Concrete looks dry long before it is dry. A slab can have a pale, dusty surface while the concrete a few centimeters down is still close to saturated. Seal that slab with a coating that does not tolerate moisture and the water has nowhere to go. It collects under the film, pushes up blisters, weakens the bond and, with some chemistries, interferes with cure.
Moisture testing is how coating and waterproofing crews decide whether the concrete is ready. This guide explains why moisture matters, how the common test methods work in general terms, when and where to test, and what can be done when the numbers are too high. It is general information. The test method, number of tests and acceptance limit for a real project are set by the coating or membrane manufacturer, the specification written by a qualified engineer or coatings specialist, and the applicable standards.
Key takeaways
- Excess moisture causes blistering, debonding, poor cure and, under floor coverings, adhesive failure. It also feeds outgassing pinholes.
- In-situ relative humidity probes (ASTM F2170) measure conditions inside the slab and are widely specified for floors.
- The calcium chloride test (ASTM F1869) estimates the moisture vapor emission rate from the surface over a set period.
- The plastic sheet test (ASTM D4263) is a simple qualitative check, not a measurement.
- Electronic moisture meters give comparative readings that are useful for mapping, not for acceptance unless the manufacturer says so.
- If a slab is too wet, the options are to wait and dry it, use a moisture-tolerant primer or system, or install a vapor mitigation layer, all with the manufacturer’s agreement.
Why moisture matters
Concrete is mixed with more water than the cement needs to hydrate. The rest has to leave by evaporation, slowly, through the pore structure. How long that takes depends on the water-cement ratio, slab thickness, whether it dries from one side or both, the air temperature and humidity, curing method and any later wetting from rain, cleaning or groundwater.
Once a coating or membrane is on the surface, the moisture behaves differently depending on the product:
| Problem | What happens | Typical products affected |
|---|---|---|
| Osmotic blistering | Water is drawn through the concrete toward soluble material at the bond line, building pressure under the film | Dense, low-permeability films such as many epoxies and polyurethanes |
| Vapor pressure debonding | Warming of the slab raises vapor pressure under an impermeable film | Thick, impermeable coatings on slabs without a vapor barrier below |
| Interference with cure | Moisture reacts with the resin or prevents proper film formation | Moisture-sensitive polyurethanes and some primers |
| Outgassing pinholes | Air and vapor escape through the fresh film as the slab warms | Most coatings, especially on porous or freshly prepared concrete |
| Adhesive failure under coverings | Alkaline moisture attacks flooring adhesives | Resilient flooring and some sheet membranes |
Ground-bearing slabs with no effective vapor retarder below are the most difficult case, because moisture keeps arriving from the ground long after the original mix water is gone. Our guide to below-grade waterproofing covers the related problem of water and vapor moving through basement walls and floors.
The common test methods
In-situ relative humidity probes
ASTM F2170 describes drilling holes to a set depth, sealing a sleeve in each hole, letting the concrete reach equilibrium and then reading the relative humidity and temperature with a calibrated probe. The depth depends on the slab thickness and whether it dries from the top only or from both sides; the standard sets it as a proportion of the thickness. The idea is to measure the humidity the surface will settle at once it is sealed and moisture redistributes.
- Strengths: measures conditions inside the slab, less affected by surface drying or recent weather, widely used for floor coatings and coverings.
- Limitations: needs equilibration time before reading, probe calibration, careful drilling and sealing, and the building at or near service conditions.
Calcium chloride test
ASTM F1869 places a dish of calcium chloride under a sealed dome on the cleaned surface for a set period. The weight gain of the dish gives an estimate of the moisture vapor emission rate, usually expressed per unit area per 24 hours.
- Strengths: simple equipment, long history, many older data sheets give limits in these units.
- Limitations: it reflects mainly the upper part of the slab and is sensitive to the ambient conditions during the test. Many specifiers now prefer in-situ RH for this reason, though some manufacturers still ask for it.
Plastic sheet test
ASTM D4263 tapes a square of clear plastic sheet to the surface and leaves it for a set period. Condensation under the sheet or a darkened surface shows moisture is moving. It is a quick indication, useful as a first look or on walls and soffits, but it does not give a number and a dry result does not prove the slab is dry enough for a sensitive coating.
Electronic moisture meters
Impedance and capacitance meters are pressed onto the surface and give a reading on a relative scale or a percentage for a given material. ASTM F2659 is a guide for using them as a preliminary, comparative evaluation. They are fast and good for mapping wetter and drier areas and for choosing where to place other tests. Readings depend on the meter, the concrete and surface conditions, and are not interchangeable between instruments.
