Years ago encountering an interior concrete residential floor happened in atypical homes, perhaps a Frank Lloyd Wright creation or a futuristic glass and steel home. Today concrete floors are considered a mainstream option for homeowners. Some say homeowners are championing the movement and for a variety of reasons: a one-of-a-kind artistic installation, a floor that does not harbor allergens, easy maintenance, or to save replacement costs.
With the advent of overlays, micro-toppings that are spray-on, trowel-down, self-leveling, and stampable, interior concrete floors are both a desirable first choice, and a solution to restore or enhance an existing concrete floor. The products range from very thin layers to 2-inch thick overlays with unlimited decorative finishes. Manufacturer's overlay products have variations that affect installation procedures. Training, following manufacturer directions, and staying with a single manufacturer will help to ensure good results.
UNDERSTANDING AND PREPARING THE SLAB
Providing for moisture vapor transmission (MVT) and doing proper surface preparation to correctly profile the concrete for your application is of great importance when topping concrete. This step significantly affects the sureness of the bond and long-term durability. Preparation can take more time than installing the topping.
There are certain questions to ask before topping concrete. If it is a new slab, has it cured for 28 days? If the slab is on grade, is there a moisture barrier installed beneath it? Is the slab's moisture vapor emissions rate (MVER) less than the typical maximum allowed of 3 pounds of water vapor per 1000 square feet in 24 hours? Is the underlying concrete strong enough for the overlay to bond to?
“Variables include if the concrete is on a flood plain, the quality of the concrete, or whether the walls and roof were in place at the time concrete was placed,” says Jonathan Shoemaker, technical services consultant, Mapei Corp., Deerfield Beach, Fla. Shoemaker advises testing the concrete to better understand its physical properties. He recommends administering a calcium chloride test to determine MVER when possible. If the MVT is too high there are moisture reduction systems to apply to the concrete that limits MVT. Vince Schrementi, owner, Everlast Concrete, Steger, Ill., scrapes the concrete with a razor-sharp knife that he carries with him while looking at jobs to bid. He does this in several areas to feel the strength or weakness of the slab. It is an indicator of the preparation he needs to include in his bid. If the concrete is weak and crumbly, he includes a densifier for the concrete. If the slab has lot of dips or a build up is required, he includes a mortar material to repair and achieve a flatter surface.
Overlay manufacturers usually include a profile number with their directions. That number indicates the degree of profile, or “tooth” the concrete needs for a successful bond and refers to the International Concrete Repair Institute (ICRI) surface preparation guidelines. ICRI shows 9 levels of profile; from an almost smooth 1 to level 9 where the profile is ¼ inch deep.
“Shot blast is a popular way to profile for most applications,” says trainer and senior technical representative, Glen Roman, Brickform. “Grinding removes coatings and laitance but it tends to polish concrete, tightening the surface instead of really opening the concrete.” A shot blast machine uses steel beads to pummel the slab. “If unfamiliar with shot blasting for overlays, try a test patch to determine the correct bead size to use,” says Amy Flanagan, director of marketing for Blastrac. “Start with the finest sized bead and then try heavier shot until you reach the desired profile.”
The strength of the concrete matrix and how it was finished impacts profiling. Workers need to be careful and use the correct size shot to remove laitance. Otherwise, the concrete can become bruised, resulting in the overlay material bonding to unsound concrete. Contractors often have shot blast equipment that is too large and difficult to use inside homes, so they turn to a combination of equipment to scarify or grind. Matthew Newman, owner, Vision Concrete Institute, Temecula, Calif., diamond grinds and even does an acid wash on the concrete surface. Here again, manufacturer's recommendations are important. Any profiling inside a house includes controlling the debris that comes from breaking up the surface concrete-mastics, sealer, and cementitious particles. An advantage of shot blast equipment, which is available in smaller sized machines, is that it's a closed system with the steel shot hitting the concrete and rebounding back into the machine. Any other equipment needs a shroud or dust collector connected to a vacuum.