Using Ground-Penetrating Radar and Dielectric Probe Measurements in Pavement Density Quality Control

Abstract

Ground-penetrating radar and capacitance-based dielectric surface probe measurements are used to measure fluctuations in voids, bitumen content, or both, in newly asphalted pavements without causing structural damage. Both methods rely on the compaction of asphalt to reduce the proportion of low-dielectricity air in the material, which increases the volumetric proportions of high-dielectricity bitumen and rock and thus results in higher asphalt dielectricity values. Ground-penetrating radar enables pavement thickness to be measured rapidly from a moving vehicle and information on variations in pavement voids content to be collected simultaneously on the basis of dielectricity fluctuations. The results can be calibrated against real void content by material sampling or by comparison of dielectric value with voids content values determined beforehand for the same material under laboratory conditions. This means that the subcontractor can be informed quickly of any values that exceed or fall below the norms and can take immediate steps to rectify such defects. Other advantages offered by the technique are the rapidity of the measurements and the immediate availability of the results. In addition, the one measurement provides simultaneous information on pavement and base thicknesses and the quality of the latter. The dielectric probe based on capacitance measurements lends itself to use in asphalt mass proportioning examinations performed at the laboratory stage, which enables the values to be used directly for monitoring in situ pavement compaction. The advantages of the dielectricity probe are rapidity of measurement, low-cost meters, and the avoidance of radiation. Thus far, the probe has been excessively sensitive to variations in the roughness of pavement surfaces. The theory behind these research methods is discussed, the methods are described, and the results of laboratory tests conducted at the Texas Transportation Institute in 1994–1995 and field tests performed in Finland in 1995 are presented.