Influence of Surface Layers Upon the Measurement of Concrete Resistivity

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Title: Influence of Surface Layers Upon the Measurement of Concrete Resistivity

Author(s): S. G. Millard and K. R. Gowers

Publication: Special Publication

Volume: 126

Issue:

Appears on pages(s): 1197-1220

Keywords: carbonation; concrete resistivity; corrosion; durability; moisture; surface layers; Materials Research

Date: 8/1/1991

Abstract:
The measurement of the electrical resistivity of concrete is a nondestructive technique that is rapidly gaining acceptance as a means to evaluate the severity of reinforcement corrosion when used in conjunction with potential mapping methods. The concrete resistivity can be determined in situ from placing four equally spaced surface electrodes in contact with the structure and passing a current between the outer electrodes. A measurement of the voltage between the inner electrodes leads to an assessment of the resistivity of the concrete. One practical difficulty in interpreting resistivity measurements is allowing for the error caused by a surface layer with a resistivity lower or higher than that of the underlying concrete. This could, for example, be due to recent wetting of the concrete or due to carbonation of the surface zone. This effect is discussed, and practical correction curves permitting a true assessment of the resistivity of the underlying concrete are given. A very dramatic error in resistivity measurement can occur when there are two surface layers with resistivities--one lower and one higher--than the underlying concrete, such as might be caused by a recent wetting of concrete already having surface carbonation. This wetting can cause a paradoxical increase in the apparent resistivity of the concrete in excess of one order of magnitude. The results of experimental and theoretical studies are presented. The reasons for this effect are discussed, and practical guidance for in situ resistivity measurement is given.