Noninvasive surface measurement of the corrosion impedance of rebar in concrete

Corrosion of steel reinforcing bars embedded in concrete generates expansive products and decreases the cross section of the bars, compromising the safety of the structure. Early detection of corrosion of reinforcing steel in concrete structures permits appropriate repair to be made to extend the life of the structures. Information on the early stages of corrosion also provides vital insight on the corrosion mechanism. Unfortunately, there is only a limited number of appropriate methods that generates reliable and fast data on the corrosion rate of reinforced concrete. Existing corrosion techniques were developed for corrosion of metals in aqueous media and require direct contact to the metal through the media. The need to establish electrical contact with the reinforcing bar in concrete requires that the concrete cover be removed and this often reduces the long-term concrete performance.; A new electrochemical technique has been developed to assess the corrosion state of a steel bar embedded in concrete. The new method uses electrodes placed on the concrete surface to measure the impedance response from the underlying corroding steel bar and it does not require to access the rebar. Experimental work was conducted in reinforced concrete blocks to measure the impedance response of a steel bar with different corrosion rates and different corrosion extents. An analytical model was constructed to simulate the impedance response for a given corrosion state of a steel bar embedded in concrete.; The experimental results show that the corrosion rate and the accumulation of previous corrosion products had a significant influence on the impedance measured on the surface of the concrete. The values of peak phase and peak imaginary part decrease as the corrosion rate increases; the peak phase frequency moves to a smaller value as the previous corrosion accumulation increases. The model simulation showed that every subtle change in interfacial impedance at the concrete/rebar interface is reflected in the impedance measurement on the concrete surface. A measured impedance spectrum is the result of the interfacial impedance, concrete resistivity, and measurement geometry. Of the measured impedance characteristics, however, the slopes of the quadrature component against frequency are independent of both the geometric effects and concrete resistivity. Both the experimental results and the simulation have shown that this new method has indeed potential for reliable non-destructive assessment of corrosion in reinforced concrete structures.