Research On The Impact-echo Response Of Prefabricated Concrete Column Near Boundaries

The impact-echo method is widely used in the non-destructive testing of concrete members.The stress wave is generated by tapping the surface of the test structure,and the echo signal received by the sensor is converted into a frequency domain signal through a fast Fourier transform（FFT）.The peak frequency of the spectrogram reflects the thickness and defects of the test members.However,in the actual test,it is found that different degrees of frequency offset will occur when the tapping point is located at the edge of the components,resulting in inaccurate test results.Based on stress wave theory,this paper analyzes the echo responses of concrete members at free and fixed boundaries through numerical simulation and field tests.The main research contents and results are as follows:（1）Summarize the development history and application status of the impact-echo method and stress wave theory,and discuss the obstacles and technical problems encountered in the practical application of the impact-echo method.（2）The ABAQUS finite element software is used to simulate the impact-echo of the small ball hitting the surface of the object to be tested to verify the rationality of the model.At the same time,the excellent frequencies of the plain concrete model and the reinforced concrete model are compared,and it is found that the frequencies are basically the same,so the influence of the reinforcement can be ignored in the simulation and actual test.（3）Under the condition of free boundary,five concrete columns with different section size are established,loads are applied in sequence at the free boundary,and the receiving point is 5 cm above the load,and the time-history curve is extracted for frequency analysis.The simulation results show that the excellent frequency will produce the phenomenon of low frequency offset at the free boundary,the amplitude carried by the low-frequency gradually decreases with the increase of the distance from the free boundary,and the amplitude carried by the high-frequency gradually increases with the distance from the free boundary,and finally tends to the thickness frequency and the phenomenon of low-frequency offset disappears.（4）Under the condition of fixed boundary,five concrete columns with different section size are established,loads are applied in sequence at the fixed boundary,and the receiving point is 5cm to the right of the load,and the time-history curve is extracted for frequency analysis.The simulation results show that the excellent frequency will produce high frequency offset at the fixed boundary,and the amplitude carried by high-frequency first increases and then decreases with the increase of the distance from the fixed boundary,and the amplitude carried by low-frequency increases with the distance from the fixed boundary,and finally tends to the thickness frequency and the phenomenon of high frequency offset disappears.（5）The typical 600×600mm,700×700mm,and 800×800mm concrete columns were tested on-site by using the impact-echo instrument.The test results were basically consistent with the simulation results.Low frequency migration occurred at the free boundary,and high frequency migration occurred at the fixed boundary.For concrete columns with three typical cross-section size,0.3 times the column thickness range should be avoided at the free boundary,and 0.5 times the column thickness range should be avoided at the fixed boundary for detection,so as to avoid errors leading to wrong judgments.Figure [60] table [30] reference [57]...