Behavior of high strength reinforcing steel rebars after high temperature exposure: Tensile properties and bond behavior using pull-out and end beam tests

Abstract

High strength reinforcing steel (HSRS) is increasingly being adopted in the construction industry in North America due to its high load-carrying capacity, yet their behavior relative to normal grade rebars under external hazard events such as fire exposure still need further research. In current work, the first goal is to investigate the residual tensile properties of different types of HSRS and normal rebars including ASTM A706, A615, A1035CS, and A1035CM with grades of 420, 550, and 690 before and after high temperatures (350 °C and 700 °C), that simulates residual capacity after different levels of fire exposure. The second goal is to explore the residual bond strength of these rebars using both pull-out and end-beam tests after heat exposure, facilitating a comparison among test methods and evaluating their reliability under fire hazard conditions. The tensile test results indicated insignificant changes in the mechanical properties of all rebars after 350 ℃ heat exposure. However, there was a significant reduction of more than 30% in both yield and ultimate strength after 700 ℃ heat exposure for high grades like grade 690 of ASTM A1035. Among all rebar types and grades, only grade 420 of ASTM A706 retained most of its ductility and strength, making it the most reliable rebar among those tested against fire hazards. In the bond tests, splitting failure was observed at both ambient and after 500 °C heat exposure. Heat exposure resulted in a 73% reduction in bond strength across all rebars for the pull-out tests, with no significant variations between rebar types. On the other hand, end-beam tests indicated a 12.6% average reduction in bond strength with a 9% improvement in bond behavior for ASTM A706 grade 420. However, for ASTM A1035 CS grade 690 and ASTM A706 grade 550 there were slight reductions in bond behavior, with decreases of 9% and 14%, respectively. Compared to the end-beam test, the pull-out test results in a significant reduction in bond strength due to its high dependency on excessive compressive strength reduction resulting in brittle splitting failure. Based on comparison with empirical models, it is recommended not to use models derived from pull-out tests to estimate bond strength, as they tend to overestimate it. Instead, empirical models based on end-beam tests are recommended, since they show consistency with test results. It should be noted that although the bond strength based on splitting failure is primarily dependent on concrete properties, changes were observed in the bond behavior among different rebars, which can be attributed to changes in friction at the interface due to morphological changes in the rebar surface after heat exposure, as well as potential variations in thermal expansion properties, which need further research.