Multiscale Investigation of Oxidative Aging in Biomodified Asphalt Binder

Oxidative aging causes major changes in asphalt binder's physiochemical and rheological properties, giving rise to pavement distress and failure. Asphalt aging occurs due to two main processes: a loss of volatile components and a reduction of the malthene phase and oxidation of certain functional groups in asphalt, increasing the concentration of asphalt's polar components. Performing SARA (saturate, aromatic, resin, and asphaltene) analysis and FTIR (Fourier transform infrared) spectroscopy on the crude oil shows that the ratio of polar components to nonpolar ones is higher in oxidized asphalt compared to virgin asphalt. However, when a biomodifier is introduced to virgin asphalt, the rate of carbonyl (as a polar functional) formation is reduced, which may indicate a delayed oxidation due to the presence of biomodifier molecules in the asphalt. To understand the fundamental origin of oxidative aging at the molecular level, this paper provides a comprehensive computational chemistry analysis conducted in conjunction with laboratory experiments. On the basis of the results of our analysis, the enhanced performance of biomodified asphalt binder stems from dual-protection mechanisms of biobinder components that defer asphalt aging: the less reactive molecular species found in biobinder that show little propensity toward oxidation and consequently are less affected by the new polar functionalities and the highly reactive components (such as α-tocopherol) that are the primary targets for oxidative attacks, acting as sacrificing elements to save key components of asphalt materials (such as asphaltenes) from oxidative agents. Polarizability calculations show that biobinder constituents are considerably less polarizable than asphalt molecules. Lower polarizability of biobinder indicates the lower tendency of these chemical species toward new polar functionalities arising from the presence of oxidative agents. In contrast, the high polarizability obtained for asphaltene molecules suggests that they are easily affected by the oxidative agents. Therefore, the presence of α-tocopherol in biobinder acting as a sacrificing element could delay asphaltene oxidation as evidenced by the lower carbonyl formation in asphalt samples containing biobinder.