Problems Underfoot

When Moisture Means Concrete Floor Covering Can’t Stay Connected

There is no more insidious problem in the flooring industry than recurring moisture problems in flooring installed over an improperly dried concrete slab. High moisture levels in the concrete can interfere with the performance of all adhesive-dependent finished flooring products.   In recent years, changes to adhesives have resulted in their contents having reduced or no volatile organic compounds (VOCs), but a consequence of the changes is that these adhesives are more moisture sensitive than in years past. Most manufacturers provide a maximum moisture value for their product (for warranty purposes) to ensure the concrete slab falls within the appropriate parameters required before the flooring is installed. To help ensure that high moisture levels do not end up causing a problem after installation, it is paramount that installers get an accurate measurement of the moisture levels in the slab.

Concrete moisture, of course, starts with the original aggregate mix. This moisture dissipates through surface evaporation as the slab cures. As moisture is drawn from the surface, the internal moisture redistributes through the slab, moving more moisture to the surface. Under ideal conditions, this process happens at a “rule-of-thumb” rate of approximately 30 days of drying time for each inch thickness of concrete for the slab to reach a moisture level that would be acceptable for some floor coverings/adhesives. The key phrase for this rule of thumb is “ideal conditions.” Notice it does not say “from the date the concrete is poured.”

The process also works in reverse. When the amount of surface moisture is lower than the ambient air, concrete can absorb additional moisture into the surface of the slab, slowing the drying process considerably. This process of moisture “movement” will continue indefinitely until the finished floor product is installed, meaning that readings of the moisture levels in the concrete slab will always be experiencing a degree of flux.

Concrete moisture can also come from groundwater sources or poor drainage in slabs in contact with the ground, capillary action (or wicking) from subsurface moisture, or in applying moisture to the surface through rinsing or moisture-rich surface additives.

Temperature variations can slow the drying process by influencing the ambient humidity above the slab and reducing the air’s capacity to accept the evaporating surface moisture. This natural time frame of drying concrete can also be exacerbated by purely human influences – fast-track construction schedules or delays by other trades that compact the flooring installers schedule.

So how can the general contractor and specifier help the flooring installer prevent future problems with flooring installations?

Like most problems, there are two approaches: one is proactive, the other reactive. Let’s look at some of the steps involved in a proactive approach:

• When pouring a new slab, make sure drainage runs away from the slab and that the grade is adequate for local conditions.
• Specify a true vapor barrier membrane under the slab to prevent moisture coming up through the ground into the slab.
• Use a concrete mixture with a lower water-to-cement ratio. Various admixtures may provide adequate workability while minimizing the initial moisture content.
• Allow the slab to dry naturally at service conditions. This means protecting it from the elements as well as avoiding maintenance or cleaning processes that will wet the slab.
• Specify, conduct, and verify — testing the slab moisture with a reliable test method before installation of the floor covering, is everyone’s responsibility!

How do we test for the moisture content of drying concrete?

Various test methods are available although not all are considered adequate for flooring installation requirements according to ASTM International. Testing should always be done with the concrete as close to service conditions as possible, and tests should be conducted at various locations throughout the slab.

Considered the industry standard, relative humidity (RH) testing measures the concrete’s RH by installing a probe or sensor within the concrete slab to a specified depth. Forty percent of the slab thickness has been found to be indicative of what the final RH conditions would be if the concrete were sealed (i.e. finished flooring installed) at that stage in the drying process. ASTM standard F2170 covers the requirements for RH testing before installing the flooring over a concrete slab.

Another test method, the anhydrous calcium chloride test (ASTM F1869) uses a measured amount of a salt compound sealed under a glass dome to determine the moisture vapor emission rate (MVER) in a concrete slab. Weight is used to calculate the amount of moisture absorbed over a 60-72 hour period. Once frequently cited by manufacturers, the calcium chloride test has recently been disallowed for testing all lightweight aggregate concrete. By its nature, the test results are limited to surface conditions and do not predict sub-slab moisture conditions.

Moisture meters use electromagnetic signal resistance or impedance to give an indication of slab moisture. While they, too, give a shallow or surface reading, they can be useful in determining the placement of RH probes by providing comparative readings across the slab. A few moisture meters use pin-style technology to determine moisture content, but for concrete these are not considered reliable.

A mat test adheres a vinyl flooring product to the concrete and seals the edges in order to test the reliability of the adhesive at the concrete’s current moisture conditions. Because it relies solely on a visual inspection after 72 hours, it is not considered a reliable indicator of the moisture conditions within the slab. It can produce false negative results.

The last test, polyethylene sheet test (ASTM D4263), is a simple test that indicates only a qualitative result. In this test, a square plastic sheet is taped to the concrete and left in place for a number of hours. The presence of condensation under the sheet is a positive indication that excess moisture is likely to be present, but it is not a certainty that the slab conditions are acceptable throughout.

If it’s too late for prevention, then the options become much more problematic. Typically, warranty claims will mean removing the flooring and adhesive to allow the concrete slab to cure more thoroughly. If the moisture conditions cannot be remediated naturally, other options might include using decorative concrete, less moisture-sensitive floor coverings, breathable floor coatings, or to install a topical moisture vapor suppression system. In extreme situations, or if the moisture conditions have caused severe damage to the concrete slab, it may need to be repoured and the flooring installed again.

Accurately measuring concrete slab conditions before flooring installation begins is the only way to ensure that moisture problems in the concrete slab don’t put your job on shaky footing.