Findings from actual fire investigations show that buildings often collapse some time after fire, which leads to severe human and economic losses. Therefore, it is of great importance to investigate how to assess the damage degree of post-fire building, determine the optimum time to repair the fire-damaged concrete structures and identify whether the structures are safe or not. This paper presents the experiment results of the influence of two different cooling methods and various standing time on the residual compressive strength of concrete after elevated temperatures. 100 mmx100 mmx100mm cubes were cured in a standard curing laboratory until 28d, and then elevated to 250 degrees C, 450 degrees C, and 650 degrees C respectively. After cooled by natural cooling method and spraying water cooling method respectively, the residual compressive strength of concrete was measured after different standing time Id, 3d, 7d, 14d, 28d and 56d. The results obtained in this experiment show that the residual strength of concrete cooled by both two cooling methods appears to degrade with the rise of elevated temperature. The minimum residual strength is closely related to cooling methods and standing time: the minimum residual strength of concrete cooled by natural method is at 14d when elevated to 250 degrees C and 450 degrees C, and is at 28d when elevated to 650 degrees C. However the minimum residual strength of concrete cooled by the spraying water cooling method is at 7d when elevated to 250 degrees C and is at 14d when elevated to 450 degrees C and 650 degrees C. After going through the same temperature, the changing pattern of residual strength is of similarity under both cooling methods. However, within 1-3d standing time, the residual strength of concrete increased sharply and then dropped to the lowest when cooled by spraying water cooling method, in which the mechanism needs further study. Furthermore the suggestion of the optimum repairing time for reinforced concrete structure after elevated temperature is presented.
