Moisture is one of the quietest threats to a new floor. A concrete slab can look and feel completely dry while still pushing moisture vapour upward. Seal it under a coating, and that vapour has nowhere to go. The result is bubbles, blisters and peeling, sometimes months after installation. Here is what facility managers should know before any resin floor goes down.
Why concrete holds moisture
New concrete contains far more water than it needs to cure, and it can take a long time to dry. Slabs on grade without an effective vapour barrier can also draw moisture up from the ground indefinitely. Ontario's freeze-thaw cycles, snowmelt and humid summers make this a real issue in older buildings and new construction alike.
How moisture is tested
In-situ relative humidity probes: sensors placed in drilled holes measure humidity inside the slab. This is the most reliable method and the basis of the ASTM F2170 standard.
Calcium chloride tests: measure the moisture vapour coming off the surface over a set period (ASTM F1869).
Plastic sheet test: a quick visual check, useful as a screen but not a substitute for proper testing.
The coating manufacturer sets the moisture limit its product can tolerate. Testing tells you whether the slab is within that limit.
What happens if the slab is too wet
You have three options: wait for the slab to dry (often not practical), install a moisture-mitigating epoxy primer designed to block vapour, or choose a system that tolerates more moisture, such as some urethane cement products. A good contractor will explain the trade-offs in writing.
Signs a floor failed because of moisture
Blisters or bubbles that contain liquid
Peeling that starts in the middle of the slab, not at the edges
White, powdery residue under lifted coating
Ask before you sign
Before accepting a flooring quote, ask whether moisture testing is included, which method will be used, and what happens if readings are high. If the answer is "we don't test", get another quote.
Moisture testing is part of every Hardbond project
We test every slab before installing epoxy, polyurethane or urethane cement, and specify moisture mitigation where readings call for it. Learn more about our floor preparation or book a free site assessment.
