Transformation Behavior and Mechanism of Sara Fractions in Aged Asphalt: A Molecular Simulation and Experimental Study

Abstract

One of the main manifestations of asphalt aging is the conversion of its light components into heavy ones. This study aims to examine the chemical structural changes during the thermal-oxidative aging of asphalt components and to explain the underlying transformation mechanisms. The separated SARA fractions were aged individually, and asphaltenes transformed from resins were precisely extracted through secondary separation. Elemental analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and proton nuclear magnetic resonance (1H NMR) were used to analyze the chemical structure of asphalt components before and after aging and transformed asphaltenes. The results indicate that although the chemical structures of different asphalt components vary, their aging processes are similar. Aging primarily increases the number of oxygen-containing functional groups rather than promoting the transformation of molecular structures into other components. The chemical structure of transformed asphaltenes is closer to resins than to the polycyclic aromatic hydrocarbon-dominated asphaltenes, but their oxygen content and functional group distribution differ significantly from resins. Molecular dynamics (MD) simulations of the aggregation behavior of resin molecules were conducted. The analysis shows that the increase in oxygen-containing functional groups leads to higher polarity, promoting intermolecular aggregation, with aged resins exhibiting stronger aggregation behavior than unaged resins. After aging, resins become less soluble in n-heptane, and the collective aggregation of molecules is identified as the fundamental cause of their transformation into asphaltenes. This study provides deeper insights into the transformation process of asphalt components during aging, offering theoretical guidance for accurate modeling and regeneration methods for aged asphalt.

Keywords: road engineering, Thermo-oxidative aging, SARA fractions, Converted asphaltenes, Molecular Dynamics