Wood is a material made up of three types of macro-molecules – cellulose, hemicellulose and lignin – whose combination gives wood a porous structure that allows water to pass between cells.

Water is found in wood in three different forms: constitutive water (water molecules internal to the cells), bound water (attached to the cell wall by adsorption) and free water (water circulating in the wood’s vessels).

When wood is dried, all the free water and some of the bound water must be removed to achieve the right moisture content for use. The first stages of drying will begin by evaporating the free water present in the vessels.

Once the free water has evaporated from the vessels, the bound water will gradually leave the wall until the desired moisture content – or anhydrous state – is reached.

During the drying process, the characteristic water content when the wood is free of free water and saturated with bound water is called the fiber saturation point (FSP). From this water content to its anhydrous state, wood is in its hygroscopic range.

It undergoes dimensional variations and changes in its properties. As the water content decreases, the thickness of the cell wall decreases, causing the wood to shrink.

These deformations of wood during drying are due to the anisotropy of shrinkage. Shrinkage within wood depends on the direction in which it is viewed. Depending on the wood’s three directions – radial, tangential and longitudinal – it deforms differently and irregularly.

Generally speaking, radial shrinkage is less important than tangential shrinkage (longitudinal shrinkage is not taken into account, as it is 0.1% or negligible), causing the wood to distort during drying.

This difference between radial (5% on average) and tangential (10% average) shrinkage also explains why cracks can appear radially in a wooden slice during drying since the periphery of the section (tangential direction) tends to shrink more than the radius (radial direction).

Shrinkage is proportional to the variation in humidity. The shrinkage coefficient α is defined by the dimensional change due to a 1% change in humidity content on the dimension of dry timber.

In the case of a log where the heart of the timber is enclosed, and referred to as the “enclosed core”, the differences in radial and tangential shrinkage mean that the periphery of a cut tends to shrink more than its radius. As a result, the timber is subjected to perpendicular tensile stresses in the tangential direction, or cracks will occur.

As the perpendicular tensile strength of timber is low, cracks occur in most cases. The width of the crack can even be predicted by calculation, and depends on the radius, the timber’s humidity variation (from the FSP), and the difference between the radial and tangential shrinkages.

The location of cracks in round timber is hard to predict. They occur at the shortest distance between the heart and the edge, or in the more fragile areas (near knots, for example).

These cracks generally have little effect on the mechanical strength of timber whose core is enclosed – except in bending, when the crack is horizontal at the support points. The fibres of the timber are dissociated but not broken.

On the other hand, they can constitute water traps that can lead to fungal growth, as well as expose part of the timber that was not sufficiently durable or protected by a preservative treatment: surface protection or partial impregnation with a chemical product.

One technique used on road barriers is to deliberately create this crack by cutting a notch in the middle of the timber. This ‘notch’ releases the deformation stresses and therefore limits the appearance of cracks in undesired areas (at assembly points, for example).

The opening of the notch varies according to the humidity variations. It is generally done towards the bottom to not create a water trap.

Some large structural elements are reconstituted and are much more homogeneous. Their stability means that deformations are greatly reduced and that there are no (or few) problems with the appearance of cracks. These products are, among others, glued laminated timber and plywood.

Source: The International Tropical Timber Technical Association. For more information, visit https://www.atibt.org/en/.

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