Humidity Level in a Concrete Block

Moisture

• The moisture in a concrete slab is not free water like you have in a water glass. It's actually adsorbed or incorporated into the concrete. Water can travel through a slab by means of wicking, much as a paper towel soaks up water. The rate at which moisture filters through the concrete depends on the relative humidity on each side. Concrete is relatively impermeable so the rate at which this process occurs is slow, but it happens nonetheless.

Sources

• The moisture in a concrete block can have many sources. It could be residual moisture, water left over from the original mixing process because concrete slabs can take as long as six weeks to a year to dry completely. It could also come from groundwater sources or damp soil. The so-called blotter layer sometimes used between concrete and vapor barrier can also harbor moisture if it's wet at some point.

Measurement

• Concrete humidity is measured in a variety of different ways. Taping a plastic sheet in place for 16 hours or so, for example, will give you an idea whether moisture is present, although a negative result doesn't guarantee moisture is at acceptable levels. Moisture meters measure electrical resistance to determine the moisture content. Pieces of concrete can also be removed, weighed then oven-dried to determine how much moisture they lost. It's also possible to apply calcium chloride -- a highly hygroscopic or moisture-absorbent salt -- beneath a sealed plastic dome then measure how much water it absorbs over a determined time frame. Finally, some builders use electronic relative humidity probes, which can determine the moisture profile right throughout the slab and monitor the rate at which it dries over time.

Problems

• If you want to avoid problems with concrete humidity, you can take several steps. The structure should be designed to ensure proper drainage so water does not accumulate around the slab. Vapor retarder membranes placed beneath the slab can help prevent water from entering. Using a concrete mixture with less water, avoiding extended moist curing and allowing enough time for the slab to dry properly will all help reduce problems with residual moisture. Finally, testing humidity before you install a floor covering will help identify potential problems.