Measurement of the electrical properties of concrete at radio frequencies
This thesis is a presentation of research into the elecrical properties of concrete at radio frequencies (1-1000 MHz). The physical properties of concrete are examined and found to be profoundly influenced by water both at gauging and at later stages of development. The merits of currently used test methods for quality assessment of concrete are discussed. The theory of polarization in dielectric materials is examined and a range of formulae suitable for frequency dispersion calculations in homogeneous and heterogeneous materials is presented. A review of previous research into the electrical properties of concrete at frequencies in and below the RF range, including a brief description of Ground Probe Radar techniques, is presented and the results discussed. A distinct shortage of data is found in the 100-1000 MHz range. The electrochemical properties of concrete are considered and their influence on the electrical properties is examined. A previous hypothesis in this area, based on the behaviour of ions in the mix water, is shown to be incorrect, but a new hypothesis relating the dielectric dispersion of fresh concrete to ionic effects at the surface of hydrating cement grains is proposed. The results from calculations based on the hypothesis are compared with available data at 1-1000 MHz and found to be in agreement. A geophysical explanation, previously derived for sedimentary rocks, is proposed as an influencing factor on the electrical properties of hardening concrete in the same frequency range. Measurement techniques at radio frequencies are considered and the application of these to concrete measurements is discussed with reference to two particular electrode systems at 1-100 MHz and 100-1000 MHz. A technique for calibrating out the effects of connector distortion is described and the results of tests presented. The results from a range of experiments on various concrete mixes in the 1-1000 MHz range are presented and discussed. The data obtained for fresh concrete agree very well with the hypothesis. The data for hardening concrete show a qualitative agreement with the relevant hypothesis. The probable effects on Ground Probe Radar signals of the dispersion observed for hardening concrete in the 100-1000 MHz range are discussed. Suggestions for further research are given.