Defect characterization in concrete elements using vibration analysis and imaging (2012)

TAE KEUN OH Advisor: Professor John S. Popovics The deteriorating national infrastructure promotes the need for reliable non-destructive evaluation (NDE) and structural health monitoring (SHM) methods for existing concrete structures. The conventional impact-echo (IE) test, which utilizes contact type sensors, is an efficient NDE method that is used to identify and characterize internal defects in concrete structures. However large testing volumes associated with the infrastructure demand many tests, which can be labor-intensive and time-consuming; contactless, air-coupled IE tests offer a solution to this limitation. In this study, effective implementation of air-coupled IE to characterize delamination defects in concrete structures is sought. Progress in this effort is reported here. First, the development and verification of a testing system using air-coupled sensors is described. Laboratory studies demonstrate that the proposed testing hardware and configuration are practical and effective for sensing leaky waves and local resonances in concrete, and thus can be used for IE tests. The proposed air-coupled sensor is cost-effective and does not require external power, signal conditioning, nor acoustic shielding against ambient acoustic noise. Second, the vibration behavior of the region around delamination defects is simulated using 2-D axis-symmetric and 3D finite element (FE) analysis. In order to reduce computation effect and render the model more practical for this work, unwanted wave-reflections from the model boundaries were suppressed using an efficient approach and air-coupled sensing configurations were simulated. The results of the FE simulation compare favorably with analytical solutions and the IE test results on concrete samples. Third, different types of resonance testing configurations are evaluated, including a coupled source-receiver configuration (similar to the IE test) and a fixed source, moving sensor configuration (standard modal test). A new approach to self-normalize individual air-coupled data sets, which uses information from the leaky surface wave pulse in the time signals, is proposed and evaluated. Such corrected test data provide improved modal and vibration frequency images. An effective NDE test method that uses the corrected air-coupled data is then proposed; the testing method has the advantage that internal damage location, size and shape need not be known in advance. Fourth, an effective data imaging technique is proposed for comprehensive interpretation of air-coupled IE data, specifically for detection of near-surface delamination defects. The necessary parameters and processing required for the method are introduced. An approach to determine the optimal values of the image parameters, based on the data set itself, is proposed. The optimized images are compared with conventional imaging methods for data from simulated and actual bridge slabs, and the advantages of the new imaging approach are illustrated. Fifth, a semi-analytical model of vibration resonance nearby delamination defects is developed. The model makes use of the edge effect concept. Based on the model, a practical straight-forward formula is proposed that predicts the flexural vibration frequencies of the region above delamination defects with arbitrary aspect ratio and side to thickness ratio, and for any mode order. Based on results from the formula, the effects of the IE test setup on the frequency spectrum are discussed, and limitations in the application of the IE test are pointed out. Finally, some strategies for practical application of air-coupled IE tests are recommended for future complimentary work efforts.