Concrete floor moisture-related failures continue to cost property owners a great of money and lost time. It pays for these parties to take charge of the process, requiring testing by independent agencies and creating detailed specifications that direct the entire construction team as to what is expected of them so potential flooring problems can be identified and solved in advance.

Testing concrete slabs for excess moisture has become a common construction requirement, particularly where flooring or impermeable membranes are to be moisture-mitigation-flooringinstalled on top of the slab. Problems with floor coverings, such as bubbles, blisters, and delamination, occur when an impermeable floor coatings trap excess moisture remaining in the slab.  EP Floors Corp. can test your concrete, to determine if the moisture content in a slab is within an acceptable range for seamless epoxy flooring, polyurethane floor coatings, and other impermeable coverings.

Nature of Concrete:  Both fresh concrete and slabs that have cured and hardened have excess water. Immediately after concrete is placed and throughout the curing period, efforts are made to prevent water from evaporating from the slab so that the concrete can properly hydrate. After the initial curing period, however, we want the concrete to dry out, and large amounts of water are given off by a newly placed slab. Concrete with a higher water-to-cementitious material ratio gives off proportionately more moisture simply because there is more free moisture in the slab and the differential vapor pressure between the concrete and the air remains high. But if the concrete is simply air drying, the rate finally decreases, which means the differential vapor pressure is greatly reduced. When the water vapor approaches equilibrium, flooring, coating, or sealing can be applied.
However, even after it has hardened, most concrete remains porous, so its moisture increases or decreases with changing temperature and humidity. Concrete’s permeability, the rate at which it will allow moisture to pass, depends on the size and distribution of the pores in the concrete matrix. Generally speaking, the lower the water-to-cementitious-materials ratio of a concrete mixture, the lower its permeability will be after it has cured. Concrete’s permeability can also be reduced by adding any of a number of products to fill in the voids in the concrete matrix.


ASTM F 2170 – Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ probes, measures moisture inside a concrete slab.

ASTM F 1869 – Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous calcium Chloride, measure vapor emission at the concrete surface.

Both testing methods provide valuable information, and it’s good idea to perform both, if possible, prior to installation, in addition to the pH test.

Calcium chloride test quantifies the moisture vapor emitted from the slab. This test has long been the standard in the U.S. since the 1950’s.    The relative humidity (RH) test measures moisture conditions inside the slab. The relative humidity test is the standard in several other countries. The calcium chloride test generally indicates moisture conditions in the top region of the concrete slab, whereas relative humidity probes tell you what’s going on in the body of the concrete which is more useful in the long term.

Measures Vapor Emission   

The calcium chloride test, sometimes called the moisture vapor emission rate (MVER) test, is simple inexpensive and widely accepted. It takes advantage of the fact that calcium chloride absorbs moisture like a sponge. A small container of salt is weighed before and after it’s exposed to the concrete slab for 60 to 72 hours. The weight difference as a percentage of the original weight of the calcium chloride used gives the moisture vapor emitted by the slab. It is usually expressed in pounds per 1,000ft of concrete per 24 hours. Most manufacturers publish the acceptable amount of moisture that their products will withstand. That amount will vary between 3 and 5 lbs. Order TEST KITS (click here).

For accurate results, this test has to be conducted with the building at the same temperature and humidity levels as during use – which can be problem if the HVAC system isn’t operational, as is often the case during construction. Also, the results reflect only the moisture in the top ½ inch to ¾ inch of the slab. Most of the moisture within slabs accumulates in the lower half (The lower half can have twice as much moisture as the upper half).

This test involves drilling holes into the concrete and inserting a probe that measures moisture deep within the concrete slab. The results, expressed as a percentage of relative humidity rather than in pounds of moisture vapor, are read on a meter connected to the probe. While the relative humidity testing apparatus isn’t as sensitive to the ambient room temperature and humidity as the calcium chloride test, it’s still important that both the test hole and the probe be allowed to acclimate to the room’s conditions.

The relative humidity test provides a more complete picture of what’s going on throughout the entire depth of the slab, the initial cost of the relative humidity test apparatus is substantially more than the calcium chloride test. It is ideal to conduct both the tests since each looks at a different part of the same picture.

Litmus Paper Test – PH TEST
This test measures the alkalinity of the surface of the concrete. Alkaline salts are a whitish-gray residue left on the surface of the concrete after the moisture has escaped. These tests are important, but can also be misleading because not all moisture problems carry high quantities of alkali to the surface.

Who should do the testing?
The Floor Covering Industry White Paper Position Statement on Moisture Emission Testing recommends independent testing.


How to Write A Good Concrete Moisture Testing Specification

Concrete Floor Moisture Vapor Mitigation- Overview
Concrete Moisture Facts


 Moisture problems can be solved through expensive moisture remediation processes;
However, the best method is prevention!