Wetting and drying in hydrophobic, macroporous asphalt structures

Abstract

Wetting and drying of a hydrophobic, macroporous medium like porous asphalt (PA) are investigated in this study under different environmental loadings. Three different types of porous asphalt, PA4, PA8 and PA11, with different pore size distributions, are used to produce specimens that are sealed on all sides except at the top surface, and are wetted by discrete water droplets or by pouring water from the top. The wet specimens are subjected at the top surface to airflow, at low (1m/s) or high (3m/s) wind speeds, and heat from a UV lamp. Gravity-driven imbibition by impinging water droplets is observed only in PA11 specimens due to the presence of large pores with diameters between 2 and 6mm. However, the drying patterns of all the three types of PA are similar i.e. even after 3.5h of forced convective and non-isothermal drying, water evaporates only near the top surface. The hydrophobicity of the solid matrix limits the influence of liquid transport on the drying process. Consequently, the dominant drying mechanism found in PA is evaporation at the free surface of water islands within the specimen, followed by water vapor diffusion to the top surface and its subsequent convection by airflow. Therefore, high wind speeds lead to a faster drying only if moisture is concentrated near the top surface of the specimen. In comparison with a previous experiment, it is seen that in PA8 and PA11 specimens, gravity-driven drainage is the most important source of fast moisture removal and has a much shorter time-scale than convective drying.