Strengthening: external post-tensioning and FRP

How bridges get built · Finishing, articulation and maintenance · Strengthening: external post-tensioning and FRP

Also called: external prestressing, CFRP strengthening, carbon fibre plates, bridge strengthening

How it is done

Assess the structure: inspect and test (cores, cover, chlorides, tendon condition, crack maps), establish the existing stress state and the deficiency (flexure, shear, fatigue, lost prestress), and set the target capacity.
Design the system: tendon layout and force, deviators and anchorage blocks, the capacity of the old concrete to take the anchorage forces (local zone, bursting, shear transfer of new blocks), and the effect of the added prestress on other sections and on the supports.
Scan for existing tendons and bars, then core through webs and diaphragms; build anchorage blocks and deviators against roughened concrete, tied in with stressed bars or bonded dowels, or use steel brackets bolted through.
Install HDPE ducts and strands or greased monostrands, stress in the designed sequence with calibrated jacks, recording force and elongation, and monitor cracks and deflection during stressing.
Protect the tendons with grout, wax or grease inside the duct, with anchorages that allow later inspection, restressing or replacement, and leave access openings.
Where only local tension or shear capacity is missing, bond FRP or steel plates instead (see FRP strengthening).
Verify by load test or monitoring where the margin is small, and update the load rating and the maintenance manual.

Key numbers

Tendonstypically 7–19 strands of 15.2–15.7 mm (1,860 MPa) in HDPE ducts, or greased and sheathed monostrands
Jacking stress limit (EN 1992-1-1 recommended values): the lesser of 0.8 fpk and 0.9 fp0.1k
Friction at deviators (EN 1992-1-1)μ about 0.10–0.12 for HDPE ducts and 0.18–0.25 for steel ducts
Programmeweeks per span, governed by coring and anchorage blocks rather than stressing
Indicative costUSD 200–500 per m² of deck strengthened

Where it fits

Load rating deficiencies, corrosion losses, seismic retrofit of piers, change of use.

Where it does not

FRP on wet or weak concrete substrates; external PT without anchorage capacity checks in the old concrete.

Choosing it

Choose external PT over FRP when the deficiency is global flexure or lost prestress over a span: it actively closes cracks and reduces dead-load stresses and live-load stress ranges, which passive FRP cannot do.
Prefer FRP or bonded plates for local flexural or shear deficits, or where there is no room for anchorages and deviators.
Check that the old concrete can take the anchorage forces before choosing external PT; if it cannot, steel anchor frames or new diaphragms add cost and time.
Prefer replacement when tendons are badly corroded across many sections, chlorides are high throughout, or the strengthened bridge would still need a weight limit.

Plant, pace and money

PlantDrilling, PT equipment, epoxy and FRP kits, access platforms.

ProductivityWeeks per span.

CostExternal PT USD 200-500 per m2 of deck; FRP USD 100-300 per m2 applied.

Risks and controls

What goes wrong

Anchorage failure in old concrete, debonding of FRP, fire vulnerability of epoxy.

Quality assurance

Pull-off tests, anchorage proof loads, stressing records.

Origins

Franz Dischinger used external prestressing on the bridge at Aue, Germany (1936–37); the technique was revived in France from the 1970s to repair and strengthen prestressed bridges and then for new segmental decks, and Long Key Bridge, Florida (1980) brought it to US practice.

Examples

Hundreds of European motorway viaducts strengthened with external PT; Japanese and Californian pier wrapping after Kobe and Northridge.

Case studies

Hammersmith FlyoverUK · 2011–15Corrosion of the internal tendons of the 1961 flyover by salt water closed it in December 2011; it was strengthened with new external post-tensioning, and its bearings and joints were replaced.

Koror–Babeldaob BridgePalau · 1996The 241 m record-span balanced cantilever box girder of 1977 had sagged by more than 1 m; remedial works with added post-tensioning and removal of the midspan hinge were completed in 1996 and the bridge collapsed about three months later. The standard warning to understand the existing stress state before adding prestress.