Detecting the interfacial bonding of concrete-filled steel tube columns after fire by two stress wave-based methods – A case study

For concrete-filled steel tubes (CFST), the steel tube can increase the stiffness of the member, while the filled concrete prevents column buckling and allows the steel tube to withstand high temperatures for longer periods of time. The bonding between the steel tube and the concrete is difficult to evaluate from the appearance. In this paper, the bonding conditions of the CFST columns of a high-rise building under construction after a fire was evaluated by two stress wave-based non-destructive test (NDT) methods in-situ. They are the Lamb wave-based method and the normalized impact-echo method. Among them, the Lamb wave dispersion curve, which is obtained by a single tapping of a steel ball and a receiver 0.4 m away, can quickly evaluate the interface bonding conditions under the measuring line. The normalized impact-echo can pin-point the precise location of interfacial debonding. The test results show that both NDT methods can accurately classify good bonding conditions in the region without fire damage. Combining the results of the two methods, we found that high temperature was likely to cause some damage, such as microcracks on the concrete surface or intermittent separation of steel from concrete, but there is no continuous gap at the steel–column interface.