The importance of substrate surface water absorptivity

July 16, 2021

Photo ©[1]
Photo ©

by Dean E. Craft, DBA, CSI, CDT, LtCol USMCR (ret.)

Often referred to as substrate “porosity,” substrate surface water absorptivity refers to the ability of a flooring substrate surface to absorb liquid relatively quickly. To better conceptualize, imagine a concrete masonry unit (CMU) block. If a drop of water was placed on the CMU, it would absorb the liquid very quickly. Now, imagine a hard, smooth surface such as steel, existing resilient flooring, or power-troweled concrete, all of which could serve as the substrate for many different flooring products. If a drop of water was placed on any one of those surfaces, it would sit there and likely evaporate before being absorbed.

Historical context
Why is this of concern for porosity and how that relates to a successful flooring installation? Before addressing this question, it is helpful to provide some historical context. In the 1970s, concerns were raised about indoor air quality (IAQ). The apprehensions grew in the 1980s, and eventually, encompassed the flooring industry, too.

On January 11, 1990, the National Federation of Federal Employees (NFFE) petitioned the Environmental Protection Agency (EPA) under section 21 of the Toxic Substance Control Act[2] (TSCA) to start rulemaking proceedings and focus on reducing emissions from new carpets.

While EPA opted to not initiate the specific rulemaking proceedings[3] it decided to “… initiate a series of actions designed to assess and, if necessary, reduce the public’s exposure to compounds which may off-gas from carpeting.” Upon mutual agreement with EPA, the Carpet and Rug Institute (CRI) agreed to conduct total volatile organic compound (TVOC) emission testing on various materials, and reported the results to EPA’s Office of Toxic Substances in the “Carpet Policy Dialogue Compendium Report[4],” published in September 1991.

Following closely on the heels of the compendium report, CRI launched a Green Label program[5] in 1992 to test carpet, cushions, and adhesives to help design professionals identify products with low VOCs. This resulted in a fundamental, rapid, and fairly radical change in the formulation of many of the adhesives and other constituent components used for flooring.

An example of a non-absorptive (non-porous) concrete surface.  Photos courtesy ISE Logik[6]
An example of a non-absorptive (non-porous) concrete surface.
Photos courtesy ISE Logik

New adhesives versus old
There is a common belief the ‘good glues’ or flooring adhesives of times past were insensitive to concrete substrate moisture. That is actually not true, and there is literature going back to the 1950s discussing this very topic. Building on that misconception, many believe the ‘newer, water-based’ adhesives, developed for their low VOCs, are the primary source of flooring installation issues. Historically speaking, the ‘newer’ glues are not really new anymore—many of them began their evolution almost 30 years ago. Further, over that time, there have been untold millions of square feet of very successful installations. However, there is a fundamental difference between many of those older adhesives and the newer ones, especially in how they behave once applied to a substrate surface. For example, wet-set water-based adhesives necessitate water loss through evaporation and/or absorption into the substrate for adhesive strength development. In contrast, a moisture-cured adhesive absorbs moisture from the air or substrate for strength development. Knowing whether the concrete surface is porous or not prior to the application of an adhesive and using the appropriate installation methods for the specific porosity conditions are crucial for a successful and durable floorcovering installation.

What drives surface porosity?
A major determining factor of substrate surface porosity is the density of the material. A common substrate for many flooring installations is concrete. In its fresh state, concrete is basically a suspension of solids in water. As consolidation occurs, the denser solids, such as large aggregates, settle. This settling, or displacement, pushes residual mix water, sand, and cement fines upward toward the surface in a process known as bleeding, which is also the process that results in the “cream” at the surface of newly placed slab. Bleeding of the free water continues until the cement paste has hardened enough to finish the sedimentation process, and often results in sheen of liquid water on the surface of the slab. Final troweling of the concrete slab surface is not supposed to occur until the concrete has set and bleeding has stopped; doing so could lead to other surface issues, such as dusting, and a weakened surface layer.

There is no definitive amount of time it takes for the bleed process to finish, as it is the result of a multitude of factors ranging from the concrete mix design to the project site environmental conditions. The amount of water in the initial mix is normally the largest driving factor and mix designs with higher water to cementitious material (w/cm) ratio, all other factors held constant, will bleed longer than a similar mix design with a lower w/cm. Mix designs with various fine supplementary cementitious materials (e.g. fly ash or silica fume) will likely bleed slower than similar mixes without such materials. Various admixtures and fibers can also significantly alter bleed rates. Air movement and temperature and ambient environmental humidity also impact bleeding, and subsequent evaporation of surface water. For example, on windy days or when temperatures are hot, or when the ambient humidity is very low, the bleed water may not even be noticeable since the evaporation rate might be greater than the bleed rate. As a result of all these factors, and more, the time from concrete placement to setting and the stoppage of bleeding, could be anywhere from 20 to 30 minutes to several hours.

Absorptive (porous) concrete surface.[7]
Absorptive (porous) concrete surface.

