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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The method described in the previous sections on research, development and application is only one of many possibilities for shear reinforcement with fiber-reinforced composites. Wrapping, U-jacketing, L-jacketing, side bonding each with discrete strips or continuous sheets are alternatives, Teng et al. (2004). Reliable anchoring is crucial for success. The method described above not only guarantees this, but the shear reinforcement is also active due to the post-tensioning. 3. Laminated carbon fiber reinforced polymer loop tendons as external strengthening method 3.1. Introductory remarks External iron tie rods are retrofitting vaulted structures since Roman times, Amici (2015). Nowadays external post tensioning is generally considered to be one of the most cost effective, durable and low impact strengthening solutions for civil structures. Corrosion resistance, outstanding fatigue properties and lightweight make CFRP the most optimal material for such applications. The low weight helps to save labor cost during installation. The non-laminated CFRP loops originally developed by Winistörfer are not the optimal solutions for many of these practical applications because a certain minimum bending stiffness is mostly required for the assembly of the tendons. For this reason, the Empa spin-off Carbo-Link started producing laminated loops more than twenty years ago. Carbo-Link has been supplying laminated CFRP crane pendants since 2003, with more than 20,000 such tendons in active service. Such elements are also perfectly suited for external post-tensioning and as replacement of corroded hangers and stay cables in bridge construction. 3.2. Production process The production process at the Carbo-Link Company is highly automated. A tape-laying machine is winding an "endless" 50 mm wide unidirectional carbon fiber/epoxy prepreg around two bolts. As soon as the required cross section is reached, the bolts are going to be replaced by milled titanium thimbles. The whole assembly is rotated around the longitudinal axis similar to a classical filament-winding machine. High strength polyester yarns tie the two layers of plies, each of rectangular cross-section, in lateral direction together into one circular cross-section. After the application of additional reinforcement in the region of the thimbles the hanger is cured in an oven at 140°C. 3.3. Pioneering applications An early application of CFRP tendons for external post-tensioning is the case of the "Verdasio-Bridge", Meier et al. (2016). This is a two-lane highway bridge and was built in the seventies. The length of the continuous two-span girder is 69 m. A large internal post-tensioning steel cable positioned in a concrete web was fully corroded. It had to be replaced in December 1998. Four external CFRP tendons arranged in a polygonal layout at the inner face of the affected web inside of the box replaced the corroded steel cross section. Each parallel wire bundle was made up of 19 pultruded CFRP wires with a diameter of 5 mm. This project is as far most interesting as in this case the sustained post-tensioning stress is in average as high as 1800 MPa. The results of the long-term monitoring are until today excellent. There is not any stress relaxation. However, there is one disadvantage with this solution. The anchorage of the parallel wire bundles is in comparison to laminated carbon fiber reinforced polymer loop tendons relatively expensive. This is why the latter are preferred today. Recently Carbo-Link supplied such CFRP hangers (Figure 6) for two major railway arch network bridges in Germany. The Stuttgart light rail bridge spans the busy A8 highway, located 9 km south of Stuttgart main station, linking downtown with the airport. The main span of the bridge is connected by an 80 m network arch and two protruding truss frames. Initially, Keil and Haspel (2021) of the sbp engineering company had planned to use fully sealed steel cables with threaded fittings as hangers for supporting the main span of the bridge. Empa's investigations into the fatigue strength of carbon tendons revealed significant potential for the material. It was discovered that the cross-section of the hangers could be reduced by 75%. This finding led to less wind-induced vibrations and increased possibilities for optimizing the bridge's network structure. Therefore, sbp recommended the use of carbon laminated loops as hangers. A test program was designed in collaboration with Prof. Urs Meier from Empa, as the expert, and