When the substrate rejects the coating

What's the best way to coat fly ash concrete?

From the experience of PQA Inspector Dave Lick

A tidy few years ago, the inspector found himself on a project that should have been a breeze: a new Arts Center with four levels of poured-in-place concrete ceilings.

The concrete surfaces were in virtually ideal condition: the concrete contractor had done an excellent job leveling and roughening the walls with no sacking or poor-quality patching work to be seen. This was a LEED job, so the spec called for a water-based primer with a strong track record approved under MPI #3 Primer, Alkali Resistant, Water Based.

The project's schedule required the painters to start by priming only a small 50-foot section prior to coating the remaining ceiling surfaces. That was serendipitous, because the next day the inspector found the primer had zero adhesion to the surface; it readily came off with very fine dust stuck to the back of the delaminated film.

Relieved at finding the problem before the whole job was primed, the inspector decided the coating had been applied over a ‘dirty’ surface and the dusty contaminants had interfered with adhesion. He showed the dusty film sample to the painting contractor and told him a cleaner surface would be required. The contractor was dubious (“we broomed the hell out of it!”) but proceeded to meticulously broom and clean the area again with wet rags; since this was an interior surface, power washing was not an option. The primer was applied …and again, the same result: zero adhesion.

The inspector asked for three new test patches, stood by to oversee the contractor’s surface prep work, and applied three other products approved under MPI #3 including a clear product the inspector knew to have exceedingly high penetration characteristics. No luck: three complete failures and an inability to penetrate the pervasive fine dust.

The inspector now grew suspicious and asked the GC, “Is there something different about this concrete?” The answer was: yes, it’s fly ash concrete.

What’s Fly Ash Concrete?

Concrete is typically composed of gravel, sand, water, and Portland cement. Worldwide, the manufacture of Portland cement has a relatively large carbon footprint; as of 2005, it accounted for the greenhouse gas equivalent of 330 million cars driving 12,500 miles per year (sustainableresources.com). Fly ash, a by-product of coal-fired electric generating plants that might otherwise end up in a landfill, can be substituted for more than 25% of the Portland cement portion of the mix. Two types of fly ash are available: Class C fly ash, which is typically light or tan colored and is produced from burning lignite or sub-bituminous coal; and Class F fly ash, which is dark grey and is produced from burning anthracite or bituminous coal.

Fly ash consists mostly of silica, alumina, and iron; its tiny sphere-shaped particles fill small voids and produce a denser concrete. So using fly ash concrete offers the benefit of a stronger, more durable product while reducing the environmental impact of concrete manufacturing and providing a beneficial re-use for fly ash. Using fly ash concrete can help a project achieve LEED credits for recycled content.

The paint inspector has a different perspective. Coal and oil are essentially the same product — they’re just different ages — so he suspects fly ash might impart oily characteristics to the concrete. And this we know to be true: a surface with an oily residue will always present a challenge to water-based coatings.

Try a Solvent-Based Primer

The GC on the project had experience with fly ash concrete, and suggested the contractor try using a solvent-based primer instead. So a product approved under MPI #69 Primer, Bonding, Solvent Based was applied to a test patch — and this time, the inspector picked an ‘unclean’ area that had not undergone the meticulous broom-and-wipe preparation used for the waterbased primed surfaces. Lo and behold, 100% penetration of the concrete surface was achieved and the adhesion was perfect; the inspector couldn’t pry it off with a knife.

It may be noted that MPI #69 is not defined as an alkali-resistant product, and its recommended applications are not specifically concrete but rather “problematic surfaces;” however, the concrete walls in the Arts Center were months old by the time painting work commenced so alkali burn was not likely to be a problem, and the particular product chosen by the inspector was recommended for use on cementitious surfaces.

Changing the primer to a solvent-based product would eliminate the architect’s opportunity to get a LEED paint point on the project, but nevertheless, the architect agreed. The priming was done over the weekend so that use of the solvent-based product wouldn’t interfere with other trades working on the building, and the walls were finished with the specified standard latex finish coat.

Déjà Vu All Over Again

Fast forward to just a few months ago: the inspector was working on a new maximum security prison with 30-foot high tilt-up concrete walls. Once again, the concrete was in ideal condition: power ground and roughened, then power washed, no sacking or patches, slightly rough and clean.

As part of his regimen, he looked for shiny spots on the surface that might indicate bond-breaker compounds that can interfere with coating adhesion. Instead, he saw telltale dark grey cloudy patches in the concrete, not just on the surface, but deep into the slab as far as he could scratch. This was a LEED project, so the older-and-wiser inspector now knew to ask the GC: “is this fly ash concrete?” And yes indeedy, it was.

Just as with the fateful arts center project years before, a waterbased primer approved under MPI #3 was specified. So the wary inspector requested a test patch be prepared first — and sure enough, complete failure with zero adhesion ensued. He tried a test patch with another MPI #3 product that had never failed him before and once again, zero adhesion. The architect and GC were on site to view this fiasco, so the inspector asked for two more test patches: one with the surface thoroughly cleaned of dust and residue, the second simply ‘as is.’ Both areas were then primed with a solvent-based primer approved under MPI #69 — and both performed perfectly.

The architect initially balked at changing the spec to the solvent-based primer, since this would eliminate his ability to get the LEED paint point. But as the inspector pointed out: what was the alternative? In theory, abrasive blasting could create a profile with sufficient tooth for the waterbased primer to adhere, but the suspected oily residue could still be present — and besides, abrasive blasting would have been prohibited on this project. The architect relented and the solvent-based primer was applied. Since this was an interior space, the GC supplied substantial ventilation to assure a safe working environment and once again, the work was done on weekends to avoid disturbing other trades on the project.

While both projects were ultimately resolved successfully from a coating performance perspective, the inspector wonders if there is a better solution out there, especially one that might enable the use of waterbased primers. So we ask our readers: do any of you have experience coating fly ash concrete? If so, what’s your solution — and keep in mind that abrasive blasting might seem like an alternative, but is rarely acceptable on many commercial/institutional projects. Email your responses to shaun@mpi.net.

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