Misusing pH to Diagnose Flooring Failures

Concrete is the number one building material in the world and as such most buildings have a concrete surface that receives a flooring system. When problems arise with a flooring installation, many times pH is blamed as a contributing factor to a failure, thus relieving the flooring manufacturer or contractor from liability. Unfortunately, there is a lack of understanding in the construction industry of what pH is, what is measured or how it affects flooring systems and adhesives. Often, liability for a flooring failure is shifted to the building owner out of ignorance over what the pH reading means. This paper will explore some of the facts related to pH to provide clarity for both the building owner and the contractor to ensure a successful flooring installation or repair.

The first step in understanding pH is to recognize that pH is a unit of measure. We recognize and use units of measure or scales daily to quantify things. For length we have inches or centimeters, for weights, cwt (hundredweight) ounces or grams, and for volume, fluid ounces or milliliters. When dealing with liquids, in addition to volume and weight, we also have chemical classification. A fundamental concept in chemistry that measures the acid or alkali range of a liquid substance is pH. This unit of measure registers the potential of hydrogen (also referred to as power of hydrogen) within a liquid. It is a logarithmic unit of measure that quantifies the strength of hydrogen ions (H+) in a solution. Based on the amount of hydrogen present, we classify the liquid as acid, neutral or base. Base is often referred to as alkaline, but for this paper we will use base for strength and alkalinity for concentration. This will be important when we evaluate readings.

So why hydrogen? The significance of hydrogen ions in pH measurement stems from the fact that hydrogen is the most abundant element in the universe and is commonly found in various compounds, including water (H2O). In an aqueous solution, water molecules dissociate into hydrogen ions (H+) for an acid or hydroxide ions (OH-) for a base. The balance between these ions determines the pH of the solution. The pH scale ranges from 0 to 14, with 7 representing neutral. Values below 7 indicate acid, with lower pH values indicating higher acid strength. Values above 7 indicate base, with higher pH values indicating greater base strength or caustic level.

It is important to note that the pH scale does not directly measure the concentration of acids or bases present in the solution, only the hydrogen strength.

Because our focus is on concrete, we will address examples from neutral to caustic (7-14) on the pH scale. Unlike linear units of measure, 1, 2, 3, 4 etc. which are evenly or equally spaced, pH is logarithmic so the spacing is a multiple such as 10, 100, 1000, 10000 etc.

This means that a one-unit change in pH represents a tenfold difference in the amount of hydrogen/hydroxide ions. For example, a solution with a pH of 10 is ten times more caustic than a solution with a pH of 9 and a hundred times more caustic than a solution with a pH of 8. This becomes particularly important when looking at the potential caustic nature of the liquid being tested. The pH scale may tell whether a liquid is acid or base, but it cannot measure the level of concentrated acid or base. This is important to understand because an acid or base pH reading will not directly indicate potential harm or safety. One common example is hydrochloric acid and Coca-Cola. Hydrochloric acid has a pH of 3 and is dangerous while Coca-Cola has a pH of 2 but is drinkable. Why?

The answer is concentration. While pH classification (acid, neutral, base) provides information about strength, it does not directly indicate the concentration of acid or base present in a solution. Concentration is the amount of acid or base diluted in a specific volume and is represented in percentage (%) of that total volume. It is the concentration of the acid or base in a solution that determines its potential effects on materials or human safety. For concrete, the percentage of hydroxide (OH-) is the most important identifier and what determines potential danger. It is important to recognize that when the water component is reduced or the element percent of fixed volume (concentration) is increased, the potential for issues is magnified. Rember that pH quantifies the nature of the solution, it does not directly measure the concentration of a substance or its alkalinity. Alkalinity is used to quantify the percent of hydroxide in solution to a specific volume and this is the real component that needs to be quantified, not pH.

Furthermore, it is worth noting that dry non-soluble solid materials have no measurable pH. If a solid does not readily dissolve in water, its pH value becomes irrelevant, as there will not be a sufficient amount of hydroxyl ions present to interact with other materials. This poses a challenge with concrete because it always retains some level of moisture, however if there are not enough soluble salts within the matrix and no pathway or open capillary matrix for moisture migration then there is no potential for an alkaline solution presenting at the bond line. Not understanding this interaction leads to further misunderstandings and added costs to projects when pH testing is done on new concrete.

Additionally, concrete surfaces carbonize and as a result the surface pH can drop as low as 8.5. Provided the surface is clean and the profile is acceptable, some contractors will proceed with flooring installation with a false assumption that they are safe. Alternately, many contractors wait and test right after surface preparations when this passive layer is removed, and the resulting pH reading can be as high as 13. Without understanding what they are reading, they will require additional remediation to lower the pH unnecessarily.

This is where problems arise. Remember that pH alone cannot indicate alkaline concentration. Surface minerals and salts cannot be in high enough quantities to result in a concentrated alkali solution. Also, while concrete is primarily made of non-soluble solids, it maintains an average pH between 12 and 13. In a dry solid state this is not a problem. Even when moisture is introduced to the surface, there may not be an issue, unless that moisture is allowed to move freely into and back out of the concrete. The mechanism for this movement is the capillary channels within the concrete matrix. This can also be referred to as the permeability of the concrete. Higher permeability equals a higher risk of osmotic attraction of the hydroxyls, specifically sodium hydroxide, within the concrete. Lower permeability reduces or eliminates this risk. Liquid moisture is the necessary element in all pH readings and is the catalyst to dissolve or attract sodium hydroxide into a concentrated solution. Remember that a dry solid is not a problem, only liquid solutions are. So where does this liquid come from?

