Wood hardness and stability
One of the most important factors conditioning the maintenance of the wooden floor is the species of tree as well as its physical and mechanical properties: stability, hardness, resistance, density, thermal conductivity, etc. These properties differ with each trees species; furthermore, the results often vary within the range of the same species of wood.
- Factors Which Influence Timber Characteristics Within The Same Wood Type
- Hardness of Wood
- Stability of Wood
- Average Hardness and Stability of Various Wood Species [!]
Factors Which Influence Timber Characteristics Within The Same Wood Type
- Tree growth area. The properties characteristic to the same species of trees may vary depending on the area and conditions of growth. As a rule, the rate of the tree growth in the colder northern regions is slower, therefore the annual growth rings are closer to each other and such wood often characterized by the higher density that conditions higher hardness and resistance of the future products.
- Tree age. The older wood generally has higher density. Though the bamboo is a grass, this rule is applicable to it as well, therefore it is desirable that the bamboos not younger than 4 years were used for the production of the bamboo floor.
- The properties of wood are also conditioned by the cutting area, i.e. what particular place of a tree wood was cut from. It has been determined that in some trees the density of wood increases in the direction from heartwood towards bark, whereas in some trees is it vice versa. The density of wood also varies depending on the height of the trunk.
- Cutting type. There are three main ways of cutting the wooden floor.
Timber cutting types
- Radial – longitudinal cutting perpendicular to the tree rings. The characteristic feature of the radial cut wooden floor is the uniform straight texture of the tree rings, because tree rings make approximately 90° angle toward the floor surface.
- Tangential – longitudinal cutting not perpendicular to the tree rings. Such floor has vivid arched pattern of the growth rings. In such floors tree rings don’t form right angle toward the floor surface.
- Transversal – wood cutting perpendicular to the tree-trunk and direction of fibre.
Board of radial cut
Board of tangential cut
It has been determined that the hardness and stability rates of wood of radial cut are slightly higher than those of the wood of tangential cut, therefore such floor will be harder and less sensitive to the climatic changes.
Considering the above listed factors all indexes of wood physical and mechanical properties, mentioned in this text, should be taken us average rates, that were obtained after a number of measurements. If these rates are known, judgements about the maintenance peculiarities of the specific species of trees can be made.
Although, timber can be characterized by a number of properties, the main of them, that influence the durability and exploitation of wooden floor, are hardness and stability.
HARDNESS OF WOOD
Hardness measuring by Brinell method
F – pressing force
D – ball diameter
d – indentation diameter
Hardness is the ability of wood to with-stand indentations caused by harder bodies (e.g. heels or furniture feet). The durability of wooden floor, resistance to wear, bumps of falling things and scratching depend on wood hardness.
Hardness of wood is usually determined using the so-called Brinell test method: the steel ball is pressed into the surface of wood with certain force, afterwards the indentation left by the ball is measured and the value of the Brinell hardness number is calculated. The harder is the wood, the higher is the number.
Hardness measure unit HB (Hardness Brinell). 1HB equals to 10 MPa or 10 N/mm². In the tables this coefficient is usually given without any dimension.
STABILITY OF WOOD
Wood stability is the property conditioning how much and how fast the dimensions of wood change if the moisture content in it changes. It should be noted that the moisture content in wood may vary due to the change in climatic conditions indoors and also due to direct contact with water (floor cleaning with wet cloth) or wet surfaces (laying floor on wet base). This can be explained by the hygroscopicity, i.e. its capacity to absorb from the ambience or release it. The absorption or release of moisture takes place until the moisture content in wood corresponds to the relative humidity and indoor temperature. Furthermore, under the conditions of constant relative humidity that corresponds to the moisture content in wood, the level of moisture content in the wooden floor will not change.
There are two types of changes in wood: shrinking and swelling.
Shrinking is the reduction of the dimensions and volume of wood due to the reduction of water content in wood. Shrinking of wood is conditioned by its storing in dry air when the bound water present within the cell walls of wood is released to the ambience. Due to the loss of bound water the thickness and dimensions of the cell walls reduce. The wooden floor shrinkage results in the gaps between individual boards.
Swelling is the increase of the dimensions and volume of wood due to the increase of moisture content in wood conditioned by wood storing in wet air or water. Such being the case, this property is inverse to shrinking, however the applicable rules are the same. Swelling results in warpage or uplift of wooden floor.
