Calcium Chloride Moisture Test vs. Relative Humidity

Concrete moisture testing methods, like calcium chloride (MVER) or the concrete moisture meter, test the surface of the slab.

But they fail to take into account an important aspect of concrete drying: the moisture gradient, or moisture differential, as it’s called in the American Concrete Institute’s Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring (ACI 302.2).

We’re going to explore the Calcium Chloride Moisture Test versus Relative Humidity Testing and discover how these methods can help prevent costly flooring failures due to excess moisture.

Let’s dive into the world of concrete moisture testing and ensure a solid foundation for your next flooring project.

• How Do You Know How Much Moisture Will Come Up Once You Install that Floor Covering?
• How Do You Know What that Future Equilibrium Moisture Level Will Be?
• Accurate Moisture Testing for Concrete
• Best Concrete Moisture Test

As concrete slabs dry, they tend to have less moisture on the surface and more moisture deep within. When a floor covering is applied on top, the moisture deep within rises to the surface and remains there since evaporation can’t happen quite as easily.

How Do You Know How Much Moisture Will Come Up Once You Install that Floor Covering?

It’s about understanding the term equivalent depth.

Research at the Technical University of Lund shows that “the moisture level at a certain ‘equivalent depth’ from the surface is exactly equal to the moisture level which will be achieved at the surface after the floor covering is laid.” (Hedenblad, “Drying of Construction Water in Concrete, 1997, p. 11–12).

That depth for a slab drying from one side is 40% (Ibid.).

That’s why ASTM F2170, the standard for relative humidity (RH) testing, requires drilling into the slab at 40% depth and placing probes there. This method will give you the most accurate picture of the slab’s future moisture condition—and confidence for a successful floor installation.

Moisture is a major cause of flooring failure. It can lead to the breakdown of floor covering adhesives, warping, or even mold growth (ACI 302.2).

But where is this moisture coming from?

Often from the subfloor, particularly a concrete one.

The American Concrete Institute’s Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring states, “As the slab dries, the concrete loses more moisture from the top than from the middle or bottom” (ACI 302.2).

This creates a moisture gradient within the slab. There is less moisture on the slab’s surface and more deep within.

However, the moisture deep within eventually rises to the surface when a floor covering is applied on top.

In Concrete Floors and Moisture, a book written on behalf of the Portland Cement Association (find it on their website), author Howard Kanare discusses this:

“When a floor covering is placed on top of a slab, it restricts evaporation from the top surface of the slab. Moisture within the slab then distributes itself to achieve an equilibrium due to temperature and chemical interactions from the top to the bottom of the slab. In the long run, adhesive and flooring are then exposed to the equilibrium moisture level at the top of the slab” (p. 48–49, emphasis added).

How Do You Know What that Future Equilibrium Moisture Level Will Be?

That’s where relative humidity (RH) testing, approved by ASTM F2170, comes in. Kanare explains that “relative humidity is a measure of equilibrium moisture level” (p. 48).

RH testing provides a look into the concrete’s future condition. By measuring with RH probes at 40% depth per the ASTM standard, you’ll know how the moisture will distribute itself through the slab once a floor is installed over it.

That way, you can get your slab to the right RH levels—and prevent major moisture issues!

Surface-based moisture tests are not recommended because they are easily affected by ambient conditions and may give misleading results.

Methods for Testing Moisture in Concrete

Accurate moisture testing is critical to understanding the complete moisture levels of any concrete slab. One test on the surface of the slab is obviously inadequate when trying to make a go or no-go decision about installing flooring.

Different areas of a slab may dry unevenly so adequate testing will test a number of different spots on each slab, and will test below the surface of the slab (at service conditions) as well.

ASTM International has provided several standards related to testing moisture conditions with two different test methods before installing flooring over a concrete slab: in-situ probes (ASTM F2170) and calcium chloride testing (ASTM F1869).

Calcium Chloride Test Method

The calcium chloride test method is used to determine the moisture vapor emission rate (MVER) from a concrete slab. Calcium chloride testing involves sealing a small dish of calcium chloride on a clean section of concrete under a plastic dome.

The salt absorbs moisture in that environment (and presumably coming from the concrete slab) and the weight gain after three days is used to calculate the MVER.

While this method is still specified by many flooring manufacturers, architects, and adhesive manufacturers, the calcium chloride test really only tests the surface conditions of the slab.

Learn more about how to conduct moisture tests for concrete floors in our more in-depth article.

(Side note: Calcium chloride testing has also been disallowed as an appropriate method for testing on lightweight concrete.)

RH Testing – The Best Concrete Moisture Test

To test the moisture conditions within the slab, the best indicator of the total moisture picture is relative humidity testing using in situ probes.

A series of test holes are drilled into the slab and a small probe is placed into the hole where it is allowed to equilibrate with the slab before readings are taken.

Research has found that placing the probe internally, at a depth of 40% of the slab’s total thickness, provides the best indicator of the moisture conditions the adhesive and finished flooring product would encounter if they were installed.

Understanding these basic concepts about concrete, and correctly monitoring its moisture as it dries, can significantly reduce the risk of moisture-related flooring problems.

From a concrete specification that suits the time frame available to the flooring contractor that must choose the adhesive best suited to the flooring and slab conditions.

With the right moisture condition information, it should be possible for every professional on a building site to prevent flooring failures.

Free Download – Which Rapid RH Sensor is Right For You?

Jason Spangler

Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.

Last updated on September 23rd, 2024