Mechanical Characterization of Normal and High-Strength Steel Bars in Reinforced Concrete Members under Fire

Abstract

This study investigates the high-temperature mechanical response of deformed steel bars used in the United States (ASTM A615 and A706, all grades) for the construction of reinforced concrete structural members that are at risk of fire exposure. Bars meeting both ASTM standards with nominal yield ranging from normal (420 MPa) to high strength (up to 690 MPa) were tested to fracture using a universal testing machine in combination with an electric split-tube furnace. A full stress–strain characterization at temperatures from ambient to 800°C was obtained, and all grades exhibited similar reductions in strength and stiffness as well as strain ductility at ultimate and fracture as a function of increasing temperature. Based on the experimental results, a modified version of the Eurocode 2 stress–strain model for hot-rolled steel rebar at elevated temperature is proposed. The reductions in steel ductility that are introduced by the proposed model are examined in a numerical study. A simple prototype floor beam, designed to have the same nominal strength using each grade of rebar, is analyzed for fire resistance according to ASTM E119 thermal and deflection criteria. The numerical results indicate that the reductions in strain ductility in the proposed model can reduce flexural performance for fire-exposed sections that use higher strength rebar grades. Also, reduced minimum cover requirements that are enabled by the use of higher strength bars with smaller diameter will allow faster temperature increases in the steel reinforcement. As a result, the fire resistance of the floor beam may be reduced in some cases below standard predictions based on nominal strength.