Effect of fire insulation on the fire performance of concrete beams strengthened with CFRP sheets bonded using by magnesium phosphate inorganic adhesive

The fire safety of carbon fiber reinforced polymer (CFRP)-strengthened concrete structures is often compromised owing to the thermal vulnerability of conventional epoxy-based adhesives. Magnesium phosphate inorganic adhesive (MPIA), an emerging high-temperature resistant binder material, has attracted increasing attention in the field of CFRP strengthening (hereinafter referred to as CFRP-MPIA strengthened system). However, few studies have investigated the fire performance of CFRP-MPIA strengthened members utilizing MPIA as fireproof coatings, and the thermo-mechanical degradation mechanism of this insulation system under fire remains unclear. To address this gap, seven specimens were fabricated with variations in three parameters: insulation material (cement mortar vs. MPIA), insulation configuration (U-shaped full wrap vs. U-shaped half wrap), and insulation thickness (10 mm, 20 mm, and 30 mm). All specimens were tested under the combined actions of sustained load and ISO 834 standard fire. The fire behaviors of the specimens, including temperature and mid-span deflection progressions, failure time of the strengthening system, fire endurance, and failure modes, were monitored throughout the test. The results showed that compared with cement mortar insulation, MPIA insulation exhibited lower thermal conductivity, a refined microstructure, and higher strength retention at elevated temperatures. These properties contributed to its excellent heat insulation performance and strong adhesion to the concrete substrate, thereby significantly enhancing the fire resistance of the CFRP-MPIA strengthened concrete beams (by over 90% compared with that of the unprotected specimen). The fire endurance of the CFRP-MPIA strengthened concrete beams with U-shaped full wrap insulation improved by only approximately 7% compared with those with U-shaped half wrap insulation, highlighting the importance of targeted protection for both the strengthening system and longitudinal tensile rebars during fire safety design. These findings demonstrate the feasibility of employing MPIA as a fireproof material and provide new insight into the protection of CFRP-MPIA strengthened concrete structures in fire-prone environments.