Flooring Forensics: The importance of cementitious substrate cleanliness – July 2024
By Walter Bell
In my position as a Forensic+QA/QC technical flooring consultant, I have seen many failed flooring systems caused by insufficient cleanliness of the substrate to be bonded to. The level of cleanliness has a direct impact on floorcoverings’ ability to adhere to the preceding layer of a flooring system, which is made up of components like primers, adhesives and floor coatings, including concrete moisture mitigation coatings, cementitious patching, skim-coating, self-leveling underlayments, overlays and toppings.
A LOOK AT THE STANDARDS
Numerous relevant industry standards have minimum requirements that concrete slabs be sufficiently clean prior to installing a flooring system. These standards are often referenced by the flooring product manufacturers’ installation guidelines and technical data sheets, which typically mandate similar minimum cleanliness requirements for their products to achieve optimal bond.
For example, “ASTM F710 Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring” states, in part: “The surface of concrete floors to receive resilient flooring shall be dry, clean, smooth and structurally sound. They shall be free of dust, solvent, paint, wax, oil, grease, residual adhesive, adhesive removers, film-forming curing compounds, silicate-penetrating curing compounds, sealing, hardening or parting compounds, alkaline salts, excessive carbonation or laitance, mold, mildew, and other foreign materials that might affect the rate of moisture dissipation from the concrete, the adhesion of resilient flooring to the concrete or cause a discoloration of the flooring from below.”
Likewise, it would be difficult to find a manufacturer that does not include mandates for minimum cleanliness requirements. A more stringent example is Tarkett’s installation instructions for rubber tile flooring, which state, in part: “All subfloors must be permanently dry, clean, smooth and structurally sound. The surface must be free of all dust, loose particles, solvents, paint, grease, oil, wax, alkali, sealing/curing compounds, old adhesive, and any other foreign material which could affect the installation and adhesive bond to the substrate. Permanent and non-permanent markers, pens, crayons, paint or similar marking tools used to mark the substrate or the back of the resilient flooring material will cause migratory staining. Subfloor contamination or markings that bleed through the flooring material causing discoloration or staining are excluded from the Tarkett Limited Warranty. All substrate contaminants must be mechanically removed prior to the installation of the flooring material.”
As prevalent as these mandated cleanliness requirements are in the flooring industry, I, too, often discover in forensic floor failure investigations that flooring installers and floor coating applicators do not follow the manufacturers’ technical data sheets, installation instructions and industry standards regarding cleanliness practices.
What is considered a subfloor contaminant relative to a flooring system? It is anything on the concrete slab or other surface that will affect the ability of the flooring system to achieve sufficient bond strength. Contaminants can include dust, paint, oil, grease, curing compounds, silicates and other surface treatments, poorly bonded cutback and residual adhesives, loose particles, and any other substance that could reduce or prevent full adhesion of the flooring product.
A number of contractors on a project site may use marking spray paint on the concrete slab to indicate where various items need to be installed, or for other reasons. These marking paints are not intended to provide a permanent bond to surfaces, so they must be removed. If bonding to paint on a subfloor, then the bond strength of the flooring system is limited to the bond strength of the paint to the concrete slab or other subfloor surface. Some marking paints are formulated for outdoor usage and may include oil-based solvents that can be a bond breaker for flooring products. I have seen numerous floor failures where there was a lack of subfloor preparation that included residual marking paint and other subfloor contaminants.
ESTABLISHING CLEANLINESS
Residual dust is the root cause for many floorcovering adhesives, coatings and cementitious patching and leveling compounds not having sufficient bond to the substrate. Dust on the substrate surface and in the pores of the subfloor can create significant bonding issues. Traditional sweeping and vacuuming of a subfloor will typically leave a bond-breaking fine layer of dust.
Sweeping is not recommended where silica dust or other hazardous respirable dust particles might be present. Most industry recommendations are to use industrial-grade vacuums that have HEPA-rated filters and an anti-clog system, and that are able to effectively collect concrete and other fine dust by maintaining adequate suction volume without the filter clogging. These vacuums should comply with OSHA regulations for respirable dust collection and containment.
