Aged Concrete Floors Can Hide a Multitude of Sins


Excerpted from MPI’s Coatings Specialist training and the experience of Paint Quality Assurance Inspector Dave Lick

Floor projects can be tricky when the slab is new and the design specifications are mindful of the best practices for floor coating application.

But the challenges multiply when an existing building is renovated and re-purposed, and its bare, aged floor is slated to receive a handsome new floor coating.

Problems can start at the bottom: if the surface was never meant to be coated, the slab may be placed directly over gravel or sand with no vapor barrier — and any coating designed to protect the surface will fail if a moisture problem already exists below.

Another concern is that the slab may have been troweled too smoothly and will require careful surface preparation to achieve the necessary profile for proper coating adhesion.

And sometimes, the floor may be sealed with non-visible surface treatments — or contaminated with years of oil and grease — all of which can cause problems with adhesion, curing, or application, and will also prevent acid etch solutions from properly roughening the surface.

Case History: From Welding Shop to Showroom

Such was the case when the intrepid inspector was called to a project where a building that housed a welding shop for 20 years was to be gutted and converted to a car dealership. The bare concrete floors were to receive a decorative finish.

Two different floors were coated by two different contractors using the same epoxy finish. One floor is still in excellent condition after years of service. The other floor failed so quickly that when the new cars were driven into the showroom, their tires started picking up sections of the peeled floor coating.

Why did one floor fail drastically while the other remains in pristine condition years later?

First, Evaluate the Surface

When evaluating the condition of the bare concrete, the inspector’s first task is to check for pre-existing treatments on the floor such as moisture-retarding sealers applied while the slab cured, or clear sealers applied to reduce dust. You can ask for records from the building owner but frequently no history is available. So a surface inspection is required. If the surface is visibly shiny, that’s a sure clue, but since heavy traffic may have worn the treatment away in some places, be sure to look in low traffic areas (corners or under equipment) as well. Also, many treatments are invisible to the eye, so a surface test may be required. The most popular method is the water drop test: if the water drop beads on the surface and is not absorbed by the concrete, there is a good chance the concrete has a treatment. A sure test is to apply a muriatic acid solution to the surface; if there’s no reaction (bubbling, spitting, foaming), there’s a treatment on the surface that must first be removed, typically by mechanical surface preparation methods.

In the case of the welding shop, the floor surface did not have a sealer — however, it was stained extensively from cutting oils and carbon residue. Such excessive contamination can wreak havoc on a coating job by…

  • preventing the acid etch solution from properly roughening the surface
  • interfering with adhesion of the subsequently-applied coating
Project 1: The Secret to Success

The first floor was washed repeatedly with an emulsion cleaner to break down the oil, and after each washing a sample of muriatic acid wash was applied to see if a reaction ensued. If the solution didn’t foam, the surface wasn’t yet clean enough, and another round of scrubbing and hosing down with the emulsion cleaner was required (done in accordance with local environmental regulations). When after repeated cleanings the surface was at last deemed clean enough, the contractor did an acid etch prep with a muriatic acid solution until the surface was dull and gritty to the touch, like 100 grit sand paper, and the surface was thoroughly rinsed, dried, and tested to verify that the pH was within an acceptable range for the coating system.

The decorative epoxy floor coating was then applied: a base coat followed by a clear coat with decorative chips broadcast into the wet surface, followed by two more clear coats.

This floor came out beautifully.

Project 2: The Formula for Failure

After the first project, a second floor in similar condition was coated by a different contractor without an inspector present — and this floor began peeling almost immediately, as soon as the new cars were driven onto it. Failure analysis determined that the only surface preparation was solvent washing in an attempt to lift the oil. That was the first red flag: hydrocarbon solvents such as mineral spirits, MEK, xylene, etc. should not be used to remove grease and oil because they may spread and transfer these contaminants deeper into the concrete. But in addition, no acid etch solution was ever applied to roughen the surface.

In our experience, inadequate surface prep is frequently the culprit when a floor coating starts to peel shortly after application. But sometimes surface preparation gets compromised because the contractor is under relentless pressure from the owner to meet the building opening deadline — and that deadline pressure is exacerbated when the construction schedule is prepared before the extent of the floor contamination is fully recognized. With a project such as this, an owner is better off allowing time for proper surface preparation; the new dealership incurred considerable delay and expense moving the cars to an offsite storage space while the peeling coating could be scraped and grinded off, the surface properly prepared, and a new coating applied.

In summary, bare aged concrete floors can hide a multitude of flaws that, if not rectified, can quickly lead to premature failure. To remove any doubts about the cleanliness of the surface, start by preparing a 3’ x 3’ test patch: apply one coat, let it cure, and then perform a crosscut adhesion tape test ASTM D3359. If the adhesion measures 5a, the rest of the job can proceed.

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