Design and evaluation of a coaxial transmission line fixture to characterize Portland cement concrete

Electrical characterization of portland cement concrete (PCC) is necessary for reliable operation of many electromagnetic nondestructive evaluation (NDE) techniques. Examples of such techniques include ground penetration radar (GPR). For that purpose, PCC specimens were cast, and their complex permittivity and magnetic permeability were measured over a wideband of frequencies (1 MHz to 10 GHz) using both time domain and frequency domain measurement techniques. As part of an ongoing research project, three set-ups to measure the dielectric properties of PCC were designed. The measurements are carried out over a frequency range of 0.1 MHz to 10 GHz. The three different set-ups are parallel plate capacitor (0.1 MHz-40 MHz), coaxial transmission line (0.1 GHz-1 GHz), and TEM horn antenna (0.5 GHz- 10 GHz). This paper discusses the development of the coaxial transmission line fixture. It was designed to allow the evaluation of large PCC specimens and obtain an average effect of PCC components. A time domain reflectometry (TDR) setup was used in the design of the fixture and in testing its performance. A calibration scheme was developed and used to calibrate the fixture. Using a two-port S-parameter model, the complex permittivity and magnetic permeability of the PCC specimens were evaluated by frequency domain measurement. Time domain measurement techniques were based on one-port S-parameter model. Evaluation of the set-up using especially molded PCC specimens showed that curing status can be detected at a lower frequency using the imaginary component of the relative permittivity.