Construction methods · Finishing, articulation and maintenance

Strengthening: external post-tensioning and FRP

When a bridge must carry more than it was built for, or has lost capacity, it is strengthened: external tendons inside the box or under the deck add prestress; bonded carbon fibre plates and sheets add tension capacity to slabs, girders and piers; steel plates and jackets do the same in heavier form.

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, find the deficiency, design the strengthening for the existing stress state.
External PT: install deviators and anchorages on the existing concrete with drilled anchors, thread and stress tendons, protect with sheathing and grease or grout.
FRP: prepare the surface, bond plates or wrap sheets with epoxy, cure, protect from UV and fire; confined columns are wrapped for seismic ductility.

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.

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.

Examples

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