Evaluation of near surface material degradation in concrete using nonlinear Rayleigh surface waves: Fid-Bau Portal

Evaluation of near surface material degradation in concrete using nonlinear Rayleigh surface waves

Gross, J. / Kim, J.Y. / Jacobs, L.J. / Kurtis, K.E. / Qu, J.

Rayleigh surface waves are used in this research to develop a field applicable technique for ultrasonic evaluation of near surface damage in concrete. The wave mixing technique for Rayleigh waves propagating along the surface shows a great potential for the evaluation of carbonation damage in concrete. The sum of two independent sinusoidal waves, driven by two different frequencies, is used to excite a low frequency wave. This wave arises through the interaction of the two ultrasonic waves with the nonlinear material, at the difference frequency of the fundamental frequencies. In this way a very low frequency wave could be generated by still using a 50kHz transducer. The reduction of the frequency has the advantages that side effects like scattering, local effects due to inhomogeneity of the material and especially attenuation can be decreased with larger wavelength, which makes the ultrasonic wave more sensitive to actually investigated material nonlinearities. A comparison between the carbonated and the undamaged reference sample is performed with the result that the nonlinearity of the carbonated sample is larger than that of the undamaged sample. However, in this work only two specimens arc considered. Concrete is a multi-phase material and its nonlinearity can have significant variability, even before the damage occurs. Nonetheless, the goal was to develop a method that is capable of reliably measuring the nonlinearity of concrete materials using Rayleigh surface waves to make it also applicable in field. The wave mixing technique shows all necessary features to meet these requirements. The nonlinearity of the samples can be reliably measured and the results can be reproduced with a good repeatability. For future work on this method it is recommended to test different coupling materials for the accelerometer, to make the measurement procedure less laborious. The material evaluation, e.g. detection of carbonation, has to be performed on more than two samples, also the nonlinearity of the specimen should be measured before and after undergoing the carbonation process to be able to determine the effect of carbonation on the nonlinearity of the material. The effect of the chosen frequency pair has to be investigated further.