The effects of fiber hybridization on the direct shear behavior of UHPC subjected to elevated temperature

An experimental investigation was conducted at Swinburne University to study the effects of fiber hybridization on the direct shear behavior and mechanical properties of ultra-high performance fiber reinforced concrete (UHP-FRC) members exposed to elevated temperatures. The effect of two key variables on the direct shear performance and mechanical properties of UHP-FRC were investigated. The key variables include: the type of fiber hybridization (hybrid of either hooked end steel fiber (HESF) and straight steel fiber (SSF) only or with jute fiber (JF) as well), and temperature applied on the specimens (room temperature, 200 degrees C, 400 degrees C and 600 degrees C). The direct shear stress-slip performance, ductility and toughness were determined. Furthermore, the thermal-mechanical properties of UHP-FRC such as residual direct tensile strength, residual compressive strength, spalling resistance and mass loss were investigated. Scanning electron microscope micrographs were used to evaluate the microstructure of UHP-FRC matrix reinforced with different types of fiber and the morphological alteration in the surface of JF under high temperatures. The tests results indicate that the residual direct shear stress decreased significantly after exposure to high temperatures. Moreover, thermal spalling occurred for specimens subjected to 600 degrees C and reinforced with hybrid of HESF and SSF only. While, using hybrid of HESF, SSF and JF was an effective method to prevent the thermal spalling under 600 degrees C and enhance the residual direct shear stress of UHP-FRC. Preliminary empirical expressions have been developed in this study to predict the shear transfer capacity of UHP-FRC as a function of compressive strength, steel fiber content, natural fiber content and temperature.