Nonlinear ultrasonic test of concrete cubes with induced crack

•The excitation voltage has non neglectable influence to nonlinear parameters.•The higher harmonic ratios are sensitive to the micro defects in concrete.•The β and γ has great potential to detect the segregation in concrete.•The increase trend of β (γ) on micro and macro crack orientation is observed.•Nonlinear parameter γ is more sensitive to the appearance of micro damage. Second and third harmonic ratios method to evaluate initial damage in concrete have been extensively studied. However, some regularities of this technique still not clear in current stage. In order to better understand the regularity of nonlinear parameters β and γ on micro and macro damage in concrete, the cracks with different damage scales are induced in concrete specimens. Three levels of test voltage are applied to intact concrete specimens, with the purpose to demonstrate the excitation voltage has an unneglectable effect on nonlinear parameters. The regularity of nonlinear parameters on crack orientation is also obtained in this study. The results of the experiment have considerable importance with respect to the method of higher harmonic ratios for concrete and other inhomogeneous materials.