Post-Fire Bond Stress Behavior in Reinforced Concrete Members

Abstract

This research investigates the post-fire bond stress behavior in unconfined reinforced concrete members. According to design codes such as Eurocode-2 (EN 1992-1-2), fib Model (2010), and ACI 318, guidelines have been developed for predicting bond, anchorage, and temperature effects on material properties of concrete and steel bars. In Eurocode-2, there are reduction factors considered to estimate the effects of fire on structural behavior whereas, fib Model (2010) and ACI 318 relate the bond strength to development length under ambient temperature conditions based on variables such as compressive concrete strength (fc'), bar diameter (db), and confinement. Nevertheless, none of these codes consider the residual bond-slip behavior of RC members. To examine the residual bond-slip performance of RC members, an experimental study was carried out with respect to variables of temperature (T), concrete strength (fc'), cover to bar diameter ratio (c/db), and fuse of intumescent paint. Compressive strengths of concrete (fc') = 30 and 50 Mpa and cover-to-bar diameter ratios (c/db) = 2 and 3 were used to simulate various resistance levels against splitting by the concrete cover. The specimens were heated in an electric furnace up to 500 and 700 °C, maintained at each temperature level for 30 minutes, and then allowed to cool down to ambient temperature (20 °C). After Cooling process, displacement-controlled pull-out tests were conducted to obtain peak bond stress, slip at the peak load, post-peak behavior, residual bond stress, and mode of failure. The results show that there is a marked reduction in peak and residual bond strength along with an increase in slip and a softer response after the peak when exposed to high temperatures. Bonding performance improves when using higher cover ratios due to more effective splitting resistance of the concrete cover. The use of intumescent coating mitigates the effect of temperature on bonding performance. The results obtained from this research provide residual bond-slip relationships for RC members, giving valuable insights for the design of RC members after fire.