A carbon fibre laminate — also called a CFRP plate, from carbon fibre reinforced polymer — is a strip or fabric of carbon fibres in a polymer matrix, bonded to the outside of an existing structure to increase its capacity. It has become one of the standard solutions in concrete repair and strengthening.
Why carbon fibre
Carbon fibre has a very high tensile strength along the fibre direction at low weight. That makes it suitable as “external reinforcement”: the laminate takes tensile forces the existing concrete or steel reinforcement can no longer carry. Advantages over a bonded steel plate:
- Light — it can be applied by hand, with no crane or propping.
- Does not corrode — no rust in the bond line, which is a known failure mode with bonded steel.
- Thin — the build-up is a few millimetres, so almost no headroom is lost.
Types
| Type | Form | Application |
|---|---|---|
| Unidirectional plate | Pre-cured strip, fibres in one direction | Flexural strengthening of beams and slabs |
| Multidirectional laminate | Fibres at several angles | Forces acting in more than one direction |
| Carbon fabric (wrap) | Flexible sheet, saturated with resin on site | Wrapping columns, shear strengthening |
| Composite beam | Laminate around a light core | Special structures |
Typical applications
Bridges carrying heavier traffic loads than they were designed for, slabs into which a new opening is cut, beams required to carry an extra storey, and columns given additional confinement by wrapping.
Conditions and limitations
- It all comes down to the bond line — the strength of the strengthening depends on adhesion to the substrate. The concrete surface must be sound; carbonated concrete, spalling and weak laitance must be removed first.
- Surface preparation — grit-blast or grind, remove dust, make flat. The pull-off strength of the substrate is tested beforehand.
- Temperature and moisture — epoxy adhesives are sensitive to low temperatures and damp substrates. Follow the manufacturer’s application data strictly.
- Fire resistance — the epoxy matrix loses strength at raised temperature. Where fire resistance is required, additional protection is needed. This is regularly overlooked.
- Design and installation — the strengthening must be designed by a structural engineer and installed by a certified applicator. This is not DIY work.
Related terms
- Concrete repair
- Reinforcement
- Composite
- Epoxy adhesive
- Supplementary reinforcement
- Concrete degradation
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