Arun Narayanan

Sensing of damage and substrate stress in concrete using electro-mechanical impedance measurements of bonded PZT patches

The influence of stress and induced damage in concrete on the electro-mechanical (EM) impedance response of bonded PZT patches is evaluated for applied compressive loading. Full field displacements obtained from digital image correlation are used to evaluate the level of stress-induced damage in concrete. Stress in the substrate produces an imposed strain on the PZT. A change in the imposed strain produces a rightward frequency shift and an increase in the amplitude of the resonant peak in the EM conductance spectrum of the PZT. An increase in the substrate compliance produces a decrease in the resonant frequency and an increase in the amplitude of the resonant peak. Changes in the resonant peak in the conductance spectrum induced by increasing substrate stress are of a significant magnitude when compared with the changes induced by damage. In the early stages of damage associated with distributed microcracking, the counteracting influences of increasing level of damage and increasing stress on the resonant peak result in no shift in frequency for measurements under applied load. There is however an increase in the amplitude of the resonance peak. When the applied stress is removed, there is a net decrease in frequency resulting from damage in the form of distributed microcracks. Measures of changes in the resonant peak based on root mean square deviation (RMSD), do not show any observable change when measurements are performed under applied loading. There is a consistent increase in RMSD values and frequency shift with increasing damage when the applied stress is removed. The centroidal measure of the normalized frequency spectrum reflects changes in substrate stress. At higher applied stress levels, there is a nonlinear increase in damage, leading to localization and cracking. The influence of damage is dominant in this region and significant changes are obtained in the RMSD values in both loaded and unloaded conditions.

Monitoring progressive changes in cementitious materials using embedded piezo-sensors

An embedded piezoelectric based Lead Zirconate Titanate (PZT) sensor is developed to monitor the early age progression of hydration and development of material stiffness of cementitious materials. The setting behavior of cement mortar is monitored continuously by analyzing the electromechanical (EM) responses of the embedded PZT sensor. The evolving mechanical impedance of the cementitious material is extracted from the measured coupled EM response of the sensor embedded inside mortars of different compositions. The changes in the EM conductance obtained from the embedded PZT sensor is shown to be sensitive to the composition of the mortar and the presence of admixtures which alter the rate of early hydration. The mechanical impedance of the mortar medium extracted from the EM measurement of the embedded sensor is shown to sensitively reflect the progressive change in the stiffness of the mortar material as it transforms from a fluid to a solid state.

Early age monitoring of cement mortar using embedded piezoelectric sensors

A piezoceramic based sensor consisting of embedded Lead Zirconate Titanate (PZT) patch is developed for assessing the progression of hydration and evolution of properties of cement mortar. A method for continuous assessment of cement mortar with different water to cement ratios after casting is presented. The method relies on monitoring changes in the electromechanical (EM) conductance of a PZT patch embedded in mortar. Changes in conductance are shown to sensitively reflect the changes in the mechanical impedance of the cementitious material as it transforms from fluid to solid state.