| Because the concrete structure is more complex,the load borne by the structure is not clear,concrete shrinkage and creep,temperature effect and other reasons lead to the theoretical analysis of the stress of concrete structure is not practical,so the research on reliable structural stress measurement method has very important engineering value.The change of coda wave velocity can reflect the change of internal structure and stress of concrete.The key is to detect the concrete under compression with high precision.Based on the coda interference technology,this paper detects the internal damage state of concrete test blocks and concrete short columns with four different strength grades of C20,C25,C30 and C40 under different load states.The feasibility of coda interference method is proved through theoretical analysis,comparative research and experimental verification.The specific research contents are as follows:(1)Comparing the acoustic elastic stress testing technology with the coda interference technology,by establishing the variation curve of the direct wave velocity under the small stress state and comparing the direct wave and coda wave waveforms under different load states,it can be found that under the load of 0-200kN,the variation rate of the direct wave velocity of the four strength concrete test blocks is less than 0.05,and the waveform has no obvious time delay,High correlation;Compared with the direct wave,the shape of coda wave not only changes in amplitude,but also has obvious time delay,indicating that the coda wave is more sensitive to the micro change of internal damage of concrete and has higher measurement accuracy.(2)The coda interference test of four concrete specimens with different strength under compression is carried out.The coda waveform is processed by waveform expansion technology,and the compression state of concrete is studied by establishing the coda velocity change rate curve.The established wake wave velocity change rate curve can be divided into three sections:0-40%ultimate load,40%-60%ultimate load and 60%ultimate load to failure.The change trend of the curve has a high correlation with the compression state of concrete.The internal damage state of concrete can be detected through the wake wave velocity change rate curve.At the same time,according to the concrete stress-strain curve,The change rate of wake wave velocity and stress state model of structural concrete under compression.(3)The influencing factors of coda wave propagation in concrete are analyzed from the aspects of concrete strength grade,curing age and coarse aggregate particle size.According to the analysis of coda wave velocity change rate,it is concluded that when ultrasonic coda wave propagates in concrete,with the increase of test block strength,the increase of curing age and the decrease of coarse aggregate particle size,the propagation path is longer and the development is better,The variation rate of coda wave velocity is greater.(4)The coda wave interference test is carried out on concrete short columns with different strength,and the coda wave velocity change rate curve is established.According to the test results,the conclusion of concrete test block test and the stress state model are verified.To sum up,as a new ultrasonic testing method,the sensitivity and accuracy of coda interference technology for micro cracks in concrete have been preliminarily verified.With the deepening of research,coda interference technology can be gradually applied to practical engineering and become one of the nondestructive testing methods of concrete structures. |
