Cast-in-place post-tensioned box and slab decks

How bridges get built · Cast-in-place concrete decks · Cast-in-place post-tensioned box and slab decks

Also called: PSC box girder, post-tensioned box, voided slab, cast-in-place PT deck

How it is done

Coordinate the details before formwork starts: tendon profiles, anchorage zones with bursting and spalling steel, vents, couplers and congested intersections, with a full-size mock-up of the worst anchorage zone.
Erect the soffit and outer web forms, fix bottom-slab and web reinforcement, and fix ducts on supports at about 0.6–1.0 m to the surveyed profile with anchorages on the bulkheads.
Inspect and survey the duct profile, and pressure-test the ducts where specified, before concreting (hold point).
Place the inner forms and top-slab reinforcement; pour bottom slab and webs first and top slab second, or in one pour with a controlled sequence, controlling heat in thick webs and diaphragms.
Cure, and at the specified transfer strength thread and stress the tendons in sequence, recording force and elongation against the calculated values.
Grout the ducts promptly with a qualified grout, check every vent, and seal and protect the anchorages.
Strike the forms after stressing, survey the deck profile and record camber as the baseline for long-term monitoring.

Key numbers

Box girdersspans 40–250 m; solid and voided slabs up to about 25–30 m
Depthconstant boxes about L/18–L/25 on continuous spans; haunched boxes about L/16–L/20 at the pier
Jacking stressabout 0.75–0.80 fpu
Elongationwithin about ±5–7 % of the calculated value
Groutingcommonly within 7 days of stressing, sooner in aggressive environments
Concrete40–60 MPa

Where it fits

All cast-in-place methods; box girders for spans above 40 m, slabs up to 25-30 m.

Where it does not

Poor grouting practice: voids in ducts are the dominant durability failure of PT bridges.

Choosing it

Choose a box girder over a voided slab above about 30–40 m spans, for curved alignments that need torsional stiffness, and for wide decks with long cantilevers.
Prefer solid or voided slabs below about 25–30 m and for skewed or irregular geometry.
Choose external tendons when inspection and replacement matter; internal bonded tendons give better crack control and ultimate capacity.
Prefer precast girders for repetitive simple spans; a cast-in-place box suits curvature, long continuous spans and appearance.

Plant, pace and money

PlantFormwork, PT jacks and grouting plant, concrete supply.

ProductivitySet by the erection method.

CostDeck USD 1,100-2,500 per m2 depending on the method.

Risks and controls

What goes wrong

Duct blockage, wrong tendon profile, grout voids, anchorage failure at stressing, thermal cracking of thick webs.

Quality assurance

Duct profile survey before pouring, stressing records (force and elongation), grout tests and vacuum grouting where required, anchorage sealing.

Origins

Freyssinet patented prestressing in 1928; Dischinger's Aue bridge in Germany (1937) and Freyssinet's Luzancy bridge in France (1946) were early prestressed bridges, and cast-in-place post-tensioned boxes became the standard long-span concrete deck in the 1950s.

Examples

The game's psc_box_cip and psc_box_cip_fcm deck types.

Case studies

Ganter Bridge, Simplon PassSwitzerland · 1980Christian Menn's curved post-tensioned box with a 174 m main span, its stay cables encased in concrete walls above the deck.

Koror-Babeldaob BridgePalau · 1977Cast-in-place post-tensioned box with a 241 m span whose creep deflection far exceeded predictions, the main reason modern designs model long-term behaviour carefully.