Creep of normal and high-strength A615 carbon-steel reinforcing bars at elevated temperature

When conventionally reinforced concrete elements are subjected to fire, their carbon-steel reinforcing bars can experience a relatively long duration of modest temperature increase (up to 600 °C) due to the insulation provided by the concrete cover before the element fails or the fire decays. Tensile tests under constant applied stress were performed on 25 M (#8 US) reinforcing bars that conform to ASTM A615 Grades 420, 520, and 690 to measure time-dependent creep at constant temperatures from 400 to 600 °C (i.e., up to the critical temperature per ASTM E119). Creep rates at 400 °C were small for all grades regardless of the applied stress level. Specimens across all grades showed an increase in secondary creep rate when applied stress and/or temperature also increased. Tertiary creep was only observed once the temperature was increased to 600 °C with a higher level of applied stress (∼40% of ambient yield). The experimental data is used to develop predictive expressions based on both the Dorn-Harmathy model and a combined time-hardening model. Both can be used as input for numerical analysis of heated reinforced concrete structures. The models are applied in two simple examples to predict cumulative creep strains when the bars are under constant stress and transiently heated from 400 to 600 °C.