Morphological Analysis of Asphalt Oxidation and Its Relationship with Changes in Its Chemical and Thermo-rheological Properties

The formation of aromatic structures and unsaturation (carbon–carbon double bonds) increases the stiffness of asphalt during oxidation. This can be explained by the fact that the presence of oxygen and double bonds in the material’s molecular structure enhances its polarity and, consequently, the number and magnitude of intermolecular interactions. It is expected that a rigid (oxidized) asphalt would be less efficient in dissipating energy through deformation and flow, as its molecules have reduced mobility compared to a less rigid (non-oxidized) asphalt. In other words, oxidized asphalts have a greater tendency to dissipate energy through fracture. The development of methodologies to enhance asphalt properties has been evolving, especially in the analysis of rheological properties. However, little has been done regarding the microstructural characterization of asphalt through scanning electron microscopy, which allows for studying variations in morphology, crystal formation, cracks, or other structures that explain the material’s behavior as it oxidizes. The objective of this paper is to study the microstructure and elemental composition of asphalts subjected to oxidation processes under ambient conditions in Costa Rica by means of scanning electron microscopy (SEM). The obtained results show significant findings regarding increased stressed areas, material roughness, changes in chemical and mechanical properties, and the cyclic evolution of its morphology. This provides evidence of the mechanism of micro-crack generation in asphalt.