## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?  
### DC Measurement of Electrical Resistivity  
#### 2.1.1 2-Electrode Configuration  
The two-point probe method, also known as uniaxial method, enables the measurement of bulk electrical resistivity. The specimen is placed between two electrodes (commonly parallel metal plates) with moist sponge contacts, then a DC electric current is injected and the consequent potential drop is measured (Figure 1).  
The standard to perform electrical resistivity measurements with the uniaxial method is the ASTM C1760 \"Standard test method for bulk electrical conductivity of hardened concrete\" [37]. The standard foresees the use of saturated cylindrical (100 mm diameter, 200 mm length) hardened concrete specimens, with a standard 28-day curing (56-day curing in the presence of supplementary cementitious materials, such as GGBFS or fly ash, since they prolong the time to dry). A 60 V DC potential difference is applied and the electric current is measured after 1 min; therefore, the electrical conductivity is obtained according to Equation (1):  
\[\sigma\ =\ k\frac{I_{1}}{V}\frac{L}{D^{2}}\ [\mathrm{mS/m}] \tag{1}\]  
where:  
\(\bullet\quad\)\(I_{1}\) is the electric current at minute 1 (\(\pm\)5 s), in mA  
Figure 1: Uniaxial method (please note that electric current is measured in series to the power supply, whereas the corresponding potential drop is measured in parallel).  
\(\bullet\)\(V\) is the applied potential difference, in V  
\(\bullet\)\(L\) is the average length of the sample, in mm  
\(\bullet\)\(D\) is the average diameter of the sample, in mm  
\(\bullet\)\(K\) is the conversion factor, equal to 1273.2  
Literature reports the use of different electrode configurations as well as different geometrical configurations of the specimen with respect to the standard (see Table 1). Indeed, the standard is rarely followed: in the last 20 years, no study has been carried out in compliance with ASTM C1760, probably because authors prefer to use their own instrumentations and to prepare specimens with molds available in their laboratories, adopting the measurement configuration that better fits their study objectives.  
Stainless steel meshes are often used as electrodes. [PERSON] et al. [38] used a wire net as electrode in concrete cubic specimens. These specimens would have been embedded in concrete structures as distributed sensors for SHM purposes. [PERSON] et al. [39] used the same configuration but in concrete prisms manufactured with carbon nanotubes and nanocarbon black filler. [PERSON] et al. [40] realized stainless steel gauzes to be embedded in cementitious prisms, whereas [PERSON] et al. [41] inserted these electrodes in concrete prisms reinforced with super-fine stainless wires.  
Also, copper is used for the electrodes; [PERSON] et al. [42] used copper mesh electrodes embedded in cubic cement paste specimens. [PERSON] et al. [43] used copper tapes, glued with silver conductive epoxy around the surface of dog-bone paste specimens manufactured with fly ash and carbon black. [PERSON] and [PERSON] [44] used silver paint and copper wires as electrodes in prismatic specimens. Brass-made wires were used as electrodes by [PERSON] et al. [45] in both prismatic and cubic specimens.  
Plates are another type of electrodes used in the literature. [PERSON] et al. [46] embedded stainless steel plate electrodes in prisms. [PERSON] and [PERSON] [47] guaranteed a good electric and mechanical contact between metal plate electrodes and specimens surface by means of a cloth moistened with NaCl solution and a clamp, testing concrete specimens manufactured with conductive carbon fibers different in terms of shape and dimensions.