Thermodynamics and kinetics of moisture transport in bitumen

[Display omitted] •A parameter optimization approach combined with the finite element method is proposed to study moisture transport in bitumen.•Clustering of water molecules occurs at high relative humidity levels, accompanied by significant decrease of moisture diffusion coefficient.•The diffusion coefficient and water cluster size of each binder type depends on the bitumen chemical and structural properties.•The Flory-Huggins analysis of moisture isotherms reveals a feasible method to measure the bitumen solubility parameter. Moisture in bitumen and at the bitumen-aggregate interface affects the cohesive and adhesive properties of asphalt mixtures, which are critical for the service performance and durability of pavements. This paper aims to investigate the kinetics and thermodynamics of moisture transport in bitumen at various temperatures and relative humidity for different bitumen types. Transport models are introduced to study the moisture transport mechanisms. A parameter optimization approach combined with the finite element method is applied to simulate moisture transport behavior. Results show salient sorption increase at higher relative humidity levels (more than 70%), indicating the occurrence of clustering of water molecules in bitumen, which can lead to a significant decrease of the diffusion coefficient. Transport models show great quality in simulating experimental results, in which the S-Cluster model provides a detailed explanation of the moisture transport mechanisms and describes better the performance at high sorption levels. The diffusion coefficient, cluster size and activation energy were determined and were found to be linked to the bitumen chemical and structural properties. The transport kinetics and thermodynamics are expected to contribute to a comprehensive understanding of moisture transport behavior in bitumen and further of pavement moisture damage at complex and interacting environmental conditions.