# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 5 Applications and Prospects in Civil SHM
### Fiber Reinforced Composite Structures  
With the increasing demand for fiber-reinforced polymer (FRP) materials in structural strengthen and renovation [148], the safety and reliability of the FRP-related materials become crucial [149]. Traditional approaches for monitoring the FRP materials were based on fiber Bragg gratings (FBGs), as well as ultrasonic sensors, which require complicated analysis processes and expensive resolving devices [150]. Moreover, an FBG sensor can only detect the strain or crack of a single point at the sensor area; thus, it is difficult to monitor the deformation and damage at different locations on FRP composites. In addition, FBGs are vulnerable to water and corrosive chemicals, so they need protective sleeves and cannot be compatible with FRP material because their diameters are very different [150]. Compared with FBG-based methods, the electrical resistivity effect of the FRP materials themselves were noticed by scientists and developed for self-sensing composites [151, 152]. Nevertheless, resistivity parameters of the rough FRP materials without other additives are hard to control, which brings negative impacts to the sensing performance. To enhance the sensing ability, carbon-based fillers were considered to improve the sensitivity and stability of the detection performance of the FRP materials [153, 154, 155]. In general, CNTs or graphene are bonded to the surface of the FRP through coating and chemical vapor deposition. Due to the tiny size of CNTs, conductive networks are formed in the interfaces of carbon fibers. Therefore, local area monitoring is achieved by regulating the deposition of CNTs to the damage-prone area of the FRP composite structure [156, 157]. As an example, [PERSON] et al. preliminarily investigated the properties of several continuous carbon-fiber epoxy-resin composites for strengthening concrete structures [158]. They improved the sensing ability of small-range strain by setting various geometric shapes and arrangement modes of the carbon fibers. As shown in Figure 10, composite materials were made on site for structural reinforcement in civil engineering. Results showed that the composite cannot only sense a small deformation of less than 1% strain, but also increase the strength and bearing capacity of the original concrete structure. Figure 9: (**a**) crack expansion process of concrete square slab, (**b**) corresponding electrical resistance tomography of sensing skin under for crack detection [147] (Copyright 2014, Elsevier Ltd.). In addition, FRP materials are usually classified as carbon-FRP (CFRP) and glass-FRP (GFRP). [PERSON] et al. fabricated a nano-composite quantum resistance sensor, which can be embedded and distributed in GFRP without affecting its mechanical properties. Fatigue tests illustrated that the closing/reopening and expansion of cracks along with the increase in cyclic loading led to damage accumulation of the GFRP material [150]. This sensor was able to track the strain and damage of the GFRP structure in both the elastic and plastic stages until it failed. On the other hand, electrical resistance measurements were applied for the fracture detection and failure prediction of cylindrical-type CFRP composites (Figure 11a). Results showed that longitudinal cracks of the CFRP cylinder were easily detected according to the signal jump of the resistivity-change ratio [159]. Moreover, the resistance-variation graph can be used to distinguish different fracture modes of the CFRP cylinder (Figure 11b). In addition, CNT-epoxy coatings were developed for large-area damage detection in GFRP-based structures [53]. A multichannel electrode network was integrated on the coating to build a two-dimensional map for multi-damage detection of a GFRP structure (Figure 12a). Figure 12c illustrates the location and quantification of a 23 mm long crack according to the normalized resistance variation of the sensing network.