RESEARCH ON THE DAMAGE CONDITION OF REINFORCED CONCRETE FILLED STEEL TUBES UNDER AXIAL LOAD USING ULTRASONIC TESTING

The ultrasonic technique was used to evaluate the damage conditions of plain concrete, reinforced concrete, concrete filled steel tubes (CFST) and reinforced concrete filled steel tubes (RCFST) subjected to axial load. The ultrasonic waveforms passing through the middle section of a column were obtained under different loads. The root mean square (RMS) amplitude and RMS amplitude ratio of waveforms are defined for analyzing the development of concrete cracks. The waveforms were transformed into the frequency domain by fast Fourier transform (FFT). According to the principle that the solid medium cracking absorbs high frequency sound wave energy that makes the ultrasonic main frequency shift from high to low frequency, the first main frequency and its amplitude variation were selected to investigate the damage condition of concrete and reinforced concrete in steel tubes. The experimental results show that the crack variation of concrete and reinforced concrete in steel tubes can be effectively reflected by the RMS amplitude, RMS amplitude ratio and the first main frequency and its amplitude. The damage condition of reinforced concrete under axial load can be divided into five stages, that is, the concrete crack generation and extension, concrete compaction resulted from steel tubular constraint, concrete crack rapid expansion, strengthening due to the steel tubular hoop effect and loss of component strength after the buckling of steel tubes. It is found that the constraint stage of steel tube to core concrete obtained from the strain analysis of steel tube is consistent with the damage evolution obtained from ultrasonic testing. Therefore, the ultrasonic testing method is an accurate tool to evaluate the damage of reinforced concrete inside steel tubes under axial load and to determine the hoop effect of steel tubes.