Do it right or pay the price. It’s an old saying that’s especially true when it comes to installing Gauged Porcelain Tile Panels (GPTP) over a concrete floor slab.

GPTP offer many benefits that help explain the material's fast-growing popularity. GPTP are strong, resistant to scratches, easy to maintain and yield a clean, seamless look that designers and architects often find highly attractive.

Once you make the choice to go with GPTP for a given floor project and you’re getting ready for the installation, you’ll want to consider several key factors. There’s the obvious, of course such as carefully selecting the right adhesives and other materials for the job, as well as paying close attention to surface preparation. But one factor could easily get overlooked. and if that happens, the price to pay could be a hefty one. The factor is this: excessive moisture in the concrete floor slab.

It can be hard to think of a concrete slab as containing excessive amounts of moisture. After all, concrete is rock-solid. To the casual observer, it typically looks dry within a week or two after it’s poured.

And yet, moisture inside the concrete -- once it gets sealed off with an impermeable floor product -- becomes serious business. It could lead to adhesive degradation, mold, mildew, discoloration of grout lines or other moisture-related issues that can be costly to address.

The fact is, one of concrete’s most essential and necessary ingredients is water. Without sufficient water, concrete won’t cure properly, meaning it will fail to achieve its characteristic strength, hardness and durability.

Many people wonder, “Can I install GPTP once the concrete’s curing process is done?” Not right away. After the curing process is complete, the concrete slab must dry to an acceptable level to be ready for the installation of any finished floor product, including GPTP. Yet no slab is ever completely dry; all concrete contains some water no matter how dry it may look at the surface.

The key is to wait for the slab to dry to a level within the moisture tolerance of the finished floor products -- that way moisture damage won’t occur.

But, how long does it take for concrete to dry? The general rule of thumb is to build into the construction schedule about one month of drying time for each inch of slab thickness. So, for example, in the case of a 5-inch slab, wait about five months for the slab to dry sufficiently.

However, it’s important to realize that this is not a hard-and-fast rule, but simply a guideline to help with project planning. Actual concrete drying times will vary widely, depending on the ambient relative humidity and temperature and whether or not the building is completely enclosed with the HVAC up and running.

Another critical thing to know about concrete moisture is what’s happening as the concrete dries. In a nutshell, the drying process involves evaporation from the slab’s surface. And as this occurs, a moisture gradient forms with considerably less moisture at or near the surface than deeper down inside the concrete.

The way that concrete dries is one reason why a concrete slab can be deceptive. It can look dry at the surface, tricking you into thinking that the slab is dry when it’s not. It’s also why you never want to rely on a surface-based, quantitative assessment of concrete moisture such as the anhydrous calcium chloride test. Surface-based tests tend to be misleading and unreliable.

Another thing to know: once the slab is sealed with a flooring product like GPTP, thereby preventing further drying of the concrete, the moisture gradient in the slab will tend to even out over time. This will bring more moisture to the surface where it can interact with the flooring product and cause significant damage.

Excess moisture in concrete may also interact with and seriously degrade certain compounds used in a self-leveling underlayment. And of course, these self-leveling products are commonly used to attain the flat surface necessary for a successful GPTP installation. Again, you’re liable to pay dearly for overlooking the havoc that can come about due to too much moisture in the concrete.

Fortunately, all that heartache can be easily prevented. A fast, reliable, proven method exists to assess the moisture within the slab. This method, known as the in-situ relative humidity (RH) test, doesn’t just rely on surface measurements that can mislead you. Instead, it fully accounts for how concrete dries. This test assesses the overall moisture condition of the slab so you can have assurance about when it’s truly ready for your installation.

Scientific studies at Lund University in Sweden and elsewhere have shown that by using RH probes to take RH measurements at 40% of the slab’s overall depth (for a slab drying from one side), you get an accurate picture of how much moisture the finished floor will “see” after it is installed.

This in-situ RH test, standardized as ASTM F2170, gives you a simple number — an RH percentage — that can quickly be compared to the manufacturer’s RH specification for a given floor product. If this number is less than the spec, the slab can be considered “dry” enough for the floor installation to proceed safely. If the number is higher than the spec, then the slab is not dry enough, and you’ll normally want to allow more time for the concrete to dry.

The most popular of today’s RH test kits is the Rapid RH L6 by Wagner Meters. Not only is it simple and economical to use, it gives a digital readout of the test data that can be wirelessly transmitted to your smartphone for complete data integrity.

Always remember this: while GPTP offer plenty of advantages, you’ll want to take some precautions to ensure a successful installation over concrete. One of the most important of these precautions is to test reliably and accurately for concrete moisture.

The concrete mix requires water for proper curing of the slab, but you must wait for the concrete to dry to an acceptable level before you install GPTP. Photos courtesy of Wagner Meters

Surface-based moisture tests such as anhydrous calcium chloride are often misleading because they fail to assess the slab’s overall moisture condition.

The in-situ RH test gives a number that can be compared to the manufacturer’s spec for easily determining when the concrete is ready for the installation.

About the author

TILE Tech Focus Jason Spangler

Jason Spangler, Wagner Meters’ sales manager, has more than 25 years of experience in sales and sales management across a broad spectrum of industries. He has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture test. Jason, who received an MBA from West Texas A&M University in November 2018, has extensive industry involvement, including the National Wood Flooring Association (NWFA), International Certified Flooring Installers Association (CFI) and serving as vice chairman of associations for the Flooring Contractors Association (FCICA).