Concrete Moisture Testing

Concrete contains far more water than it needs to hydrate, and much of the excess leaves only slowly as vapor through the top surface. When a low-permeability coating covers that surface, moisture and the alkaline salts it carries collect at the bond line. The results include osmotic blisters, loss of adhesion and, during application, pinholes and outgassing. Moisture testing tells you whether the slab is ready, or whether you need a moisture-tolerant primer or a vapor mitigation system.

Why moisture matters for coatings

• Osmotic blistering: soluble alkalis and salts at the interface draw water through the slab and create pressurized, liquid-filled blisters.
• High pH at the bond line: moisture carries alkalinity to the surface, where it can degrade some adhesives and coatings.
• Adhesion loss: continuous vapor drive can push a coating off even without visible blisters (see delamination).
• Application defects: moisture and air leaving the slab as it warms can cause pinholes, bubbles and poor cure in fast-setting systems.

Where slab moisture comes from

The main sources are excess mix water, ground moisture beneath slabs on grade without an effective vapor retarder, and re-wetting from rain, wet curing, washing or leaks. Drying is slow and depends on slab thickness, the water–cementitious materials ratio, curing method, temperature, humidity and airflow. Slabs on grade without a vapor retarder may never get dry enough for moisture-sensitive systems. ACI 302.2R gives guidance on designing slabs that receive moisture-sensitive floor finishes. Lightweight-aggregate concrete and slabs on metal deck can take especially long to dry.

Test methods compared

F2170 and F1869 both call for a minimum number of tests based on floor area: three tests for the first 1,000 ft² (about 93 m²) and at least one for each additional 1,000 ft². Place them to include the areas most likely to be wet.

In-situ relative humidity (ASTM F2170)

F2170 measures the relative humidity inside the concrete, which shows how much moisture will redistribute toward the surface once it is sealed. Many in the industry regard it as the more reliable indicator for slabs that will be covered.

1. Condition the space. Bring the building to service temperature and humidity, and hold it there for the period the standard requires before testing (48 hours).
2. Drill the hole. For a slab drying from the top only, such as a slab on grade, drill to 40% of the slab thickness. For a slab drying from both top and bottom, such as an elevated slab, drill to 20%.
3. Clean and sleeve. Remove dust from the hole and install the sleeve or probe so that only the bottom of the hole is open to the sensor. Cap it.
4. Equilibrate. Allow at least 24 hours for the hole to reach equilibrium before taking the reading. Earlier editions of the standard required 72 hours.
5. Read and record. Use a calibrated sensor that has reached the temperature of the slab. Record RH, temperature, depth, location and ambient conditions.

Coating manufacturers commonly set limits somewhere around 75–85% RH. Moisture-tolerant and moisture-mitigating systems are often rated much higher, sometimes up to saturation. Use the value on the product data sheet.

Calcium chloride test (ASTM F1869)

A pre-weighed dish of anhydrous calcium chloride is sealed under a plastic dome on a prepared area of bare concrete. After 60–72 hours, the dish is reweighed. The weight gain, divided by the exposed area and the test time, gives the moisture vapor emission rate (MVER) in pounds per 1,000 ft² per 24 hours. One lb/1,000 ft²/24 h is about 56.5 µg/(s·m²).

The test area must be cleaned down to bare concrete beforehand, typically at least 24 hours before setup. The space should be within the conditions the standard specifies, roughly 65–85 °F (18–29 °C) and 40–60% RH. F1869 mainly reflects the upper portion of the slab and is sensitive to ambient conditions, which is why many flooring and coating specifications now prefer F2170 or require both. Published coating limits are commonly in the range of about 3–5 lb/1,000 ft²/24 h for standard systems, with higher allowances for moisture-tolerant products.

Screening tests

The plastic sheet test (ASTM D4263) tapes an 18 × 18 in (457 × 457 mm) sheet of polyethylene to the slab for at least 16 hours. Condensation under the sheet or darkened concrete indicates moisture. A dry result does not prove the slab is dry enough, because the test is affected by temperature and does not measure deep moisture.

Electronic meters used per ASTM F2659 give quick relative readings, useful for mapping wet zones and choosing where to place quantitative tests. Do not use them alone for acceptance.

Interpreting results and options

If results are within the manufacturer’s limits, proceed with surface preparation and coating. Prepare the surface promptly after testing, because conditions can change. If results exceed the limits, the usual options are:

• Wait and dry: run HVAC, dehumidify and ventilate, then retest. This works when the moisture comes from mix water rather than from the ground.
• Moisture-tolerant primers: some epoxy primers are formulated for damp concrete or elevated RH.
• Moisture vapor mitigation systems: purpose-made coatings, often high-solids epoxies, applied over prepared concrete to limit vapor transmission under the finish system. They are rated for specific RH or MVER levels.
• Fix the source: address drainage, leaks or missing vapor retarders where practical.

Frequently asked questions

Which is better, F2170 or F1869?

They measure different things. F2170 measures internal RH and is less affected by surface conditions. F1869 measures surface emission over a few days. Use whichever test the coating manufacturer’s limits are written for, or both if the specification requires it.

How long does concrete need to dry before coating?

There is no reliable fixed time. Drying depends on mix design, thickness, curing, vapor retarder and environment. Test against the product’s limits.

Is the plastic sheet test good enough?

It is a useful quick check. Because it is qualitative and can give false “dry” results, it should not be used alone to accept a slab for a moisture-sensitive coating.

Can I test before the building is closed in?

You can screen early, but F2170 and F1869 require the space to be at service conditions. Results taken in an open, unconditioned building may not reflect how the slab will behave once coated.