A hypothesis for explaining the differential anisotropicshrinkage behavior of wood has been proposed,and it was based on the differences in the cell wall ultrastructure.The starting point of the consideration is thatwood shrinkage is governed by its chemical composition,ultrastructure, and gross anatomy. It is also well knownthat the transverse shrinkage anisotropy of earlywood(EW) is more pronounced than that of the latewood (LW).In the paper, the cell wall ultrastructure and shrinkageanisotropy has been related to each other, and to thispurpose, a set of crystallographic texture descriptorswas applied. The descriptors are based on X-ray diffraction(XRD) experiments conducted on matched EW samplesfrom different growth rings of Scots pine. The rangeof the microfibril angle (MFA) was identified. The ratio ofthe maxima of inverse pole figures (IPFs) of both the tangential(T) and radial (R) directions was determined. Theratios quantify the inhomogeneity of the spatial arrangementof the ordered areas. The results of the study clearlyindicate that the transverse shrinkage of wood is governedmostly by a specific ultrastructural organization of moderatelyorganized cell wall compounds.

The anisotropy of wood properties is related to the ultrastructural organization of wood cell walls. The mean microfibril angle (MFA) is the most obvious parameter quantifying the ultrastructure. Various methods for the MFA measurements have been developed. However, the direct microscopic techniques (both light and electron ones) as well as the indirect X-ray methods were dominating. However, the helical arrangement of cellulose fibrils in wood cells around the longitudinal anatomical direction results in spatial changes of orientations of the lattice planes. Such misorientation between the longitudinal anatomical direction and the microfibril axes has a spatial character and therefore, it can not be correctly described by a single parameter only, i.e. by MFA. The most comprehensive description of the spatial distribution of orientations of cellulose crystallites can be obtained when defining a set of parameters consisting of the rotating axis (given by the polar coordinates θ and ψ and referred to the sample framework) as well as the angle of rotation (ω) around the axis. In order to analyze of wood anisotropy a stereographic projection of the rotating axes on the base of the (010) plane of the lattice cell of cellulose is recommended. Regarding the crystallographic system of the monoclinic lattice of cellulose and the two-fold symmetry of the <010> axis, the projection plane corresponded to the a-c plane of the elementary cell [3]. An example of the projection and the distribution of the rotation axis characterizing the spatial organization of wood microfibrils is given in Fig. 1. The θ, ψ and ω parameters were determined with the original computer program SpaceWood. The parameters were determined regarding the crystallographic symmetry and the parameters of the unit-cell of cellulose given by Zugenmaier [3].

Densification of wood in the transverse direction increases the density. One of the reasons for densifying wood is to produce high quality timber from timbers of low quality. However, densified wood shows an undesired behaviour, i.e. a tendency to return to its initial dimensions when it is subjected to heat and humidity, i.e. shape memory. There are several methods to overcome the problem of shape memory.

This study uses a three-layered cross-laminated wood panel where one of the layers is of densified wood and the other two are of normal wood together these will mechanically restrain the shape memory of the densified layer.

The study includes three stages:

• Densification of clear wood in the radial direction
• Manufacture of a three-layered cross laminated composite product with densified wood as a service layer
• Testing of the shape stability when the composite was subjected to variations relative humidity (40-85 % RH at 20°)

The result of this study reveals the significance of service to bottom layer thickness ratio on the shape stability of the cross laminated composite. Consequently, the performance and the shape stability of the cross laminated composite were significant when the service to bottom layer thickness ratio increases. Therefore, it appears feasible to disclose the appreciable degree of shape stability, hardness and wear resistance of the product. Accordingly, cross laminated composite can be considered as one of the promising mechanical methods for improving moisture movement of densified wood product.

Den utvändiga fasaden ska ge byggnaden ett uttryck genom utformning och kulör. Fasaden ska också skydda de isolerande skikten i väggen från yttre påverkan. Dessa funktioner kan uppfyllas av i stort sett alla material. Om trä ska trä vara konkurrenskraftigt måste trämaterialet, fasadkonstruktionen och ytbehandlingssystemet väljas och samverka på ett sådant sätt att fasaden får en lång livslängd med litet och lågt underhåll. Därigenom blir träfasaden ekonomiskt och estetiskt attraktiv för brukaren i vid mening.

