Link slabs and joint elimination

How bridges get built · Finishing, articulation and maintenance · Link slabs and joint elimination

Also called: link slab, jointless deck conversion, debonded link slab, continuity slab, joint closure

How it is done

Survey the joints, bearings and girder ends, and check that the substructure can take the longitudinal forces of the longer continuous unit and that the bearings suit the new articulation.
Design the link slab for the end rotations of the adjacent spans under live load and temperature gradient, with a debonded zone over the girder ends so it acts as a flexible hinge, and reinforce it to control crack widths.
Close lanes in stages, remove the joint, break out the deck ends over the girders, keeping existing bars for lapping where possible, and clean the girder tops.
Place the debonding layer over the girder ends, fix new top and bottom reinforcement lapped or coupled into the existing deck, and adjust or replace bearings for the new movement pattern (one fixed pier per unit).
Cast the link slab in fibre-reinforced concrete, or in ECC or UHPC where crack control must be tight, and cure carefully to limit early shrinkage cracking.
Convert the bridge ends to semi-integral abutments, or move the joint to a sleeper slab at the end of the approach slab where a leak does no harm.
Seal the link slab top with a membrane or sealer, map crack widths after the first summer and winter, and update the articulation in the bridge records.

Key numbers

Debonded lengthabout 5 % of each adjacent span on each side of the gap (Caner and Zia)
Link slab thicknessusually the same as the deck, 200–250 mm
Design crack widthabout 0.3 mm
ECC tensile strain capacityabout 2–5 %, against roughly 0.01 % for plain concrete

Where it fits

Multi-span simply supported precast or steel girder bridges with leaking pier joints.
Deck rehabilitation projects that are already closing lanes.
Short to medium overall lengths with piers flexible enough to share the movement.

Where it does not

Long units whose end movements would exceed what the remaining joints can take.
Stiff piers fixed to the deck that would attract large restraint forces.
Bearings that cannot accept the new movement pattern and will not be replaced.

Choosing it

Choose a link slab over renewing a failed pier joint when spans are short to medium, girder ends and bearings are serviceable, and the piers can take the longer unit's forces; the joint problem is removed, not renewed.
Choose full structural continuity (cast-in-place diaphragm and continuity reinforcement) over a link slab on new construction or when the girders need negative-moment capacity; link slabs keep the girders simply supported and are easier to retrofit.
Choose ECC or UHPC over conventional concrete when rotations are large or crack width limits are tight; the volumes are small, so the higher unit cost matters little.
Keep the joint where the unit length, skew or curvature would make the remaining joints and bearings too large, or the piers are too stiff to share the movement.

Plant, pace and money

PlantSaws and breakers or hydrodemolition, small mixer or ready-mixed concrete, deck screed, traffic management.

ProductivityIndicative: one to three joints per week under staged traffic management.

CostIndicative: of the same order as one strip-seal joint replacement per location (tens of thousands of USD per joint), with no recurring joint cost afterwards.

Risks and controls

What goes wrong

Link slab cracks wider than designed.
Restraint forces on piers and bearings.
Poor bond or laps to old reinforcement.
Leak moved to the abutments if the bridge ends are not dealt with.

Quality assurance

Inspection of debonding layer and laps, curing records, crack mapping after the first seasonal cycle, bearing condition checks after conversion.

Origins

Making simple-span precast girders continuous for live load became common US practice from the 1960s. Link slabs that connect only the deck, leaving the girders simply supported, were developed in the 1990s (Caner and Zia, 1998), and the first ECC link slab in the US was built in Michigan in 2005.

Examples

Used by several US state transportation departments to remove pier joints from multi-span girder bridges during deck rehabilitation.

Case studies

Grove Street Bridge over I-94, Ypsilanti, MichiganUSA · 2005First ECC link slab in the United States, built by Michigan DOT with the University of Michigan to replace a pier expansion joint.