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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Table 1. Characteristics of the 12 mm wide CFRP tape. E-modulus 130 GPa Strain at failure 1.2% Breaking load for a loop with 25 layers 125 kN Thickness of one tape layer 0.16 mm Cross section of one tape layer 1.92 mm 2 Stirrup cross-section for 2 x 25 layers 96 mm 2

Fig. 1. Post-tensioning process.

On the upper side, the steel pad elements were elevated with the assistance of a hydraulic actuator (Fig.1d). Before releasing the post-tensioning force, multiple steel shimming plates were inserted (Fig. 1e) between the top of the beam and the steel pad, enabling the transfer of the post-tensioning force as per Table 2, once the actuator was removed, into the concrete beam. The beams were composed of two load application blocks and a disk-shaped central section, which was the focus of the research. The dimensions of the disk were as follows: length of 2240 mm, height of 1200 mm, and thickness of 150 mm. The blocks and the disk formed a continuum. The aim of the tests was to load the disk area in such a way that a shear failure would occur there. Either the compression field forming in the concrete or the shear reinforcement - consisting of steel stirrups and CFRP loops in four of the five beams - was to fail without the longitudinal reinforcement reaching its yield point. The static system of the test specimens corresponded to rigid clamping on both sides. The type of loading was identical for all tests and was applied by vertically displacing one of the two restraints, whereby a constant shear field was generated, neglecting the dead weight of the beam. The moment load was therefore antimetric with respect to the center of the beam. No normal forces were applied, and any that occurred during the test were eliminated by controlling the horizontal servo-hydraulic actuators. In the first experiment, beam #1 (Fig. 2a, 3a and 3b), post-strengthened with external post-tensioned CFRP loops was loaded to failure by continuously increasing the shear force in one load direction. The second test, in which the not post-strengthened beam # 2 was loaded in the same way until failure, served as reference. In contrast to the first experiment, the beam #3 was fitted with slack external CFRP loops so that the influence of the post-tensioning could be investigated. In the fourth test, the initially not post-strengthened beam #4 was loaded until large shear cracks developed (75 % of the breaking load of reference beam #2) before the load was released again. After applying post tensioned CFRP loops, the beam was loaded until failure, with the loading direction being the same as the initial direction. In the final fifth experiment, the beam #5 post-strengthened with post-tensioned loops was also loaded until major shear cracks occurred. This load level corresponded to around 67% of the breaking load of beam #2. The load direction was then reversed and the load was increased until the beam failed. The test program is summarized in the Table 2 and the results are presented in the Figures 2. In all experiments, shear failure occurred before the longitudinal internal steel reinforcement reached its yield point. The failure of all beams in which external post-tensioned CFRP stirrups were used was triggered by the tensile strength of one of these external stirrup elements being reached. As expected, the actual failure process was brittle for all beams, including not post-strengthened beam #2. However, with the exception of test beam #4, the failure of the loop elements occurred in all tests with pronounced advance notice in the form of clearly visible and audible tearing of smaller carbon fiber strands. In all of these cases, the load-bearing capacity of the concrete was almost reached, which could be clearly seen from the crack patterns. In beam #5, only the pronounced expansion of the concrete compression zone led to the breakage of the CFRP stirrups located there. In each case, the fracture of a CFRP stirrup resulted in the immediate failure of at least one other neighboring CFRP stirrup. The redistribution of the shear load to the steel stirrups also led to their instantaneous failure with an accompanying widening of one or two existing diagonal cracks. The strong influence of the post-tensioning on the load-bearing behavior was clearly demonstrated by tests beams #1 and #3. As expected, the non-post-tensioned CFRP stirrups in beam #3 only took over significant parts of the load bearing mechanism of the disk element at the time when the internal steel stirrups failed. There was no interaction