Moisture Vapor From Below: The Coating Killer

Three relative humidity probes went into a 620 square foot shop floor on a Thursday and read 94, 96 and 91 percent. The coating crew was booked for Monday. That slab was eleven months old and its 28-day cylinders had broken at 4,610 psi against a 4,000 psi spec — a good slab by every number anyone had asked for. It was also soaking wet inside, and nothing in the strength report said so.

I run the tests and hand over the report; whether it changes a schedule is not my call. I have watched crews coat over numbers like those. They call back in eight to eighteen months.

Strength and dryness are unrelated

A cylinder break tells you the paste hydrated enough to carry load. It says nothing about the free water still in the capillary pores. A cubic yard is batched with 250 to 330 pounds of water, and only a quarter to a third combines chemically with the cement. The rest leaves through the top, because a slab on grade dries in one direction.

The planning figure is one month of drying per inch of thickness, and that assumes the building is closed, near 75 degrees and 50 percent humidity, with a capillary break underneath. A 4 inch slab needs four months. On wet ground with the door open, it may never get there.

Two tests, measuring different things

Both are standardized and quoted interchangeably.

Calcium chloride, the anhydrous dome test. A dish of salt under a sealed dome for 60 to 72 hours, weighed before and after, reported as pounds per 1,000 square feet per 24 hours. Most coatings want 3 or less. It only reads the top three quarters of an inch, so a slab whose surface dried in a windy week passes while the core is saturated.
In-situ relative humidity probes. A sleeved hole drilled to 40 percent of slab depth, capped and left to equilibrate at least 72 hours. Limits run 75 to 85 percent depending on the product. At that depth the reading approximates where the whole slab settles once you seal the top, which is what a coating does.

When they disagree, the humidity probe is telling you what happens after the film goes down. The guidance on slabs receiving moisture-sensitive flooring from the American Concrete Institute covers both methods.

The conditions the test needs

A number is worthless without the conditions attached.

The space sits at service temperature and humidity for 48 hours before testing and throughout. Testing an unheated garage in January produces a meaningless number.
Curing compound, sealer and old adhesive come off the test spot first; a membrane suppresses the dome test badly.
Coverage is three tests for the first 1,000 square feet, plus one per additional 1,000.
Put a probe near the overhead door, near any wall with irrigation on the far side, and wherever the floor dries slowest. Averages hide the failure.

Testing runs $50 to $150 per location; a written report on a small floor lands around $300 to $700. Against a $4,000 coating that gets peeled and redone, it is cheap information.

Where the water comes from

Sometimes the slab is just young. More often it is fed: no vapor retarder underneath, or one torn by boots before the pour, a high water table of the kind routine across South Florida, a driveway pitched back toward the door, irrigation throwing at the foundation wall. In dry high-altitude air the top inch dries so fast that a taped-down plastic sheet comes up clear while a probe at 40 percent depth reads in the nineties.

Under a new slab the answer is a vapor retarder of at least 10 mil, laps sealed, directly under the concrete with no sand blotter. It cannot be retrofitted, so an existing floor gets a mitigation system instead — a high-solids epoxy barrier rated to hold back 95 to 100 percent internal humidity, at $2 to $6 per square foot on top of grinding and the coating. Which topcoat goes over that matters less than people think, though it still matters for surviving hot tires and jack stands.

Frequently asked questions

Does a 4,000 psi break mean the slab is ready to coat? No. Compressive strength and internal moisture are independent numbers. A slab can hit design strength at 28 days and still read 95 percent internal humidity a year later. Ask for a moisture test, not a strength report.

Which test should I ask for, calcium chloride or humidity probes? In-situ relative humidity on any slab on grade, since it reads the depth that matters. Calcium chloride is still useful for comparing areas of the same floor, and some manufacturers name it in their warranty language.

Can heaters and fans dry a slab faster? Somewhat, and only if the moisture has somewhere to go. Heat plus dehumidification plus air movement helps; heat alone in a closed garage just moves water around. None of it works while the source is active, so fix irrigation, downspouts and grade first, then retest.

Soil data: USDA Web Soil Survey.