| As civil infrastructure ages it is pertinent that existing structures are evaluated on the basis of both performance and durability. Additionally, new materials and construction methods must be similarly evaluated in order to promote research and development in civil engineering. A literature search shows that current practice relies primarily on destructive testing for performance evaluation and life span measurements and/or forensic evaluation of failed structures for durability. An investigation of the failure mechanisms in concrete show it to be especially prone to deterioration due to the development and propagation of microcracking, which reduces its integrity and thus its resistance to numerous other mechanisms of deterioration. Current non-destructive testing methods permit flaw detection, but are not sensitive to the gradual deterioration of concrete that 239 begins as soon as the structure is placed into service, especially micro-cracking at interfaces between the components that make up concrete. It is this deterioration that eventually gives way to detectable flaws and shortly thereafter to failure.; In pursuit of a means of evaluating early deterioration in concrete, we have developed a non-destructive testing method that utilizes an acoustic perturbation in the test region to focus the dispersion of energy in the frequency domain. Amplitude modulated acousto-ultrasonic testing uses the intermodulation of pressure waves to generate narrow band responses at predictable frequencies due to their interaction through contacting interfaces. This predictable response provides a strong basis for filtering, and therefore excellent signal to noise ratios. Analytical modeling has been used to explain the energy dispersion on multiple levels including harmonic frequency generation, fundamental frequency distortion, and (the focus of this dissertation) sideband generation through amplitude modulation. This model has been supported at each level by experimentation, which has also been presented herein. |
