Modal testing as a tool for non-destructive testing of concrete structures

This paper presents experimental results from vibration tests of reinforced concrete beams subjected to progressive cracking. Three reinforced concrete beams of 6 meters length have been examined. Cracks are artificially induced in these beams by static loading tests in a certain number of load-steps. After each static load-step, a complete experimental modal analysis is performed in order to determine the dynamic characteristics of the beams. The dynamic characteristics are determined by performing curve-fitting procedures on series of measured frequency response functions of the structure. The influence of the nonlinear behaviour of the concrete beams is investigated by performing dynamic measurements at different excitation amplitudes. The influence of progressive cracking on eigenfrequencies, modal damping ratios and mode shapes is investigated. The results show that progressive cracking of the beams results in a measurable frequency shift. The evolution of modal damping ratios is discussed and commented. The non-linear behaviour is observed, even at the very low amplitude at which the modal analysis is performed. However, the influence on resonant frequencies and mode shapes is negligible. This is not the case for the modal damping ratios.