Ultrasound diffusion for crack depth determination in concrete: Fid-Bau Portal

Ultrasound diffusion for crack depth determination in concrete

Ramamoorthy, S.K. / Kane, Y. / Turner, J.A.

The use of high-frequency ultrasound for inspection of concrete slabs is discussed. For frequencies above 100 kHz, the scattering from the microstructure is significant. In the limit of many scattering events, it is shown that a two-dimensional diffusion equation may be used to accurately model the scattering process. Experiments on unreinforced concrete slabs were conducted using varying source/receiver separation distances over a frequency range of 100-500 kHz. It is shown that both the elastic wave diffusivity and the dissipation may be extracted from these measurements. In addition, the use of ultrasound diffusion for determining the depth of surface breaking cracks is discussed. Current methods for measurement of crack depth are dominated by coherent field methods such as by surface waves. These methods have the limitation that the penetration depth is limited to the wavelength. For the case of ultrasound diffusion, the arrival of the peak diffuse energy at the receiver is delayed relative to the uncracked case due to the presence of the crack. This delay is the prime indicator used for determining crack depth. Experiments were carried out on pre-cracked concrete specimens of varying crack depth (0 - 40 % of the specimen thickness). Contact transducers were placed at the specimen surface with source and receiver separated by the crack. Tone bursts over a frequency range of 400-600 kHz were used. The delay in energy arrival is shown to scale with the square of the crack depth. Thus, this technique offers improved detectability for deeper cracks. Aspects for practical implementation, in terms of real cracks, signal to noise ratio, and data acquisition are also discussed.