Structural Strengthening of Concrete with Fiber Reinforced Cementitious Matrix (FRCM) at Ambient and Elevated Temperature — Recent Investigations in Switzerland

Abstract

This paper presents recent experimental investigations on structural strengthening by means of (Carbon) Fiber Reinforced Cementitious Matrix (FRCM) in Switzerland. A first test series deals with full-scale reinforced concrete slabs strengthened with one or two composite reinforcement meshes embedded in a shotcrete layer. Static load tests up to failure show the efficiency of the strengthening in terms of increased yield and ultimate load compared to the reference specimen. Due to the initially necessary straightening of the textile, the contribution at lower deflection levels is limited. Only with advanced cracking and crack opening, the mesh develops its full contribution. Ultimate load is reached after a prompt relative slip of the mesh in the shotcrete. In the post-peak domain, failure by concrete crushing was observed. To study the residual tensile strength of the carbon reinforcement after exposure to high temperatures, various tensile tests on small rovings previously cut out of a composite mesh were performed. The specimens were heated to temperatures of 300 °C, 500 °C, 700 °C, and 1000 °C, kept at that level for 30 minutes, and finally cooled down to room temperature. The subsequent tensile tests performed at room temperature revealed a significant drop in the residual tensile strength for exposure temperature higher than 300 °C. A final test was performed on a reinforced concrete slab strip strengthened with a shotcrete layer including a composite mesh as tensile reinforcement. Under a constant service load, the slab was exposed to fire with a temperature rise according to a European standard curve (ETK) for two hours. The slab could withstand the applied loads for the full two hours, during which the composite mesh reached a temperature of about 440 °C. This observation is consistent with the results from tensile tests on filaments, clearly indicating a residual tensile strength after exposure at a similar temperature. The temperature in the internal steel reinforcement did not trespass a critical value of 500 °C as proposed by current design recommendations.