Carbon Composites
1. Introduction
Carbon composites, also known as carbon fiber-reinforced polymers (CFRP), are advanced composite materials consisting of carbon fibers embedded in a polymer matrix such as epoxy, polyester, or vinyl ester resin.
They combine exceptionally high strength and stiffness with low weight, making them ideal for strengthening and retrofitting civil engineering structures.
In civil applications, carbon composites are primarily used for:
- Structural strengthening and rehabilitation of existing RC members.
- Corrosion-resistant reinforcements in aggressive environments.
- Lightweight and durable structural components.
2. Composition and Constituents
| Component |
Description |
Function |
| Carbon Fibers |
Produced by pyrolysis of PAN (polyacrylonitrile) or pitch at high temperatures (1000–2000°C) |
Provide high tensile strength and stiffness |
| Matrix Resin |
Epoxy, vinyl ester, or polyester resin |
Transfers stress between fibers and protects them from environment |
| Fillers & Additives |
Silica, alumina, or coupling agents |
Enhance bonding and performance |
| Surface Treatment (Sizing) |
Application of coupling agents on fibers |
Improves adhesion with resin |
3. Types of Carbon Composites
| Type |
Description |
Characteristics |
Typical Applications |
| CFRP Sheets |
Thin, flexible sheets (0.13–0.5 mm thick) |
Easy to apply, high tensile strength |
Strengthening beams, slabs |
| CFRP Laminates / Plates |
Rigid strips prefabricated under controlled conditions |
Excellent quality control, easy to bond |
Flexural strengthening of girders |
| CFRP Fabrics / Wraps |
Unidirectional or bidirectional woven fabrics |
Conform to complex shapes |
Column confinement, seismic retrofitting |
| CFRP Rods / Bars |
Pultruded carbon rods |
High tensile modulus, non-corrosive |
Replacement for steel bars, prestressing |
| Carbon Fiber Tendons |
Bundles of fibers for prestressing |
Lightweight and corrosion-free |
Prestressed bridges, cables |
4. Key Mechanical Properties
| Property |
Typical Range |
Unit |
Remarks |
| Tensile Strength |
3000–5000 |
MPa |
5–10× steel |
| Tensile Modulus |
150–250 |
GPa |
Comparable to steel |
| Density |
1.5–1.8 |
g/cm³ |
~¼ of steel |
| Ultimate Elongation |
1.2–1.8 |
% |
Brittle in tension |
| Coefficient of Thermal Expansion |
Negative or near zero |
— |
Dimensionally stable |
| Fatigue Resistance |
Excellent |
— |
Ideal for cyclic loads |
| Corrosion Resistance |
Excellent |
— |
Ideal in marine/industrial environments |
5. Advantages of Carbon Composites
- High strength-to-weight ratio
- Corrosion and fatigue resistance
- Ease of installation and handling
- Minimal increase in member dimensions
- Compatibility with various substrates
- Durability in aggressive environments
6. Limitations
- High initial cost
- Brittle failure mode
- Specialized installation and quality control required
- Limited ductility compared to steel
- Temperature-sensitive epoxy matrices
7. Applications in Civil Engineering
| Application Area |
Purpose |
Example |
| Flexural Strengthening |
Increase bending capacity of beams/slabs |
Bonded CFRP laminates on soffit of beams |
| Shear Strengthening |
Improve shear resistance |
Wrapping CFRP strips around beam sides |
| Column Confinement |
Enhance ductility and load capacity |
Full wrapping of circular/rectangular columns |
| Seismic Retrofitting |
Improve performance during earthquakes |
Confinement and flexural strengthening |
| Crack Repair and Control |
Arrest propagation of cracks |
Surface bonding CFRP sheets |
| Corrosion Rehabilitation |
Replace corroded steel reinforcement |
Externally bonded CFRP strips |
| Bridge Strengthening |
Increase load-carrying capacity |
Strengthening girders and decks |
8. Selection of Suitable Carbon Composite System
| Selection Factor |
Consideration |
| Structural Function |
Flexural, shear, or axial strengthening |
| Substrate Condition |
Surface cleanliness, moisture, cracks |
| Environmental Exposure |
Temperature, UV, chemicals |
| Bonding Surface |
Concrete, steel, masonry |
| Design Life |
Desired durability and maintenance frequency |
| Ease of Application |
Site accessibility and complexity |
| Economic Feasibility |
Material cost vs long-term benefits |
9. Design Principles
Design is based on limit state concepts and guidelines such as ACI 440, FIB Bulletin 14, or IRC:SP:80.
