Using Ground-Penetrating Radar for Real-Time Quality Control Measurements on New HMA Surfaces (1702-5)

Segregation and poorly compacted longitudinal construction joints are major problems with new HMA (Hot Mix Asphalt) surface layers. These defect locations typically have higher air voids and greater permeability than the well-compacted areas of the mat. They can permit moisture to enter the lower HMA or base layers and they can initiate severe reductions in pavement life. The causes and remedies of pavement uniformity problems have been under investigation for many years. One major concern has been that there is no technology that can rapidly locate and quantify these defects. Recent research projects at the Texas Transportation Institute have demonstrated that Ground Penetrating Radar (GPR) technology has potential to assist in this area.|This report describes a real-time GPR data acquisition and processing system called RADAR2000. GPR sends pulses of radar waves into the pavement structure and captures the reflections from the layer interfaces within the structure. From the reflection amplitudes and the time delays between peaks the RADAR2000 system computes and displays in real time a) the HMA surface dielectric, b) the base dielectric, and c) the surface thickness. For uniformly compacted HMA layers, the computed surface dielectric should be relatively constant; significant decreases have been found to be associated with high air void locations. The surface dielectric is related to the density of the HMA layer and can be converted to air voids content by the use of calibration cores. The base dielectric can also be converted to base moisture content by the use of laboratory generated calibration curves.|The RADAR2000 system is demonstrated on two new construction projects. The first project involves an evaluation of two different HMA mixes placed on IH 20 in the Odessa District. The GPR data indicated that both materials contained segregated areas. The second project was newly constructed flexible pavement with a thin HMA surface layer. This section was performing poorly after only a few days in service. The GPR indicated that part of the problem was a poorly compacted longitudinal joint which was permitting moisture to enter the granular base layer. The air void content close to the joint was twice that found in the well-compacted areas of the mat.