Blisters in a New Concrete Floor: What They Tell You

Wally Bates

Sep 26

2 min read

The floor looked perfect at the walkthrough. Six weeks later there are blisters the size of quarters. Cut one open and there is water inside. This is the most common concrete coating failure there is.

Newly coated concrete floor in a large facility

Cause 1: Moisture vapor drive

Blisters in a coated concrete floor almost always mean moisture coming up from below. A slab on grade sits on damp ground, and water vapor moves up through it constantly. Most high-performance epoxies have low permeability, which is good for protection but means the vapor stops at the coating. Pressure builds under the film until the bond gives way.

Cause 2: Osmosis

Sometimes the mechanism is osmotic. Salts or contaminants at the surface pull water through the coating and swell into blisters, at pressures well above what vapor alone produces.

Cause 3: Outgassing

Coat a warm slab in the afternoon and the air in the pores expands as the temperature rises, pushing up through the wet film. The fix is timing, applying as the slab temperature is falling, and a suitable primer.

Prevention starts with testing

In each case the coating itself did not fail. The slab was not tested. ASTM F2170 (in-slab relative humidity probes) and ASTM F1869 (calcium chloride) exist for this reason, and flooring manufacturers publish the moisture limits their products tolerate.

When results exceed the limit, there are options:

A moisture-mitigating primer.
A breathable coating system.
Correcting drainage or vapor retarder issues at the source.

What to check in your spec

Moisture testing per ASTM F2170 or F1869 before any coating, with results recorded.
The manufacturer's moisture limit as the acceptance criterion.
A mitigation primer as an alternate if limits are exceeded.
Application timing guidance to control outgassing.

Further reading: Tnemec Technical Bulletin 97-05, Water Vapor Emission on Concrete Substrates. Standards: ASTM F2170, ASTM F1869, ASTM D4263.

Talk to a consultant. Atlantic Coating Consultants helps owners, engineers and contractors in PA, NJ, DE, MD, DC, WV and OH with specification writing, on-site condition assessment, training, product recommendations and field support. Call 484-644-5303 or email atlantic@tnemec.com.