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Title: Application of piezoelectric materials to monitor modulus and strength development of cementitious materials
Authors: Lu, Xubin
Keywords: DRNTU::Engineering::Civil engineering
Issue Date: 17-Sep-2018
Source: Lu, X. (2018). Application of piezoelectric materials to monitor modulus and strength development of cementitious materials. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Strength monitoring of early-age concrete improves the efficiency of construction as it provides information on the optimum time for shoring removal and pre-stress transferring. Electromechanical impedance (EMI) technique has been proven to be a useful tool for strength monitoring of cementitious materials. One of the key limitations of this technique is the lack of physical models, which resulted in its strong reliance on statistical measures to quantify the strength of structure being monitored. As the key originality of this thesis, a novel concept of “Smart Probe” is proposed to parametrically estimate the modulus and strength development of cementitious materials using the principle of EMI technique, overcoming the critical shortcoming of the traditional EMI technique. Instead of directly bonding a lead zirconate titanate (PZT) patch on the host structure, a PZT was first surface-bonded on a pre-fabricated aluminium beam termed Smart Probe, which was then partially embedded into cementitious materials for strength monitoring. The structural resonance frequencies of the Smart Probe can be identified from the conductance signatures throughout the curing process serve as strength indicator. An analytical model is developed to predict the dynamic modulus of elasticity of cementitious materials based on structural resonance frequencies. Experimental study was carried out on a mortar slab specimen to verify the model and to investigate the performance of the Smart Probe. It was found that the dynamic modulus of elasticity of the host structure could be predicted from the conductance signatures using the proposed model. Compressive strength assessment was achieved by establishing an empirical relation with the dynamic modulus. The proposed technique for strength monitoring of cementitious materials is parametric with high repeatability. The analytical model of the Smart Probe is further improved by explicitly formulating the distributed stiffness and mass of the embedded segment of the Smart Probe in the governing equations, allowing more realistic predictions of the actual dynamic characteristics of the coupled Smart Probe-cementitious material system. A 3-D coupled field finite element (FE) model is also established to predict the conductance spectrum. Multiple resonance peaks in the conductance spectrum computed by FE simulation are in good agreement with the experimental data. The effectiveness of both models in characterizing the resonance peaks are verified with experiment. Parametric study of the Smart Probe shows that the dimensions and position of the PZT patch on the Smart Probe influence the magnitude and the sharpness of the resonance peaks in the admittance spectrum. In order to overcome the shortcomings of the traditional resonant frequency method (RFM) to measure dynamic modulus of concrete, such as the requirement of bulky and expensive equipment, this thesis proposes a new EMI technique for determining the dynamic modulus of cementitious materials, based on similar working principle of RFM. In this technique, a PZT patch is surfaced-bonded on a specially designed specimen of cementitious material, termed “Miniature Prism”. Experimental results show that, combining the technique with FE analysis, the dynamic modulus of the cementitious materials can be non-destructively measured. The technique possesses advantages, such as simplicity of data acquisition procedure and higher repeatability of measurement, that make it suitable for real-time monitoring.
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