AN EMBEDDED PZT-BASED TRANSDUCER FOR MONITORING CURING OF CEMENTITIOUS MATERIALS IN STRUCTURES

Abstract

Curing concrete during construction is crucial to ensure proper strength gain for structures made of concrete. Monitoring the real-time strength gain during construction can indicate whether curing is appropriately done and can help take corrective measures on time. Some standard methods to estimate the setting time of cement mortar or concrete are the Vicat penetration test and ASTM C-403 penetration resistance test. Currently available, curing monitoring methods, such as the maturity method, cannot monitor the long-term hydration of concrete. Although inaccurate in estimating the setting times, the heat of hydration curves obtained from isothermal calorimetry could be used to indirectly indicate cement matrix densification or stiffness gain, even beyond the final setting time. They are, however, lab-based and expensive. The present research mainly aims to address these issues by proposing a dual lead zirconium titanate (PZT)-based embedded transducer to ensure proper curing during construction. This can be achieved by continuously monitoring concrete stiffness gain or setting throughout the curing period, right from casting. The transducer works on the principle of the electromechanical impedance (EMI). Many researchers have explored EMI techniques to monitor the setting of cementitious materials. During the hydration of cement mortar, the synergetic effect of the chemical reactions, shrinkages, and pore reduction effect causes variation in the conductance signature of the bonded or embedded PZT patch. Researchers have studied the signature for changes in amplitude, damping ratio, peak prominence, and peak frequency while monitoring hydration. Similarly, computationally intensive statistical indices such as cross-correlation, correlation coefficient deviation (CCD), mean absolute percentage deviation, and root mean square deviation have also been explored. The performance of the proposed transducer has also been investigated using statistical metrics, considering the EMI signature of the transducer before embedding it in the mortar specimen as a reference. The results have been presented in this study. In the proposed transducer system, the peak shift in the flexural and axial modes of operation can provide information regarding the progress of curing using a simple peak tracking technique. The results are similar to those obtained from CCD. The current study also shows that the frequency shift curves correlate with the isothermal calorimetry heat curve. Other factors reported in the literature that could affect the EMI signatures are temperi ature, water-cement ratio (w/c), curing conditions, and compaction. The current work has conducted the experiment in a temperature-controlled lab environment, with uniform compaction and w/c for each mortar cube specimen. To confirm the effect of temperature on the proposed transducer system, it was placed in a Bio-Oxygen Demand incubator, with both heating and cooling options, allowing it to be set to a fixed temperature. The study measured the EMI signatures around the resonant peak range at different temperatures in steps of 5 C. Although, the overall EMI signature showed variation, the resonant peak frequency reduced by just 0.54% for a temperature rise of 5 C, which may be considered within acceptable error.