Understanding Osmotic Failure: When Moisture Vapor Treatments Aren't Enough
While most flooring professionals take precautions against elevated slab moisture, ionic salts remain an often overlooked threat to floor coating integrity. Even when using traditional ASTM F3010-compliant moisture vapor treatments or cementitious urethanes — products rated to handle up to 99% slab relative humidity (ASTM F2170) or moisture vapor emission rates up to 24 pounds — your project can still fail due to elevated salt levels at the bond line.
The Perfect Storm: Four Elements That Create Osmotic Failure
Osmotic failure requires four specific elements to occur:
- A semi-permeable membrane (the concrete)
- Water
- A low or non-permeable topping
- Salts (primarily Potassium, Sodium, Chlorides, and Sulfates)
When you install a low-permeability floor coating on concrete, moisture within the slab equalizes, increasing moisture concentration near the surface. As this moisture migrates toward the bond line, hydrophilic salts attract it and begin to hydrate. The result? A mineral sludge that typically manifests as blisters. In some cases, the pressure from hydrated salts pushes through the coating entirely, leaving either brownish liquid or hard mineral deposits.
New vs. Old Slabs: Different Risks, Similar Concerns
New Concrete: Not Immune to Problems
While new slabs generally have a lower risk of osmotic failure, they're not entirely safe. The use of Sodium or Potassium Silicate "waterproofing," hardeners, or curing agents can introduce problematic salts into concrete. When floor coatings are planned, these treatments should be avoided. Additionally, adhesive residue, snow melt chemicals, and other contaminants can contribute to salt buildup.
This is precisely why carefully reviewing concrete specifications and mix design submittals is crucial for successful installations.
Older Slabs: High Risk Factors
Older concrete slabs face greater risks due to the natural migration of salts through the slab over time. Elevated salt levels are commonly found along "ghost lines" where previous flooring was removed. If you're working with older slabs — especially those showing visible efflorescence, darkened contraction joints, excessive dusting, or contamination — you should have cores taken and evaluated to determine the appropriate mitigation strategy.
How To Protect Against Salt-Related Failure in Resinous Concrete Floor Coatings
Cementitious urethane-based systems typically offer better protection against salt-related failures than most epoxy-based moisture vapor primers, but even they have limits. When selecting a mitigation material, it's essential to know the acceptable salt threshold it can withstand.
In extreme cases where salt levels are exceptionally high, more aggressive intervention may be necessary, including milling the top layer of concrete (sometimes up to ¼ inch).
The Value of Proper Testing
Core samples represent an inexpensive and straightforward way to prevent potentially disastrous flooring failures. This small investment in testing can save thousands in repairs and prevent project delays, making it an essential step in your flooring preparation process.
Resinwerks: Your Partner in Problem Prevention
At Resinwerks, our decades of industrial coating formulation experience have given us unique insights into preventing osmotic failure before it starts. Not sure if your project is at risk? Contact our technical team today for a consultation and discover how our specialized systems can protect your investment against hidden salt contamination.
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