Overlaying is simply a matter of laying a concrete topping of only a quarter-inch or so over an existing slab and stamping that new surface. Aesthetically, you can end up with the same coloring and texture that mimics the look of brick, stone, marble or whatever texture you’re after on a newly poured, thicker slab.
The condition of the slab and the preparation work done on it is key to victory or doom of the overlays. The substrate should be on stable ground; if the substrate preparation is bad then the overlay can crack or delaminate just like regular concrete. Proper surface preparation includes removing any coatings or impurities on the concrete slab as well as leveling and profiling the concrete so the overlay can bond with the surface.
Every job is different, and the required surface preparation will vary from one surface to the other. If you want to avoid overlay problems and call backs, a thorough, complete surface preparation process is the best start to a trouble free adhesion. And when in doubt, call an expert. Always ask for help before moving on, talk to the overlay material manufacturer and find out what surface prep they recommend.
Some of the steps for overlaying include;
- Before starting any surface prep, the concrete must be structurally sound, repair any cracks as not all concrete is good for an overlay.
- Identify any possible bond breakers. These could be easily visible like paint stains or a floor covering or invisible like a cure and seal or grease spots. You can use a water test to check for invisible bond breakers. Pour a little water onto the surface, and if it beads up or sits on top of the concrete there is something there that will prevent the overlay from bonding.
- Bond breakers can either be removed through mechanical or chemical means. The method used will depend on what you are removing and what type of profile is needed. Using a chemical on the concrete will strip off the bond breaker without profiling the surface. Use an acid etches as well as profile that can create a more porous substrate for the overlay. Remove impurities and acid in case you use chemical etching, neutralize the concrete before applying the overlay.
- Mechanical methods include grinding, sanding, shot blasting and scarifying. This may be the method of choice if you need a heavier profile or when working indoors where chemicals may cause more damage or bring up other challenges. Handheld grinders can be used to remove impurity spots, be careful not to over grind or dig deep gouges with the grinder as it will result in an uneven floor. Clean properly the surface to get rid of any dust that came up due to the bond breaking process before beginning the profiling step.
- If a concrete surface only has bond breakers in spots – not covering the entire slab – a degreaser may be used. Although, degreasing of the entire concrete slab can be done in case there are spots that were overlooked.
- Determine the type of concrete surface needed for the overlay to adhere before profiling. This will depend on the overlay material as well as the depth of the material being applied. For example, epoxies typically require a more aggressively profiled floor than some other overlay materials.
- The aggressiveness of concrete surface profiling will also influence the choice of equipment. Hand grinders, pressure washers and maybe a walk-behind grinder or a vacuum system may be used if a contractor applies a lot of overlays indoors. The equipment can be purchased or rented.
- After profiling, properly clean the surface one more time to make sure all the dust and impurities are removed.
Check for moisture
- Whether on old or new concrete, a vapor transmission test should always be done. A vapor transmission test measures the amount of vapor forced out of the concrete, measured in pounds per thousand square feet. Moisture testing prior to overlay application is essential since too much moisture can negatively affect an overlay. A typical industry standard is between 3 and 4 pounds per 1,000 square feet. In case of more than these you can use a vapor barrier or vapor dissipating epoxy underneath the overlay.
- Contractors also need to test the amount of moisture in the concrete using either a calcium chloride test or a probe in the concrete or you can use both to be very sure. While the calcium chloride test has been used for moisture testing for years, the newer probe test will measure deeper into the concrete. But it is also essential to refer to the manufacturer’s recommendations or specifications as to the maximum amount of what moisture vapor transmission should be for their product to work.
The overlay advantage
- For floor maintenance, it is less expensive to install overlays than to tear out the existing concrete and start over. Affordability is always an issue, and contractors who can overlay have a built-in advantage. A lot of times people don’t have the money to tear something out. But as long as it’s stable concrete, we can go over the top of it with the polymer concrete. Moreover, it’s more expedient to go over the top as long as you have a good substrate.”
