October 12/19; Volume 30/Number 8
By Scott Tarr
It is critical to properly prepare concrete slabs prior to flooring installation. After all, it is the surface of the slab that the material bonds to, so the better that surface, the better the bond.
The industry standard for surface preparation for flooring is ASTM F710 Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. While the document is specifically written for the installation of resilient flooring, many of the requirements are useful to the installation of other materials like carpet and wood. However, the standard doesn’t include sufficient direction on the requirements due to the wide range in types of flooring materials and their associated needs.
First, the top surface of a concrete slab is not the only surface that requires proper preparation for successful performance; the bottom surface of the slab should include an effective moisture vapor retarder. While ASTM F710 requires the vapor retarder be a minimum thickness of 0.010 inches and have a maximum permeance of 0.1 perms, the permeance of vapor retarders should be less than that of the flooring system.
Regarding the top surface of concrete slabs, ASTM F710 requires them to be “dry, clean, smooth and structurally sound.” Unfortunately, these requirements are qualitative and the specific degree of dryness, cleanliness, smoothness and structural soundness is often dependent on the specific flooring being installed. It is recommended to discuss requirements with the manufacturer but I have compiled a brief, general rundown of each.
Dryness of the concrete is measured quantitatively following standard tests (ASTM F1869 and F2170). It is highly recommended that independent, certified testing technicians be retained to perform the tests. The International Concrete Repair Institute (ICRI) manages a national certification program and database (icri.org for reference).
Cleanliness of the slab surface is often an issue. ASTM F710 requires the surface be free of materials such as dust, paint, oil, residual adhesive, adhesive removers, curing compounds (including the dissipative type) or any foreign materials that might affect adhesion of flooring to the concrete or cause discoloration. To achieve optimal cleanliness, dry mechanical methods such as shot blasting are recommended. However, it must be verified the method doesn’t cause micro-fracturing (bruising) of the surface as this creates a structural soundness issue. If an aggressive method is used, a lighter, secondary method removes fractured areas. For final cleaning, sweeping or blowing causes dust to become airborne and settle while thorough vacuuming permanently removes all dust from the exposed pores.
Smoothness of the slab surface is an issue if the flooring shows imperfections. Specifications should require the appropriate concrete floor flatness/levelness (FF/FL). However, this is measured within 72 hours after placement and if the slab is not designed to remain flat, drying from the exposed surface results in curling/warping at joints and cracks. As this develops over the first year, its correction should be a separate bid item in specifications performed just prior to flooring installation. Further information on warping relaxation is available from ICRI.
Tarr will present “Do it Right or Do it Over! Preparing Concrete Surfaces to Receive Finish Flooring” at TISE in Las Vegas on Jan. 20. Registration is open now at TISEwest.com.