Civil Engineering Reference
In-Depth Information
dampness, temperature and acidity. Dampness in organic material need to be
from 18-25 per cent. Dampness quotients above and below these figures are not
attractive for these spores. The majority of fungi, however, survive long dry peri-
ods. A temperature between 20 and 35°C makes an attack possible - attacks can-
not happen below 5°C. Disintegrating fungus does not strike in environments
with a high alkaline content, i.e. with a pH over 6.0. One exception is the fungus
Merulius lacrymans
.
There are four principal ways to avoid attack from insects and fungus:
• Use of high quality material in exposed locations
• Structural protection of exposed materials
• Preventive treatment of materials and passive impregnation
• Use of impregnating substances: active impregnation
Impregnating substances are usually divided into insecticides and fungicides.
Their main task is chemically to prevent or kill vermin and micro-organisms. If
the guiding principle for creating the substances was simply to make surfaces
uncomfortable for insects or fungus, then impregnation would not be a cause of
environmental concern. But the whole concept behind them is the creation of bio-
logical poisons that kill, something that has caused unforeseen consequences for
other animal species, not least mankind. The main task of this chapter is to show
how impregnation can be avoided. The main subject will be timber, as this is the
most widely-used organic material in the building industry, especially in north-
ern Europe. Other organic products will also be discussed. Fungicides in paint
are discussed in the previous chapter.
Choosing quality material
In old trees a large part of the trunk consists of heartwood, which has a strong
resistance to fungus and insects. Not even the house longhorn beetle can pene-
trate the heartwood of pine. Heartwood was traditionally used in log construc-
tion and external panelling, and until the nineteenth century in windows and
doors.
Initially pine was thought to be more durable than spruce, but this conclusion
has been modified. The core of pine has almost no moisture absorption capacity,
whereas the sapwood has a moisture absorption, lengthwise in the cells, 10 times
greater than that of spruce. Pine cladding from the young core is therefore less
protected than spruce. Birch cladding is even weaker, with a permeability about
1000 times greater than that of spruce. Generally speaking, the absorption of
moisture increases in relation to the breadth of the growth rings.
