China has made significant strides in the research and practical application of structural reinforcement and strengthening using carbon fiber composites. Common methods used in engineering include section enlargement, steel jacketing, prestressing, steel plate bonding, and carbon fiber composite bonding. Traditional reinforcement methods are generally outdated, with complex construction processes and techniques that can impact the structure's weight and usable area. In contrast, the carbon fiber composite bonding method offers advantages such as high strength, efficiency, corrosion resistance, ease of construction, and no increase in structural dimensions, making it widely used in engineering projects.

1. Principles of Carbon Fiber Composite Reinforcement

Carbon fiber reinforcement of concrete structures is a modern method that began research in the 1980s and was introduced to China in 1996. It quickly gained attention in the engineering community, becoming a hot topic for research and application. This method involves bonding carbon fiber composites to the surface of concrete structures, enhancing their strength and improving their performance through the collaborative work of the fibers and the structure.

2. Carbon Fiber Composite Materials

The primary materials used for reinforcing and repairing concrete structures with carbon fiber composites are carbon fiber fabrics and compatible resins. Carbon fiber composites are known for their high strength, high elastic modulus, light weight, and excellent corrosion resistance, with tensile strength about ten times that of ordinary steel bars. The compatible resins include base resins, leveling resins, and bonding resins. These resins improve the bonding quality of carbon fibers and help form a composite material body with the concrete, enhancing the structure's bending and shear resistance.

2.1 Carbon Fiber Fabrics

Carbon fiber fabrics can be categorized based on their raw materials into PAN-based, viscose-based, and asphalt-based fabrics. They also vary by specification, including 1K, 3K, 6K, 12K, and 24K or larger tow carbon fiber fabrics, and by carbonization process into graphitized, carbonized, and pre-oxidized fabrics. Additionally, they differ in weaving methods, such as woven, knitted, braided, and pre-impregnated fabrics.

2.2 Bonding Materials

The adhesives used for carbon fiber composite reinforcement include three types: primer (base resin), repair resin (leveling material or putty), and impregnating resin. The primer strengthens the concrete surface, enhancing the bond between the concrete and carbon fiber composites. The repair resin levels the concrete surface, facilitating the adhesion of carbon fiber sheets. The impregnating resin binds the carbon fibers together and to the concrete, forming a composite material that resists external forces. The performance of the impregnating resin is crucial for the effective reinforcement of concrete structures.

3. Superior Performance of Carbon Fiber Composite Reinforced Concrete Structures

• High Strength and Elastic Modulus: Carbon fiber composites offer high tensile strength, approximately ten times that of steel, and an elastic modulus comparable to steel.

• Corrosion Resistance and Durability: Carbon fiber composites are chemically stable and do not react with acids, alkalis, or salts, providing excellent corrosion resistance and durability for reinforced structures.

• Low Thermal Expansion Coefficient: Carbon fiber composites have a very low thermal expansion coefficient, almost zero in the fiber direction.

• Ease of Construction and High Efficiency: Reinforcing with carbon fiber fabrics does not require heavy machinery or large equipment, occupies minimal space, and allows for flexible cutting and quick installation.

• Guaranteed Construction Quality: Carbon fiber fabrics, being flexible, can adhere well to uneven surfaces after repairs, achieving an effective adhesion rate of over 95%.

• Minimal Impact on Structures: The light weight and thin profile of carbon fiber composites do not significantly increase the weight or dimensions of the original structure, maintaining usable space.

• Wide Range of Applications: Suitable for reinforcing different types of structures, shapes, materials, and various weak points in structural elements.

4. Applications of Carbon Fiber Composite Reinforcement in Concrete Structures

In practical engineering, carbon fiber fabrics are primarily used for reinforcement. The technology is mature and widely applied in several areas:

• Enhancing Shear Capacity: Carbon fiber fabrics contribute to shear resistance similar to stirrups and help prevent premature yielding of main reinforcement by restraining concrete and carrying tensile stress.

• Increasing Bending Capacity: By bonding carbon fiber fabrics to the tensile surface of structural elements, their bending capacity is significantly improved.

• Improving Seismic Performance: Carbon fiber fabrics can enhance the ductility and energy absorption capacity of concrete elements, especially in beam-column joints and columns under axial load, providing excellent seismic reinforcement.

• Boosting Fatigue Resistance: Reinforced concrete beams and prestressed concrete beams strengthened with carbon fiber fabrics maintain strength and stiffness after repeated loading cycles, significantly extending their fatigue life and reducing deformation.

5. Limitations of Carbon Fiber Composite Reinforcement in Concrete Structures

• Brittleness: Carbon fibers are linear elastic and brittle, requiring significant deformation of the structure to fully utilize their properties.

• Adhesion Issues: Carbon fiber reinforcement can experience bonding failures, leading to brittle failure modes in the reinforced concrete structures.

• Limited Research on Long-Term Performance: There is a lack of research on the performance of carbon fiber reinforced concrete under long-term and impact loads.

• Theoretical Gaps: While there is extensive research on the shear and bending reinforcement mechanisms of carbon fiber materials in beams and columns, there is limited understanding of their application in shear walls.

Overall, carbon fiber composite reinforcement technology offers significant economic, social, and environmental benefits. With the development of carbon fiber materials, reduced production costs, and ongoing research, this technology will see increasing application in concrete structure reinforcement, promising a bright future.