**5. Conclusion**

This study evaluated the application of PZT sensors for monitoring the strength development of high strength concrete. The applicability of the conventional impedance measuring technique, which is normally used to detect damage, was extended to monitor the curing process of concrete. The impedance signals were obtained at six different curing ages. The compressive strengths of the test concrete cylinders were also evaluated by considering the resonant frequency variations and cross-correlation coefficient. Based on the experimental results, the resonant frequencies in the impedance signals shifted gradually to the right side with increasing curing time, which confirms the applicability of impedance measurements to monitor the strength development of concrete. The largest deviation of the resonant frequency shift was observed between days 3 and 5, and the change decreased with time. In addition, the 1-CC values increased due to strength development during the curing process. A wireless impedance system showed similar results to that of the wired impedance system. Therefore, a wireless system that can improve the applicability to a construction site can be used to monitor the strength development of concrete. Consequently, the wireless strength development monitoring system for concrete can be employed comfortably in construction sites. Furthermore, piezoelectric sensors that monitor the strength development can be used for structural health monitoring (SHM) after construction. In addition, embedded curing monitoring and a SHM system for high strength concrete can be developed to improve the applicability and efficiency of this system.
