Experimental research on fire resistance of the concrete beams strengthened with the CFRP sheets pasted by magnesium phosphate inorganic adhesive

Abstract

The fire safety of CFRP-strengthened concrete structures has raised much concern due to the sensitivity of epoxy-based bonding agents to high temperature and failure of strengthening system. Hence, replacing traditional epoxy adhesives with the inorganic adhesives with better high-temperature resistance provides a chance to improve the safety of the strengthened concrete structures under fire. At present, there are few studies concerning the fire behavior of the concrete beams strengthened with the CFRP sheets using magnesium phosphate inorganic adhesive (MPIA) as adhesive agent, and the failure mechanism of the CFRP-MPIA strengthening system under fire is unknown. To fill this gap, eleven specimens were designed and fabricated in this study, and the standard fire tests under constant load were carried out. During tests, the fire responses of the specimens, including the temperature and mid-span deflection developments, the failure time of strengthening system, fire resistance, and failure mode were measured and analyzed. Also, the effect of load level, adhesive type and fire insulation on the fire resistance of the strengthened concrete beams was investigated. The results showed that the MPIA, served as bonding agent, possesses excellent high-temperature resistance, and the “binder-CFRP sheet-binder” sandwich design can thus play a role of thermal insulation for CFRP sheets, which postpones the failure time of CFRP-MPIA strengthening system effectively, and the excellent mechanical properties of the CFRP sheets under anoxic conditions can also be fully utilized. These make the fire resistance of the CFRP-MPIA strengthened concrete beams without fire insulation are slightly higher than those of the CFRP-epoxy strengthened beams with a fire insulation of 20 mm, achieving the integrated design goal of strengthening and fire resistance. Additionally, with the combined use of the fire insulation and the “binder-CFRP sheet-binder” sandwich strengthening system, the fire resistance of the CFRP-MPIA strengthened concrete beams are over 50 % higher than those of the CFRP-epoxy strengthened concrete beams.