In the last post, we have already known about advantages of wood as a building material. However, there are some disadvantages of wood but they are easy to disregard, and eliminate as long as the cause is known.
Shrinkage and Swelling of Wood
Wood is a hygroscopic material. This means that it will adsorb surrounding condensable vapors and loses moisture to air below the fiber saturation point.
Deterioration of Wood
The agents causing the deterioration and destruction of wood fall into two categories: Biotic (biological) and abiotic (non-biological).
Biotic agents include decay and mold fungi, bacteria and insects.
Abiotic agents include sun, wind, water, certain chemicals and fire.
Biotic Deterioration of Wood
Woods are organic goods. Like any organic good, wood is a nutritional product for some plants and animals. Humans can not digest cellulose and the other fiber ingredients of wood, but some fungi and insects can digest it, and use it as a nutritional product. Insects drill holes and drive lines into wood. Even more dangerously, fungi cause the wood to decay partially and even completely.
Biological deterioration of wood due to attack by decay fungi, woodboring insects and marine borers during its processing and in service has technical and economical importance.
It is necessary to give some short information about fungi agents to take measures against the wood deterioration.
Physiological requirements of wood destroying and wood inhabiting fungi:
A favorable temperature.
The temperature must be 25-30°C for optimum growth of most wood rotting fungi. But some of them can tolerate temperature between 0-45°C.
An adequate supply of oxygen
Oxygen is essential for the growth of fungi. In the absence of oxygen no fungi will grow. It is well known that storage of wood under water will protect them against attacks by fungi.
Generally wood will not be attacked by the common fungi at moisture contents below the fiber saturation point. The fiber saturation point (FSP) for different wood lies between 20 to 35% but 30% is accepted generally.
It is recommended that wood in service must have a moisture content at least 3% less than FSP to provide desirable safety against fungi.
Wood is an organic compound and consists of 50% carbon. That means that wood is a very suitable nutrient for fungi because fungi derive their energy from oxidation of organic compounds. Decay fungi wood rotters can use polysaccharides while stain fungi evidently require simple forms such as soluble carbohydrates, proteins and other substances present in the parenchyma cell of sapwood. Additionally, the presence of nitrogen in wood is necessary for the growth of fungi in wood.
Insects are only second to decay fungi in the economic loss they cause to lumber and wood in service. Insects can be separated into four categories: Termites, powderpost beetles, carpenter ants and marine borers.
There are two types of termites: Subterranean termites damage wood that is untreated, moist, in direct contact with standing water, soil, other sources of moisture.
Dry wood termites attack and inhabit wood that has been dried to moisture contents as low as 5 to 10%. The damage by dry wood termites is less than subterranean termites.
Powderpost beetles attack hardwood and softwood. At risk is well seasoned wood as well as freshly harvested and undried wood.
Carpenter ants do not feed on wood. They tunnel through the wood and create shelter. They attact most often wood in ground contact or wood that is intermittently wetted.
They cause damage primarly to unpainted wood by creating large tunnel in order to lay eggs.
They attack and can rapidly destroy wood in salt water and brackish water.
...[to be continued]
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