Investigation of permeability of open graded asphalt mixture considering effects of anisotropy and two-dimensional flow

The objective of this study is to investigate the anisotropic effect of permeability and measure permeability of open graded friction course (OGFC) mixtures under the combined horizontal and vertical water flow. The anisotropic effect of permeability of OGFC mixture was investigated using a customized testing method. The ratios of maximum and minimum permeability coefficients were found in the range of 1.4-1.7 for different mixtures. In addition, considerable differences were observed for the permeability measured in the opposite directions with the same water flow path. A special permeameter was developed to measure permeability coefficient and surface runoff coefficient under two-dimensional water flow considering the effect of rainfall intensity and transverse cross slopes. The testing results show that the permeability of OGFC mixture is affected by rainfall intensity and transverse cross slope in addition to the air voids. These factors affect the effective air voids available for water flow and thus water saturation level in the OGFC mixture. The results indicate that the traditional one-direction flow method may not be adequate to accurately evaluate the permeability of asphalt mixtures with large air voids.