Deck slabs on girders: cast-in-place, precast panels and stay-in-place forms

How bridges get built · Precast concrete decks · Deck slabs on girders: cast-in-place, precast panels and stay-in-place forms

Also called: composite deck slab, precast deck panels, stay-in-place formwork, full-depth precast deck

Deck slabs on girders: cast-in-place, precast panels and stay-in-place forms — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Survey the girder tops and set the haunch (build-up) over each girder to the deck profile, allowing for the remaining camber and the deflection under the slab weight.
Install the forms: overhang brackets on the exterior girders and, between girders, removable forms, steel stay-in-place forms or partial-depth precast panels on bedding strips.
Fix the bottom and top mats on chairs, check cover and shear connectors, and set the screed rails for the finishing machine.
Pour in the planned sequence (positive moment regions first and regions over the piers last on continuous spans), using a finishing machine and a retarder for long pours.
Wet-cure for about 7 days, starting fog sprays or evaporation control as soon as finishing allows.
For full-depth precast panels, set the panels on levelling devices, then make the joints and fill pockets and haunches (see the full-depth panel method).
Inspect cover and cracking at 28 days and seal cracks before waterproofing or surfacing.

Key numbers

Slab thickness200–250 mm typical (AASHTO LRFD minimum 175 mm)
Top cover50–65 mm where de-icing salts are used
Partial-depth precast panels80–100 mm thick with a cast-in-place topping
Curingabout 7 days of continuous wet curing
Evaporation limit while placingabout 0.5–1.0 kg/m2/h
Costcast in place USD 150–250 per m2; full-depth precast USD 300–450 per m2

Where it fits

All girder bridges; full-depth panels for accelerated construction and deck replacement.

Where it does not

Cast-in-place in freezing or very hot weather without protection; full-depth panels on girders with uncontrolled camber.

Choosing it

Choose a cast-in-place slab on removable forms when time is available: cheapest, and the simplest to make continuous and smooth.
Prefer stay-in-place steel forms or partial-depth precast panels to cut formwork time over traffic or water; steel forms hide the soffit from inspection, and panels can show reflective cracking over their edges.
Prefer full-depth precast panels for redecking under short closures and for accelerated construction, provided girder cambers are controlled.

Plant, pace and money

PlantFormwork, finishing machine, curing, cranes for panels.

ProductivityCast-in-place 2-3 weeks per span; full-depth panels a few days.

CostCast-in-place USD 150-250 per m2; full-depth precast USD 300-450 per m2.

Risks and controls

What goes wrong

Early cracking from shrinkage and restraint, low cover over the top mat, panel seating gaps.

Quality assurance

Cover surveys, curing records, panel haunch grouting, crack mapping at 28 days.

Origins

Composite cast-in-place slabs on girders became standard once welded shear studs were adopted in the 1950s; stay-in-place steel forms and partial-depth precast panels spread in the US from the 1960s and 1970s to cut formwork labour, and full-depth panels grew with deck replacement and accelerated construction programmes.

Examples

Full-depth panels are standard for US deck replacements and Korean ABC.

Case studies

Woodrow Wilson Bridge (original), Washington DC areaUSA · 1983The deck was replaced with full-depth precast panels, an early large-scale use of panels for redecking a busy crossing.