# Applications of Cement-Based Smart Composites to Civil Structural Health Monitoring: A Review
## 2 Fabrication and Operational Issues
### Environmental Effects and Durability  
Environmental actions on civil structures due to temperature and moisture may affect the long-term sensing performance of the installed CSC elements. In general, due to hot expansion and cold contraction of the cementitious matrix, the spacing of functional fillers changes and the tunnelling effect is modified. As a result, the electrical resistivity tends to decrease with increasing temperature until a limit value is reached above which the water present into the micro-structure pores evaporates causing a sudden increase of the resistance [64]. On the other side, repeatability and stability of sensing measures change if the external temperature changes. Under temperature range between 0 \({}^{\circ}\)C and 40 \({}^{\circ}\)C, adequate results in terms of stability are obtained (with the best performance in the range 20-40 \({}^{\circ}\)C), whereas measures become less repeatable at higher temperature (above 60 \({}^{\circ}\)C) or below 0 \({}^{\circ}\)C [65]. Extreme climate conditions such as freeze and thaw cycles might cause phase transformation of water, between solid and liquid state, causing micro-cracks into the matrix and irreversible increase of resistivity combined with low repeatability [66]. Nevertheless, investigations about the sensing ability of CSCs at very high temperature (larger than 200 \({}^{\circ}\)C, representative of the temperatures in fire-damaged structures) are in progress and the first results have been recently published in the literature [67]. These studies might have significant impact on the applications of CSCs to civil SHM but further studies are needed to reach well-established results and firm conclusions.  
Change of ambient humidity may yield modification of the CSC internal moisture content which affects the sensing performance. Since water is an electrical conductor, saturated CSCs are systematically characterized by resistivity considerably lower than the dried ones. In fact, as known, conductive paths within CSCs are mainly due to overlapping fillers conduction, tunnelling effect conduction and ionic conduction. Moisture content essentially affects the latter, which offers a contribution, although generally limited, to the piezoresistive response. Furthermore, the presence of pore water with ionic concentration causes severe polarization affecting the piezoresistive performance in terms of repeatability [68].  
The influence of moisture depends on type and amount of conductive filler. CB particles can absorb water with a subsequent increase of the electrical resistivity [69]. Conversely, for CSCs doped with CFs and CNTs, the electrical resistivity decreases as internal moisture increases due to the larger ionic conduction [68]. When SFs are used as conductive filler, particular attention should be paid to corrosion effect, for which no studies are available in the literature. Regarding the relationship between moisture and filler amount, for concentration lower than the percolation threshold an increase in CSC conductivity can be observed when moisture increases, even if with a consequent reduction in sensitivity due to the disturbance effect of ionic conduction, whereas high presence of fillers reduces the influence of moisture on sensing performance [70]. The main actions aimed at internal moisture reduction are air-drying and oven-drying. The latter proved to be more effective, although oven temperature and drying time are key parameters that need for appropriate experimental evaluation. For CSCs with 0.1%-weight of CFs, 3 days at 95\({}^{\circ}\) C may be recommended [63].