The Layman’s Guide to Testing Moisture In Concrete

by Charles Milledge
Some days I think I spend about half of my work day answering questions about concrete moisture and test procedures.

Anhydrous Calcium Chloride Moisture Vapor Testing – The CaCl Test

ASTM – F1869-11 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor using Anhydrous Calcium Chloride is the guide that needs to be followed to correctly set a CaCl test. Here are the boiled down basics for doing this correctly.

  1. The proper number of tests to set for your floor is 3 for the first one thousand square feet and 1 additional kit per 1000 sf. over the the first thousand.
  2. Once the sites to be tested are selected, a 20×20″ test site area should be ground to a CSP (Concrete Surface Profile) of 1-2. The test site has to sit after being ground for a minimum of 24 hours.
    Thus endeth the first day
  3. Weigh the container and place the container and hood on the prepared site per the kits instructions. The kit should sit for 60-72 hours after placement.
    Thus endeth the second day
  4. Return and remove the hood and immediately seal the CaCl container per the kits instructions. The container should be weighed with a gram scale accurate to .1 grams. (A food gram scale is not accurate enough). If you don’t own such a scale and don’t want to send it in to be weighed, I suggest taking it to a local pharmacy. They will generally weigh the container for you. Although you should be prepared with a good story about why you are having a white powdery substance weighed at a pharmacy to avoid any unwanted police inquires.
  5. With an accurate weight in hand follow the kits instructions on how to do the calculation to arrive at the Moisture Vapor Emission Rate (MVER).
  6. Compare the result with the maximum MVER allowed in the installation instructions of the product you wish to install. This will be stated in pounds / 1000 sf / 24 hours. Your test result should be lower. If it’s higher then you don’t install!

Now, all that being said, here are some issues with using CaCl to determine suitability.

  • It only measures the first 3/4″ or so from the top of the slab down. Moisture in concrete is in a gaseous or vapor form. As such it disperses across a pressure gradient from the bottom to the top of the slab. When the slab has a porous floor covering such as carpet on it, the moisture vapor passes through the carpet and dissipates into the air causing no problems along the way really. However, if it meets a non-porous floor such as vinyl, upon exiting the concrete it condenses back to liquid (actual water). If the non-porous floor has been glued to the slab, this water along with the alkalinity salts that it carries is condensing at the adhesive layer. The combination of the water and the alkalinity salts attack the adhesive and cause it to re-emulsify (they turn it to mush) and lose bond, i.e. big time floor failure. In the case of a floating, non-porous floor, this water lays there until it very slowly evaporates. This can result in mold and mildew growth that can be harmful and also smelly.
    • A little known fact even within our industry is that there actually is a maximum amount of exposure that even a floating product will allow before it too has problems.
  • CaCl can be effected by many factors, for example, direct sunlight on the test kit during testing can skew the result. Large commercial dehumidifiers can be brought in to quickly dry the surface of the slab so that CaCl will pass, but because the test only measures 3/4″ or so down, when the floor is covered the moisture equalizes across the pressure gradient and causes the MVER to increase and potentially results in a failure. High ambient relative humidity in the room can causes invalid test results. Setting the test immediately after grinding the slab can cause a false high result. And on and on and on…
  • Because of all these issues the resilient industry as a whole is moving away from CaCl and towards Rh (In-situ Relative Humidity) testing. While both tests are still acceptable by most manufacturers, many are starting to state a preference towards the Rh. You will find that they will state, in many cases, that if both types of tests are done the Rh test should be used as the ultimate determining factor when proceeding with installation.

Alkalinity Testing – the pH Test

ASTM F710-11 is the guide for the standard test method to measure concrete surface pH.

When you do moisture testing you should also take a pH test to measure the pH of the slab at each test site as well. This is important because the alkalinity is really what causes the breakdown of an adhered installation. The moisture (water) is merely the vehicle that carries the concentrated alkalinity salts up to the adhesive layer. Once in contact the alkalinity is what starts breaking down the glue.

If you’ve ever seen grout in a ceramic tile or the edges around a vinyl composition tile installation become covered with white crystals (efflorescence) over time, that’s what’s happening. The moisture is carrying the salts up through the slab and then through the cementitious grout. Once at the top of the grout the moisture evaporates leaving behind the concentrated alkalinity crystals on the surface.

Relative Humidity – the Rh Test

ASTM-F2170-11 is the Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes.

Here’s the Reader’s Digest version.

  1. Same minimum quantity of tests per square footage as F1869-11: 3 for the first one thousand square feet and 1 additional kit per 1000 sf. over the the first thousand.
  2. For a slab that is open to ambient conditions from 1 side (This is what you have in a basement), a hole is drilled to a depth of 40% of the overall slab depth.
    • Can’t stress enough that the correct hole depth is critical to the accuracy of the test result.
  3. A sleeve or probe (depending on what brand of equipment you’re using) is inserted into the hole.
    Thus endeth the first, second and third day.
  4. After 72 hours a reader or probe (again depending on the equipment) is inserted and read. This will give you the internal Rh of the slab at 40%.
  5. This test result is then compared to the manufacturers requirements for Rh to determine whether the floor can be installed. You want the test result to be lower than the manufacturers guidelines to proceed with the install.

Let me just qualify and say that for all of the above information, I’m giving you a very condensed version of concrete moisture testing procedures that should be relevant in most residential basement and slab construction situations. The competent flooring professional will have access to complete documentation published by ASTM International.

So… once you get an accurate result of either CaCl or Rh and of course their sidekick pH, we can get you going in the right direction with what you may or may not need to do regarding floor prep and installation of your flooring. Hopefully, I didn’t get to sciency for you, but it’s a complicated issue and there can be a ton of variables to account for. The pro members and myself look forward to helping you with your flooring projects, as well as helping each other stay current with the standards and best practices of providing consumers with quality products and services.

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