Even with a fine layer of dust that may seem insignificant, the flooring product will not properly bond to the subfloor. Many floor failure investigations have revealed that the flooring product adhered predominantly to the dust, with little bond to the subfloor. Depending on the percentage of area bonded to the subfloor, traffic on the finished floor and other factors, this lack of sufficient bond can cause the flooring to start failing within weeks or months. While it may take longer, the aftereffect is that the flooring systems will eventually fail entirely. It’s like using adhesive tape on a dusty surface.
Many installers will use sandpaper on a rotary buffer to debond surface contaminants on a cementitious subfloor, creating significant dust and other particulates. With wood, cork and other products that may require sanding and staining, fine dust particles from sanding are created. Prior to applying a finish floor coating, recoat and intercoats, each layer is typically abraded with an abrasion pad on a buffer, which also produces fine dust particles.
With each of the above processes, traditional sweeping and vacuuming leave dust on the surface and in the pores of porous surfaces. After sweeping and vacuuming, I recommend using damp microfiber mop pads-changed out frequently, as needed-to pick up the residual dust. This creates a level of cleanliness that can greatly increase the bond strength of an adhered flooring system. This cleaning process works well for smooth concrete slabs that have been ground or polished prior to each step in the finishing process.
Commercial floor cleaning machines for smooth surfaces that sufficiently extract the cleaning water also work well. Floor scrubbers or cleaning machines used for this purpose should be dedicated for use with clean water only-no detergents or other chemicals. Rental machines are not recommended because of unknown residual chemicals that could contaminate the subfloor. Adequate time should be given for the surface to dry to the flooring manufacturer’s requirements.
For slabs that have been shotblasted or otherwise have a rough profile, microfiber mop pads do work. After profiling and vacuuming, it is recommended to use a commercial/industrial floor scrubber designed for rough surfaces that will get into the valleys of the profile and extract and capture the dust. This additional step will increase the bond strength of floor coatings, toppings, overlays and underlayments to the concrete slab. The recommendations are the same as with the smooth-surface machines in terms of clean-water use only and potential issues with rental cleaning equipment.
TESTING FOR CLEANLINESS
During our quality assurance (QA) inspections after a subfloor has been prepared by the installation contractor and is awaiting the next step, we wipe the subfloor with a damp microfiber cloth to visually determine the level of cleanliness prior to installing the flooring product. On smoother surfaces, clear adhesive tape can be applied to the cleaned surface, then transferred to black or dark paper to visually see the amount of dust transferred to the tape. With improper cleaning, the tape will often not adhere to the paper due to the amount of dust collected.
These QA procedures are conducted with each layer of a multi-component flooring system.
With certain flooring system products, such as moisture mitigation and finish floor coatings, overlays, patching compounds, toppings and underlayments, our QA process often includes the installation of mockups. On these mockups, tensile-strength bond tests are conducted in accordance with ASTM test methods to provide a quantifiable bond strength of the flooring products. We utilize an electronically controlled hydraulic pull-testing device that measures the bond failure in pounds per square inch (PSI). With proper subfloor prep, the tensile bond strength of the layers of flooring products will either surpass the tensile strength of the concrete slab or max out the testing device, at which that strength surpasses manufacturer specifications.
NACE No. 6/SSPC-SP 13, Surface Preparation of Concrete states, in part: “Prepared concrete surfaces may be inspected for surface cleanliness by lightly rubbing the surface with a dark cloth or pressing a translucent adhesive tape on the surface. The test method and acceptable level of residual dust shall be agreed on by all parties.”
Prior to being a technical flooring consultant, I owned a specialty floor contracting company. As part of the subfloor preparation, we always conducted the final cleaning using damp microfiber mop pads. This provided a dust-free surface for optimal bond, and as a result, we never had a project with bonding issues. The takeaway is that the level of cleanliness of a substrate can have a significant impact on the success or failure of a flooring system. This is not the only factor, but it is one that is too often overlooked.
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