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This paper deals with the issue of detection of internal defects and failures of concrete structures with emphasis on the use of non-destructive methods. All parts of the structure that reduce its service life or bearing capacity can be considered as internal defects and failures. These are, for example, gravel nests, cracks, delamination, caverns etc. Due to the prevalence of concrete structures, the development of this part of diagnostics is very important both for the safety of their users and to reduce the economic costs of their future repairs.

This article deals with defects in solid fired bricks caused by repeated freezing and defrosting. Defects can be divided into surface (visible) such as cracks or chips of the test element. The second type of defects are those in the internal structure (invisible on the surface). The defects in the internal structure can be the easiest detected by non-destructive methods such as the resonant pulse method or the ultrasonic pulse method. This article deals with the possibility of detecting defects in the internal structure of solid fired bricks and their relation to surface defects. The test elements were repeatedly frozen and defrosted, and after a certain number of cycles, changes in the internal structure were monitored by the resonance method and by the ultrasonic method. The variable for monitoring the failure of the internal structure is the relative dynamic modulus of elasticity (RDM).