A simplified procedure for evaluation of damage-depth in concrete exposed to high temperature using the impact-echo method

Concrete is widely recognized as a material capable of withstanding the intrusion of high temperatures during fires. However, under different high-temperature conditions, concrete can still experience strength reduction, cracking, or spalling, which can significantly impact the safety and durability of concrete structures. Conventionally, the wave refraction technique was used to detect the depth of this damage layer. However, the wave refraction technique is a time-consuming point-by-point detection method. In order to increase detection efficiency, this paper proposes a simplified method based on a single-point test. Numerical analysis of the thermal conduction of a concrete slab exposed to elevated temperature was performed first to investigate the temperature distribution within the concrete slab. Subsequently, the wave refraction technique was numerically simulated to evaluate the damage depth of the concrete slab. According to the refracted wave propagation path, a simplified procedure is proposed for the detection of the damage depth of concrete under high temperature. In the simplified procedure, a receiver is placed at an adequate distance from the impact source so that the first arrival wave at the receiver will be a wave refracted from the interface between the damaged layer and the sound layer inside the concrete. To verify the applicability of the proposed simplified procedure, concrete slab specimens subjected to an elevated temperature of 600 °C were tested in this study. The experimental results indicate that the simplified method proposed in this paper can indeed be used to detect the depth of high-temperature damage in concrete. In addition, the experimental results show that under the same high-temperature exposure conditions, the depth of fire damage increases with a decrease in the water-cement ratio. This can be attributed to the higher thermal conductivity coefficient of concrete with a lower water-cement ratio.