Concrete exhibits remarkable nonlinear acoustic behaviors that originate from its complex microstructural characteristics at multiple length scales (from cm to nano meter). The nonlinear behaviors are observed in a variety of different ways including harmonic generation, nonlinear mixing, shift of resonance frequency and damping parameter, etc. This research exploits the harmonic generation characteristic in propagating Rayleigh waves to evaluate microstructure evolutiondamage in a concrete specimen under room temperature compressive creep load. A measurement setup that uses a wedge transmitter (50 kHz) on cylindrical specimen surface and a non-contact air-coupled receiver (100 kHz) is proposed. In situ nonlinear ultrasonic measurements are performed on the creep specimen for three months – this simulates an ultrasonic inspection of damage in a field concrete structure under a significant long-term compressive load. Microcracking first occurs which lead to changes in nonlinearity parameter which is followed by a macrocrack that leads to a shear fracture. Microscopic study is conducted to measure density and morphological features of microcracks.

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