With regards to newly placed concrete, the primary influencer as to whether the substrate surface is porous or non-porous is how the concrete was finished. The accepted standard for interior slabs is for them to receive a light-steel trowel finish which will, in most cases, render the surface non-absorptive. This is simply due to the densification effect of the trowel blades on the surface concrete paste.

Even older concrete can have a non-porous substrate surface. It is often said concrete gets stronger throughout its life. This is true as long as sufficient moisture and reactive ingredients are present. So, even a decades old basement residential slab may have a non-porous substrate, and be very dense, simply due to the concrete continuing to gain strength and densify over the years.

However, the density of the substrate surface will drive the absorptivity of the substrate surface, and concrete is not the only substrate encountered. Other acceptable substrates that may be encountered are polymer terrazzo, ceramic tile, existing well-adhered resilient flooring, and even steel. Each of these would be non-porous and would likely require additional surface preparation before installing new flooring.

The role of the flooring contractor
Once the jobsite is ready for the installation of floorcovering, it is the floorcovering contractor’s responsibility to determine whether the substrate surface is porous or not, and to proceed with the correct methods and compatible products for the determined porosity. This is a crucial factor in the proper installation of many different flooring system components, such as flooring adhesives, cementitious underlayments, and primers.

This is not new. For years, most flooring manufacturers have said adhesive spread rate and open time, or product installation, was dependent on substrate porosity. What was missing, however, was an industry accepted standardized method for determining substrate surface porosity in the field. In response to this, ASTM F3191-16, Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring was developed[8]. ASTM F3191-16 assists the installation contractor in identifying how an adhesive should be applied. It can also help the design professional to specify the type of adhesive and where; or at least to ensure such a determination is clearly required in the project specs.

With the proper specification language in place, should the substrate surface be determined to be non-porous, the only adjustments needed might be to use a smaller trowel size, and perhaps a reduced open (‘flash’) time, as many of today’s adhesives can be used on both porous or non-porous substrates. However, in other cases, an entirely separate adhesive may need to be used, or an entirely separate substrate preparation protocol may be required, as not all adhesives are suitable for installation on non-porous substrates. To avoid costly change orders and flooring installation delays, the design team should take great care in ensuring the specified adhesive is actually suitable for the project’s substrate surface absorptivity conditions, as not all flooring manufacturers have adhesives for both. Language such as “…use an adhesive recommended by the manufacturer” may result in unexpected delays and expensive slab profiling and self-leveling, when all that may have been needed was a different adhesive. Language such as “…use an adhesive suitable for substrate conditions and compatible with flooring backing” could greatly expand the project’s options. The importance of addressing porosity in the specification cannot be overstated. Installing an impermeable floorcovering on a non-porous substrate with the wrong adhesive for the porosity conditions, or with an improperly applied adhesive for the substrate surface porosity conditions, can lead to early bond failure, adhesive oozing through seams, adhesive displacement, a moisture-related flooring failure, etc.

Determining surface porosity
The first step in evaluating whether a particular substrate surface is porous or not is to consult the written instructions of manufacturers of resilient flooring, adhesive, primer, and underlayment, or combination thereof, for their acceptable test methods and time limits. In the absence of written instructions, the industry now has ASTM F3191-16. It lays out a very simple and quick process to assess substrate surface absorptivity/porosity. To begin, substrates need to be at the service temperature and relative humidity expected during normal use, or at the conditions required for installation of the floorcovering material per the relevant manufacturer’s specifications. This is so the area tested best replicates the conditions that should exist when the adhesive (or primer, underlayment, etc.) is applied. To achieve these conditions, the interior space will likely need to be climate controlled for the test to proceed and produce accurate results. In new construction, achieving and maintaining the proper climate-controlled environment is often neglected, even during the installation of floorcoverings.

Once the space is appropriately acclimated, the substrate surface where the test is to be performed needs to be prepared in the exact manner as planned or as required for each specific floorcovering material installation. Given that many projects have different flooring systems, assessing substrate surface absorptivity/porosity is not a one-size-fits-all approach. Some areas may first require grinding to achieve the proper concrete surface profile, while other areas will only need to be clean, smooth, and surface dry.

After the space has been appropriately conditioned and the substrate surface properly prepared, the test is very straightforward. Simply place a single drop of potable water (approximately 0.05 mL) on the substrate surface using a pipette, water dropper, straw, etc., taking care that the drop is not placed from such a height that causes it to splatter, and wait. The waiting period is very brief with the cutoff time between porous or non-porous within F3191-16 being exactly one minute. If at or before a minute the drop of water absorbs into the substrate surface, then the surface is to be considered porous. If, however, a full minute has elapsed and the water has not been absorbed, then the substrate is to be considered non-porous.

It is important to understand F3191-16, or perhaps a manufacturer’s own substrate surface porosity test, is not a “pass/fail” type of test. Instead, it is a qualitative assessment of substrate water absorption (porosity) and whether or not that substrate should be regarded as porous/absorptive or non-porous/non-absorptive as these terms relate to the installation of resilient floorcoverings, adhesives, self-leveling underlayments, primers, and other products. It is an evaluation to help the project team determine the proper substrate surface preparation and adhesive or adhesive spread rate for the materials to be installed. As mentioned before, depending on how the specification is worded, the resulting determination may be easily addressed simply by changing the trowel size or selecting a different product.

