Installing overlay cement finishes can solve many problems and provide a wide variety of finishes. But they all have one basic requirement: The slab beneath must be structurally sound with a bondable surface.
Overlay cement applications can be used to restore the condition of a floor surface, sometimes followed by diamond polishing to reduce maintenance or provide a shiny look that’s appreciated by customers. But they are also used for decorative applications, including stained, textured, stamped, patterned, or stenciled to report fine detail and color.
Overlays can be paper-thin or several inches thick. This article will focus on applications ¼-inch-thick and less.
The thickness of the overlay defines the method of slab preparation. Preparation must create a surface profile free of bond-inhibiting material. The depth of the profile increases with the thickness of the application. If you are in this business, you should own Guideline No. 03732 prepared by the International Concrete Repair Institute (ICRI), Des Plaines, Ill. You can buy it online at www.icri.org along with plastic surface profile samples. Table 1. illustrates the range of choice for both the depth of profile and the method to achieve it. Whatever your decision, be sure the profile won’t reflect through the overlay material you are using.
The key ingredients of overlays include cement, high-range water reducing admixtures, fine aggregate, and polymer. “Good overlay cement is composed of the right blend of fine aggregate sizes. Well-graded mixes have more even setting characteristics and perform better because there are fewer voids to be filled between aggregates, reducing the amount of shrinkage and increasing the strength,” says Joe Garceau, co-owner of Butterfield Color, Aurora, Ill. The top size of the aggregate is also important because it determines how thick an application can be.
There are many polymer resins on the market, each having an effect on performance. Manufacturers don’t usually reveal which they use, so your experience with workability and product durability must be your guide.
There are products rated for interior and exterior use. The primary difference is the inclusion of gypsum in the mix for interior work.
How to install
Overlays can be sprayed, placed with a hopper gun, or trowel applied. As a self-leveling overlay, they can be placed with a gage-rake, squeegeed, or mixed with decorative aggregate and placed as terrazzo. Given the wide range of application, there are few rules for how to install them. You will need to discuss the process with your product sales representative and perhaps attend a seminar.
Mixing the product is one of the constants. Overlays require the exact amount of water specified by the manufacturer. Garceau recommends accurately measuring water into a bucket and drilling a hole in the side on the water line to ensure that each batch will be the same. Many products must also be “slaked”—mixed, left undisturbed for about 5 minutes, then mixed again.
When thin overlay products fail, it’s almost always because of inadequate sub-base preparation. It accounts for most of the time spent on installation. Some contractors think pressure washing a slab provides a bondable surface, but Garceau cautions against that. He recomends opening the surface by diamond grinding with 100-grit sizes or below. Base slab surfaces should be saturated surface-dry, with no standing water anywhere. It’s wise to check the relative humidity of the concrete beforehand.
Products rated for micro-topping have very fine aggregate in them, so they can’t handle the abrasive wear that thicker toppings can. They should be sealed with an abrasive-resistant finish such as a polyaspartic or urethane sealer.
When mixing material in a bucket with a drill, use a box-type mixing paddle and choose a heavy-duty drill that doesn’t exceed 600 rpm.
Of all concrete products in the marketplace, thin overlays are the most versatile. They are the ideal product for creative minds to play with.