This research considers the problem for vibration based damage diagnosis in reinforced concrete slabs. It suggests the analysis of the time domain measured accelerations for the purposes of fault detection and quantification. The measured accelerations from different damage states of the slab are first subjected to initial transforms to bring them to vectors with reasonable component number. These vectors are further transformed using principal components analysis (PCA), which brings their coordinates down to two. In addition to reducing the number of measured acceleration points, PCA clusters the new vectors making distinguishable the different damage states. The results from the application of the suggested method convincingly demonstrate that the method can be applied for fault detection as well as for estimating the damage extent.