BOND BEHAVIOUR OF STEEL IN CONCRETE UNDER FLEXURE WHEN EXPOSED TO FIRE

Abstract

Reinforced concrete structural elements are generally subjected to elevated temperatures during an event of fire break out. The elevated temperature received at the surface of the structural element raises the temperature of the concrete and steel across the section. The most affected part of the section at any time during an event of a fire is the concrete and the reinforcing steel adjacent to the exposed surface. The elevated temperature reduces the material properties of both concrete and steel and degrades the stress transfer capacity between steel and concrete or bond behaviour between steel and concrete. The present study focusses on investigating the effect of elevated temperature on the bond behaviour between reinforcing steel and concrete. A three phase experimental investigation was conducted to address the problem defined in the thesis. The first phase was dedicated to assess the bond behaviour between steel and concrete in splitting and pull out mode of failure after exposure to elevated temperature. The assessment in the splitting mode of failure includes testing of 80 bottom cast scaled splice beam specimens considering the parameters: thickness of the concrete cover (20mm - 40 mm), diameter of the bar (12mm - 20 mm) and level of confinement. All the specimens were cast, cured and then heated in an electric furnace to achieve a steady-state temperatures of 350 ºC, 550 ºC and 750 ºC. The splice beam specimens were then tested under four-point bending in displacement control mode to obtain bond stress in splice and the relative slip between the reinforcing steel and the surrounding concrete. Further in order to investigate the effect of elevated temperature on the pull out failure of the reinforcing bars (12 mm, 16mm and 20mm) in concrete under flexure a total of 24 Rilem beam specimens were tested. Similar to splice beam specimens the Rilem beam specimens were also heated to the different temperatures stated above and were then tested under four-point bending to obtain bond stress vs slip behaviour. The cracking and failure mode during all the testing were also observed.