Dependency of Relative Humidity and Temperature on Diffusion Parameters for Water Vapor Diffusing into Asphalt Mixtures

Relative humidity (RH) and temperature are two major influences on water vapor diffusion in asphalt pavements. However, the combined effect induced by RH and temperature has not been qualified. This study evaluated the RH dependency of Phase I water vapor diffusion and then explored the combined effect caused by RH and temperature. A series of Phase I water vapor diffusion tests at various RH levels were designed and carried out on asphalt mixture specimens using a Gravimetric Sorption Analyzer (GSA). Diffusivity ($D$) and water vapor retention capability per unit mass under unitary gas of water vapor with pressure $P_w$ ($\theta$) were determined by applying a more advanced three-dimensional (3D) diffusion model. $D$ and $\theta$ were then converted into their corresponding values at 1 atmosphere with the same water vapor pressure ($D_{atm}$ and ${\theta }_{atm}$) based on two developed conversion equations. A quadratic function–type model was established between RH and ${\theta }_{atm}$ theoretically based on Henry’s law. A similar quadratic function–type model was also developed between RH and $D_{atm}$ based on the definition of diffusivity in porous materials. Furthermore, two comprehensive models that took both RH and temperature into account were theoretically established and were able to describe the RH-temperature dependency of $D_{atm}$ and ${\theta }_{atm}$. The combined effect of RH and temperature was presented in forms of multiplication.