Axial compression performance of steel tube columns filled with slurry-wrapping recycled concrete after exposure to high temperatures

Due to frequent fire accidents and stone scarcity, fire protection design and recycled aggregates are issues that needed to be solved in the construction industry. Steel tubes and slurry-wrapping reinforcement treatment can effectively improve the load-bearing capacity of recycled concrete. This paper reports on the mechanical behavior of slurry-wrapping recycled aggregate concrete-filled steel tube (CFST) columns after exposure to high temperatures. Three series of 30 CFST columns were designed with the coarse aggregate type and exposure temperature as variation parameters: natural aggregate concrete-filled steel tube (NACFST) column, recycled aggregate concrete-filled steel tube (RACFST) column, and slurry-wrapping recycled aggregate concrete-filled steel tube (SRACFST) column. The results show that the failure pattern of the specimen is less affected by the exposure temperature and the coarse aggregate type. The compressive strength of the RACFST columns is significantly increased after the slurry-wrapping treatment. With the temperature increasing, the ultimate strength tends to increase initially and then drop. Since the recycled concrete has more pores, the compressive strength loss rate of the RACFST columns is lower than that of the NACFST and SRACFST columns at 800 degrees C. The lateral deformation factor is maintained until the exposure temperature reaches 400 degrees C and then begins to decrease as the temperature increase. It is not accurate to predict the strength of CFST columns after high temperatures by ultrasonic pulse velocity. The predicted strength of CFST column after high temperatures was compared with design codes, and an equation considering the material strength variation was proposed.