Evaluation of concrete degradation depending on heating conditions by ultrasonic pulse velocity

As the mechanical properties of concrete subjected to high-temperature heating decrease due to continuous degradation, it is necessary to evaluate the integrity of concrete in real time. Hence, the degradation monitoring (continuous integrity evaluation) of concrete subjected to high-temperature heating was examined by employing ultrasonic pulse velocity. In this study, the mechanical properties and the ultrasonic pulse velocity in concrete subjected to high-temperature heating were evaluated for ordinary-strength to ultra-high-strength concrete. To measure the ultrasonic pulse velocity in concrete during high-temperature heating, transducers were contacted to the top and bottom surfaces of the concrete specimens using SUS bars. This enabled monitoring of concrete degradation due to heating. In a range of 200-300 °C, it was confirmed that the thermal strain coincides with the static fracture strain at room temperature. The cracks generated during heating and the expansion of the cracks after cooling were clearly confirmed by continuous measurement of the ultrasonic pulse velocity. The cracks generated at temperatures up to 300 °C had little effect on the decrease in compressive strength. However, it was confirmed that the elastic modulus continuously decreased, because the cracks generated during heating and expanded after cooling led to strain increase at the peak stresses. Therefore, the evaluation method proposed in this study is anticipated to allow the integrity evaluation of concrete at high temperature heating.