The development of a diagnostic system is a complex engineering problem aimed at preventing damage caused by deteriorating properties of the materials and structures. This article presents the method of nondestructive testing of change control of concrete structures. The analysis of literature data shows that the overwhelming majority of studies are devoted to the change in electrical resistance of concrete in the collection of its strength, and practically no analogous studies regarding its aging are presented. Theoretical substantiation of the change in electrical concrete characteristics (conductivity, resistance) is given with a change in its strength. A work over-bridge in operation more than 30 years has been surveyed. The experimental part consisted in measuring the electrical potentials on the bridge structures, as well as in determining the strength of the material at the same points. Correlation dependence between the measured electric potential value and the strength of the samples taken from the bridge structures were obtained.