Usually for verification of compliance, the fatigue resistance of prestressing steel is determined from tests of
naked specimens at 2 million cycles. However, for design the fatigue resistance of tendons cast into concrete,
is substantially lower. To verify the resistance of existing older prestressed concrete bridges and for the
design of new bridges, S-N curves of prestressing steel in curved steel ducts embedded into concrete are
needed. In bridges, the load cycles due to heavy vehicles may rise up to about 10E8 cycles or even more.
Previous tests with curved tendons in steel ducts primarily cover a range of up to about 20 million cycles.
Thereby no real endurance strength has been estimated jet. Hence the S-N curves given in Eurocode 2 and
Model Code 2010 are defined hypothetically for a range from 10^6 up to 10^8 and are not based on test results.
The reason is that experimental investigations in a range up to 10 8 cycles are very expensive and also demand
a very long duration.
Essential progress results from the development of an optimized test setup that allows a frequency of 10Hz
for the applied load cycles. Therewith, the experimental investigations up to 10^8 cycles have been done by
means of prestressed concrete beams with embedded curved tendons in steel ducts.
Furthermore, procedures to also forecast the lifetime in the case of very low stress ranges respectively the
remaining lifetime of a running test had been developed in conjunction with an interdisciplinary research
project. The procedures are based on refined statistical analysis of the extensively measured data including
increase of crack width, strains, sound emission etc. Additionally the analysis of the latter leads to some
interesting new perceptions
