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چکیده
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Evaluating the concrete-steel composite action and bond behavior in structures with concrete-filled steel tube (CFT) members is an essential part of the design phase of these structures, especially for exposure to fire as a potential risk in the service life of any structure. In this research, the natural bond strength and bond stress-slip behavior of lightweight concrete-filled steel tubes (LCFTs) containing rock wool waste following thermal loading at elevated temperatures were investigated. For this purpose, 40 CFT specimens consisting of circular steel sections filled with lightweight concrete containing several volume contents of rock wool waste (0, 2.5, 5, 7.5, and 10%) were made and thermally loaded at 20, 200, 400, and 600 C. Afterward, all the lightweight concrete-filled steel tube (LCFT) specimens were subjected to up to four loading cycles in the load-reversed push-out test. The first loading cycle was conducted to investigate the heat-induced bond strength loss of the LCFT specimens and the impact of rock wool waste on this parameter, while the remaining cycles were conducted to assess the macrolocking component (after the loss of chemical adhesion and microlocking components). The findings demonstrated that raising the temperature to 600 C decreased the bond strength of the LCFT specimens by up to 97% relative to that at the room temperature, while the presence of rock wool relatively compensated for this severe loss. The incorporation of rock wool into the lightweight concrete mix increased the bond strength values of the LCFT specimens after thermal loading at 200, 400, and 600 C by up to 20.2, 18.2, and 7.2%, respectively, relative to that of the reference specimen (lacking rock wool). In the macrolocking assessment, the first cycle was compared with the third one, and the second cycle was compared with the fourth one. It was observed that at the room temperature, the bond strength of the LCFT specimens in the third cycle decreased by 40–52% relat
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