Research On Concrete Stress Measurement Based On Coda Wave Interferometry

Stress is one of important indicators of structural condition. At present, the majority of stress measurement techniques applied to concrete structure are based on Hooke’s law, by measuring strain to indirectly measure stress. The results of these stress measurement techniques often have some differences with the real stress in concrete structures. Thus, development of reliable stress measurement methods is important to accurately assess the performance of structure. Coda wave interferometry technology is a new technology that can detect weak wave velocity variations induced by small perturbations. Because concrete is a complex multi-phase medium, Coda wave in concrete is more apparent than in other media, Thus it has a broad prospect that ultrasonic coda wave interferometry technique is applied to concrete. This paper aims to study stress measurement method based on coda wave interferometry techniques. The main work of this paper includes the following:(1) The ultrasonic wave propagation law in concrete and influence factors are analyzed firstly. This paper thinks that concrete base maintain flexibility under stress, wave velocity change is mainly caused by the closure and expansion of micro-cracks. Concrete is regarded as isotropic body which contain randomly distributed micro-racks with the same shape, then establish velocity model under unloaded state.(2) Considered the velocity change induced by micro-cracks, then establish a model for calculating wave velocity change induced by parallel micro-cracks in the concrete. On this basis, wave velocity change caused by micro-cracks closure is equivalent to that caused by a set of original horizontal cracks; wave velocity change caused by micro-cracks expansion is equivalent to that caused by a set of original vertical cracks. Theoretical model of the relationship between coda wave velocity and stress is established.(3) By coda wave testing, we studied the coda wave propagation law in concrete and conducted calibration between stress and wave velocity under the conditions of step loading. The results showed that the higher frequency of the sound source, the more developmental coda wave is and the longer waveform is; when the receiver disposed on the side of blocks, sound source emitted from the top have an apparent coda wave than emitted from the opposite side. Then fit the theoretical model parameters using least squares so that the relationship between coda wave velocity and stress is established. The results show that the theoretical model can well explain the result of wave velocity variation with stress. In the closing stages of the crack, the axial velocity increased significantly, while weak lateral velocity changes; in the crack propagation stage, the axial velocity reduced weak, and the transverse velocity reduced sharply.