Ultrasonic evaluation of surface-breaking crack depth in steel-fiber reinforced concrete

In practice, the commonly used P-wave based ultrasonic inspection tests for concrete are adequate to evaluate either material thickness or its elastic properties. In general, the test configurations of such methods are not suitable for the estimation of the depth of surface-breaking cracks, which is crucial to predict the residual load capacity of a structural member. The methods based on the ultrasonic surface waves (UWS), on the other hand, have the potential to realize such inspection. Since the surface waves propagating in a medium display variation when they encounter with a vertically aligned crack, it is possible to estimate its depth by monitoring the changes in the wave characteristics. Based on this principle, hereby a UWS based diagnostic procedure is presented for the estimation of depth of surface-breaking cracks in concrete elements. The diagnosis is demonstrated on six lab-scale steel-fiber reinforced concrete (SFRC) beam specimens, which are subjected to the crack-controlled three-point bending test in order to create a specific crack depth in each one. The beams are then examined by implementing a multi-channel ultrasonic test configuration. The recorded surface waves propagating in the beams are processed by utilizing the signal processing techniques, including wavelet transform and two-dimensional Fourier transform to extract two diagnostic features related to the wave attenuation and dispersion in phase velocity. Finally, the reliability of these two features are demonstrated by examining their sensitivities with respect to the visually measured crack depths.