Concrete slabs may seem dry and solid, but due to its makeup of water, sand, cement, and aggregate, and its porous nature, it can hold moisture. Over time, the water will be released, but when you’re planning on laying a coating down, knowing how to test for moisture allows you to make the right call on how to treat the slab. If you lay an impermeable coating over concrete that’s giving off moisture, it will bubble and blister, or mold and mildew can grow. In order to prevent these complications, we’re sharing some ways to identify and test for moisture so you can get the best possible results.

Concrete Moisture Myths Debunked

First, there are some common misconceptions related to moisture and how it affects concrete, so we want to debunk those.

  • Myth: Just because the concrete is dry at the surface, the slab is dry all the way through. Nope. Airflow, temperature, and how the concrete is laid all affect how quickly the water within evaporates and what happens at the surface is no indication as to what lies within.
  • Myth: All coatings tolerate moisture the same. Every manufacturer makes their coatings slightly different and how well a urethane or epoxy coating tolerates prolonged moisture and humidity can vary.
  • Myth: Old concrete is dry. Even after a few years, concrete can still retain and give off moisture, particularly in high humidity areas, a compromised vapor barrier below the slab, or a water leak from somewhere within the space.
  • Myth: All Relative Humidity (RH) tests are the same. There are several types of RH testing, including ones that have sensors installed into the slab and others that measure vapor emissions.

Types of RH Tests for Concrete

The easiest way to test for vapor emission is the “plastic sheet method” and, while it seems simple, it was developed by the ASTM Committee for Protective Coatings and Lining Subcommittee on Application and Surface Preparation. Basically, a square of plastic sheeting is tightly taped directly to the concrete slab, and after 72 hours, the plastic is raised slightly in order to take a humidity reading using a hygrometer.

The second direct test is the ASTM F 1869 which measures moisture vapor emission using anhydrous calcium chloride. Sections of concrete are sealed in plastic, but then, using calcium hydroxide is used. A sealed packet of calcium hydroxide is weighed, then placed under the enclosure and left for 72 hours. If there is an increase in weight, it indicates how much moisture was emitted. Test results are measured in pounds per square feet over 24 hours, and there are kits you can purchase to do this test.

Additionally, you can test the humidity within the slab using a moisture meter that has a probe that is inserted into a hole drilled into the slab and then sealed in place. After 72 hours, readings can be taken to determine the relative humidity.

To determine which kind of test is right for the coating you plan on installing on the concrete, contact the coating manufacturer. They will be able to tell you which test and what conditions they recommend.

Contact Us for Concrete Coatings and More

We have a wide variety of urethane and epoxy coating designed to withstand high traffic and heavy use as well as the tools and equipment you may need to improve your concrete slab. To learn more, reach out to us today at  (815) 472-9754 or by filling out our contact form to get started!