Long-term strain-ageing effects on low-carbon steel reinforcement

Abstract

Modern seismic codes are based on capacity design and hierarchy of strength principles that allow the formation of plastic hinges in sacrificial elements within a structure. An accurate estimation of the mechanical properties of the steel reinforcement is required to achieve a ductile failure mechanism. After being subjected to plastic deformation, many types of low-carbon steel experience a time- and temperature-dependent phenomenon known as strain-ageing. This phenomenon causes changes in the mechanical properties such as increase in yield and reduction in ductility. Therefore, in the aftermath of an earthquake event, the hierarchy of strength between beams and columns in a subassembly and/or between flexure and shear within the same structural element of a damaged structural system could potentially be compromised. In this paper, the extent of strain-ageing in New Zealand Grade 300E and Grade 500E reinforcement is investigated through experimental testing in which reinforcing bar samples were pre-strained up to selected strain limits and then subjected to natural (at 15 °C) and accelerated ageing (at 100 °C), at different time intervals, up to one year. The changes in mechanical properties were monitored and quantified. These changes must be accounted for during post-earthquake assessments and repairing design and are recommended to be incorporated in future code provisions.