Resistance of prestressed concrete structures to fatigue in domain of endurance limit

Abstract

Due to the disproportionately increase of traffic over the last decades, the number of load cycles from heavy trucks imposed on bridges has grown as well. In particular existing older bridges were generally not designed for current and future traffic load. Therefore, in many cases, deficiencies concerning the ultimate resistance, the resistance to fatigue and the serviceability limit states have been identified. As a result of the high increase of traffic, the number of load cycles up to 10E8 might be reached over the lifetime of a bridge. The S-N-curves were empirically determined under constant amplitude loadings according to recent valid standards. However, in the past the conducted experiments on fatigue behavior of post-tensioned prestressing steel in steel ducts were limited to 2x10E7 load cycles. In the course of various research projects at the TU Dortmund University, eleven large scale tests on prestressed concrete beams have been conducted in total. One of the test girders reached a maximum of 108,250,000 number of load cycles with a stress range of 50 MPa. The ensuring result of the test series was, that even with a low stress range (50 MPa), it was not possible to determine the endurance limit. These findings bring into question the assumption for slope coefficient (k2 = 7) of second branch according to current standards. Beside the experiments under laboratory conditions, investigations via monitoring on an existing bridge with a deficient fatigue strength has been started as well. The long-term objective is the installation of sensors on bridges for the observation of the fatigue behavior. If damage accumulation results in a critical state, prompt initiation of necessary measures can be conducted.