Climatic influences on vertical asphalt aging: A two-year field study and molecular-level analysis

To elucidate the vertical gradient of asphalt aging under natural environmental exposure, six field observation stations were strategically established across climatically diverse regions. Asphalt binder specimens were deployed at these sites and subjected to natural aging over a two-year period. Following exposure, comprehensive assessments were conducted, including mechanical performance testing, fatigue life evaluation, and chemical compositional analysis. Complementing these field investigations, we employed molecular dynamics (MD) simulations to probe the thermodynamic behavior of naturally aged asphalt and elucidate the molecular transformations underpinning macroscopic performance changes. The results demonstrate that natural asphalt aging exhibits a clear gradient from the surface to deeper layers, significantly influenced by regional climate. In hot climates, elevated temperatures enhance binder diffusivity and reaction kinetics, resulting in deeper and more uniform aging, whereas cooler regions show aging primarily confined to surface layers. Two distinct vertical aging mechanisms were identified: a surface-limited, UV-driven process in cold, high-altitude areas, and a diffusion-assisted, thermally accelerated process in warm, low-altitude regions, each producing different mechanical and chemical profiles. Moreover, natural aging leads to systematic increases in cohesive energy density (CED) and solubility parameter (SP), driven by the accumulation of polar functional groups that strengthen intermolecular cohesion. Importantly, CED proves to be a robust thermodynamic indicator of asphalt aging severity, correlating with higher stiffness, reduced phase angle, and diminished fatigue life. These findings not only deepen our understanding of asphalt aging under various environmental conditions but also hold significant implications for the development of more targeted and efficient pavement design and maintenance strategies. • Field-Aging Investigation Across Six Climatic Regions • Two distinct vertical aging mechanisms were discovered • Multi-Scale Analysis Unveils Molecular Aging Mechanisms • Systematic Increases in CED and SP During Natural Aging • CED as a Thermodynamic Indicator of Aging Severity