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Concrete is a hygroscopic material, and adsorbs
moisture from adjacent soils. This moisture
wicks through the concrete where it will diffuse into the
interior of the building, and may lead to excess humidity, chronic
dampness and mold.
Concrete
footings, located at the bottom of the excavation, can be exposed to water
ponding, which is the severest form of moisture contact. The small
particle size of cement allows the concrete to 'pull' moisture from
surrounding soils with an intermediate particle size. For example, concrete has
no difficulty in pulling water from damp sand. However, concrete in
contact with water ponds encounters no opposing 'pull', and rising damp becomes a
very significant water source inside the building.
This web page
examines water ponding in excavations, how it occurs, and
how it can be
prevented.
SOIL TYPES
Certain soils
enable water ponding:
-
Impervious Soils: These soils, for example, clay, silt and shale,
allow water ponding to occur, which may lead to high levels of rising damp.
-
Pervious
Soils: These soils, for example, sand and gravel, allow water to
pass through to the water table below, and water ponding cannot occur.
WATER
PONDING ON IMPERVIOUS SOILS
The diagram
below shows water ponding making contact with the footing concrete, leading
to high levels of
rising damp.

The following errors
were made by the contractor:
-
Fastfoot®
Footing Membrane Not Installed - soil moisture is wicking into the
footing concrete;
-
Drain
Tile Located Too High - the top of the drain tile must be located
below the bottom of the footing to ensure water ponds cannot contact the
concrete;
-
Drain
Tile Not Correctly Sloped - the tile must be sloped to the storm
outlet;
-
No
Drainage Layer - a 3" to 4" granular drainage layer should be
installed over the site to ensure water ponding cannot touch footing
concrete and to create pathway to drain tiles.
To prevent water ponding,
please click here.
For picture of foundations suffering from
rising damp,
please click here.
For a printable report on Drainage & Rising
Damp,
please click here.
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