Ground penetrating radar (GPR) technology has been used for the past 30 years for a variety of applications to assess pavement performance and structure. The main issue after all these years remains: "How well does GPR work and under what conditions?" Results show that GPR works well for some situations, but is not an appropriate tool for other situations. GPR is currently not used on a routine basis by the US Departments of Transportations (DOTs) due mainly to difficulties encountered in data interpretation, as well as the expenses involved for conducting GPR surveys. Data interpretation difficulties are mainly attributed to the fact that images obtained from the reflected signals are dependent on the GPR frequency used, and the dielectric properties of the structural materials. To calibrate GPR systems and to better interpret collected data (signals), a project is currently underway at the Virginia Smart Road in Southwest Virginia. Twelve different flexible pavement sections and one continuously reinforced concrete pavement section, comprised of different layers/materials, are incorporated in the test facility. These sections provide a unique opportunity to explore the feasibility of using GPR to assess pavements, and to verify its practicality. Thirty-one copper plates are placed at different layer interfaces throughout the pavement sections. The copper plates serve as a reflecting material and thus allow the determination of the dielectric constant of the layers over the GPR frequency range. Two GPR systems were considered in the preliminary evaluation: an air-coupled system with a pair of horn antennae operating at a center frequency of 1 GHz, and a ground-coupled system operating at a center frequency of 900 MHz. This paper presents preliminary results from both GPR systems. Early results revealed the capability of each system's penetration and their capability to distinguish between layers. Due to the low dielectric contrast between pavement layers, especially base/subbase layers, the copper plates serve as means to indicate that adequate energy reached those deep layers. Reflection at the interface between layers with similar dielectric constants will be used to fine-tune GPR data analyses as the layer materials' dielectric constants and layer thicknesses are determined at the Smart Road.
