Elastic wave-based non-destructive test (NDT) methods are effective for detecting flawsin concrete structures. With the recent developments in computing hardware and software,imaging techniques have become very popular in NDT applications. However theapplication of elastic wave-based imaging methods for concrete structures is severely limitedby the physical coupling between sensors and concrete surface, which reduces testingefficiency. In this report, the air-coupled sensing technique is proposed as a solution toimprove the efficiency of elastic wave-based test methods for concrete structures.Theoretical analyses are first conducted to study the propagation of leaky Rayleighwaves in fluid-solid half spaces. Closed-form solutions of the Green???s function are derivedfor pressure and displacement in both the fluid and solid. This analysis provides theoreticalbackground necessary for practical air-coupled sensing of leaky Rayleigh wavesin concrete. The theory is also extended to underwater NDT applications.Two applications of air-coupled sensing are considered. One is air-coupled leakysurface wave sensing in concrete. A laboratory study and field tests demonstrate thatair-coupled sensors are very effective for sensing leaky surface waves in concrete. Thesensitivity and accuracy of air-coupled sensors are comparable to contact sensors. Aircoupledsensors are suitable replacement for contact sensors in SASW and MASW testsand moreover help improve test efficiency. In addition, the contact-less nature of aircoupledsensing enables the study of the effect of defects on wave attenuation. The experimentalresults show leaky Rayleigh waves are sensitive to the existence of cracks inconcrete when waves propagate across cracks; the crack positions are clearly located in a2-D scanning test image.The second application is air-coupled impact-echo. Two reinforced concrete slabscontaining different types of defects were inspected using an air-coupled impact test testingscheme. 2-D scanning impact-echo tests were conducted over the slab containingvoids and delaminations. The 2-D scanning image clearly shows the location of embeddeddefects, and their depths are also determined. Air-coupled impact-echo is also appliedto examine the grouting condition of embedded ducts. The poorly-grouted and ungroutedsections are identified within the metal duct.
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