# Applications of Cement-Based Smart Composites to Civil Structural Health Monitoring: A Review
## 2 Fabrication and Operational Issues  
CSCs basically have a multi-phase nature. Macroscopically, two phases can be assumed, namely the cementitious matrix and the dispersed conductive filler. The cementitious matrix is characterized by electrical resistivity values ranging between \(10^{6}\) and \(10^{9}\)\(\Omega\cdot\)cm, so it may be referred to as the non-conductive phase. Such component is often cement paste, sometimes mortar, rarely concrete. The conductive phase usually consists of materials such as steel fibers (SFs), carbon fibers (CFs), carbon nanotubes (CNTs), carbon black (CB) or less frequently other nanomaterials: they are either used alone (single-doping) or combined into hybrid solutions (multi-doping) to enhance performance [9]. SFs represent the early functional filler for cementitious matrix. Originally developed as reinforcement solution in order to enhance the tensile strength of the base material [14], the SF capacity of improving electrical conductivity of the matrix was recognized afterwards: since then, the strain-sensing attitude of SF-doped RC has been investigated [9; 15]. Typical SFs are characterized by diameter (D) ranging between 0.025 to 0.6 mm and length (L) from 3 to 30 mm depending on the intended use [16; 17]. Despite the good mechanical and electrical properties of SFs, the sensing capacity was strongly affected by corrosion, so the use of carbon-based materials for CSCs has been considered as an alternative. CFs are characterized by several excellent properties including high electrical conductivity and corrosion resistance, due to the huge carbon content (larger than 92%-weight) [18] and high aspect-ratio [19]. Typical dimensional ranges (min-max) of CFs are 7-18 \(\times\)\(10^{-6}\) mm (D) and 3-30 mm (L). CFs are currently available on the market at relatively low cost, and they represent the most studied functional filler in the literature [13; 20; 21; 22]. On the other hand, CNTs have recently gained more and more popularity as functional filler for CSCs since they are characterized by better mechanical and conductive properties with respect to CFs [23; 24; 25]. Discovered by [PERSON] in 1991 [26], CNTs consist of carbon atoms bonded in a helical crystalline structure. Two different forms are available, namely single-wall (SWCNTs) and multi-wall (MWCNTs) carbon nanotubes, consisting of one cylindrical lattice of carbon atoms and multiple concentrically positioned helical lattices, respectively. SWCNTs have thickness of a single atom and diameter (D) ranging from 0.4 to 3 nm, whereas MWCNT diameter (D) ranges between 1.4 and 100 nm. CNTs are characterized by very high aspect ratio able to ensure high electrical conductivity. Nevertheless, the main obstacles to the wide application of CNTs in CSCs are the high dispersion complexity and the higher cost with respect to CFs [27]. Differently from fiber-shaped materials, CB is a powder composed of conductive particles able to reduce the electrical resistance and enhance the density of cementitious matrix, although with negligible capacity in improving tensile mechanical properties [28]. The lower cost and dispersion complexity make CB an attractive solution for CSCs even if the low aspect ratio implies less effectiveness in enhancing the electrical conductivity of the composite: as a result, CB is often used in hybrid mixings in combination with fibrous filler [29; 30]. Actually, there is frequently a third component in CSCs, the dispersant agent. Indeed, the dispersion of micro- or nano-scale filler into the cementitious matrix is commonly recognized as a challenge, especially for those conductive phases characterized by high aspect ratio with considerable attractive area forces and high probability of agglomeration [31].