Deck waterproofing and drainage

How bridges get built · Finishing, articulation and maintenance · Deck waterproofing and drainage

Also called: bridge deck membrane, waterproofing, deck drainage

Deck waterproofing and drainage — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Fix the system at design stage: membrane type, protection layer, falls, gully positions, upstands at kerbs and joints, and drip details; check that crossfall and longitudinal fall carry water to the outlets without ponding.
Prepare the deck once the concrete is old enough for the system: remove laitance and curing compound by shot or grit blasting, fill blowholes, and confirm surface moisture and that the deck is above the dew point.
Prime and apply the membrane: sprayed methyl methacrylate or polyurethane in two coats of contrasting colour, or bitumen sheets with lapped and sealed joints; turn it up at kerbs and into joints and drain outlets.
Test before covering: film thickness, spark (holiday) testing of sprayed systems, adhesion pull-off tests and inspection of laps; repair defects.
Place the protection layer or first asphalt course at controlled temperature, keeping tracked plant and sharp aggregate off the bare membrane.
Install gullies at low points and on the upstream side of joints, edge drains to remove water trapped on the membrane, and carrier pipes discharging away from piers, bearings and the ground below.
Flow test the drainage, record membrane areas and batches, and put gully cleaning into the maintenance manual.

Key numbers

Sprayed membranes (MMA, polyurethane)about 2–3 mm dry in two coats; MMA cures in about an hour
Bitumen sheet membranesabout 4–5 mm, laps of 100–150 mm
Deck fallscrossfall typically 2–2.5 %; avoid longitudinal grades below about 0.5 %
Outputabout 1,000–3,000 m² per day for sprayed systems
Indicative costUSD 30–70 per m² installed

Where it fits

All concrete decks with asphalt surfacing; steel decks under polymer overlays have their own systems.

Where it does not

Membranes applied to damp or dusty decks; drains that discharge onto the pier.

Choosing it

Choose a sprayed membrane over sheets on complex geometry, around many upstands, or where a fast cure matters; choose sheets where the crew and climate suit torch-on work and the deck surface is rough.
Choose a membrane under asphalt over a bare deck wherever de-icing salt or marine chloride is present; asphalt without a membrane traps salt water against the concrete and is worse than no surfacing.
Prefer a bonded concrete or polymer overlay to a membrane where the deck must carry traffic directly without asphalt, or where an existing bare deck is being preserved.

Plant, pace and money

PlantBlasting, spray rigs, thickness gauges.

Productivity1,000-3,000 m2 per day.

CostUSD 30-70 per m2.

Risks and controls

What goes wrong

Blistering, pinholes, damage from paving plant, blocked drains.

Quality assurance

Adhesion and thickness tests, holiday testing, drainage flow test.

Origins

European concrete decks have been waterproofed under asphalt since the mid-20th century, first with mastic asphalt and bitumen sheets, later with sprayed MMA and polyurethane systems; the UK made certified membranes a requirement for trunk road bridges (BD 47, now CD 358). Much of North America instead kept bare decks and relied on concrete quality, cover, sealers and, from the 1970s, epoxy-coated reinforcement.

Examples

Standard on Korean, European and Gulf decks; US decks often use bare concrete with sealers instead.

Case studies

Ynys-y-Gwas BridgeUK · 1985A 1953 precast segmental post-tensioned bridge collapsed without warning after chloride-laden water passed through the deck at the segment joints and corroded the tendons; it pushed UK practice toward watertight decks and joints and led to the 1992–96 moratorium on grouted post-tensioned bridges.

Midland Links motorway viaductsUK · 1970s onwardSalt water leaking through deck joints onto the crossheads and columns of the elevated M5/M6 around Birmingham caused decades of costly concrete repair and cathodic protection, the standard UK example for keeping water off substructures.