## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### AC Measurement of Electrical Resistivity
#### 2.2.2 4-Electrode Configuration
[PERSON] and [PERSON] [109] used the Wenner technique with a modified electrode array, with two copper-copper sulphate reference electrodes to give a stable surface potential for the measurement of electrical potential difference caused by the excitation given through the outer electrodes; the same configuration was used in [110, 111, 112]. Moreover, [PERSON] [113] used the four-point Wenner array in the frequency range of 50-1000 Hz on slab specimens; the area where the measurement was made had been prior wetted with tap water, to optimize the electrical contact. A grid of measurement points was defined on the specimen surface, to obtain a resistivity map. [PERSON] et al. [28] used surface electrodes to perform electrical resistivity measurements on concrete prisms; they guaranteed a good electric contact by means of conductive gel based on potassium hydroxide. [PERSON] et al. [114, 115, 116] used a four-electrode array on prisms and performed the measurement in the 0.01-1000 Hz band. Also, [PERSON] et al. [117] used the 4-point probe method, but they did not consider a constant spacing among electrodes. [PERSON] et al. [118] used a specific square probe (50 mm spacing) on prisms reinforced with rebars (10 mm diameter), repeating the measurement twice for each tested location; the same probe is used by [PERSON] et al. [119]. A square array is used also by [PERSON] et al. [120], employing an excitation signal of 1 V (peak amplitude) at 100 Hz to scan a series of 12 electrodes placed around a cylindrical concrete specimen, in order to average the results on 12 measurements. They used brass electrodes, inserted for 10 mm in the specimen. The measurement was performed 30 min after the cast. The square array was reported to be sensitive to the specimen anisotropy. Also attached or embedded electrodes can be used for 4-electrode configuration. [PERSON] and [PERSON] [121] used copper electrodes attached with silver paste on cylindrical specimens, as well as [PERSON] et al. [122]. [PERSON] et al. [123] used copper tapes as electrodes, attached with conductive silver colloidal paste. [PERSON] et al. [124] used two platinum electrodes and two reference electrodes, controlling the field stability in the specimen. [PERSON] et al. [125] used copper electrodes, with insulating coating at the extremities. [PERSON] and [PERSON] [23] tested 4-electrode configuration with different materials, both embedded or coupled externally to the specimens. In particular, they used copper electrodes, lead plates, and copper wires. They tested three different electrodes configurations: 4 copper electrodes (internal electrode distance of 80 mm, external electrodes distance of 120 mm), 2 copper electrodes (internal, at a distance of 120 mm) and 2 lead plates (external, clamped on the specimens), and 4 copper wires (internal electrode distance of 80 mm, external electrodes distance of 120 mm). They reported that the most reliable system is the one composed by 4 copper sheets. Two different types of electrodes were tested by [PERSON] et al. [126]: square sheets of conductive film and cylindrical urethane sponges penetrated by a gold-coated needle along the longitudinal axis and containing water; the electrode spacing was equal to 75 mm. Also, [PERSON] et al. [80] tested two different electrode configurations: four stainless steel rod-electrodes with heat-shrink sleeving (keeping uncovered a 10-mm tip), and a spacing of 30 mm; two internal rod-electrodes and two external stainless steel plate-electrodes with a polymethyl methacrylate backing. They performed the measurements at 1 kHz on cubic concrete specimens (150 mm side), noticing that the latter configuration is not suitable at higher resistivity values, when the boundary effects at the concrete/air interface become significant, overestimating the measured resistivity.