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.
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