How to Avoid Sacking Havoc on Concrete Walls
The Case of the Peeling High-Rise
"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. Mis-applied 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 done during cold weather is always suspect: sacking must be applied at a minimum air temperature of 41° F/5° C and maximum 80% humidity. But problems with the mix or application under even ideal ambient conditions can still lead to dramatic failure.
The perils of coating a sacked surface - and a few simple precautions that can help assure the project proceeds smoothly - are illustrated in the following story.
A general contractor building a new 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 prior to paint application to remove any residual dust, and allowed the surface to dry. He then sprayed and backrolled the walls with two coats of the specified coating – without first checking to see if the sacking was intact. That was his first mistake. And if the painters on the job had looked closely, they might have noticed a fine residual grit gathering in their rollers or in the paint they were rolling with, and concluded something was wrong. But they didn’t.
Shortly after the coating cured, the workers installing the balcony railings noticed that anywhere their tools impacted the wall, the paint started to peel — in fact, the paint was peeling off the entire surface.
The inspector called in to conduct an adhesion test was able to easily peel the new coating off in 5-foot x 5-foot sheets. What did he find? The back of the coating film was caked with sacking material. The sacking that looked okay to the contractor and had survived a power washing in reality had little adhesion to the concrete surface 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 the paint off the entire 32-story building – soffits, walls, 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 since poor quality sacking is epidemic on many new concrete construction projects, it’s critical that the painting contractor himself check the integrity of the sacking prior to accepting the surface.
Ideally, the painting contractor and general contractor would perform this inspection together: run a knife across randomly selected areas of sacked surface to see if the sacking can be easily removed. Alternatively, simply rub the sacking with your fingers. If it’s loose, you’ll see residual powder all over your hand and indentations in the sacked surface -- and if the sacking is sound, you’ll be bleeding! Or certainly you’ll leave 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 it’s cured, the GC and painting contractor should verify the quality and integrity of this mock-up section using the procedure above. If the sacking is unsound, there’s a problem with the mix or ambient conditions, and the problem must be remedied before sacking work can continue.
But ultimately, it is the painting contractor’s responsibility to catch the problem before commencing with painting work, and if necessary inspect the sacking himself. If it’s unsound, he must show the GC, the architect, 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.