Civil Engineering Reference
In-Depth Information
Since forestry has been industrialized, the quality of timber has reduced con-
siderably, and the need for impregnating substances has rocketed over the last
few years. New fashions in architecture, which include exterior timber struc-
tures, have accelerated this trend.
The following functional qualities are expected of a good impregnating sub-
stance, independent of the organic material it is to protect:
• Enough poison to prevent attack from fungus and insects; wood ants
(
Camponotus herculeanus
) are not usually deterred, whatever poison is used
• Not poisonous to people or animals
• Ability to penetrate into the material
• Resistant to being washed out or vaporized from the material
• Free from damaging technical side-effects such as miscolouring, corrosion of
nails, etc.
An impregnating substance with all these qualities does not exist. Effective poi-
sons such as metal salts have particularly damaging effects on humans. Less
damaging substances such as bark extract and cooking salt are at the same time
less effective.
Preventive impregnating agents must be differentiated from biological poi-
sons, which are used after the material has been attacked. The same material can,
however, often be used in both cases. In Table 19.3, both main groups are treated
as one group. The poison categories 'medium' and 'high' represent strong bio-
logical poisons. There is generally a clear connection between a poison's strength
and its effectiveness. Different impregnating poisons are used in larger or small-
er proportions in different mixtures, often in reciprocal combinations. To make
the mixtures fully effective, both fungicide and insecticide may be needed in the
same mix. They are dissolved in water or solvents. The substances are applied to
the timber by pressure impregnation or by brushing on. About 90 per cent of
pressure impregnation uses water-soluble metal salts; the rest uses solvent-based
creosote. For external application, solvent-based derivatives of oil are most com-
monly used.
Apart from creosote, permetrine is the most common oil derivative and has
superseded such derivatives as pentachlorophenol, which were phased out dur-
ing the 1980s and 1990s because of environmental and health risks. It is used
mainly to protect timber, but also to protect against moths in woollen blankets.
The most important salts for pressure impregnation are arsenic, chrome and
copper. There are different classes of impregnating substances; timber in contact
with the ground requires strong substances in large doses, but in well-ventilated,
outdoor cladding a much weaker mix will be effective enough. There is a clear
tendency today to choose an undifferentiated all-round impregnating agent,
