Nonlinear Ultrasonic Testing with Resonant and Pulse Velocity Parameters for Early Damage in Concrete

Many types of damage or deterioration in concrete cause microcracking. Damage usually progressed from evenly distributed microcracks, to major cracks, to eventual failure of the concrete. This article reports on a study undertaken to compare the effectiveness of a nonlinear ultrasonic testing method with established ASTM testing methods in detecting early damage in concrete. In this study, geometrically identical concrete specimens were evaluated with nonlinear ultrasonic techniques. When the fundamental ultrasonic frequency interacts with a material, harmonics are generated. As damage increases, the magnitude of the nonlinear interaction increases, causing a greater portion of the fundamental frequency to be converted to higher harmonics. The authors included specimens of three distinct water-cement ratios which were compressed at regular load increments to induce damage. At each load increment, no cracking was visually observed. However, with increases in damage to the specimens, increases in harmonic ratios were measured. These increases were markedly larger as compared with changes noted with other nondestructive ASTM tests performed. The authors conclude that the nonlinear ultrasonic method shows considerable promise as a sound, nondestructive evaluation technique for detecting early damage in concrete. Sensitivity of this method generally increased with increasing water to cement (w/c) ratio.