Influence of quasi-static strain rate on the stress-strain characteristics of modern reinforcing bars

Abstract

While it has been known for decades that even for quasi-static loading, increments in strain rate lead to increased stresses in the inelastic range, this effect is often ignored. However, accurate knowledge of the stress-strain characteristics of reinforcing bars is an indispensable prerequisite for the safe design of new structures and the realistic assessment of the structural safety of existing ones, and the strain rate dependency of the reinforcing steel characteristics should thus be accounted for in many situations. An exemplary case is quality control: Reinforcing bars produced today are periodically tested to check their conformity with specifications, determining their stress-strain characteristics in standard tensile tests. However, the applied quasi-static strain rates may vary considerably, but are not commonly reported. Hence, the results are subject to considerable uncertainty. Another relevant case is the structural safety assessment of existing structures affected by local corrosion of the reinforcement: Their cross section (and hence, stiffness) varies considerably along the bar axis and consequently, the strain rate in corroded sections is significantly higher than in non-corroded sections, leading to higher yield stress and tensile strength. This study investigates the effect of quasi-static strain rates on the stress–strain characteristics of modern reinforcing bars based on a comprehensive experimental campaign. In four series of experiments, 41 tensile tests on three different types of reinforcing bars were conducted, applying strain rates between 0.004 ‰/s and 1.0 ‰/s. Compared to the static stress, an increase of up to 8% in the dynamic stress was observed, depending on the type of reinforcing bar. Based on these observations, a simplified model for the strain rate dependency was developed and validated against experimental data, showing excellent agreement.