Why Coated Floors Blister
Slab moisture, the two ASTM tests that predict it, and what real preparation involves.
The coating did not fail. The slab did.
A coating that lifts in blisters with clean concrete on its underside was not badly made and usually was not badly applied. It was pushed off by water vapour moving up through the slab. Concrete is porous; a slab over damp ground moves vapour continuously; a coating turns that slab into a sealed surface and the pressure has to go somewhere.
The important part is that this is predictable. Two standard test methods exist precisely so nobody has to find out the expensive way.
| ASTM F1869 | ASTM F2170 | |
|---|---|---|
| What it measures | Moisture vapour emission rate from the slab surface | Relative humidity inside the slab |
| How | Anhydrous calcium chloride placed on bare concrete under a sealed dome | A probe drilled into the slab and read after it equilibrates |
| Reported as | Pounds per 1,000 square feet per 24 hours | Percent relative humidity |
| Depth | Surface measurement | 40% of slab thickness for a slab drying from one side; 20% for two-side drying |
| What it tells you | Whether the surface is emitting moisture now | What the slab will equilibrate to once it is sealed |
Grinding is not a formality, it is most of the labour.
The second reason coatings fail is bond. A coating adheres to whatever it touches, so paint, sealer, curing compound, old mastic or a previous coating all have to come off before anything new goes down. Published figures for fully contracted concrete grinding run from about $1.50 to $6.00 per square foot, and the spread is the point: light profiling of clean concrete sits at the bottom, and stripping thick epoxy, hardened black mastic or levelling severe high spots sits at the top.
That range is a published national market figure from a surface-prep cost guide, not a rate from this site. What your slab needs is a question of what is on it.
What good preparation looks like
- Existing coatings and sealers ground back to sound concrete
- Cracks and spalls repaired before coating, not filled with topcoat
- A consistent surface profile across the whole floor
- Slab moisture tested by F1869 or F2170 where there is any doubt
- Dust extraction, because contamination in the profile is a bond-breaker
Slab moisture FAQ
What is ASTM F1869?
The calcium chloride test. Anhydrous calcium chloride is placed on bare concrete under a sealed dome and weighed, giving a moisture vapour emission rate in pounds per 1,000 square feet per 24 hours — used to judge whether a slab is dry enough to receive a covering.
What is ASTM F2170?
The in-situ relative humidity test. A probe is set into a hole drilled to 40% of the slab thickness for a slab drying from one side, or 20% for two-side drying, and reads the relative humidity inside the concrete once it equilibrates.
Which test is better?
They answer different questions. F1869 reads what the surface is emitting now; F2170 reads what the slab will equilibrate to once it is sealed, which is what actually matters under a coating.
Does a new slab need testing?
New concrete is the most likely to hold construction moisture, so it is where testing earns its keep most obviously. An old slab over damp ground is the other case.
Can I skip preparation if my floor looks clean?
Looking clean and being coatable are different things. Sealers and curing compounds are invisible and are bond-breakers, which is why the slab gets profiled rather than just swept.
How much does preparation add?
Published national figures for contracted concrete grinding span roughly $1.50 to $6.00 per square foot depending on what has to come off. The cost guide on this site collects the published ranges with sources.
Had a floor lift on you before?
Call and describe what happened and what the slab looks like now.
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