What would the world be without wood? Almost everything made by Man, from the protective fire of cavemen to elegant parquet floors, reminds us of our ancient dependence on this natural element. That is why in this work we will scrutinize beyond the rings of his time, to know everything about it.
Table of Contents
- 1 Concept and description of wood
- 2 history of wood
- 3 Structure or composition of wood
- 4 Physical properties and uses of wood
- 5 Thermal properties of wood
- 6 electrical properties
- 7 wood hardness
- 8 wood weight
- 9 wood stability
- 10 smell of wood
- 11 Thermal and acoustic insulation
- 12 Mechanical properties of wood
- 13 Wood types
Concept and description of wood
Sure you've ever wondered what is wood. In such a way that we will start this post trying to clarify that doubt. To be very precise, we can define wood as a resource that is extracted from the wood of trees. A resource that is used in countless construction elements, as well as being the oldest of all fuels.
While we can describe wood as an element made up of an intricate set of tissues that in turn make up the mass of tree stems, ignoring their bark.
In order to have a clear idea of its historical value, the description that is usually given to it as the lightest, most ductile and resistant construction element, used by man since time immemorial, is useful.
history of wood
As we anticipated in our introduction, the history of humanity is closely linked to the use of wood.
It turns out that wood was the first element for construction that was available to man. Apart from being used as fuel and a weapon for hunting, it was also of great value in our ancestors' quest to find shelter.
So it was that the hut with wooden supports and a roof of branches provided shelter against the elements. But that was only the beginning, centuries later it would be used in the construction of bridges and ships.
So ancient is our relationship with wood that the art of lamination related to the ornamental use of this element was known by the Egyptians in the year 3000 BC. of C
This originated due to the absence in the area of woods with attributes for construction, which prompted them to create veneer and inlay techniques.
An art perfected
From those remote beginnings until the XNUMXth century, this Egyptian art of plating maintained its artisanal condition. This was due to the fact that it required a high knowledge of the benefits of wood, as well as a meticulous work of cutting and gluing.
Thus, in the XNUMXth century, modern plate-cutting techniques appeared. Later, at the beginning of the next century, this trade will take another important leap with the arrival of new snags. It is then that we know the plywood sheet, with the characteristics that remain to this day.
This sheet is so ductile that it can be easily bent, accepting almost any shape, which adds value to the skill of the construction.
Finally, on this point we can add that the wood, either compact or laminated, was used in the manufacture of both cars and aircraft. As well as in the boat factory.
The new preservatives and adhesives, children of the industrial development of the end of the XNUMXth century and the following one in its entirety, have boosted the wood applications, turning it into durable, strong and ductile elements, which we can now see forming part of almost everything in our environment. But, despite all its uses, it is necessary to have a lot of Environmental awareness so as not to completely deplete this natural supply.
Structure or composition of wood
To advance in this article, it is necessary to talk about the wood structure. A fundamental issue to understand this necessary element.
The first thing to note is that wood is made up of cells that come together and intertwine. They are cells with a tubular appearance and of different lengths.
In such a way, that when making a cross section the following parts can be observed:
Medulla and medullary rays
It represents the central area, which is also the oldest.
This part of the wood is formed by the effects of drying and resinification. It adopts the shape of a cylinder in the axis of the plant. It is composed of circular cells that show obvious pores in their bond angles.
It is a botanical term that refers to the area of the trunk that integrates part of the woody tissue, which is the hardest part of the wood.
It is located in the central part of the stem and branches, being made up of dead cells, such as the outer layer that surrounds it.
More precisely it is the area immediately to the core of the tree, made of hardwood. It is bathed in some of the plant's own compounds, such as tannin, which give it its peculiar pink colour.
The main function of this part of the wood is to provide the plant with a fairly solid structure so that it can support the weight of the trunk and its foliage.
It is also the origin of the ability of wood to act as a structural element in architecture, something that is often compared to that same characteristic of steel.
The term refers to the newest part of the wood. In this sense, it is located in the last growth rings of the plant, something that is produced by the action of the vascular cambium in the tree trunk.
It is the area of the plant trunk that is located under the husk, where the newest growth rings are counted.
It differs from the heartwood in that it is lighter in color and softer in shape. It is also more permeable and moister than its neighbor.
It collaborates in the support of the structure, as well as in the flow of sap and in the collection of reserve elements.
This is the generating mantle, which is found under the shell made up of cavities with very narrow panels, capable of changing shape thanks to successive cell multiplications. This forms the inner faces of the new wood, which also becomes the outer layer of phloem.
The layers of this new wood are made up of spring wood, which is light in color. But it also shows a soft texture, as a result of greater vegetative activity in the spring period and part of the summer.
Also known as rhytidome. It is the layer that externally covers the stems and roots of woody trees.
