Mechanical tensile testing and metallurgical investigation of the residual properties of stainless steel reinforcing bar after exposure to elevated temperatures

Abstract

This thesis examines the behaviour of stainless steel reinforcing bar following exposure to elevated temperature. Stainless steel reinforcement in concrete is an increasingly popular structural solution for a variety of applications where corrosion resistance, excellent mechanical properties and long life-cycles with little maintenance are required. Prior to this work, there was no information available in the published literature on the post-fire properties of stainless steel reinforcing bar, although this data is vital for an engineer wishing to study the structural integrity of a reinforced concrete component or system following a fire. Accordingly, this thesis presents a detailed analysis and discussion on both the tensile and metallurgical behaviour of stainless steel reinforcement, following exposure to various levels of elevated temperature of up to 900°C and also three different cooling methods, rapid-cooling in water, natural-cooling in air and slow-cooling in a furnace. The study includes austenitic stainless steel reinforcement in grades 1.4301, 1.4401 and 1.4436 as well as both hot-rolled and cold-worked duplex stainless steel bars in grade 1.4362, with a comparative carbon steel B500B grade also assessed. Within the post-fire temperature segment of this thesis, an increase in strength of up to 15% is noted in the stainless steel following cooling after elevated temperature exposure, with consistent responses between the different cooling methods. The austenitic grade reinforcement in particular presented a very stable microstructure across the various testing regimes, whilst the duplex reinforcements manifested a more unstable microstructure in the post-fire temperature testing. In the final part of this thesis, elevated temperature tests are conducted under isothermal conditions on austenitic grade 1.4301 and duplex grade 1.4362, to study how their mechanical behaviour evolves with increasing levels of temperature exposure. The study covers the most common stainless steel reinforcing bar grades available on the market and focuses on recreating practical and realistic fire scenarios where possible. The tests conducted show how the current design standards, currently based on carbon steel, are inadequate for efficient design of stainless steel RC structures and could benefit from independent guidance. To conclude, this thesis presents independent recommendations and guidance for engineers to assist in calculating the structural integrity of a component or system following a fire.