What Is Carbon Fibre Polymer Strengthening and How Is It Applied?

• Ahmet Toka
• 7 August 2026
• Structural Strengthening
• 5 min read

Carbon fibre polymer strengthening makes it possible to increase the capacity of a reinforced concrete member with almost no enlargement of its section. A fabric less than a millimetre thick, bonded in the right direction and in the right way, raises the flexural or shear capacity of the member significantly.

In this article we explain how the method works, the difference between fabric and laminate, how the application proceeds step by step, how quality control is carried out and where the method falls short.

The strength of carbon fibre lies not in the fabric but in the bond. If the surface has not been correctly prepared, even the strongest fibre in the world carries no load at all.

Ahmet Toka, General Manager

How Does It Work?

Carbon fibre is a material whose tensile strength is many times that of steel but whose weight is very low. On its own it is simply a fabric; its structural function emerges when it is bonded to concrete with epoxy resin. The epoxy both holds the fibres together and transfers load from the concrete to the fibre.

Carbon fibre works only in tension and is strong only in the direction of the fibres These two statements determine the whole of the application: the fabric is placed according to where the member is required to carry tensile stress and the direction of that stress. A fabric laid in the wrong direction contributes nothing at all.

Application to columns works on a different principle: when the fabric is wrapped around the column, it confines the concrete laterally. This confinement effectincreases both the axial strength of the concrete and its capacity to deform without fracturing (its ductility). In seismic strengthening this is one of the most valuable contributions.

Fabric or Laminate?

There are two basic product forms and they are used in different places.

• Fabric: Flexible, cut from a roll and applied. It conforms to curved surfaces, column wrapping and complex geometries. Its thickness is formed on site together with the epoxy, which increases the sensitivity to the applicator.
• Laminate (plate): Rigid strips of constant thickness manufactured in a factory. Used on flat surfaces, particularly on the underside of beams for flexural strengthening. The thickness and fibre content are guaranteed at the factory; application is faster but it is suitable only for flat surfaces.

In practice, laminate is preferred for the flexural strengthening of beams and fabric for shear strengthening and column wrapping. Details: carbon fibre polymer strengthening.

Application Steps

1. Surface preparation. Plaster and paint are removed and the concrete surface is opened up by grinding or blasting. No laitance layer, oil or loose material should remain on the surface.
2. Repair. Voids, breakouts and areas of exposed reinforcement are repaired with repair mortar; if necessary concrete damage repair is carried out. If there are active cracks, they are first filled with injection .
3. Rounding the corners. In column wrapping, sharp corners are rounded to a specified radius. If this step is skipped, the fabric is cut through and fails at the corner; whether the wrapping works at all depends on this detail.
4. Primer. Epoxy primer is applied to saturate the pores of the concrete and raise the bond strength.
5. Levelling with putty. Small irregularities in the surface are made good with epoxy putty; this is necessary to ensure no air voids remain beneath the fabric.
6. Applying the fabric. The fabric is laid onto the saturated epoxy, pressed with a roller to release air and allow the resin to work in between the fibres. Lap lengths are left at the value specified in the calculation.
7. Sealing and protection. A second coat of epoxy is applied over it. Where fire protection or visual concealment is required, suitable plaster or cladding is applied.

Quality Control

The performance of a carbon fibre application cannot be fully assessed by eye, which is why inspection methods matter.

• Pull-off adhesion test: The bond strength is measured by carrying out pull-off tests at specified points on the applied surface. Failure is expected to occur within the concrete; failure at the interface indicates a bond problem.
• Void check: Areas containing voids are identified by lightly tapping the surface. Voids are filled by epoxy injection.
• Record of ambient conditions: The temperature and relative humidity during epoxy application must be recorded; adhesion falls in conditions close to the dew point.
• Mixing records: The ratio of the epoxy components and the mixing time must be documented.
• Lap and direction check: The direction of the applied fabric and the lap lengths are checked against the design.

The Limits of the Method

Carbon fibre strengthening is a powerful tool but it is not the solution to every problem. Knowing its limits is necessary for correct design.

• The increase in stiffness is limited. If the structure has an excessive drift problem, carbon fibre does not solve it on its own; methods that increase stiffness, such as adding shear walls, are required.
• Fire resistance is low. Epoxy softens at high temperatures; fire protection must be planned separately.
• It depends on the quality of the concrete. The system transfers load into the concrete. If the concrete strength is very low, the bond strength also remains low and the method becomes ineffective.
• It depends on access to the surface. It cannot be applied to surfaces of a member that are clad or inaccessible.
• It is exposed to UV and mechanical impact. A protective coating is required in exposed applications.

For a comparison with other methods see structural strengthening page.

Where Is It Used?

Carbon fibre strengthening stands out wherever enlarging the section is not possible or not wanted. The uses we encounter most often on site are as follows:

• Beam flexural strengthening: In buildings whose use is changing (residential to office, office to archive), when the floor load increases the capacity of the beams is raised with laminate.
• Column wrapping: To give ductility to old columns with insufficient stirrups; one of the most widespread applications in seismic strengthening.
• Strengthening around slab openings: Recovering the capacity lost around lift, services or stair openings cut at a later date.
• Shear strengthening: Applying a U-shaped wrap to beams showing diagonal cracks in the support regions.
• Equipment loads in industrial buildings: Increasing the point load capacity of slabs where new machinery is to be installed.
• Hoop stress in water structures and silos: Raising the tensile capacity of circular structures with a circumferential wrap.
• Buildings requiring conservation: Preferred because it barely changes the appearance or the section.

What these uses have in common is this: the lack of capacity is at member level. If there is a stiffness problem across the structure as a whole, carbon fibre becomes only part of the solution.