Strengthening — Jacketing, FRP & External Prestressing — Bridge Rehabilitation & NDT Notes

Last reviewed 30 Sept 2026 · 8 min read

Repair versus strengthening

Repair restores the original capacity and durability.
Strengthening (retrofitting, upgrading) increases the capacity above the original — to carry heavier loads, to correct a design or construction deficiency, to meet new code requirements (such as seismic), or to compensate for damage that cannot be repaired.

Strengthening must be designed: the extra capacity has to be transferred to the existing structure, and the effect on the whole system (higher loads on foundations, reduced ductility, change of stiffness) must be checked. The existing condition (concrete quality, corrosion) must first be repaired: strengthening a damaged member without repair is not durable.

Concrete jacketing

An RC jacket is a layer of new reinforced concrete cast around an existing member.

Purpose: increases the section size, the axial and flexural capacity, the shear strength and the confinement (ductility). Used for columns, piers, pier caps, beams and foundations.
Detailing: the old surface is roughened and cleaned; dowel bars (epoxy-anchored) or shear connectors tie the jacket to the old concrete; new longitudinal bars and stirrups (or spirals) are placed; a high-workability, non-shrink concrete (micro-concrete) is placed by formwork or shotcrete. The thickness is typically 75–150 mm.
Behaviour: the jacket works with the old member only if the interface is bonded; design assumes full composite action if the interface shear is resisted, or a reduced contribution otherwise.
Effect on the structure: a bigger, stiffer member attracts more seismic force and increases the weight, so the foundation is checked.

Steel jacketing

A steel jacket (two half-shells welded together, or steel plates around a column) wraps a column or pier; the gap is filled with grout. It provides very high confinement and shear strength, is fast to install and does not increase the size much. It is widely used for seismic retrofit of bridge piers (steel jackets of circular or elliptical shape) and for corrosion-damaged columns. Corrosion protection of the steel jacket is essential.

Section enlargement and added members

Enlarging the depth of a girder by a new soffit or top layer, connected with dowels.
Adding a new girder or a stringer, or additional cross-girders and diaphragms, to distribute load.
Adding supports (a new pier or a bent under a span) to reduce the span.
Converting to composite action by adding shear connectors and a deck overlay (for a steel–concrete or PSC girder).
Replacing the deck by a lighter one, reducing the dead load.

Externally bonded steel plates

Steel plates (about 5–8 mm thick) are epoxy-bonded to the tension face (flexural strengthening) or the sides (shear strengthening) of a concrete beam and often anchored with bolts at the ends. The plate adds tensile capacity. Points to watch: surface preparation, the epoxy thickness (about 1–3 mm), corrosion of the plate, the weight and handling of long plates (limited length, jointing), and the risk of peeling (debonding) at the ends where the stress concentrates. It has been widely superseded by FRP because of these problems.

Fibre-reinforced polymer (FRP) strengthening

FRP is a composite of high-strength fibres (carbon, glass or aramid) in a polymer matrix (epoxy). It is light, very strong in tension, does not corrode and is easily installed.

FRP	Properties
CFRP (carbon)	Very high strength and stiffness (E ≈ 100–250 GPa), excellent durability; the choice for flexural strengthening; costly
GFRP (glass)	Lower stiffness (E ≈ 20–50 GPa), cheaper; used for confinement and shear; alkali sensitive without protection
AFRP (aramid)	Good toughness and impact resistance

Forms

Sheets and fabrics wrapped and bonded with resin (wet layup) — for confinement of columns, shear strengthening of beams (U-wraps or full wraps) and flexural strengthening.
Pre-cured plates (laminates) bonded with epoxy adhesive — for flexural strengthening of beams and slabs.
Near-surface-mounted (NSM) bars or strips — placed in grooves cut in the concrete cover and embedded in epoxy; better bond and protection than surface plates.
FRP wraps for confinement — increase the strength and ductility of columns by confining the concrete (seismic upgrading).

Design principles

FormulaDesign of FRP for flexure

The FRP acts as additional tension reinforcement attached to the tension face, working at a limited strain — an effective strain $ε_{f e}$ (of the order 0.004 to 0.008, usually limited to about 0.004 or lower) to prevent debonding; it never reaches its rupture strain in practice.
The added moment capacity $Δ M \approx A_{f} f_{f e} (d_{f} - \frac{a}{2})$ with $f_{f e} = E_{f} ε_{f e}$ .
The total capacity is the sum (existing steel + FRP); the ductility falls, so codes limit the strengthening ratio — the existing member must be able to carry at least a certain part of the load without the FRP (in case of fire or vandalism), for example the service load with the unstrengthened section.
Shear: U-wraps or side plates; the effective strain is limited to about 0.004; FRP contributes $V_{f} = \frac{A _{f v} f _{f e} d _{f} ( sin α + cos α )}{s _{f}}$ .
Confinement: the FRP wrap increases the confined concrete strength; the design is based on the confining pressure $f_{l} = \frac{2 n t _{f} E _{f} ε _{f e}}{D}$ for a circular column.

Worked ExampleExample — FRP flexural strengthening

A CFRP plate 100 mm wide and 1.2 mm thick ( $E_{f}$ = 165 000 N/mm²) is bonded to the soffit of a beam. Take the effective strain $ε_{f e}$ = 0.004 and a lever arm about 450 mm.

$f_{f e} = 165000 \times 0.004 = 660$ N/mm².
Plate area $A_{f} = 100 \times 1.2 = 120$ mm².
Tensile force $T_{f} = 120 \times 660 = 79200$ N = 79.2 kN.
Additional moment $Δ M = 79.2 \times 0.45 = 35.6 kN \cdot m$ per plate.

To gain 100 kN·m, three such plates (or a wider or thicker one) are needed. The plate stress of 660 N/mm² is well below its rupture strength (about 2800 N/mm²) — the strain limit from debonding governs the design.

Installation and limits

Surface preparation is the most critical step: the concrete is ground or blasted to remove laitance, cracks are repaired, corners are rounded (radius about 15–20 mm) for wraps, and the surface is dry and clean.
Environment: temperature and humidity limits for the epoxy; UV and fire protection coatings; the glass transition temperature of the epoxy (about 60–80 °C) limits performance near fire or in hot climates.
Anchorage: end anchorage (U-wraps, bolts or FRP anchors) to delay debonding.
Quality control: pull-off tests, tap tests for voids, thickness checks, coupon tests of the cured composite.
Durability: FRP does not corrode, but the adhesive and the interface must be protected; monitor after installation.

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