Baseline-free electromechanical impedance technique for material characterization and delamination detection in concrete

Abstract

Concrete is the most used construction material in modern society, and it is complex because of the anisotropy and inhomogeneity caused by its multiphase characteristics. As one of the smart materials, piezoelectric materials provide new opportunities to measure and understand concrete properties. When used together with piezoelectric materials, the electromechanical impedance (EMI) technique is a promising tool for localized monitoring/detection because of its high operational frequency range, high flexibility in sensor installation, and convenient implementation. In practice, the EMI technique can quickly and easily measure the electrical impedance of piezoelectric sensors, which are coupled with the mechanical impedance of the structures and the piezoelectric sensors. Baseline-dependent EMI techniques, which attempt to correlate the variation of signals and changes of the structure via statistical indices obtained before and after damage, are popular in real practice. Although these techniques are easy to conduct and model-free, their results are greatly affected by sensor selection, bonding status, and temperature changes, thus limiting their applicability in actual structures. In contrast, baseline-free EMI techniques, which can utilize the mode shapes and corresponding resonant frequencies of target structures to evaluate structural features quantitatively, have rarely been reported, especially in the concrete field. In this thesis, baseline-free EMI techniques were systematically developed for concrete by using different types of sensors. Fixed and detachable piezoelectric sensors were employed to characterize concrete materials and detect delamination in the concrete cover, respectively. Taking advantage of the easy installation of the surface-bonded sensor (SBP), a baseline-free EMI technique was proposed for the first time to quickly and accurately measure the modulus of elasticity of standard concrete cubes. The generality, repeatability, stability, and accuracy of the proposed method were validated regarding different piezoelectric sensors, bonding status, temperatures, and concrete mixes. Then, the mode shapes of a concrete cube were carefully investigated, and special modes that are insensitive to material anisotropy and inhomogeneity were selected. Subsequently, a novel SBP installation strategy for extracting the target modes and corresponding assessments was developed to accurately measure the elastic constants (including modulus of elasticity and Poisson's ratio) of concrete materials. The effectiveness and accuracy of the proposed method were validated on standard concrete cubes, and the method was compared with the traditional dynamic resonance method. Given that an embeddable piezoelectric sensor (EBP) can be installed in advance, vibration modes of concrete cubes that are easy to identify and measure even at a high damping ratio were selected. The corresponding sensor installation and signal processing were developed to measure the concrete elastic properties at a very early age, which is a stage that traditional dynamic resonance methods can hardly cover. Effectiveness and accuracy of the proposed method were validated by comparing it with traditional dynamic resonance methods after demolding. EBP was further utilized to widen the applicability of the baseline-free EMI technique in terms of element size. The length-insensitive vibration modes of a prism were selected, and the corresponding sensor installations and signal processing were developed to measure the modulus of elasticity. Effectiveness and accuracy of the proposed method were validated in comparison with that using global fundamental vibration modes. Finally, the feasibility of the detachable sensor installation was investigated to increase applicability of the EMI technique to damage detection in a large area, and a baseline-free assessment was developed to detect the delamination of concrete cover. Effectiveness and robustness of the proposed method were validated by conducting laboratory tests and on-site trials. Through systematic, comprehensive numerical and experimental studies, this thesis explores a series of innovative applications of the baseline-free EMI technique in concrete. The outcomes of this thesis offer an enhanced understanding of concrete materials/structures and pave a new avenue for applying EMI techniques.