Investigation of effect of temperature on water vapor diffusing into asphalt mixtures

This study investigated the effect of temperature on Phase I water vapor diffusion in asphalt mixtures. A test protocol with a series of Phase I water vapor diffusion tests was designed and performed on hot mix asphalt specimens and fine aggregate mixture specimens. A Gravimetric Sorption Device was utilized to perform the diffusion tests on three replicate specimens of each mixture type at five temperatures. A relative humidity differential was established between the inside and the outside of the test specimen and drove water vapor to diffuse into the specimen until reaching equilibrium. The mass of the test specimen was continuously measured at a frequency of every 5 s using the magnetic suspension balance. The mass of water molecules diffused into the specimen at any time point was calculated to be the measured mass of the specimen at that time point less the initial weight of the specimen. A previously developed 3-D diffusion model was applied to the test data of all replicate specimens to determine the Phase I water vapor diffusivities and the moisture retention capabilities per unit mass under only water vapor pressure at various temperatures. These two parameters were converted into the corresponding values under 1 atmosphere, which were increasing with the increase in temperature. It was found that water vapor diffused into the asphalt mixtures at a significantly lower speed under 1 atmosphere than under only water vapor pressure. It was also observed that an asphalt mixture had a much smaller moisture retention capability per unit mass under 1 atmosphere than under only water vapor pressure. The Arrhenius equation was selected based on theoretical justification to quantify the effects of temperature on diffusivities and moisture retention capabilities per unit mass. Excellent goodness of model fit was achieved for every replicate specimen with an R2 value larger than 0.95. The Arrhenius equation was demonstrated to be an appropriate theoretical model that was able to accurately characterize the temperature dependency of both diffusivities and moisture retention capabilities per unit mass.