## 3 Properties of Multifunctional Cemertitious Composites
### Piezoresistivite Performance  
The phenomenon where electrical resistivity varies in response to mechanical strain under an applied load is known as piezoresistivity [145]. The piezoresistive performance under applied load can be utilized to assess the sensing properties of multifunctional cementitious composites. In this context, the fractional change in resistivity (\(\frac{\Delta\theta}{\rho}\approx\frac{\Delta R}{K}\)) is commonly employed to evaluate the sensing performance of these composites [146]. The relationship between fractional change in resistivity and stress is used to determine stress sensing, while the correlation between fractional change in resistivity and strain is applied to investigate the strain sensing capability, also known as the gauge factor (\(\frac{\Delta R}{\epsilon}\)) [89]. Depending on the type of loading, the trends in fractional changes in resistivity (FCR) vary [147]. Under monotonic loading, the FCR trend may decrease due to the compaction of voids and the closing of gaps between conductive fillers, remain balanced due to no change in microstructure, increase due to small cracks, and drastically increase due to the widening of cracks and damages, as observed in Figure 11a [148]. Conversely, under cyclic loading, the FCR decreases during loading due to the compaction and closing of gaps between conductive fillers and increases during unloading due to the reversal of the microstructure to its original state, as depicted in Figure 11b [149]. Figure 11b also indicates the superior sensitivity of multifunctional cementitious composites in dry conditions compared to wet conditions [149]. This may be attributed to polarization effects caused by ion movement in wet conditions [140]. It should also be noted that the sensitivity of multifunctional cementitious composites is higher in dry conditions than in wet conditions when the concentration of conductive fillers is within the threshold region [150]. However, there will be hysteresis behavior in FCR curves under cyclic loading if the microstructure variation in self-sensing cementitious composites is significant, and the reversal of the initial state of the microstructure condition cannot be achieved after each unloading stage [11]. It has been revealed that there is no drift in the FCR of multifunctional cement-stabilized sand under cyclic loading before the initiation of cracks. Nonetheless, when cracks and degradation begin with increasing stress levels, the FCR trend under cyclic compressive loading exhibits a hysteresis pattern. Generally, the FCR under cyclic loading is repeatable without any drift  
Figure 10: Schematic illustration of conductive mechanisms: (**a**) zone A (insulation phase); (**b**) zone B (percolation zone); (**c**) conductive pathways in zone B (percolation zone); (**d**) zone C (electron conductive-dominated phase); (**e**) conductive pathways in zone C (electron conductive-dominated phase) [143, 144], published by Elsevier (2019) (2023). when the stress level is within the elastic region. On the other hand, the FCR under loading is not repeatable when the stress level is within the plastic region due to the appearance of irreversible microstructural changes [151]. A similar trend has been reported in another study as well [152]. The sensing capability of multifunctional cementitious composites depends on various factors, including the concentration of conductive fillers, the type of conductive fillers, the aspect ratio of conductive fillers, loading conditions and levels, electrode material and configuration, and the measurement system [45]. The sensing performance of multifunctional cementitious composites increases with the increasing concentration of conductive fillers, as illustrated in Figure 12[154]. In reference to Figure 12, it is evident that the noise-to-signal ratio decreases with the increasing carbon black (CB) concentration, indicating the profound influence of the conductive filler concentration on sensing performance.