The purpose of wood in constructions

Service Description

The purpose of wood in constructions

The wood is one of the great natural treasures of our country. It is a main construction material which should be used with care, in order not to be scattered. Saving the wooden material is an important task of our building sites.

To be used in construction, the wood must accomplish certain quality conditions. The properties of wood should be well known on site, in order to distinguish between the good wood and the one with defects.

The wood properties

The wood properties depend largely on the composition of it. Some properties can be observed through a exterior wood research (weight, moisture, appearance, colour). These are called physical properties.

Other properties can be observed by processing or experimenting the wood (Strength, durability, elasticity, wear, resistance). These are called mechanical properties.

The wood weight

The weight of a cubic meter of wood is called specific weight.

Because the wood stacked in a cubic meter shows the gaps between them, this weight is also called specific weight in stack. If the wood of a cubic meter are placed without gaps between them, the weight of wood is called the apparent specific weight (in this case the wood has pores and channels).

The weight of the wood varies by its essence, by the way of tamping the wooden material after his composition. It also depends on the amount of water contained by the wood (moisture degree).

The wood is generally lighter than water and therefore it floats on water. The soft wood (the spruce, the fir, the pine) weigh 450-600 kg/mc in dry condition and in wet condition to 750 kg / mc.

The hard wood (oak, beech, hornbeam, ash, black locust) weighs in dry condition 600-800 kg / mc and in wet condition up to 1000 kg / mc. In a wagon of 10 tonne sit can be loaded typically 22-24 mc of fir or spruce dry timbers and 14 to 16 mc of oak tree.

The wood humidity

The amount of water which the wood contains is called moisture. It is calculated in percentage unlike the weight of wood in dry condition.

In terms of moisture, the wood is split as follows:

– Green construction wood with humidity greater than 35%

– Semi-dry construction wood, with humidity of 20-35%,

– Air-dried construction wood with 12-20% humidity

– Completely dry wood, with humidity of 0% (it can not be get than in laboratory).

The fresh cut wood has a moisture percentage of about 50%. The woodcut and left outdoors loses moisture until it gets equal to the air humidity. In this condition it is obtained theair-dried wood.

During hot summers, the moisture of the wood decreases to 12%, namely until the lowest humidity to the air. The normal humidity for our climate is 15% and it represents the quantity of water contained by the air-dried wood. During the autumn the moisture of wood increases because it absorbs some part of the air humidity.

The wood kept in natural conditions can not be completely dry, because the air humidity is never lacking. The changing of the wood humidity is accompanied by enlargement or reduction of volume.

The wood has the ability to decrease its volume (decrease) when it is kept in a dry place, or to increase its volume (swell) when it is kept in a wet place.

Reduction of the wood, called the wood contraction is due to evaporation of contained water. In this way, the wood vessels are lowering, leading to reducing its size. This contraction is most visible at drying the boards.

Due to uneven drying of the layers of wood, its cracking occurs. The outer layers quickly lose the moisture, lowering faster the volume. The inner layers lose hard the moisture, keeping close its same size. In this way the cracks are produced from the surface to the core.

The dilatation of the wood is due to humidity that the dry wood absorbs from the air. Thus vessels and fibres are larger, which leads to increased wood size. Inflation may cause the warp of the wood, but in the opposite direction of drying. It can show the essence and the quality of the wood.

Also it shows us if the wood is healthy or if it has defects.

A vivid colour, clear and uniform indicates a healthy wood. A bluish colour which suddenly changes from the middle to the periphery of the wood indicates an unhealthy wood.

The smell also can give indications about the quality of the wood. A musty or rotting smell shows that the wood is unhealthy.

The sound may also give a sure indication of the wood quality. Hit with a hammer, the wood must give a clear and uniform sound. A braised sound looks that the wood has defects.

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