In turn, it is made up of three sub-layers: the phloem, the phloem and the vascular cambium.
Despite being just an outer layer, it can represent up to 15% of the total weight of the plant.
Its main function is the isolation and protection of plant tissues from atmospheric elements.
Physical properties and uses of wood
This is one of the most important aspects to take into account, if we want to understand how wood can be used in the best way in construction work or in the production of handicrafts.
Nor should we confuse these physical and structural properties with those of medicinal, food, ornamental or many other origins. To be more precise, in the cases that we will address below, we will mention the properties that concern, especially, the construction industry.
About this it is necessary that you know that the properties of the wood will always be a function of its progress, longevity and moisture content, as well as the different kinds of soil in which it is found and the different parts of the log.
But let's see what are the most important physical properties of wood.
Note that the physical properties of wood are not always the same for all paths through a given vertex. In such a way that we can define three fundamental directions in which these properties can be defined and measured, namely:
- Axial: Occurs parallel to the direction of plant growth, which is also known as the fiber direction.
- Radial: This runs perpendicular to the axial, cutting the axis of the trunk.
- Tangential: It can occur in the form of the previous two.
Since wood has hygroscopic conditions, it can absorb or give off moisture, depending on environmental conditions.
The water that is released runs off completely in a certain time. But there remains a part – along with the water of constitution. This is the saturated water, which corresponds to the environmental humidity that surrounds the wood, until a balance is reached.
About what to say that the wood is dried in the open air.
You should also know that the humidity of this natural element can vary in very wide parameters. For example, freshly cut wood has a humidity that can range between 50 and 60%.
This is important, given that variations in humidity allow the wood to expand or contract, thereby changing its volume and density.
The volume of the wood usually changes as its humidity levels vary, which generates, as we have already said, the expiation and contraction, which means a degree of deformity.
Let us remember that wood is an absorbent material, which means that the variation in humidity in the direction of the fibers is almost imperceptible. Although this changes in the transversal sense.
The secret of these changes in the proportions lies in the ability to capture the water in the walls of the woody structure, where the liquid accumulates between the cells, with the effect of separating or attracting them.
Where the saturation point of this series of nerves corresponds to the moisture content, by the time the walls of these woody nerves have absorbed all the water they can absorb. This is the point of greatest cell expansion, so the wood gets maximum volume, which is the same as 30% moisture.
But curiously, the wood is capable of continuing to increase its level of retained water, although this is not reflected in an increase in its volume, since the liquid in this case occupies the capillary area and the tracheids of the woody system. This is what is known as free water.
In addition, the deformations that occur due to changes in the humidity of the wood will be governed by the position that the section in question occupies in the plan. In such a way that different deformations can be observed, both radial and tangential.
Regarding this property of wood, we can say that what is known as actual Density, it is ostensibly the same for all species. In such a way that a common term of 1,56 can be defined.
While apparent density it changes according to the species, although this can also occur in the same species. This phenomenon is determined by the level of humidity and its location in the plant.
Now let's see what those variations are according to the species:
- Wild Pine: between 0.32 and 0.76Kg/dm3
- Black Pine: 0.38 – 0.74Kg/dm3
- Tea Pine Wood: 0.83 – 0.85Kg/dm3
- Fir: 0.32 – 0.6Kg/dm3
- Larch: 0.44 – 0.80Kg/dm3
- Oak: 0.71 – 1.07Kg/dm3
- Oak: 0.95 – 1.20Kg/dm3
- Beech: 0.60 – 0.90Kg/dm3
- Elm: 0.56 – 0.82 Kg/dm3
- Walnut: 0.60 – 0.81 Kg/dm3
To close this point, you must bear in mind that all woods are classified according to their apparent density, as follows:
- heavy woods
- Very light
Thermal properties of wood
Like all materials, wood expands in the heat and constricts in the presence of cold. However, such a phenomenon is not usually observed with the naked eye, since the rise in temperature goes hand in hand with a reduction in humidity.
Then with the reduction of humidity the other becomes imperceptible.
But the movements in the perpendicular direction of the woody nerves also rise. Then the heat exchange will be related to humidity, the specific weight and the species of the plant.
However, a more efficient transmission will be recorded when it runs in the direction of the fibers, instead of following the perpendicular directions.
An important aspect that you should know about wood is that when it is dry it is an excellent insulator of electricity.
It is known that the moisture resistivity level will depend on the direction, which is lower when it is in the direction of the fibers. But it will depend on the species of wood, which is superior in those logs that have oils and resins.
Another factor that influences this variation is the specific weight, because being larger it increases the capacity of wood enlistment.
The hardness of this important raw material can be defined as the tenacity that opposes both wear and scratching, nailing and a long etc… As you can imagine, the older and more rigid it is, the higher the resistance it opposes.
