Surface opening cracks often occurs in concrete structures. They may appear as a
consequence of several degradation mechanisms such as repeated loading, differential
settlement, chemical attacks, drying shrinkage, and freeze-thaw cycles, among others. While
in some cases, surface opening cracks may only affect the aesthetics of the concrete surface,
in most cases they are an indication of structural distress and/or decreased durability [1]. In
order to evaluate the damage of the concrete structure due to cracking, it is important to
quantify the crack geometric parameters including width, extension, and more importantly the
depth of penetration. Depending on the type of structure, the nature of the cracking, and the
crack penetration depth, surface opening cracks need to be repaired.

Crack depth determination can be performed non-destructively by the time-of-flight
diffraction technique [2-4]. In this technique, stress waves are generated on one side of the
crack, with wave arrival times monitored by a transducer placed on the opposite side of the
crack. Stress waves can be generated by a mechanical pulse, such as given by ultrasound
equipments, or by mechanical impact, as in the impact echo technique. Crack penetration
depth is determined assuming a particular wave propagation path.

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