| With our country increase the infrastructure investment after the reform and opening up. all kinds of large geotechnical project are constantly emerging. We all know the main material of modern engineering construction is reinforced concrete, and under the influence of various internal and external effect, concrete is easy to crack, which affects the quality of the concrete, a lot of damage to buildings. Therefore, it must be tested for these cracks, grasps the crack condition and related parameters, it can help us to determine the hazard degree and study the corresponding reinforcement measures to save people’s lives and property safety.Concrete crack detection has been a technical problem in engineering construction. Caused by a lack of detection theory and method, at present, the deep vertical fracture has not been an effective detection method. In order to reasonably evaluate the influence of the crack of concrete structure safety and formulate follow-up reinforcement scheme, it must be accurate to determine the status, development and the cause of the cracks, the crack depth, length and width is a very important indicator. Especially crack depth is a key indicator. In the actual detection work, crack depth detection is much more difficult than the length and width detection.Crack width detection divides into three categories:feeler or crack width contrast card (used for coarse, low accuracy), crack microscope (precision in0.02～0.05mm). crack width tester. At present the main method is crack microscope, but the crack width tester was invented, it avoid the artificial close range to adjust focal length requirements, reduce the labor intensity.Crack depth detection is the most direct and accurate method of drilling, but this method is not only time-consuming, laborious and it can cause damage to the building structure, when the crack depth is larger or in special area, it’s difficult to drill sampling. Followed the method is by the sound waves to measure, in the side of the parallel cracks to stimulate the acoustic signal, in the other side to reception of the transmission signal, by comparing the no cracks and crack of transmission signal or through transmission tomography to determine crack depth, however, this method needs a lot of time to crack on both sides of the hole to decorate observation system, it’s also a damage detection.Nondestructive testing technology is more be used at present, Mainly it can fall into ultrasonic method and the impact of elastic wave method, Both of the two methods should observation system in the concrete surface, according to the crack at the bottom of the reflected signal characteristics and propagation path of geometric relationship to calculate the depth of the crack. Ultrasonic method is through the ultrasonic probe and the receive signal, the ultrasonic signal has high frequency but low energy, it’s generally applicable to the shallow crack. Shock elastic wave method is using hammer source, signal low frequency but high energy, which suitable for deep fracture detection. But the problem of the two methods is when cracks exist in the filler, most of the energy transferred by filler, acoustic signal cracks at the bottom of the reflection signals are often difficult to judge, especially for deep cracks.In recent years, the development of a new method of cracks in nondestructive testing-Rayleigh wave method, and also in the concrete surface observation system, using hammer source, by receiving the Rayleigh potter to calculate the depth of the crack. Ultrasonic method and the impact of elastic wave method is to use the body wave in elastic wave detection, but most of the energy of elastic wave that is excited by surface is Rayleigh wave, and in layered medium, the Rayleigh wave has dispersion characteristics and the wavelength of Rayleigh wave is different, the penetration depth is also different. Rayleigh wave method has high resolution, wide application range, little effect by site, simple testing equipment and fast testing speed, economy and a series of advantages, it’s widely used in geotechnical engineering investigation play and disaster geology survey, etc.This paper briefly reviews the traditional geophysical exploration methods used in the application of crack detection and Rayleigh surface wave nondestructive testing was carried out on the fracture. Which leads to the research topic-using the Rayleigh surface wave detect the vertical cracks of concrete. In Rayleigh surface wave principle introduce how to use dimensionless real transfer matrix algorithm of FAN you-hua forward for the dispersion curve to calculate, through the staggered-mesh finite difference method and the free boundary condition of Rune Mittet treat and exponential attenuation absorbing boundary conditions are numerically simulated. Using tau-p transform algorithm for the energy spectrum of dispersion curve to calculate. Then compare the simulation of dispersion curve value to the actual theoretical value. And we can obtain the feasibility of numerical simulation method. Next, proceed the numerical simulation of the fracture medium, establish a simulation study compared with no cracks, and to establish different depth of crack is studied. And we can get four conclusion. Finally we establish the vertical fracture numerical simulation compared with the measured data of the three gorges dam. Found that both of the body waves and surface waves will produce reflection and transmission through crack, the energy of reflection wave is stronger than the transmitted wave, and the energy will spread along the crack, the scattering at crack tip. And it can find in the fracture medium, surface wave don’t have higher order mode. It is important to note if cracks are air, surface wave and longitudinal wave will have reflection and scattering, but it don’t have transmission. Which means in the actual detection, one side of the crack will only receive the scattering wave. However, Rayleigh wave detecting concrete vertical fracture theory is not perfect, and the finite difference method and τ-p transform algorithm also has some defects, which cause the overall detection accuracy is not high, so it still need the workers of the surface wave remain to further study. |
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