Concrete and Moisture: Strategies for moisture testing and mitigation – May 2023

Interview by Kemp Harr

Moisture in concrete is as big a problem as ever, due to everything from subpar technicians to lack of education and coordination to flawed concrete pours and tight production schedules. Floor Focus reached out to industry expert George Donnelly, founder of George Donnelly Testing and Inspections, to take a closer look at the issue and discuss strategies for moisture control and mitigation.

Q: What are the typical moisture issues you encounter, and how do you resolve them?
A: Within the term “issues,” I would differentiate between remodel/reuse slab-on-ground, remodel/reuse suspended concrete, and new construction-slab-on-ground and suspended slab.

With new construction, whether slab-on-ground or suspended, insufficient dry time would typically be the number one issue. There is an old Portland Cement Association rule of thumb: Concrete will dry at the approximate rate of one month per inch of thickness, when the ambient temperature is not less than 70º F, the ambient relative humidity is below 50%, and there is constant air movement. These are not the conditions that most construction sites experience! Fast-track construction is the norm.

Compounding the lack of sufficient time being allotted, a great number of concrete finishers are burnishing floor slab surfaces. Large ride-on power trowels deliver floors that meet Ff (floor flatness) and Fl (floor levelness) specifications, but this equipment densifies slab surfaces and retards drying. The American Concrete Institute (ACI) publication 301.1R, “Guide to Concrete Floor and Slab Construction,” suggests that floor slabs intended to host adhered floorcoverings should be treated to a light-steel trowel finish. Hard-steel troweling and burnishing should be reserved for warehouse floors.

Beyond finishing, the use-or misuse-of curing compounds further retards drying. My preference is the use of curing blankets that can be set on slab surfaces and removed after a matter of days. If a curing compound is to be applied, it should comply with the performance requirements of ASTM C309 and be naturally dissipative. However, even self-dissipating curing compounds may require mechanical removal when they are applied too thick or have simply not dissipated. The use of non-dissipative compounds, regardless of chemistry, should be avoided where concrete is intended to host directly adhered floorcoverings. ASTM F710, “Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring,” is listed as a reference document in Division 9 of most project manuals. Under the general guidelines of F710, “The surface of concrete floors to receive resilient flooring shall be free of … film-forming curing compounds, silicate-penetrating curing compounds, sealing, hardening or parting compounds.”

Resolution of the issues noted above are best addressed at the design level. There is continuing need for education and coordination that would have Division 3 and Division 9 specifications be compatible at the time of bidding.

Dealing with remodel/reuse slabs-on-ground typically causes a discussion that owners do not want to have. Most slabs of extended age are not directly underlaid with an effective sub-slab moisture vapor retarder, which is likely the most important component of ASTM F710. ASTM E1745, “Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs,” was originally approved in 1996, and the current performance criteria was published in 2011.

Whenever I am asked to moisture test an existing slab-on-ground, my first question is whether there has been any coring or trench work at the site, and then, whether a membrane of any kind was encountered. If the answer is that no membrane or clear plastic sheet was encountered, I know that my test data cannot be considered predictive and will be subject to change without notice, based on changes in sub-slab soil moisture content and/or the moisture content of the slab as a result of placing a vapor retarder (aka floorcovering) on the surface of the concrete.

The floorcovering industry is not alone in recognizing this problem. ACI 302.2R, “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Material,” contains the following caution: “Warning-A moisture test should not be used to predict future concrete drying behavior, to provide evidence that moisture criteria are satisfied, or to establish expected floorcovering performance if the concrete slab has not been placed directly on a vapor retarder/barrier.”

Ultimately, these floor slabs and their owners are best served by installing a mitigation system that often causes an expensive change order to be issued.

Dealing with remodel/reuse of suspended slabs is typically a non-issue. But there have been any number of floor system distress claims and failures when these slabs are not properly evaluated and were wet when floors were installed or became wet after floorcoverings were installed. There are suspended slabs sitting over un-vented crawl spaces. There are expansion projects that turn parking garage slabs into the floors of occupied spaces. There have been roofs removed when additional stories are added to a building and a few instances where the exterior walls have been removed when taking a structure to the bones. Moisture testing concrete slabs under these conditions often reveals unexpected results.

Q: How big a problem is moisture in slabs? How does it impact flooring installations?
A: Clearly, the greatest threat to floorcovering systems is moisture passing through a concrete floor slab. The problem is big, historic and will continue until all stakeholders, in every project, are willing to deal with this issue.

Studies dating back to 1954 have recognized that moisture passing into building envelopes from the ground negatively impacts floorcoverings. Too often, we see permeable floorcoverings (e.g., broadloom carpet, asphalt tile) that were not adversely affected by moisture transmission replaced by highly impermeable floorcoverings (aka vapor retarders), and distress or failure is the result.

Residual construction moisture in a concrete slab can be problematic, affecting floorcovering materials and the adhesives that bond them to the concrete. With the introduction of “high moisture” adhesives, we see too many installations occurring without a full evaluation of the floor slab or the building’s service environment. For example, limestone large aggregate may contain between 3% and 5% of its weight in moisture versus granite aggregate, which will hold only about 0.5% of its weight in moisture. Water in limestone slowly releases into the cement paste, and this has been the root cause of multiple failures, primarily across the South and into Texas and Oklahoma.

There have also been issues when facilities are only partially occupied, and HVAC setbacks become common. It takes only a few degrees of setback to cross the dew point under a floorcovering when concrete in-situ relative humidity is 95%!

