The importance of substrate surface water absorptivity

Absorptive (porous) concrete surface.
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. 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.

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