FRP strengthening: bonded laminates, sheets and near-surface mounted bars

How bridges get built · Finishing, articulation and maintenance · FRP strengthening: bonded laminates, sheets and near-surface mounted bars

Also called: CFRP strengthening, externally bonded FRP, carbon fibre wrapping, NSM FRP, fibre-reinforced polymer strengthening

How it is done

Assess the member: concrete strength and cover condition, existing reinforcement and stress state, corrosion and chloride levels (which must be dealt with first), and the capacity needed; check the unstrengthened member for the load case the code applies if the FRP is lost, for example in fire.
Design to the governing guide (ACI 440.2R, fib Bulletin 90, Concrete Society TR 55): debonding strain limits, anchorage length, end anchorage by U-wraps or FRP anchors, and service stress limits in the existing steel.
Repair the substrate: remove unsound concrete, treat corroded bars, patch, inject cracks wider than about 0.25 mm, and round corners that will be wrapped.
Prepare the surface by abrasive blasting or grinding to expose aggregate, check flatness, moisture and dew point, and prove the substrate with pull-off tests.
Apply: prime, then bond pultruded laminates with thixotropic epoxy and roll out the excess; or saturate sheets by wet lay-up ply by ply with the fibres oriented as designed; for near-surface mounted bars, cut grooves in the cover and embed the bars in epoxy.
Cure under protection from rain, dust and low temperature, then protect against UV, impact and, where required, fire with a coating or board.
Inspect by tap test or thermography for voids, test witness panels or the work by pull-off, record plies and orientation, and add strain gauges and a load test where the strengthening is critical.

Key numbers

CFRP laminates1.2–1.4 mm thick, 50–150 mm wide, modulus about 165–210 GPa, tensile strength about 2,500–3,000 MPa
CFRP sheetsabout 200–600 g/m² of fibre per ply
Substrate (ACI 440.2R)concrete pull-off strength at least 1.4 MPa; cracks wider than about 0.25 mm injected before bonding
Strengthening limit (ACI 440.2R)unstrengthened capacity at least 1.1 × dead + 0.75 × live load effects, so loss of the FRP does not cause collapse
Temperaturemaximum service temperature at least 15 °C below the epoxy glass transition temperature
Wrapped corners rounded to at least 13 mm radius (often 25 mm)

Where it fits

Flexural and shear strengthening of slabs, T-beams and box girders for heavier loads or lost reinforcement.
Confinement of columns for seismic ductility and axial strength.
Repair of impact-damaged prestressed girders where strand losses are within the limits of the guide.

Where it does not

Weak, wet or chloride-contaminated substrates that have not been repaired.
Members that need more fire resistance than the unstrengthened section provides.
Cases where existing dead-load stresses or deflections must be reduced (use external post-tensioning).
Surfaces exposed to repeated vehicle impact or abrasion without protection.

Choosing it

Choose FRP over bonded steel plates where access is tight, weight must not increase or plate corrosion is a concern; prefer steel plates where impact or fire resistance matters and handling is easy.
Choose FRP over external post-tensioning for local flexural or shear deficits; choose external PT when dead-load stresses or deflections must be reduced, which passive FRP cannot do.
Prefer near-surface mounted bars or strips to surface-bonded laminates on surfaces exposed to wear, or where debonding governs, provided the cover is deep enough to cut the grooves.
Do not use FRP on chloride-contaminated concrete without first stopping the corrosion; the wrap hides the damage while the substrate fails behind it.

Plant, pace and money

PlantAbrasive blasting or grinding, epoxy mixing and application kit, laminate rollers, groove saws for NSM, access platforms or under-bridge units, pull-off tester.

ProductivityIndicative: tens of square metres of sheet, or roughly 50–150 m of laminate, per crew shift once the substrate is ready; surface preparation often takes longer than the bonding.

CostIndicative: USD 100–400 per m² of bonded sheet or laminate area installed, plus access, substrate repair and any fire protection.

Risks and controls

What goes wrong

Debonding at plate ends or at intermediate cracks; control with strain limits and end anchorage.
Failure in the substrate concrete rather than the adhesive.
Loss of strength in fire or at temperatures near the epoxy glass transition temperature.
Galvanic corrosion where carbon fibre touches steel; use an insulating glass layer.
Hidden corrosion continuing behind the wrap.

Quality assurance

Substrate pull-off tests, surface profile, moisture and dew point records, material certificates and witness panels, void survey by tapping or thermography, records of plies and orientation.

Origins

Bonded steel plates were used to strengthen concrete bridges from the 1960s, notably in France. Urs Meier and Empa replaced the steel with CFRP laminates, first on the Ibach Bridge near Lucerne in 1991, and the seismic programmes in California and Japan after 1989–95 made FRP column wrapping routine.

Examples

Bonded CFRP is now routine for adding shear and flexural capacity to concrete girders and slabs, wrapping piers for seismic ductility, and repairing impact-damaged prestressed girders over highways.

Case studies

Ibach Bridge, LucerneSwitzerland · 1991First use of bonded CFRP laminates to strengthen a bridge, by Empa (Urs Meier), in place of the bonded steel strips used until then.

Horsetail Creek BridgeUSA · 2000A 1914 reinforced concrete girder bridge on the Historic Columbia River Highway, short of shear and flexural capacity for modern loads, was strengthened with bonded carbon and glass FRP instead of being replaced.