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In order to strengthen and repair existing concrete structural elements, fibre-reinforced polymer composites are often externally bonded using structural adhesives. It is thus desirable to monitor the in situ performance of the sandwiched adhesive layer in such fibre-reinforced polymer–strengthened systems via its stiffness and strength gain throughout the curing process. The electromechanical impedance technique, which relies upon the utilisation of piezoelectric sensors, offers this capability. Although the technique has been verified experimentally in the laboratory, no known electromechanical impedance–based modelling study has been reported. This study, therefore, proposes the first electromechanical impedance–based finite element and analytical models to monitor the curing of structural adhesives. The dynamic elastic modulus of structural adhesives during curing can be determined from the developed models via a model updating process. Semi-empirical relationships were then developed to determine the tensile strength of structural adhesives from the resonance frequency obtained from the electromechanical impedance technique. This was made possible by correlation between static tensile tests on structural adhesives and the dynamic elastic modulus. These electromechanical impedance–based models were found to perform equally well when compared to the previously developed wave propagation–based models. This study shows the robustness of the electromechanical impedance technique for non-destructively predicting the dynamic elastic modulus and tensile strength of adhesives throughout the curing process. [ABSTRACT FROM AUTHOR]

The ability to monitor the strength development of early-age concrete is an important capability in the laboratory and in the field. Accurate and reliable in situ measurements can inform the appropriate time for removal of formwork and loading of structural elements, as well as determination of batch quality. The piezoelectric-based electromechanical impedance technique is emerging as a viable option for such monitoring needs. The first research articles on the topic started to appear in 2005, and since then, the research field has advanced and has grown in popularity. This article therefore presents the first state-of-the-art review of the topic to date. In this article, existing research is reviewed and sorted into key themes while critical developments as well as knowledge gaps are identified. The topics addressed range from miniaturization of hardware, methods of installation, incorporation of wireless technology, modeling, data interpretation, signal processing, influence of curing, and environmental conditions to a wide range of other practical issues. Previous studies have indicated that the electromechanical impedance technique has the potential to be developed into an autonomous and remote monitoring system, capable of predicting the strength development of early-age concrete structures in real time. An end game is therefore the realization of this capability. Appropriate comments are therefore also provided in this article about this goal. Researchers interested in venturing into this research area shall find this article a useful introduction as well as a state-of-the-art assessment. In addition, the identified research gaps can inform projects for more experienced research teams. [ABSTRACT FROM AUTHOR]