Repairing high-strength concrete two-way solid slabs exposed to elevated temperature using NSM-CFRP ropes

Exposure of reinforced concrete (RC) structures to elevated temperatures results in significant degradation of their mechanical properties and overall structural integrity, necessitating the development of effective repair strategies to restore their load-bearing capacity and long-term durability. This study introduces a novel approach through both experimental and theoretical investigations into the efficacy of using Near-Surface Mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) ropes to repair two-way high-strength concrete (HSC) solid slabs subjected to elevated temperatures of 600 °C for a duration of 3 h. A total of eight slabs, each measuring 1050 × 1050 × 70 mm, were tested, comprising two normal-strength concrete (NSC) slabs and six HSC slabs. The study examined three primary variables: the number of CFRP ropes (2 or 3), their orientation angles (0° or 45°), and their configuration patterns (radial star or concentric squares). The key performance indicators evaluated included load capacity, failure modes, stiffness, and ductility. The experimental results indicated that the NSM-CFRP rope repairing technique significantly enhanced the structural performance of heat-damaged slabs. Load capacity improved by 12 % to 35 %, stiffness by 260 % to 343 %, and ductility by 127 % to 324 % when compared to unstrengthened slabs. Notably, the configurations of one rope in a radial star pattern around the column (R-SR) and three ropes arranged in concentric squares at a 45° angle (3R-CS 45°) demonstrated the highest recovery efficiencies, restoring the pre-fire load capacity by 10 % and 1 %, respectively. Theoretical analysis revealed that the models by El-Gamal et al. and Ospina et al. provided close alignment with the experimental findings, with average experimental-to-theoretical ratios of 1.09 and 1.12, respectively. In contrast, the ACI 440.2R-22 model was more conservative, yielding a ratio of 1.22, while the JSCE-97 model significantly overestimated the punching shear capacity, exhibiting the least accuracy among the models analyzed, with a mean ratio of 1.81 and a standard deviation of 0.126. The findings of this research underscore the viability of NSM-CFRP ropes as an efficient and economical method for restoring heat-damaged concrete slabs. This approach provides a flexible repair solution that requires minimal disruption, positioning it as an ideal option for industrial and infrastructure rehabilitation projects.