Methodology Research On Concrete Ultrasonic Computerized Tomography

Because of the complexity of concrete material and deficiency in design, construction and maintenance, defects or damage of concrete components may emerge in construction or operation, which seriously affects the safety and even causes serious accidents. Therefore, it becomes particularly important to fast and accurately retrieve the inner quality condition of concrete components to provide a precise basis for evaluating concrete performance. In recent years, the ultrasonic computerized tomography technology was introduced in concrete nondestructive detection to display the internal structure, defect size and distribution of the concrete under test in the form of image accurately, vividly, and completely, which makes up limitations of traditional concrete detection method and becomes a new development trend of concrete structure detection. In this doctoral dissertation, my in-depth and systematic research in the aforementioned direction is introduced, including the principle of concrete ultrasonic testing, concrete ultrasonic tomography imaging theory and forward inversion method based on ray theory, concrete ultrasonic tomography method, image post-processing method for chromatography imaging results, and main factors influencing tomography results based on Fresnel volume, etc. The main research results are as follows:(1) An improved ray tracing method, based on Snell’s law and combining with the perturbation method, was proposed aiming at certain characteristics rules of random distributions of wave velocity and, defect within concrete, which was suitable for the forward characteristic of concrete ultrasonic computerized tomography. Test results of different models show that, the improved ray tracing path is more close to the real acoustic path. And the improved perturbation method for ray tracing based on Snell’s law can be applied in forward process of concrete ultrasonic computerized tomography imaging.(2) Through the study of iterative inversion algorithm for conventional concrete ultrasonic computerized tomography and the least square algorithm, two improved ART algorithms as well as the improved conjugate gradient algorithm and improved LSQR algorithm for concrete ultrasonic computerized tomography were proposed. Concepts of velocity classification and progressive block in imaging unit were introduced in the two improved ART algorithms. A preprocessing matrix was introduced in the improved conjugate gradient algorithm for projection matrix preprocessing to reduce the condition number of the projection matrix. A damping factor was added to the improved LSQR algorithm to suppress the impact caused by errors. The improved algorithms and the original algorithms were compared and analyzed via the numerical model and concrete block test. The results show that the proposed algorithm can improve the speed and accuracy of calculation. The algorithm effectiveness was verified by the imaging results analysis of two groups of concrete specimens’ internal defect. The imaging results show that the improved algorithm can better reflect the location and distribution of the defects in concrete specimens.(3) Aiming at the ill-posed problem caused by the Ray sparse and incompleteness in the concrete ultrasonic computerized tomography based on ray theory, a concrete ultrasonic computerized tomography method based on Fresnel volume was proposed. Ultrasonic energy propagation is actually inside the first Fresnel volume, so using Fresnel volume instead of ray to conduct Concrete Tomography would increase the coverage area of concrete test region, and thereby increase the speed and efficiency of the tomographic inversion, and improve the resolution and reliability of the tomography result figure. Numerical models and concrete specimen test proved the superiority of concrete ultrasonic computerized tomography method based on Fresnel volume.(4) The image post-processing of the concrete ultrasonic computerized tomography and the main influencing factors of ultrasonic computerized tomography were studied. Hierarchical clustering analysis was used in the image processing of concrete ultrasonic computerized tomography. Concrete specimen tests show that hierarchical clustering can suppress artifacts, highlight the defects and improve resolution, reliability, and readability of the resulting images. So it can improve the ability to identify defects through resulting images. In addition, a variety of influencing factors of ultrasonic computerized tomography was investigated and analyzed, which would facilitate more stable and suitable design of concrete ultrasonic computerized tomography.