) value. In contrast, aromatics with benzene rings experience enhanced polarity and intermolecular interaction due to conjugated electron pairs forming pi-pi interactions, leading to reduced FFV values and D values. Moreover, the increase in molecular weight of aromatics significantly reduces both intermolecular interactions and D values. The carboxyl group in fatty acids leads to higher polarity, resulting in stronger intermolecular interactions and lower FFV values and D values. However, after esterification, the properties of fatty acid esters show improvement. Glycerol, with its hydroxyl group having the strongest polarity, exhibits a dense arrangement with the highest density and solubility parameters, which may exceed that of asphalt and may not be suitable as a rejuvenator. The investigation reveals that the polarity of regenerating agents is positively correlated with density, cohesive energy density, and solubility parameters.