Subsurface crack determination by one-sided ultrasonic measurements

Corrosion of steel reinforcement in concrete is a common type of damage. The cracks propagate from the steel bar to the surface without giving any visual sign prior to surface crack formation. As long as the surface material is intact, the sensitivity of the longitudinal wave velocity to the subsurface cracks is doubtful. In this paper, cracks were created in steel fiber reinforced concrete specimens by four point bending. Wave measurements took place on the intact surfaces (compression side) using common acoustic emission transducers. Although there was no visual sign of the crack, Rayleigh as well as longitudinal wave velocities clearly decreased relative to those of the sound material. Other parameters like the amplitude and the experimental scatter of the waves were much more sensitive to damage. Numerical simulations were conducted in order to make a parametric study concerning the depth of the sound layer, the propagating wavelength and the measured wave parameters and propose a firm methodology. It is concluded that by scanning a surface with simple acoustic one-sided measurements, the identification of the location of the subsurface damage is possible, while the propagating wave gives information about the depth of the crack.