Bond behavior of lightweight concrete-filled steel tubes containing rock wool waste after exposure to high temperatures

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 degrees 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 degrees 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 degrees 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% relative to that in the first cycle (with the average macrolocking contribution of 53%), while this reduction reached 21-27% in the fourth cycle relative to the second one. Furthermore, by increasing the temperature to 200 degrees C, the bond strength values in the second to fourth cycles increased by up to 71.2% relative to the corresponding values at the room temperature, while for the target temperatures of 400 and 600 degrees C, the corresponding bond strength values dropped by up to 96.6 and 98.9%, respectively. In the end, a prediction model for the bond strength of LCFT specimens containing rock wool waste was proposed as a function of temperature. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigated the bond behavior of lightweight concrete-filled steel tubes containing rock wool waste at elevated temperatures. The findings showed that rock wool waste could compensate for the loss of bond strength in LCFT specimens under high temperature conditions.