Interaction of ultrasonic waves with partially-closed surface-breaking cracks in concrete structures has been studied. Measurements have been conducted on a reinforced concrete beam containing various mechanical-load-induced cracks and compared with the baseline measurements at those locations. Influence of crack width, incident angle of waves with cracks, and distance from the cracks on travel time and amplitude of the waves have been investigated when the beam was unloaded. It has been observed that a measurable part of the waves propagate through the cracks due to the acoustic coupling between the crack faces, although attenuation can be relatively high. The travel time has shown a nearly independent behavior from remaining crack opening in the measured range of 0.05 mm to 3 mm. Measurements in directions orthogonal and parallel to the crack suggest that there is substantial anisotropy in the cracking zone. Furthermore, an effective width of the micro-cracking area around the cracks has been estimated from the measurements.

Asphalt materials generally exhibit temperature-dependent properties, which can influence the performance of fatigue crack inspection and monitoring systems for bridge deck structures. For a non-intrusive fatigue crack sizing methodology applied to steel decks using ultrasonic guided waves, the effect of asphalt has been investigated. A higher-order spectral finite element model has been implemented to capture the propagation characteristics of guided waves in the multi-layer waveguide. Experimental evaluation has been performed on an aluminum plate covered with bitumen. The results reveal that the extent of the effect of asphalt on propagation of guided waves depends on the type and layout of the asphalt. For a particular asphalt type of interest, i.e. open asphalt surfacing, however, this effect appeared to have limited influence on the performance of the crack sizing methodology, i.e. in the order of a few percents. In the analysis, temperature variation, and uncertainties in the material properties and asphalt bonding condition have been taken into account.