Study On The Regularity Of Hidden Crack’s Influence On Wave Propagation Characteristics In Concrete

Due to the factors such as long-term loading, environmental corrosion, material aging etc, the progressive damage will appear inside the concrete during its service period, which will decrease load capacity and durability of the structure, and finally,makes the life of the structure reduced. How to predict damage evolution of the concrete structure in service effectively is one of the key issues to be solved in current concrete study. In view of this, to provide theoretical basis and application suggestions for detecting concrete material, this article based on the linear and nonlinear acoustic wave theory studied the relationship of the concrete crack parameters and wave propagation characteristics. The main research contents and conclusions are as follows:1)conduct ultrasonic test on the cured concrete specimens with different morphological parameters, use signal processing technology such as fast fourier transform and wavelet analysis to extract acoustic signal parameters, then, the influence of cracks on these parameters under the state of zero stress is studied. The result shows that, under zero stress, wave velocity of concrete with cracks is lower than the concrete without crack, but the variation is not apparent when the crack is small. Besides the crack width has the most remarkable influence on wave velocity,then is the crack depth, but the fracture strike and the angular separation have no impact on ultrasonic wave velocity.2)under the state of zero press, use fast fourier transform and spectral analysis to get acoustic parameters of concrete. Compared to the specimen without crack, the shape parameters of crack have significant influence on the maximum amplitude,when the fracture strike is coincident with acoustic wave propagation direction, the amplitude is the smallest, while when they are vertical the amplitude is the largest.Except the basic frequency, the change rules between each parameter and the shape parameters of crack are similar. Both the maximum amplitude and spectrum area are decrease with the increase of crack size.4)the weighted spectrum area considered weightiness filters high-frequency interference to some extent, and is very sensitive to stress. During loading procedure,the weighted spectrum area of specimens with different shape parameters have the same change rule, that is, with the increase of stress, the larger the crack size the more apparent the weighted spectrum area changed. The weighted frequency amplitude andweighted spectrum area have almost the same tendency with the change of stress.when the stress exceed the 70% of ultimate bearing capacity, they both decrease rapidly, which might due to the crack runs through the whole concrete, the specimen is close to damage.5)the basic frequency has nothing to with the concrete internal structure and the wavelength of ultrasonic wave. The crack have a little impact on basic frequency under the state of zero stress, and the basic frequency is almost constant even the stress grows under the load.6)based on the analysis of the relationship between defect scale and ultrasonic wavelength under zero stress, the characteristics of ultrasonic wave velocity caused by different defect distribution under this state is established. By analyzing the extension mechanism of microcrack and the concrete damage constitutive relation under single microcrack, the constitutive relation of concrete damage and ultrasonic parameters is obtained. Compare the theoretical result with test result, it turns out that the concrete damage constitutive relation express by ultrasonic wave velocity can reflect the influence of stress on wave velocity well.