Efficiency and sensitivity of linear and non-linear ultrasonics to identifying micro and macro-scale defects in concrete

This study describes a useful contribution to the ultrasonic non-destructive evaluation of both micro and macro-scale defects or damages induced in concrete under initial and peak level load applications, respectively. The presented research findings are based on measuring linear and non-linear ultrasonics parameters. For this purpose 45 cubes of 150. ×. 150. ×. 150. mm and nine cylinders of 100. ×. 200. mm were produced using three distinct water-to-cement ratios (w/. c) i.e. 0.40, 0.50, and 0.60, respectively. The cylindrical specimens were used for measuring the 28-days concrete compressive strength. The cubic specimens were loaded in various steps representing different damage levels up to failure, selected from the measured compressive strength of the tested cylinders. The assessment of the specimens at each loading step was performed by linear and nonlinear ultrasonic techniques with several output power levels. The time-domain, frequency and normalized spectra were plotted for the wave signals, obtained at each loading and power levels. The arrival time and harmonic amplitudes were carefully measured in order to estimate the pulse velocity, wave attenuation, and harmonic generations. The measured values were compared and discussed for different power and damage levels in the tested specimens.