Summary: | A design scheme of multi-element sensor which included electrical resistivity probes, multiple Cl<sup>−</sup> selective electrodes, and a steel corrosion monitoring system was proposed in this work. Embedding this multi-element sensor in concrete enables the real-time and non-destructive monitoring of internal electrical resistivity, free Cl<sup>−</sup> (Cl<sub>f</sub>) contents in the concrete pore solution at different depths, and steel corrosion parameters. Based on the monitoring data obtained by the multi-element sensor, the freezing-thawing (F-T) damage degree, the Cl<sub>f</sub> diffusion coefficient, the quantitative relation between F-T damage degree and Cl<sub>f</sub> diffusion coefficient, the initiation period of steel corrosion, and the critical content related to steel corrosion are determined. To conclude, the multi-element sensor provides key durability parameters for the establishment of the Cl<sub>f</sub> diffusion model, the assessment of health condition, and the prediction of service life of concrete under the coexistence of the F-T cycle and Cl<sup>−</sup>.
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