# Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring
## 1 Introduction
The electric potential changes immediately when the strain changes, leading to high sensitivity and fast response, which are valued in dynamic strain monitoring [19]. However, piezoelectric sensors cannot measure static or pseudo-static strain because the potential difference in the material will soon return to zero when the deformation process stops. Flexible capacitive sensors measure strain by translating the external deformation into capacitance change of the conductive material. Benefits of capacitive sensors include high sensitivity to tiny deformation, high linearity and low hysteresis [20]. Nonetheless, the fabrication process for capacitive sensors is more difficult than that for other kinds of sensors. Compared with piezoelectric and capacitive sensors, piezoresistive sensors present easier fabrication, high sensitivity and long durability. As a result, piezoresistive-based flexible strain sensors are the most widely used flexible sensors [21]. Piezoresistive strain sensors are composites that usually comprise two-phase materials, namely, the conductive phase and the substrate phase. The conductive phase provides the sensing properties of the sensors, and the substrate phase provides the shape and structural integrity of the sensors [22]. The constituents of these two-phase materials cooperate with and complement each other to achieve the functions of flexible strain sensors. One of the most important things in designing a flexible piezoresistive sensor is the choice of suitable conductive and substrate materials that meet both the sensing and mechanical requirements for practical application. Researchers tend to adjust the concentration of the conductive and substrate materials to improve the functional properties of the piezoresistive strain sensor or add extra phase materials to the composite to endow extra functionality [23]. Nevertheless, existing composite materials still encounter challenges in practical application, such as low sensitivity for tiny strain, degradation of sensing performance caused by material fatigue and plastic deformation of the polymer substrate during large cyclic deformation [24]. In practice, civil structures often suffer time-variant environments and different service conditions, such as seasonal temperature changes, construction loads, fatigue loads, typhoons and earthquakes [25, 26, 27]. Under these conditions, the effectiveness and stability of strain sensors in the long term are difficult to guarantee [28]. Therefore, sensors installed on structures are required to have integrated performance of high sensitivity, large stretchability and durability [29]. Specifically, measurement of the full strain range from the elastic stage to plastic buckling and then to crack propagation is very difficult for traditional strain sensors. Therefore, novel piezoresistive flexible strain sensors with improved structural design and fabrication technology are promising for realizing accurate monitoring of the complete deformation process, which is significant to intelligent civil SHM [30]. To explore advanced methodologies for monitoring the full process of structural deformation from small strain to crack propagation, this study focuses on the state-of-the-art research progress of piezoresistive-based flexible strain sensors in civil SHM. The keywords in the Abstract are used for searching and retrieving related papers before the writing of this review, namely the piezoresistive mechanism, flexible strain sensor and civil structural health monitoring. The methodology of the selection of appropriate papers for this review includes three criteria. The first criterion is that the selected papers should be highly related to the topic and keywords of this review. The second criterion is that highly cited papers and papers from high-authority journals are preferable for selection. The third criterion is the publication time of the paper. Papers published in the recent five years are more valuable because of the rapid growth and improvement of piezoresistive technologies.