The production of concrete works in the construction of monolithic frames of buildings and structures is associated with a large number of technological operations and the corresponding dependence of the quality of the products obtained from the culture of construction production and the level of its control. In many cases, concreting is carried out with some deviations from the technical regulations governing the preparation and laying of the concrete mix, which has a negative impact on the consumer characteristics of the structure, first of all, on strength. It is known that the most common methods of industrial control of the quality of concrete work are destructive tests of concrete samples laid in control cubes together with the constructed structure and non-destructive testing of concrete of the strength that has gained strength. However, even qualitatively and timely carried out control measures do not guarantee the achievement of design-controlled design strength throughout the volume, since control cubes are prepared for testing under ideal conditions of laying and concrete set of strength not observed in the design, and non-destructive methods determine the strength of near-surface layers of concrete, which allows judging reliably the strength of the material throughout the volume of the structure. Especially these circumstances relate to the most massive and, at the same time, the most important for the structural safety of the whole building frame structures – monolithic foundation slabs. The difference between the actual strength of the slabs extracted from the slab during the production of the construction and technical expertise of concrete cores depends significantly on the depth of core extraction, while the design of the slab is always carried out on the as-sumption of an equal strength of the concrete along the depth of the structure, which is obviously the source of the potential limited availability of foundation plates and the cause of the appearance of numerous defects in the construction and operation of structures of the above-foundation part of the frame. The paper presents experimental studies of the authors to determine the actual difference in concrete strength from the depth of foundation slabs and quantifies it.