Before applying an epoxy coating to a slab, concrete moisture has to be measured with a standardized test, and the result compared with the limit given in the manufacturer's data sheet. If the concrete exceeds that limit, you wait, deal with the water source or install a compatible moisture barrier. Coating concrete that is too wet almost always leads to blisters and delamination.
Concrete looks dry on the surface long before it is dry in depth. Water rising through the slab builds pressure under the epoxy film, which has low vapour permeability: the coating swells, blisters and then lifts in sheets. This guide is for maintenance managers, plant managers and project managers preparing an industrial epoxy floor in Quebec and Ontario. It explains which tests to ask for, how to read them and what options exist when the slab is not ready.
Why does moisture make an epoxy floor fail?
Moisture makes an epoxy floor fail because it attacks adhesion from below, where nobody sees it. Concrete holds free water in its pores. As long as that water can evaporate, it causes no harm. Once the slab is covered by a tight film, vapour condenses at the interface and creates pressure that pushes the coating upward.
Three mechanisms come up on site:
- Vapour pressure. Differences in temperature and humidity between the slab and the room air drive vapour upward. Under a low-permeability film, it accumulates and forms blisters.
- Osmosis. Soluble salts in the concrete or at the interface draw water through the film. Osmotic blisters often contain liquid and come back after repair if the cause is not addressed.
- Alkalinity. Migrating water carries alkalis that can degrade the adhesion of some primers at the interface.
The outcome is the same in all three cases: delamination, blisters, opening joints. Our article on coating failure modes shows how to tell a moisture blister from other failure modes.
Which tests measure concrete moisture?
Three standardized tests are used in practice, and they do not measure the same thing. The choice depends on what the manufacturer's data sheet requires: it names the reference test and the acceptance limit.
| Test | What it measures | Typical use | Main limit |
|---|---|---|---|
| ASTM D4263 (plastic sheet) | Presence of moisture rising to the surface, qualitatively | Quick first screening, before quantitative tests | Gives no numerical value; sensitive to ambient conditions |
| ASTM F1869 (calcium chloride) | Moisture vapour emission rate at the slab surface | Required by some data sheets and specifications | Reflects only the zone near the surface; needs controlled ambient conditions |
| ASTM F2170 (in situ probes) | Relative humidity inside the slab, at a depth set by the standard | Increasingly the method manufacturers specify | Requires drilling holes and an equilibration period before reading |
The number of test locations, their placement, the exposure time and the air conditions are set by each standard. A badly installed test, or one run in an unheated building, gives a value that means nothing. Each reading is recorded with the date, location, air temperature and humidity.
How is the plastic sheet test (ASTM D4263) done?
The plastic sheet test is done by taping a clear sheet to the concrete, edges sealed, for the period the standard sets, then checking for condensation under the sheet or darkening of the concrete. If either appears, the slab is emitting moisture and coating is not yet an option.
It is a good screening tool, simple and inexpensive, but it remains qualitative. A dry sheet does not prove the slab meets the manufacturer's limit; it only shows there is no obvious moisture rise at the time of the test. On an industrial floor project, it is almost always followed by the calcium chloride test or in situ probes. It is also repeated at several locations, especially near drains, pits, exterior walls and areas where the slab sits on grade.
Calcium chloride or in situ probes: which to choose?
The right choice is the one the manufacturer's data sheet names. When it accepts both, in situ probes (ASTM F2170) give a better picture of the moisture that will return to the surface once the slab is covered, because they measure inside the concrete rather than at its surface.
- ASTM F1869, calcium chloride. A weighed dish of calcium chloride is placed under a sealed dome on a prepared surface. After the set period, it is weighed again: the weight gain gives a vapour emission rate. The test is sensitive to air temperature and humidity, and it reflects only the top layer of the slab.
- ASTM F2170, in situ probes. Holes are drilled to the depth set by the standard, sealed sleeves are installed, the air is allowed to equilibrate, then relative humidity is read with a calibrated probe. The test takes more preparation, but it is less influenced by the surface condition.
