## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Chloride Penetration
Even if this test is relatively simple and rapid (despite requiring an extensive specimen preparation), some issues have been raised, such as absence of steady-state conditions, heat evolution (its increase enhances the total passing charges, besides increasing the microstructure damages and possibly changing the pore solution chemical composition; especially in younger concretes or with high w/c ratio [181]), and pore solution alteration in the presence of pozzolanic materials [182]. [PERSON] et al. [101] reported a decreasing exponential relationship between RCPT and SR test results, with a high correlation (R\({}^{2}\) = 0.92) and a quite low variability (5.3%). [PERSON] et al. [92] found a power relationship between chloride concentration and SE (y = 25.724-x\({}^{-0.5}\), with R\({}^{2}\) = 0.9568). Lower electrical resistivity values can indicate an easier chloride penetration through pores; in particular, they state that for \(\rho>17\) k\(\Omega\cdot\)cm the chloride penetration is lower, whereas it becomes very low for \(\rho>41\) k\(\Omega\cdot\)cm and negligible for \(\rho>220\) k\(\Omega\cdot\)cm. on the other hand, there is a very high chloride penetration for \(\rho<5\) k\(\Omega\cdot\)cm. [PERSON] et al. [100] found a strong power relationship (y = 67998x\({}^{-1.028}\), with R\({}^{2}\) \(\approx\) 0.90) between RCPT and SR for a wide range of concrete specimens also containing SCMs, such as natural pozzolans (pumice and tuff), rice husk ash, silica fume, and metakaolin. The relationship between electrical resistivity and total passing charge would be linear if the specimen temperature was kept constant during the test (R\({}^{2}\) \(\approx\) 0.99) [181]. Similarly, good correlations were found by [PERSON] et al. [104] (y = 5801.2x\({}^{-0.819}\), with R\({}^{2}\) \(\approx\) 0.95) and [PERSON] et al. [96] (y = 7.535x\({}^{-0.88}\) with R\({}^{2}\) = 0.95 and y = 15.712x\({}^{-0.90}\) with R\({}^{2}\) = 0.98, for bulk and surface resistivities, respectively), measuring surface/bulk electrical resistivity according to AASHTO T358 and AASHTO TP119, respectively. Therefore, SR can be considered an indicator of permeability and represents a valuable alternative (faster and more precise) to RCPT test, changing the type of measured data (electrical resistivity instead of total passing charge) but remaining a measurement linked to concrete electrical resistance. Surface resistivity can be used as a quality control predictor of the chloride penetration resistance, but not of **diffusion behavior**, requiring dedicated long-term diffusion tests. However, according to [PERSON] et al. [183] the **AC impedance measurement** is a simple, reproducible and therefore valuable technique for determining the diffusion coefficient in concrete. Similarly, [PERSON] [5] stated that electrical resistivity can be used as an indirect control of chloride diffusivity thanks to the Nernst-Einstein's equation, correlating electrical resistivity and ion diffusivity (y = 49.13x\({}^{-0.91}\), R\({}^{2}\) = 0.97). Connectivity and pores play a fundamental role: fewer pores with lower connectivity, higher electrical resistivity and lower diffusivity are obtained. When chlorides penetrate, the material microstructure evolves, changing porosity and tortuosity. Electrical resistivity mirrors these variations, decreasing together with the ionic concentration [86]. Electrochemical Impedance Spectroscopy (**EIS**) can be used to monitor reinforced specimens during chloride ingress, allowing the detection of chloride arrival on the reinforcement as well as the steel depassivation, the corrosion onset, and the chloride exit [143]. Finally, [PERSON] et al. [79] found a linear correlation between conductivity and **chloride migration coefficient**, (y = 1195x, with R\({}^{2}\) = 0.88); therefore, electrical resistivity measurement can be considered as a quicker and cheaper alternative method to the standard Rapid Chloride Migration (RCM) test [184].