Molecular dynamics modeling and simulation of bituminous binder chemical aging due to variation of oxidation level and saturate-aromatic-resin-asphaltene fraction

Bituminous binder's chemical aging process leads to significant changes in its mechanical and rheological properties. The two main outcomes of chemical aging are the oxidation of molecules and changes in the binder's saturate-aromatic-resin-asphaltene (SARA) fractions. The binder components’ reaction to oxygen results in the formation of polar viscosity-building molecules, while changes in the SARA fractions disturbs the binder's balance, giving it brittle properties. As both of these factors affect the binder at the molecular level, molecular dynamics (MD) simulations can improve the fundamental understanding of binder aging. Therefore, nine MD models were built (one model that represents unaged binder and eight different aged binder models) in this study for two specific purposes: to compare the MD simulation results with the experimental results and to conduct a parametric analysis of the MD simulations to investigate the effect of each aging outcome on the properties of the binder. A comparison among binders with different aging levels showed that the MD simulations and experiments had the same rank order in viscosity values, but they had significantly different magnitudes, which may be partly attributed to the high shear rates used in the MD simulation. The parametric analysis indicated that the dominant aging mechanism in the laboratory aged binder was the disturbance of the SARA fractions, while the oxidation of the molecules appears to be a more dominant mechanism in the field aged binder.