EL HOMBRE Y LOS TRES MUNDOS

In this section it will be analysed the pathologies that usually appear in construction systems made of wood. These can be divided into two main groups; structural and durability pathologies.

The structural pathologies come from the wrong design and sizing of the elements that form part of the construction system, such as timber frames, bearing walls made of CLT (Cross Laminated Timber) or wood structures made of posts and large laminated beams in order to cover large spans. The mechanical resistance of wood is known and it differs depending on the direction of the fibres and the presence of noodles, which are catalogued as defects.

Apart from structural ones, durability pathologies are also detected due to the response of material to biotic and abiotic agents.

The biotic agents are the weather agents, which affect to exposed materials. Wood is hygroscopic material and due to that it absorbers and releases humidity when there is a moisture source. Because of that, when it is raining it swells and when it stops raining the material is contracted due to drying, which produces drying fissures and cracks parallel to the fibres, it is also twisted and curved.

This characteristic behaviour, movements and deformations, have to be into account in the design of construction wood elements which are more susceptible to be in contact with water. For that, strategies such as the design of joints which allow wood movement and air spaces where moisture can be accumulated in order to make easier its ventilation and water evacuation are usually followed.

Apart from the rain water, there is other deterioration agent, the solar radiation. This phenomenon (photo degradation) affects superficially the material. The ultraviolet (UV) rays destroy the wood lignin, which makes it to become grey gradually, and the infrared (IR) radiation makes it to release resin and other material substances by exudation.

These two agents are usually presented in our climate along year. Consequently, the material deterioration takes place from the outer face of the material toward the interior.

On the other hand, there are the biotic agents. Between them, they are distinguished the fungi and the larval cycle insects.

Inside the group of fungi, there are two classes; chromogenic and rot fungi. The chromogenic fungi, as their name points, are characterized by colorizing wood, although they do not affect to the mechanical resistance of the material. However, rot fungi are harmful. These fungi eat the wood cellulose and hollow the interior of the material setting aside the lignin, which surrounds the cellulose.

The last agents are larval cycle insects. Between them, the most common are; powder-post beetles, death watch and drugstore beetles, weevils-snout beetles, longhorn beetles, and termites. The last insect, in contrast to the other insects, also eats wood when it is adult, the rest of insects only eat wood during their larval phase. The termites are very destructive and their behaviour differs from the rest, being harder to remove.

1.3.1 Wood protection

The wood protection is not always necessary as it depends on its natural durability and Class of Use. This classification is based on five degrees from 1 to 5, and it is based on its grade of exposition to weather agents, which deteriorate the material.

Some species do not need protection treatments for some Classes of Use, due to their natural durability. In case of protection treatments, they have to be suitable for the material considering its impregnability, which at the same time depends on the type of wood, softwood or hardwood, and specimen.

In the Basque Country (Spain), the most common species are: Radiata-pine (Pinus Radiata, D.Don.), Douglas-fir (Pseudotsuga Menziesii, Mirbel), Oak-tree (Quercus Robur, L.), Lawson- cypress (Chamaecyparis Lawsoniana, A. Murray), and larch-tree (Larix Decidua, Mil.).

If the kind of specie is relevant in order to know the absorption capacity of the protection treatment, the permeability to impregnation also differs depending on the specimen, heartwood and sapwood, being the sapwood the most impregnable part [20]. In contrast, the heartwood is the part that has the most natural durability, which means that it has more resistance to deterioration agents, biotic and abiotic.

An important factor to take into account when a product is applied to wood is that its mechanical strength is reduced in major or minor quantity depending on if it is superficial or deep protection and the type of product or substance, such as CCA (chromated copper

arsenate), ACQ (alkaline copper quaternary), Tanalith-E, Wolmanit CX-8 or CDDC (copper dimethyldithocarbamate) [21].

The chemical products used to protect wood from exterior climate and increase its durability cutting down its degradation, alter the physical properties of the material which varies its mechanical strength [22].

In addition to the most common chemical treatments, such as copper components, they are in study others based on nanoparticles of TiO2 (titanium dioxide) and clay nanoparticles, with optimal results in terms of wood ageing giving rise to nanomaterials as future application in the field of wood protectors [23].

Another alternative to conventional chemical treatments to protect wood is the heat- treatment. This method consists on introduce wood into a heat chamber to 190 and 210 °C temperature, depending on wood specie and grade of exposition to weather. This method extracts the wood resin which is the most vulnerable part of this material against the majority of biotic agents. However, there is a variation of the physical properties of the material such as the rigidity, hardness and superficial roughness, after this process [24].

The variation of the wood physical properties is also presented during the natural drying process when it is exposed to climate. Both heat and mass transfer deform materials and produce tensions in the material [25]. In case of the wood heat-treatment, as well as in the case of natural drying, exothermic reactions appear and depending on the distribution of the wood products these reactions are not distribute in the same way. This calculus is complex and has been presented in various studies [26], trying to simulate the real behavior of each piece of wood when they dry.

If during the dry process of wood deformations and tensions appear, during the wood life cycle, when this material is exposed to climate, it also suffers constant dry and wet processes due to weather agents, such as rain, wind and sun. In case of the design of wooden façades, there are different designs and compositions taking into account that this material is curved and twisted after these processes, overall in locations such as Bilbao where rainy days are frequent. That is, it is important to stablish a design which considers the swelling and contraction movements of this material [27, 28].

Apart from that, the rain water and solar radiation, which are abiotic agents, also accelerate the deterioration and ageing of wood. If both weather effects are harmful when they take place in short periods of time, solar radiation mainly alters the most superficial exposed part of the material, and it is revealed by means of colour loss, turning it into grey.

This effect supposes a larger perception of deterioration, which can be attenuated with stabilizers that protect wood slowing down the loss of colour and brightness [29].

They exist some treatments which are applicable to heat-treated wood, such as acrylic paints made of polyurethane with extracts of bark and lignin stabilizers, this last substance gives to wood its characteristic colour avoiding its degradation due to abiotic agents, although its efficiency also depends on the wood species [30].

In case of the north hemisphere, the façades which are oriented to the south are more exposed to solar radiation. Because of that, the loss of colour in wooden façades is higher when these are placed facing the south.

If the behaviour of façades made of timber frame and intermediate insulation are analysed, it is known that the northern façades are more prone to suffer harms, which are derived to the condensations produced due to the thermal jump between the interior and the exterior. In case of enclosures orientated to south, this effect takes place in less measure as it is more exposed to solar radiation, that is, the superficial cladding absorbs the direct solar radiation and it is released in form of heat. On the other hand, the façades which are facing the north are more susceptible to suffer condensations. Due to that, materials with high inertia and thermal mass are recommended, as they absorb heat and release it gradually when the exterior temperature goes down, avoiding a high heat jump in specific points. In case of the current solutions, which are highly insulated and made of various material layers, this phenomenon has to be studied in order to avoid interstitial condensations [31].