Decorative Floors With Polymer Overlays, continued
Mixing and Applying Polymer Concrete
Like conventional concrete, polymer concrete is made by mixing wet and dry ingredients. Some overlay products come bagged in dry ready-mix form, with the polymer already in the dry ingredients — all you add is water. In other cases, the polymer comes in a liquid form and is field-mixed with water and typical dry ingredients — cement and sand, as well as crushed stone or aggregate when the overlay is exceptionally thick. We use such a thick overlay when we're leveling floors that are seriously out of whack. Some products can be built up in layers, but it's easier to do a continuous pour and put aggregate in the material that goes in the low spots. This is possible because overlay is mixed on site in small batches.
Polymer toppings cure quickly, which is a major selling point when there's not much time to do the job. Some of the products I use can support foot traffic in 3 or 4 hours and develop compressive strengths of 3,000 psi in 3 hours, 6,000 psi in 24 hours. Overlay products come in two types, trowel-applied and self-leveling. It's usually more economical to use trowel-applied toppings: They generally cost less up front, and unlike self-leveling products, they can be applied in a thin even coat. It usually takes more material to do a self-leveling overlay because the topping might be 3/8 inch thick at one end of the room and 3/4 inch thick at the other.
Spreading self-leveling toppings has to be done with care. It's important to work out any air bubbles, but the material shouldn't be touched once it starts to set.
A gauge rake is a handy tool for spreading trowel-grade material. Adjusting the wire bales on each end raises or lowers the blade, making it easier to spread an even coat of material.
Trowel-grade materials also have longer working times; some can be stamped or textured before they set. To maintain a consistent thickness, we spread the overlay with a gauge rake or notched trowel. Afterward, we go over it with a regular trowel to produce the texture and finish we want. Some trowel-applied materials are referred to as microtoppings because they can be applied in layers as thin as 1/16 or even 1/32 inch.
Self-leveling overlays set up very quickly, which makes them tricky to use. It takes some experience to know when to stop spreading self-leveling overlay. The material starts to set in 10 or 15 minutes, so you have to work fast to maintain a wet edge as you work your way across the floor. Spreading puts the material where you need it and removes air bubbles that can cause imperfections. But you shouldn't touch the surface after the polymer starts to set. If you do, it leaves a mark that's hard to repair. If you do it right, the floor will be dead flat and have a glossy sheen.
Self-leveling overlay is soupy when it's wet, so you should make sure there are no leaks or holes in the substrate. We recently missed a small nail hole in a bathroom floor and ended up with a cone-shaped depression where the overlay leaked out. We happened to be using a product that's difficult to patch. The client was very particular, so we ended up tearing out and redoing that part of the job.
Coloring and Top-Coating
There are many decorative options. One of the most popular with our clients is to treat the partially cured surface with acid stain. This produces subtle variations in color. Integral color can also be created by adding pigment to the overlay before it's applied, or dry color hardener can be worked into the topping after it has been spread and troweled.
Polymer overlays cure so quickly that they can be stained and finished a day or even hours after they're applied. Here, acid stain sprayed onto the surface leaves a pleasing mottled effect.
Note the decorative grid of scored control joints. Though it looks like a colored slab, the concrete topping is only 1/2 inch thick.
Similar techniques are used to color conventional concrete, but because polymer overlays cure so fast, you can complete a job in much less time. For example, a conventional slab is supposed to cure for 21 days before acid stain is applied. We know from experience that we can do it sooner, but if we do it too soon, the stain will eat away the surface layer of cement and expose the sand. By contrast, we can acid-stain a typical polymer topping the day after it's applied. Some overlay products have to be stained within a few hours after application; otherwise, the fully cured material will be too dense for the stain to take. When that happens, the colors are not as bright or intense as they could be. Sometimes we lightly sand the floor to open up the surface so it will accept the stain. The sanding also removes minor blemishes like popped air bubbles.
Acid stain can be brushed, sprayed, or rolled onto the surface. We allow it to sit for 8 to 12 hours, then wash it off and neutralize it with water that contains baking soda or ammonia. We then rinse the surface with clean water and allow it to dry before applying a clear sealer coat. A basic sealing job is done with wax or acrylic lacquer. A top-of-the-line job gets a coat of waterborne epoxy followed by a coat of polyurethane resin and multiple coats of acrylic wax.
Polymer toppings cost $1 to $2 per square foot for a 3/8-inch layer. Self-leveling material is typically more expensive than trowel-grade material because it contains costlier types of polymer. Pre-mixed bagged overlays contain everything except the water, so they cost more than the material you make by adding liquid polymer additive to dry ingredients such as Portland, sand, and aggregate that you supply yourself. Installation labor runs about a dollar or two per square foot, including a basic decorative treatment. The final price would include an additional markup for overhead, profit, and the degree of difficulty for the particular job.
Tom Ralstonis a third-generation concrete contractor in Santa Cruz, Calif. Photos for this article were provided by Tom Ralston Concrete.