WOOD - Materials for Civil and Construction Engineers - page 419

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Fungi
Most forms of decay and sap stains are the result of fungal growth. Fungi
need four essential conditions to exist: food, proper range of temperature,
moisture, and oxygen. Fungi feed on either the cell structure or the cell con-
tents of woody plants, depending on the fungus type. The temperature range
conducive for fungal growth is from 5°C to 40°C (40°F to 100°F). Moisture
content above the fiber saturation point is required for fungal growth. Fungi
are plants and, as such, require oxygen for respiration. Fungi attack pro-
duces stains and/or decay damage .
To protect against fungal attack, one of the four essential conditions for
growth needs be removed. The most effective protection measure is to keep
the wood dry by using coatings or by correct placement during storage and
in the structure. Fungi can also be contained by treating the wood fibers
with chemical poisons through a pressure treatment process.
Construction procedures that limit decay in buildings include the
following:

10.11.1

1. Building with dry lumber that is free of incipient decay and excessive
amounts of stains and molds
2. Using designs that keep the wood components dry, using wood treated
with preservatives
3. Using a heartwood from decay-resistant species in sections exposed to
above-ground decay hazards
4. Using pressure-treated wood for components in contact with the ground.

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10.11.2 Bacteria

Bacteria causes “wet wood” and “black heartwood” in living trees and a gen-
eral degradation of lumber. Wet wood is a water-soaked condition that oc-
cupies the stem centers of living trees and is most common in poplar,
willows, and elms. Black heartwood has characteristics similar to those of
wet wood, in addition to causing the center of the stem to turn dark brown
or black.
Bacterial growth is sometimes fostered by prolonged storage in contact
with soils. This type of bacteria activity produces a softening of the outer
wood layers, which results in excessive shrinkage when redried.

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10.11.3 Insects

Beetles and termites are the most common wood-attacking insects. Several
types of beetles, such as bark beetles, attack and destroy wood. Storage of the
logs in water or a water spray prevents the parent beetle from boring. Quick
drying or early removal of the bark also prevents activity by beetle attack.
Damage can be prevented by proper cutting practices and dipping or spray-
ing with an appropriate chemical solution.