Moisture Behavior of Slab-On-Ground Structures
Leivo, V.; Rantala, J. (2003)
Tampere University of Technology
2003
Rakennustekniikan osasto - Department of Civil Engineering
This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-2011041510613
https://urn.fi/URN:NBN:fi:tty-2011041510613
Tiivistelmä
The thermal and moisture conditions of slab-on-ground structures differ from other structures. The ground structure is in contact with warm and moist drainage and fill soil layers. The subsoil or drainage layer around the ground structures becomes a moisture source under normal thermal and moisture conditions. The bonding and transforming mechanism of moisture and especially the amounts of transformed moisture depend on the temperature field of the structure. Therefore temperature and moisture related phenomena should be examined together.
The drainage and fill soil layers are either in direct or indirect contact with large water storages of subsoil, ground water or upper ground water tables and gravitational water layers. Coarse soil layer containing fine fractions can convey large amounts of water from these water sources by capillary transformation not only vertically but also horizontally. The propagation speed of the horizontal capillary water front from the gravitational water layer in the drainage soil satisfying requirements of class 2 drainage material is about 30 cm per hour, and the amount of transformed water is up to 150 kg/m3. The capillary movement of water in soil layers contacting ground structures should therefore always be prevented using a coarse and sufficiently thick capillary breaking layer. The relative humidity of the pore air in drainage soil is always close to the saturated volume (RH = 100%) because the pore structure of the soil is in contact with moisture sources and there is always some gravitational or capillary water transforming at the soil layers. The temperature of the drainage soil layer under the slab-on ground structure in a heated building is about +15…+17°C, depending on the used thermal insulation and indoor temperature. The temperature behavior of the drainage soil layer follows the temperature changes of the indoor air and ground slab quite precisely. Annual changes in outdoor temperature does not affect the temperatures of the drainage soil layer very much.
A temperature and moisture content difference forms between parts of a slab-on-ground structure. These differences tend to steady with flows through the structure. The water vapor content of the warm and moist drainage soil layer (Tt≈+16°C, RHt≈100%) is higher than the water vapor content of common indoor air (Ts≈+20°, Rhs=20...40%). Therefore the water vapor diffusion flow at the slab-on-ground structure is almost in every case upwards from the subsoil towards the dryer indoor air. Increasing diffusion flow increases the risk of saturation, or the critical moisture content of some material layers can be exceeded. In many cases the most critical part of the structure is the lower surface of the floor covering, especially if the used covering related has a too low water vapor resistance or it is too water vapor tight.
Three different states should be examined in the moisture design of slab-on-ground structures:
1. The drying phase when the structural moisture from the cast slab should be able to dissipate to the indoor air or subsoil. During this phase the moisture source is the structure itself.
2. The operating phase when the warm and moist subsoil and drainage soil layers are the moisture source of the slab-on-ground structure. The increase of relative humidity and the risk of saturation should be taken into consideration. The structure should be able to evaporate the moisture diffusing from the subsoil.
3. Damage situation (for instance plumbing leak) when the moisture source in inside the structure and the excess water should be able to escape either to the subsoil or indoor air.
The drainage and fill soil layers are either in direct or indirect contact with large water storages of subsoil, ground water or upper ground water tables and gravitational water layers. Coarse soil layer containing fine fractions can convey large amounts of water from these water sources by capillary transformation not only vertically but also horizontally. The propagation speed of the horizontal capillary water front from the gravitational water layer in the drainage soil satisfying requirements of class 2 drainage material is about 30 cm per hour, and the amount of transformed water is up to 150 kg/m3. The capillary movement of water in soil layers contacting ground structures should therefore always be prevented using a coarse and sufficiently thick capillary breaking layer. The relative humidity of the pore air in drainage soil is always close to the saturated volume (RH = 100%) because the pore structure of the soil is in contact with moisture sources and there is always some gravitational or capillary water transforming at the soil layers. The temperature of the drainage soil layer under the slab-on ground structure in a heated building is about +15…+17°C, depending on the used thermal insulation and indoor temperature. The temperature behavior of the drainage soil layer follows the temperature changes of the indoor air and ground slab quite precisely. Annual changes in outdoor temperature does not affect the temperatures of the drainage soil layer very much.
A temperature and moisture content difference forms between parts of a slab-on-ground structure. These differences tend to steady with flows through the structure. The water vapor content of the warm and moist drainage soil layer (Tt≈+16°C, RHt≈100%) is higher than the water vapor content of common indoor air (Ts≈+20°, Rhs=20...40%). Therefore the water vapor diffusion flow at the slab-on-ground structure is almost in every case upwards from the subsoil towards the dryer indoor air. Increasing diffusion flow increases the risk of saturation, or the critical moisture content of some material layers can be exceeded. In many cases the most critical part of the structure is the lower surface of the floor covering, especially if the used covering related has a too low water vapor resistance or it is too water vapor tight.
Three different states should be examined in the moisture design of slab-on-ground structures:
1. The drying phase when the structural moisture from the cast slab should be able to dissipate to the indoor air or subsoil. During this phase the moisture source is the structure itself.
2. The operating phase when the warm and moist subsoil and drainage soil layers are the moisture source of the slab-on-ground structure. The increase of relative humidity and the risk of saturation should be taken into consideration. The structure should be able to evaporate the moisture diffusing from the subsoil.
3. Damage situation (for instance plumbing leak) when the moisture source in inside the structure and the excess water should be able to escape either to the subsoil or indoor air.