## 1 Introduction
The literature reports some reference values of electrical resistivity related to cement type and exposure conditions [25; 26]; however, it is worth considering that each mix-design would provide different electrical resistivity values. In reinforced concrete structures (RCS), another parameter affecting electrical resistivity is the short circuit effect related to the embedded steel reinforcement [27]; in fact, reinforcement, being metallic, attracts the electric current [28], distorting the measurement result. Besides the intrinsic characteristics of concrete, also the environmental conditions, particularly temperature (T) and relative humidity (RH) [29], impact concrete electrical resistivity. Concerning T, a higher value increases the ions mobility, making \(\rho\) decrease. Concerning RH, since water has a good electrical conductivity, the material electrical conductivity increases with its saturation degree. In this regard, also aging of concrete influences the electrical resistivity, since it gives a reduction in the moisture content due to the continuous hydration of the cement paste, which modifies the paste microstructure. Electrical resistivity also correlates with concrete degradation status, represented by different durability indicators as saturation degree, porosity, and chloride penetration [30]; electrical resistivity of concrete makes it possible to detect the penetration of aggressive agents prior to the onset of the degradation process [31]. If porosity is high, permeability increases and, consequently, so does the ingress of aggressive substances. This ingress mines the integrity of concrete and reinforcements, especially if the pore interconnection is high. In addition, thanks to the piezoresistive behavior of concrete (piezo-resistivity is a physical characteristic of electrically-conductive materials, whose electrical resistivity changes with deformations [32]), electrical resistivity measurements make it possible to monitor the stress/strain behavior of concrete [33]. The electrical resistivity of concrete can be measured by direct current (DC) or alternating current (AC). The type of current used (DC or AC) and the electrodes configuration (e.g., electrode spacing [34; 35]) adopted influence the measurement result, especially in terms of accuracy. A minimum of two electrodes is needed to excite the material with an electric current (or potential) and to measure the corresponding electrical potential difference (or current). Another configuration can adopt 4-electrode measurement points, separating excitation and measurement electrodes in order to avoid sharing electric current and potential. Also, the electrodes positioning, which the current path depends from [36], heavily affects the measurement results. Different types of electrodes are used in the literature, such as wires, plates, meshes, bars, tapes, and conductive paints. However, one of the major problems regarding electrical resistivity measurement in concrete is that no widely accepted standards are currently available, and many different methodologies are reported in the literature. Therefore, this review paper aims at providing the reader with an overview of the different approaches suggested by the scientific community when addressing electrical resistivity measurements, as well as highlighting some issues that may be tackled by the experimenter as well as the real missing item related to the topic, i.e., measurement uncertainty. Also, several target applications of this measurement method are discussed, with a focus on inspection and monitoring of cement-based structures. Finally, suggestions on how to perform electrical resistivity/impedance measurements, either at lab-scale or in-field, are provided.