Flexural stiffness of RC beams with high-strength steel bars after exposure to elevated temperatures

In this paper, the flexural stiffness of the reinforced concrete (RC) beams reinforced with high-strength steel bars after exposure to different elevated temperatures was investigated experimentally and theoretically. The effects of the elevated temperature (room temperature, 200 degrees C, 400 degrees C, 600 degrees C, 800 degrees C, and 1000 degrees C), high-temperature duration time (0 h, 1 h, and 2 h), and steel bar types (HRB600 and HRB500) on the load-deflection response, load-stiffness response, and crack distribution of tested beams were investigated. The results showed that when the temperature is below 600 degrees C, the effect of high temperature on the stiffness reduction of high-strength RC beams is not significant, while the beams reduced the stiffness when the exposure temperature reached 800 degrees C and 1000 degrees C, regardless of the type of the steel bars. The elevated temperature accelerated the development of the deflection of RC beams, and the bending stiffness of the RC beams decreased more severely as the increase of duration time. By using appropriate degradation factors, the residual stiffness of the beams was predicted well by the analysis calculation method and the effective inertia moment calculation method, for example, the prediction accuracy is over 91% and 85.8% for the cases under and over 800 degrees C, respectively. However, for the beams at 1000 degrees C, a correction coefficient (0.652) was applied to improve the accuracy of the calculative stiffness of the HRB600 RC beams at the service load stage.