Flexural toughness characteristic of steel fiber reinforced concrete under high temperatures: Experimental, numerical, and machine learning study

Investigating the post-fire flexural toughness of steel fiber reinforced concrete (SFRC) is crucial for ensuring structural safety and reliability. Therefore, flexural toughness tests were conducted on 48 SFRC specimens, with temperature ( T = 20 °C, 200 °C, 400 °C, and 600 °C) and steel fiber (SF) volume content ( V sf = 0%, 1%, 1.5%, and 2%) as variables. The effects of T and V sf on the flexural strength and flexural toughness of SFRC were first analyzed. Additionally, detailed numerical simulations were conducted to explore the impact of the length-to-diameter ratio ( l / d ) of steel fibers on their mechanical properties at high temperatures. Finally, the Bayesian neural network model was developed to systematically predict the flexural strength based on 204 data points from this study and existing literature, and evaluate its influencing factors. The results indicate that high temperatures significantly reduce the flexural strength of SFRC, resulting in a loss of approximately 90% at 600°C. However, increased fiber content significantly enhances the post-cracking load-bearing capacity and energy absorption of SFRC under elevated temperatures, thereby improving its flexural toughness. Moreover, the simulation results align well with the experimental data. Increasing the l / d ratio of steel fibers positively influences the flexural capacity and residual strength of SFRC. Within the range of 25 to 80, the most pronounced enhancement is observed at a ratio of 63. Shapley Additive explanation reveals that temperature, SF type, and l / d ratio are the most influential factors affecting SFRC’s flexural strength at high temperatures. • Flexural strength of SFRC may decrease by up to 90% at 600 °C while its toughness improves significantly. • The effect of l / d on SFRC’s flexural behavior at high temperature was analyzed using mesoscale finite element method. • The BNN model is best suited to comprehensively predict SFRC’s flexural strength. • SHAP analysis reveals that temperature and steel fiber properties significantly effect the flexural strength of SFRC.