轴压荷载下钢管钢筋混凝土损伤状态超声检测研究

在轴压荷载下分别对素混凝土、钢筋混凝土、钢管混凝土和钢管钢筋混凝土的损伤状态进行超声检测，得到不同荷载下超声通过柱的中截面的波形图，定义波形图均方根振幅和均方根振幅比，并分析混凝土裂隙的变化过程。通过对波形图进行快速傅里叶变换得到频谱图，基于固体介质裂化吸收高频声波能量，使超声主频向低频移动的原理，应用频谱图的第一主频及其幅值变化分析钢管内混凝土和钢筋混凝土的损伤演变过程。结果表明:声波波形均方根振幅、均方根振幅比和第一主频及其幅值可以有效反映钢管内混凝土和钢筋混凝土的裂隙变化情况。轴压荷载作用下钢管内钢筋混凝土的损伤呈五段式变化，分别为核心混凝土裂缝产生并发展、混凝土在钢管约束作用下逐步密实、混凝土裂缝迅速扩展、钢管套箍作用加强段和钢管屈曲后构件丧失承载力。通过钢管外壁的应变分析，表明钢管对核心混凝土的约束作用阶段与超声检测判定的混凝土损伤演化过程一致。超声波技术可以准确检测轴压荷载作用下钢管内钢筋混凝土的损伤状态并确定钢管对混凝土套箍作用的发挥阶段。

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.