Whether you’re an installer, retailer, or sales associate you’ve more than likely been asked to install over lightweight concrete, or a gypsum based substrate/subfloor (Photo 1). And more than likely, you either sold or installed product that was adhered directly to these types of substrates. Installing any type of flooring product to lightweight substrates requires extra precautions. I don’t know of one manufacturer that advocates a glue-down or thin-set “direct bond” over a gypsum-based substrate, yet thousands of square feet of flooring is installed on a daily basis in this manner. And so starts the roulette game. Some installations will be successful while others fail.
Lightweight substrates are typically utilized in above-grade installations such as multifamily housing, condominiums, hospitality, and radiant systems. So why does the industry use a lightweight substrate? Weight is a major factor, along with sound dampening properties and fire ratings. The industry has labeled gypsum-based pours as “Gypcrete”, much like Formica for laminate countertops. Gypcrete is actually a brand of gypsum and a registered trademark of the Maxxon Corporation. Gypsum pours do not have as high of a compressive PSI (pounds per square inch) rating as concrete substrates. On average, a gypsum-based substrate will attain a compressive psi up to 2,000, while concrete will average compressive psi of 3,000 and up. Manufacturers of gypsum-based substrates state that a 3/4” pour will have a dry time of 5 to 7 days and a pour of 1 ½” will have a dry time of 14 days. Low temperatures or high humidity will lengthen the dry time. I have spoken with several installers who went by these numbers only to have a failure. To this I always ask, did you do any type of moisture testing? The usual answer is no. Keep in mind that moisture testing of a gypsum substrate is different from that of concrete. A calcium chloride test (ASTM 1869), is not considered a quantitative test. The in-situ relative humidity test (ASTM 2170) is the test procedure the industry looks to for moisture testing of these substrates. If using an electronic meter to get a qualitative reading, make sure it’s one that is recommended by either the supplier of the gypsum-based pour or the flooring manufacturer.
The best installation method for a gypsum-based substrate is a floating type of installation but, since we all know that flooring is being directly adhered to a gypsum substrate, what are installers doing to achieve the best bond? Gypsum substrates are very porous and if a water-based adhesive is used to install flooring, the moisture can be absorbed into the substrate and the adhesive sets to quickly, creating a weak bond. There are manufacturers that have primers that can be applied to the surface of the gypsum substrate. Now, keep in mind that these products do not absorb the entire depth of the gypsum substrate, but only the surface or slightly under. And, if the adhesive and product have more strength and pull (shear strength), there will be potential to fail where the primer and the gypsum interface. So here are some pointers when dealing with a gypsum-based substrate.
• First, try to install as a floating installation.
• Run a screwdriver or nail across the surface to see how deep or how easily the tip will penetrate to give you an idea of what you are faced with. If the surface gouges/flakes easily, your only option may be a floating installation.
• If glue-direct is the way the installation is headed, contact the manufacturer of the flooring product to determine best course of action. Let them make the call whether or not they feel their product will be suitable for this type of installation and follow manufacturer’s recommendations. • If the manufacturer states no to this type of installation, just remember you are on your own if there are potential issues.
• Conduct proper moisture testing and don’t rely on the 14-day dry time.
• If floor patching is required, use a gypsum (white) patching compound. Gypsum substrate and white gypsum patch are compatible. Portland based (gray) patching compounds are not recommended directly over a gypsum substrate. The same goes if a self-leveling underlayment is required. There are gypsum-based self leveling products suited for installation over a gypsum based substrate with much higher compressive strength than normal gypsum subfloors.
• Use a recommended primer that will be compatible with the flooring being installed.
• If installing tile or stone you can refer to F180-11 of the TCNA handbook for Ceramic, Glass, and Stone Tile Installation.
• Document the condition of the substrate prior to initiating work with photographs.
• If you are bidding on removal of flooring over a gypsum substrate, you may encounter extensive damage from removal which could lead to a complete re-pour or the use of self-leveling underlayment so bid carefully. (Photos 1-4)
Lightweight concrete is different from gypsum in that appearance wise, Gypsum is white while lightweight concrete looks from all appearances, the same as concrete. Lightweight concrete can also achieve psi of 3,000 or higher. If it looks like concrete, is hard like concrete, what are the differences? It is called lightweight concrete because it uses different aggregate than concrete. Lightweight concrete can consist of several aggregate materials depending on the specification requirements and availability. Expanded clay, slate, shale, crushed brick, volcanic materials such as perlite, pumice, and fiber reinforcing are some of the products used. The aggregates used in lightweight concrete are porous, allowing for absorption of up to 20 percent while normal weight aggregate used in concrete will have absorption rates less than 5 percent. Being lightweight reduces the dead load weight in structural applications enabling less material to be used for the structure. Lightweight concrete averages 90 to 120 pounds per cubic foot. Normal weight concrete is typically 135 to 150 pounds per cubic foot. So what are the concerns and issues with lightweight concrete with flooring? Moisture; lightweight aggregate is kept saturated prior to being mixed with cement, so at that time, it has absorbed as much moisture as it can hold. When placement is done, the moisture in the aggregate starts to absorb into the concrete as it hydrates and slowly evaporates. With the inherent higher moisture content, it’s this factor that causes issues for the installation of flooring. Lightweight takes longer to dry, and that is why moisture testing is so critical. Also remember that calcium chloride testing (ASTM 1869) is not recognized as a test method for determining moisture content for lightweight concrete. In-situ relative humidity testing (ASTM 2170) is the recognized test method. Here are some points to remember with lightweight concrete.
• Conduct proper moisture testing.
• If you don’t know what type of concrete, ask.
• On new construction, make sure the lightweight concrete meets the flooring manufacturer’s requirements.