## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Corrosion Risk of Reinforcements  
The corrosion of steel rebars is the principal cause of structural deterioration of concrete [31], heavily impacting the service life of a structure. The corrosion rate depends on both the oxygen availability and the electrical resistivity of concrete, which mirrors the ion mobility in concrete and, consequently, the corrosion speed [7]. A low resistivity is generally linked to the concrete susceptibility to corrosion, since it is related to more rapid chloride penetration and corrosion rate [25]. Indeed, during corrosion, local electrical cells continuously generate a very little current, which can cause great harm, especially if the electrical resistivity of concrete is low. The electrical resistivity of concrete, being related to the ion mobility in the solution and to the microstructure itself, can act as an indicator for the ingress of CO\({}_{2}\) and chlorides [185]. This holding, electrical resistivity is directly linked to structural durability [31]. The literature proposes several threshold values; however, these thresholds are very variable. For instance, [PERSON] et al. [105] indicated active corrosion when \(\rho<10\) k\(\Omega\)-cm, whereas low corrosion probability when \(\rho>30\) k\(\Omega\)cm. [PERSON] [5] highlighted the importance of controlling moisture and temperature on specimens to obtain reliable and comparable data. Despite such different ranges, concrete electrical resistivity is considered as a valuable parameter for the evaluation of **corrosion risk**, provided that the concrete composition influencing the \"baseline value\" of electrical resistivity is considered. The quantification of corrosion risk is decisive for maintenance, protection, and repair decision-making [186; 187]. Moreover, corrosion can occur only in a small range of temperature, w/c ratio, and relative humidity; [PERSON] et al. [188], developed a probabilistic evaluation method for the estimation of corrosion risk by means of electrical resistivity measurements, taking into account the corrosion rate and identifying different corrosion risk levels (negligible, low, moderate, and high). [PERSON] [113] proposed a methodology to evaluate the corrosion probability in concrete slabs by using electrical resistivity measurements (according to the four-point Wenner array method) in combination with half-cell potential method (defined in ASTM C876 [189]). The latter requires direct access to the steel reinforcement for the connection of a high impedance voltmeter and a reference half-cell, to obtain a map of the potentials that could be associated with areas of active corrosion. Among the other factors, corrosion probability depends on the ionic conductivity. Ionic conductivity can be estimated by measuring the electrical resistivity of concrete [190], therefore the two quantities may be correlated. The combined evaluation of electrical resistivity and corrosion potential may improve the quantification of the probability of corrosion, which seems higher when \(\rho<5\) k\(\Omega\)-cm. In fact, the corrosion potential sharply increases for \(\rho<4\) k\(\Omega\)-cm. On the other hand, [PERSON] and [PERSON] [34] considered 10 k\(\Omega\)-cm as the upper limit for the corrosion probability in concrete. However, both the methods have their own disadvantages and provide better results when used in combination, even if they do not directly assess the instantaneous corrosion rate [110]. [PERSON] [191] found a reasonable linear relationship between corrosion probability and concrete electrical resistivity (y = \(-0.11\)x + 118, with R\({}^{2}\) = 0.89), indicated by steel potential after 20 weeks of salt/drying cycles. However, this does not imply a direct relationship between electrical resistivity and critical chloride content (i.e., the chloride quantity that is believed tolerable before corrosion starts); electrical conductivity is strongly related to chloride penetration, as previously demonstrated [192].