## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### AC Measurement of Electrical Resistivity
#### 2.2.2 4-Electrode Configuration
By realizing more resistivity profiles at increasing depths, a pseudo-section of the apparent resistivity can be obtained through inversion algorithms: this is known as electrical resistivity tomography [98]. A system suitable for this aim was employed by [PERSON] et al. [99], using a multi-electrode probe consisting of 14 electrodes equally spaced at intervals of 20 mm; four different electrode spacing are possible, namely 20, 40, 60, and 80 mm. It is worth underlining that the measurement results are influenced by cracks, reinforcing rebars, humidity, and voids. Commercial systems are also available: [PERSON] and [PERSON] [12] used a commercial resistivity meter, RESI (whose current version is Resipod by Proceq), intended for the assessment of the risk of corrosion. This meter exploits the Wenner method (with a measurement frequency of 40 Hz) and gives an estimation of corrosion rate, besides evaluating concrete homogeneity and curing conditions. [PERSON] et al. [100] used a surface resistivity meter [97] with four equally spaced electrodes and an excitation signal at 13 Hz with a peak-to-peak amplitude of 25 V (trapezoidal wave), as indicated in [101]. They took measurements at 4 quaternary longitudinal locations and considered the average value. They stated that the four-point Wenner array probe is one of the best methods to measure electrical resistivity, being non-destructive, easy to perform, and fast. Also, [PERSON] and [PERSON] [16] used a commercial resistivity meter based on 4-point probe method, using an electrode spacing of 30 mm and a low-frequency (13 Hz) AC excitation current. They considered both cubic (150 mm side) and cylindrical (100 mm diameter, 200 mm height) specimens; for the former, two measurements on three perpendicular surfaces were performed, whereas for the latter two opposite headings (180\({}^{\circ}\)) measurements parallel to the length of the cylinders, 120\({}^{\circ}\) apart from each other. [PERSON] et al. [102] applied a commercial surface resistivity meter on cylindrical specimens; [PERSON] and [PERSON] [103] used a commercial system based on the four-point Wenner probe (50 mm spacing) on cubic specimens of 250 mm per side (cast with three different w/c ratios of 0.4, 0.5 and 0.6) after 28 days of curing in a moist chamber and then in unsaturated condition in a laboratory environment (air-dried, T = 22 \(\pm\) 3 \({}^{\circ}\)C, RH \(\pm\) 65%) for two years. CNS RM MKII commercial resistivity meter was used by [PERSON] et al. [104], who considered three different sets of concrete specimens; [PERSON] model 400 soil resistivity meter was used also in [105], considering cylindrical test specimens, with 4 rebars segments. [PERSON] et al. [35] used a resistivity meter to investigate the effect of the probe spacing on resistivity by testing cylindrical and prismatic specimens. [PERSON] and [PERSON] [88] used a commercial surface resistivity meter (50 mm electrode spacing) on cylindrical concrete specimens. Surface electrodes are widely used in 4-point probe AC configuration. [PERSON] and [PERSON] [106] considered prisms, as well as [PERSON] et al. [31]. [PERSON] et al. [94] used the four-point Wenner probe (50 mm spacing) according to the standard UNE 83988-2 \"Concrete durability. Test methods. Determination of the electrical resistivity. Part 2: Four points or Wenner method.\" [107], averaging the results on three measurements spaced at 120\({}^{\circ}\). Cylindrical specimens (manufactured with the addition of fly ash, high-volume fly ash (HVFA), and fine limestone powder) were considered also by [PERSON] et al. [101] and the same method is used by [PERSON] and [PERSON] [98], adopting an increasing spacing of the electrodes in order to perform a vertical sounding of the tested prisms. [PERSON] et al. [4,108] compared the results obtained with the Wenner method with those obtained with 2-point probe method.