By Ray Thompson, Jr.
After a concrete slab has been tested for and met the moisture requirements it is time to begin preparing the slab to receive the floor covering material. A concrete slab should meet the standards set forth by ASTM F-710 Standard Practice for Preparing Concrete Slabs to Receive Resilient Flooring. When dealing with concrete floors, there are several things that must be examined in order to meet these standards:
The surface of the concrete must be flat 3/16 in 10’ (4.8mm in 3m). One of the best ways to check the surface is to place a 10-foot straight edge on the surface of the concrete. You should not be able to slide three quarters stacked upon one another beneath the straight edge. The American Concrete Institute (ACI 302.1R) recommends a floor flatness/levelness composite flatness and levelness of FF 35/FL 25.
All concrete slabs to receive a floor covering needs to be free of any type of sealer, curing compound or parting compound. Visually checking a slab to see if it contains one of these coatings is difficult. One of the best ways to check a slab is to broadcast some water on to it and watch to see if the water is absorbed into the concrete as illustrated in the photo above. If the water is absorbed into the slab, then it can be assumed there is no coating on the slab.
Concrete floors to receive resilient flooring shall be free of dust, solvent, paint, wax, oil, grease, residual adhesive, alkaline salts, excessive carbonation or laitance, mold, mildew, and other foreign materials that might prevent adhesive or patching compound’s bond. The slab’s surface must be cleaned of all loose and soft material. The best way to accomplish this is to buffer sand the surface of the concrete with a buffer/sander with a coarse paper or metal disc.
Cracks that are wider than 0.035″ (the thickness of a credit card) lose their aggregate interlock and will often lead to show-through unless special crack preparation is undertaken.
Intersecting cracks like the ones illustrated in Photo #5 are an indication of either the control joints are too far apart or it’s a poor-quality slab. Cracks like these need to be chased and epoxy injected, then prepared.
There are basically two types of joints: active (moving) and dormant (non-moving). When preparing concrete surface cracks, grooves, depressions, control joints or other non-moving joints, irregularities should be filled or smoothed with a cementitious patching compound recommended by the resilient flooring manufacturer for filling or smoothing, or both. Patching or underlayment compound shall be moisture-, mildew-, and alkali-resistant, and for commercial installations, shall provide a minimum of 3,500 psi compressive strength.
Active joints such as expansion joints, isolation joints, or other moving joints in concrete slabs should not be filled with patching compound or covered with resilient flooring. Consult the resilient flooring manufacturer regarding the use of an expansion joint covering system. Installing a floorcovering over an active joint will lead to additional problems due to the movement and will be found on the deficiency (punch) list.
When preparing a control joint (saw cut), it’s imperative the entire depth of the saw cut be cleaned and vacuumed before filling. By filling the total depth of the saw cut, it helps eliminate the bulging of the patch and/or tunneling of the material due to minimal slab movement.
Most installers have found that when doing floor preparation work on concrete, they need compressive strength from their patching compound. Therefore, they use water to mix their patch, as latex additive, will lower compressive strength. They’ll then power mix, as the high-speed mixing will break the surface tension of the water allowing the patch to become smoother and thinner without adding additional water. The biggest mistake installers make is adding too much water to the patch prior to patching concrete joints and cracks. Too much water will lower the compressive strength of the patching compound.
When applying the patch, dampen the concrete ahead of the area to be troweled. This will stop the concrete from absorbing the water out from the patch too quickly and will make the patch easier to work down into the control joint (saw cut). You’ll also get more working time out of the patch. This works well without sacrificing the compressive strength by overwatering of the patching compound.
It’s amazing how many claims there are over concrete floor preparation or the lack of it. Most successful installations are a direct result of the efforts made by the installation team’s attention to preparation detail.