Stress and strain behavior monitoring of concrete through electromechanical impedance using piezoelectric cement sensor and PZT sensor

In this paper, a nondestructive testing method using piezoelectric sensors to monitor the stress–strain relationship of concrete by using the electromechanical impedance technique is proposed. Two piezoelectric sensors—a lead zirconate titanate (PZT) sensor and a piezoelectric cement (PEC) sensor—were investigated. A direct compression test was performed and a resistive strain gauge was used to measure the stress–strain curve of concrete at an age of 28 days. Furthermore, the impedance–frequency spectrum of a piezoelectric sensor embedded in the concrete was measured at every load in increments of 0.1 fc'. The results indicated that, as the load increased, the concrete conductance as measured by the PEC and PZT sensors in the applicable frequency decreased. Therefore, both sensors could be used to monitor the stress and strain during loading. The applicable frequency range for monitoring the stress and strain of concrete was near the second resonance peak as measured by the PZT sensor and at a high frequency exceeding 800 kHz as measured by the PEC sensor. The applicable frequency range of the PEC sensor was broader than that of the PZT sensor, and the use of the PEC sensor afforded the advantages of a low conductance spectrum fluctuation and short analysis time. A regression analysis of the conductance root-mean-square deviation GR and the stress–strain relationship of concrete indicated that the correlation between stress and GR was logarithmic and that between strain and GR was linear. Overall, the results indicated that the proposed approach can be used to evaluate the stress–strain behavior of concrete.