Generally speaking, there are few reported problems associated with the construction of decorative vertical finishes using prepackaged vertical overlay mixes or by the occasional contractor who makes their own mix. Contractors use them to install stamped or carved artificial rock, brick, wood, or handcarved art pieces and the material can be applied as much as several inches thick. They also can be micro-thin, trowel-finished, plaster-type finishes used to flatten walls or add color. Designers of tilt-up buildings sometimes add architectural interest to the flat walls by gluing expanded polystyrene board moldings to increase profile depth and then covering them with colored vertical overlay concrete.
In their simplest form, overlay concrete mixes contain cement, small aggregate (sometimes with large aggregate added), water, and latex polymer additives. There are five families of latex polymers, which each have hundreds of variations: acrylic, styrene acrylic, vinyl acetate ethylene, polyvinyl acetate, and styrene-butadiene resin. Each admixture has positive and negative attributes but acrylics tend to be preferred for most installations because cost is reasonable, bonding characteristics are good, and they allow moisture vapor to pass through them—reducing problems that can follow an installation over time.
One of the challenges formulators have is to get the right amount of polymer additive into a mix. Ken Lipovsky, a Cast Polymers applications specialist for Reichold, Research Triangle Park, N.C., says, “Resin rich and resin starved create problems—balance is the key.”
What latex polymers do
Steve Hermanson, a product segment specialist for wall coatings for BASF, Shakopee, Minn., says his company’s latex acrylic polymer should be diluted with water at a minimum rate of one part acrylic to three parts of water added to concrete. Other manufacturers may require different amounts; it’s the percent of solids in the mixture that counts.
Latex polymers in concrete improve the bond between the concrete and the substrate. They also facilitate better curing by retaining water in the concrete.
How much latex polymer should be used?
There are no guidelines to tell you how much latex polymer should be added to a mix because there are many different polymer products on the market, each with its own requirements. They come packaged dry (added to the powder mix ingredients) and wet. If they are wet, the percentage of solids mixed with water varies between products and contractors can decide how much or little to add. One thing is clear—if too little latex polymer is added, you run the risk of having your mix dry out too much with crusting and shrinkage being the result. Some contractors are tempted to do this in order to reduce material costs.
When dry latex polymer is added to prepackaged mix ingredients, contractors must be careful to add the specified amount of water.
Shrinkage is related to the amount of water in a mix and as such has little to do with the amount of latex polymer added. However, less water-of-convenience can be added to mixes when latex polymer is added, facilitating placement and reducing the amount of shrinkage as a result.
The thickness of an installation is related to shrinkage as well. For example, microtopping applications rarely have shrinkage-related problems while very thick ones have a greater potential for developing shrinkage cracks. Sometimes contractors add large pea gravel aggregate to their mixes to help minimize shrinkage after initial set occurs. However, Jerry Garceau, co-owner of Butterfield Color, Aurora, Ill., cautions against this, saying the increased weight of the mix can cause it to slump when it’s applied. He suggests building up thickness with more than one application, allowing 24 hours or more between applications.
Ake Grunditz, owner of Fine Design, Alameda, Calif., says he seldom has trouble with shrinkage even though he builds up as much as 8-inch thicknesses. “I add significantly more polymer to my mix than usual,” he says. “The cost for this is that my mix is stickier but I’ve learned how to work with this.” The added latex polymer helps him when ambient conditions cause surface crusting, though he also sets up windscreens and sunscreens to minimize the problem.
All vertical overlay cement applications must bond to something, whether it’s concrete or polystyrene foam. It’s important to follow established guidelines for concrete surface preparation. Follow the International Concrete Repair Institute’s book titled Guideline for Surface Preparation that provides information about the type of preparation needed for different thicknesses of overlay material.
Many manufacturers of overlay products recommend that you apply a bonding agent to the substrate before the overlay product is applied. A usual requirement is that the overlay material be applied before the bonding agent dries, lest it become a bond breaker. So the placement of the mix usually happens at the same time the bonding agent is being applied. Bonding agents have two benefits: they enhance the bond to the substrate and the bonder forms a vapor barrier, helping to retain water in the mix during the curing period.
If you are using a manufactured overlay product, most problems are related to substrate preparation and ambient conditions. If you are making your own mix or changing the ratio between latex polymer and cement, then troubleshooting afterward can involve such issues as shrinkage, curling, curing, and bonding issues.
For additional information about polymer-modified concrete, read ACI 548.3R “State-of-the-Art Report on Polymer-Modified Concrete.” For more information about preparing surfaces for overlays, contact the ICRI at 847-827-0830 and ask for their technical guideline number 03732, “Selecting and Specifying Concrete Surface Preparation for Sealers, Coating, and Polymer Overlays.”