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After a concrete slab has been tested for and met the moisture requirements it is time to begin the preparation of 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, in order to meet these standards, must be examined:  

Photo 1 - Straight edge

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 ten-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.  

Photo 2 - Broadcasting with water 

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 and watch to see if the water is absorbed into the concrete as illustrated in the photo.  If the water is absorbed into the slab, it can be assumed there is no coating on the slab.

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Photo 3 - Buffer/sander cleaning of the concrete

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 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.

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Photo 4 - Width of crack

Cracks that are wider than 0.035” (the thickness of a credit card) lose their aggregate interlock and will often lead to show show-through unless special crack preparation is undertaken.

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Photo 5 - Intersecting cracks

Intersecting cracks like the ones illustrated are an indication of either the control joints being too far apart or a poor quality slab.  Cracks like these need to be chased and epoxy injected, then prepared.

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Photo 6 - Control joint (saw cut)

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, and other 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 p.s.i. 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. Covering an active joint will only lead to additional problems due to the movement and will be found on the deficiency list.  

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Photo 7 - Vacuuming the saw cut to get out all debris

When preparing a control joint (saw cut) it is imperative the entire depth of the saw cut be cleaned and vacuumed out 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.    

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Photo 8 - Power mixing

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.  The next thing, they will 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 when patching concrete joints and cracks are adding too much water to the patch.  This too will lower the compressive strength of the patching compound.

Photo 9 - Application 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 you patch too fast and will make the patch easier to work down into the control joint (saw cut), plus you will get more working time out of the patch.  This works well without sacrificing the compressive strength of the patching compound.

It is 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.