Experimental study on the fatigue resistance of prestressing strands at low temperature

Cold-drawn high-strength steel for prestressing applications and stay cable bundles is suspected to have reduced fatigue resistance at low temperature between -20° and -50°C (Nürnberger 1981a, Kobrin 1974). For strands as tensile elements of a stay cable, high stress range results from combined low dead loads and high traffic loads and fatigue loading is decisive for the cable design (Svensson 2011). An experimental research project at Ruhr-Universität Bochum investigated the fatigue behavior of 7-wire strands (Rm = 1860 N/mm2) at low temperatures. In total 108 fatigue tests were carried out. The tests were equally distributed at room temperature and at low temperature [-30°C to -50°C]. The specimens were anchored particular in a laboratory anchorage which takes failure on the free length. Similar tests with equal fatigue parameters were carried out with a standard wedge anchorage to investigate the behavior in the anchorage region. In addition to 88 single-stage fatigue tests 20 multi-stage fatigue tests were carried out at room temperature and at low temperature. The multi-stage fatigue tests can be separate in structural durability program tests and in tests with a progressively increasing load range. Latter can be used for an alternative calculation of the endurance strength of strands (Prot 1948). The single-stage fatigue tests at room temperature showed the expected differences between laboratory and wedge anchored strand specimens. Moreover the results showed a tendency to a reduced fatigue strength for laboratory anchored strands at low temperatures. The fatigue resistance decreased to the level of the anchorage zone at room temperature. A superimposed effect for the fatigue resistance of wedge anchored strands at low temperature was not found. The multi-stage fatigue tests confirmed the reduced fatigue resistance of laboratory anchored strands at low temperature. Moreover an underestimation of the material for the cumulative damage calculation with the linear miner rule can be observed.