Concrete Members with Prestressed CFRP Reinforcement: Investigations on Stress Redistribution

Non-metallic reinforcement made of carbon-fibre-reinforced polymer (CFRP) offers the advantage of corrosion resistance, allowing the concrete cover to be reduced to the minimum required for structural purposes. CFRP reinforcements are available as grids, rebars or strands. Seven-wire strands are commonly used for pre-tensioning, while additional non-prestressed reinforcement (e.g., meshes or rebars) is required to ensure economic efficiency and serviceability. Due to differences in the bond characteristics and tensile stiffness of strands and rebars, load transfer in mixed-reinforced components differs accordingly. Understanding the resulting stress-redistribution is therefore essential to achieve economical designs and to avoid premature failure caused by the linear-elastic behaviour of CFRP. In this study, the stress redistribution in the tension zone of bending beams reinforced with mixed CFRP reinforcement, consisting of pre-tensioned strands and reinforcing bars, was investigated. First, the concrete cover c and transfer length of 7.9 mm prestressed strands were determined by testing small high-strength concrete (HSC) beams. These parameters are essential for the design of the expansion body used in the stress redistribution tests. ... mehrThe prestressing force of the strands had to be transferred to the surrounding concrete without inducing cracking. To quantify stress redistribution, an HSC expansion specimen incorporating two reinforcement elements (Ø 8 mm rebar and Ø 7.9 mm strand) was fabricated and loaded in tension at its center Distributed fibre optic sensors (DFOS) were employed to measure the strains in the reinforcement. These strain data were used to derive the forces and stresses in the rebars, revealing the stress redistribution pattern. The results indicate that the reinforcing bars carry a larger proportion of the load. This redistribution, governed primarily by bond behavior, leads to increased load transfer to the bars. However, equilibrium between the bars and the strand is gradually restored towards the end of the test.