## 2 Electrical Resistivity/Impedance Measurement: Is It Really that Simple?
### AC Measurement of Electrical Resistivity
#### 2.2.1 2-Electrode Configuration  
The measurement setup defined by the uniaxial method of Section 2.1.1 can be also adopted with AC excitation (see Table 3). However, there are no general statements on optimal frequency to be used and both sine and square waves are deployed. In fact, it is worth noting that materials electrical properties are frequency-dependent [69] and, consequently, the measurement frequency deeply affects the results. AASHTO TP 119 [70] suggests the use of an AC 60 V voltage, a testing period of 6 h, and a 100 \(\times\) 50 mm (or 150 \(\times\) 75 mm) cylindrical specimen, where two stainless steel plates connected to the Wenner probe tips (short-circuiting WE and SE electrodes and RE and CE electrodes, respectively) are applied as electrodes. As for uniaxial test of ASTM C1760 [37], this procedure is not widely adopted, and everyone prefers to set up its measurement bench configuration. Electrodes in pin/bar forms are often used for this configuration: stainless steel pins [71], also covered with heat-shrink tubes apart from the 10 mm tip [72], brass electrodes [73] or screws can be used to improve the contact between electrodes and material, at different depths to obtain information on different layers [74]. Also, gauzes can be used, in different materials, such as copper (with water-saturated paper towel strips to ensure a good electric contact between electrodes and specimen surface) [75], titanium [22], stainless steel [21], and iron [76]. Plate electrodes were employed [77], maintaining the contact through different methods: clamps [78], wet sponges [11], absorbent cloth with a soap diluted solution by placing 5 kg on the top face of the specimen [79], sponges saturated with Ca(OH)\({}_{2}\) solution [80], or saturated with 20 wt.% NaCl solution [81], applying an external pressure of 6 kPa on the top electrode to improve the electrode/specimen contact [82]. [PERSON] and [PERSON] [23] used G-clamps or wood cubes to ensure a good contact between electrodes and specimens; furthermore, they spread a paste of ethyl alcohol and carbon dust on the specimen ends to apply the excitation signal. Other electrodes used in the literature are stainless steel circular electrodes [83; 84], red-copper electrodes with thin absorbent sponges to guarantee the contact [85], and graphite electrodes [86]. Commercial systems are also used to measure electrical resistivity. For example, [PERSON] et al. [87] used a commercial conductivity meter. As electrodes, parallel wires of mixed metal oxide activated titanium were embedded in the concrete specimens. [PERSON] and [PERSON] [88] employed the commercial Merlin meter (working at 325 Hz) to perform electrical resistivity measurement in cylindrical concrete specimens (100 mm diameter, 200 mm height). The electric contact between the clamp electrodes and the specimen faces was guaranteed by means of interposed sponges. Finally, [PERSON] et al. [89] used an analogue sensor (65 \(\times\) 8 \(\times\) 3 mm) embedded in prismatic concrete specimens (300 \(\times\) 200 \(\times\) 110 mm) to measure temperature and electrical resistivity at 100 kHz.