Elastic wave-based non-destructive test (NDT) methods are effective for detecting flaws in concrete structures. With the recent developments in computing hardware and software, imaging techniques have become very popular in NDT applications. However the application of elastic wave-based imaging methods for concrete structures is severely limited by the physical coupling between sensors and concrete surface, which reduces testing efficiency. In this thesis, the air-coupled sensing technique is proposed as a solution to improve the efficiency of elastic wave-based test methods for concrete structures.; Theoretical analyses are first conducted to study the propagation of leaky Rayleigh waves in fluid-solid half spaces. Closed-form solutions of the Green's function are derived for pressure and displacement in both the fluid and solid. This analysis provides theoretical background necessary for practical air-coupled sensing of leaky Rayleigh waves in concrete. The theory is also extended to underwater NDT applications.; Two applications of air-coupled sensing are considered. One is air-coupled leaky surface wave sensing in concrete. A laboratory study and field tests demonstrate that air-coupled sensors are very effective for sensing leaky surface waves in concrete. The sensitivity and accuracy of air-coupled sensors are comparable to contact sensors. Air-coupled sensors are suitable replacement for contact sensors in SASW and MASW tests and moreover help improve test efficiency. In addition, the contact-less nature of air-coupled sensing enables the study of the effect of defects on wave attenuation. The experimental results show leaky Rayleigh waves are sensitive to the existence of cracks in concrete when waves propagate across cracks.; The second application is air-coupled impact-echo. Two reinforced concrete slabs containing different types of defects were inspected using an air-coupled impact test testing scheme. 2-D scanning impact-echo tests were conducted over the slab containing voids and delaminations. The 2-D scanning image clearly shows the location and depths of embedded defects. Air-coupled impact-echo is also applied to examine the grouting condition of embedded ducts. The poorly-grouted sections are identified within the metal duct.
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