The bearing capacity of sub-asphalt soils, which is a critical parameter for assessing pavement conditions and guiding pavement maintenance, is greatly influenced by soil water content. In this study, ground-coupled ground penetrating radar (GPR) techniques were used to non-destructively monitor the volumetric water content in sub-asphalt aggregate layers during an extended infiltration experiment. Water was injected over a period of several months into two differently layered pavement test sections, one of which contained a sub-asphalt drainage layer. GPR travel time data were used to estimate the water content in each aggregate layer and the variations in water content with time, and GPR amplitude data were used to indicate areas of high water content immediately beneath the asphalt layer. The GPR data revealed significant variations in the water content of the aggregate layers in both the horizontal and vertical directions and over time. Comparison of the water content estimates from GPR travel time data and from gravimetric water content measurements showed that the difference between the two techniques was approximately 0.02 cm3/cm3. These results suggest that GPR techniques can be used for accurate, non-invasive water content estimation in sub-asphalt aggregate layers and for assessing the efficacy of pavement drainage layers.