Post-fire mechanical properties of low-relaxation hot-dip galvanized prestressed steel wires

Low-relaxation hot-dip galvanized prestressed steel wires are the basic component materials of high-strength steel cables, which have wide applications as key load-bearing members in prestressed steel structures. Provided that the general appearance of the prestressed steel structure is acceptable after a fire event, how the behaviors of the steel cables in these structures have been affected must be estimated accurately to ensure safety. Therefore, a series of experiments with a total of 360 specimens was conducted to investigate the post-fire mechanical properties of low-relaxation hot-dip galvanized prestressed steel wires with various grades, namely, 1670, 1770, 1860, and 1960. Tensile coupon tests were performed on specimens after exposure to 13 preselected temperatures up to 1000 °C, where two different cooling methods, namely, air cooling and water cooling, were considered. The post-fire stress-strain curves, elastic moduli, yield strengths, ultimate strengths, and ductility were obtained. Additional tests were also conducted to study the effects of cyclic heating-cooling. The post-fire mechanical properties of the studied steel wires changed significantly after exposure to temperatures exceeding 400 °C, and the change characteristics of different steel wire grades were similar. Moreover, the influences of different cooling methods were notable: the water-cooled steel wires lose most of their ductility and strength when the exposure temperature exceeded 700 °C; whereas the effects of cyclic heating-cooling were insignificant. Thus, predictive equations that incorporate the influences of different cooling methods were developed to evaluate the post-fire mechanical properties of the steel wires studied.