Inversion of non-invasive resistivity measurements in fourelectrodes configuration for corrosion characterization in reinforced concrete samples
Résumé
In the context of corrosion diagnosis in reinforced concrete (RC), non-invasive four point electrical measurements represent, by many aspects, an interesting alternative to classical three-point characterization with rebar connection. As a matter of fact, the four-point surface measurement without connection to the rebar, is known to be sensitive to the rebar corrosion. Yet, this rebar effect was first seen as a drawback to the method, initially meant to provide a reliable estimation of the structure durability by assessing the concrete resistivity. The first advantage of this method, namely its absence of rebar connection, comes with a complexification of the current paths within the RC structure. Part of the current diffuses only in the concrete while a residual part will penetrate in the rebar depending on its corrosion current density jcorr. The determination of the rebar corrosion through that kind of measurement is therefore less straightforward than for three-point measurements. However, by multiplying the acquisition points and running an Electrical Resistivity Tomography (ERT), it becomes conceivable to spatially image the RC structure. The four-point resistivity data may be inverted in order to recover both the instantaneous corrosion rate of the rebar as well as the matrix electrical properties and heterogeneities. We developed a laboratory ERT experiment (CorImager®-device from IRIS Instruments) designed to sequentially run multiple four-point measurements on prismatic reinforced mortar specimens of dimensions 10 x 10 x 40 cm3 . The device, composed on the one hand of 96 electrodes distributed over the 4 faces of a corset like antenna (see Fig.1), and on the other hand of an acquisition central enabling multi- channel measurement and switching, provides us with a 3D investigation of our probe. Our samples collection gathers about thirty specimens, most of them armed, differing in the mix or in the rebar preparation. Some have been specifically designed to test the widely reckoned ability of this method for heterogeneous matrix characterization, on our unusually small sample dimension.
Preliminary results on such a heterogeneous sample, bearing two polystyrene resistive inclusions of dimension 4 x 4 x 3 cm3 in a regular CEMI mortar matrix such as w/c = 0.5 and s/c=3, have already shown that this method can concomitantly provide, by data inversion, an image of the matrix heterogeneity (see Fig.2) together with an estimation of a uniform corrosion value. This jcorr-value of 16mA/m² obtained through inversion of ERT data, was comparable to the values of 8 ± 2 mA/m² determined by classical three points measurements.
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