## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Effect of Reinforcements in Electrical Resistivity Measurements
Hence, they suggest using a geometry correction factor (e.g., in cylinders the ratio of sample area to sample length). As for the electrode resistance, which was addressed using a series model, they showed that, for the materials used in their evaluation, the needed correction is quite small. Indeed, the variabilities reported by [PERSON] et al. can become even higher (10-40%), as discussed in [149, 150], due to both concrete heterogeneity or measurement system metrological characteristics [151], just to cite some. The concept of uncertainty associated with resistivity/impedance measurements becomes even more relevant when in-field testing is considered. In fact, in in-field applications, the whole measurement chain (instrument to inject/measure electrical quantities, wires, electrodes, concrete) related to the resistivity/impedance measurement becomes difficult to be controlled, mainly because it is highly affected by the conditions of concrete (it is indeed non-stationary) and the interaction of the whole chain with the environment (e.g., temperature, relative humidity, etc.). [PERSON] et al. [152] discussed the decrease of electrical resistivity with the increase of temperature and RH. As RH is concerned, [PERSON] et al. identified a linear relationship between concrete resistivity and RH (correlation coefficient \(>0.83\)). The higher the RH level, the higher the moisture content in concrete. Hence, the easier the flow of current within concrete. The results provided by [PERSON] et al. well match with the findings previously discussed in their paper of 2002 [153], in which they investigated the effect of moisture content on concrete resistivity on both air-dried specimens and oven-dried specimens. They demonstrated that resistivity decreases with an increase in w/c ratio, but for the saturated concrete specimens with high w/c ratios (w/c \(>0.55\)) the resistivity difference is small. Mostly important, they showed that with consistent water loss ratio (\(>\)3%) concrete resistivity increases no matter the mix adopted. This is indeed justified by the fact that current is carried by ions [152], and ions constitute the conductive phase of concrete as pore solution, whose electrical resistivity is linked to concrete resistivity through the empirical law of [PERSON] [154]. As for temperature, electrical resistivity and temperature are linked through the Arrhenius equation, involving the empirical computation of activation energy [155], which in turn is linked to moisture content itself [154]; when temperature rises, the movement of the charged ions dissolved into the pore solution increases according to a linear function [152]. Given this high-variability of concrete resistivity/impedance, it becomes clear that in-situ calibration becomes relevant when addressing in-field applications. Despite this is a recognized need, it is not widely addressed in the literature. [PERSON] et al. [156] proposed to use LCR meters to derive proper correction factors to be applied to the results to improve their accuracy. [PERSON] et al. [155] combined numerical models with laboratory tests to evaluate the uncertainty associated with the measurement and using water (instead of concrete) as a reference for calibrating the sensor to be used in-field. Recommendations on calibration are available mostly for in-lab tests, given that in-field measurements are much more complicated and often just connections and contacts are verified [25]. Indeed, calibration of the measurement system is carried out in-situ more as a performance verification. \"Dummy cells\" (electric circuits with high precision components of known value) are typically used for this purpose. However, the characteristics of these reference devices may differ quite a lot from those of the target structure.