Humidity Can Cause Moisture Problems

Question:
Chris, I am doing a stained concrete project using an acetone-based stain. I plan on sealing the stained concrete with a water-based urethane. This is a large 8,000-square-foot floor in the cafeteria of a local manufacturer, and it receives heavy traffic. I just completed a moisture test (using a calcium-chloride test). I did three tests, with results that ranged from 6.13 to 6.58 pounds. But I understand the recommendation is no more than 4 pounds. This floor is on grade, and the concrete is over two months old with a vapor barrier under the slab. The interior humidity has been high (around 70%) due, I think, to rainy weather and wet drywall mud.

Here is my question: Because of the high moisture readings, should I do this job? Is 4 pounds a strict guide, or are there some tolerances? For example, is 6.58 pounds acceptable but something higher, like 12 pounds, too high? This is the first time I have done a test like this.

Answer:
According to the ASTM standard for conducting a calcium-chloride test (ASTM F 1869), a reading above 4 pounds—the amount of moisture flow in pounds per 1,000 square feet over a 24-hour period—is considered a no-go for sealing. This is especially true for nonbreathable coatings, like the polyurethane you plan to use. That being said, you have a slab that should eventually drop below 4 pounds because of the vapor barrier.

You hit the nail on the head in regard to humidity. This is a very real environmental factor often missed when contractors prepare to seal, especially interior floors. Concrete is a sponge, and if conditions are right it will absorb and hold moisture, both in liquid and gas forms. The process of osmosis now comes into play—the movement of a liquid or gas from an area of high concentration to low concentration until balance is achieved. If the air above your floor has a high concentration of water vapor, or humidity, then the concrete will absorb moisture until it contains water equal to what is in the air. The practice of closing doors and windows to “dry-in” a room or building can actually trap moisture and create a steam room of sorts, making the problem worse or creating a problem where one didn’t exist. Drywall mud, paint and rainy or hot, humid weather can cause high moisture contents indoors, which will cause concrete to absorb moisture. This moisture will stay in the concrete until the moisture content of the air above the slab abates. Whether the humidity is manmade or natural, it can cause short-term high moisture readings, especially in the spring and fall, when days are warm and nights are cool.

Wait for a dry spell, open as many windows and doors as possible, use a fan to move air across the floor, and take another test after 24 to 36 hours of drying. If the HVAC system in the building is operational, run the air conditioner to help remove moisture. When the moisture readings drop low enough for you to seal the concrete (at or below 4 pounds), apply the sealer in the afternoon or evening, when humidity is lowest.