Laser-Based System for Non-Destructive Inspection of Concrete Structures

A laser-based system for remote non-destructive inspection of concrete structures has been developed. The system is intended for the location of inner flaws in concrete structures, mainly in concrete transportation tunnels. The technique is based on the initiation and detection of standing Lamb waves in concrete layers. Vibration in concrete is initiated by laser impact provided by a pulsed Nd:YAG laser operating at wavelength of 1064 nm. A CW Nd:YAG laser radiation of 532 nm wavelength is used for detection of initiated vibration. The detection is performed with the use of a laser photorefractive interferometer. The interferometer uses principles of dynamic holography via two-wave mixing in photorefractive crystal and homodyne detection. A photorefractive crystal — Bismuth Silicon Oxide (BSO) — is used as a nonlinear medium for the recording of the dynamic hologram. Concrete samples examined in the laboratory experiments reproduce real concrete material used in transportation tunnels. Detection of various types of inner defects like voids, slits and honeycombs has been demonstrated. Accuracy of defect location is better than 2 cm. A mobile model of the system has been assembled and demonstrated in the field conditions.