Effect of thermal shock due to rapid cooling on residual mechanical properties of fiber concrete exposed to high temperatures

An experimental investigation was conducted on the effect of thermal shock during cooling on residual mechanical properties of fiber concrete exposed to elevated temperatures from 200 to 800 °C. Various cooling regimes were used including natural cooling, spraying water for a series of durations from 5 to 60 min, and quenching in water. The temperature determination results prove that the rapid cooling regimes such as quenching in water, or water spraying for 30 min or more, caused an action of “thermal shock” to concrete under elevated temperature, characterized by a high temperature decreasing rate ranged from 25 to 44 min/°C. The experimental results indicate that, compared with natural cooling, thermal shock induced by water quenching and spraying water caused more severe damage to concrete, in terms of greater losses in compressive strength, tensile splitting strength, and fracture energy. The fact that the impact of spraying water for 30 min or more on mechanical properties was almost the same as that of water quenching, indicates that spraying water for 30 min or more could cause thermal shock to a similar degree to water quenching, which is in good agreement with the results of the temperature determination. Hybrid fiber (steel fiber and polypropylene fiber) can enhance residual strength and fracture energy of concrete subjected to thermal shock induced by rapid cooling from high temperatures up to 800 °C to room temperature.