Q: Some new construction projects in my area are developing visible cracks in their exterior concrete walls within a year or two, despite using elastomeric wall coatings. Is this due to poor coating quality? If so, how can contractors be certain of the quality of the elastomeric wall coating they use?
A: Thermal changes and ground settlement are some of the reasons concrete surfaces crack. This is inevitable with all types of concrete and concrete surfaces, ranging from smooth and split-face concrete block to poured-in-place, precast, and tilt-up walls.
As water migrates out of the concrete during curing, the resulting hardened material does not have the flexibility to expand and contract with routine temperature changes. Barely perceptible hairline cracks in the summer can become four times wider when they contract during the winter. Also, new buildings settle, creating additional stress.
High-quality elastomeric wall coatings (EWC) applied at high film-build will bridge these inevitable concrete surface cracks and hide them from view. They also resist moisture, mildew, and dirt damage while maintaining an attractive, uniform appearance on the concrete surface.
Unfortunately, the success of EWC performance over the last several decades has led some coating manufacturers to cut corners on materials and properties such as elongation and film thickness. Contractors are often seduced by lower prices without understanding the potential ramifications of using lower-grade EWCs.
There are several important criteria to check before specifying an EWC for your future projects. Unfortunately, many of these criteria are not indicated on the product label, so you must spend some time reading technical data sheets on manufacturers’ websites to get a complete picture. Try to find a coating that satisfies all, or most, of the following:
1. 100% acrylic: Choose an EWC formulated with 100% acrylic elastomeric binders. Acrylic binders are more waterproof than alternative binders, and they offer the most robust ultraviolet stability and long-term durability. Substitutions are made to lower cost, not to improve performance.
2. Elongation properties: You want an EWC that can stretch by at least 300% from its original size, and up to 600%. Check if the manufacturer has elongation test results for its recommended dry film thicknesses. This should not be hard to find.
3. Dirt resistance: The elongation capability needed in a good EWC also makes it softer than conventional paint. Softer products with adhesion properties tend to get dirtier. The best EWC formulations use resins that deliver excellent dirt pick-up resistance without sacrificing elongation. Look for a product that indicates “good dirt pick-up resistance.”
4. Microbial resistance: The importance of resistance to mildew growth varies by geography, but all high-quality EWCs should have anti-microbial properties. Most exterior products will feature mildew or microbial resistance on the label. If not, check the technical data sheet.
5. Volume solids percentage: The percentage of solids in the formulation dictates how much film remains on the surface after the coating cures. Look for coatings with a range of 40% to 52% solids. Anything less will behave more like paint.
6. Recommended wet-mil and dry-mil film thicknesses: Millage means thickness, and increased thickness means better performance. In tandem with the volume of solids, this is the ultimate key to elongation performance and long-term durability. A conventional paint is applied at a film thickness of 1 to 6 wet mils, but it dries to just 1 to 3 mils. EWCs are typically applied 10 times thicker, resulting in a finish with a dry thickness of 15 to 20 mils.
Achieving 15 dry mils cured thickness in a 50% solids coating will require application at 30 wet mils thickness. To attain 15 dry mils thickness in a 40% solids material requires application ~40 wet mils.
7. Moisture vapor transition rate: The objective of every EWC is not to block water, but to allow water vapor to pass through the coating so it breathes. Water carries soluble salts and other water dispersible materials in the concrete to the surface, resulting in efflorescence, which good EWCs prevent.
However, trapped water vapor can cause peeling, bubbles, and blisters in the coating, which eventually leads to coating failure. Look for high “perm inches” numbers on technical data sheets as an indication of good moisture vapor transition rates.
8. Manufacturer/product reputation: Ask your peers about their experiences. It’s very likely you will hear the same brand names and company names repeated relative to reliability and long-term performance. How long has the manufacturer been in business? How often do they introduce innovative new coatings? What is the track record of their main EWC product lines?
Make sure that you follow best practices when applying EWCs:
- Never apply an EWC directly to fresh, highly alkaline surfaces.
- Apply two coats of EWC rather than one thick coat to help ensure a uniform film thickness.
- Look for a complete checklist on product labels or at the Paint Quality Institute website, www.paintquality.com.
Contributed by Dow Construction Chemicals. Visit www.dowconstructionchemicals.com for more information.