Executive Summary

The degradation of reinforced concrete (RC) infrastructure necessitates the implementation of high-performance strengthening solutions. Recent research highlights Fiber Reinforced Polymer (FRP) as a premier material for enhancing both shear and torsional capacities. This article synthesizes findings from two pivotal studies: one focusing on Near-Surface Mounted (NSM) FRP rods for shear reinforcement and another on Externally Bonded (EB) CFRP wraps for torsional strengthening.

1. Torsional Enhancement via Externally Bonded CFRP Wraps

Research conducted examined the efficacy of Carbon Fiber Reinforced Polymer (CFRP) in mitigating torsional failure. Torsion often presents a critical challenge in edge beams and curved structural members.

Keys:

• Configuration Impact: Multiple wrapping schemes are evaluated, including fully wrapped and strip configurations. Full wrapping proved most effective, significantly increasing ultimate torque capacity.
• Capacity Gains: Experimental results indicated an increase in ultimate torque of up to 47% compared to reference beams when using specific CFRP configurations.
• Failure Mechanisms: While reference beams typically fail through concrete crushing and steel yielding, CFRP-strengthened beams shifted failure modes toward CFRP debonding or rupture, indicating that the composite material effectively redirected structural stresses.

2. Shear Strengthening via Near-Surface Mounted (NSM) FRP

The NSM technique involves embedding FRP rods into pre-cut grooves in the concrete cover, bonded with high-strength epoxy. This method offers superior protection against environmental degradation and mechanical damage compared to externally bonded sheets.

Keys:

• Groove Optimization: Bond testing revealed that a groove size of 1.5 to 2.0 times the rod diameter is optimal for maximizing the bond strength between the FRP, epoxy, and concrete substrate.
• Material Comparison: Both Carbon (CFRP) and Aramid (AFRP) rods were tested. CFRP generally exhibited higher stiffness and shear capacity enhancement, though AFRP provided a more ductile failure profile.
• Interaction with Internal Stirrups: The research emphasizes that the efficiency of NSM strengthening is influenced by the existing internal steel stirrups. The total shear capacity (V_n) is a combined function of concrete (V_c), steel (V_s), and FRP (V_f) contributions.

3. Engineering Implications for Infrastructure Management

1. Strategic Selection: Engineers should select NSM rods for environments where surface protection is paramount (e.g., parking garages, high-traffic zones) and CFRP wraps for members requiring high torsional stiffness.
2. Bond Integrity: The success of both methods relies heavily on the quality of the adhesive interface. Surface preparation and epoxy application must adhere to strict volumetric ratios to prevent premature debonding.
3. Design Formulations: Current analytical models (such as ACI 440) provide a conservative baseline, but experimental data suggests that the synergy between internal reinforcement and FRP requires nuanced calculation to avoid overestimating the contribution of the composite.

Conclusion

Fiber Reinforced Polymers represent a transformative shift in structural rehabilitation. Whether through externally bonded wraps for torsion or near-surface mounted rods for shear, these materials provide a lightweight, non-corrosive, and high-strength alternative to traditional steel jacketing, ensuring the longevity and safety of global infrastructure.