PSI - Issue 64

Urs Meier et al. / Procedia Structural Integrity 64 (2024) 29–39 Meier/Winistörfer / Structural Integrity Procedia 00 (2019) 000 – 000

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cost-effective solution for extending the service life of aging infrastructure. The successful application of CFRP at the Ibach Bridge demonstrated the feasibility and effectiveness of using advanced composite materials for structural strengthening purposes. It paved the way for the wider adoption of CFRP strengthening techniques in civil engineering and bridge construction projects around the world. Nearly four decades have elapsed since Meier's proof in 1987 that reinforced concrete structures could undergo effective post-strengthening with externally bonded CFRP strips. Today, we witness the construction of large network arch bridges for railways that rely entirely on CFRP hangers for the suspension of the bridge deck, Meier et al. (2021). Within the last three decades, the use of CFRP in construction is continuously growing. According to Zhang et al. (2023), the construction industry currently accounts for 3.6% of global carbon fiber demand, or approximately 6500 tons per year. 2. Non-laminated carbon fiber reinforced polymer loop tendons as external post-tensioning In the 1990s, as flexural post-strengthening of structures with externally bonded CFRP laminates gained increasingly attraction in practice, research and development efforts focused on enhancing shear post-strengthening. The work of Conen (1966) sparked a transformative concept: the use of multiple layers of thin, non-laminated carbon fiber-reinforced polymer tapes for external post-tensioning, presenting an innovative alternative to traditional laminated reinforcement methods. Winistörfer (1999) carried out a comprehensive study of tendons where the load was applied to non-laminated CFRP loops using two bolts as anchorage. When these tendons are formed, layers of thin tape are wrapped around the bolts. The outermost tape layer is fusion bonded to the next outermost layer, which forms a closed ring that can carry hoop tension. However, the inner tape layers remain non-laminated. The loop shaped tendons were tensioned until failure. The study verified the principle that, in the region of the bolts, the frictional forces generated between the tape layers are sufficient to induce force in the tape layers and to prevent the loops from unravelling. However, the relative sliding of the inner tape layers allows an equalization of force in the layers. Strain gauge readings confirmed that each non-laminated layer carried a similar amount of force. An equivalent laminated system failed at a much lower load due to through thickness stress concentrations. Lees et al. (2002) explored the viability of Conen and Winistörfer's concept as external post-tensioned stirrups at Empa through a series of large-scale experiments on T-beams, yielding highly satisfactory results. Stenger (2001) investigated the applicability of the concept as external post-tensioned CFRP stirrups through a cooperative project between ETH and Empa. The study involved a series of large-scale experiments conducted on the Beam Element Tester developed by Kaufmann and Marti (1996). The objective of the test series of Stenger was to examine the impact of the non-laminated carbon fiber reinforced polymer tendons as external post-tensioned shear reinforcement (stirrups) on the load-bearing capacity and deformation behavior of steel reinforced concrete beams with minimal internal steel shear reinforcement. 2.1. Basic experiments Stengers experiments focused on two key factors: the level of post-tensioning and the presence of pre-existing damage in the beams. Each test parameter was meticulously altered one at a time, beginning with the baseline test of the not post-strengthened beam #2. This method allowed for immediate observation of the effects of each parameter change. All beams tested shared identical geometries and inserted steel reinforcement. The research aimed to provide insights into the performance and behavior of the concrete shear walls under varying external reinforcement conditions, shedding light on the effectiveness of external post-tensioning with non-laminated carbon fiber reinforced polymer tendons as a method for enhancing structural integrity. The carbon fiber-reinforced loop elements utilized to form the stirrups comprised Toray T 700 unidirectional carbon fibers embedded in a polyamide 12 matrix, with an average fiber volume fraction of 58 percent. The stirrups were fashioned from 12 mm wide continuous tapes, wound around the elliptical steel pad elements (blue in Fig. 1) at four cross sections spaced 500 mm apart. Each stirrup consisted of 25 layers of tape, with the two outermost layers welded over a length of 90 mm after encircling the concrete cross-section. Characteristics of the thermoplastic CFRP tape are given in Table 1.