Change in impact-echo response during fatigue loading of concrete bridge T-girder

Impact-echo tests were performed concurrently with a full-scale fatigue load test to nondestructively assess the change in condition of a reinforced concrete bridge T-shaped girder having aspect ratios of B/D = 1.08, D/b = 2.33, and B/t = 5.1. The initial condition of the girder and the condition for six subsequent intervals of fatigue loading were assessed by analyzing the cross-sectional modes of vibration and changes in apparent P-wave velocity. The first three cross-sectional modes of vibration were determined to be 1820, 2900, and 3800 Hz for the initial condition of the girder. Damage resulting from fatigue loading was indicated by a frequency shift in the cross-sectional mode frequencies and the appearance of new peaks in the response spectra corresponding to wave reflections from damage. The highest level of damage was observed for tests located in the vicinity of the loading plate and correlated well with visual observations of cracking on the exterior surface of the girder.