## 3 Properties of Multifunctional Cemertitious Composites
### Integrated Self-Sensing and Self-Healing  
SHM encompasses various levels categorized according to functionality and complexity [159], as depicted in Figure 14. Establishing an SHM at the highest level necessitates the utilization of numerous sensors and related components to achieve optimal SHM efficacy. As discussed earlier, the primary limitation of conventional, sensor-based SHM lies in the deployment of congested sensors. In addressing this issue, multifunctional cementitious composites offer a potential solution, eliminating the need for densely placed spot sensors. Previous studies have classified SHM into five levels based on [PERSON]'s suggestion [160], and this classification can be applicable to the SHM approach which incorporates multifunctional cementitious composites, as illustrated in Figure 14. While the self-sensing features of multifunctional cementitious composites exhibit the potential to fulfill the requirements of the five SHM levels [161], a comprehensive guideline still needs to be developed. Consequently, there is a need to explore SHM strategies based on the self-sensing performance of multifunctional cementitious composites in order to establish a standardized framework. However, the standalone use of multifunctional cementitious composites cannot achieve the efficiency required for SHM at level 6. Therefore, the integration of an intrinsic self-sensing feature and self-healing feature in multifunctional cementitious composites introduces a novel concept in SHM, as indicated in Figure 14. Analyzing Figure 14 reveals that the self-sensing feature of multifunctional cementitious composites can be considered within level 5. Nevertheless, a new level (level 6) may be added to the existing SHM concept after integrating the self-sensing feature with a self-healing capability in multifunctional cementitious composites. The self-healing mechanisms in cementitious composites are broadly classified into two distinct types: autogenous and autonomous. Autogenous self-healing primarily involves healing minor cracks through the hydration of residual unhydrated cement particles. This category also benefits from the incorporation of various additives such as carbon fiber, carbon nanofibers, carbon nanotubes, fly ash, ground granulated blast furnace slag, and other nanomaterials into cementitious composites. These additives enhance the formation of cement hydration products and CaCO\({}_{3}\), thereby bolstering the autogenous self-healing capabilities of multifunctional cementitious composites [162]. For example, [PERSON] et al. [163] observed enhanced self-sensing and self-healing characteristics in multifunctional cementitious mortar with the inclusion of MWCNT and mineral-based admixtures. Similarly,  
Figure 14: Differentiation of SHM levels based on their respective efficiency ratings. [PERSON] et al. [164] noted significant autogenous self-healing effects in a multifunctional cementitious composite that incorporated fly ash and polyvinyl alcohol (PVA) fibers, aiding in crack repair, as illustrated in Figure 15a. This figure demonstrates crack closure after a 14-day healing period, causing variations in electrical impedance spectroscopy (EIS) signals, as shown in Figure 15b. Referring to Figure 15b, the expansion of the Nyquist plot due to 2% strain from tensile strength is evident, followed by a reduction as a result of the healing process over time. The healing of cracks leads to increased electrical conductivity, thus contracting the Nyquist curves, as depicted in Figure 15b. While the autogenous self-healing method does not require the embedding of additional materials like shape memory alloys and capsules, its effectiveness is limited to the healing of very small cracks. Conversely, autonomous self-healing in cementitious composites necessitates the integration of additional embedded materials, such as embedded shape memory alloys, electrodeposition technology, capsules, vascular networks, and bacteria, resulting in the healing of larger cracks [162].