1Katedra stavební mechaniky, ²Experimentální centrum, Stavební fakulta, České vysoké učení technické v Praze, Thákurova 7, CZ – 166 29 Praha, Česká republika.

Gradual degradation happens to the bearing structure of a concrete bridge. The reasons of this effect are especially a repeated great static loading, a dynamic loading, alkali silica reactions and a frost-thaw influence. The importance of an observation on real structures in order to obtain the experimental results necessary for the development of theories for prediction of a service life increases. The paper describes the results of periodic change monitoring of modal parameters of two road bridges made from prestressed concrete.