Corrugated steel culvert coatings: rehabilitating the invert in place

A corrugated steel culvert can be rehabilitated in place when the invert steel is worn but still continuous: the water is diverted, the surface is blast cleaned, thinned areas are repaired, then a thick abrasion-resistant coating is applied to the invert and lower walls. The work follows confined space rules, and the selected system must not reduce the hydraulic capacity the structure was designed for.

Galvanized corrugated steel culverts, and sometimes aluminized ones, sit under access roads, private roads, mine sites, industrial parks and rail lines. Their crown often stays in good condition while the invert, at the bottom, loses its zinc and then its steel. Replacing the structure means excavation, road closure and heavy permitting. A well-chosen coating, applied in time, extends the life of the culvert without touching the backfill. This guide is written for infrastructure and maintenance managers in Quebec and Ontario. It builds on our protective coatings reference page.

Why does a culvert invert wear out before the crown?

Because the invert takes every form of attack: it receives sand, gravel and debris carried by the water, it stays wet all the time and it collects de-icing salt from the road. The crown sees only humidity and occasional flow.

The mechanisms involved:

• Bedload abrasion. Sediment wears away the zinc layer, then the steel, especially in fast streams and during spring runoff.
• Erosion-corrosion. Abrasion removes the corrosion product layer as fast as it forms, which speeds up metal loss; see our article on erosion-corrosion.
• Aggressive water. Acidic, soft or chloride-laden water consumes zinc faster.
• De-icing salt. Road runoff enters the culvert; our guide to de-icing salt corrosion explains its effect on steel.
• Waterline zone. The usual waterline alternates between wet and dry and often corrodes faster than the rest of the wall.

How do you assess a culvert before choosing a solution?

The assessment combines a visual inspection of the interior, a measurement of the remaining steel thickness at the invert and a check of the structure's shape. A coating does not restore structural strength: it protects steel that is still load-bearing.

Points to check:

• Perforations. Number, size and location, especially at seams and bolt rows.
• Remaining thickness. Measured by ultrasound at the invert and compared with the nominal sheet thickness.
• Deformation. Flattening, buckling or sagging of the crown, which call for a structural review rather than a coating.
• Backfill loss. Material washed out around the culvert, often through perforations, creates voids under the road.
• Ends. Bevels, headwalls and riprap, which govern flow at the inlet and outlet.

The assessment also records the stream itself: how fast the water runs, how much sediment it carries in spring and whether debris tends to jam at the inlet. These conditions drive the choice of coating thickness and type as much as the condition of the steel.

Our coating inspection service produces this assessment with photos and measurements, and it becomes the basis of the quote.

What coating-based rehabilitation options exist?

Three families come up most often: a thick abrasion-resistant coating on the invert, a rebuilt invert in concrete or mortar, and a protective coating over the full circumference. The table compares their uses.

A polyurea coating is often selected for its abrasion resistance and fast set, which helps when the diversion window is short. On a galvanized culvert that is still sound, a zinc-compatible system is applied, as for a duplex system on galvanized steel.

How does in-place rehabilitation work?

It is done during low-flow periods, with the water diverted out of the work area: the diversion is installed, the culvert is cleaned, blast cleaned and repaired, the coating is applied and inspected, then flow is restored after the cure time.

1. Flow diversion. Cofferdams and pumping, or a temporary pipe, depending on flow and on the permits the owner has obtained.
2. Cleaning. Removal of sediment, debris and vegetation, then hydro blasting or abrasive blasting to the specified grade.
3. Repairs. Holes closed with plates, inserts or a suitable filler; corroded seams and bolts restored.
4. Application. Coating applied in coats per the data sheet, with particular attention to the corrugations, where film thins on the crests.
5. Inspection. Dry film thickness readings, holiday testing if the specification calls for it, visual inspection.
6. Return to flow. After the cure time stated by the manufacturer, with temporary works removed.

Work in or near a watercourse follows the applicable environmental authorizations, which the owner obtains beforehand. Our maintenance and rehabilitation team coordinates these steps.

How do crews work safely inside a culvert?

A culvert is generally a confined space: entry happens only with a hazard assessment, an entry permit, atmospheric monitoring, mechanical ventilation and a rescue plan. CSA Z1006 serves as the reference for managing confined space work, along with provincial regulations.

Hazards specific to culverts:

• Sudden rise in water. Rain upstream can fill the structure quickly; weather and water level are monitored and an evacuation signal is set.
• Atmosphere. Gases from decaying sediment, oxygen deficiency, solvent vapours and blasting dust.
• Restricted access. Crouched work, slippery corrugated footing, exit possible only through the ends.
• Traffic overhead. Road vibration and noise, signage near both ends.

Low-solvent products make ventilation easier. Workers wear respiratory protection suited to the products and to blasting.

Does the coating change the culvert's hydraulic capacity?

A thin coating barely changes the flow area, but it changes wall roughness; a concrete invert, on the other hand, reduces the flow area. In both cases, the owner has the responsible designer confirm that the design flow and, where relevant, fish passage are still met.

What to keep in mind:

• Roughness. A smooth wall moves water faster, which can increase scour at the outlet or hinder fish passage in some streams.
• Flow area. Any thickness added to the invert slightly reduces clearance; a concrete slab reduces it more.
• Slope and ends. A coating does not correct an unsuitable slope or outlet scour, which are handled by other means.

Our role is to apply a system that fits the hydraulic requirements set by the owner, not to redefine them. Our article on internal pipe lining for water covers related questions for pressure pipes.

Where do we work and how do you get a quote?

Our crews and qualified subcontractors work from LaSalle across Quebec and Ontario, including Saguenay-Lac-Saint-Jean and Mauricie and Centre-du-Québec, on industrial, mining and forestry road networks.

Pricing is on consultation: it depends on culvert diameter and length, condition, the diversion required, access, the selected system and the work period. Send photos, dimensions and location through the quote request: reply within 24 business hours.

Frequently asked questions

Can a culvert that is already perforated be coated?

Yes, if the holes are localized and the surrounding steel is still load-bearing: they are closed before the coating is applied. If the invert is perforated over a long section or the structure is deformed, a rebuilt invert or replacement is more appropriate.

Does flow have to stop completely?

The work area must be dry, but the stream does not have to stop: a pumped diversion or a temporary pipe carries the water past the work.

Does the coating bond to galvanized sheet?

Yes, with preparation and a primer compatible with zinc. Remaining zinc is cleaned and profiled; areas where it has disappeared are blast cleaned to bare steel.

What season is best for the work?

The low-flow period, usually late summer or early fall, when flow is low and temperatures allow the products to cure. The exact window depends on the stream and on the permits.