Self-leveling overlays have been around for decades, but they were initially used for purely utilitarian purposes. In the late 1980s, these versatile overlays started coming of age as a legitimate floor finish alternative to pricier high-end materials such as slate, granite, and marble. Not only are self-leveling overlays less expensive than these alternatives, they may even outlast them when properly sealed and maintained.
Self-leveling overlays offer numerous advantages for both the installer and end user:
- They provide a flat, durable surface that can serve as a blank canvas for a wide variety of decorative embellishments.
- They can smooth, level, and restore worn or uneven concrete. Because these overlays are typically applied at thicknesses of ¼ inch or greater, they effectively cover minor flaws and compensate for height variances.
- They offer quick installation and rapid drying times. Some systems are ready for light foot traffic 4 to 6 hours after installation.
There are a few limitations, however. Because of their application thickness and the nature of the ingredients used, self-leveling overlays are less flexible after drying than microtoppings and spray-down systems, which can make them more prone to cracking. They are also prone to skinning over with any air movement across the surface, which jeopardizes their workability. For these reasons, self-leveling systems are generally restricted to interior applications.
Preparing the surface
Successful installation of a self-leveling overlay starts with proper surface preparation:
- If there are existing cracks, assess whether they are static or working or moving. This will help to determine the best products and methods of crack repair to use (see go.hw.net/crackrepair for more advice or repairing cracks). Some overlay manufacturers offer patching products specifically for use with their systems.
- Most overlay manufacturers recommend honoring all contraction, construction, and isolation joints in the concrete substrate, which often means you will need to mark their locations (such as on an adjacent wall) so that later, when the overlay dries, you will be able to make sawcuts over the top of the existing joints.
- On newly placed concrete, self-leveling overlays require a minimum of 28 days curing time prior to installation. Conduct moisture-vapor tests (see go.hw.net/vaportesting for more on moisture vapor testing) to ensure that the emission rate is not excessive. If the test results indicate that the rate is too high, you may need to cover the floor with a surface-applied moisture control product prior to overlay installation to bring the emission rate within industry tolerances. Check with the manufacturer for recommendations.
- Shotblasting is the preferred method for profiling concrete floors in preparation for self-leveling overlays. According to the International Concrete Repair Institute guidelines, you should aim for a concrete surface profile of 4 to 6 (click here for more information on surface preparation.)
After shotblasting the surface to obtain the desired profile, you must apply a primer. Don’t neglect this important step. With self-leveling overlays, many failures and imperfections such as delamination, pinholing, and air bubbles are due to improper priming.
Priming serves two important functions. First, it seals the pores of the concrete so it won’t absorb moisture from the newly placed overlay. If water seeps into the pores of the concrete, it will displace air. As the air rises to the surface to escape, it can produce pinholes and air bubbles (sometimes referred to as “fish eyes”) in the fresh overlay. The second reason for using a primer is to ensure good adhesion of the over lay to the substrate. Most manufacturers recommend applying a minimum of one and in some cases two or three coats of primer, depending on the product used and the porosity of the floor. Manufacturers usually sell primers designed for use with their systems.
The type of primer will determine the method of application. Primers for some self-leveling overlays are thin enough to be sprayed onto the substrate using a pump-type sprayer. After spray application, you should use a brush or roller to work the primer into the pores of the concrete. These primers usually require three to four hours of drying before you can install the overlay.
Other systems may require the use of an epoxy primer—a thicker material that requires application with a notched squeegee followed by a roller to work it into the surface. When using an epoxy primer, it’s usually necessary to hand broadcast sand into the wet epoxy to ensure adhesion of the overlay. Epoxy primers can take 12 to 24 hours to dry depending on jobsite conditions, such as air temperature and humidity.
In the next issue we will cover mixing and placing of self-leveling overlays.