# Applications of Cement-Based Smart Composites to Civil Structural Health Monitoring: A Review
## 2 Fabrication and Operational Issues
### Mix Design  
CSC mix design concerns the definition of the optimal proportions among the different components. First of all, an appropriate concentration of functional filler has to be defined, taking into account the percolation threshold [33]. If an insufficient amount of filler is used, no conductive network is created into the matrix microstructure, and negligible changes of conductivity can be measured. Adding filler while remaining under the percolation threshold, the electrical resistivity strongly decreases with respect to the original value (corresponding to the matrix alone). In fact, increasing the relative amount of filler yields shorter and shorter conductive paths until the percolation threshold is exceeded; after that, appreciable changes of resistivity are no more observed [20; 34]. In the presence of fine aggregates within the matrix, double percolation has to be considered [29] involving both filler and cement paste percolations: with increasing sand/cement ratios, the filler percolation threshold considerably increases. It is worth noting that the piezoresistive behavior of CSCs does not depend only on the amount of filler, but also on its dispersion into the cementitious matrix. In fact, even in the presence of relatively low filler concentration, if the gap between two generic adjacent filler particles ranges between 1 nm and 10 nm, conduction occurs through the cementitious matrix because of the transition of electrons through particles immersed into an electrical field (tunnelling) [35]. The best sensing performance of CSC is therefore achieved for filler concentrations corresponding to the coexistence of both conditions (effective dispersion, and concentration lower than the percolation threshold), although filler type and aspect ratio considerably affect the quantification of the optimal filler amount: in fact, the longer the conductive elements, the lower the percolation threshold and the tunnelling effect. Typical values of filler concentrations reported in the literature are about 5% for CFs and SFs, 1% for CNTs, and 10% for CB [36].  
The mix design of the matrix starts from the conventional proportion for the considered typology (cement paste, mortar or concrete). However, the introduction of functional filler as well as dispersant agent has to be properly accounted for since both components affect workability at fresh state along with rheology and mechanical properties [37]. The key parameter governing those aspects is the water-to-cement (w/c) ratio. Theoretically, higher w/c values improve workability and effectiveness of fillers dispersion within the matrix. On the other side, large w/c ratios affect the mechanical properties of CSCs. Due to the complexity of the involved phenomena, a trial-and-error approach is generally adopted in the literature to obtain the best mixing proportions for assumed components [38]. Typical w/c ratios are set within the range 0.4-0.6, depending on the effective mixing composition [39]. Another major parameter related to mix design of CSCs is the possible presence of aggregates, fine or coarse, and the amount with respect to cement. The addition of aggregates to the cement paste decreases the electrical conductivity of the matrix and such effect is proportional to the aggregate diameter; thus, the sensing capacity decreases as aggregate dimension increases. In addition, coarse aggregates provide detrimental effect on dispersion effectiveness and create obstacles to the conductive passage [40]. Nevertheless, if sand amount is properly defined, higher electrical conductivity can be obtained due to the reduction of voids in the matrix [16]. Similarly, some studies report the addition of fly ash and silica fume to minimize matrix porosity and enhance the sensing properties of CSCs [41]. Alternatively, or in addition to the mineral admixture, plasticizers can be introduced during the mixing process in order to ensure sufficient workability of the paste and preserve both mechanical and rheological behavior [42].