## 4 Self-Sensing Geosynthetics
### Carbon-Fibre-Based Self-Sensing Geosynthetics  
Various carbon fibres (CFs) such as continuous carbon fibres, carbon short fibres, carbon micro-fibres, and carbon nano-fibres (CNFs) are widely used as a conductive phase in self-sensing geosynthetics. Indeed, CFs have also been braided or distributed in polymer and fabric composites to develop self-sensing geosynthetics. Continuous carbon fibre polymer composites are fabricated by combining various carbon fibre arrangements such as textile or unidirectional knitted or woven tows with polymer resin under controlled conditions [144]. These composites exhibit complex features including low density, high stiffness and strength, high damping dispersion energy, strong impact and corrosion resistance, and tuneable thermal expansion [145]. In addition, hybrid or multiple carbon and inorganic fibre composites (e.g., aramid, polypropylene, and glass) have been developed to improve the composite sensing capability and strength [146; 147; 148]. The composite structural element configurations determine the strain-sensing capability and attributes of continuous carbon fibre polymer composites [149; 150; 151; 152]. The strain sensitivity of continuous unidirectional carbon-fibre-reinforced epoxy composites has been reported in the fibre direction. The tensile force reversibly decreases the longitudinal electrical resistance, whereas strain increases the transverse electrical resistance because changes in the fibre alignment change the electrical connections of continuous carbon fibres. When a tensile load is applied to the composite, the fibres align more in the loading direction, thereby potentially increasing the electrical connections and reducing the resistance [152; 153]. However, further increasing the tensile loading reduces the fibre cross-sectional area and consequently increases the electrical resistance. Continuous carbon fibre polymer composites could exhibit GFs ranging from \(-35.7\) to \(-37.6\) and from \(+34.2\) to \(+48.7\) in the longitudinal and transverse directions, respectively. Consequently, continuous carbon fibre polymer composites could be very useful for developing sensing applications and self-sensing geosynthetic geomaterials and for building and maintaining civil infrastructure, particularly railway foundations. However, carbon fibre polymer composites usually exhibit low ductility. Consequently, multiple or continuous carbon fibre/epoxy  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Composite Type** & **Fabrication Method** & **GF** & **Maximum Strain (\%)** & & **Description** & **References** \\ \hline \multirow{8}{*}{Graphene/rubber} & \multirow{8}{*}{Template-induced assembly} & \multirow{8}{*}{82.5} & \multirow{8}{*}{100} & \multirow{8}{*}{\(\blackcheck  
hybrid composites have been investigated with other materials to enhance the self-sensing and mechanical properties [154; 155]. The fibre characteristics, constituent fibre proportion, and composite fibre organisation all impact the composite ductility and sensitivity. Glass fibres, one of the most common fibres, have piqued the interest of researchers to produce hybrid CF/GF polymer composites exhibiting improved ductility and sensitivity [156; 157]. These composites exhibit an internal carbon fibre core externally covered by a glass fibre bundle, as shown in Figure 16.  
[PERSON] et al. also reported proper sensing capability and piezoresistivity for this composite under cyclic and monotonic tensile loading [156; 158]. By increasing the CF content, an alarm signal was achieved practically at the failure load (2.4% CF and 49% GF). They also reported drastically increased electrical resistance at loads much lower than the failure load when the CF content was reduced (48% GF and 0.2% CF; 48% GF and 0.6% CF) [156; 158]. However, the hybrid CF/GF epoxy composites could not identify early damage because the fractional changes in the electrical resistance were approximately 1% at strains below 0.6%.