Important Factors In The Physics Of Building Materials

In every building project it is important to have a clear picture of a material's physical properties. There will be different demands on the different groups of materials. The following are the central technical specifications (see also Table 4.3):

• Weight indicates what structural loading can be anticipated in the building, which building techniques can be used, etc. Each material has its characteristic density.

• Compressive strength is an expression of how much pressure the material tolerates before failing, and is of particular importance in the design of columns, arches and other vertical structural elements.

• Tensile strength expresses how much a material can be stretched before collapsing. This is important for the calculation of horizontal structural elements such as beams or floors, and suspended structures.

• Thermal conductivity describes a material's ability to conduct heat. It describes the insulating properties that can be expected of this material as a layer within an external wall for example. The conductivity of a material is dependent upon the weight of the material, the temperature, its moisture content and structure.

• Thermal capacity of a material is its ability to store heat or cold. This often helps to even out the temperature swings in a building and also in many cases reduce energy consumption. Thermal capacity is directly proportional to a material's weight.

• Air permeability indicates how much air will move through a material under different pressure conditions. It depends upon a material's

Table 4.3 Aspects in the physics of building materials

Structural materials

Climatic materials

Surface materials

Surface treatment

Weight

x

x

x

Compressive strength

x

(x)

(x)

Tensile strength

x

(x)

(x)

Thermal conductivity

(x)

x

(x)

(x)

Thermal capacity

(x)

x

(x)

Air permeability

(x)

x

(x)

Vapour permeability

(x)

x

(x)

(x)

Moisture regulating quality

(x)

x

x

(x)

Notes: x: primary aspect; (x): secondary aspect.

Notes: x: primary aspect; (x): secondary aspect.

porosity - the size and the structure of its pores. The moisture content of the material also plays an important role since water in the pores will prevent air passing through. Air permeability is particularly important when making a building airtight.

• Vapour permeability gives the equivalent picture of water vapour penetration under different pressures. This can vary according to the material's moisture content and temperature, and is a decisive factor in the prevention of damage caused by damp.

• Moisture regulating quality describes a material's ability to absorb and release humidity. A measurement often used for this is the moisture buffer value (MBV). It is closely related to the vapour permeability, but also to density, porosity and sorption properties. Materials with a high moisture buffer value can be used positively to reduce the humidity in the indoor climate. This can both improve health and reduce building damage caused by humidity, including growth of moulds and fungi as well as humidity-induced chemical reactions (Rode et al., 2005). In order to be effective, the paint or surface treatment on such materials must also have a high vapour permeability.

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