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Best Practices: How to Avoid
Sacking Havoc on Concrete Walls

Thursday, April 17, 2014

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Sacking—the practice of filling bugholes and other voids in concrete walls by rubbing a mixture of cement, sand, and water through a sack—can be very risky business.

Misapplied sacking can wreak havoc on a coating project. The sacked surface may appear sound to the eye, but if you rub it with your hands, the sacking will come off down to the bare concrete—and any coating applied over the sacked surface will almost surely fail prematurely, sometimes drastically.

Sacking
MPI

Misapplied sacking can wreak havoc on a coating project, and there's no easy fix.

Sacking done during cold weather is always suspect. Sacking must be applied at a minimum air temperature of 41°F/5°C and maximum 80 percent humidity. But problems with the mix or application, even under ideal ambient conditions, can still lead to dramatic failure.

The perils of coating a sacked surface—and a few simple precautions that can avert them—are illustrated in the following story.

The Case of the Peeling High-Rise

A general contractor building a 32-story high-rise condominium told the painting contractor that the extensively sacked exterior concrete surfaces were ready for painting. As is common, the painting contractor power-washed the walls to remove any residual dust and allowed the surface to dry.

He then sprayed and backrolled two coats of the specified coating—but did not check first to see if the sacking was intact. That was his first mistake.

Now, had the painters on the job looked closely, they might have noticed a fine residual grit gathering in their rollers or in the paint and realized that something was wrong. But they didn’t.

Painting concrete high-rise
iStock / © lawcain

After power washing and allowing the surface to dry, the painting contractor should personally ensure that the sacking is intact before painting.

As it was, shortly after the coating cured, the workers installing the balcony railings noticed that anywhere their tools impacted the wall, the paint started to peel.

A Costly Failure

In fact, the paint was peeling off the entire surface.

An inspector was called in and conducted an adhesion test that easily peeled off the new coating in 5-by-5-foot sheets. What did he find? The back of the coating film was caked with sacking material.

The sacking that had looked OK and had survived a power washing turned out to have little adhesion to the concrete and readily came off under the stress of the cured coating.

This turned out to be a costly failure. The painting contractor had to scrape all of the paint off the entire 32-story building—soffits, walls and balconies—and the general contractor had to grind off all the loose sacking, leaving a surface pockmarked with bugholes.

Then the entire surface had to be repainted.

How Could This Have Been Prevented?

The GC or concrete/masonry contractor is responsible for first examining the texture, color, and adhesion of the sacked surface.

But because poor-quality sacking is epidemic on many new concrete construction projects, it’s critical that the painting contractor personally check the integrity of the sacking before accepting the surface.

Sacking
MPI

Adhesion testing before painting can avert a massive coating failure later.

Ideally, the painting contractor and general contractor would perform this inspection together. They may run a knife across randomly selected areas of sacked surface to see if the sacking can be easily removed. 

Alternatively, they could simply rub the sacking with their fingers. If it’s loose, they’ll see residual powder all over their hands and indentations in the sacked surface. And if the sacking is sound, they’ll be bleeding! Or certainly leaving some skin behind.

The Importance of Mock-Ups

It is important to note that the inspection and evaluation of sacking work should be done before the entire building is sacked. The concrete/masonry contractor who’s doing the sacking should start with a small test area.

Once the area has cured, the GC and painting contractor should verify the quality and integrity of this section using the procedure above. If the sacking is unsound, that means that there’s a problem with the mix or ambient conditions. Whatever the cause, it must be remedied before sacking work can continue.

Ultimately, however, it is the painting contractor’s responsibility to catch the problem before painting work and, if necessary, inspect the sacking personally. If it’s unsound, the painter must show the GC, the architect and the owner—whoever is involved with the job—and make clear to them the perils of applying paint over the surface.

To avoid premature coating failure, all unsound sacking should be removed—even if it’s over the entire building. Sacking on exterior surfaces may be removed via high-pressure water cleaning at a minimum of 4000 psi; interior surfaces may be grinded with a diamond grinder.

All around, it’s costly and time-consuming work, and a waste of time and resources that could be avoided with the simple precautions described above.

   

Tagged categories: Adhesion; Coating failure; Commercial Buildings; Concrete defects; Concrete repair; Good Technical Practice; Master Painters Institute (MPI); Peeling; Quality control

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