Can carbon fiber plates reinforce circular concrete columns?

Based on ACI 440, the answer is Yes.

Unidirectional CFRP laminates (carbon plates) are permitted as external longitudinal tensile reinforcement for circular columns deficient in flexural capacity or large eccentric compressive resistance.

Primary function of vertical CFRP plates: Supplement internal longitudinal steel rebars to boost the flexural strength and enlarge the P-M interaction envelope under eccentric axial loads.

BUT

Critical mandatory constraint from ACI 440.2R: Longitudinal CFRP plates cannot be installed without transverse FRP confining wraps. Unrestrained vertical plates violate prescriptive detailing rules; full design credit for CFRP tensile/compressive strength is disallowed in formal strength calculations if no transverse confinement is provided.

Is there meaningful structural benefit when only vertical CFRP plates are bonded to a circular column (no transverse FRP wrap)?

Marginal theoretical benefit (only temporary, non-permanent remedial use)

Vertically oriented CFRP fibers resist axial tension only. Under pure bending or large tension-controlled eccentric loading, the plates share tensile force on the tension side of the section, marginally reducing flexural crack width and slightly raising nominal flexural strength for short-term static service conditions. 

Severe code-limitated critical drawbacks

1) No concrete confinement; negligible improvement in concentric/small-eccentric axial capacity 

2) Zero shear capacity enhancement

3) Early CFRP debonding and delamination governed by bond strain limits 

4) Insufficient ductility, non-compliant for seismic retrofits

So combining vertical precured CFRP plates + full transverse FRP wrapping (ACI standard compliant retrofit scheme)

CFRP plates+wrap columns eliminating all deficiencies of vertical-only plate bonding while simultaneously upgrading flexure, axial compression, shear, and seismic performance.

1) Independent role of vertical precured CFRP plates

Supplement external longitudinal reinforcement to raise nominal flexural strength for bending and large eccentric compression; expands the tension side of the column’s P-M interaction curve.

Restrain opening of vertical flexural cracks under lateral cyclic loads, reducing service-level deflection and improving stiffness stability.

Offset design deficiencies when internal longitudinal steel reinforcement is corroded, undersized, or damaged.

2) Multifunctional mandatory contributions of transverse FRP wraps

• Passive confinement for boosted axial compressive capacity

Lateral expansion of compressed concrete activates tensile stress in circumferential FRP fibers, applying uniform passive confining pressure, significantly elevating peak compressive strength and ultimate compressive strain of core concrete, drastically improving concentric and small-eccentric load resistance.

• External shear reinforcement to suppress brittle shear failure

Circumferential FRP fibers act as continuous external transverse stirrups, directly resisting diagonal shear tension. Eliminating the risk of premature shear collapse under lateral and seismic loading.

Full transverse wrapping clamps vertical CFRP plates tightly against the cylindrical substrate, suppressing out-of-plane buckling of compression-side plates and lowering interfacial shear stress below the debonding strain threshold. This allows full design utilization of CFRP tensile strength without excessive strain reduction factors.

• Ductility and seismic performance upgrade

Confined concrete exhibits a long post-peak plastic stress-strain plateau, enabling large plastic lateral drift and hysteretic energy dissipation. Internal longitudinal rebars are restrained against compressive buckling. The failure mode transitions from sudden brittle collapse to ductile tension-controlled flexural yielding, satisfying all ACI seismic ductility and drift limits for retrofitted columns.

• Durability and corrosion protection

Continuous transverse FRP wrapping seals surface microcracks, blocking ingress of moisture, chloride ions, and carbonation agents. The FRP barrier protects internal steel reinforcement from corrosion, allowing relaxed environmental reduction factors in long-term strength calibration.

3) Synergistic combined performance (1+1 > 2)

• Full-spectrum capacity upgrade: Flexure, eccentric axial load, concentric compression, and shear strength are all quantifiable via standardized ACI equations.

• Compatible composite action: Transverse wraps resolve the fatal debonding/buckling flaws of standalone vertical CFRP plates, ensuring full composite interaction between FRP and concrete substrate.

• Controllable ductile failure hierarchy: Guarantees flexural yielding precedes concrete crushing or shear rupture, aligning with ACI’s core design principle of ductile structural response under extreme loads.

• Code-validated design workflow: Every performance metric (flexure, shear, confinement, debonding strain, seismic ductility) has explicit calculation procedures, satisfying third-party review, inspection, and acceptance criteria for structural retrofits.

Comparative Summary Table