Characterization of early-stage material damage in cement-based materials using nonlinear ultrasound
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The assessment of civil infrastructure like concrete containment vessels, dams, and bridges requires ability to nondestructively characterize their damage state, and thus reducing rehabilitation costs, and enhancing public safety. A critical step in achieving this goal is the ability to monitor microcrack development, since these microscale defects directly affect the early-stage material performance, and thus the entire service life of concrete civil infrastructure. Although excellent progress has been made in the development of nondestructive evaluation (NDE) and structural health monitoring (SHM) techniques for concrete, the evaluation of microstructural condition is still challenging due to the low sensitivity of the conventional NDE parameters. Nonlinear ultrasonic (NLU) and nonlinear acoustic (NLA) techniques have attracted significant attention in the last decades, since unlike conventional linear parameters such as wave velocity and attenuation, nonlinearity parameters have great sensitivity to changes in the microstructure of cement-based materials. Previous research has helped establish the feasibility of nonlinear techniques for monitoring the early-early damage state of cement-based materials [1–6] and successfully demonstrated that the NDE of microscale degradation in concrete is possible. In spite of significant progress in the area of NLU and NLA, several issues on the NDE of concrete are still open for research. For instance, it is not fully validated that the existing nonlinear techniques are indeed effective for monitoring in-service concrete structures since the feasibility of these techniques is only limited to small size concrete specimens – the detection of microscale defects in the full-scale concrete structures still poses problems. Another issue is that those concrete structures allow only limited access due to their complex geometry; a local measurement technique should be implemented in this complex measurement condition. To overcome the current issues in the application of NLU to concrete, this research develops an SHG technique using Rayleigh surface waves in the frequency range of 40 to 120 kHz. A non-contact, air-coupled detection technique is developed and implemented to characterize the damage state in cement-based materials. One objective of this research is to demonstrate the feasibility and applicability of the proposed SHG setup for monitoring microcracking development in concrete. Then, using the acoustic nonlinearity parameter, β from the SHG setup developed, five independent tasks are conducted: (1) monitoring the effect drying shrinkage on microcracking development, (2) understanding the role of SRA in mitigating drying shrinkage, (3) quantifying the state of carbonation in concrete specimens; (4) examining the state of alkali-silica reaction (ASR) induced damage at two different times in large-scale concrete slabs, and (5) developing an in situ nonlinear technique to quantify the development of microcracks induced by creep and cyclic loading. Through a comparison with the experimental results, a comprehensive understanding of the early-stage material state in concrete is reviewed by the acoustic nonlinearity parameter, β.