9.1 Flexural Strengthening:
- CFRP applied at tensile zone (soffit) of beams.
- Acts as additional tensile reinforcement.
- Design ensures proper bond length and debonding control.
9.2 Shear Strengthening:
- CFRP strips or wraps applied on sides of beams at angles (45°/vertical).
- Shear resistance due to fiber orientation and bond.
9.3 Axial Confinement (Columns):
- Confinement pressure increases compressive strength and ductility.
- Effective for seismic upgrading.
10. Methods of Application
| Step |
Process |
Description |
| 1. Surface Preparation |
Remove laitance, dust, oil |
Ensure clean and sound concrete surface |
| 2. Primer Application |
Epoxy primer coat |
Promotes adhesion between concrete and CFRP |
| 3. Putty/Filler Application |
Level uneven surfaces |
Smooth bonding surface |
| 4. Resin Application |
Epoxy resin on surface |
Acts as adhesive layer |
| 5. Fiber Placement |
Lay CFRP sheet/laminate |
Ensure fiber alignment along load direction |
| 6. Impregnation / Rolling |
Apply saturant resin and roll |
Remove air bubbles and ensure full saturation |
| 7. Curing |
Room temperature or heat-assisted |
Achieves full bond strength |
| 8. Protective Coating |
UV-resistant coating |
Protects CFRP from environmental exposure |
11. Testing and Quality Control
| Property |
Test Method |
Standard |
| Tensile strength of CFRP |
Flat strip test |
ASTM D3039 |
| Bond strength to concrete |
Pull-off test |
ASTM D4541 / ACI 440.2R |
| Flexural test of strengthened beam |
4-point loading |
ASTM D7264 |
| Impact resistance |
Drop-weight test |
ASTM D7136 |
| Durability tests |
Exposure to UV, salt spray, moisture |
ASTM D570 / D5229 |
12. Relevant IS / International Standards
| Code / Standard |
Description |
| IRC:SP:80-2008 |
Guidelines for the use of externally bonded fiber-reinforced polymer systems for rehabilitation of concrete structures |
| IS 14858:2000 |
Requirements for bond between FRP and concrete (reference) |
| ACI 440.2R-17 |
Guide for the design and construction of externally bonded FRP systems |
| FIB Bulletin 14 |
Externally bonded FRP reinforcement for RC structures |
| ASTM D3039 / D3030 |
Tensile properties of polymer matrix composites |
| ISO 10406-1:2015 |
FRP reinforcement for concrete – Test methods |
13. Safety and Handling Precautions
- Use gloves, goggles, and masks during resin mixing and application.
- Maintain adequate ventilation to avoid inhalation of fumes.
- Avoid exposure to open flames or direct sunlight during curing.
- Dispose of epoxy waste and resin containers as per environmental regulations.
14. Practical Insights
- Bridge Strengthening:
Use of CFRP laminates increased load capacity of RCC bridge girders by 25–30%.
- Column Retrofitting:
Carbon fiber wraps enhanced ductility and seismic resistance of RC columns.
- Slab Rehabilitatio:
CFRP sheets bonded under slabs reduced deflection and restored serviceability.
15. Summary
Carbon composites (CFRP) are high-performance materials revolutionizing structural rehabilitation and strengthening due to their superior mechanical properties, corrosion resistance, and ease of installation.
Proper selection, design, and application in accordance with relevant standards ensure long-term performance and sustainability in modern civil infrastructure.