It has been demonstrated that water vapor consistently transports in asphalt pavements. The relative humidity differential, which exists between the subgrade below and the atmosphere above the pavement, is a major contributor to the water vapor diffusion. However, the effect of the relative humidity differential has not been quantified on the water vapor diffusion in asphalt mixtures. This paper designed a laboratory experiment to investigate the water vapor diffusion in asphalt mixtures under a number of relative humidity differentials. The designed experiment consisted of five individual water vapor diffusion tests, each of which was performed at a specific relative humidity differential in the pre-vacuumed measuring cell of the test equipment. Two diffusion models were developed based on the Fick’s second law of diffusion. It was found that the two-dimensional model provided an accurate characterization of the water vapor diffusion in both radial and axial directions of the cylindrical test specimens.

A linear model was established between the relative humidity differential and the total diffused mass based on the real gas law. An inverse proportionality was identified between the relative humidity differential and the diffusivity in both radial and axial directions. The determined diffusivity values were further converted to the corresponding diffusivity values under 1 atmosphere, which were in agreement with the diffusivity data reported in the literature. A linear model was derived for the relationship between the relative humidity differential and the diffusivity under 1 atmosphere, which indicated faster water vapor diffusion in asphalt pavements when subjected to a larger relative humidity differential between the subgrade and the atmosphere. The diffusivity in the radial direction was always larger than that in the axial direction despite the variation of the relative humidity differential. This fact implied the anisotropic distributions of air voids and aggregates in the asphalt mixture specimens, which assisted water vapor in diffusing more easily in the radial direction than in the axial direction.