Photo courtesy of Sundek of Washington
Looking for a cool idea for the pool deck or backyard patio? A knockdown finish will fit the bill, say experienced professionals.
A knockdown finish on horizontal concrete is not dissimilar to the knockdown finishes common on drywall. It involves a cementitious material that is dispensed by hopper gun, applied with spatter brush, or in some cases screeded, troweled and textured with rollers or stamping tools. The textured material is “knocked down” with a trowel shortly after application, when the material is partially but not totally dry, to level the high points. This produces a more uniform, less-rough surface that is comfortable to the bare foot while providing resistance to slip.
Using the knockdown method to apply an overlay gives visual appeal to otherwise gray or aged concrete by adding color and texture.
What’s more, knockdown veterans say the finish makes the surface cooler on hot summer days.
“By using a texture that imparts peaks and valleys, you can lessen the surface area that the bare foot comes in contact with,” said Todd Fisher, of family-owned Unique Concrete NJ, located in West Milford, N.J. “The valleys then act as a cooler zone filled with air, leaving only the peaks for the foot to come in contact with.”
Though knockdown finishes are relatively uncomplicated in design and installation, expert practitioners of the knockdown art emphasize the importance of proper practices and techniques to ensure the finish meets target durability and performance parameters.
“The most important things are preparation and accurate measuring of the product,” says Dick Krach, CEO of Sundek of Washington, a decorative concrete contractor based in Chantilly, Va.
Important preparation procedures include inspection to determine if the surface is clean and free of existing coatings or sealers. Consider whether a hard-troweled surface should be ground or scarified to open up the concrete to ensure the overlay will penetrate and adhere. Also, adjacent areas should be masked for protection from overspray.
Krach said a typical system installed by Sundek includes a polymeric coating-type material that is applied as a bonding agent or primer, a base coat of the cementitious product — often a polymer-modified material, and a finish coat using the same cementitious material but applied with a hopper gun that dispenses globules over most, but not all, of the surface. Protective sealers, based on acrylic, polyurethane or other chemistries, can be applied to the finished knockdown surface.
The cementitious mixture is supplied as a dry material in bags and mixed with water on-site. The mixture is a combination of portland cement, sand, other specialized additives, and a polymer that acts as a binding agent — the glue, says Krach.
Not all such mixtures contain the polymer component. Fisher, of Unique Concrete, says his company applies cementitious materials that are not polymer-modified, but contain a colloidal silica admixture that he says reduces the potential for efflorescence, enhances strength and density, and decreases concrete shrinkage. At the job site, the company adds the liquid colloidal silica to the mix water, then completes the cementitious mixture with cement, sand, integral color, and PVA (polyvinyl acetate) fiber for added strength and shrinkage reduction.
Texture being applied to a slab with a textured roller.
Photos courtesy of Unique Concrete of NJ
The knockdown of a texture, done with a trowel by hand. The applicator can control the degree of knockdown.
Tips and techniques
Unique Concrete of NJ takes a different tack with its knockdown finishes than many contractors, Fisher says, as it does not employ a hopper gun to dispense the finish material. Rather, Unique Concrete’s typical approach is to pour, screed and trowel the overlay in a conventional way, then use specially designed rollers to create texture ridges that are then knocked down with the troweling methods.
“To give variation and uniqueness you can knock it down in different ways,” Fisher says.
The creation of texture using the rollers does not vary greatly on Unique Concrete jobs. “What does change is the timing of when you knock down the finish. You can adjust that. For example, doing it sooner when the mixture is softer will make the surface more suited for a project where barefoot traffic will be common, while still providing traction due to having peaks and valleys.”
By contrast, for a project on a driveway, knockdown troweling is done later, when the placed mixture is more set up. Thus, less material is removed from areas of raised texture, leaving behind more relief and more grip.
Also, different troweling methods are used to give variety to the texture, Fisher says. These techniques include moving the trowel in different directions rather than just left to right, or perhaps using an S-type curving motion to produce “movement,” creating a variegated texture rather than a consistent or repetitive knockdown.
Knockdown is not an inherently widely variegated texture, “so we try to move the trowel in different directions to create more variation,” he says.
Tom Ralston, owner of Tom Ralston Concrete, in Santa Cruz, Calif., calls knockdown finishes a “bulletproof” decorative option for pool decks that is relatively difficult to get wrong if a few basic guidelines are followed:
- Make sure the mixing ratios are accurate, as too much water will weaken the mix design.
- Move quickly when applying the product, to ensure consistency.
- Don’t apply if the surface is too hot or too cold, as air temperatures between 60 F and 70 F are considered optimal.
- Ensure all tools are in good working order before starting, including clean mixing buckets, the mixing drill apparatus, and the hopper gun, hose, compressor and trowels. “If all is not in order, issues will arise,” he says.
Ralston says slip resistance is a key selling point for knockdown finishes, in particular for commercial applications. The ability to renew aged or plain gray concrete with color and visual appeal are also important marketing attributes.
Adds Sundek’s Krach: “That’s really what we’re selling. We say we beautify ugly concrete.”