Non-destructive evaluation of concrete in damaged and undamaged states

This paper presents findings on nonlinear ultrasonic testing of concrete. The study was focused on testing cubic concrete specimens. It consisted of the non-destructive evaluation of concrete cubes using nonlinear ultrasonic technique with different frequency transducers. Concrete cubes of 150 mm × 150 mm × 150 mm in size and three distinct w/c 40%, 50%, and 60% were examined in damaged and undamaged states. Prior to inducing damages, the concrete cubes were ultrasonically evaluated in undamaged conditions. The damages were induced into the concrete cubes under compression testing. The transducer used at the wave-transmitting end had a broadband frequency of 100 kHz. However, at the receiving end, transducers of 200 and 300 kHz broadband frequency were used. The test results obtained for each concrete cube, in both damaged and undamaged conditions, were plotted and compared. The ratios of harmonic generations, nonlinear material parameters, and attenuation or loss in fundamental amplitude were calculated. The performance obtained by using transducers with different frequency as receivers in measuring the higher harmonics was also examined. The nonlinear ultrasonic test method using low frequency transducer as receiver exhibited greater sensitivity over high frequency transducer. Similarly, the third harmonic ratio was found more sensitive to concrete cracking than the second harmonic ratio. As concrete is a high attenuation material, the change in fundamental amplitude or attenuation was also found sensitive to using transducers with different frequency. The test results were also analyzed to find correlation between w/c, wave attenuation and higher harmonic generation based nonlinear parameters. These findings are very helpful for understanding the behavior of damaged concrete sections under nonlinear ultrasound testing.