- The concrete overlay floors are durable
- Overlays can be a solution when working on a difficult terrain. It may be difficult to pull out the existing concrete but much easier to overlay the top if it’s in good condition.
After installing Westcoat’s TC-6 overlay tinted with Sahara stain and antiqued with a Mocha-colored release, contractor Mike McAnulty stamped this driveway with an Old Granite pattern from Proline. It was then sealed with Arizona Polymer’s Poly 250. Photo courtesy of Vegas Hardscape
Noe Serna says he’s been using overlays for nearly 15 years inside his home and on his driveway. He now has it in his store, where forklifts are all over it. He’s never had a problem with any of it. Photo courtesy of Southbay Decorative Concrete Supplies
“We’ll go in one day and do the prep work, pressure washing and priming the slab,” says Boyce. “Then we’ll go in the next day and set up the mix station and, while one guy’s getting all the colors accurate, we’ll have two guys applying and one stamping. On the third day, it’s really a half day as far as going in and washing the slab down, getting it ready for sealing. Really, we can do a 600- to 800-foot space in a three-day process without any problem.”
During the prep stage, says McAnulty, “we’ll make sure there won’t be a moisture problem in the substratum. We’ll use a vapor barrier or a moisture control system if we think there might be. For assurance, we almost always use an epoxy with a full broadcast of sand as a primer after diamond grinding. We’ll conduct calcium chloride testing on interior concrete substrate jobs to test for moisture. We’ll also prepatch cracks, if necessary. Then we’ll start with diamond grinding the surface to take out surface stains and provide a good bond.”
For the overlay stage, McAnulty uses Westcoat TC-6, a dry polymer-based cement that comes in 50-pound bags. “We mix it with five quarts of water, and apply it to the surface with a 3/8-inch gauge rake to maintain consistency, and use a magic trowel and steel trowels to remove the gauge and spike shoe marks,” he says.
When it comes time for stamping, Boyce recommends a generous application of a liquid release first. “(The concrete mix is) almost a peanut-buttery consistency, so if you get on it too soon (stamping) or you don’t have enough release material applied, you’ll start pulling on it and the stamp will stick to it.”
Koury advises making sure all edges are terminated effectively and properly beveled. Tires ramming continuously against an unbeveled edge could crack the surface.
Contractor Jim Boyce created this rustic wood-looking patio by stamping an overlay with Butterfield’s Gilpin’s Fall Bridge Plank pattern. The topping was colored with Butterfield’s Burnished Taupe and antiqued with Storm Gray and Deep Charcoal. Photo courtesy of Rhino Concrete
For coloring or tinting, you’ve got two basic options. “You can add integral color which is pretty simple now,” says Serna, adding that his store carries 20 to 30 different colors. “It comes in one bag and all you do is add water.”
The other way, he says, “is to get the overlay down and then dust the color onto the surface before stamping.”
Serna says that the integral colors are mostly available in the mid-range tones. If you’re looking for a color that’s particularly light or bold, you should go with the dusting method.
When it comes to coloring, Boyce advocates using one very experienced person to do all the mixing. “You don’t really want to start having multiple people mixing because they’ll get different colors and consistencies,” he says. “I want it to look like a stamped concrete hardscape, not like an overlay. If you don’t control your mixes you can get blotching really quickly and you’re not going to be able to get it out.”
Boyce adds additional steps to give his customers a more vintage look.
“Typically, we’ll add three or four different colors in our antiquing schemes to provide good depth on the overlays,” he says. “Especially on the wood (look). We’ll put two broad antiquings over the top and then I’ll go back and hand-antique the knots in for a realistic look.”
Do the job right, experts maintain, and you’ll end up with stamped concrete surfaces that fool the eye, please the wallet and win you referrals.
A formerly drab concrete driveway comes alive with an Italian slate pattern.