## 3 Results and Discussion
### Effect of Chloride Ions on Electrical Resisitvity  
Figure 11 represents the variation in electrical resistivity depending on chloride concentration. In the figure, the Y label, the electrical resistivity ratio between the concrete with chloride and concrete without chloride, was calculated from Figure 8. The variation in electrical resistivity of the concrete with the _w/c_ ratio was not monitored; however, the electrical resistivity had decreased for concrete with a high chloride concentration. Based on the Figure 11, the average electrical resistivity of concrete contaminated with the same chloride concentration was calculated. Figure 12 shows the correlation between the chloride concentration and ratio of the average electrical resistivity of concrete with chloride concentration. The variation with the _w/c_ ratio was not monitored. It is obvious that the chloride concentration leads  
Figure 11: Chloride concentration vs. resistivity of the concrete. Figure 10: Gradient in the linear regression deriving from the resistivity ratio vs. chloride concentration. to reduced resistivity of the concrete. The determinant coefficient was 0.97, which is meaningful in determining the variation in electrical resistivity of concrete with chloride ion content, as follows:  
\[\frac{\rho_{CI}}{\rho_{watt}}=1-0.163\,C_{CI} \tag{3}\]  
where \(\rho_{CI}\) is the electrical resistivity of the concrete with chloride ions; \(\rho_{watt}\) is the electrical resistivity of the concrete without chloride ions, \(C_{CI}\) is the chloride ion in the concrete (by weight of the cement). Figure 13 represents the result of measuring the carbonation depth of concrete mixed with chloride ions. The higher the chloride ion, the higher the carbonation depth. This trend was more evident for concrete with a high _w/c_ ratio. Figure 14 shows the average value of the electrical resistivity of the carbonated concrete mixed with chloride ions and a clear experimental trend was not observed. Although concrete without chloride ions should show a higher electrical resistivity than concrete with chloride ions at the beginning, the opposite trend was noticed as time elapsed. Electrical conductivity is decreased by carbonation, revealing the complex nature of the microstructure change [28], while the chloride-binding capacity is decreased due to consumption of the chloride adsorbent, C-S-H phase, and Friedel's salt [29]. These two have mutually contradictory effects on the electrical resistivity of carbonated concrete. At the early stage, the effect of carbonation on resistivity is not significant, thus the electrical resistivity decreases because of only the influence of the chloride ions on the electrical conductivity. However, it showed a somewhat opposite trend as time elapsed. This means that electrical resistivity should be influenced by carbonation with elapsed time because of the reduction in porosity [30]. In particular, because the bound chlorides can be converted into water-soluble chloride ions due to carbonation [31], electrical resistivity should be further lowered. Nevertheless, the opposite result was examined. It is believed, therefore, that the reduction in porosity due to carbonation has a greater effect on the electrical resistivity than the increase in conductivity due to chloride ions. Figure 12: Effect of chloride concentration on the resistivity of concrete. Figure 13: Carbonation depth of the concrete with various chloride concentrations. Figure 14: Electrical resistivity of the carbonated concrete pre-mixed with chloride. Figure 15 shows the electrical resistivity ratio of the concrete under combined deterioration to normal concrete without carbonation and chloride ions. The Y label, the electrical resistivity ratio between the carbonated concrete and normal concrete, was calculated from Figure 14. The electrical resistivity ratio was clearly governed by the chloride concentration and carbonation depth. The higher the carbonation depth and the chloride concentration, the higher the electrical resistivity ratio.