Evaluation of internal cracks in RC beams using vibration testing and wave propagation analysis

The effectiveness of forced vibration testing to detect cracks within reinforced concrete (RC) beams was examined. Quasi-static load was applied to induce flexural cracks within RC beam specimens. Resonant frequency was measured from longitudinal standing vibration of the beams. Results indicated that the resonant frequency decreased as the cracks increased and propagated, showing that damage level (deformation) of beams can be evaluated from forced vibration testing. Moreover, the crack region within the beams was identified in the local-thickness vibration tests. Wave propagation analyses were then used to interpret the experimental results. The calculations indicated that the waves diffracted around the cracks during standing vibration, which in turn decreased the resonant frequency. Comparisons of the test and calculation results found that a flexural concrete crack with a width above 0.03 mm can be modeled as a simple notch for evaluating the resonant frequency, without the need to consider the influence of aggregate contacts within a crack. These findings are useful in improving simulation methods and furthering progress in damage evaluation of concrete structures.