| Concrete is the main structure materials in construction engineering, during the construction, due to many uncertain factors such as construction, the defects of concrete interior may occur. Concrete structures are damaged, it will influence the safety of the engineering, therefore, the non-destructive testing in the defects of concrete interior is becoming more and more important. Ultrasonic computed tomography has some advantages such as image with high resolution, sharp images and good reliability, it can more accurately determine the size and position of internal defects in concrete and it becomes an important method in concrete testing. This article was based on Snell's law ray-tracing method that was introduced to computed tomography, and studied on concrete testing results as well as the major factors affecting imaging, and developed corresponding application software on engineering. Main research contents and results of this article was as follows:(1) The rectangular models of concrete( 0.4m?0.5m) with low velocity, high velocity and mixing velocity were established in this article, the results were analyzed and compared in computed tomography based on Snell's law. Through the establishment of three models above, the paths of line tracing, once ray tracing and secondary ray tracing, ART inversion and speed mean-square errors of various models were analyzed. The results demonstrated that line tracing chromatography imaging was less than once ray tracing and secondary ray tracing,the paths of once ray tracing and secondary ray tracing could basically obtain the location of defects, and the paths of secondary ray tracing did not occur larger mutation, smooth transition, its speed mean-square errors were relatively more minimum, the profile of anti-played figures were more clear, and theory model were more anastomosis, using this algorithm effective to improve imaging of precision.(2) The main factors affecting the ultrasonic imaging based on Snell's law were analyzed in this article. A case study of concrete model of low velocity anomaly, analysis of mesh generation, ray tracing, lines density, defects effect of velocity on CT imaging. The results showed that in the case of forward horizontal and vertical grid of the division of equal number of meshes, the most suitable inversion for selecting was 290/forward grid number, when the left and right detection method was used. Right network grid on improving CT imaging of precision has a big help; Secondary ray tracing imaging was more than line tracing and once ray tracing; Measuring lines distribution were more intensive, imaging of effect were better;Defects speed were more close background model speed, pseudo shadow interference was smaller, errors was smaller, imaging of effect was better.(3) Ultrasonic computed tomography imaging software based on Snell's law was developed and applied in practical engineering cases. In the LabVIEW platform,chromatography imaging software was developed including reading sound time, and displaying ray tracing path, and designing anti-played reconstruction. Method had line tracing,once ray tracing and secondary ray tracing. Ray path displayed in two dimension figure, and exported amendment of sound time data. Anti-played method had BPT, and ART, and SIRT algorithm. Imaging figure displayed in speed distribution figure, and saved speed of each network grid unit. The TH-504 pile quality acoustic CT instrument developed by our research group was used to collect the datas for the bored concrete piles, then analyzed by developing software, the results showed that using ultrasonic tomography method based on Snell's law testing concrete piles were reliable.This research provided an important basis for internal defects in ultrasonic testing of concrete and some reference value for practical engineering testing. |
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