Behavior and Mechanical Properties of Hybrid GFRP-Steel Reinforced Concrete Beams Immediately After Exposure to Fires

This study investigated the behavior and mechanical properties of hybrid glass fiber reinforced polymer (GFRP)-steel reinforced concrete (RC) beams immediately after exposure to fires. These hybrid beams were characterized by the ratio ρ f / f s of GFRP area to the total area of GFRP and steel. Experiments were conducted on 16 GFRP-steel RC beams, divided into four groups by ρ f / f s varying from 0 to 1. In each group, four beams were exposed to ISO 834 fire for 0 (control), 15, 30, and 45 min, immediately followed by loading. Results indicated that the fire duration and ρ f / f s were important parameters that governed the failure mode, mechanical properties, and behavior of the tested beams. Steel yielding and concrete crushing were the failure modes of beams without fire exposure. Beams with a high ρ f / f s exhibited a low residual deformation. At room temperature, the ultimate load increased by 35.9% and the elastic stiffness decreased by 44.2% when ρ f / f s increased from 0 to 1. Reductions in the ultimate load of beams with ρ f / f s of 0 and 1 exposed to 45 min of fire were 29.2% and 67.3%, respectively. The elastic stiffness of beams exposed to 45-min fire reduced by 45.2%–61.3% compared with that of their control beams. The increase in ρ f / f s and fire duration reduced the high ductility to moderate ductility. An analytical model was proposed to calculate the residual strength of GFRP-steel RC beams in fire. The model exhibited reasonable and safe estimations for beams with low ρ f / f s , while it underestimated the ultimate moments for beams with high ρ f / f s .