Below-Grade Coating for Steel and Concrete: Foundations, Pilings and Buried Structures

Once the backfill goes in, the coating on a foundation wall or buried steel member is out of reach for the life of the structure. Digging it back up to fix a failed coating costs many times what it would have cost to do it right the first time. That's why below-grade coating is one place where cheap prep and thin coats are a false economy.

For decades the go-to answer has been coal tar, most often Carboline's Bitumastic 50 coal tar mastic or Bitumastic 300M coal tar epoxy. Both are still widely used. But they're not the only option, and they're not interchangeable. Here's how to choose.

What Below-Grade Coatings Are Up Against

• Constant moisture from groundwater, perched water, and wet backfill.
• Soil chemistry: chlorides, sulfates, acidic soils, and in some sites road salt runoff. In Michigan, winter de-icing salt is a real factor along foundations near drives and loading docks.
• Soil bacteria, including sulfate-reducing bacteria that attack buried steel.
• Soil stress: backfill settling and freeze-thaw movement that pull on the coating.
• Mechanical damage from rocks in the backfill and handling during installation.

Coal tar's low water vapor permeability and resistance to soil chemicals and bacteria are exactly why it became the standard for this work.

Damp-Proofing vs. Waterproofing

This distinction matters when you're writing or reading a spec. Damp-proofing resists moisture moving through a wall from damp soil. It is not designed to hold back water under hydrostatic pressure. Waterproofing is a membrane system designed to stop water under pressure, like a foundation sitting below the water table.

Bitumastic 50 is sold as a below-grade damp-proofing coating. If the foundation sits in groundwater or the basement has a history of leaking under pressure, you need a waterproofing membrane system, not just a thicker coat of mastic.

Foundation Walls and Concrete Below Grade

For damp-proofing concrete and CMU foundation walls, a single-component coal tar mastic like Bitumastic 50 is simple and economical:

• Concrete cured 28 days at 75°F / 50% RH before coating.
• Surface cleaned and abraded per ASTM D4258 and ASTM D4259. Laitance, form oil, and loose material removed.
• First coat thinned so it soaks into the concrete.
• Total film of 18 mils dry, applied in one or two coats and kept between 12 and 30 mils.
• Backfill no sooner than 7 days after the final coat, with the surface kept above 60°F.

Because Bitumastic 50 is designed to stay soft and self-healing, small scuffs during backfill tend to flow closed. That doesn't make it rock-proof. Keep sharp stone and debris out of the first lift of backfill against the wall.

“Below-grade coating is the one job you never get to redo cheaply. Blast it, build it to thickness, test it, then bury it.”

Buried Steel and Underground Tank Exteriors

For buried structural steel, pipe, and underground storage tank exteriors, the choice depends on whether the steel will see abrasion or cathodic protection:

• Bitumastic 50 works on buried steel that won't see abrasion. It is self-priming and meets MIL-C-18480-B and Bureau of Reclamation CA-50.
• Bitumastic 300M is the better choice when the steel needs a hard, abrasion-resistant film or runs with cathodic protection. The data sheet lists compatibility with controlled cathodic protection, plus references to SSPC-Paint 16, Corps of Engineers C-200/C-200a, AWWA C210 for exterior, and STI-P3 for underground steel tanks.

Steel prep for either should be at least SSPC-SP6 commercial blast for buried service. Carboline allows SP2/SP3 as a minimum for non-immersion, but buried steel in wet soil behaves a lot like immersion, and SP10 near-white blast is the safer spec. Target a 2–3 mil profile.

Steel Piling

Pilings are harder than they look because one pile passes through several exposure zones: atmospheric at the top, a splash or wet/dry zone at grade or waterline, and full burial or immersion below. The splash zone is where corrosion is worst.

For piling, coal tar epoxy is the traditional choice because it cures hard and handles abrasion during driving better than a soft mastic. Coat before driving where possible, apply the full film thickness, and let it cure before handling. If the top of the pile stays exposed to sunlight, expect the coal tar to chalk and brown. Where appearance matters above grade, switch to an epoxy and urethane system above the splash zone.

Test It Before You Bury It

Every square foot of below-grade coating should be checked for thickness and pinholes before backfill. Per NACE SP0188, use a low-voltage wet sponge detector below 20 mils DFT and a high-voltage spark tester above 20 mils. Repair holidays, let them cure, and retest. That inspection takes an hour or two. Excavating a failed foundation coating takes weeks.

When Coal Tar Isn't Allowed

More owners, agencies, and general contractors now write specs that exclude coal tar because of its PAH content and worker exposure concerns. When that happens, the usual substitutes for below-grade steel are high-build coal-tar-free epoxies, often called “coal tar epoxy replacements.” For foundation damp-proofing, asphalt-based (non-coal-tar) damp-proofing products are common. See Coal Tar Regulations and Coal-Tar-Free Alternatives for more.

For the full application procedure, including recoat windows and cure times, see How to Apply Bitumastic.

Frequently Asked Questions

What is the best coating for below-grade concrete foundations?

For damp-proofing, a high-build coal tar mastic such as Bitumastic 50 applied at about 18 mils dry is a proven choice. If the foundation is under hydrostatic pressure from groundwater, a waterproofing membrane system is needed instead.

What is the difference between damp-proofing and waterproofing?

Damp-proofing resists moisture moving through a wall from damp soil. Waterproofing is designed to stop water under hydrostatic pressure, such as a foundation below the water table.

How long before you can backfill over Bitumastic 50?

Carboline allows backfilling 7 days after the final coat, provided the surface temperature has stayed above 60°F.

What coating should be used on steel pilings?

Coal tar epoxy such as Bitumastic 300M is a traditional choice because it cures hard and resists abrasion. Where coal tar is prohibited, a high-build coal-tar-free epoxy is used. Portions exposed to sunlight should get an epoxy and urethane system.

Is coal tar epoxy compatible with cathodic protection?

Bitumastic 300M is listed by Carboline as compatible with controlled cathodic protection.