Q: In general, is the problem caused by Division 3 contractors, or are there other factors?
A: I would not blame Division 3 contractors, unless they are not performing to the project’s plans and specifications. Their focus is to meet a design strength requirement and deliver a slab that meets a specified flatness and levelness. They are not required to deliver a “dry” floor slab that meets floorcovering contractors’ needs. There is a continued lack of coordination that should occur at the design stage, with input from both Division 3 and Division 9 contractors. Problems can be avoided when all parties understand the issues and work together.

Q: Why does it seem this problem never goes away, or are the incident rates for moisture-related failures on the decline?
A: I believe problems in new construction are diminishing as people who have suffered failures have learned the basics-the hard way. However, so many slabs-on-ground were placed without the benefit of an effective sub-slab vapor retarder that remodel/reuse problems will likely be with us forever.

Q: Have innovations around moisture testing and mitigation eased these challenges to any degree?
A: Proper testing and analysis of test data have benefited new construction projects, but too often the improper analysis of data from testing existing floor slabs has led to distress and/or failure claims. As reported above, even ACI recognizes that an effective sub-slab vapor retarder is required under the floor slabs if we are to place a predictive value on concrete moisture tests. Soils under a slab-on-ground will achieve saturation, or near saturation, regularly enough to cause concrete to become saturated, or near saturated, regardless of test data that indicates a picture in time.

Mitigation, based on ASTM F3010, has clearly benefited countless projects and become a profit center for manufacturers and installation contractors. But anyone dealing with mitigation knows the cost is high.

Q: Are there effective surface treatments that keep moisture away from the adhesives that hold the floor down on the slab?
A: I have been a longtime proponent of two-component, resin-based mitigation systems meeting the performance requirements of ASTM F3010. However, as with anything else, they must be properly applied, ensuring the required dry film thickness has been achieved across the entire floor surface. Further, you can never be certain that a floor slab surface will host a mitigation system unless you test it. ASTM F3010 requires surface tensile strength testing of prepared concrete surfaces prior to application of the mitigation system and requires a mock-up of the installed mitigation product with secondary tensile strength testing. These steps are often omitted, but when properly performed they can reveal deficiencies that would otherwise lead to latent bond failure.

We are seeing an increase in one-component mitigation system offerings, and I believe many will work-if properly applied.

I would again emphasize the need to review available construction documents, comprehensively test and consider all of the factors that might cause the best plan to go awry.

Q: What is the best advice you give commercial flooring contractors to make sure they avoid claims related to moisture issues?
A: I would always suggest proper testing and full documentation of test results. I am regularly sent concrete in-situ RH test data and asked for advice. Test data reporting should include concrete in-situ temperature, ambient temperature and relative humidity measurements, noting the concrete slab’s design thickness and the presence or lack of a sub-slab vapor retarder. I would suggest a floorcovering contractor ask and note the vapor retarder’s manufacturer and the color of the membrane. Ask and note how the concrete was cured and perform water droplet tests and surface pH tests. If they are not qualified to evaluate the data, develop a resource that can evaluate it for them.

My strongest recommendation is not to let the general contractor’s problem or the owner’s problem become their problem.

Q: Beyond moisture, what are the biggest challenges with concrete projects?
A: When we say “biggest challenges,” I think “biggest cost issues,” and there are numerous. Three that come to mind immediately are,
• Alkali-silica reaction (ASR and/or NASR), or other expansive contaminants in the concrete (lignite, coal, wood chips, etc.). Unless a reaction has already caused concrete slab surface cracking or “pop-outs,” the offending aggregate may be lying passively in the concrete waiting for sufficient moisture to trigger an expansive reaction.
• Excessive surface dusting (excessive carbonation) caused by improper venting of propane-fired heaters. This can occur anywhere in the U.S. but is most common in Northern states during winter months.
• Slabs that are neither flat nor level. Concrete contractors measure Ff and Fl within 72 hours of placing and finishing the floor slab. Concrete slabs will change shape during the drying process and may cause the need to level beyond “normal, minor surface prep.” Concrete contractors will stand by their test data, and the argument over a change order ensues.
These issues are beyond the scope of a floorcovering contractor, but all will affect their work.

Q: What are some of your favorite products and suppliers when it comes to concrete work?
A: While I am open to new products and innovations, my first goal is to ensure that an effective sub-slab vapor retarder is going to be properly placed. Any product to be considered should meet ASTM E1745 performance requirements and needs to be installed per the guidance of ASTM E1643. High quality sub-slab vapor retarders are offered by Stego Industries, Reef Industries, Raven and WR Meadows, just to name a few.

Reef and Raven offer re-useable concrete curing blankets, and WR Meadows offers a dissipative, membrane-forming curing compound. Without doubt, there are many other fine products on the market; these are just a few I commonly recommend.

Where available, I believe self-desiccating concrete is an answer to fast-track schedules. U.S. Concrete offers Aridus concrete, which I have seen perform as advertised.

In summary, there is a need to recognize that concrete is happy to be wet. Concrete with internal relative humidity in excess of 90% will continue to cure with no upward limit on the compressive strength it will achieve.

As far as concrete is concerned, there is no moisture problem. Conversely, floorcovering systems prefer permanently dry concrete. Owners, architects, interior designers, general contractors, concrete contractors and floorcovering contractors can successfully balance the needs of all stakeholders-if they work together.

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