Just about every new construction project has a concrete slab. It’s easy to get tempted to complete a job faster by rushing to finish the concrete. Ignoring some basic facts about working with concrete could lead to the flooring, and your reputation, being ruined.
Concrete moisture testing is the only way to know that concrete is ready to be finished and have the flooring installed. Testing also helps you control your schedule and budget. Learning eight truths about concrete moisture makes you a better general contractor.
- 75% Relative Humidity(RH) or below is acceptable for finishing a concrete slab. Different floor coverings can handle different levels of RH. A moisture test is the only way to know if a flooring will succeed on your concrete.
- All concrete has some moisture content. Most of the water in concrete comes from the mixing process. You can apply a floor covering to the concrete once it is free of excess moisture.
- Water flows through concrete. While drying, concrete forms pathways called capillaries that water moves through. Moisture is more concentrated deeper in a slab. Water from outside the concrete can enter and slow the drying process.
- At the equilibrium point, water stops moving through concrete. After you seal concrete, the moisture inside that can’t escape balances evenly throughout the slab. If the moisture content is excessive, it can cause the sealer and the flooring above it to fail.
- Moisture in concrete won’t move faster than conditions allow. Ambient temperature and air circulation determine how fast concrete dries. Some products claim to help concrete dry faster, but they may not work. A common way to estimate drying time is 30 days per inch of concrete. Once you pass your estimated drying time, you can have a concrete moisture test done to know if it is dry or not.
- Your target moisture content is based on your flooring. Each project will aim for a different amount of concrete moisture content. Your target equilibrium point is based on how much moisture the flooring you will install can handle.
- It’s impossible to know the concrete moisture content without testing. Estimating the drying time for a concrete slab can’t be relied upon for determining whether it is ready to be coated. You must have professional testing done to get an accurate picture of your slab’s readiness.
- Surface moisture is not the same as moisture content. The RH in-situ probe testing method is the best way to know that your slab has the correct moisture content. You can’t rely on the plastic sheet or calcium chloride method, which only tests surface moisture.