Bond performance of steel rebar in alkali-activated slag-based concrete after exposure to elevated temperature

Abstract

Bond strength deterioration between reinforcing bars and surrounding concrete is critical to structural performance loss during a fire. However, available studies on the bond behavior between alkali-activated concrete and reinforcing bars at elevated temperatures are insufficient. This study conducted pull-out tests on alkali-activated slag-based concrete (AAC) specimens with three concrete strength grades (C30, C50, and C70) and rebar diameters (14, 18, and 22 mm) at ambient temperature and after exposure to 200, 400, 600, and 800 ℃. The experimental and analytical results showed that AAC pull-out specimen using normal-strength concrete was more prone to splitting failure than Portland cement concrete (PCC) specimens and required greater lateral confinement. The absolute and relative values of residual bond strength for AAC and PCC specimens after high-temperature exposure were similar, which corresponds to their similar thermal degradation of concrete strength. After exposure to 200 ℃, 400 ℃, 600 ℃ and 800 ℃, their rebar bond strength declined rapidly to about 80%, 50%, 15%, and less than 10% of their respective bond strength at ambient condition, respectively. Compared with the counterpart PCC specimens, AAC specimens exhibited greater peak slip and relatively smaller bond stiffness after exposure to elevated temperature due to smaller elastic modulus of AAC. Further, the concrete strength and concrete cover-to-rebar diameter ratio have a significant effect on the failure mode and residual bond strength of AAC pull-out specimens. To evaluate the residual bond property of reinforced AAC after exposure to elevated temperature, an empirical model of bond strength versus temperature and a simplified bond-slip constitutive equation were proposed.