Investigation of electrically isolated capacitive sensing skins on concrete to reduce structure/sensor capacitive coupling

Damage to bridges can result in partial or complete structural failures, with fatal consequences. Cracks develop in concrete infrastructure from fatigue loading, vibrations, corrosion, or unforeseen structural displacement. Effective long-term monitoring of civil infrastructure can reduce the risk of structural failures and potentially reduce the cost and frequency of inspections. However, deploying structural health monitoring technologies for crack detection on bridges is expensive, especially long-term, due to the density of sensors required to detect, localize, and quantify cracks. Previous research on soft elastomeric capacitors (SECs) has shown their viability for low-cost monitoring of cracks in transportation infrastructure. However, when deployed on concrete for strain monitoring, a structure/sensor capacitive coupling exists that may cause a significant amplification in the signal collected from the SEC sensor. This work provides a detailed experimental study of electrically isolating capacitive sensing skins for concrete structures to reduce the structure/sensor capacitive coupling of an electrically grounded sensor. The study illustrates that the use of rubber isolators effectively decreases the capacitive coupling between concrete, which inherently has capacitive properties, and sensors such as the SEC that utilize capacitance measurements. In addition, the required thickness of isolation for accurate strain monitoring using the SEC with geometry described in the paper is investigated and better strain correlation is observed between the rubber of isolation thickness 0.30 mm and 0.64 mm with rubber of isolation of approximately 0.40 mm having the best response. Tests were conducted on small-scale concrete beams, and results were validated on full-scale reinforced concrete bridge decks recently taken out of service. This study demonstrates that with proper isolation material, the SEC can accurately transduce strain from concrete within a 10 $\unicode{x03BC} \varepsilon$ error for strain levels beyond 25 $\unicode{x03BC} \varepsilon$.