Why Calcium Chloride Moisture Tests Don’t Tell the Full Story

Posted by ARIDUS Rapid Drying Concrete on Aug 7, 2017 1:13:49 PM


When placing concrete, especially concrete slab foundations, it’s obviously important to have moisture-testing procedures in place. By getting a constant read on the moisture levels in the concrete slab, contractors can determine the best time to install flooring.

Not testing for moisture properly could lead to a premature installation of flooring, resulting in devastating damages like curling and cupping down the road.

Typically, a drying schedule is allotted to give the concrete enough time to cure and dry to target moisture levels (80 percent relative humidity).

A method once used to test for moisture - one that the industry has moved away from - is the calcium chloride test. The test has since been replaced by relative humidity (RH) tests after the industry called into question calcium chloride’s reliability.

What Is the Calcium Chloride Test?

The calcium chloride test is one of the most commonly used concrete moisture measuring methods, but has fallen by the wayside as more improved moisture tests have risen. This test measures moisture vapor emission rate (MVER) and is executed by placing a small dish of calcium chloride under a plastic covering.The calcium chloride in the dish then absorbs the moisture in the environment and a reading is taken after three days of absorption to determine the MVER at that moment in time.

The MVER is calculated in pounds and is used to help contractors determine the best time to install flooring. Unfortunately, as popular as the test is and was, the results can be misleading.

Only Scratching the Surface

Concrete moisture levels can vary wildly depending upon the conditions surrounding the placement of the concrete and within the concrete mix itself. Factors such as climate, humidity levels and water-to-cement ratios (w/c) can dramatically affect moisture readings at the surface of a concrete slab.

Most importantly, the calcium chloride test only approximates the moisture content at the surface of the slab, but can’t accurately present a reading of the moisture content inside the slab. Often, the test can over exaggerate the moisture content of a concrete slab with higher w/c ratios or in warmer environments.

With lower permeability concrete slabs, such as those containing fly ash, the test can underestimate the moisture content when in reality it is dangerously high.

By not measuring the moisture content internally throughout the entire cross-section of the slab, moisture that is still trapped could be left undetected and will eventually become a liability.

While ARIDUS Rapid Drying Concrete dries uniformly, most concrete slabs dry from the top down. As a result, contractors can often install flooring well before a concrete slab has reached acceptable moisture levels.

Additionally, a humid climate can throw the calcium chloride test well off the mark as ambient moisture is taken falsely into account.   

Why Relative Humidity Concrete Slab Tests Are Superior

Whether an owner, designer or contractor uses rapid-drying concrete or another moisture mitigation solution, they are better off using an RH test.

For an RH test, holes are drilled at approximately 40 percent of the concrete’s thickness. Probes are then inserted to get a moisture reading, which can then determine if the interior moisture content of the slab has reached the target required before flooring is installed.

Critics of the RH test would say that RH doesn’t account for moisture movement, but the test still provides the best viewpoint of concrete moisture content and when a concrete slab is safe for flooring.


Topics: Moisture Mitigation