Monitoring the concrete early-age strength gain at any arbitrary time from a few minutes to a few hours after mixing is crucial for operations such as removal of frameworks, prestress, or cracking control. This paper presents the development and evaluation of a potential active wireless USB sensing tool that consists of a miniaturized electromechanical impedance measuring chip and a reusable piezoelectric transducer appropriately installed in a Teflon-based enclosure to monitor the concrete strength development at early ages and initial hydration states. In this study, the changes of the measured electromechanical impedance signatures as obtained by using the proposed sensing system during the whole early-age concrete hydration process are experimentally investigated. It is found that the proposed electromechanical impedance (EMI) sensing system associated with a properly defined statistical index which evaluates the rate of concrete strength development is very sensitive to the strength gain of concrete structures from their earliest stages. 1. Introduction Since accurate field measurements of early-age concrete properties, such as setting time, in-place strength gain, and shrinkage stresses, are crucial to in situ quality control of concrete, various techniques have been proposed including Windsor and pullout probe tests, ultrasonic pulse velocity, impact-echo method, microwave method, and maturity method [1]. Those nondestructive techniques interactively measure certain mechanical properties of the concrete from which information on the strength is derived. However, although mechanical wave velocity methods such as the ultrasonic pulse velocity and impact-echo methods are widely used to test in a nondestructive manner, they have some limitations that restrict practical applications. Thus, mainly due to their serious drawbacks they received little interest from the construction community. Besides, these methods use extensive wiring systems to operate and necessitate special equipment to gain access to the structure during construction. In addition, these techniques perform localized measurements, and the monitoring of large concrete structures requires an extensive amount of time and effort leading to costly usage. Substructures such as concrete foundations and piles are inaccessible and their early-age strength development cannot be evaluated using these monitoring systems. The advent of smart materials, such as piezoelectric materials, shape-memory alloys, and optical fibers, has attracted interests among researchers and engineers to develop new
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