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A concrete moisture meter is a handheld tool, usually with a digital readout, used for measuring the amount of water present in concrete, usually in building construction to test concrete floor slabs. These meters are generally one of two types, surface meters and probe-type meters, which require a test hole to be made in the concrete. Surface meters tend to be less useful as they can only give readings that reflect the relative humidity (RH) of the concrete very close to the surface of the slab, no more than 1 inch (2.54 cm) deep. A probe-type concrete moisture meter requires small test holes but gives a much more accurate reading as it can measure the RH at the center of the slab, which is much more useful.
Concrete slabs, as they dry and cure, do not lose moisture evenly. They dry from the surface down, meaning that at any given time after pouring, the surface will have less moisture than the interior, and the most moisture will be at the bottom of the slab. This process of drying stops when the slab is covered, usually by flooring, but even a coating of urethane directly on the concrete itself will halt the loss of moisture. After the slab is covered, the moisture equilibrates, which means that the moisture spreads out evenly. An equilibrated slab will have an even moisture content from top to bottom.
Surface meters can measure the moisture content in a concrete slab without any need for drilling holes in the slab. They are only capable, however, of reading the moisture in the portion of the slab that is very close to the surface, generally no more than 1 inch (2.54 cm) or so deep. When dealing with thick concrete slabs that are common in building construction, this information is not useful as an indicator of the overall moisture levels throughout the entire slab. Many factors can influence the rate at which the slab dries, which also affects the readings by these types meter, including the type of cement, size of aggregate, and how the slab was troweled or pressed. A surface type concrete moisture meter may give a reading that indicates that the slab is dry enough for floor covering installation, but once the slab equilibrates after the flooring is installed, it may be to wet and cause damage to the floor or other problems.
A probe-type concrete moisture meter requires the drilling of one or more test holes in the concrete slab. These holes can be very small, are non destructive, and can be filled when no longer needed. The probe is placed into the test hole, sometimes in a perforated sleeve and gives a reading from the interior of the slab. The RH of a slab at a depth of approximately 40% to 50% of the total will give a reading that reflects how much moisture will be present in an equilibrated slab. This reading is much more useful when determining whether the slab is dry enough for flooring.