Editor’s Note: The following article is taken from the Spring 2016 issue of FCICA’s The Flooring Contractor magazine. To view the full issue of TFC, please see FCI’s March digital edition. Visit the Magazine tab at www.fcimag.com for more information.

The past 20 years have seen a rapid increase in awareness of and solutions for the various moisture-related issues that have plagued flooring installations since the discontinuation of cutback adhesives. Even though advances in adhesive technology have led to improved adhesion, moisture resistance and alkalinity tolerance, there are still numerous scenarios where moisture mitigation products are needed to properly treat concrete moisture. However, the prevalence of moisture mitigation products also presents a new challenge for flooring installers: installing flooring materials over non-porous substrates.

While there are a number of different moisture mitigation products on the market, a majority of them are resinous in nature. Whenever a resinous coating is applied to concrete, it turns an extremely porous membrane into a non-porous substrate. This creates a problem for many adhesives, especially wet-setting adhesives that are water-based or moisture-cured.

The combination of a non-porous substrate and an impermeable or mostly impermeable flooring material will keep moisture out and in, which is disastrous depending on the adhesives. Water-based wet-set adhesives require water loss through absorption or evaporation in order to develop adhesive strength and, conversely, moisture-cured wet set adhesives require the absorption of ambient moisture from the substrate or air to develop strength. If these conditions are not met, these adhesives will likely never completely cure and harden, causing adhesion issues that will manifest themselves in a number of unsightly ways.

Resinous coatings are not the only products that effectively turn concrete into a non-porous substrate. Hydrocarbons, sweeping compounds, sealants, sodium silicate sprays or admixtures and other surface contaminants can also render the surface of concrete non-porous. While it is easy to tell when concrete has been coated with a resinous system, it can be difficult to spot these contaminants prior to installation. In fact, in many cases, the concrete will appear normal. If adhesives are improperly installed or are not suitable for this substrate, it could lead to similar adhesive issues.

How can these issues be avoided? First and foremost, know your substrate. Whether installing on new concrete or removing and replacing an existing floor covering, it is the installer’s responsibility to understand the substrate they are installing over and make recommendations and adhesive selections appropriately.

If you’ve determined the concrete is non-porous due to hydrocarbon contamination, additional forensic core testing may need to be performed to determine the extent of the contamination. Luckily, there are a number of products on the market that can treat this type of contamination.

However, if the contamination is only on the surface of the concrete—such as a curing compound or sealant—these can usually be removed by mechanically preparing the concrete. In other cases, a sodium silicate topical treatment or admixture may have been used to treat the concrete. Aside from concerns about the efficacy of these products, these products can receive flooring adhesive. It’s important to ensure that the manufacturers of these products will warranty adhesion in the event of a complication.

The concrete appears porous, so is it safe to proceed? Even if concrete appears porous and there is no reason to suspect that there is contamination, it is still recommended that you determine the porosity of the substrate. Luckily, testing porosity is easy and inexpensive—simply pour a 3/4" wide droplet of water (using a water bottle cap, about the size of a nickel) onto the substrate and wait 60 seconds for the water to absorb into the substrate.

Some materials, such as cementitious patches or underlayments, may not absorb water as quickly as concrete will, but as long as it does absorb water it is considered porous. If, after 60 seconds, a significant amount of water is left behind and there is no evidence of any water absorbed, it is recommended that this substrate be treated as a non-porous substrate.

 The substrate is non-porous, but suitable for installation. Now what? If the substrate flatness is within tolerances or only minor repairs are needed, you may want to install directly over the substrate rather than pour self-leveling, which can add additional costs and extend project completion times. Thankfully, there are still several options available that will allow for this installation method.

As always, the first step is referring to the technical information for the flooring material or preferred adhesive. In most cases, there will be an alternate adhesive recommendation or procedures for installing over a non-porous substrate, which usually involves allowing the adhesive to flash off for an extended period of time or installing the adhesive as a pressure-sensitive adhesive.

It’s important to note that when flash times extend, working times decrease, leaving less time to get the floor covering into the adhesive. For this reason, it is important to work in small sections in order to not exceed working times. If you’ve exhausted all options and the porosity of the substrate was determined prior to purchasing the adhesive, contact the flooring or adhesive manufacturer for a recommendation.

In many cases, flooring installations may have patching, skim coats or cementitious underlayment in the specification or budget already. In these cases, installation can proceed as normal. However, installations over non-porous substrates are becoming more and more common, especially as budgets shrink and construction times condense. As long as the proper care is taken, the correct adhesive is used and the correct installation method is followed, there’s no reason that flooring installations over non-porous substrates cannot be successful.