Geometry measurement of subsurface cracks in concrete from ultrasonic images

Abstract

Geometry measurement of concrete subsurface damage is as crucial as detection and localization for condition evaluation. Ultrasonic reflection-based imaging can map subsurface reflector geometry, however direct measurements from images are inaccurate. In this study, amplitude of imaged reflectors is used to accurately measure geometry of horizontal and inclined reflectors in concrete subsurface. Four statistical parameters have been tested for suitability of measurement by studying their performances with respect to crack geometry (length and angle) and positions in concrete using simulations. These tests show that the ‘max’ parameter measured reflector geometry with the least error. The technique is frequency independent in the commonly used 50 to 150 kHz range of concrete inspection. For validation of the technique, experimental studies were performed on three concrete specimens with various embedded linear reflectors and one specimen with multilinear reflectors. Isolated horizontal reflectors and isolated inclined reflectors in concrete were measured with minimum 98%, and 94.5% accuracy, respectively. Compared to the prevalent pixel count approach and decibel drop techniques in medical field and metal NDT, the proposed technique performs significantly better. Measurement of multilinear and multiple-originating cracks achieved at least 90% accuracy. The technique was also tested on an arbitrary oriented crack in a three dimensional setup with significant accuracy.