مهدی نعمت زاده افروزی

Since the literature regarding the axial stiffness of concrete-filled steel tube (CFST) columns containing waste materials and their behavioral changes after thermal loading is very limited, this research attempted to evaluate the elastic modulus of rubberized fiber-reinforced concrete-filled steel tube (RuF-CFST) stub columns following thermal loading. To achieve this goal, 57 RuF-CFST specimens were constructed and various types of elastic moduli including the initial, real, secant, steel yielding point, and peak point elastic moduli were examined after thermal loading at elevated temperatures. The variables of the research for the RuF-CFST specimens with the high-strength concrete core included the thermal loading temperature (20, 250, 500, and 750 °C), percentage of crumb rubber aggregate substituting for sand by volume (0, 5, and 10%), steel fiber volume ratio (0, 1, and 1.5%), and the ratio of the steel tube external diameter to thickness (25.4 and 43). Based on the results, by increasing the thermal load, the extent of the elastic modulus drop in the specimens became notable (between 50 and 70% at 750 °C), and also, adding crumb tire to the concrete mixture led to a drop in the elastic modulus of the RuF-CFST specimens (between 7 and 18% per adding crumb tire by 10%). On the contrary, via raising the ratio of fibers in the specimens with and without thermal loading, the elastic modulus increased by 5–10%, and additionally, via raising the tube thickness, the elastic modulus increased by 25–34%. Ultimately, models to capture the elastic modulus of RuF-CFST columns after thermal loading were proposed, which were then evaluated against the empirical data of the present study and those of others, which showed a good correlation between the results.