Bond behavior of reinforcing steels in steel fiber reinforced sulfoaluminate concrete after elevated temperature exposure

Forty-eight specimens arranged into sixteen groups were used in experimental testing to evaluate the bond behavior between reinforcing steel and steel fiber reinforced sulfoaluminate cement concrete (SFRSC), focusing on the effects of elevated temperatures, steel fiber volume fractions (SFVFs), and concrete strength grades. The experimental results indicated that the peak bond strength of SFRSC decreased by 45.9%, 74.7%, 76.0%, 84.9%, and 97.2% after being exposed to elevated temperatures of 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C, respectively, in comparison with the unheated condition. This deterioration is primarily caused by the decomposition of hydration products at relatively low temperatures, the reduction in steel fiber strength, and the weakening of the fiber-bridging effect. Scanning electron microscopy (SEM) observations showed that exposure to elevated temperature induced cracking in the interfacial transition zone (ITZ) surrounding the steel fibers in the SFRSC matrix, and the cracking became more pronounced with increasing temperature. Under room temperature conditions, increasing the SFVF to 1.0% and 1.5% enhanced the peak bond strength by 18.0% and 44.9%, respectively. Meanwhile, increasing the concrete strength also significantly enhanced the bond strength of SFRSC. Under elevated temperature conditions, the strengthening effects of increasing SFVF and concrete strength were significantly reduced. On the basis of the thick-walled cylinder model, the fictitious crack model, and the present experimental results, a calculation model for predicting the bond strength of SFRSC was developed, and the corresponding bond stress – slip curves after elevated temperature were proposed. Comparative analysis indicated that both the predicted values and experimental results, as well as the predicted curves and experimental curves, showed good agreement, accurately describing the bond characteristics of SFRSC after exposure to elevated temperatures.