Residual bond-slip behavior in reinforced concrete members exposed to elevated temperatures

Many reinforced concrete (RC) structures survive fire events, after which investigation of their residual performance becomes essential. The elevated temperatures experienced by structural members during a fire negatively impact the mechanical properties of concrete and steel, as well as their mutual bond. This paper investigates the bond behavior in RC members exposed to elevated temper-atures to help designers check the safety of RC structures after a fire. Thirteen modified beam-end specimens were designed and fabricated with two diameters of longitudinal bar (14 and 22 mm), two concrete strengths (30 and 50 MPa) and two confinement levels (0.8% and 1.6%). After being exposed to a variety of heat-ing regimes and naturally cooling down to room temperature, the specimens were tested by applying a monotonic pull-out force to their eccentric longitudinal bar. The results showed that after short-term exposure to 400, 600, and 700 degrees C, the residual bond strength may decrease by 23%, 58%, and 65%, respectively. The equations for the pull-out failure proposed in the fib Model Code 2010 were found to yield realistic results in terms of upper and lower bounds for the bond behavior under monotonic loading, and the dependency of the residual bond strength on the square root of the residual compressive strength of the concrete was con-firmed. Results were used to develop recommendations for the evaluation of the residual bond strength and of the development length in RC members exposed to elevated temperatures.