Application of surface wave transmission measurements for crack depth determination in concrete

It has been established in previouspapers that stress wave attenuation meas urements performed on concrete are very sensitiveto thepresence of damage l ocated along the wave path. In this paper, a modified form of this concept is introduced in a self-compensating measurement scheme: surface wave trans mission is obtained across a range of frequencies, between two sensing poin ts on the surface of a concrete specimen. The obtained signal transmission values are independent of the type of wave transmitter and receiver used. A n experimental test series on a range of concrete slabs containing notches and controlled cracks of varying depths is described. The cracks are genera ted by subjecting the slabs to flexure using a closed-loop loading-procedur e. The actual depth of each generated crack is determined with the laser-ba sed phase measurement interferometry technique. Self-compensating wave tran smission measurements are performed across the discontinuities (notches, op en cracks, and closed cracks) and are shown to be repeatable and sensitive to the presence of notches, and even tightly-closed cracks. A relationship between the wave transmission and the crack depth normalized with respect t o propagating wavelength is noted. This relationship appears to be largely unaffected by the type of concrete and the nature of the discontinuity. The results demonstrate that self-compensating wave transmission measurements show excellent potential for sensitive detection and sizing of surface-brea king cracks in concrete structures.