How much may the parameters of wood change, depends on wood type. The following table shows the dimensional change of some wood types.
| Table 1. Average change rates of unglued timber parameters, measured when wood moisture changes from 8 to 9 percent and vice versa. | |
| Type of Wood | Average Change of Wood Parameters % |
| Oak | 0,25 |
| Ash |
0,22 |
| Beech |
0,32 |
| Maple | 0,30 |
| Birch | 0,24 |
| Merbau | 0,21 |
| Cherry | 0,23 |
| Walnut |
0,20 |
Usually, shrinking and swelling of wood indirectly depends on the wood density: the larger is the number of cell walls in the volume unit of wood, the larger is the amount of bound water that can be absorbed or released.
The factors of wood shrinking and swelling also depend on the way of wood cutting: these rates are lower for the floor of radial cut than for the floor of the same wood but of tangential cut. It should be noted that the dimensions of wood change mainly in the transverse direction of boards, and in the longitudinal direction it is hardly felt.
Shrinking and swelling process takes different periods of time for each species of trees depending on the rate of moisture absorbability (for example, oak – 30 days, ash – 20 days, beech – 14 days, maple – 12 days).
Based on the stability level all types of wood can conditionally be divided into three groups: low stability, medium stability and high stability. If the stability of the specific wood is known, judgements can be made about how much the laid wooden floor can expand or contract, what the likelihood of warpage or gap occurrence.
The following table shows the approximal changes of wood moisture, depending on conditional air moisture and temperature.
| Table 2. Relative change of moisture content in wood, %, depending on the climatic conditions according to Kailvert (the figures in the table show the moisture content in wood) | |||||||
| Conditional Air Moisture % | Temperature | ||||||
| 10°C | 15°C | 20°C | 25°C | 30°C | 35°C | 40°C | |
| 90 | 21,1 | 21,0 | 21,0 | 20,8 | 20,0 | 19,8 | 19,3 |
| 85 | 18,1 | 18,0 | 18,0 | 17,9 | 17,5 | 17,1 | 16,9 |
| 80 | 16,2 | 16,0 | 16,0 | 15,8 | 15,5 | 15,1 | 14,9 |
| 75 | 14,7 | 14,5 | 14,3 | 14,0 | 13,9 | 13,5 | 13,2 |
| 70 | 13,2 | 13,1 | 13,0 | 12,8 | 12,4 | 12,1 | 11,8 |
| 65 | 12,0 | 12,0 | 11,8 | 11,5 | 11,2 | 11,0 | 10,7 |
| 60 | 11,0 | 10,9 | 10,8 | 10,5 | 10,3 | 10,0 | 9,7 |
| 55 | 10,1 | 10,0 | 9,9 | 9,7 | 9,4 | 9,1 | 8,8 |
| 50 | 9,4 | 9,2 | 9,0 | 8,9 | 8,6 | 8,4 | 8,0 |
| 45 | 8,6 | 8,4 | 8,3 | 8,1 | 7,9 | 7,5 | 7,1 |
| 40 | 7,8 | 7,7 | 7,5 | 7,3 | 7,0 | 6,6 | 6,3 |
| 35 | 7,0 | 6,9 | 6,7 | 6,4 | 6,2 | 5,8 | 5,5 |
| 30 | 6,2 | 6,1 | 5,9 | 5,6 | 5,3 | 5,0 | 4,7 |
| 25 | 5,4 | 5,3 | 5,0 | 4,8 | 4,5 | 4,2 | 3,8 |
Table 2 shows how the moisture content in wood changes depending on the relative humidity and temperature changes. The table shows that the changes in the relative humidity in winter condition much higher variations in the moisture content than the changes in temperature: when the relative humidity changes from 45 to 60 percents, the moisture content in wood will change by about 2.5 percents. Whereas when the air temperature changes from 10°C to 25°C, the moisture content will change only by about 0.5 percent.
Referring to the European standards, the wooden flooring should be dried to 7 – 11 percents of moisture content. During the maintenance of such floor, the relative humidity should be approximately 40 – 60 percents, and the temperature - 18 - 24°C. Under such conditions the change in the parameters of the wooden floor having 7 -11 percents moisture content will be minimal.
These rates are not accidental, since the 40 to 60 percents relative humidity is optimal for human health. Living in such relative humidity strengthens immunity and stress resistance, improves health, maintaines skin and hair moisture balance, decreases the risk of chronical illnesses and alleriates their course.
AVERAGE HARDNESS AND STABILITY OF VARIOUS WOOD SPECIES
Chart of Average Brinell Hardness and Stability of Wood Types (chart is made under descriptions of wood types, data given by producers and also by our own experience).
