Contractors often ask how to respond to the results of concrete moisture tests, especially when they've used more than one test method. Why? Because different concrete moisture tests can return conflicting results, leaving contractors unsure how to proceed. This is especially true when testing older concrete floors where the contractor may have little idea of what was done when the concrete was originally poured.
A story recently shared by one customer illustrates perfectly the pitfalls of using less reliable moisture testing methods as well as the challenges of testing older concrete floors. In this case, our customer was tasked with addressing a wet basement with a concrete floor that's possibly 20 years old.
After sharing this customer's experience, we will dig into three key lessons:
- Why the in-situ relative humidity (RH) test is so much more accurate than any surface-based test.
- Why old slabs may need an investigation into whether a vapor barrier was installed and how to go about doing this.
- The pros and cons related to the use of concrete treatments or other mitigation options.
What Happened in That Basement?
The basement had clearly been accumulating excess moisture. The question was how this had impacted the moisture condition of the concrete slab. The contractor brought in dehumidifiers to address the basement’s wetness. Then, after the basement seemed to have dried out a bit, he tested the moisture condition of the floor slab using the surface-based calcium chloride test (ASTM F1869). Test results indicated very low moisture vapor emission rates in the old floor. This seemed to defy common sense, given the basement’s wet condition.
To validate the CaCl test results, the contractor then conducted an in-situ RH test by installing several RH sensors at the recommended 40% depth in the concrete slab (ASTM F2170). The RH sensors, which were measuring the moisture condition deep within the concrete slab, indicated the floor was holding a high amount of moisture. Now the contractor was even more confused, so he contacted Wagner Meters for guidance.
What We Can Learn from This Contractor's Confusion
Bottom line: First, the dehumidifiers drew moisture directly from the slab’s surface. This significantly interfered with the CaCl test results since it is a surface-based test and does not assess the moisture condition within the slab. Second, older floors require investigation and a little patience to verify their moisture condition.
Ambient Conditions Invariably Mess with Surface Testing
Surface-based tests, by definition, only take moisture vapor readings at the surface of the concrete. This makes them highly sensitive to the ambient conditions around the slab. And this is one important reason we see inaccurate results from surface-based moisture tests all the time.
The dehumidifiers used by this contractor did their intended job. That is, they pulled moisture out of the air, reducing the ambient humidity. The surface of the slab, in turn, dried out some due to the lower humidity in the air, which contributed to low moisture readings using the CaCl test.
The way to avoid inaccurate test results due to ambient conditions is to use an in-situ internal RH test to assess the overall moisture condition within the slab—well below the surface. An in-situ RH test, such as the Rapid RH test, is the only proven reliable method for assessing moisture conditions below the slab’s surface.
How to Investigate the State of Old Concrete Floors
The contractor's original intention was to use the dehumidifiers to dry out the concrete floor. However, without knowing a bit more about the floor, it can be difficult to know the best steps to take for getting the slab truly ready for the application of a flooring product. If you have access to the original construction records (a big if), you may get important information such as slab thickness, or confirmation of the presence, or not, of a vapor retarder.
In this case, the contractor didn't know whether this old concrete floor had a vapor retarder installed at the time of construction. Without a vapor retarder, one can expect moisture to continually seep into the concrete from below. This means, of course, that no amount of drying time would get the slab to a sufficiently dry condition. Truth be told, depending on the age of the basement floor, an old vapor retarder has probably started to break down and may no longer be preventing moisture intrusion into the slab. The only way to truly know is by core sampling the slab.
If a core sample is completed and the vapor retarder is deemed to no longer be effective, there are still options for getting a flooring finish installed. In this case, the best approach is to use a moisture mitigation system (which is always an option right up front, by making the assumption that the slab lacks a functional vapor retarder).
Moisture Mitigation Options – Timeline and Budget Challenges
For an older concrete slab, mitigation options are limited. Here, the contractor had already tried adjusting the ambient conditions (through the use of dehumidifiers) to accelerate drying, but without success. The only other moisture mitigation option is to apply a topical product on the concrete slab that diminishes the ability for moisture to interact with the installed flooring finish. For jobs where time is the major constraint, applying such products may be the first and best option.
When choosing a mitigation product, one has multiple options, ranging from sheet products that can be rolled out to epoxy coatings. A flooring contractor should take the following factors into consideration when deciding among the options:
- Floor preparation needed to properly apply the product. Some may require minimal floor prep while others may need some sanding, and others may even require shot-blasting.
- Any limitations that the new coating may place on the type of finished floor product that can be applied over it.
- Cost of the moisture mitigation system, from the sealant itself to the labor required to apply it. This can run anywhere from $3 to $10 per square foot. No small budget overrun.
- Are there any warranty exclusions for product use on slabs without an intact vapor retarder? This is critical when reading warranties.
The building owner and the contractor will need to decide whether they prefer to pay with dollars or time and what level of confidence they need to ensure a successful installation.
Moving Forward
Old flooring and old concrete slabs will always present challenges. The sooner you do in-situ RH testing on the old slab, the sooner you'll have an accurate picture of the slab’s existing moisture condition. If it turns out the slab is too wet, you've just saved yourself considerable time and money, because you won’t be expending time, labor, and materials on guesswork solutions that may fail.
