# Smart Sensing of PSC Girders Using a PC Strand with a Built-in Optical Fiber Sensor
## 1 Introduction  
### PC Strand with Built-in Optical Sensor  
The tendon is a critical member introducing prestressing force in concrete structures to improve its deflection and crack characteristics. However, the prestressing force may reduce when the strands in the tendon experience corrosion because of a variety of reasons and lead to significant loss of the structural performance, which may even result in the collapse of the structure if not treated in due time. Figure 1 presents examples of bridges using PSC (prestressed concrete) rods and strands that have recently collapsed [1, 2]. Managing the prestressing force is thus primordial to prevent such accidents. The prestressing force in PSC structures experience changes due to diverse causes. However, there are very few technologies enabling monitoring of prestressing force in the long-term since its introduction is noteworthy. Some examples considered the attachment of electrical resistance gages to the strand itself or the installation of load cells at the anchor of the tendon, but these solutions necessitate a connecting wire for each of the sensors and may face the problem of the increasing resistance of the connecting wire if the distance to the measuring point becomes longer, which in turn will generate problems in data processing. In addition, long-term durability may also be problematic if the sensors are exposed to the external environment. Accordingly, research on sensor-type PC strands is being led in numerous countries to overcome such problems and achieve a strand that can play the role of sensor, apart from its natural application as a member introducing prestressing force in the structure [3, 4, 5]. Figure 2 shows some examples of sensor-type PC strands that have been developed. Among the sensor-type strands shown in Figure 2, the one developed by [PERSON] et al. [3] has difficulty in fabrication because it involves inserting the optical fiber inside a hollow steel bar and renders the ensemble monolithic by using a resin, which results in early rupture of the optical fiber. The strand by [PERSON] et al. [4] made use of a FRP (fiber reinforced polymer) rod integrating both optical fiber and glass fiber, which provided a system developing low tensile strength and inappropriate as a PC strand because of the mechanical properties of glass fibers, like relaxation and creep. The strand by [PERSON] et al. [5] installed an optical fiber along the helical gap between the wires composing the strand, which could not measure exactly the prestressing force developed in the longitudinal direction of the strand. Furthermore, when bundles of strands are used simultaneously, there is the risk of damaging the sensor because of the interference with the neighboring strands. Apart from these examples, [PERSON] et al. [6] and [PERSON] et al. [7] led studies on the accuracy and transfer length using the strands developed by [PERSON] et al. [4] and [PERSON] et al. [3]. As a sensing technology for evaluating the behavior of structures, optical fiber sensors show many possibilities. [PERSON] et al. [8] presented an optical fiber twist sensor based on surface plasmon resonance excitation, which showed high sensitivity in both gaseous and liquid media. Also, [PERSON] and [PERSON] [9] and [PERSON] et al. [10] showed the possibility of long-term monitoring. The optical sensor PC (OSPC) strand, developed by the Korean Institute of Civil Engineering and Building Technology (KICT) in 2013, adopts the method of measuring the prestressing force by a CFRP (carbon fiber reinforced polymer) core wire incorporating an  
Figure 1: Examples of prestressed concrete (PSC) bridge collapse: (**a**) Morandi bridge (Italy, 2018) [1]; (**b**) Nanfang’ao bridge (Taiwan, 2019) [2]. Figure 2: Research on prestressed concrete (PC) strands with measurable prestressing force: (**a**) fiber Bragg grating (FBG) sensors encapsulated into seven-wire steel strand by [PERSON] et al. [3]; (**b**) Smart steel strand by [PERSON] et al. [4]; (**c**) Optical fiber-embedded strand by [PERSON] et al. [5].