Evaluating the Interfacial Mechanics of Distributed Strain Sensors Mounted to GFRP Reinforcing Bars

Abstract

In-service reinforced concrete (RC) structures in harsh environments face significant challenges such as corrosion and cracking, which undermine their durability and resilience. To address these issues and reduce maintenance costs, a novel solution is proposed that integrates glass fibre reinforced polymer (GFRP) reinforcing bars with distributed fibre optic sensors (DFOS). This multifunctional system incorporates continuous structural health monitoring (SHM) capabilities into the electromagnetically neutral, high-strength, and lightweight properties of GFRP bars, while protecting sensors from cracking and fracture within the concrete matrix. Emerging sensing systems, such as optical frequency domain reflectometry (OFDR), offer sub-millimetre resolution in distributed strain measurements over long distances. This enables precise detection of strain peaks associated with concrete cracks at various locations along the RC structure throughout its service life. Despite considerable prior research and development, the strain transfer efficiency and monitoring stability of OFDR-based DFOS bonded to GFRP bars under various stress conditions had not been thoroughly investigated. To fill this gap, this study examined the performance of DFOS in capturing tensile strain in GFRP bars under cyclic and sustained loading. Experimental testing and numerical analysis were conducted to identify key influencing factors and define limits for efficient and stable monitoring. Strain profiles from multiple test specimens were then analyzed using a developed closed-form model that accounts for elasto-plastic strain transfer from the host material to the sensor through the adhesive interface. The experimental results demonstrated that OFDR-based DFOS reliably captured strain profiles along GFRP bars under varying loading conditions, including within the GFRP bar's serviceability limits and up to strain levels approaching 1.3%. Beyond this threshold, sustained high-stress exposure triggered plastic response in the adhesive interface, leading to gradual bondline degradation, permanent interfacial slip, and progressive strain reading anomalies (SRAs), observed by loss of accuracy in strain values measured along the DFOS-bonded length as it degraded over time. The numerical analysis closely aligned with experimental data, effectively capturing strain transfer behaviour during bond damage progression. This provides a practical framework for evaluating the quality of dynamic host–sensor interaction and potential monitoring efficiency loss under severe mechanical loading during service. Overall, the findings establish effective monitoring ranges and emphasize the importance of better understanding the strain transfer mechanisms of bonded DFOS systems, identified as a primary source of measurement error, for enabling reliable long-term strain monitoring in demanding environments.