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The development of an embeddable sensor, which can be placed inside concrete for continuous monitoring of changes in the material brought about by hydration is presented. The embedded sensor relies on monitoring changes in the electrical impedance (EI) measured from a piezoelectric lead zirconate titanate (PZT) patch placed inside hydrating concrete. The EI measurements over a range of frequencies is associated with the dynamic response of the PZT patch with distinct resonant frequencies. The EI of the PZT sensor is reflective of the mechanical impedance to its motion from the surrounding concrete medium. Changes in the EI measurements from an embedded PZT sensor with time relate with the kinetics of cement hydration. The increase in the resonant frequency in the EI spectrum is produced by the increase in the Young's modulus of the concrete medium. The changes in the resonant conductance amplitude are sensitive to changes in the stiffness of medium associated with a change in the state and the setting behavior of concrete. The role of the epoxy coating provided as protection to the PZT element is to restrict the motion in the early stages of hydration and to allow relative motion of the PZT element when the concrete gains in stiffness. An idealized representation of the embedded PZT sensor is developed for representing the dynamic impedance to its motion from the epoxy and the surrounding concrete medium. The changes in the EI resonant amplitude of an embedded PZT provides a sensitive measure of the setting behavior and early stiffening of the concrete post-setting. The changes in the resonant frequency of the embedded PZT sensor provide a continuous measure of the evolution of stiffness of the concrete throughout the setting and hardening process.

An application is developed with surface mounted Lead Zirconate Titanate (PZT) patches 9 for sensing damage in the form of a stress-induced crack in a concrete substrate. A localized 10 crack is introduced in a controlled manner using a fracture test. Full-field displacements 11 obtained using digital image correlation are used for crack penetration and crack width 12 measurements. Electrical impedance (EI) measurements are obtained from the individual PZT 13 patches, which are attached at different locations relative to the crack. Stress wave transmission 14 measurements are performed using the PZT patches as actuator-receiver (AR) pairs. The EI 15 measurements indicate that small, quantifiable changes in the mechanical impedance of the 16 substrate are experienced by the PZT patch in the vicinity of the localized crack, which 17 sensitively detect crack initiation. The stress wave-based measurements are very sensitive to 18 the presence of physical discontinuity created by a localized crack in the stress wave path. A 19 measure of stress wave attenuation, the attenuation factor is developed, to quantify the 20 measured changes in the stress wave produced by the physical discontinuity in concrete upon