Methodology of ultrasonic investigation of a natural crack or purposely-formed slot is based on taking advantage of the effect of wave diffraction at crack end. In reinforced concrete structures steel reinforcement placed across the crack is an element of medium continuity, and is a cause of difficulty in investigations. Direct transmission of ultrasonic pulse along the reinforcement is producing the earliest reception effect. This is interfering with time-amplitude recording of waves diffracted at the slot bottom or in crack decay zone and presents an important difficulty for investigations. The subject of this paper is indication of filtering-off possibility of this interference in crack depth investigations on the basis of two independent measurements. Eccentric arrangement of transducers relative to the slot is preferred, as well as two pulse transmission measurements over the same path, but in opposite directions. Pulse reflection effects by the opposite and side walls of the investigated element do not cause additional interference in time measurements of the longitudinal wave diffracted at the crack end.
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