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Best Practices:
When No Good Deed Goes Unpunished (I)

Wednesday, July 23, 2014

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First of Two Parts

Coating exterior CMU walls gets a lot of attention, but here’s a case involving interior concrete block.

File this one under the “no good deed goes unpunished” category. What seemed like a creative and well-founded solution caused a much bigger problem down the road because all the pieces of the puzzle (that is, the many ways walls can make paint fail) weren’t considered.

CMU Walls
Photos: MPI

Just one year after coating, the specified system had failed completely.

Two years ago, a shiny new laboratory was constructed with concrete block (CMU) interior walls.

The architect had consulted his MPI Architectural Painting Specification Manual, which describes the recommended coating system options for 25 different substrates.

Under Division 4.2 Concrete Masonry Units, Interior, he selected system INT 4.2F: “Epoxy, (tile-like) over latex block filler, for dry environments.” The first coat in the system is latex block filler MPI #4, followed by two coats of solvent-based gloss epoxy MPI #77.

The Devil in the Details

In retrospect, that choice was a mistake. This was a “clean room” laboratory that would be subjected regularly to rigorous disinfection by fumigation with a fog of hydrogen peroxide.

If a qualified paint inspector had been consulted for this job, he would have taken note of that and advised the architect to instead choose MPI system INT 4.2G “Epoxy (“tile-like”) for wet environments.” There, the first coat is epoxy block filler MPI #116 instead of latex, followed by the two coats of the same solvent-based gloss epoxy MPI #77.

But even that was not the error that ultimately led to the dramatic failure that followed.

Holy Concrete Block, Batman!

When the walls were ready for coating, the architect didn’t like the look of the block; it was too porous.

We see this problem too often when concrete block is the construction material of choice. Different density blocks have very different appearances.

Lightweight block has larger, more numerous holes; standard density block is “tighter,” with fewer and smaller holes. So lightweight block may not have the desired appearance for a wall where aesthetics are an issue.

Furthermore, if a wall is constructed using different density blocks, the difference will be noticeable. And when the wall is inside a brand-new lab intended as a hygienic, scientific showcase, the uneven appearance and texture will be unacceptable.

Shearing

With shearing failure, the epoxy topcoat readily delaminated from the block filler beneath it.

For projects where CMU walls must be aesthetically appealing, we recommend that the specifier and GC assure that different density blocks not be intermingled on the same wall. No matter how much paint or block filler you apply, you’re going to see the differences in the block.

Sacked

So how could the architect solve this problem and achieve the uniform surface the owner wanted? He had the blocks sacked. Sacking is a common practice for rendering a more uniform appearance for concrete walls, but we have not often seen it done on block.

Sacking is also difficult to do well. We've seen many a sacked wall where the end result was poorly adhering material that readily delaminated, taking the coating system with it.

In the lab, however, the sacking was done extraordinarily well. The finished blocks looked relatively smooth and uniform—light years ahead of the original appearance.

The specified system (latex block filler, followed by two coats of epoxy) was then applied.

One Year Later…

Just over one year later, however, cracks formed in the coating and the edges started to lift.

The inspector was called in, put a knife to the lifted edges and, lo and behold, both the block filler and the epoxy topcoat readily came off in sheets right down to the sacking.

We call that “shearing” failure: Applying stress causes intercoat delamination, and the epoxy topcoat readily separates from the coat of filler beneath it.

There were numerous loose areas, and sheets of coating readily came off each time the inspector put his knife to a lifted edge.

So what happened?

Tomorrow: What Went Wrong

About the Author: This article was written by PQA Inspector Dave Lick and is reprinted with permission from the MPI (Master Painters Institute) newsletter. MPI content describes best practices for commercial, institutional, and light industrial painting.

 

   

Tagged categories: Coating failure; Coating inspection; Concrete masonry units (CMU); Failure analysis; Master Painters Institute (MPI); Specifiers

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