Identification of subsurface damage in concrete using one sided wave measurements

Surface deterioration is the most common type of concrete damage. In the case of subsurface damage the difficulty of characterization increases as there is no visual evidence of the crack. Additionally since the close to surface layer of the material is intact, the sensitivity of the longitudinal wave velocity, which is typically measured for inspection purposes, is questionable. In the present paper, cracks were created in steel fiber reinforced concrete specimens by four point bending. Wave characteristics were then measured on the intact surfaces (compression side) using common acoustic emission transducers. It was seen that although there was no visual sign of the crack, Rayleigh as well as longitudinal wave velocities were influenced showing clear decrease relatively to the sound material. Additionally other parameters such as the amplitude or energy of the waves were much more sensitive to damage. In order to explain the results, numerical simulations were conducted making a parametric study between the depth of the sound layer, the propagating wavelength and the measured wave parameters. 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 form and depth of the crack.