Concrete is classified as being hygroscopic, meaning that it will receive and hold moisture. This also means that it releases moisture slower than it takes it in. Because of this, the flooring industry has recommended the use of under slab vapor retarders to prevent ground water from migrating up through the slab. While this practice has ancillary benefits for the concrete, it has not eliminated moisture related failures. In some instances, improper installation or placing sand on top of the membrane actually increases the risk of failure by creating a water reservoir under permeable concrete.

There are many potential sources for liquid presenting at the bond line of the flooring system: from the soil, trapped in the reservoir underneath, from within the concrete, from topical application due to water-based adhesives and cleaning, or from atmospheric condensation. Applications that utilize the under-slab vapor retarder and elevated concrete deck can exclude soil moisture as a contributing factor. Each of the remaining methods can be affected by the concrete’s internal capillary structure.

The capillary structure is the mechanism that allows moisture and moisture vapor to move into and out of the concrete element. This is referred to as the permeability of concrete. These capillary structures only form during the curing state of the concrete life cycle. Knowing how large or small these capillaries are will determine the potential danger of liquids that come in contact with concrete. In general, high water to cement ratios and low hydration rates of the cement results in high permeability.

This is precisely why the permeability of the concrete must become part of the evaluation process. The flooring industry continues to either ignore or misunderstand the relevance of concrete permeability. This is unfortunate because if liquid moisture is not able to materialize from within the concrete, the pH of the concrete cannot affect the adhesives or flooring. Permeability is the mechanism that allows transference of moisture into and out of concrete. Unfortunately, there is currently no single test method to determine the permeability of concrete. However, there are proven ways to reduce or prevent concrete from becoming permeable, and this will be addressed in a future article.

So why do we still have flooring failures after we “pass” the pH test? A typical construction process will be something like this. The concrete is placed, hopefully cured properly and then allowed to dry. Several weeks or months later, a flooring installer arrives and follows ASTM (American Society for Testing & Materials) F710, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. The installer will try to verify the concrete is clean, dry, sound, and free of contaminants. They remove the passive layer by profiling the surface and after obtaining “acceptable” moisture and pH test results, they proceed with installation. In many cases, the floor is wet mopped and then a water-based adhesive is installed, and a floor covering is placed over the adhesive. This trapped moisture, not moisture from the concrete, then dwells on the concrete releasing or attracting soluble hydroxides. Furthermore, as we now have an encapsulated space, the hydroxide can increase in concentration, resulting in a caustic solution that destroys the adhesive, provided the permeability of the concrete is high enough.

Now we end up with a flooring failure and retesting shows increased pH or moisture. This is then presented as a change of substrate conditions nullifying any warranty and placing the burden of repairs on the owner. All because we did not test for hydroxide concentration or concrete permeability before installation. Additionally, we forgot that the contractor added moisture from the adhesive and if the building was not conditioned properly, higher humidity will have been trapped under the flooring thus providing to liquid required to attract the sodium hydroxide causing the increased pH and moisture readings.

Currently there is no single test to measure concrete permeability in the field. This makes it imperative to know what contributes to higher permeability and how we can prevent or control it. High water to cement ratios above .50 and improper curing are the two largest contributing factors to concrete permeability. Because concrete is a one-way linear chemical reaction, controlling each phase of its life cycle is imperative to improving overall quality. The lower the water to cement ratio and the longer concrete is kept in a state of curing, the fewer capillaries there will be.

The capillaries are what allow moisture and moisture vapor to migrate in and out of the concrete. The only way moisture or moisture vapor can come from the concrete is through the capillary structure. It is the size and structure of the capillary matrix that determines the permeability of the concrete. If concrete has extremely low permeability, the mechanism for migration will not exist. This means that moisture at the bond line will have to have come from the atmosphere, cleaning, or from the adhesive.

So, what do we now know? We know that pH is a logarithmic unit of measure that quantifies the acid or base strength of a liquid substance based on the strength of hydrogen/hydroxide ions. We know that the pH reading does not quantify the concentration of the solution or determine its potential for harm or safety.

We know that a solid, regardless of its pH reading, that does not dissolve readily in water does not pose a problem as the hydroxide (OH-) cannot become a concentrated alkali solution. We know that the permeability of concrete directly correlates to the ability of soluble materials and hydroxides to present at the bond line. Finally, while a high pH alone cannot indicate if the moisture is coming from or through the concrete, a neutral or low pH reading definitively excludes the concrete from being a source of moisture as it is impossible for moisture to pass through it without attracting sodium hydroxide. The means the source of moisture must be topical or atmospheric.

In conclusion, understanding what pH is and what it measures is a first step in solving a flooring problem. But it is not the last or only step. Understanding that other influences such as topical or atmospheric moisture and water-based adhesives all introduce moisture into the equation. Properly cured, low water to cement ratio concrete cannot contribute to a flooring failure because there is no mechanism of transport to the bond line. It is the contractor’s responsibility to verify the substrate conditions, ensuring that the concrete is acceptable to receive the flooring. This means not only that it is clean, dry, and sound, but there is proper surface profile, and the concrete has been tested for soluble hydroxides (OH-), not just for a pH strength. Finally, regardless of pH strength readings, the permeability of the concrete must be evaluated. The lower the permeability, the less likely it is there will be any failures related to the concrete, pH, or concentration. The building owner should no longer allow the flooring installer to walk away from a “moisture problem” considering they likely introduced the very moisture in question.