Q:

Why does a concrete floor sweat?

A:

Quick Answer

If a concrete floor is sweating, the likely culprit is a condition known as sweating slab syndrome, in which the interior concrete slab's surface occasionally develops moisture as a result of dew point condensation. This makes the surface of the concrete slippery, making it dangerous to move or lift heavy objects while on that surface.

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Full Answer

When dew point condensation causes a concrete floor to sweat, humid, warm air makes its way into a structure through open windows, vents and doorways. As the air moves around, it condenses onto any surface it finds that is at or beneath the dew point, which is frequently the floor. When the ambient temperature goes up rapidly, along with the relative humidity, a sweating slab is likely to happen, because the humid air entering the space takes a lot less time to heat the air inside the space than it does the floor. This often leaves the slab cold enough for condensation to happen.

In some cases, when salt deposits are present on the concrete floor, the floor sweats as well. Salt draws moisture from the surrounding air as well as from the concrete. Sometimes moisture develops inside the concrete, pushing the salt in the mixture to the surface, where it attracts more moisture.

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