Non-linear ultrasonic evaluation of damaged concrete based on higher order harmonic generation

Non-linear ultrasonic testing, based on higher order harmonic generation, is a true mean for nondestructive evaluation of concrete allowing damage detection at early stages of damage. Using conventional non-destructive testing methods like pulse echo, velocity and impact echo for evaluation of such damages is difficult because poor sensitivity of these methods to early damage occurrence. In this paper an experimental investigation of 18 cubic concrete specimens, caste with three different water-cement ratios, using non-linear ultrasonic technique is presented. The specimens were ultrasonically evaluated both in damaged and undamaged conditions. The specimens were damaged progressively by loading them under compression in several steps up to their ultimate load bearing capacity. At the end of each loading step ultrasonic evaluation was performed and time domain waveforms were recorded at different power levels. Frequency spectra were prepared by performing Fast Fourier Transformation of the recorded time domain waveforms. The frequency spectra were used to obtain the first three harmonic amplitudes. The captured harmonic amplitudes were used to calculate the second and third harmonic ratios. Comparison of the harmonic ratios has shown the extraordinary sensitivity of the non-linear ultrasonic method, used in this study, to early damage detection. It was also observed that this sensitivity further increases as water cement ratio increases.