Most epoxy floor failures (peeling, blisters, hot-tire lifting) trace back to two things: poor surface prep, or moisture moving up through the slab. Testing before you coat takes a few days and can save a floor.
Why moisture matters
Concrete is porous. Moisture vapor moving up through the slab can push against a coating and break its bond. Slabs without a vapor barrier underneath, new slabs that haven’t dried enough, and floors in humid climates are the most at risk.
Test 1: plastic sheet (quick screen)
ASTM D4263 describes taping a square of clear plastic sheet to the slab, sealing all edges, and checking it after at least 16 hours. Condensation under the plastic, or darkened concrete, points to moisture. It’s a rough qualitative check. A pass doesn’t prove the slab is dry enough.
Test 2: calcium chloride (MVER)
ASTM F1869 measures the moisture vapor emission rate. A dish of calcium chloride is sealed under a dome on the cleaned slab for a set period and then weighed. Results are reported in pounds per 1,000 sq ft per 24 hours.
Test 3: in-situ relative humidity
ASTM F2170 uses probes placed in holes drilled to a set depth in the slab to measure internal relative humidity. Many flooring manufacturers now prefer this method because it reflects conditions inside the slab, not just at the surface.
What to do with the results
- Compare against the coating manufacturer’s published limits for the specific product. Limits vary, and following them protects your warranty.
- If moisture is too high, use a moisture-mitigating primer made for that purpose, or choose a breathable coating system.
- Also test for oil contamination, and do mechanical prep (grinding or shot blasting) as the system requires.
Related: epoxy calculator · cold weather curing · safety basics