January 4, 2021 chair
For the long term durability of historical wooden buildings, constructors and users who deal with this subject have to know wood properties exactly.
Wood is an organic, hygroscopic and anisotropic material. Its thermal, acoustic, electrical, mechanical, aesthetic, working, etc. properties are very suitable to use it is possible to build a comfortable house using only wooden products. With other materials, it is almost impossible. But wood has some disadvantages too. Following is some very short information about this subject.
BENEFITS OF WOOD
As we know, many, materials change in size and volume as the temperature changes. They expand with increasing of the temperature. This means linear and volumetric expansion. The expansion. The expansion causes decrease in the strength of materials. Steel, which is inorganic and non-combustible and therefore has an advantage against fire, but when used in buildings, it expands and collapses as a result of increase in heat.
Wood does not practically expand against heat. On the contrary, by the effect of heat, it dries out and gains strength. The only time wood expands a little is when the humidity level is below 0%, and this is only scientifically significant. In practice, the humidity level of wood does not drop under 5% even in the driest climate.
The coefficient of thermal conductivity of the wood is very low. Aluminium transmits heat 7000 times, steal 1650 times, marble 90 times and glass 23 times faster than wood. For this reason, wood is used for making matches, handles of hardware equipment, ceilings and wall coverings.
Specific heat of wood is high. That means high amount of energy is needed to increase and decrease the temperature of one-kilogram of wood. Wood requires almost twice amount of heat energy than stones and concrete; similarly, three times of energy is needed for heating or cooling steel.
Sound isolation is based on the mass of the surface. Wood, as a light material, is not very perfect for sound isolation; But it is ideal for sound absorption. Wood prevents echo and noise by absorbing sound. For this reason it is extensively used in concert halls.
Sound velocity is faster in woods than gases and liquids, and it is close to that of metals. Sound energy loss as a result of friction is also significantly low in woods due to its lightness and structure. Because of such properties, wood is extensively used in musical instruments.
Resistance to electrical current of a completely dry wood is equal to that of phenol formaldehyde. An oven dried wood is a very good electrical insulator. To some extent air dried wood is the same. Unfortunately electrical resistance of wood is lowered by increasing the moisture content. The resistance of wood saturated with water. Static electricity that is dangerous for human health is not observed in wood unlike metal, plastic and other materials. For this reason wood is preferred as a healthy material.
Although wood is a light material, its strength is quite high. For instance, while the tensile strength of wood with 0,6/cm3 specific gravity is 100 N/mm2, the tensile strength of steel with 7,89/cm3 specific gravity is 500 N/mm2. Dividing tensile strength by specific gravity gives the breaking length and quality of material. This figure means the breaking length of the material, when hung as a result of its own weight. While the breaking length of steel is used for construction is 5.4 km, chrome mobile steel is 6.8 km, hardened bow steel is 17.5 km, breaking length of spruce wood is 19.8 km, and laminated wood made of beech is 28.3 km. For this kind of properties, wood and laminated wood is used in wide-gap constructions like health centers and sport halls.
Wood is a decorative material when considered as an aesthetic material. Each tree has its own color, design and smell the design of a tree does change according to the way it is sliced. It is possible to find different wooden materials according to color and design preference. It can be painted to darker colors of varnished, and can be given bright or mat touches.
Although wood has oxidation characteristics in some way, it is not the kind of oxidation seen in metals. Metals get rust, wood doesn’t. For such characteristics, use of wood is preferred to avoid rust when necessary.
It is easy to repair and maintain wood. While old woods can be renewed by special touches other materials are highly difficult and costly to maintain and to repair. Therefore they are usually disposed of.
There are more than 5000 kinds of woods in the world. Their specific gravity, macroscopic and microscopic structures are different. Accordingly, their physical, thermal, acoustic, electrical and mechanical properties are also different. Because of this variety, it is possible to find wood suitable for needs. For instance, for heat isolation and sound absorption woods in lightweight are used. Similarly, heavy ones are used for construction purposes.
DISADVANTAGES OF WOOD AND WAYS TO ELIMINATE THEM
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 attack most often wood in ground contact or wood that is intermittently wetted.
They cause damage primarily 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.
Minimizing the Problems of Wood:
Most of the commonly employed strategies for protecting wood involve drying, coating and or impregnation.
Careful selection of wood
Some species have naturally decay resistant heartwood. Such species include sweet chestnut (Castanea sative Mill.), oak (Quercus spp.), juniper (Juniperus spp.). Sapwood is never naturally durable species has little or no decay resistance and must be treated if long-term durability is desired.
Coating provides protection to wood used both indoors and outdoors. Coating prevents rapid uptake and loss of moisture and reduces shrinking and swelling that can lead to surface cracking and other problems. But coating does not totally prevent changes in moisture content. Coating slows, but does not halts moisture level. Coating with solid color or pigmented stains protects wood against ultraviolet rays.
The addition of fungicides to coating provides some protection against development of decay and mold fungi.
Deteriorating paint film actually increases the decay hazard. Cracked paint allows moisture to come into contact with wood surface, and poses a barrier to rapid and complete redrying.
