Macro-crack characterization in concrete by diffuse ultrasound under low frequency dynamic loading
Résumé
In concrete, a crack reveals a morphology comprised of an external part with a surface opening, and a completely or partially closed part below the surface with potentially partially opened zone. Conventional ultrasound methods defined by ASTM or ISO normalization are founded on low frequency waves time of flight measurement. Consequently, they are unable to detect a few centimeter scale surface breaking crack. At higher frequencies, strong multiple scattering phenomena occur due to the presence of aggregates, making impossible standard waveform analysis. The main issue of this paper is to use higher frequencies and to take advantage of multiple scattering by the study diffuse waves transport and its interaction with partial contacts along the crack. The ability of diffuse ultrasound to characterize the opened zone of the crack is shown. However, the closed part of the crack remains imperceptible. To overcome this difficulty, a low frequency dynamic loading is superimposed to diffuse waves in order to alternatively open and close portions the crack. Experimental results associated with numerical simulations show the validity of the method and should subsequently provide physical interpretations about the morphology of a real macro-crack in concrete.