## 3 Properties of Multifunctional Cemertitious Composites
### Contact and Non-Contact Electrical Measurement System  
Electrical measurements for cementitious composites are conducted using contact (electrode-based) and non-contact (electrodeless) methods [93]. The electrode-based approaches, which involve directly attaching electrodes to the composite, frequently encounter challenges such as increased contact resistance and the potential for electrode detachment, thereby undermining their reliability. In contrast, the non-contact electric  
Figure 5: Application forms of self-sensing cementitious composites [92], published by Elsevier (2015). resistivity technique, developed by [PERSON] et al. [94] and patented in 2003, addresses these issues by circumventing problems associated with electrodes in the assessment of cement hydration. This method operates based on a transformer principle, where a primary coil generates a current in the specimen that functions as a secondary coil of the transformer, as depicted in Figure 6[95]. Figure 6 provides a schematic representation of the non-contact resistivity measurement setup. [PERSON] et al. [99] introduced an enhanced non-contact resistivity measurement system designed to assess porosity and pore connectivity in cured cementitious composites after 28 days, as depicted in Figure 7. Unlike the predecessor non-contact measurement system, this modified system features a ring pipe filled with 3% NaCl solution, serving as a secondary coil within the transformer system. Despite this alteration, the operational mechanism remains akin to other non-contact resistivity measurement systems: a constant-frequency voltage applied by the primary coil induces a toroidal current in the secondary coil, measured by the leakage current meter (Figure 7b). In contrast to the previous system using a ring specimen, the modified setup utilizes a slice specimen saturated with NaCl  
Figure 6: Non-contact resistivity measurement system: (**a**) photograph [96; 97], published by Elsevier (2008) (2022); (**b**) schematic illustration [98], published by Elsevier (2020); (**c**) specimen cross section (drawn by the authors). solution (Figure 7c). While innovative, this modified system is limited to measuring the electrical resistivity of saturated specimens, presenting a notable drawback in that it cannot be used for dried self-sensing cementitious composites. Non-contact systems are classified into two types: resistivity and impedance measurements. A limitation of the non-contact resistivity measurement system is its inability to produce a Nyquist curve (i.e., correlation of imaginary and real impedance over frequency variation), attributable to the use of a constant frequency. Despite the widespread use of non-contact resistivity measurement systems in assessing hydration, porosity, and pore structure evolution, this system's low sensitivity hinders the tracking of late cement hydration [100]. Consequently, it has two drawbacks: an inability to simulate frequency-dependent pore structure evolution caused by hydration and a limitation in providing comprehensive information on pore structure evolution in cementitious composites [101]. To address the limitations of non-contact resistivity systems in evaluating pore structure, [PERSON] et al. [102] introduced the non-contact impedance measurement system in 2012. This system (Figure 8) effectively monitors pore structure changes during hydration stages across a broad frequency range, which is crucial for understanding the frequency-dependent characteristics of pore structure evolution [93]. Although the non-contact impedance system performs better than the resistivity system, it exhibits a lower sensitivity level than the  
Figure 7: Modified non-contact resistivity measurement system: (**a**) the picture of the modified non-contact measurement system; (**b**) details of the modified non-contact measurement system; (**c**) specimen used in the modified non-contact measurement system [99], published by Elsevier (2018). contact impedance system.