## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Chloride Penetration  
Chloride penetration is the main degradation cause of reinforced concrete structure. When a critical threshold is overcome, degpassivation induces the corrosion of reinforcements. Therefore, a proper service life design of concrete infrastructures, verifying the durability requirements for concrete exposed to aggressive environments due to seawater or de-icing salts [173; 174], is required for minimizing repair and maintenance costs. Good correlations has been found [5] between concrete **electrical resistivity**[74] and **chloride ingress**[175] and this relationship can be used as a durability indicator, also for quality control and concrete classification [176]. In general, the chloride diffusion coefficient is inversely proportional to the electrical resistivity of concrete [177]. Similarly, [PERSON] et al. [178] evidenced a linear correlation (\(\mathrm{R}^{2}=0.93\)) between the diffusion coefficient and the electrical conductivity of concrete. Electrical resistivity reflects the ions mobility in pore solution; therefore, the relationship with chloride penetration is reasonable [100]. Indeed, electrical resistivity can be used as a NDT for the evaluation of the service life [92], since it reflects charge flow and ions mobility, easing the corrosion process [103]. Chloride ions modify the electrical impedance response of cementitious materials; in particular, as chloride migration proceeds, first the bulk electrical resistance increases, then decreases [124]. [PERSON] et al. [77] monitored concrete blocks (\(300\times 200\times 200\) mm) exposed in a marine site in three different locations: just above the low-water mark, just below the high-water mark, and well above it. They employed multielectrode arrays to evaluate the advancement of NaCl in the concrete cover, given that the increasing ionic concentration in the pore fluid, due to seawater penetration, decreases the concrete electrical resistivity. Several works in the literature aim at correlating electrical resistivity and chloride penetration [7]; however, the relationships found are very different from each other, and this can lead to a difficult interpretation of measurement results, also considering that in-field measurements cannot benefit from controlled conditions, contrary to laboratory setups [92]. In fact, electrical resistivity mainly depends on the chemical composition of the pore solution and the presence of ions different from chlorides could be misleading in the evaluation of chloride permeability through electrical resistivity assessment. As a matter of fact, [PERSON]'s law provides the estimation of chlorides through multilinear regression considering the relationship between concrete electrical resistance and porosity, saturation degree, and the interstitial fluid resistance [30]. The **Surface Resistivity (SR) Test** has been judged as a promising alternative to Rapid Chloride Permeability Test (RCPT), since it is easier, more rapid, does not require sample preparation, and with results affected by lower variability [101]. However, SR Tests needs to be accurately set-up, as electrode configuration (Figure 2), contact force between electrodes, and specimen might severely affect the results of test [179]. In fact, the electrode-specimen contact area influences the measurement, with a maximum error in electrical resistivity estimated at 6% when passing from 1 mm to 40 mm electrode diameter (it is worth underlining that this effect is much more problematic in 2-electrode configuration, also limiting the suitable measurement frequency range) [155]. RCPT method, specified by ASTM C1202, is really a measure of electric conductivity, since it monitors the amount of electric current passing through a cylindrical specimen when a 60 V DC electrical potential difference is applied for 6 h [165]. The total charge is obtained by integrating the measured current over time, and corresponding levels of penetrability to chlorides are given (high, moderate, low, very low, or negligible) [180].