Q: The painted surfaces of some of the tilt-up concrete walls on our projects seem to develop blisters, peeling, or flaking shortly after application. Sometimes this occurs after it rains. Is there anything different about tilt-up walls that makes this happen?
A: As with any paint or coating project, the key to good results is paying attention to the three Ps: Prep, Prime, and Paint. Procedures for painting and coating concrete surfaces are different from other surfaces, and tilt-up concrete surfaces do require special preparation.
Surface Preparation: The most likely reason for blistering and peeling in a tilt-up wall, especially if it occurs shortly after it rains, is failure to remove the mold release agents and bond breakers used for casting the concrete component.
This is the big difference between tilt-up concrete and other forms: It is cast on the jobsite in concrete forms. After the concrete wall or column has cured, a mobile crane tilts the piece up and moves it into place, where it is braced into position and secured. Painting contractors may not always be fully aware of this because they arrive on the jobsite after the walls are in place.
To prevent the concrete from adhering to the molds, contractors apply release agents, or bond breakers, to the mold before the concrete is poured. These release agents can be solvent-based, water-based, oil-based, silicone-based, silicone-free, silicone water-based and many other proprietary combinations. But they all perform the same function—they create lower surface energy between the concrete form and the concrete to mitigate adhesion. The objective is to be able to lift the cured concrete from the casting mold smoothly and cleanly.
Unfortunately, release agent residue also can inhibit adhesion of coatings and paint to the concrete surface. Most painting contractors are aware of this, and paint companies do a good job of educating painters about the need to remove release agent residue before painting or coating tilt-up concrete surfaces. Power washing at the specified pressure using the specified cleaning solvent should do the trick, but there are two cautions.
One, be methodical and thorough when power washing. Two, there is a trend toward making release agents and paints/coatings more compatible, but it is always prudent to power wash the surface first. Even with “compatible” release agents, if too much was applied to one area, it could swamp the system and adversely affect coating adhesion.
All concrete surfaces must be washed before coating to remove dirt, dust, and excess sand anyway, so always take the extra step of power washing tilt-up concrete to remove any release agent residue, whether they are compatible or not.
Also, check for any signs of mildew, which can be removed by washing the surface with a 3:1 water-bleach mixture, leaving it on for 20 minutes, and adding more as it dries, as needed.
In addition, any material containing portland cement experiences some measure of efflorescence. This is caused when soluble salts and other water-dispersible materials rise to the surface of concrete as a result of low temperatures, rain, dew, condensation, or water added to the surface of fresh concrete to facilitate troweling. Typically white, if efflorescence is present, remove it by wire-brushing by hand. Efflorescence will cause discoloring of painted concrete walls, but probably not the blistering and peeling you have seen.
Priming: Using a good acrylic or acrylic/styrene masonry sealer or primer on concrete surfaces shortly after installation is essential to facilitate maximum adhesion, durability, and resistance to efflorescence and mildew.
This is especially true because few contractors have the luxury of waiting a month for concrete to fully cure. All acrylic masonry primers and sealers are designed to handle the higher alkalinity (pH) of fresh concrete. Most primers or sealers are designed to be applied at a few mils dry film thickness. Elastomeric coatings are designed for the 10 to 20 mils referenced here.
Painting: The most likely reason for flaking after application to a masonry surface is a paint that is not 100% acrylic or acrylic/styrene, both of which are highly durable and water resistant. Most alternative paints will hydrolyze (decomposition of a compound due to reaction with water) in moist conditions, causing the paint to flake apart.
The issue seems to be proliferating. This suggests that either the contractor is not aware that tilt-up is concrete, or that alternative paints and coatings are engineered for cost reductions, but not for performance.
It’s always less costly to do it right the first time. Always prepare tilt-up concrete surfaces thoroughly, removing all mold release agent residue. Be sure to prime the concrete surface with a high-quality acrylic masonry primer or sealer. Finally, be sure to use a 100% high-quality acrylic masonry paint for a durable, colorfast surface.
Contributed by Dow Construction Chemicals. Visit www.dowconstructionchemicals.com for more information.