What is The Best Test for Concrete Moisture?

Concrete moisture remains the number one reason for flooring failures and therefore, moisture testing and/or mitigation strategies are always recommended to ensure a successful installation. But what are the methods of testing and are they reliable? Here are a the most common testing methods:

Hand-Held Moisture Meters
Contractors often ask if hand-held moisture meters work. The answer is yes, they do, however, it’s important to note that no hand-held concrete moisture meter of any type can give consistently accurate concrete moisture meter readings. Here are few of the reasons why:

  • Each concrete slab has different mix designs (aggregate sizes, content, metal reinforcement, etc.) The meter sends an electromagnetic wave into the concrete and the sensor interprets how much moisture is in the concrete. Aggregate density and concentration as well as any metal reinforcement close to the surface can impact how the signal is reflected.
  • Hand-held meters only measure the top half inch of the slab. This is problematic because the moisture in the slab, more often than not, is more heavily concentrated in the bottom 50% of the slab. Once non-permeable flooring is installed, any moisture within the slab will redistribute to reach equilibrium. This means that the top 50% of the slab will get wetter causing a flooring failure.
  • They can be dramatically impacted by the relative humidity ambient environment. So, a drier environment may yield a lower reading or conversely you could get a false positive if the test is conducted in high humidity environment. The calcium chloride (dome test) also suffers from a lot of these same flaws.
  • If you’re requiring a moisture test that is non-destructive to the slab and fast to use, this is your best choice.

Calcium Chloride (Dome) Test
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. Here’s what you need to understand about the Calcium Chloride testing method:

  • The building must be acclimated and have the HVAC system running for at least 24 hours prior to starting the test.
  • Similar to hand-held meter testing, results can be highly impacted by relative humidity and temperature of the environment.
  • If you’re on a tight budget, the Calcium Chloride test is relatively inexpensive to perform with no training or specialized equipment.
  • These tests can be easily damaged by other trades on active construction sites making the results invalid.

Relative Humidity Probe Test

The most reliable moisture testing option is the Relative Humidity Probe Test. It is conducted through the use of in-situ probes which measure RH within the slab. Their depth-specific application has proven to accurately represent moisture conditions in a concrete slab. Their very location within the concrete means they are less susceptible to environmental or ambient changes at the surface.

  • Unlike hand-held meters and calcium chloride testing methods, this type of test measures the internal relative humidity of the concrete, accurately accounting for moisture in the bottom half of the slab.
  • The test requires holes be drilled into the concrete that must be repaired after testing is complete, so although it is the most accurate, it is also the most invasive.
  • This test requires specialized equipment and some staining in order to preform test.
  • The Relative Humidity Probe Test is the most accurate and dependable data about the moisture content of the concrete irrespective of environmental conditions.

Mitigation Strategies When Humidity is Too High
In the event that you have a concrete slab with high moisture levels, you will likely have to have a unwelcomed conversation with your end user or general contractor. You will need to explore mitigation strategies to ensure the success of your project. More often than not, the proposed flooring system will need to be reevaluated and the overall cost of the project will increase. Here are some of your options when this happens.

  • You can propose that the customer consider a vapor permeable flooring option such as polished concrete. This is often a viable solution, provided there are no other issues with the concrete such as ASR.
  • You can install a more moisture tolerant flooring system such as Urethane Cement.
  • You can install a specialized moisture mitigation epoxy isolation coating.
  • Depending on your specified finished floor you may also need to install a Cementous topping as well, to enable the use of water-based adhesives.