This hardness can be classified as follows:
the very hard
- Holm oak
- pear tree
the soft ones
the very landas
- white poplar
This is another element to consider when selecting the right wood for a building. This will vary according to different factors:
- Humidity: All freshly cut wood weighs more than that which has had time to dry.
- Resin: the resinous wood has a higher weight than the one that does not have this compound.
- tree age: The heartwood of mature plants is thicker and heavier than that of young plants.
- Growth rate: The plank of the plant that develops slower is always stronger and heavier than the one that develops quickly.
- Sapwood existence: This is lighter than the heartwood, so that a piece with sapwood will weigh less than the same piece made up solely of heartwood.
- Density: The more solid the wood, the more woody system and less air the dry sample will show. For this reason, a piece of carob will weigh considerably more than one of identical proportions, but made of a variety of wood that maintains large spaces between the ducts, since these are filled with air in the dry wood. For a better example: balsa wood is extremely light, since more than 90% of its dry volume is air.
Freshly cut wood loses moisture to balance in this aspect with the environment.
The air-drying process can take weeks or even months. This will depend on the level of density of the wood, in addition to its thickness, average humidity of the environment and the speed of the air that circulates between the planks.
In the case of more stable woods, such as teak and mahogany, the shrinkage is less during drying, so they keep their best shape. While those that are not so stable, among these the mamey contract more, so they tend to arch and twist, in addition to presenting the dreaded cracks.
To prevent damage, freshly cut lumber should be placed on pallets and in a shaded spot, where it will not be affected by rain or excessive drafts.
On less stable woods
In the case of less stable wood, the drying process is slower, requiring it to be sawn into thin strips and protected from the wind.
Keep in mind that the stability of the wood will also depend on the growth of the plant, as well as the position of any boards inside the log.
This means that if boards are cut from branches or a log that has grown leaning, then the wood on either side of the center will be different in density. Such a phenomenon will generate an internal tension that can lead to bowing and fraying of the boards.
Another factor that will affect the stability of the wood is the cut that the board received. These must be sawn in a radial direction, taking into account that those woods whose growth rings are shown perpendicular to the surface of the board, are more stable than those cut in a tangential direction. In these cases the rings are more or less parallel to the surface.
smell of wood
Some logs give off a particular aroma when cut. This smell can vary in intensity depending on the location where the tree grew.
As in the case of color, the smell of wood is due to the chemical elements it stores, especially in the heartwood.
For many connoisseurs of the subject, cedar wood is the one that gives off the greatest and best aroma when it is freshly closed. Its scent is derived from the essences of the sap. This has become so famous that it serves as the basis for some of the most renowned perfume companies in the world.
Some even combine it with cinnamon or cloves, to enhance its exotic value.
It is also known that the smell of cedar is used as a decongestant, when nasal problems occur.
Thermal and acoustic insulation
The holes in the wood interrupt the movement of heat through it. This gives it qualities of extraordinary thermal insulation.
In addition, despite its indisputable capacity for combustion, it is known that it can delay the passage of fire, in the case of thicker beams.
While in terms of sound, its insulation properties are not very high, especially when compared to other more efficient materials.
Mechanical properties of wood
Here the most important elements are grouped from the construction point of view. Its due study and its rigorous application will result in a greater stability of the buildings, that is, they will be safer for man.
In this case, different factors act, such as humidity, which must be located below the saturation level of the fibers, which is 30%.
It is noteworthy that the compressive strength will become higher when the humidity level drops. However, from that 30% the resistance becomes constant.
It also influences the direction of the effort. The maximum resistance will be related to the effort exerted in the same direction of the fibers, but which will decrease as it moves away from that direction.
At this point, the most important thing is to know that the fracture in compression is verified by the distance of the wooden columns and their individual arching.
Wood is one of the most suitable materials for tensile work. Its use in elements exposed to this force is only minimized by the difficulty of transferring the traction energy to them.
The anisotropic nature of this wood natural resource also has to do with this particularity. In such a way that the resistance in the parallel direction will be much higher than in the perpendicular direction.
Tension fracture usually occurs suddenly. So in this aspect it can be said that wood is a fragile material.
It could also be ensured that the wood is not at all resistant to bending stress, both radially and tangentially. Although the same does not happen if this effort is applied perpendicular to the fibers.
This is how an element subjected to a bending force deforms, generating a reduction in the dimensions of the upper fibers, while there is an elongation of the lower ones.
When we project any wooden element that will be exposed to bending, in addition to considering that it resists the loads that will act on it, it will be necessary to prevent an exaggerated deformation that could generate cracking of the coating.
For this it would be enough to raise the edge or length of the piece, which increases the rigidity.
To finish, we will see how the trees are grouped according to the classification given according to the different types of wood:
- Holm oak
- Fruit trees
Tropical or African woods