Non-Contact Damage Detection in Concrete Using Laser Doppler Vibrometry and Various Excitation Methods

A substantial share of reinforced-concrete infrastructure assets has reached an age where deterioration mechanisms such as cracking, delamination, and voiding may develop, potentially increasing safety risks and maintenance demands. Conventional condition assessment commonly relies on localized intrusive testing (e.g. coring) and manual sounding, which can be disruptive, labour-intensive, and partly subjective. Vibration-based non-destructive testing (NDT) provides an alternative by exciting the structure and evaluating changes in its dynamic response. In contrast to previous studies, which typically assess a single excitation method in isolation, this study provides a systematic side-by-side comparison of three vibration-based NDT excitation approaches: mechanical impact using a custom compressed-air impact device, acoustic excitation, and shaker excitation. All three methods were evaluated under identical measurement conditions. The vibration response is measured using laser Doppler vibrometry (LDV), enabling non-contact acquisition of frequency-response signatures. A custom mechanical excitation device was developed and evaluated, and the results indicate that it provides stable and repeatable excitation with good defect discrimination. Experiments on specimens with representative defect types show that mechanical impact and shaker excitation yield the most repeatable and discriminative response features, whereas acoustic excitation provides insufficient signal-to-noise ratio (SNR) for the smallest tested specimens (150 x 150 x 150 mm). Among the evaluated setups, the electrodynamic shaker and the compressed-air impact device offer the most promising low-noise measurements. The goal is to enable efficient and scalable inspection methods for safer and more reliable monitoring of reinforced-concrete infrastructure.