Concrete moisture testing before epoxy: the tests, the limits and what to do

Test concrete for moisture before you coat it, because moisture moving up through the slab is what lifts epoxy into blisters and peels it off. The two tests that give numbers are the calcium chloride test, ASTM F1869, which reports pounds of water per 1,000 sq ft per 24 hours, and in-slab relative humidity probes, ASTM F2170. Standard coating systems allow about 3 lb or 75–80% RH, and a calcium chloride kit costs about $12–36 per test.

This article covers why moisture matters, how each test works, how many to run, the limits coating systems set, what to do when a slab fails, and what to write down. It’s one of the steps in starting an epoxy flooring business that pays for itself the first time it stops a bad job.

Why moisture matters

Epoxy and similar resinous floors are non-permeable: moisture vapor rising through the slab gets trapped under the film. Excess moisture discolors coatings, interrupts their cure, and delaminates them. When a coating peels from moisture, the concrete underneath may look dark and damp. That’s delamination, and the fix starts with removing the coating.

New slabs are the obvious risk, but not the only one. New concrete can look dry well before it’s ready to coat, and moisture can vary from one area of the same slab to another. Old slabs on grade without a vapor barrier underneath can pass in a dry month and fail in a wet one.

The tests compared

Plastic sheet (ASTM D4263)

Tape a sheet of plastic, 12 by 12 inches or larger, tightly over bare concrete and leave it 24 hours. Condensation under the plastic, or concrete darker than the area around it, means the slab likely holds too much moisture for a standard system. It’s the least precise test and isn’t always accepted under product warranties; ASTM’s current edition, D4263-24, treats it as a practice for indicating capillary moisture, nothing more.

Calcium chloride (ASTM F1869)

A pre-weighed dish of calcium chloride sits under a sealed dome on bare concrete and absorbs the moisture coming out of the slab. After 60 to 72 hours you reweigh it and calculate the calcium chloride test result in pounds per 1,000 sq ft per 24 hours. The method can’t be used over coatings, sealers, reactive penetrants, patching or leveling compounds, or on lightweight aggregate or gypsum concrete, so test bare, ground concrete that has been open to the room for at least 24 hours.

In-slab relative humidity (ASTM F2170)

You drill holes, insert sleeves or probes, let them equilibrate, and read the relative humidity inside the concrete. The key requirements:

• The slab and the air above it held at service temperature and humidity for at least 48 hours first.
• Holes 40% of the slab’s thickness deep if it dries only from the top, 20% if it dries from top and bottom.
• A 24-hour equilibrium period in each hole, and readings that drift no more than 1% RH over 5 minutes.

Older guidance may say 72 hours; that was the 2002 edition of F2170. Work from the current edition.

Meters

Hand-held meters cost about $25–760. Under ASTM F2659 they read the comparative moisture condition of the top inch of the slab and are not a basis for accepting a floor. They’re good at one thing: walking the slab to find the wettest areas, which is where your F1869 or F2170 tests should go.

How many tests to run

Both quantitative tests use the same density: three tests for the first 1,000 sq ft and one more for each additional 1,000 sq ft, with a partial 1,000 counting as another test.

Put that cost in the quote. Moisture testing is cheap next to a callback.

Typical limits

Go by the current technical data sheet for the exact product. Older guides and store listings sometimes quote different numbers; the current data sheet is the one the product warranty follows. If you’re unsure, call the manufacturer’s technical line before you install.

Timing and acclimation

• New concrete. Wait at least 28 days. The 28-day convention comes from lab studies showing about 95% of the excess water has left a slab by then; cooler or more humid conditions take longer.
• Service conditions. Test the floor in the conditions it will live in. When that isn’t possible, ASTM F1869 allows 65–75°F and 40–60% relative humidity, held for 48 hours before and during the test. Keep test sites away from windows and direct sun.
• The season. A garage tested in a dry August may read differently in March. Without a vapor barrier under a slab on grade, seasonal changes in ground moisture can push excessive moisture through the slab no matter what the earlier tests showed.

What to do when a slab fails

1. Retest after drying. Raising the room temperature and increasing ventilation helps a drying slab, such as new concrete, give up its excess moisture before flooring goes down.
2. Use a moisture-vapor-barrier primer. A moisture vapor barrier primer goes directly on the concrete under the system. Simiron’s MVB, for example, reaches its 25 lb / 95% RH rating only at its full 16-mil thickness, needs sound concrete (at least 3,500 psi compressive and 300 psi tensile strength), must be overcoated, and won’t prevent failures from alkali-silica reaction or from contaminants left by old hardeners and sealers. At about $82.50 a gallon and 100 sq ft per gallon, that’s about $82.50 ÷ 100 = $0.83 per sq ft of extra material.
3. Choose a different system. Some products are built for wetter slabs, such as moisture-tolerant epoxy base coats rated around 9 lb and urethane cement systems that can go on new, damp concrete. Ask the manufacturer for a system rated above your readings.
4. Walk away. If the readings are far above every product you can get, or the slab is flooding from outside, don’t coat it.

What to document

Every test is evidence. Record, for each one:

• Date and time placed and read; the test method and edition.
• A sketch of the floor with each test location marked, the total area and the number of tests.
• Slab temperature, air temperature and relative humidity during the test.
• Kit lot numbers, starting and ending weights (calcium chloride) or meter serial number (RH).
• The result for each location, the product's limit, and the decision you made.
• Photos of each test in place.

Keep it with the job file and the product data sheets. If a floor fails later, this is what tells you and the manufacturer whether moisture was the cause. The job-day checklist has a place for it, and epoxy floor warranties covers why documentation matters when a callback turns into a claim.

Where this fits in the job

Moisture testing comes first, then grinding to the right concrete surface profile, then the system itself; see flake floor installation for the full sequence and epoxy cure time, temperature and humidity for the other conditions you’ll measure on install day. If a floor does fail, why epoxy floors fail walks through the causes.