In both cases, the building has to be close to service conditions (heating on, envelope closed) for the period the standard requires. Testing a slab in mid-winter in an unheated warehouse, then occupying it heated, can completely change how moisture behaves.
How do you recognize hydrostatic pressure?
You recognize hydrostatic pressure when the water does not come only from residual moisture in the concrete, but from an outside source that pushes continuously: water table, saturated soil, failed drain, no vapour retarder under the slab. Typical signs are stains that come back after drying, white efflorescence, water in cracks or joints, and readings that do not drop from one test to the next.
This changes the strategy. Waiting solves nothing, since the water source does not run out. A surface moisture barrier can resist vapour emission, but not all are designed for liquid water pressure. The source has to be dealt with first when possible (drainage, repair of active cracks, waterproofing), then a system chosen whose data sheet explicitly covers hydrostatic pressure. Through-cracks are often treated by concrete crack injection before coating.
What if the concrete is too wet?
If the concrete exceeds the manufacturer's limit, there are three ways forward: wait and dry, install a moisture barrier, or choose a moisture-tolerant primer. The choice depends on the measured value, the cause, the work window and the floor's exposure.
- Wait and dry. On new concrete, the safest route is often to let the slab dry, with heating and ventilation running, then test again. It rarely fits a short production shutdown.
- Moisture barrier. A specially formulated epoxy system, applied to prepared concrete, limits the vapour reaching the topcoat. Its data sheet sets the maximum value it tolerates, the preparation and the thickness. It is part of the system, not an improvised add-on.
- Moisture-tolerant primer. Some primers bond to damp concrete without surface water. They do not replace a barrier when emission is high; they widen the acceptable range within the limits of their data sheet.
In every case, mechanical preparation (shot blasting or grinding) remains mandatory to open the surface and remove laitance. A barrier applied over laitance that was not removed will come off with it. For slabs exposed to chemicals, the choice overlaps with that of a secondary containment coating.
How do these tests fit into the flooring project?
Moisture tests fit into the project at the planning stage, not the day before application. A standardized test takes time to install and read, and an unfavourable result can add a moisture barrier, meaning materials, a coat and time.
A simple approach:
- At the site survey. Identify risk areas (slab on grade, pits, drains, exterior walls), run a plastic sheet screening and gather the slab's history (age, vapour retarder, known leaks).
- At the quote. State which test will be run, to which standard, and which option is planned if the value exceeds the manufacturer's limit.
- Before application. Run the quantitative tests under the conditions the standard sets, record the results and compare them with the data sheet.
- In the close-out file. Hand over the moisture readings with the other checks (preparation, temperature, thickness), since they will be useful if a claim arises.
Selecting the complete system falls under our protective coatings service and the concrete protection page. Our crews and qualified subcontractors work from LaSalle, in Montreal, Montérégie and elsewhere in Quebec and Ontario, in industrial settings only (no garages or homes). Send photos, the floor area and the slab's history through the request a quote form: pricing is on consultation and the reply comes within 24 business hours.
Frequently asked questions
How long should new concrete cure before coating?
There is no single duration that applies everywhere: drying depends on slab thickness, the mix, curing, heating and ventilation. The only reliable answer comes from a standardized test whose result meets the limit in the manufacturer's data sheet.
Is a surface moisture meter enough?
No, not to accept a slab. A surface meter helps locate wetter areas and decide where to place the tests, but it does not replace ASTM F1869 or ASTM F2170 when the data sheet requires one of them.
Does a moisture barrier solve hydrostatic pressure?
Not always. Some barriers are designed for vapour emission, others for water pressure. Check that the data sheet explicitly covers hydrostatic pressure and, ideally, deal with the water source first.
Are moisture tests needed on old, dry concrete?
Yes. An old slab may have lost its vapour retarder, sit on damp soil or have absorbed oils and wash water. A plastic sheet screening, then the test the manufacturer requires, avoids discovering the problem after application.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.