Experimental study on the effects of external strengthening and elevated temperature on the shear behavior of ultra-high-performance fiber-reinforced concrete deep beams

Abstract

The shear behavior of deep beams, especially of ultra-high-performance fiber-reinforced concrete (UHPFC), is rather intricate, as it depends on several factors, including its depth, concrete strength, amount of flexural and shear reinforcements, the ratio of shear span to depth, and the bearing area under the load and at the support. Moreover, these beams may be exposed to elevated temperatures requiring strengthening, which has not been adequately studied. This study examines experimentally the shear capacity of UHPFC deep beams strengthened using carbon fiber-reinforced polymer (CFRP) sheets for improving the shear strength of deep beams after exposure to an elevated temperature. Eight specimens were cast in four groups of two specimens each. Testing of the beams of the first group was performed at the ambient temperature, whereas the beams of the second group were tested after being exposed to a temperature of 450 °C. For the third group, the deep beams were strengthened by CFRP U-wrap strips and tested at the ambient temperature, whereas the beams of the fourth group were subjected to the elevated temperature of 450 °C, followed by strengthening using CFRP U-wraps and then testing to collapse. The test results indicated that the post-peak behavior was affected significantly by the elevated temperature. Moreover, the experimental results indicated the excellent performance of strengthening schemes in improving the post-cracking behavior of UHPFC deep beams, enhancing shear strength (up to 16 %), energy-based ductility index (39 % to 648 %), and deflection ductility index (68 % to 144 %). An analytical model is developed to predict the load-carrying capacity of UHPFC deep beams strengthened after exposure to elevated temperature.