A Wave-Propagation-Based Approach to Estimate the Depth of Bending Cracks in Steel-Fiber Reinforced Concrete

Estimation of the depth of surface-breaking cracks caused by bending in concrete has crucial importance in order to predict the remaining load capacity of the structural member. In practice, ultrasonic tests are the most common methods to assess the condition of concrete. However, the commercial ultrasound-based methods focus on the estimation of thickness of the structural element rather than the crack depth. The cracks cause dispersion and attenuation in the propagating waves, and thus by monitoring the changes in these wave characteristics, diagnostic indexes correlated with the crack depth can be defined. This paper explains this approach performed on six laboratory-scale steel-fiber reinforced concrete beams (50 × 10 × 10 cm3). Each beam is loaded under the crack-controlled three-point bending test until a specific crack depth is reached. These beams are then subjected to ultrasonic testing to acquire the propagating surface waves. The recorded signals are analyzed by utilizing signal processing techniques, including discrete wavelet transform and frequency-wavenumber analysis in order to obtain the attenuation and dispersion behavior of the surface waves. Finally, these wave characteristics are exploited to extract diagnostic features which estimate the crack depth. Herein, the preliminary results for the crack depth estimation based on these proposed diagnostic indexes are presented.