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  • Building Forensics International

What Causes Concrete Slab Moisture?

It's time to talk about concrete slab moisture. At Building Forensics International we have had our share of projects where concrete slab moisture is the topic of conflict. BFI just finished a two year investigation and litigation case involving concrete slab moisture.

This blog entry will focus on the causes of concrete slab moisture. "What causes concrete slab moisture?"

The first source would be ground water sources. This would be when the concrete slab is in contact with saturated ground, or where there is poor drainage. Moisture moves to the surface of the slab through capillaries or what is referred to as wicking. Just like a candle wick draws the oil up to the lamp to keep the flame burning. Wicking is a descriptive term for the ground moisture being drawn up throughs the pores of the concrete to the surface of the slab.

A second source is water vapor form damp soil will spread over a portion of the slab and pool or appear as droplets on the slab surface. This usually occurs in cooler temperatures or lower humidity due to vapor pressure.

Third, Wetting of the blotter layer (fill course) will on occasion trap moisture. The blotter layer is a thin layer of coarse or fine sand, or pea gravel used as an absorbant layer between the concrete slab and plastic sheeting. The blotter layer if over saturated with moisture can cause moisture on the slab surface.

Fourth, residual moisture in the slab from the original concrete mixing water will move toward the surface. This can take anywhere from four weeks to over a year.

Finally, there are some factors that can affect the drying rate of the concrete slab. Original water content of the concrete, type of curing, humidity and temperature during the drying period. Any wetting of the slab after final curing will increase moisture levels in the slab itself and cause the drying period to last longer.

BFI Staff Writer

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