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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.

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A simple laboratory test for evaluating unstabilized granular base material is introduced. The test consists of monitoring the capillary rise of moisture within a 300-mm-high cylinder of compacted aggregate. The moisture conditions at the aggregate surface are monitored with a dielectric probe. A graph of surface dielectric versus time is used as the basis for performance classification. The poorest-performing materials are those that rapidly reach saturation and exhibit high surface dielectric values. The dielectric is a measure of the “free” or unbound water within the aggregate sample. It is not a simple measure of the moisture content of the material but an assessment of the state of bonding of the water within the fine aggregates. It is this unbound water that is thought to be directly related to the strength of the material and to its ability to withstand repeated freeze-thaw cycling. This test was developed by the Finnish National Road Administration and successfully used to investigate a major flexible base failure. The test is now under evaluation in Texas, where several Panhandle districts have reported cold-weather pavement cracking problems that they have attributed to the flexible bases. The test setup and equipment used are described, and test results from several base materials are presented. Also discussed are laboratory results from a recently completed project comparing eight Texas and four Finnish aggregates. One of the Finnish aggregates was classified as a poor performer; the remaining three were good performers. All of the Texas aggregates were rated as inferior to the high-quality Finnish aggregates. If this test is found to be successful in discriminating good- from bad-performing materials, it could be used to flag potentially poor performers and to evaluate the effectiveness of different base improvement techniques such as chemical stabilization or fines replacement.