## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Effect of Reinforcements in Electrical Resistivity Measurements  
In case of **embedded steel**, the short circuit effect can be minimized by making the excitation current flow orthogonally with respect to the rebars (e.g., placing the sensing probe perpendicular to the rebars, in case of [PERSON]'s method [102]). [PERSON] et al. [27] examined this effect both through experimental and numerical tests highlighting its dependence on both the specimen geometry and the electrode spacing; they introduces a rebar presence factor quantifying the effect on the measured resistivity. Also, [PERSON] et al. [99] used numerical simulations to evaluate the effect of steel rebars on electrical resistivity measurements, carrying out parametric studies to evaluate the effect of different geometries and sensor positioning. As a rule of thumb, the distance between the Wenner's array and the reinforcement rebar should be at least twice the electrode spacing. [PERSON] et al. [80] have found that also the sponge used for electric contact can contribute to the measured result, increasing the electrical resistivity; therefore, particular attention should be paid to each component of the measurement chain. [PERSON] et al. reported numerous factors causing deviations from the real resistivity value when using the Wenner array [145], such as size and geometry of specimens, rebars positioning, and presence of surface layers with different conductivity. [PERSON] et al. [4, 108] compared the results obtained with the Wenner method with those obtained with 2-point probe method; they highlighted the necessity of performing the measurement as far as possible from the metallic reinforcement of concrete structures or adopting a sufficiently small electrode spacing (in relation to the cover depth), concluding that the Wenner method is suitable for a reliable quality control of concrete durability also during the construction process. Uncertainty and Calibration in Electric Resistivity/Impedance Measurements: The Missing Items in Literature  
Uncertainty [146] is a key aspect in every measurement system. Yet, it is also one the most missing item even in scientific documents targeting applications that involve measurement systems. This is also the case for electrical resistivity measurements. Indeed, to the best of the authors' knowledge, the uncertainty associated with an electrical resistivity measurement has been rarely tackled by the scientific community dealing with this approach in concrete. Actually, very few studies face this issue. [PERSON] et al. analyzed the uncertainty associated with resistivity measurement for a coastal bridge [147]. The uncertainty was assessed based on measurements repeatability (1800 measurements) and considering local material anisotropy. The Wenner 4-probe approach was adopted to perform the measurements. The authors also tried to correlate the results of the uncertainty analysis with the corrosion risk from RILEM TC154 recommendation [148]. This way, they suggested a parameter, namely probability of wrong assessment (PWA), that was adopted, together with uncertainty, to help the bridge owner in the maintenance of the infrastructure. [PERSON] et al. [81] focused on the variability of bulk electrical resistivity measurements (uniaxial method) on concrete cylinders. They considered both within and multi-laboratory variability exploiting the coefficient of variation (COV) as parameter addressing the variability. They claimed that, in both conditions (within and multi-laboratory), variability increases over time. Operator variability, variability of specimens, and the inherent variability in the mixture are all associated together into the within-laboratory variability, which does not exceed 4.36%. As the multi-laboratory variability is concerned, this was evaluated from ten laboratories and twelve differing mixtures, resulting in a variability of 13.22%. An important aspect underlined by the authors is that specimen geometry can greatly influence the results of an electrical test.