**5. Conclusions**

Despite meeting the requirements for industrial floors tested by the standard method, local fluctuations in compressive strength values in surface cross-sections are a frequent reason for their failure conditions.

In industrial floors made of concrete without surface hardening, the weakest layer is the top layer 10–50 mm thick. The strength of such thin layers can be tested using the ultrasound method with spot head on borehole materials taken from the structure.

Determination of the distribution and dynamics of concrete compressive strength changes throughout the entire cross-section allows, for example, for the establishing of the thickness of the layer which must be removed during repair of the floor. The reconstructed layer must be laid on concrete having the necessary strength (typically fc ≥ 20 MPa).

Concrete floors analyzed in the article can be divided into three types based on the curve showing the distribution of strength across their thickness. In this case, the one-parameter evaluation of concrete in the floor, based on the standard destructive compressive strength tests, is not sufficient. The addition of a parameter, which is a strength gradient, was proposed. Floor concretes can be controlled based on the tests of concrete compressive strength class and its gradients across the floor thickness.

**Author Contributions:** Conceptualization, B.S. and T.K.; methodology, B.S.; validation, B.S. and T.K.; investigation, T.K. and B.S.; resources, T.K. and B.S.; writing—original draft preparation, B.S. and T.K.; writing—review and editing, T.K. and B.S.; visualization, T.K.; supervision, B.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
