Application of the Langmuir-type diffusion model to study moisture diffusion into asphalt films

Abstract

The objective of this study was to investigate the behavioral characteristics of moisture diffusion into asphalt films and to analyze the correlations between diffusion parameters and asphalt components. A Langmuir-type diffusion model was successfully established and applied.Experimental moisture uptake data of four asphalt films immersed in distilled water for 258 h were measured at intervals by the gravimetric method, and the diffusion parameters involved in the Langmuir-type model were exactly fitted by the least square method. Then, a component analysis of the asphalt was conducted by following the solvent precipitation method. Thus, correlations between diffusion parameters and asphalt components were further analyzed.The results show that the behavioral characteristics of moisture diffusion into asphalt films can be well explained by the theory of the Langmuir-type model and that the water molecules absorbed in asphalt films are in mobile and bound phases. The process of moisture diffusion in asphalt films can be divided into two stages: in the first stage, both adsorption of water molecules on the surface of the asphalt (transformation from the mobile phase to the bound phase) and transport of water molecules in the mobile phase through diffusion channels within the asphalt occur simultaneously; in the second stage, adsorption proceeds on the surface of the asphalt, and moisture uptake in the mobile phase becomes constant. The ratio of the adsorption coefficient to the desorption coefficient exhibited an excellent positive correlation with the moisture uptake in the bound phase, as well as with the total equilibrium moisture uptake (R2＞0.99). Among the four components in asphalt, the content of asphaltene was significantly and positively correlated with the adsorption coefficient, while polar aromatics were inversely correlated.