Generally wood will not be attacked by the common fungi at moisture content below the fiber saturation point (FSP). FSP for different wood lie between 20-35%, but 30% is accepted generally: Fungi can not attack wood used indoor and in heated rooms, since the equilibrium moisture content (EMC) is much more below than FSP. e.g. 6%
If wood is soaked in water, wood absorbs water and is saturated with it. Finally there will be no more oxygen in wood. In this situation fungi can not grow in them. This is the main reason why woods are kept in water for a while. Besides underwater constructions, it is impossible to use woods completely wet; so when they are used out of water, they have to be completely dried out to EMC in order to protect them against fungi attack. In heated rooms, where the EMC lie between 5-10%, fungi can not survive on them.
One of the most effective ways to prevent degradation of wood is to thoroughly dry it and keep it dry. The last case is very important since even wood that has been kiln dried will readily regain moisture if placed in a humid environment.
Wood can be dried in air or in some type of dry kiln. Air drying alone is not sufficient for wood items which are used in heated rooms. Therefore kiln drying is necessary. Kiln drying has many advantages: One of them is the killing of staining or wood destroying fungi or insects that may be attack the wood and lower its grade.
Wood that will be used indoor need only be dried to provide for long term protection against rot.
Treating With Wood Preservatives
We can prevent decaying of wood by treating it with wood preservatives. But some of the wood preservatives may harm humans and other creatures. For this reason if wood is used outdoor in situations where it is often wet or in close proximitly to liquid water, then wood must be treated with wood preserving chemicals to achieve long term durability.
Wood preservatives are divided into two groups: Waterborne and oilborne chemicals.
About %75 of wood that is commercially treated today is treated with waterborne salts, and CCA is the compound used in treating for the greatest volume of wood.
Only creosote and pentachlorophenol are effective protecting wood in direct ground contact. These are also the only two oilborne preservatives that provide general protection against decay causing fungi, termites, marine borer and other insects.
Oil based or oilborne preservatives are generally used for treating of wood used outdoors in industrial applications; such as ties, piling and poles.
In a serious situation, wood is treated with waterborne preservatives for example chromated copper arsenate and, after thorough seasoning, is retreated with creosote.
Wood in service must be periodically retreated by brushing or a variety of other methods.
Retreatment of wood window frames, door frames and wood timber and beams is sometimes carried out by drilling holes in areas where decay has begun and filling these holes with a suitable treating compound. Treating compound in the form of solid rods are mostly preferred since it provides a slow release of active ingredients.
Retreatment of wood used in ground contact must be realized by application of pastes and wrapping with preservative impregnate bandages.
Abiotic Deterioration of wood:
Another disadvantage of wood is that it easily catches fire. Wood consists of organic compounds which are composed mainly of carbon and hydrogen. They can combine with oxygen and burns. Because of these properties, wood is classified as a combustible material.
If the temperature of a inflammable gas is between 225°-260°C, it burns with a touch of flame. After the withdrawal of flame it will stop burning. If the temperature increases to 250°-270°C, it burns with a touch of flame and goes on to burn without a flame. If the temperature increases to 330°-520°C, wood begins to burn spontaneously. Chemical materials, especially extractives in woods structure cause the burning point to change. For example, a resinous piece of pinewood can catch fire in lower temperatures. In addition to this, specific gravity and surface mass (m2/kg) affect the duration of flame. Wood burns harder when the specific gravity and surface mass and moisture content increase, and vice versa.
Using thick wood as a structure element is another way of extension of burning point. Outer surface burns and turns into charcoal. Charcoal, which forms on the surface of wood as it burns is a very effective heat insulator. Therefore large timbers burn very slowly. In addition to this, wood is very good heat insulator too. The outer surface of the wood is 1000°C and the interior part is still 40°C when a piece of thick wood is burning. For this reason, buildings with thick structure elements such as beams and columns do not collapse easily on fire. On the other hand, in steel constructions, as heat increases, steel faces deformation, and their resistance decreases and collapses, where wood is used preventive measures must be taken for safety against fire. In this case wood is not a dangerous material.
It is impossible to make wood noncombustible like inorganic materials. In order to prevent potential dangers, wood can be processed in some fire retardants.
Fire retardants may be divided into two categories: Coating and chemicals-water soluble salts-that are impregnated into the wood structure.
Coatings are used to reduce the formation of volatile, flammable gases by promoting rapid decomposition of the wood surface to charcoal and water. They also protect wood surface against high temperature water soluble salts e.g. diammonium phosphate, ammonium tetraborate, sodium acetate, alkali silicates, borax are used against fire hazards in wood. Wood can be impregnated by these chemicals. This type of process can contribute to the increase of the burning point and retard spread and penetration of flame.
Fire retardants only reduce the flammability of wood and slow or eliminate progressive combustion. They do not prevent burning totally in the presence of an external source of fire. In this case, wood does not go on burning once an external source of flames is removed.
Prof. Dr. Ramazan ÖZEN
President, Zonguldak Karaelmas University