How to Test for Water Vapor in a Concrete Floor Slab

How to Test for Water Vapor in a Concrete Floor Slab

For all its strength and endurance, concrete has a tendency to wick moisture in the ground. Some concrete slabs remain moist, but some puddle. Not all slabs draw moisture. Some remain completely dry. Bad drainage, proximity to groundwater sources and the thickness of the water table all exacerbate the issue. When concrete slabs maintain a high moisture content, floor adhesives have difficulty sticking, which restricts flooring options.

Plastic Sheet Method

While not a quantitative test, a sheet of plastic stick to the concrete slab for 16 hours or more reveals whether the slab is more prone to water vapor difficulties. Polyethylene is typically the plastic used, and also a sheet about 18 inches square will be all that’s needed. Duct tape, or even something similar, must fasten the sheet securely on all four sides. If the slab is moist under the sheet following the subscribed period of time, then the concrete includes moisture. This process does not give a measurement of how much moisture the slab contains. Another downside to this testing method is that a dry slab under the plastic sheet does not indicate whether humidity problems live deeper in the slab.

Calcium Chloride

Most flooring manufacturers recommend putting a cup featuring a prescribed quantity of anhydrous calcium chloride under a sealed, plastic dome on the slab. The tester then measures the quantity of moisture absorbed from the minerals in a period of 60 to 72 hours. That measurement enables the tester to figure out the total amount of water vapor emitted from the bunker in a 24-hour period. This inexpensive test compares the calculated vapor emission into a maximum acceptable limit of 3 to 5 pounds of moisture per 1,000 square feet of concrete. The test is determined by the temperature and relative humidity inside the room, plus it only records the quantity of water vapor emitted in a narrow window of time.

Humidity Probe

While somewhat damaging, a humidity probe gives a very clear and quantitative number to the quantity of water vapor in a concrete slab. The probe process requires a hole drilled to the masonry to a specified thickness. Inserting the instrument into the opening supplies more information than surface testing lonely. The probe provides moisture amounts at various levels throughout the slab. In addition, the probe has access to the substrate below the slab. Since water vapor measurements may be run through the access hole at any moment, the humidity probe method enables testers to see how fast the slab dries.

Other Alternatives

Other testing procedures are used and may continue to be accessible, however they produce unreliable results, use damaging procedures or are too expensive for many applications. Moisture meters, for instance, do not provide accurate indications of the true moisture conditions of the full slab. A gravimetric method expects that part of the floor be ripped away for testing and weighing. A nuclear detector and radio frequency test is true, but the equipment is expensive and may only be controlled by licensed individuals.

See related