# Applications of Cement-Based Smart Composites to Civil Structural Health Monitoring: A Review
## 4 Discussion and Open Issues
### Sensing Applications  
The first studies on CSC applications for SHM of medium- and full-scale structural elements concerned RC members completely or partially doped with conductive fibers, specifically CFs and SFs [75; 76]. These studies qualitatively confirmed the promising applicative perspectives of multifunctional CSCs for civil SHM. Nevertheless, the large costs and fabrication complexity make the corresponding functionalization approaches inconvenient for civil structures; as a result, an alternative approach has been pursued by installing small self-sensing CSC elements into RC members. This is also the only implementable solution in the case of existing structures, which are the main focus of SHM due to their obsolescence. Very different sensor dimensions have been adopted mainly depending on sensing application and filler typology. Specifically, volumetric ratios (the ratio between the volume of the CSC sensing element and that of the tested structural member) typically considered in the collected literature range between 5.1 \(\times\) 10\({}^{-5}\) and\(5.2\times 10^{-2}\). No information about the required number of sensors is reported with respect to reliability of measurements, redundancy of the SHM system, or SHM targets. Generally, CSC sensors are embedded close to the external edge of the member. Specifically, the sensor is placed within the framework before casting operations. A few studies consider different installation methods namely external attachment by means of highly adhesive resin [77] or mechanical connection by a steel device able to ensure structural continuity and electrical insulation of the CSC [54], or embedment with the addition of corrugated steel dowels to improve the strain transfer [81]. Nearly all methods yielded reliable measurements. However, embedding is the preferred solution due to the complete inclusion of CSC, which ensures good performance in terms of strain transfer from the monitored member. Conversely, the external installation appears more feasible for SHM of existing structures, even if embedment could be still carried out by perforation of the member and use of slightly expansive cement-based materials to restore the original continuity. In any case, further studies aimed at the evaluation of the influence of installation methods on the sensing performance of multifunctional CSCs are needed. CSCs are typically applied to measure monodirectional strain induced by externally applied loads. Specifically, pure axial- or bending-induced strain have been investigated, with sensors installed in constant-stress regions in the latter case; more complex stress states are still under-investigated. The sensing performance of CSCs as dynamic sensors have been very recently investigated at large-scale obtaining promising results. This opens interesting applicative perspectives for the application of CSCs to full-scale members. However, in view of SHM applications, some issues are still far from being overcome. For instance, repeatability and stability of measurements are significantly affected by the applied stress levels. Under cyclic excitation, indeed, stable data can be obtained until a maximum stress level of 30% of the strength of the material. Beyond that limit repeatability problems have been observed. Mechanical compatibility between CSCs and monitored RC members has been also recognized to play a critical role [71], especially when CSCs are used for monitoring high-stress levels close to failure. Unfortunately, very few studies report the mechanical properties of CSCs and the material of the structural prototype; thus, it is impossible to draw any firm conclusion. About the electrical configuration, the four-probe method is mostly adopted because of its accuracy, but a few studies provide information about current type (i.e., AC, DC and biphasic DC) and, in particular, quantitative parameters such as intensity and frequency for AC measurements.