Performance of Normal Strength Concrete Slab Strengthened with High-Performance Concrete After Exposure to Elevated Temperature

Reinforced concrete as the most important construction material suffers from long-term deterioration due to different exposure conditions. Fire attack is a critical exposure condition as it can lead to complete collapse of the structure. On the other hand, the repair and strengthening of existing structures have become necessary both technically and financially. Moreover, since high-performance concretes (HPCs) are extensively used as repairing or strengthening materials for different structures, their performance after exposure to elevated temperature needs to be investigated. Therefore, this study is directed to investigate the post-fire flexural behavior of RC slabs cast with traditional normal strength concrete (NSC) and strengthened with HPC. Twenty-one RC slabs were prepared and tested including casting the full thickness with the same mixture (single-concrete slabs) and composite slabs (cast with NSC and HPC). Different variables were considered; using high strength concrete, 30% fly ash, 30% slag, 0.5% polypropylene, 0.5% steel fibers, hybrid fibers (0.5% steel + 0.5% polypropylene), reinforcement ratio, the side exposed to elevated temperature (tension or compression), and joining the HPC layer to the NSC (shear studs or epoxy resin). The slabs were exposed to the required temperature of 600°C for 2 h. The results show that strengthening the RC slab in tension or compression by using HPC remarkably enhanced the slab’s performance after exposure to elevated temperature. Specially, composite slabs containing hybrid fibers in tension side when exposed to elevated temperature from the tension side recorded the highest cracking load, ultimate load, stiffness, toughness, and ductility index as compared to the NSC slab, with increases of 92.8%, 116%, 157%, 335%, and 86.9%, respectively.