Measurement of the electrical resistivity of concrete is a non- destructive technique that may be used in conjunction with potential mapping as a means of evaluating the severity of reinforcement corrosion. Concrete resistivity can be determined by placing four equi-spaced electrodes in contact with the structure to be measured, and passing an alternating current between the outer electrodes. Measuring the potential difference across the inner electrodes enables the resistivity to be calculated. A source of practical difficulty in measuring the resistivity in this way is the error caused by a surface layer with a resistivity lower or higher than that of the underlying concrete. This might be the result of, for example, salt ingress or carbonation of the surface. The effect is analysed experimentally and theoretically, and practical correction curves are given, which permit an assessment of the true resistivity of the underlying concrete. Substantial error can be caused where there is a second surface layer with a relatively low resistivity on top of a high resistivity surface layer. This can occur, for example, with surface wetting of carbonated concrete. This low resistivity layer can cause a paradoxical increase in the resistivity measurement. The reasons for this effect are discussed, and practical guidance for in-situ resistivity measurement is given. (A)
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Millard et al. (1992) studied this question.