Nondestructive evaluation of reinforced concrete T-shaped beams by frequency spectrum analysis of impact-echo waveforms

Numerical and experimental work has been performed to determine the transient dynamic response of undamaged and damaged concrete T-shaped beams when subjected to transient impact. The numerical component of this research included modeling of the transient dynamic response of laboratory-scale concrete "T-beams" with varying dimensions and aspect ratios. Modal analyses, resonant analyses and transient dynamic analyses were performed on two-dimensional plane strain and three-dimensional finite element models using Ansys v. 8.0. Experimental verification of the numerical results was carried out for three of the modeled T-beams. From the experimental and the numerical work performed on the laboratory T-beams, frequencies of the first six cross-sectional modes of vibration dominating the response were determined. Relationships between the fundamental frequencies of the T-beams and the frequencies of higher modes of vibration were determined. Relationships between the shape factor of the T-beams and the aspect ratios were determined as well.; The experimental component of this research included testing four full-scale reinforced concrete "T-girders" using the impact-echo method in conjunction with monotonic and fatigue load testing. Impact-echo tests were first performed on the girders to assess their initial condition and then carried out concurrently with the fatigue and the monotonic load testing to assess the condition of the girders as loading progressed. The T-girders included unretrofit specimens to serve as controls and specimens retrofitted with carbon fiber reinforced polymer strips (CFRP). The experimentally observed dominant cross-sectional modes of vibration were verified by the results of the numerical work performed on the scaled T-beams. The load induced crack depths were interpreted for a number of testing locations on the fatigue loaded control specimen and agreed well with visual observations. The change in the apparent P-wave velocities as loading progressed was determined for the T-girders by monitoring the change in the fundamental frequencies at each testing point. In general, the CFRP retrofit specimens had a lower percent reduction in the apparent P-wave velocities than the unretrofit specimens. Results of impact-echo testing of the fatigue loaded T-girders were used to develop a methodology to calculate the effective bending stiffness of the T-girders based on impact-echo measurements. In addition, a method is proposed to predict the remaining service life of the T-girders based on impact-echo measurements.