Nonlinear Ultrasonic Nondestructive Evaluation of Damaged Concrete Based on Embedded Piezoelectric Sensors

We studied the acoustic signal responses of the concrete in undamaged and damaged conditions and different depths of sensors embedded in the concrete. The acoustic signal responses of the concrete under different applied loads were analyzed by using time domain waveform and frequency domain spectra. The results showed that the change of the ultrasonic wave amplitude was sensitive to the crack of the concrete surface at the microscopic level when the depth of sensors embedded in the concrete was 20 mm. The first pulse amplitude level, the peak-to-peak value, and the third harmonic amplitude reduced by 5%, 20%, and 35%, respectively, when the applied load to concrete was 10 N·mm−2. When the sensors were embedded in concrete with a depth of 60 mm and applied with various loadings, via analyzing the acoustic signals, the internal cracks and extensive damage of concrete were detected. When the applied loading reached 35 N·mm−2 and the maximum value, the peak-to-peak values of waveform were reduced by 15% and 60%, respectively. In addition, the harmonic ratios () considerably increased with increasing applied loadings.