## 4 Results and Discussion  
### Influences on the Interfacial Effect between the Concrete Surface and the Counter Electrode  
In EIS measurements, which use an external counter electrode, electrical contact between the working and counter electrodes should be properly maintained. To improve the electrical contact, a synthetic sponge saturated with various solutions is typically used. It was confirmed that the concrete resistance was affected depending on the types of solutions used as the contact media [31; 32; 33]. Moreover, the contact pressure can affect considerably the determination of the concrete's resistance [31]. As the ohmic resistance is typically considered as concrete resistance for the calculation of the corrosion rate, it is necessary to choose a proper contact medium that can affect the concrete resistance between the counter (35 \(\times\) 200 mm) and working electrodes. The EIS measurements were conducted by using an external counter electrode with various solutions as the contact media, as mentioned above. The electrical impedance spectra for active (corroded steel; LS in the sample) and passive (non-corroded steel; SS in the sample) states are shown in Figure 6. For clarity, in all the figures, solid marks indicate every decade within the used frequency ranges, whereas curves are drawn by plotting all the data points. Additionally, the impedances at 100 Hz were omitted in all impedance spectra as the values spiked in all the studied sample cases. The resistance at 1 kHz, corresponding to a cusp point, multiplied by the exposed area is estimated as ohmic resistance in this study. In a passive state corresponding to SS, ohmic resistances were obtained: 13.0 for tap water, 13.2 for a saturated Ca(OH)\({}_{2}\) solution, 13.0 for 0.55 M NaCl solution, and 14.2 k\(\Omega\cdot\)cm\({}^{2}\) for a simulated cement pore solution, respectively. On the other hand, ohmic resistances in an active state were 14.8, 14.9, 15.0, and 16.5 k\(\Omega\cdot\)cm\({}^{2}\) for tap water, a saturated Ca(OH)\({}_{2}\) solution, 0.55 M NaCl solution, and a simulated cement pore solution. Even concrete resistances were estimated within one sample, but the values for the passive state were approximately 15% lower than for the active state. The difference results from the inhomogeneity of the concrete. Although it was expected that lower ohmic resistance in the active state is lower due to higher contamination of chloride or ionic movements between the cathode and anode, these local effects were not observed. Although various contact solutions were used, it was observed that the concrete resistances with the contact media were similar (less than 3% difference) except for that in the simulated pore solution. The electrical resistances of the contact solutions themselves were not considered, as the concrete between the surface and the steel depth may have been highly contaminated by chlorides. In other words, as the pore solution in concrete attained an adequately high conductivity due to chloride ingress during the wetting period, it would be reasonable that an interfacial effect by a contact medium could be canceled out, even though a solution with a high electrical resistance (such as tap water) was used. Figure 5: Flow chart of the experimental procedure. It was observed that the resistances using the simulated cement pore solution at 1 kHz in both steel conditions were approximately 10% higher than those of other solutions. It may seem that the electrical resistance of the contact medium increased by the diffusion of dissolved alkali ions into the concrete sample, and led to a decrease in ionic contents which were present in the contact medium. The conductivity of KOH or NaOH solutions was higher than that of the NaCl solution at the same concentration, but the influences on the bulk resistance were marginal as the sample used in this study had already been highly contaminated by chloride, and, given the high solubility of the alkali solution, only led to an increase in resistance.