Concrete moisture meters measure water in concrete for building construction. Surface gauges only measure humidity close to the surface, while probe gauges give more accurate readings by drilling small test holes. Concrete slabs dry unevenly, and equilibrate after being covered. Surface meters are not useful for thick slabs, while probe meters require test holes but give more accurate readings. RH at 40-50% depth indicates equilibrated moisture levels.
A concrete moisture meter is a hand-held instrument, usually with a digital readout, used to measure the amount of water in concrete, usually in building construction for testing concrete slabs. These gauges generally come in two types, surface gauges and probe gauges, which require you to drill a test hole in the concrete. Surface gauges tend to be less useful as they can only give readings that reflect the relative humidity (RH) of the concrete very close to the slab surface, no more than 1 inch (2.54 cm) deep. A probe concrete moisture meter requires small test holes but gives a much more accurate reading as it can measure relative humidity in the center of the slab, which is much more useful.
Concrete slabs, when they dry and harden, do not lose moisture evenly. They dry from the surface down, which means that anytime after pouring, the surface will have less moisture than the inside, and most of the moisture will be at the bottom of the slab. This drying process stops when the slab is covered over, usually by pavement, but a urethane coating directly over the concrete itself will also stop moisture loss. After the slab is covered, the humidity equilibrates, meaning that the humidity is distributed evenly. A balanced slab will have uniform moisture content from top to bottom.
Surface meters can measure the moisture content in a concrete slab without the need to drill holes in the slab. I am able, however, to only read the moisture in the portion of the plate that is very close to the surface, generally no deeper than 1 inch (2.54 cm) or so. When dealing with thick concrete slabs that are common in building construction, this information is not useful as an indicator of overall moisture levels throughout the slab. Many factors can affect how quickly the slab dries, which also affects the readings of these types of meters, including the type of concrete, the size of the aggregate, and how the slab was troweled or tamped. A surface-type concrete moisture meter may give a reading indicating that the slab is dry enough for floor covering installation, but once the slab equilibrates after floor installation, it could get wet and cause damage to the floor or other problems.
A probe-type concrete moisture meter requires one or more test holes to be drilled into the concrete slab. These holes can be very small, non-destructive, and can be filled when no longer needed. The probe is inserted into the test hole, sometimes into a perforated sleeve, and gives a reading from inside the slab. The RH of a slab at a depth of approximately 40% to 50% of full will give a reading that reflects the amount of moisture present in an equilibrated slab. This reading is much more useful in determining if the slab is dry enough for paving.
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