Repainting steel bridges with containment

How bridges get built · Finishing, articulation and maintenance · Repainting steel bridges with containment

Also called: maintenance painting, lead paint removal, bridge repainting, overcoating, abrasive blast cleaning with containment

How it is done

Survey the coating: condition rating, adhesion and thickness, lead and chromate content, and areas of section loss; choose spot repair, overcoating or full removal on life-cycle cost.
Design the containment to the class needed (SSPC Guide 6), checking wind load and added weight on the bridge, clearance for traffic, rail or navigation below, and negative air pressure with dust collection for abrasive blasting.
Erect access and containment section by section: suspended platforms or scaffolding wrapped in impermeable sheeting, with ventilation and filtration.
Prepare the surface: abrasive blast with recyclable steel grit to Sa 2½ (SSPC-SP 10) for full removal, or power-tool clean and feather the edges for overcoating; wash off soluble salts and verify salt level and profile.
Apply the new system before flash rust appears: typically a zinc-rich primer, epoxy intermediate and polyurethane or polysiloxane topcoat, with stripe coats on edges, bolts and welds; measure dry film thickness after each coat.
Collect, test and dispose of the waste, as hazardous where it fails leaching tests, and monitor airborne lead, workers' blood lead, and soil and water around the site.
Dismantle the containment, inspect the finished section, and record the system, thickness and date for the next maintenance cycle.

Key numbers

Surface preparationSa 2½ (ISO 8501-1) or SSPC-SP 10, blast profile about 50–100 µm for zinc-rich primers
Soluble chloride on the blasted steelcommonly limited to about 3–7 µg/cm²
Three-coat zinc, epoxy and polyurethane system: about 250–350 µm total dry film thickness
Durability (ISO 12944)'high' class 15–25 years and 'very high' over 25 years to first major maintenance
Airborne lead (OSHA 1926.62)action level 30 µg/m³, permissible exposure limit 50 µg/m³ (8 h average)
Applicationsteel at least 3 °C above the dew point

Where it fits

Steel bridges at the end of a coating cycle.
Bridges carrying lead or chromate paint.
Marine and de-icing environments where coating breakdown leads quickly to section loss.

Where it does not

Open abrasive blasting of lead paint.
Overcoating weakly bonded old systems.
Painting when the steel is less than 3 °C above the dew point or humidity is high.

Choosing it

Choose spot repair or overcoating over full removal when the existing coating is well adhered and failures are local; it avoids most of the hazardous waste, but test patches must first show the old paint can carry new coats.
Choose full removal and a new system when breakdown is widespread, adhesion is poor or section loss must be repaired; it resets the cycle for 20 years or more.
Use full enclosure under negative pressure rather than open blasting wherever lead or chromate is present or the bridge crosses water, roads or housing.
Consider metallising (thermally sprayed zinc or zinc-aluminium) under the paint on marine or splash-zone members when the next repaint must be as far away as possible; it needs better surface preparation and costs more.

Plant, pace and money

PlantContainment structures, dust collectors, abrasive blasting and recycling units, airless spray, dehumidifiers, coating inspection instruments, waste handling.

ProductivityIndicative: a few hundred square metres of full removal and repainting per containment per week; large bridges are repainted in campaigns lasting several years.

CostIndicative: overcoating USD 30–80 per m²; full removal and repainting with containment USD 100–300 per m² of steel surface, more where traffic, rail or water govern access.

Risks and controls

What goes wrong

Lead exposure of workers and the public.
Wind loading on the containment and the bridge.
Release of debris to water or soil.
Premature coating failure from salts or poorly coated edges.
Fire inside the containment.

Quality assurance

Soluble salt and profile tests, dry film thickness readings to ISO 19840 or SSPC-PA 2, adhesion tests, air and blood lead monitoring, waste classification records.

Origins

Lead-based paints protected steel bridges for about a century. As their hazards were recognised in the 1970s–90s, US rules (OSHA 1926.62 in 1993) and SSPC containment classes turned repainting into an enclosed, monitored operation, while zinc-epoxy-polyurethane and glass flake epoxy systems extended maintenance cycles to 20 years and more.

Examples

Long-span steel bridges in the US, UK and Australia are repainted in multi-year campaigns inside enclosed containment; robotic blasting reduces worker exposure in confined areas.

Case studies

Forth BridgeUK · 2002–11The 1890 cantilever bridge was blast-cleaned to bare steel inside encapsulated scaffolding and recoated with a glass flake epoxy system expected to last about 25 years, ending the idea of endless repainting.

Sydney Harbour BridgeAustralia · 2013Abrasive blasting robots developed with the University of Technology Sydney were brought in to remove lead paint in confined parts of the arch, reducing workers' exposure.

Golden Gate BridgeUSA · 1965–95The original lead-based paint was progressively removed and replaced with a zinc-rich primer and new topcoat system; painting continues as routine maintenance.