# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 5 Applications and Prospects in Civil SHM
### Fiber Reinforced Composite Structures
As indicated from Figure 12d-e, the CNT-epoxy coating showed a varied GF from 3 to 12 within a 0-2.5% strain range, which is higher than the tradition foil sensor. The excellent sensitivity was attributed to the susceptible destruction and reconstruction processes of the conductive  
Figure 11: (**a**) Long CFRP cylinder specimen during flexural test; (**b**) resistance variation graph under different fracture mode of CFRP cylinders [159] (Copyright 2016, Elsevier Ltd.). Figure 10: (**a**) textile-pattern strain sensor; (**b**) concrete specimens reinforced with carbon fiber epoxy resin composites [158] (open access, MDPI). networks in the ultrathin coating. In addition, the multichannel electrode network detected the damage length with a sensitivity of 0.007%/mm (Figure 12f). Last but not least, Table 4 gives a summary of the aforementioned piezoresistive strain sensors used for the SHM of civil structures. As shown in the third column of Table 4, the detectable strain ranges of most of the sensors are below 5%, which is much smaller than the piezoresistive sensors shown in Tables 2 and 3. This is because the elastic and recoverable strain range of civil structures is usually below 1-3%, according to the construction materials. For instance, the FRP-based structure will crack near a 1% \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Materials** & **Maximum** & **Gauge Factor** & **Linearity** & **Cyclic Times for** \\  & **Filler + Substrate** & **Detectable Range** & **(Sensing Range)** & **(Srain Range)** & **Durability** \\ \hline
[137] & Carbon Particle + PI & 0.6\% & 25 & Near linear (\(\epsilon<0.6\%\)) & \\
[138] & MWCNTs + Resins & 20\% & 2.6 (\(\epsilon<1\%\)) & Linear (\(\epsilon<1\%\)) & 10 (\(\epsilon<1\%\)) \\
[139] & MWCNTs + Epoxy & 0.14\% & & Linear (\(\epsilon<0.14\%\)) & 1000 (\(\epsilon<0.14\%\)) \\
[140] & CB + Epoxy & 1.2\% & 11–42 (Depend on & Nonlinear & \\  & & & filler content) & & \\
[142] & MWCNTs + PDMS & 50\% & 1.5–6 & Nonlinear & \\
[143] & rGO/Cellulose nanofiber + Epoxy & 55\% & 34–71 & Nonlinear & 100 (\(\epsilon<4\%\)) \\
[144] & RGO-CNF + WEP & 4\% & 16–52 (Depend on & \\  & & & filler content) & & \\
[146] & Gold nanoparticles + PDMS & 30\% & 0.04 & Near linear & \\
[151] & MWCNT/FRP + Epoxy & 1.23\% & 5–6 & Nonlinear & \\
[154] & CN/Is Glass Fiber + Epoxy & 1.2\% & 2.25 & Linear (0.5\% \(<\)\(\epsilon<1\%\)) & \\
[155] & MWCNTs/Glass Fibers + Epoxy & 2.5\% & 3.8 (\(\epsilon<0.2\%\)) & Near linear (\(\epsilon<0.2\%\)) & \\
[156] & CNT/Glass Fibers + Epoxy & 2.2\% & & Nonlinear & \\
[157] & Graphene nanoplate + Glass fiber & 2\% & 840–16,400 & Nonlinear & \\
[158] & Carbon fibers + Epoxy & 3.5\% & 10 & & \\
[159] & Carbon fibers + Epoxy & 1\% & 0–0.25 & Nonlinear & \\
[53] & CNT + Epoxy resin & 2.5\% & 3–12 & Nonlinear & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Piezoresistive strain sensors for civil SHM. Figure 12: (**a**) Electrode disposition on the CNT/epoxy coating; (**b**) damages on the slab; (**c**) two-dimensional electrical resistance mapping of crack detection; (**d**) normalized resistance variation under tensile strain; (**e**) gauge factor variation under tensile strain; (**f**) normalized resistance variation with damage length [53] (Copyright 2017, Elsevier B.V.). strain (Figure 11b). Hence, the mission of the strain sensor changed to detect cracks or bulking rather than measure strain when the structural strain is beyond the recoverable range. Nonetheless, little consideration has been paid to the long-term durability of the piezoresistive sensors, as indicated from the blanks in the last column of Table 4.