Bonding properties between steel-basalt hybrid fibers reinforced cementitious composites and existing concrete at high temperatures

Steel-basalt hybrid fibers reinforced cementitious composite (SBFRCC) is a potential reinforce-ment and repair material for engineering structures with fire risk due to its excellent mechanical properties exposed to ambient and high temperatures. However, the bonding properties between SBFRCC and existing concrete are still unclear, especially at high temperatures. To fill this gap, the bonding properties between SBFRCC and existing concrete at high temperatures were investigated based on two situations by using the slant shear and splitting tensile tests. First, the existing concrete was heated at high temperatures and then bonded with SBFRCC and strengthening concrete to form combined systems, respectively. The effects of the interface roughness, high temperature duration, and post-fire-curing on their bonding properties were investigated. Second, the existing concrete was bonded with SBFRCC and then heated at high temperatures, and the effects of fly ash substation rates on the bonding properties were studied. Besides, the backscattered-electron (BSE) was used to observe the interface between the exiting concrete and SBFRCC exposed to various temperatures. Results show that the first bonded and then heated specimens are slide-failed and separated at the bonding interface from room tem-perature to 600 degrees C, while the damage is generated within the existing concrete at 800 degrees C. SBFRCC has better high temperature resistance than the existing concrete. The slant shear and splitting tensile strengths between PC-A and SBFRCC are 9%-33.3% higher than that of PC-A and strengthening concrete at all temperatures. The appropriate roughness at the interface by pol-ishing can significantly improve the bonding properties between the existing concrete and SBFRCC. The bonding strength is recovered by more than 18.2% and 29.8% after the post-fire curing of 28 and 90 d, respectively. The increase of fly ash substitution rates has a certain contribution to the bonding properties above 600 degrees C.