## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Corrosion Risk of Reinforcements
They found that imperialist competitive algorithm (ICA) provides higher accuracy and flexibility than genetic algorithms (GA). [PERSON] and [PERSON] [12] underlined the existence of an inverse correlation between corrosion current density and the electrolyte resistance, as reported in Equation (4):  
\[I_{corr}\propto\frac{1}{\rho} \tag{4}\]  
where \(\rho\) [\(\Omega\cdot\)m] is the electrical resistivity of concrete. This indicates that the rate of corrosion increases together with decreasing electrical resistivity of concrete. [PERSON] [175] stated that the relationship between the corrosion current density and the electrical resistance (and resistivity, if anodic and cathodic sites are considered fixed) can be considered almost linear for a wide range of corrosion current density values. However, many times cathodic activation control (electrical charge transfer) dominates on concrete resistance in driving the corrosion resistance, as well as oxygen diffusion can play a significant role in the cathodic control (particularly in wet environments). EIS can provide information on the corrosion kinetics, indicating the corrosion mechanism (activation, concentration, or diffusion), monitoring the situation over time [201]. [PERSON] et al. [202] investigated the electrical resistivity of concrete through the cover region to the reinforcement rebar, at intervals of 5 mm. Considering that the electrical resistivity of pore water, salt solution, and pore fluid mixture can be represented by parallel conduction elements (equivalent electric circuit), the concentration of NaCl in the pore water after salt solution exposure can be estimated. [PERSON] et al. [126] used the Wenner method to measure the electrical impedance of concrete for corrosion detection, without connecting to the rebars, by placing the electrodes just above the reinforcements. They found that the impedance decreases as corrosion proceeds, enabling a clear discrimination between healthy and corroded rebars. Therefore, the impedance difference in a certain frequency range (10-100 Hz) can be considered as a suitable parameter for monitoring reinforced concrete structures. However, they stressed that corrosion detection is more difficult if the rebar is located more deeply and that above 1 kHz the passive film formation on the rebars cause a sharp increase in electrical impedance. [PERSON] et al. [139] identified the frequency range between 10 Hz and 1 kHz as the best to investigate the corrosion degree of reinforcing steel. Also, [PERSON] et al. [116] aligned the electrodes with the reinforcement rebar, stating that polarization resistance and double layer capacitance are clearly visible in the measured electrical impedance. This approach provides results comparable to standard EIS method without the need of contact with the reinforcement rebar and faster thanks to the use of higher frequency values. However, the result heavily depends on the geometry, the electrodes positioning, the concrete cover depth, the diameter of reinforcement rebars and the concrete electrical resistivity. [PERSON] et al. [105] investigated corrosion on concrete specimens exposed to seashore and submerged. In the former scenario, the electrical resistivity values are approximately three times greater than in the latter and, in both cases, electrical resistivity increases over time. It worth noting that chloride concentration does not significantly influence concrete electrical resistivity; reinforcement steel rebars are probably in an active corrosion state when electrical resistivity is \(<\)10 k\(\Omega\cdot\)cm, whereas they remain in their passive state when \(\rho>30\) k\(\Omega\cdot\)cm. Electrical resistance is important not only for estimating the corrosion rate but also in the design of cathodic protection systems against corrosion [203].