Dynamic nonlinearities for identification of the breathing crack type damage in reinforced concrete bridges

Reinforced concrete structures experience nonlinear vibration response because of softening in concrete due to damage, which results in unstable vibration. Thus, considering the nonlinearity in vibration for assessment of damage is imperative to identify the actual damage scenario. Nonlinearity is predominant during the initial stage of damage when cracks start developing in the concrete exhibiting nonlinear breathing-crack mechanism. By suitably extracting nonlinear features from ambient/forced vibration, information on initial stage of damage (primarily breathing-crack) in large concrete structures can be obtained. In the present study, evaluation of nonlinear dynamic features from an experimental model of a reinforced concrete bridge girder-deck system is investigated. The damage scenarios in the bridge girder is experimentally simulated by monotonically applied static load at different levels. Vibration responses are acquired using the accelerometers positioned at regular intervals, so that the global system behaviour can be assessed. Damage features at various damage levels are evaluated based on the signal statistics, amplitude dependency, harmonics, phase-plane information and so on. The breathing-crack behaviour of the bridge girder during initial damage stages is investigated and the existence of sub- and super-harmonic components in the frequency spectra is distinctly evidenced and their amplitudes are quantified. The change of skewness pattern of the frequency spectra with increasing damage level is also evaluated. A distinct variation in phase space portrait is observed with increase in damage levels. It is found that, in spite of presence of inherent material nonlinearity, nonlinearity caused through system damage, even at initial stages of damage, is predominant and could be distinctly captured without any baseline information. The identified nonlinear vibration characteristics showed a remarkable relevance in detecting breathing-crack type damage in weakly nonlinear bridge. The investigated damage features from this study would pave the way for real-time baseline free assessment and identification of damage in concrete structures.