Professor Yong-Rak Kim
Date of this Version
Farshad Fallah, "Molecular Dynamics Modeling and Simulation of Bitumen Chemical Aging" Master's thesis, University of Nebraska-Lincoln, 2017.
Chemical aging of asphalt binder leads to significant changes in its mechanical and rheological properties, resulting in poor pavement behavior and distress. Various laboratory methods have been used to simulate asphalt aging during the service life of the pavement. However, controversy exists regarding the capability of these methods to predict field aging, as various factors interact with the pavement during service and the mechanism behind aging is not fully understood. The two main outcomes of chemical aging are oxidation of asphalt molecules, and change in asphalt SARA (saturate, aromatic, resin, and asphaltene) fractions. Reaction of oxygen with asphalt components forms polar viscosity-building molecules, while the change in SARA fractions upsets the balance in asphalt, giving it brittle properties. As both these factors affect the binder at the molecular level, molecular dynamic (MD) simulations can provide core insights into understanding the fundamentals of asphalt aging. This study used MD simulation to investigate the effect of each aging process on the properties of asphalt binder. In that respect, a representative model of virgin binder and eight different models of aged binder were built, including models for laboratory- and field-aged binder. Properties such as density, bulk modulus, glass transition temperature, and viscosity from different MD models were compared to distinguish between the effect of each aging mechanism on binder properties. The results showed that both aging mechanisms lead to an increase in density, bulk modulus, and viscosity of the binder. The SARA fractions of laboratory-aged binder were found to affect bulk modulus and viscosity more dramatically compared to field aged binder. However, when the oxygen content of models was considered, field-aged binder showed a larger increase in bulk modulus and viscosity compared to laboratory-aged binder. This indicates that the dominant mechanism from laboratory aging process is upset in SARA fractions, while oxidation of molecules might be a more dominant mechanism for field-aged binder. No statistically significant difference was observed between the glass transition temperature values for virgin and aged binder.
Advisor: Yong-Rak Kim