Construction methods · Precast concrete decks

Precast girder casting and prestressing beds

I, U and bulb-T girders are pretensioned on long beds: strands are stressed between abutments, concrete is cast around them, and when it reaches strength the strands are cut and the girder cambers up. The girder's quality, camber and durability are set here, not on site.

How bridges get built · Precast concrete decks · Precast girder casting and prestressing beds

Also called: pretensioning bed, precast yard, girder fabrication

How it is done

Stress strands on a 100-150 m bed with a jack and abutments; drape strands with hold-downs where required.
Set side forms, pour high-strength concrete (50-70 MPa), steam or moist cure to 35-45 MPa within 18-24 hours.
Release the strands, measure camber, store on supports at the design bearing points.

Where it fits

All pretensioned girders: viaducts, overpasses, marine trestles, rail bridges.

Where it does not

Storage on the wrong points (creep and sweep), site casting without a proper bed.

Plant, pace and money

PlantPrestressing bed, steel forms, batching plant, steam curing, gantry cranes.

ProductivityOne girder per form per day.

CostUSD 600-1,000 per m3 of girder.

Risks and controls

What goes wrong

End-zone cracking at release, camber variation between girders, strand slip.

Quality assurance

Strand force records, release strength tests, camber measurement and tracking to erection.

Examples

PCI-certified plants in the US; Korean and Japanese precasters supply most girders by truck.