Fire Resistance Ramifications of High-Strength A615 and A706 Rebar

Abstract

Fire is one of the most serious threats that a structure may experience during its service life. Thermal expansion, extreme temperature gradients, and degrading material properties can lead to structural failure. Structural fire resistance is addressed in building codes, and has grown more robust over the years. Past research has studied normal strength (Grade 60) rebar, but information regarding high strength rebar is incomplete. To ensure that the current standards of practice accurately predict the behavior of hot-rolled, high strength rebar at elevated temperature, an experimental study was conducted at Lehigh University. Samples of #8 ASTM A615 Grades 60, 75, and 100 and ASTM A706 Grades 60 and 80 reinforcement were tested in tension using an electrically heated ceramic furnace and a universal testing machine. The stress-strain behavior along with the elastic modulus, yield strength, and ultimate strength were determined through constant temperature tests. A metallurgical microstructure analysis was conducted on samples from testing to observe changes in the steel microstructure. The test methods were modeled after past research and the rate of loading and sample sizes conformed to current ASTM standards. The test results showed good agreement with the current reduction values for elastic modulus and yield strength found in Eurocode. The test results showed poor agreement with the current reduction values for yield strength found in ACI 216 (2014). Eurocode assumes ultimate total elongation to be constant in its material stress-strain model. The test results showed poor agreement with the current total elongation values found in Eurocode. A computational analysis was performed to determine the effect on reinforced concrete beam elements containing rebar with the experimental strength reduction properties. Performance was compared with beams using ACI 216 and Eurocode steel reduction properties. The current data shows that the Eurocode material model for hot-rolled steel at elevated temperature does not fully predict the stress-strain behavior of these bars. Modifications to the Eurocode material model are proposed. The new model is suitable for a performance-based approach to structural fire resistant design of reinforced concrete members