## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### Freezing/Thawing  
In cold regions, the formation of frost inside cement-based structure is of great concern, since rapid cooling causes significant temperature and moisture gradients, thus resulting in considerable internal pressure caused by water confined in pores, as well as the crystallization pressure, giving possible damages. This is mirrored also by a compressive strength loss and cracks formation. Electrical resistivity, being related to moisture content and temperature, can provide information on the ice content; moreover, ice alters the porosity and connectivity of the material, which induce changes in the electrical resistivity. [PERSON] et al. [72] studied the influence of freezing/thawing on electrical impedance of concrete. They adopted specific thermal cycling, passing from 120 \({}^{\circ}\)C to \(-\)70\({}^{\circ}\)C (at a rate of 10 \({}^{\circ}\)C/h), maintaining the temperature for 3 h to reach thermal equilibrium, then they increased the temperature of 20 \({}^{\circ}\)C (at the same rate) till equilibrium; the cycle was repeated twice in a day. Both the real and the imaginary parts of impedance change with temperature; in particular, the impedance increases with the ice formation in the pore network [90; 204], making the pore-water volume decrease, besides increasing slightly ionic concentration in the unfrozen pore water [83]. Moreover, the ions mobility decreases when temperature decreases, consequently decreasing electrical impedance as confirmed by [PERSON] et al. [65]. Furthermore, the maximum value of \(\rho\) increases with the number of freezing/thawing cycles, indicating progressive damage caused by frost [205; 206]). After ice nucleation, a linear relationship between the logarithm of electrical resistivity and temperature can be identified. This curve provides valuable information on the effect of frost on concrete durability [83]. [PERSON] et al. [90] estimated the ice content of mortar through electrical impedance measurements. If the electrical impedance variations are not constant over cycles, it means that some frost damages have occurred. The freezing point is identified at approximately \(-7\)\({}^{\circ}\)C; above \(0\)\({}^{\circ}\)C, there is a linear relationship between the logarithm of electrical resistivity and temperature (no ice formation), whereas the increase of electrical impedance is sharp below the freezing point (which increases with cycles). The ice content is higher during thawing than freezing, reasonably because of the natural supercooling effect of the material (during freezing, water cannot form ice because of lack of ice nucleation crystals). [PERSON] and [PERSON] [83] highlighted that, during freezing, ice formation increases salt concentration in the unfrozen pore water, making its electrical resistance decrease. Therefore, another resistance component, linked to the paste-pore water interface, which increases with temperature decrease, appears. Moreover, after melting, there is a residual expansion in cement pastes causing cracking and pore deformation, possibly changing the micro-scale structure. [PERSON] et al. [66] analyzed cement pastes with different moisture contents, highlighting that electrical resistivity is very sensitive to nucleation and growth of ice crystals in pores of cement-based materials. As already said, electrical resistivity increases with temperature decrease (because of a decrease in ion mobility); at first, the decrease proceeds slightly, then rapidly, indicating crystallization of the pore solution. The variation of electrical resistivity is stronger in thawing than in freezing as a consequence of moisture redistribution. When moisture content is higher, both freezing and melting temperatures increase; moreover, electrical resistivity is lower. It is possible to find a log-linear relationship between electrical resistivity and temperature that is due to changes in both pore solution molecular activities and conductive pathways.