PSI - Issue 64

Mingyang Xi et al. / Procedia Structural Integrity 64 (2024) 515–522 Mingyang Xi et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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1. Introduction The economic progress and development of a country significantly depend on the effective promotion of transportation infrastructure construction. Bridges are an important component of transportation infrastructure. For instance, there are currently over one million bridges in China, most of which are prestressed reinforced concrete beam bridges. The prestressing tendons in these bridges are usually strands, and during the construction and operation phases, prestress losses can occur due to factors such as strand relaxation and concrete shrinkage. Without effective monitoring, these issues could affect the long-term performance of the structure and even lead to bridge failure (2013). Therefore, there is an urgent need to monitor the prestressing strands in reinforced concrete beam bridges. To address the difficulty of effectively monitoring the internal conditions of prestress in actual engineering projects, researchers like M.G. Li (2021) et al. proposed a self-sensing strand technology embedded with Fiber Bragg Grating (FBG) sensors. This technology has proven through practical application to be capable of precise and effective real time monitoring of internal stresses in the cable body. Qin (2020, 2021) et al., by applying self-sensing strands in the cables, further achieved intelligent monitoring of cable force, demonstrating that FBG sensors with groove encapsulation technology have a high survival rate, large monitoring range, small error, and good stability, thereby providing technical support for the application of fiber optic grating sensors in cable force monitoring. Obaydullah (2016) et al. provided an effective means of prestress force monitoring through self-sensing strands with embedded sensors, such as Fiber Bragg Grating (FBG) sensors. These technologies not only enable real-time monitoring but also help to improve the reliability and safety of the structure. C.N. Dang (2016) et al. showed through research that prestressed concrete beams using self-sensing strands have better structural performance than traditional prestressed concrete beams, including enhanced load-bearing capacity and reduced crack width. Li. F (2018) et al. proposed a method for monitoring the tension of intelligent twisted carbon fiber reinforced polymer (CFRP) rods based on embedded FBG sensors, proving the method to have good measurement stability and durability. G.X. Fan (2020) et al. studied the strain monitoring technology of beam corrosion prestressed strands based on fiber Bragg grating sensors, verifying its effectiveness in measuring the strain of strands. Jan Koch (2014) et al. proposed a fiber Bragg grating-based fiber optic strain sensor capable of real-time monitoring of steel cables, while H.Y. Qin (2020) et al. achieved effective life monitoring of strands under fatigue load by embedding fiber Bragg grating sensors into the longitudinal grooves of the strand's central wire. Although self-sensing strand technology offers a new method for monitoring prestressed concrete beams, it also faces many challenges, including the long term stability of the sensors, the accuracy of the data, and cost-effectiveness. Future research needs to address these issues to promote the wider application of self-sensing strands in engineering practice (Hsiao, 2016). The aforementioned studies demonstrate the feasibility of combining fiber Bragg gratings with strands, proving that the integration of fiber Bragg gratings with strands can enable real-time monitoring of the stress in strands. To meet the engineering application requirements in prestressed reinforced concrete beam bridges, this research proposes an self-sensing strand embedded with fiber Bragg gratings, which possesses both load-bearing and sensing capabilities. It was applied in an actual prestressed reinforced concrete box girder bridge, monitoring the prestress loss during the construction phase of the bridge. 2.1. Self-sensing Strand Design and Encapsulation The self-sensing strands mentioned in this article are created by setting grooves on the central wire and embedding Fiber Bragg Grating (FBG) sensors into these grooves. A specially formulated adhesive is used to attach the FBG sensors within the grooves. Additionally, the sensor ’s bonding length and encapsulation materials are optimized to enhance the strain transfer efficiency and monitoring range. After encapsulation, the central wire is retwisted with the outer wires to form the self-sensing strand. An analysis of the strain transfer rate between the embedded FBG sensors and the strand matrix was conducted to determine the impact of the groove dimensions, sensor bonding length, and the elastic modulus of the encapsulation material on the strain transfer rate. Extensive experiments have shown that groove depths of 0.2-0.5 mm, widths of 0.5-1.2 mm, a diameter ratio between the central and outer wires, along with a bonding length of 40 mm and an 2. Embedded Fiber Bragg Grating Self-sensing strands