Use correct trowel size and allow sufficient flash time.[9]
Use correct trowel size and allow sufficient flash time.

With a flooring adhesive, the correct method over a porous surface may be as simple as an application rate and open time difference versus a non-porous surface, instead of a change in the product. Installation data sheets for flooring adhesives generally include a table or chart recommending trowel size, application rate, and “open time” or “flash time” for both porous and non-porous surfaces. The working time of the adhesive may be reduced as the flash time increases, so the floorcovering installer will need to manage it accordingly. The non-porous surface flash time of the adhesive is greater than the duration for a porous surface (i.e. another 10, 20, or 30 minutes), and is influenced by concrete surface temperature, ambient relative humidity and temperature, and airflow. This is regardless of whether the adhesive is being applied over a porous or non-porous surface. This flash time facilitates the evaporation of moisture from the adhesive, and if flash time is insufficient, the moisture could be trapped when installing an impermeable floorcovering. This can compromise adhesive cure and strength development, thereby jeopardizing the integrity of the floorcovering bond to the subfloor surface. Adhesive migration may also develop between the seams and transition areas to the surface of the floorcovering.

The importance of evaluating substrate surface porosity is not limited to adhesives, and can encompass many other products, such as primers, thin sets, and cementitious underlayments. For such products, and even for some adhesives, a concrete surface profile (CSP), as described within the International Concrete Repair Institute[10] (ICRI) Technical Guideline No. 310.2R-2013, of CSP 1 or more may be required if the pre-existing substrate tests as “porous.” When this occurs, the generally accepted method for preparing the substrate surface is by mechanical means. ASTM D4259, Practice for Preparation of Concrete by Abrasion Prior to Coating Application[11], is a good resource in which such mechanical methods are described in detail. In some cases, it may also be possible to install a primer specifically designed for non-porous substrates that might preclude the need to mechanically prepare the substrate surface. Also, there may be other products that do not require a porous substrate. If such products are available, they should be given significant consideration as it would reduce labor and time costs for surface preparation, and perhaps, more importantly, minimize the production of respirable silica dust, a material regulated by the Occupational Safety and Health Administration[12] (OSHA).

The role of the architect
As the author of the specifications constituting part of the contract documents, the architect (and their consultants) has considerable impact on this issue. All too often, under which adhesive to use—buried within various 09 Divisions—the phrase, “as recommended by flooring manufacturer,” is embedded into the specs. Though seemingly in the best interest of the project, that short phrase may actually limit the options for addressing a non-porous substrate surface, and end up causing the project significant schedule delays and cost overruns, especially if the manufacturer of the specified flooring does not offer an adhesive product that can be used over both porous and non-porous surfaces.

This recently occurred with an 18,581-m2 (200,000-sf) hospital project in central Florida. The project specification for resilient flooring required, in part, the use of an adhesive as recommended by the manufacturer. The manufacturer, unsurprisingly, recommended one of their adhesive brands. Upon closer analysis, their adhesives were only for porous substrates. The flooring manufacturer’s installation instructions said that if the concrete substrate surface was non-porous per ASTM F3191-16, the concrete substrate would need to be bead-blasted and self-leveled, so their porous substrate-only adhesive could be used. Upon discussion with the architect, that initial adhesive specification language was changed to “… an adhesive compatible with flooring material backing and suitable for substrate conditions.” Seemingly a simple word change, but had that change not been made, the entire project may have required bead-blasting and self-leveling. Such a process would likely have necessitated a change order on the magnitude of $600,000 to $700,000 and weeks of additional time. By thinking through the ramifications of the initial specification language in conjunction with the specified flooring, the design team was able to proactively address this potentiality through a slight alteration of the specification language. This directly and materially contributed to construction team being able to stay on budget and schedule; and all applicable warranties conveyed to the project.

Success is achievable
If the submitted floor system material that will be in direct contact with the substrate surface—irrespective whether the material is a primer, adhesive, thin set, or cementitious product—is unsuitable for direct installation on a non-porous substrate surface, the project may face extra costs and time during substrate preparation; thereby creating a delay in overall project delivery. Due to this, it is advisable to research the installation requirements for such products as it relates to substrate surface porosity, and perhaps alter any corresponding specification language to allow for the selection of alternative products because, in many cases, such alternatives are readily available without compromising warranties. Regardless of the approach, the importance of assessing concrete substrate porosity before installing flooring system materials cannot be overstated and has reached the level of being balloted for inclusion within ASTM F170, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring.

  1. [Image]:
  2. Toxic Substance Control Act:
  3. specific rulemaking proceedings:
  4. Carpet Policy Dialogue Compendium Report:
  5. Green Label program:
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  7. [Image]:
  8. Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring was developed:
  9. [Image]:
  10. International Concrete Repair Institute:
  11. Practice for Preparation of Concrete by Abrasion Prior to Coating Application:
  12. Occupational Safety and Health Administration:
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