Methods for Testing Concrete Moisture

Relative Humidity vs. Calcium Chloride

Internal Relative Humidity Testing (RH)

(ASTM F 2170) This process involves measuring the relative humidity of the concrete, at a specific depth from the slab surface. This is done by drilling and properly preparing a hole in the concrete slab and inserting an “in-situ” probe, which when connected to an electronic reader, the internal temperature and relative humidity of concrete are displayed. The probes will remain in the concrete and can be checked periodically to monitor the drying of the concrete and provide information leading to predictions of future moisture conditions. Relative Humidity is the ratio of the amount of water vapor actually in the air compared to the amount of water vapor required for saturation at that particular temperature and pressure, expressed as a percentage. Relative humidity testing can determine the internal moisture in a concrete slab, rather than test the moisture level at the top of the concrete, where it is typically the driest. This is important because the internal moisture will try to dissipate through the surface, and if the slab is covered with a floor covering before sufficient time to dry, the moisture could have devastating effects on the installed floor covering. You must check the floor covering manufacturer's and installation system requirements before you begin any installation. These requirements may vary depending on the floor covering products. NOTE: This method of testing has been used in Europe for many years and over the last few years has become the go-to method for many floor covering manufacturers in the United States.

Anhydrous Calcium Chloride Testing

(ASTM F 1869) This process consists of a measured by weight amount of Anhydrous Calcium Chloride placed in a cup/dish and sealed beneath a 1ft² sealed dome, which is placed on a properly prepared area of concrete. The test will remain in place between 60 to 72 hours. During that time moisture vapor emitting from the concrete surface is absorbed by the calcium chloride. The cup/dish is then sealed and measure by weight again. The difference between the starting weight and finish weight is calculated into a formula which will determine the pounds of moisture per 1000 ft² in a 24 hour period, or moisture vapor emission rate (MVER). The required quantities of tests are 3 for the first 1000 ft² and 1 test for each additional 1000 ft² or fraction thereof. This method of testing determines only a portion of the free moisture at a shallow depth of the concrete near the surface of the slab. The test is sensitive to the temperature and humidity of the building, requiring the building to be at the temperature expected during normal use at least 48 hours prior to and during testing, or 75° ± 10° and 50% ± 10% relative humidity. Calcium chloride testing does not predict the sub-surface conditions as ASTM F2071 Relative Humidity testing does. This method of testing is not recommended for lightweight concrete.

pH Testing

(ASTM F 710) The pH is a measure of the concentration of acid ions in a solution. A reading of 7 is considered neutral. New concrete will typically have an approximate pH of 12 to 13. This is mostly due to calcium hydroxide, which is a normal by-product of cement hydration. As a concrete surface reacts with carbon dioxide in the air, the pH of the surface is gradually reduced through a process called carbonation. So if the concrete has not had proper time or conditions to cure, you may have high pH readings. If the concrete has access to a continuing water source, such as direct contact to moist earth below, dissolved alkalies and hydroxides can be carried to the surface resulting in a high pH level. If the surface of the concrete has been removed by bead-blasting, grinding, or other means you may have high pH readings. You need to check your concrete surface pH readings with the floor covering manufacturer and/or installation system guidelines and instructions before proceeding with the installation of any floor covering or floor coating. Different flooring manufacturers, flooring products, and installation systems vary in the allowable pH level.