# Applications of Cement-Based Smart Composites to Civil Structural Health Monitoring: A Review
## 2 Fabrication and Operational Issues
### Electrical Configuration and Measurements  
The CSC fabrication as self-sensing element includes the arrangement of electrodes according to the selected electrical measurement method. It is well known that CSC resistance mainly consists of the resistances of the composite material, of the electrodes (generally negligible), and of the contact between electrode and CSC material. Moreover, CSCs are characterized by the presence of positively and negatively charged ions which, under the externally applied voltage, move towards the opposite voltage, thus creating a polarization potential. If polarization occurs during resistivity measures, particularly under direct current (DC), the recorded resistivity values tend to increase with time, so that an appropriate correlation between change in resistivity and strain can hardly be defined. Alternatively, the execution of resistance measures by using alternate current (AC) is able to strongly reduce polarization effects although impedance analysis is required, thus making the interpretation of measurements less straightforward [20]. Therefore, the definition of the electrical configuration directly affects the CSC sensing performance. Basically, two or four electrodes can be installed on each CSC sample following the two-probe or four-probe method, respectively. With the four-probe method, four electrodes are placed on the same side of the CSC: the two external probes are used for current application whereas the others are used to measure the voltage. The two-probe method uses just two electrodes working as current as well as voltage terminals at the same time. The four-probe method is able to reduce the contact resistance improving the accuracy of measurements, unless the CSC element has very little dimensions. Nevertheless, such methodology is characterized by higher complexity with respect to the two-probe method [58]. The materials adopted for CSC electrodes are characterized by high conductivity and resistance to corrosion. Accordingly, copper or stainless steel are the most used; other metals, such as silver or galvanized steel, are also seldom applied [59]. Two different installation approaches are typically used, namely embedding or surface attachment; the first proved to be sounder and more reliable for SHM applications, as well as characterized by lower polarization, higher durability and limited contact resistance [60]. Due to capacitance effect, the electrical resistivity measured between two electrodes increases as the spacing among them increases until a limit value is reached beyond which a constant trend is kept. Thus, it is recommended to place the electrodes at high distance. Increasing the size of embedded electrodes is another option to reduce capacitance. Nevertheless, it is less preferable since the higher the embedded section of the electrode, the higher the material-electrode contact resistance which affects measurements [9]. An appropriate design of electrodes has to take into account also other characteristics of the electrical circuit, such as the current type. In this regard, both AC and DC current may be used for resistivity measurements. As previously mentioned, DC measures of resistivity are generally simple, but they are affected by a time-based drift. On the contrary, AC current might be preferable since it induces very low polarization in the CSC. However, the AC frequency has to be properly defined. A commonly adopted criterion consists in increasing the frequency until the measured electrical resistance remains constant independently from its value. Another critical limit of AC measurements for SHM applications is the difficulty in the simultaneous acquisition of data from multiple sections of a CSC sensors; this is also the case when different impedance measurement devices are adopted because of the interaction among the electrical signals. In order to overcome the limits of AC and DC systems, a new technique has been recently proposed [61], the so-called biphasic DC measurement approach.