In this study, test with single impactor and receiver will be conduct on reinforced concrete structure or pavement structure and the spectrogram of group velocities will be obtained via short-time Fourier Transformation and Reassigned method. For the problem of surface degrading of concrete, the inverse methods developed for SASW method will be applied to calculate the stiffness profile from the group-velocity and wavelength relationship of surface waves. For problems with interior voids or cracks, as the inclusions may cause wave refraction, flexural vibration or changes on modal vibrations other than fundamental modes, identification techniques will be developed to transfer the complicated spectrogram into simple indicators for quick assessment of the type and dimension of inclusions. For the pavement structure, the previously mentioned methodology will also be applied to obtain the layering thickness and stiffness. For the instrumental setup of test on pavement, the interval distance between the impactor and receiver will be increased and the heavy impactor will be used to obtain the stiffness profile of pavement and base layers. As the project will develop software to automatically identify the stiffness profile or the simple indicator for abnormal signals, not only the speed of the test can be increased via single impactor and receiver setup. The testing results can be transferred into the physical characteristics that normal engineers can follow. Speedy tests can be performed in large area to detect the problems in RC structure and pavement stratified structure.
Two types of damages can be categorized for concrete plate like structure: Plate contains voids and delamination or hollow. Voids exist underneath the plate-like structure. In the study, lamb waves generated by applying an impact on the surface are used to evaluate the damage problems. The test is performed with one receiver positioned away from the impact. As different lamb wave modes traveled with different group velocities, spectrogram of the response of the receiver can be obtained by Short-Time Fourier Transformation and reassigned technique. As the technique only requires single impact and single receiver, large range and fast evaluation can be achieved. The project will be carried out in three years. For the first year, the dimensions of the impactors and the interval between the impactor and the receiver to obtain the optimized spectrogram for different plate-thickness would be explored. The characteristic spectrograms of plate containing delaminated cracks with different dimensions will be studied. For the second year, the effect of poor concrete quality, such as voids, honeycombs and unfilled prestressed tube, to the spectrogram would be explored. For the third year, the problem of voids under concrete plate-like structure will be examined. Software which can show the spectrogram real time would be developed also would the guidelines for fast identification of problems for concrete plate-like structure. Three subjects have explored using the finite-element modeling- proper choices for the impact duration and distance between impactor and receiver, the comparison of the spectrograms obtaining from a plate with and without the underlying soil layer and the response for two-layered concrete plate with weak upper layer.
Structures strengthened by steel plate are popular in Taiwan. As full bonding between the steel and matrix concrete is essential for effectively strengthen the RC structure, the epoxy must be fully filled inside the gap of steel plate and concrete. During the serving lifetime of the steel strengthened structure, the possible defects inside the structures, which can not be observed from its appearance, may be induced by peeling of the steel plate, epoxy partially filling, or corrosion of steel bar inside concrete. The aim of the study is to apply multiple stress-wave-based nondestructive testing methods for evaluating the deterioration of structures strengthened by steel plate. In this study, the larger area of a strengthened structure is examined first by the dispersion curves of the Lamb modes generated in steel plate. The locally defected area is then evaluated by the simulated transfer function obtained from the impact-echo responses. The internal defect of concrete inside the shelled steel plate is evaluated by the SASW method. To achieve the aim of speedy testing, the spectrogram is obtained by single receiver using the short-time Fast Fourier Transform with reassigned technique. The project will be carried out in three years. For the first year, following the 1-year research project conducted on 2008, the feasibility of using single receiver to obtain the spectrogram will be evaluated by the finite-element method. Tests on steel plates with various thicknesses will also be conducted. For the second year, solid concrete plate specimens strengthened by steel plate were constructed for developing the proper testing procedures to obtain correct spectrogram. The effect of unbonded area between steel plate and concrete to the diagram will be studied. For the third year, beam specimens will be constructed. The specimens will be damaged by fire or mechanical loading and strengthened by steel plate. After strengthening, the specimens will be damaged again by four point bending test to create debonding between the steel plate and concrete. The non-destructive testing will be performed at each stage.
Keywords: concrete, steel plate, Spectrogram, Lamb wave, impact-echo method, SASW.