Local defect detection using structural health monitoring with semi-non-contact and fully automated instrumentation

An efficient and fully automated global health monitoring approach to detect and quantify local defects in reinforced concrete slabs with minimal equipment and without the need of baseline information is proposed as an alternative to presently used non-destructive test (NDT) methods. Such defects are generally detected locally through a sequence of measurements over a grid at the surface of an element with stress-wave or electromagnetic-based NDT. It is shown that the delineation of local defects can also be obtained by using structural health methodologies, on high-frequency measurements, similar to those from stress-wave NDT methods but with higher efficiency. In this application, data are obtained by using an automated roving laser vibrometer and an automated modal impactor with load cell, eliminating the need to physically move the impactor and sensor over each location of the grid. Experimental measurements were performed on a reinforced concrete slab with built-in defects consisting of delaminations at different depths, debonding, and honeycomb. The location of the automated impactor is kept at a fixed position while the laser beam measurements are performed sequentially at a grid of locations on the surface of the plate. A comparison of the frequency response functions (FRFs) at each point indicates that for resonant peaks above 700 Hz, higher responses are obtained at locations above defects. The spatial contour plots of the FRFs at respective resonant frequencies (i.e. 796 Hz, 1327 Hz, and 1854 Hz) are shown to correspond to the locations of the debonding, shallow delamination, and deep delamination. The defect detection and delineation is compared to results obtained from the C-scan generated with ultrasonic shear-wave tomography, and from impulse-response test data analyzed with a recently proposed statistical pattern recognition procedure. The detection and localization of the local defects with the more efficient automated health monitoring approach is shown to be comparable to those obtained with the two selected local NDT methods. The proposed approach provides a viable process to implement quality control in prefabricated concrete elements that are not currently in-practice.