Multiscale Investigation of Moisture-Induced Structural
Evolution in Asphalt–Aggregate Interfaces and Analysis of the
Relevant Chemical Relationship Using Atomic Force Microscopy and Molecular
Dynamics
posted on 2020-03-12, 14:11authored byZhiyang Liu, Liping Cao, Tao Zhou, Zejiao Dong
Chemical changes
and intermolecular interactions in asphalt dominate
the molecular reorganization and cause the evolution of micro- and
mesostructures. Given the lack of knowledge regarding the molecular
chemistry–microstructure relationship of the asphalt–aggregate
interface, the moisture-induced adhesive failure occurring at this
interface has not been fully understood. This study investigates the
multiscale structures of the asphalt–aggregate interfaces exposed
to water and establishes the relationship between the structures and
molecular interactions. The meso- and micromorphologies of two types
of treated interfacial asphalts were observed via optical microscopy
and atomic force microscopy. The results show an undulated surface
and boundary retreat in asphalts because of the overall interfacial
tension. Dispersed microbumps measuring tens of nanometers in height
progressively grow until they merge into large bumps with increasing
water exposure depending on the types of asphalt and aggregates. Fourier
transform infrared (FTIR) spectrometry results show enriched polar
components at the surface of the treated interfacial asphalt and water
diffusion driven by complex intermolecular forces. The molecular behavior
simulated by molecular dynamics calculations reveals that aliphatic
molecules amalgamate into nonpolar clusters, while polar molecules
migrate out and act as a surfactant to stabilize the asphalt–water
system driven by the interfacial tension gradient. Internal coalescence
of nonpolar components results in protrusion of the asphalt’s
surface, and the migration of polar components to the surface accounts
for the increased absorption peaks of the polar groups. This phenomenon
could explain the FTIR spectra and formation of microbumps. The state
of absorbed water and nanostructures of the interfacial asphalt are
dominated by intermolecular interactions among asphalt, water, and
aggregates. This study provides deep insights into the structural
evolution of asphalt from the chemical and molecular perspectives.