Other methods
Some countries and manufacturers use a hygrometer under an insulated hood sealed to the surface, or the calcium carbide method, in which a sample of crushed concrete is reacted in a pressure flask to give moisture content by weight. Some European data sheets state a maximum moisture content by weight, often a few percent. Use the method the manufacturer names, because limits only apply to the method they were written for.
Comparing the methods
| Method | What it gives | Typical use | Main caution |
|---|---|---|---|
| In-situ RH (ASTM F2170) | Relative humidity at depth | Floor coatings and coverings | Equilibration time and service conditions |
| Calcium chloride (ASTM F1869) | Emission rate at the surface | Older specs, some floor products | Mostly reflects the top layer |
| Plastic sheet (ASTM D4263) | Yes or no indication | Quick check, walls and soffits | Not quantitative |
| Moisture meter (ASTM F2659 guide) | Comparative reading | Mapping and screening | Instrument-specific scale |
| Carbide method or hood hygrometer | Moisture content or surface RH | Where national practice or data sheets use them | Limits tied to the method |
When and where to test
Timing
The test methods expect the space to be at or near the temperature and humidity it will have in service, for a period before testing. Testing a slab in an unheated shell in winter, or right after rain blew in through open walls, gives results that will change. Schedules should allow time for the building to be closed and conditioned before final tests.
A common old rule of thumb says concrete dries at about 25 mm (one inch) per month. It is unreliable: drying slows sharply with depth, low ambient humidity, cold, thick slabs and dense mixes, and it ignores moisture from below. Do not plan a program on it. Test instead.
Locations and numbers
Test methods and specifications set a minimum number of tests per area. Beyond the minimum, test where trouble is likely:
- Near exterior walls, doorways and roof drains.
- Over areas that were ponded, washed or wet during construction.
- Low points, slab edges and areas without a vapor retarder below.
- Patched or repaired areas, which may contain fresh mortar.
Record the location, time, ambient conditions, instrument serial number and calibration date with every result. These are inspection records, as described in coating inspection basics.
After preparation
Shot blasting, grinding and especially water jetting change the surface. Water jetting and hydrodemolition add water, and preparation exposes concrete that may be wetter than the old surface. Moisture should be checked after preparation, immediately before priming, as part of the routine in concrete surface preparation and CSP profiles.
What to do if the slab fails
A failed result is information, not a disaster. The options usually are:
- Wait and dry. Close the building, run heating and ventilation or dehumidification, keep water off the slab and retest. This is the simplest option when time allows.
- Use a moisture-tolerant primer or system. Many manufacturers offer primers designed for damp or high-RH concrete, sometimes called moisture vapor mitigation primers. They have their own limits, also stated on the data sheet.
- Change the system. Breathable systems, such as some cementitious coatings or vapor-permeable products, may suit damp concrete better than an impermeable film. Our guide to protective coating systems compared outlines the trade-offs.
- Investigate the source. Rising moisture from the ground, a leaking pipe, a failed roof drain or condensation each need a different fix. Coating over an active leak only moves the failure.
Whatever route is chosen, record the decision and get the manufacturer’s agreement in writing, especially where a warranty depends on meeting a moisture limit.
Moisture and waterproofing membranes
Liquid-applied waterproofing membranes on decks, roofs and podiums have the same issue. Moisture under a membrane can blister it, particularly on sunny exposed surfaces where the substrate heats up quickly. Bridge and parking decks also see wetting from rain and cleaning between preparation and application. Our guide to parking deck traffic membranes discusses the cure and timing questions that follow, which also apply to bridge decks under asphalt.
Frequently asked questions
Can I convert an RH reading into an emission rate? No. They measure different things and there is no reliable conversion. Use the method the manufacturer specifies.
The surface feels dry. Do I still need to test? Yes. The surface dries first. Problems come from moisture deeper in the slab or from below.
Is a moisture meter enough for acceptance? Usually not, unless the coating manufacturer names that meter and gives a limit for it. Meters are good for mapping and screening.
How long does a new slab need? It depends on the mix, thickness, curing, drying conditions and moisture from below. Many manufacturers also state a minimum concrete age. Plan for testing rather than a fixed number of weeks.
Bottom line
Moisture is one of the most common hidden causes of coating and membrane failure on concrete, and the surface is a poor guide to what lies beneath. Use the test method the manufacturer specifies, test under service conditions, in enough locations and after preparation, and record the results. If the slab is too wet, dry it, change to a moisture-tolerant system or deal with the source, with the manufacturer’s agreement. The methods and limits for a given system come from qualified engineers, the manufacturer’s data sheets and the applicable standards. For what happens when moisture is ignored, see why coatings fail.