Integrity of reinforced concrete slabs by the analysis of response waveforms

The dynamic response of a reinforced concrete slab is dependent on its mass, stiffness, damping, geometry and support boundary conditions. Laboratory tests on a series of fully clamped RC slabs were subjected to incremental loading up to failure, and the dynamic response was measured as a function of progressive damage at appropriate intervals. It was found that for a grade 45 concrete slab specimen, the resonant frequencies of the slab for the first, second and third modes of vibration prior to failure, were reduced by 24.0, 4.5 and 8.8 per cent respectively, when using an impact hammer. This general trend was confirmed when burst chirp excitation was employed, where the first and third modes of vibration were similarly reduced. Of the three regions identified in the loading process, a region of almost constant frequency was noted for the first two modes of vibration. The results suggest that for the third mode, the rate of frequency change was more even throughout the load range. The third mode of vibration could thus be utilised for dynamic characterisation, to provide a reliable measure of the internal damage state of the slab.