If you don’t have a solid understanding about concrete moisture and how concrete dries, it’s hard to make the right call about when it’s appropriate to proceed with your flooring project. The consequences could be serious, leading to ugly and dangerous tile flooring failures. This is particularly true for tile projects when using the thin-set method of installation and/or working with impermeable materials, such as resin agglomerate tiles, large-format porcelain tiles, epoxy adhesives and various specialty products that are sensitive to moisture.

The most critical piece of information you need for making your decision to proceed is the moisture condition in the concrete. Proper moisture testing of the concrete is the only way to know if the concrete slab is dry enough to put down the flooring. For proper testing, here are the six simple truths you must know:

1 Moisture moves through concrete

Every slab of concrete has moisture and retains moisture for years — decades even — after it’s been poured. Therefore, you’re never going to achieve 100% dry concrete. What you’re looking for is a moisture condition that’s acceptable.

The ratio of water to cement in the batch of concrete varies and is one of the most influential factors affecting how long a concrete slab needs to cure and dry. Once the concrete slab is poured and cured, time is needed for the excess moisture to evaporate. If your tile floor is applied too soon, you may face bonding failures or other types of moisture-related damage to that floor.

During the drying process, the concrete batch of water, cement, aggregate and other admixtures creates tiny pathways within the concrete called capillaries. Once cured, the excess water in the concrete moves through these capillaries to release the moisture. Since most concrete floors have a vapor retarder below them, this moisture moves upwards through the concrete in order to escape.

The significant movement of water within the slab means that the moisture level will vary by depth. Generally, the surface of the slab is drier, while deeper in the concrete, the moisture content is higher.

One more important note: Moisture doesn’t just flow out of a concrete slab. The flow of moisture is actually a two-way street. External sources of water present during the drying process can potentially increase a slab’s moisture content.

2 There’s a critical point in moisture movement

In truth, moisture never stops moving in concrete. It will always react relative to its environment. However, it does reach a point of equilibrium once the slab is sealed.

Just as a vapor retarder beneath the concrete stops moisture from escaping through the bottom of the slab, a floor installation restricts the flow of moisture evaporating at the top surface. The outcome of closing off both routes of evaporation is that the remaining moisture will spread out more or less evenly throughout the slab.

The resulting point of equilibrium is critical because this is the moisture level that the tile, thin set and bonding compounds will “see” over the long term. If the expected point of equilibrium isn’t appropriate for the project’s specific flooring application, you are at risk for a significant flooring failure.

3 The moisture isn’t going to move any faster than it wishes

The common wisdom of how long it takes concrete to dry is 30 days per inch of slab depth. However, numerous factors affect how much time a concrete slab needs to dry. In addition to internal and external water sources, other environmental conditions such as ambient temperature and air circulation impact drying time.

While the 30-day-per-inch wisdom may be common, don’t rely on it. The specifications and environmental conditions of each project are unique. This rule of thumb is never enough to give you a solid quantitative basis for making your decision about when the concrete slab is ready.

4 The target point of equilibrium is specific to your project

The first three truths about concrete all lead to this next important truth: You must know what point of equilibrium in the concrete is right for the specific floor that you will apply on top of the slab. The manufacturers of each flooring product have developed recommendations based on the moisture tolerance of their specific products.

The levels of moisture in the slab at present and at the expected point of equilibrium are your ultimate guidelines that you must rely on when you decide whether the concrete is ready for your project.

5 You can’t know what you don’t measure

Several methods for measuring concrete moisture are available, but not all are reliable. In the U.S., two test methods are most commonly used.

The first is the anhydrous calcium chloride (CaCl) test, or ASTM F1869. It is conducted by placing a desiccant material on top of the slab and sealing it with a cover to block any influence from the surroundings. The material is weighed 72 hours later on the assumption that any increase in weight of the CaCl crystals translates into the “moisture vapor emission rate” of the slab.

The in-situ relative humidity (RH) test, or ASTM F2170, uses sensors to measure the relative humidity at a specific depth: 40% of the slab’s thickness for a slab drying from one side or 20% for a slab drying from two sides. Scientific research has determined that at these depths, the readings will most accurately predict the slab’s point of equilibrium, and therefore the true moisture condition that will exist after the flooring installation.

6 Surface moisture is misleading

The key, of course, is properly measuring the concrete’s moisture condition. As we’ve seen, based on what we know about how a slab dries, it’s important to collect data that accurately predicts the point of equilibrium after the flooring installation. Only the in-situ RH test has been scientifically proven to provide this information.

You see, the CaCl test only measures moisture at the surface of the concrete slab. Since we know that moisture within a slab varies by depth, any test that only measures moisture at the surface can’t provide a reliable picture of the slab’s overall moisture condition. Today, ASTM International no longer permits use of the CaCl test for lightweight concrete applications.