This paper studies the dielectric dispersion of high frequency radar wave in concrete in early-aged and hardened concrete specimens. Frequency-dependent spectra of phase velocity ν(ω) were measured to deduce the spectra of real part of dielectric permittivity ε'(ω). The dispersion was measured by three high nominal ground penetrating radar frequencies (1.5, 1.6 and 2.6 GHz), experimenting on two steel bars with concrete cover 50 and 100 mm. It was found that ν(ω) and ε'(ω) dispersed at lower frequency, but became stable at high frequency regions, which agrees with the classical GPR plateau. The same frequency components at different nominal antenna frequencies show a close range of ν(ω) and ε'(ω) in concrete of different ages. The results in this paper warrant further investigation of using GPR wave to study material properties.
This paper studies the spatial and temporal spectral absorption of reflector signals of a 1.5 GHz ground penetrating radar (GPR) during a drying process of a brickwall from initial wet to later dry state. The non-stationary GPR signals were processed with short time-Fourier transform (STFT) and wavelet transform (WT) in a novel spatial-timefrequency (STF) domain. Spatial distribution of peak frequency at the direct wave (DW) across the antenna and a backwall reflection was studied to characterize the mechanism of spectral absorption of GPR wave. Results from WT were shown to be more preferred to those from STFT because the WT offers multiple resolutions to cope with both low and high frequency components in GPR wavelets but STFT does not. In addition to the traditional GPR signal interpretation in time-domain and our previous works on time–frequency domain, the analysis method operated in the STF domain provides another possibility of material characterization by GPR in large and field scale.