# Applications of Cement-Based Smart Composites to Civil Structural Health Monitoring: A Review
## 2 Fabrication and Operational Issues
### Dispersion  
The filler dispersion is the most critical stage of the CSC fabrication since its effectiveness strongly affects both mechanical and sensing properties. This task is particularly challenging for several reasons. Indeed, due to the size of functional fillers (generally, micro- or nanoscale), such conductive particles, even if present in very low amount, are characterized by huge superficial areas and, therefore, high Van der Waals interaction forces among them develop [43]. Because also of their hydrophobic nature, carbon-based fillers tend to form agglomerations into the matrix microstructure, especially when conductive materials with high aspect-ratio are used and the mixing is characterized by considerable viscosity [44]. In order to solve problems related to functional filler dispersion into the cementitious matrix, different techniques have been proposed in the literature. Those methods can be classified into two main categories: mechanical dispersion and chemical dispersion. Mechanical methods include all the techniques aiming at the physical dispersion of fillers, such as mechanical stirring, ball milling and sonication (or ultrasonication) bath. Stirring is an ordinary step of mixing and it is enough for ordinary reinforcing fibers. However, in the case of nanomaterials its efficiency is limited since the separation among them is temporary, and when the stirring stops the particles tend to agglomerate again. Only shear stirring at high-speed proved to be effective to uniformly distribute nano-sized fillers, even if the integrity of long particles is not guaranteed with direct consequences on percolation threshold and conductivity capacity [45]. Ball milling uses the action of impacting grinding balls for breaking the inter-particle bond, avoiding agglomeration. Nevertheless, also this technique is not adequate for long particles [46]. Sonication and ultrasonication are based on the use of sound and ultrasound waves, respectively, to win the bond forces. They are currently considered the most effective among the mechanical dispersion techniques: this particularly applies to ultrasonication of carbon-based conductive materials. Typical ultrasonic frequency used for dispersion is 20-30 kHz, causing high energy cavitation of the liquid mixture with homogeneous dispersion, even if proper input control has to be ensured to avoid damage to filler particles. In the case of long conductive fibers, low power bath sonication is recommended with input power lower than 130 W [8]. Sonication time represents another key parameter, since insufficient time (below 1 h) prevents adequate filler dispersion; conversely, too long sonication (higher than 5 h) tends to break the particles and reduce conductivity [47]. Chemical methods enhance filler solubility by means of the modification of the particle external shell. Two possible surface modifications are reported in the literature: covalent and non-covalent. The first is characterized by the use of neat acids, often combined with high temperature mixing, leading to an oxidation reaction between oxygen and carbon atoms with subsequent bonding of negatively charged carboxylic groups to the CNT surface [48]. The electrostatic repulsion forces among those groups ensure separation and make possible the effective filler dispersion. However, neat acids may have significant detrimental consequences on the mechanical and electrical response of carbon fillers and, as a consequence, of CSCs [49]. Conversely, non-covalent methods use the absorption property of molecules without any consequence on the material structure. This is generally pursued by adding surfactants to the mixture [50]. The most frequently adopted surfactants are methylcellulose (MC), polycarboxylate-based (PC) superplasticizers, anionic sodium dodecyl benzene sulfonate (SDBS) and lignocellulosic (LS).