Monotonic and low-cycle fatigue properties of earthquake-damaged New Zealand steel reinforcing bars. The experience after the Christchurch 2010/2011 earthquakes

Abstract

The 2010 and 2011 Christchurch seismic events have highlighted the limitations of the current knowledge in assessing the residual capacity of earthquake-damaged reinforced concrete (RC) buildings. An important challenge during the assessment phase was determining the residual ductility and the remaining low-cycle fatigue life of damaged rebars. Low-cycle fatigue is a possible failure mechanism of steel reinforcing bars when subjected to large-amplitude cyclic loads, such as due to earthquakes. While a single seismic event may not cause rebar failure, the low-cycle fatigue life will be reduced due to plastic strain. Also, New Zealand (NZ)-manufactured Grade 300E is prone to strain ageing. This phenomenon causes a change in mechanical properties, such as increase in yield and ultimate tensile strength, return of a discontinuous yield point, reduction in ductility and rise in the ductile/brittle transition temperature, and must be considered in damage assessment.This paper discusses the effects of strain ageing on the monotonic and cyclic steel mechanical properties. Low-cycle fatigue tests were conducted on Grade 300E steel rebars. Reinforcing bar samples were subjected to constant and fully-reversed strain amplitude cycles. Strain amplitudes ranged from 0.5% to 3%. The strain-fatigue life curve for the un-aged steel was determined. The strain ageing effects on the fatigue life of Grade 300E were then investigated. Specimens were cyclically tested up to the 33% and 66% fatigue life previously determined and “artificially” aged at 100°C. Finally, they were cyclically tested until failure.The experimental data were analyzed and low-cycle fatigue models were calibrated using the Coffin-Manson empirical relationship. Fatigue lives of the un-aged and aged samples were then compared. Preliminary observations suggested that strain-ageing triggers a premature crack initiation which propagates until failure.