By Scott Tarr and Roland Vierra
The title of this article, “The Solution is Avoiding the Solution,” is a phrase that deserves your full attention: the damage that comes from the relationship between concrete and your floors can be mitigated fully if you just pay attention to a few key facts.
A recent project was in the throes of a massive failure: resilient flooring glued to an on-grade concrete slab was plagued with bubbles throughout; hardwood glued to the same slab was loose and exhibited cupping. Sectional removal of both floor coverings showed similar conditions: an accumulation of moisture along the bond lines and a substantial deterioration of the adhesive.
During the preliminary interview with the installation contractor, it was asked if there had been any moisture testing prior to the installation of the flooring. He stated no testing was done because no testing was needed. He noted that the concrete slab was over a year old, and it looked dry; hence testing would be unnecessary.
It’s not the first time this logic has been followed, and unfortunately, it’s not likely the last. So, let’s first talk about water. A water molecule is made up of three atoms that are bonded together: an oxygen atom and two hydrogen atoms. Together, the three make a molecule with the formula that most of us are familiar with: H2O. We also know H2O can exist as a solid (ice), liquid (water), and gas (vapor). These forms of H2O are called phases.
First of all, a water molecule is very small. So small that a tiny drop of water (.05 mL) contains about 1.5 sextillion water molecules1. That’s a one, with a comma, then a five, followed by 20 zeroes. In other words, a huge number of tiny molecules in a single small drop of water. Given that scale, how many molecules need to be gathered in one place to actually see the water that they create?
Condensed vapor is visible beneath the plastic of this non-adhered wood flooring system.
Now let’s go to the opposite side of the scale. How many molecules does it take to solubilize a salt molecule? Twenty-one2. Based on a recent theoretical model, just 21 water molecules, properly arranged and bonded, are all it takes to create water. With 21 molecules, the resultant water will perform exactly the way any amount of liquid water will perform: it can freeze, boil, evaporate, and act as a solvent. Except we can’t see it because it’s too small to see. We can barely see 1.5 sextillion molecules, so how are we ever going to see 21 molecules?
With this in mind, it’s important to understand how a little water can impact flooring adhered to concrete slabs. Concrete is a mixture of four basic ingredients: cement, water, rocks, and sand. Additional ingredients, such as secondary cementitious materials or a variety of chemical admixtures, also can be added to achieve specific performance requirements. Cement (usually portland cement) is a powdered material that forms a paste when mixed with water. Water hydrates the portland cement in a chemical reaction called “hydration,” which causes the paste to harden and gain strength over time. If sand is added to the paste, it forms a mortar. If rocks are added to the mortar, it forms concrete. As it hardens, the paste binds the sand and rocks together into a single mass. Concrete is the most widely used construction material in the world as it mostly is made from materials found locally, gains substantial strength and durability characteristics, and can be cast to whatever shape or dimension is required.
In order to make concrete workable and finishable, most mixtures have twice as much water than is needed to hydrate the cement. So, half of the water in a concrete mixture chemically combines with the cement, and the other half (referred to as water of convenience) needs to evaporate. When concrete slabs are placed, a portion of the extra water bleeds to the surface. This bleed water brings any soluble materials to the slab surface. The cement contains abundant soluble salt compounds such as potassium-, sodium-, and calcium-hydroxide, which are dissolved by the concrete mix water and transported to the slab surface in bleed water. These salt compounds are highly alkaline in solution, which is why personal protective equipment is required for those who work with fresh concrete to avoid cement burns to skin and eyes due to the extremely high pH.
Bleed water that rises to the slab surface must be allowed to evaporate before the concrete is finished. As the water evaporates, the alkali salts remain at the slab surface in concentrated amounts. Once the surface is finished and cured (kept warm and wet for a minimum period of time to promote the hydration of cement and corresponding strength gain), the exposed surface dries relatively quickly. When dry, the soluble alkali salts are no longer in the solution – but they are still there. Without a solution, there is no pH. However, if the surface concrete resaturates, the soluble salts dissolve again and form another high pH solution.
Concrete is a mixture of four basic ingredients: cement, water, rocks, and sand. Additional ingredients, such as secondary cementitious materials or a variety of chemical admixtures, also can be added to achieve specific performance requirements. Cement (usually portland cement) is a powdered material that forms a paste when mixed with water. Water hydrates the portland cement in a chemical reaction called “hydration,” which causes the paste to harden and gain strength over time. If sand is added to the paste, it forms a mortar. If rocks are added to the mortar, it forms concrete. As it hardens, the paste binds the sand and rocks together into a single mass. Concrete is the most widely used construction material in the world as it mostly is made from materials found locally, gains substantial strength and durability characteristics, and can be cast to whatever shape or dimension is required.
Adhesive breakdown under failed flooring due to formation of high-pH solution.
