Wood as a Construction Material

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Wood is an organic material, and has a wide range of physical properties that can vary significantly from species to species or even within species depending on the conditions under which the wood was grown. Degradation of wood can be grouped into two broad categories: biological deterioration from fungal decay or insect attack, and mechanical deterioration.

Biological Deterioration
Fungi are classified into three categories depending on the nature of the degradation they cause: molds grow on the surface of wood with little effect and can be easily removed; staining fungi penetrate the cellular structure damaging cell contents and walls, reducing strength and stiffness; and decaying fungi can significantly reduce wood's strength by penetrating its cellular structure, and can destroy its chemical composition by consuming cell contents.

In order to grow and propagate, decaying fungi require adequate supplies of oxygen, food and moisture, and temperatures between 20 and 30 degrees Celsius. Areas prone to fungal decay include situations on which water can collect, such as horizontal surfaces, or checks and splits in the wood. Wood that is in direct contact with the ground, with water, or with concrete or asphalt, is also vulnerable, as are locations where two or more wood members butt tightly together, and where debris, bird or animal droppings accumulate.

Wood affected by some decaying fungi will lose structural strength before the decay is even visibly evident. Active advanced decay can be detected and identified by looking for damage to the wood, including cross cracks, stringy and fibrous appearance, staining, brown cubical deterioration, and change of colour and odour. Symptoms of decay can also include staining on paint coatings, local crushing of the wood, and paint failure. The presence of fungal fruiting bodies on the surface of wood is also a clue.

If the spores from fungal fruiting bodies fall on a moist wood surface and meet other favorable growth conditions, they are capable of developing and producing new fungal plants. Thus, disease can be spread from one piece of wood to another without direct contact between sound and infected material.

A number of insect species (such as beetles, carpenter ants, termites, wood wasps, and carpenter bees) can significantly damage wood and reduce its strength. The typical process is initiated when a female insect lays eggs in or on the surface of the wood. The eggs hatch into larvae which then tunnel through and feed on the wood. Upon maturity, the adults emerge, leaving the wood surface perforated with small, round 'flight holes'. The feeding actions of the insects create voids in the wood and can severely degrade the structural integrity of the wooden element. Detecting infestations of some insects can be difficult as their flight holes are very small, and determining if a flight hole is from an active or past infestation is almost impossible. Frass from the flight holes, which looks like fine sawdust, can indicate activity.

Conditions that are favorable to insect infestation (such as damp or wet wood and accumulations of rotting organic debris) can be eliminated. Preventive measures using insecticide and proper maintenance of the wood elements to deny suitable habitat and access, are usually the most effective approaches to preventing insect damage. When an infestation is present, the first step is to find the location of the colony, and then to determine why it is there. When it is time to eradicate the colony, use a 'poisoned' bait that the insects will carry back, pass around, and eventually destroy the colony.

Mechanical Degradation
Mechanical degradation of wood can include weathering, mechanical wear, and structural failure. The degradation process of wood can be influenced by the presence of naturally occurring growth defects within the wood, by problems related to the conversion process from logs to dimension lumber, or by defects introduced into the wood by the seasoning or drying processes used in preparing the wood. The use of wooden members with spiral or diagonal cross grains is undesirable for flagpoles, frames, crosses and plaques because those types of grains generate twisting of the material and reduce strength and stiffness along the long axis of the wooden component.

Weathering is a generic term for degradation from exposure to atmospheric elements including ultraviolet radiation, moisture, temperature, chemical gasses, and windborne grit. Cracking and splitting of wooden components can result from various processes initiated by extended exposure to ultraviolet radiation, moisture, temperature changes, and temperature extremes. Protective coating can be damaged by atmospheric gasses that accelerate natural oxidation. Weathering often results from combinations of these factors. The application and maintenance of paint, stain or other protective finishes will prevent weathering.

Mechanical wear results in the loss of material. It can by caused by human traffic, windborne grit, sand or dust, impacts from maintenance equipment, deliberate vandalism, cables and ropes swinging in the wind, and animals or birds chewing or tearing at the wood. Protection against mechanical wear can include the application and maintenance of protective coatings, and the restriction or control of damaging activities.

Structural failure occurs when wood is subjected to stress levels that exceed its strength. The natural strength of wood can be reduced by any of the wood degradation processes discussed above. Indicators of structural failure include: sagging, splitting, or crushing of wooden members; leaning structures; and the appearance of new openings or gaps between different parts of a structure.

Additional information can be found in the following sources:

Canadian Building Digest, published by the National Research Council of Canada. Of particular interest are: CBD-85, Some Basic Characteristics of Wood; and CBD-111, Decay of Wood. These documents are available online at: www.irc.nrc-cnrc.gc.ca/cbd/cbd-e.html

Building Performance Series, published by Canadian Wood Council. These documents are available online at: www.cwc.ca

Wood Durability, published by Forintek Canada Corp. This document is available online at: www.durable-wood.com