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Garland, Damien

Mechanical Engineering

2022

M.S.

Reinforced concrete bridge decks are high performance structural elements that must survive decades of harsh mechanical and environmental loads. The structural health monitoring of bridges is a primary enabler of safe and cost-effective performance, relying on frequent inspections and repairs as needed. The undersides of concrete bridge decks are oft-overlooked areas of inspection, owing to a hard to reach and difficult location to access. Exacerbating factors to the degradation of concrete bridge decks are the freeze-thaw cycle and salt water related corrosion common in the world's Northern climates. As concrete weathers, it decays. Corrosion induced cracking and delamination are primary damage modes.Moreover, budgets are forced to contend with the seasonal roadway damage from frost-heaves and snowplows, which can lead to delays in the inspection and repair of the undersides of bridge decks. Years of neglect have led to a growing need for assessment and repair. Despite this need, the inspection of the undersides of bridge decks is laborious and costly, requiring the shutdown of roadways, the use of specialized equipment, and an above-average operator risk. Ironically, while the cost of early detection of subsurface cracks is prohibitive, the cost of repair rises when early detection is not feasible. The key is knowing where the defects are, and how extensive the damage. In recent decades, numerous methodologies for concrete crack and delamination detection have become commercially available, however the undersides of bridge decks continue to pose a challenge. Often, detection is not financially feasible until cracks have reached the surface, or concrete has fallen off the structure. Reports of vehicles struck by falling concrete are not uncommon, demonstrating the need for an affordable robust early detection methodology for failing concrete. In this thesis, a technology down-select is performed to determine the optimal solution to detecting subsurface delaminations in the underside of concrete bridge decks. The solution alighted upon is an active acoustic sensor (AAS) mounted on an unmanned arial vehicle (UAV) platform to tap and listen to the underside of bridge decks. A mechanical tapping mechanism is developed to acquire the tap data remotely using a high frequency acoustic sensor. Embedded void concrete samples of 12" and 24" form factors are used as test beds in laboratory conditions. Subsequent post-processing using the Fast Fourier Transform (FFT) and the Welch Power Spectral Density (PSD) and Hilbert-Huang Transform (HHT) are used to differentiate a solid slab from one with a void. This research has shown the methodology to be feasible, and has laid the groundwork for additional effort to refine the design and bring it to a readiness level robust enough for in-situ testing in the field.