Nonlinear Ultrasonic Second Harmonic Assessment of Concrete Defects Based on Embedded Piezoelectric Sensors

In this work, the second harmonic generation (SHG) technique based on embedded piezoelectric (PZT) sensors is developed for the defect assessment of concrete samples. Three different imperfection forms of concrete including internal capillary void during the curing process, cracking damage due to compression, and bending loads are examined and analyzed. Clear second harmonics are observed by Fourier transform of time domain signal, and the square of fundamental signal amplitude shows a linear relation with the second harmonic amplitude as theoretically expected. The nonlinear parameter presents a good distinction for samples in different states of three cases, indicating the well-round feasibility of SHG technique based on embedded sensors. In addition, the nonlinear parameter presents an excellent correlation with the variation of internal capillary void and cracks in concrete. The high sensitivity of the developed SHG technique is further validated through a comparison between the nonlinear parameter and two traditional linear parameters, namely the phase velocity of Rayleigh wave and the resonance frequency of vibrations. Experimental results in this study demonstrate that the embedded sensors are promising for the nonlinear ultrasonic nondestructive evaluation of concrete structures, particularly as a low-cost alternative of commercial ultrasonic transducers.