Diffuse ultrasonics for inspection of concrete

The propagation and scattering of high-frequency ultrasound in concrete is discussed. Frequencies above 100 kHz have wavelengths short enough for sensitivity to microcracking. However, the heterogeneous composition of concrete causes the ultrasound at such frequencies to scatter considerably. Theoretical descriptions of the scattering attenuations based on a stochastic wave equation are discussed. These expression require information about the two-point spatial correlation function. The form for this function is proposed and confirmed experimentally. Finally, ultrasound diffusion experiments are discussed. In the limit of many scattering events, the ultrasonic energy density in circular cylinders of concrete is shown to evolve in accordance with a one-dimensional diffusion equation. The ultrasonic diffusivity was measured experimentally over the frequency range of 100-900 kHz. Theoretical descriptions of the diffusivity are in accord with the experimental values. Such frequencies are well above typical frequencies used for concrete inspection. Thus, it is anticipated that the use of these higher frequencies will result in new techniques for characterizing material properties and damage in concrete structures.