# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 5 Applications and Prospects in Civil SHM
### Concrete Structures  
Different from steel structures, concrete structures crack under large tensile and compressive deformation. Researchers developed piezoresistive strain sensors using carbonaceous fillers or metallic fillers. In terms of carbonaceous fillers, [PERSON] et al. prepared CB/epoxy composite film sensors to monitor the overall stress and deformation process of concrete columns and reinforced concrete beams [140]. In this case, this film sensor showed a sensing strain range of 1.2% and a GF higher than 11, leading to the accurate measurement of strain and the preliminary location of the crack in the monitoring area [141]. In addition, [PERSON] et al. designed a composite polymer film by blending solutions of MWCNTs and PDMS, and then investigated the sensing properties of the polymer film for the deformation and crack monitoring of concrete structures. This polymer film showed a large sensing range of 50%, but a nonlinear strain-resistance relationship with various GFs from 1.5 to 6 [142]. [PERSON] et al. fabricated a flexible sensor using graphene oxide as conductive filler, cellulose nanofiber as dispersant, and waterborne epoxy as polymer matrix. This sensor showed large stretchability of a 55% strain, a GF of about 34, and good linearity and repeatability within a 4% strain range. Full-scale tests demonstrated the sensor's capability to monitor the strain and detect cracks on a concrete surface (Figure 8) [143; 144].  
On the other hand, in terms of metallic fillers, [PERSON] et al. proposed a crack-width monitoring approach by screen printing CuNWs with different ultimate strains on an epoxy-resin structural surface. They established a multilayered stress-transfer model to quantify the proportional relationship between the crack width and the ultimate strain of the CuNWs, and then redesigned a smart film to assess cracks on a practical large-scale bridge [145]. Besides, [PERSON] et al. prepared a AuNPs/PDMS composite by an in situ reduction method, and then studied its optical properties under tensile strain. This composite showed a linearity between the light absorption rate and the tensile strain within a proper strain range [146]. Moreover, [PERSON] et al. applied conductive silver glue to fabricate a multi-electrode sensing skin, the resistivity of which was monitored by electrical resistance tomography [147]. By using a differential imaging scheme, it is worth noting that this skin was able to locate the through crack of a concrete beam and track the process of crack propagation, as shown in Figure 9. Nevertheless, the sensitivity of the skin is relatively low because of the very tiny variation of the resistance of the conductive silver glue, and the quantitative relationship between resistance variation and crack length/width  
Figure 8: Deformation and crack monitoring [143] (**a**) the positions of the film sensor and resistance strain gauge; (**b**) four-point loading chart; (**c**) strains of composite film sensors (SA, SB, and SC) and traditional resistance strain gauge (TA) during loading. (open access, MDPI).  
was not provided in their work. Additionally, the resolution and accuracy of crack sensing is highly related to the quantity of the electrodes, whereas it is unpractical to set many electrodes on a sensing film in field monitoring.