PSI - Issue 64

Mehdi Aghabagloo et al. / Procedia Structural Integrity 64 (2024) 1516–1523 Mehdi Aghabagloo/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Strengthening reinforced concrete (RC) structures, like beams, columns, or slabs, with fiber reinforced polymer ( FRP) relies on a robust bond behaviour between the composite and concrete surface. This bond is facilitated by an adhesive, ensuring shear stress transfer between the FRP and the concrete. Externally bonded reinforcement (EBR) is one of the most effective methods for strengthening concrete structures (Teng et al., 2002, Zhou et al., 2017). However, premature debonding poses a significant challenge for EBR systems, a phenomenon well-documented in the literature (Malek et al., 1998- Sebastian, W., 2001). To delay premature FRP debonding, various methods have been proposed (Kalfat et al., 2013), including surface treatments for concrete (Iovinella et al., 2013) and other additional strategies (Kalfat et al., 2013), such as FRP U-jackets (Chen et al., 2012 – Garden et al., 1998), fiber anchors (Smith et al., 2011 – Ali et al., 2014), and metallic anchorage systems (Napoli et al., 2013 – El Maaddawy et al., 2008). These methods, categorized into mechanically fastened FRP (MF-FRP) (Napoli et al.,2013 – El Maaddawy et al., 2008), hybrid-bonded FRP (HB-FRP) (Guan et al., 2013 – Wu et al., 2008), and end-anchorage techniques (Galal et al., 2009), effectively control premature FRP debonding, delaying or even preventing it. Among these methods, the HB-FRP technique stands out as the most effective, with reported bond strength improvements at the FRP-concrete interface exceeding 200% (Kalfat et al., 2013, Wu et al., 2009). Understanding the impact of compressive stresses on FRP-concrete joints, especially in systems utilizing HB-FRP for external reinforcement, is essential for comprehending system performance. Numerical modeling provides valuable insights into these interfaces. In a recent study, Zhang et al. (2022) examined the bond behavior of EB and HB-FRP-to -concrete joints assuming bilinear and threefold bond-slip models, respectively. They predicted the bond capacity of HB-FRP system with multiple anchors and proposed a numerical method based on the partial interaction model to predict interfacial stress development. The focus of this study is to experimentally and numerically investigate the bond behavior of EBR and HB Carbon FRP (CFRP)-to-concrete interfaces. For the experimental assessment, single shear tests on EBR and HB-FRP precured laminate specimens are conducted. Based on experimental results, a numerical procedure, based on the finite difference method and a metaheuristic optimization algorithm, is employed to derive the bond-slip law describing the anchoring system's behavior (Aghabagloo et al., 2024). Additionally, the study separately analyzes the contributions of adhesive joint cohesion and friction induced by compressive stresses normal to the composite surface. Experimental characterization of decohesion and friction behaviors leads to obtaining separate cohesive and friction stress-slip laws. A comparison is made between the experimental behavior of the full anchoring system and the combination of separated cohesive and friction contributions. 2. Experimental methodology and results This section presents the experimental methodology for studying the bond performance of EBR and HB-FRP precured laminates-to-concrete joints, along with the experimental results. 2.1. Experimental program A summary of the test matrix is presented in Table 1. All specimens featured external reinforcement with a single CFRP laminate measuring 50 mm × 1.4 mm ( b f × t f ). The experimental program comprised single shear tests on concrete specimens involving two distinct strengthening configurations: EBR and HB-CFRP. For the EBR-CFRP strengthened specimen, a single shear test was conducted until failure. In contrast, two single shear tests were performed on the same HB-CFRP strengthened specimen. In this latter case, the “HB” test continued until the adhesive bond completely failed and subsequent unloading to zero. Following this, the “HB post - failure” test was conducted right after the “HB” test on the same specimen to obtain the load -slip curve, relying solely on the friction bond as the adhesive bond had entirely failed. For all specimens, the bonded length remained consistent at 260 mm, with a uniform 70 mm unbonded distance at the top of the block. In the case of HB-CFRP specimen, a large metal plate (260×148×10 mm) featuring 8 bean-shaped holes, each with a diameter of ∅ 13, was fastened with anchor bolts and a torque of 4 N.m.