## 4 Let's Wrap Up
### Conclusions  
This review paper aims at providing the interested reader with hints and suggestions on how to approach electrical resistivity/impedance measurements on cement-based structures. Indeed, it was one of the authors' priority to demonstrate that, depending on target applications, several variables should be considered to ensure reliability and accuracy of results, e.g., electrode configuration or excitation type (DC vs. AC), just to cite some. Literature can indeed provide useful recommendations in approaching electrical resistivity/impedance measurements in concrete [106], in particular:* The use of an AC excitation signal (electric current or potential difference) is important to avoid the Faradic effect of charges (ions) separation in the material;
* Frequencies greater than 1 kHz should be used to fully avoid the alignment of dipoles consequent to the excitation;
* A low-frequency (less than hundreds of MHz or some GHz) AC excitation current should be applied to avoid spurious mode voltage;
* The 4-point probe method should be used to avoid any influence from the contact surface area;
* A good electrical connection between each contact and the concrete surface should be ensured;
* A contact spacing of at least 1.5 times the size of the maximum aggregate size in the concrete should be used;
* A Wenner contact spacing less than or equal to one quarter of the concrete section thickness should be used;
* Concrete resistivity should be measured at a minimum distance of twice the contact spacing from the edge of the concrete section;
* An electromagnetic cover meter or bar locator should be used to locate the reinforcement rebars;
* A contact spacing less than or equal to two thirds of the concrete cover should be used where the proximity of steel reinforcements is unavoidable, in order to minimize the error due to the presence of the rebar;
* A contact spacing of at least 4 cm should be used where surface wetting effect might be expected;
* The measurement should be performed at least 24 h after a rainfall has occurred;
* A contact spacing not less than eight times the thickness of this layer should be used if the presence of a low-resistivity surface layer is unavoidable;
* A temperature compensation of +1 kW-cm per 3 \({}^{\circ}\)C fall in ambient temperature might be used to convert resistivity measurements to a standard temperature. Moreover, it is worth noting that electrical resistivity is a parameter varying within the volume, since there is no homogeneity in concrete elements; in addition, those layers close to the surface are more sensitive to variations of climate conditions (e.g., temperature changes or rain events). For this reason, the measurement should be considered as a local measurement. This suggests that a proper monitoring of a concrete structure should be approached targeting a distributed monitoring system using several sensing nodes. These nodes should be places on those parts of the target structure that are considered as the most critical to concrete degradation. On the other hand, electrical resistivity measurements can be applied also targeting inspection. No matter the final approach, i.e., monitoring or inspection, the addition of self-sensing material would produce positive effects on a resistivity measurement. The calibration of the measurement devices is a key aspect that should always be considered, especially in a measurement for which environmental variables, like temperature and relative humidity, or material characteristics, like moisture content, act as disturbing inputs and consequently contribute also as sources of uncertainty. For in-lab tests, calibration can be performed with liquid of known electrical conductivity and on standard concrete specimens. The procedure is, however, difficult for in-field measurements. Hence, calibration turns out in a check on cabling and connections or a performance verification with reference devices (e.g., electronic circuits, concrete specimens of known resistivity, etc.)