Unfortunately, even after the bleed water has evaporated from the slab during placement, there is still plenty of water in the concrete that needs to dry out. This drying process can take a long time, especially when the slab surface is given a densified hard trowel finish to meet the flatness/levelness requirements of the flooring system. While “rules of thumb” such as a month per inch of slab thickness have been used, the drying time depends on the concrete mixture, when it was placed, the degree of the surface trowel, and the ambient conditions above the slab. If the walls/roof aren’t finished, and the air above the slab isn’t dry, the slab won’t dry. And, as discussed in a previous article3, if the slab does not have an effective vapor retarder immediately beneath it, moisture vapor will continue to rehydrate the concrete on its path from the groundwater table, regardless of its depth, to the clouds in the sky, regardless of their height, as part of the hydrologic cycle4. The same rules that govern the hydrologic cycle govern vapor transmission through a concrete slab. An effective vapor retarder/barrier must reduce the water vapor transmission rate (WVTR) to less than that of a densified concrete surface and the installed flooring system.
Without an effective vapor retarder/barrier immediately beneath the slab, adequate drying conditions and sufficient time, water within the pore/capillary system of the concrete changes phases and redistributes to rehydrate the slab surface region after the floor covering is installed. Even if cleaning water is prevented from penetrating the joints of adhered flooring, if the temperature in the surface region of the concrete slab is at or below dewpoint (such as when building interiors are air-conditioned), vapor will condense to a liquid under the flooring. At the instant the condensate forms, it has a neutral pH. However, the liquid water immediately begins to dissolve soluble materials present in the area. As discussed previously, the slab surface region is abundant in soluble alkali salts transported there by the bleed process during concrete placement. While plain water doesn’t usually affect adhesives, if a solution with a high pH forms, it can break down many flooring adhesives. Most current VOC-compliant adhesives can tolerate a pH of only about 10 or 11, and the pH of solutions formed in concrete can exceed 12. In addition to breaking down the adhesive, if the flooring is moisture-sensitive, the liquid formed also may cause dimensional changes resulting in issues such as warping, cupping, and buckling.
While a liquid solution often is visible under failed flooring materials that are not absorptive, wood flooring often absorbs the solution after it has broken down the adhesive.
How do we know the concrete is dry enough to install glue-down flooring? Twenty-one molecules of H2O are not necessarily enough to break down an adhesive or cause a dimensional change in flooring. However, the critical amount of vapor or liquid water in the concrete still is not visible, so while the slab may appear dry, quantitative testing is required to secure the manufacturer’s warranty. Typically, established moisture testing using drilled sensors (ASTM F2170)5 or domed calcium chloride kits (ASTM F1869)6 is required to meet the allowable moisture limitations of a specific adhesive or flooring system. The manufacturer also may set a limit on pH, but while it is the high alkalinity of the solution that causes the breakdown, without liquid water creating a solution, pH does not exist. The pH is not a property of a dry material. To run the pH test, distilled water must be added to the slab surface to create a solution. If water is supplied to dry concrete, an alkali solution will form. But if there isn’t enough moisture (liquid or vapor) within the concrete to redistribute to the surface and form a solution after flooring installation, the soluble alkali salts cannot contribute to a high pH.
The moisture condensation and solution-forming process explain why we often see liquid water or liquified adhesive under failed flooring. It is rarely a hydrostatic pressure issue as concrete is relatively impermeable to liquid water. Liquid can penetrate joints and cracks in concrete slabs under pressure below the water table, but water vapor can penetrate concrete far above the water table and condense to a liquid within the slab. If liquid water forms, soluble alkali salts in concrete create a solution that breaks down flooring adhesives. This is why the solution to avoiding moisture-related flooring failures on concrete substrates is to “avoid the solution!”
Scott Tarr is a consulting engineer and president of North S.Tarr Concrete Consulting, P.C. based out of Dover, New Hampshire. He can be reached at firstname.lastname@example.org. Roland Vierra is president of Flooring Forensics Inc., an independent consulting firm specializing in the science and forensic evaluation of floor covering performance failures, including concrete moisture testing. Based in San Jose, California, he can be reached at email@example.com.
- Calculating the Number of Atoms and Molecules in a Drop of Water;
Anne Marie Helmenstine, Ph.D.; Article in ThoughtCo.com, updated August 28, 2019.
- How many water molecules are needed to solvate one?; Alessandro Rognoni,
Riccardo Conte and Michele Ceotto; Chem. Sci., 2021, 12, 2060–2064;
Royal Society of Chemistry.
- Tarr, Scott, “Making Concrete Substrates More Concrete,” Hardwood Floors
Magazine, National Wood Flooring Association (NWFA), Chesterfield, MO,
- Pidwirny, M. (2006). “The Hydrologic Cycle.” Fundamentals of Physical Geography,
2nd Edition. http://www.physicalgeography.net/fundamentals/8b.html.
- F2170-19a Test Method for Determining Relative Humidity in Concrete Floor Slabs
Using in situ Probes, ASTM International, 100 Barr Harbor Drive, PO Box C700,
West Conshohocken, PA, 2019.
- F1869-16a Test Method for Measuring Moisture Vapor Emission Rate of
Concrete Subﬂoor Using Anhydrous Calcium Chloride, ASTM International,
100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 2017.