Effect of molecular structure on low-temperature properties of bitumen based on molecular dynamics

Abstract

Low-temperature cracking is one of the most prevalent distress in asphalt pavement, closely related to the bitumen low-temperature properties. However, the molecular mechanism of low-temperature properties of bitumen is still ambiguous. This paper focuses on studying the effects of molecular structure on the low-temperature properties of bitumen by molecular dynamics (MD) simulation. The low-temperature viscoelasticity and glass transition temperature of bitumen were measured by dynamic shear rheometer (DSR) and differential scanning calorimeter (DSC) experiment. The MD models containing 20 kinds of molecules were constructed to represent the bitumen and validated by density, shear modulus and glass transition temperature. Furthermore, the effects of molecular structure on the low-temperature properties were analyzed by relative density, mean square displacement (MSD), radial distribution function (RDF), and potential energy. The DSR and DSC results showed that the low-temperature properties ranking of the bitumen is HY90 > LH50 > LH90 > LH70. The relative density and RDF curves showed that the increasing aromatic carbons in resins and aromatics reduce the asphaltenes aggregation degree and improve the low-temperature properties by promoting the formation and dispersion of asphaltene-resin aggregates. The MSD curves indicated that the bitumen with a high average diffusion coefficient has good low-temperature properties. The increase of aromatic carbons improves the diffusion coefficient by forming molecules with low molecular mass. Moreover, non-bond energy positively correlates with the low-temperature properties of bitumen. The low-temperature properties of bitumen can be optimized by increasing the resins and aromatics, or adding molecules with high aromatic carbon content and diffusion coefficients.