Concrete Moisture: Know it's There & Know How to Deal with It

Contributed by Eric D. Lussier

I’m quickly approaching eleven years working in and around indoor flooring, focusing mainly on sport and synthetic surfaces. Eleven years of projects of all shapes and sizes ranging from 250 square foot residential basements to 30,000 square foot college field houses. Eleven years of existing conditions, renovations, rehabilitations and new construction and the one constant that rears its ugly head on almost each job are substrate conditions, and especially concrete moisture.  Conversely, said moisture issues are seemingly new news to whomever I am working with: whether that is architects, construction managers, general contractors or end users.

There are more than a few instances that can lead to high moisture in a concrete slab. Whether it is an over-watered pour, a lack of a quality vapor barrier, a compromised vapor barrier, or a missing one entirely (either from degradation or lack of placement), a fast track installation with insufficient time for the concrete to dry, an inoperable or missing HVAC system or a handful of other events. No matter the occurrence, it can all equate to the same headaches after the fact. Normally fingers are pointed, voices are raised, materials are ripped out and unnecessary time and money is spent to potentially repair or replace flooring that perhaps should have never been installed to begin with. Industry-speak may call it “flooring failure” but most of the time the flooring is performing exactly as it is supposed to. The adhesive on the other hand, may be completely failing.

New construction technologies have our buildings tighter than ever. With the use of a proper vapor barrier removing the ground from the equation, concrete moisture has no place to go but up and through the slab. When placing a fully adhered, non-breathing floor, such as a heat-welded sheet vinyl on the slab, concrete moisture in an untreated slab travels up and out, trying to push through the adhesive and new floor in the process. Even though the norm in the industry has raised from 3 lbs. of moisture to 5 lbs., as per ASTM F1869-11 (Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride), that limit can take substantial time to achieve when it comes to new construction.

Speaking of norms in the industry, thankfully most flooring manufacturers have moved away from recognizing calcium chloride testing (which is more of a snapshot of what is happening emanating from just the top of the slab) towards in-slab relative humidity (RH) testing (what is going on inside the slab). Testing as per ASTM F2170-11 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In-Situ Probes (has become easier over the last handful of years with developed equipment, including testing probes that can be left in the slab and reusable digital probes.  It is always recommended that an independent third-party is specified to test the concrete for moisture and not the General Contractor or flooring contractor themselves.  It could be viewed that each party has a vested interest in ensuring that results are swayed their way.  If you are looking for a certified concrete moisture testing party, the International Concrete Repair Institute offers a moisture testing certification program and you can search the certified testers here.

What is extremely important to note is that any concrete moisture testing is purely a snapshot in time as to what is going on in (or, on) the slab at that point in time only and passed results does not guarantee that concrete moisture will not become a problem in the future.

Are you looking to cure the concrete moisture blues? There are solutions on the market. The cheapest, yet perhaps slowest solution is normally waiting for the slab to dry, however, you may have heard of the expression time is money.  Abrading the top layer of the slab may help speed up this process, however most floor coverings require a smooth, steel trowel finish for installation, so the abrading would have to be treated in some way, which may include being patched with a Portland-based cement. Solutions also include topical moisture mitigation systems (reference ASTM F3010-13 Standard Practice for Two-Component Resin Based Membrane-Forming Moisture Mitigation Systems for Use Under Resilient Floor Coverings) and some of these topical solutions can be installed within days of concrete placement and up to 100% relative humidity.  Flooring manufacturers have also adjusted to the moisture issues in the industry by offering solutions such as on slab moisture barriers, flooring with textile-backing & adhesive systems, 98% adhesive free installations or adhesives that allow a very high rate of moisture vapor emissions.  One of the newly recognized solutions is what is being deemed a scientifically engineered rapid-drying concrete. Please note that concrete admixtures are not listed as a solution herein, nor are they recommended by this author, nor most flooring manufacturers.

The one true method to ensure a proper floor installation is informing yourself and knowing your trusted advisors.  You need to know the flooring you've specified, know the flooring manufacturer's approved adhesive, know the threshold of concrete moisture vapor emissions, know the moisture testing methods and protocols and companies that provide them, know the time constraints and perhaps most importantly, know the flooring manufacturer’s representative and the flooring installation contractor. Flooring manufacturers’ and professional reputable flooring contractors will have a business reputation and it is in their best interest to uphold it. The manufacturers and contractors should be grilled, have references checked and should be able to corroborate their claims.
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There is no tried and true answer or solution when it comes to concrete moisture. Please know that moisture is ALWAYS present in concrete slabs and by accepting it is there and knowing how you can treat it or live with it is your best bet.