Investigating the Diffusion Dynamics of Moisture in Thermally Aged Bitumen

Moisture susceptibility is a major contributor to asphalt pavement deterioration, leading to increased maintenance costs and reduced service life. Despite the known importance of moisture damage, the molecular mechanisms by which oxidative aging influences water diffusion in bitumen─the primary binder in asphalt─remain insufficiently understood. This study aims to bridge that knowledge gap by examining how thermal aging alters the moisture resistance of bitumen through changes in its molecular structure and interfacial behavior. We employed differential Fourier transform infrared (D-FTIR) spectroscopy to monitor real-time water diffusion in thin films of thermally aged and unaged bitumen. Additionally, we used density functional theory (DFT) calculations to quantify water–bitumen interactions and identify how aging-induced chemical modifications affect bitumen’s polarity. D-FTIR analysis revealed that thermal aging significantly increased water uptake across all bitumen grades tested, with aged samples showing faster and more sustained diffusion. The spectra also showed codiffusion of polar bitumen components─particularly resins and asphaltenes─toward the water-exposed surface, while nonpolar molecules migrated away. DFT results confirmed that these polar compounds exhibit higher interaction energies with water, and that oxidation introduces polar functional groups (e.g., carbonyl and sulfoxide) that enhance hydrogen bonding and electrostatic interactions with water molecules. Overall, this work demonstrates that oxidative aging promotes water adsorption and accelerates diffusion by increasing molecular polarity and inducing surface redistribution of bitumen components. These findings offer important insights for the development of more durable, moisture-resistant bitumen formulations, ultimately supporting longer-lasting and more sustainable asphalt pavements.