Analysis and Treatment Guide for Voids in Carbon Fiber Reinforced Polymer (CFRP) Strengthening

Carbon Fiber Reinforced Polymer (CFRP) strengthening, recognized for its mature technology and significant effectiveness, has become a common method in building structural reinforcement projects. Its core principle involves bonding carbon fiber fabric to the concrete surface using specialized structural adhesive, forming a composite structure. This enables the CFRP and concrete to work together synergistically, thereby improving the load-bearing performance of the component or structure (e.g., reducing deformation, lowering stress, inhibiting crack propagation).

However, in practical applications, "voids" (delamination) are the most prevalent issue. According to the requirements of the GB50550 Code for Construction Quality Acceptance of Building Structure Strengthening, voids indicate that the carbon fiber fabric has not fully bonded to the concrete, preventing effective composite action. If the void area exceeds 5% of the total bonded area (i.e., the effective bonded area is less than 95%), the strengthening work is deemed non-compliant and must be redone.

I. Primary Causes of Voids

Inadequate Substrate Preparation:

Concrete bonding surface not ground flat.

Necessary grinding omitted (especially failing to round corners to the specified radius).

Substrate is damp, contaminated with dust/grease, or cracks not repaired and leveled beforehand.

Defects in Adhesive Application:

Uneven application of resin, resulting in inconsistent thickness.

Adhesive not spread uniformly, with insufficient quantity, particularly in complex areas like corners.

No re-coating or uneven re-coating after fabric placement, leading to insufficient two-way saturation of the carbon fiber fabric.

Non-standard Fabric Placement Operation:

Fabric not pulled taut or aligned properly during placement.

Failure to use a roller/squeegee sufficiently and effectively to expel air trapped beneath the fabric.

Inadequate rolling/squeegeeing operation, failing to ensure the fabric is flat and tightly bonded.

Material Quality Issues:

Use of carbon fiber fabric that is too stiff or loosely woven, exhibiting poor pliability and difficulty conforming to the substrate.

II. Void Treatment Methods

Small Voids (Individual void area ≤ 100 cm² / 10,000 mm²):

Needle injection of repair resin can be attempted to fill the void.

Note: This method has limited effectiveness. Difficulties injecting resin or achieving a complete repair are common, making its application relatively infrequent.

Large Voids (Individual void area 100 cm² / 10,000 mm²):

Cut out: Completely remove the CFRP fabric in the voided area.

Repair: Level the concrete substrate using repair resin.

Re-patch: Apply a new piece of carbon fiber fabric to repair the area.

Lap Requirements:

Load-bearing direction: New-to-original fabric lap length ≥ 200mm per end. If the original number of layers ≥ 3, lap length ≥ 300mm.

Non-load-bearing direction: Lap length ≥ 100mm per side.

Critical Construction Practices:

Must use a roller/squeegee to apply firm, repeated pressure (back and forth is best) to thoroughly expel air.

Ensure resin is applied evenly and sufficiently to achieve complete two-way saturation of the carbon fiber fabric.

Key Prevention & Operational Points:

Substrate preparation is fundamental: Strictly grind to achieve a level, dry, and clean surface. Corners must be rounded to the specified radius. Pre-repair all cracks.

Adhesive application is core: Guarantee even, sufficient adhesive application (especially in corners), ensuring both the substrate before placement and the fabric surface after placement are fully saturated.

Placement technique is key: Pull the fabric taut and align it correctly. Use a roller/squeegee to apply strong, repeated pressure, thoroughly expelling air to ensure the fabric is flat and tightly bonded.

Material quality is the foundation: Use qualified, highly pliable, tightly woven, premium carbon fiber fabric and compatible structural adhesive.

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