Q: About a year ago, we placed concrete for a big box warehouse. The concrete has developed an occasional problem with moisture appearing on the surface of the floor slabs. We suspect the condensation forms partly because this area of the building has no climate control and very poor circulation of air. At first the client tried to blame the problem on our work, but one expert recently told me it could be a case of sweating slab syndrome. What is this and how can we prevent it on future projects?
A: Sweating slab syndrome (SSS) is a phenomenon in which moisture intermittently develops on the surface of an interior concrete slab. It shows up most often in big box warehouses without climate control. SSS can make a concrete surface more slippery and threaten the safety of workers and material transport operations.
SSS is serious and can be challenging to diagnose and correct. However, it can be remedied once the actual causes are determined. Moisture is the underlying cause of sweating slab syndrome, typically as a result of dew point condensation. Warm, humid air enters the structure through open doorways, windows, and vents. As this air diffuses throughout the structure, it condenses on any surface at or below the dew point temperature—often the floor surface.
On some projects, salts drawn from within the concrete and deposited on the slab surface have led to SSS, by attracting moisture from the air. Other contributing factors can include the density and cleanliness of the slab. Hard-troweled slabs treated with surface densifiers, as well as slabs covered with black rubber tire marks or exhaust deposits, will exhibit reduced surface permeability. A less permeable slab absorbs less absorptive moisture from above. Furthermore, if there isn't an effective vapor retarder beneath the slab, moisture levels within the concrete and near the surface will increase over time.
To avoid SSS, the first step is to remove any type of deposits from the surface of the slab. The next is to examine air movement within the building. Facilities with little air movement can exhaust interior air through roof vents and create negative pressure in the building. This condition will allow warm, humid outside air to enter the building quickly.
The use of large ceiling fans can help reduce or eliminate SSS by minimizing ceiling-to-floor temperature differentials and by increasing the surface evaporation rate. Commercial dehumidifiers also can help by altering the building's interior environment.