Moisture has a considerable influence on the structural performance of unbound granular (UBG) pavements. Multioffset ground-penetrating radar has the potential to quantify moisture within these materials by measuring the permittivity of pavement layers. To enable moisture estimates from these field measurements, relationships between permittivity and volumetric moisture are required. This paper describes the use of a modified free-space (MFS) laboratory approach to measure the permittivity of compacted UBG material samples and to develop these relationships. Material samples were compacted within form ply boxes; various density and moisture conditions were targeted. A vector network analyzer was used to measure the phase shift between a fixed pair of ground-coupled dipole antennas as a result of sample insertion and an initial ply sheet reference measurement. Frequency-dependent permittivity values for the samples were then determined over the range of 1.0 to 2.0 GHz on the basis of these measurements. Mean values over this range were then related to the volumetric moisture content of the samples. An indicative moisture–permittivity relationship was proposed on the basis of MFS measurements of samples from several quarries. Overall, the permittivity results from the MFS approach showed reasonably good agreement for samples at higher moisture contents compared with relationships in the literature that are based on time domain reflectometry. However, for drier samples, the MFS permittivity values were higher than the literature predictions. Possible reasons for these differences are discussed, and an overview of the advantages and limitations of using the technique for characterizing UBG materials is given.
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