High Strength Concrete

The production of high strength concrete relies on three main effects (Gielen,1997):

1. A reduced water/cement ratio always increases the strength. This is done by reducing the tension of the water by adding small amounts of waterglass, natron and/or soda, see page 68.

2. With smaller size particles used as aggregates, the total bonding surface with the cement is largerand less porous.

3.The use of additional fly-ash, silica dust or other pozzolanas can increase the bonding strength.

When the strength of concrete is improved, the weight of concrete building elements can be reduced. A well-mixed high strength concrete can, for example, reduce the amount of concrete needed for columns with axial loads by 30%. For floor elements the effective span can be increased by 40% (Kram etal., 200').

Reinforcement

Steel is the material almost universally used to reinforce concrete. The steel content ranges from 40 kg per m3 up to 200 kg per m3 of

Table 14.4 Moisture buffer value (MBV) of different materials used on indoor surfaces.

Material/product

MBVpractical (g/(m2 %RH))

Dp 1% RH for 24 h (mm)

Concrete

0.35-0.42

6

Cellular concrete (aerated)

0.96-1.11

52

Gypsum

0.57-0.69

33

Brick (well fired)

0.39-0.69

104

Spruce boards

1.15-1.22

16

Birch panels

0.91-1.03

14

Dp decribes the actual penetration depth per 1% change In relative humidity (RH) during 24 hours. All materials are untreated. (Source: Rode, 2005)

Dp decribes the actual penetration depth per 1% change In relative humidity (RH) during 24 hours. All materials are untreated. (Source: Rode, 2005)

The moisture regulating ability of a material is defined by its moisture buffervalue (MBV) (Table 14.4). The moisture buffer value indicates the amount of water that is transported in or out of a material per open surface area, during a certain period of time, when it is subjected to variations in relative humidity in the surrounding air. When the moisture exchange during the period is reported per open surface area and per % RH variation, the result is the MBVpractica|. The unit for the MBVpractica|is kg/(m2 %RH) (Rode et al., 2005). One must remember that most surface coverings and treatment materials such as paints can block the MBV drastically and should be avoided (see Chapter 18).

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