Embedded electrical impedance-based PZT sensor for monitoring hydrating concrete: development and evaluation

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 electrical impedance 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