Denna studie belyser kunskapsfronten för utomhusanvändning av träslagen tall (Pinus sylvestris L.) och gran (Picea abies L. Karst.) ovan mark. Specifikt studeras användning i fasader utifrån aspekterna materialval, fasadkonstruktion, ytbehandling samt återvinning.

Marknaden efterfrågar träfasadsystem. De behov som marknadens aktörer, dvs. byggherrar, fastighetsförvaltare, arkitekter, konstruktörer, stomleverantörer, entreprenörer och representanter för småhusindustrin, framhäver kan sammanfattas i följande punkter:

• Behov av specificerad livslängd och givna tidsintervall för underhåll av träfasader. (Ska vara i nivå med konkurrerande material)
• Det är önskvärt att leverantören av ett fasadsystem ikläder sig ett långsiktigt ansvar för underhåll.
• Flexibilitet, leverantören ska kunna byta ut eller renovera fasaden vid behov.
• Byggkrav, träfasadmaterial måste kunna samverka med andra, speciellt brandklassade, material.
• Fasadsystem skall vara utseendemässigt attraktivt.

Den primära marknaden för nya fasadsystem bör vara flerbostadshus, men inte nödvändigtvis flerbostadshus av trä. Fokus ska ligga på fasadsystemets flexibilitet i arkitektoniskt uttryck och i relation till andra material och system. Nybyggnation är viktigt, men miljonprogrammet, renovering och tillbyggnad (ROT) samt energieffektivisering är också viktiga områden.

Den svenska marknaden är liten (idag ca. 70 000 m³ trä för fasader), men bör inledningsvis ändå prioriteras och därefter de nordiska länderna, samt Schweiz, Österrike och Tyskland.

I litteraturen beskrivs mer eller mindre välgrundade rekommendationer för att förlänga träfasaders livslängd och öka dess underhålls-intervall. Vissa av rekommendationerna är dock direkt motstridiga.

När aspekterna materialval, fasadkonstruktion och ytbehandling studeras finns det många detaljer som har betydelse för träfasadens beständighet. Det är svårt att sära ut de mest väsentliga faktorerna, men utan att ta hänsyn till aspekter som kostnader, tillgång, eller andra av praktiskt karaktär viktiga faktorer kan följande nyckelfaktorer identifieras för en miljöriktig och beständig fasad av tall eller gran:

Materialval

• Hög andel kärnved, helst uteslutande kärnved
• Virket ska ha stående årsringar
• Hanteringen ska utföras så att virket inte får mekaniska skador, får mikrobiella angrepp, eller blir uppfuktat eller nedsmutsat, dvs. snabb och rätt hantering, samt god emballering.
• Från marken – fasaden ska börja minst 30 cm ovan marken.
• Ventilation – utforma fasadbeklädnaden så att fukt snabbt kan torka ut. Ventilera utrymmet bakom fasaden vilket är ett enkelt sätt för att möjliggöra detta.
• Vattenavrinning – inga horisontella ytor.
• Flexibilitet – ska gälla både konstruktion och arkitektoniskt utförande. Fasadsystem som kan ”hängas på” befintliga byggnader efterfrågas.
• Förseglat ändträ – försegling av ändträytor för att förhindra fuktupptagning i träet är helt avgörande för trämaterialets livslängd. Spikning kan öppna nya ändträytor och bör därmed utföras omsorgsfullt och med eftertanke.
• Rundade virkeskanter – ger bättre täckförmåga hos färgen och minskar risk för mekaniska skador på fasadbrädorna.
• Val av ytbehandling – spelar en nyckelroll för fasadens prestanda. En träfasad ska levereras som en del av ett komplett underhållspaket.

Hantering från skog till fasadKonstruktionYtbehandling

För ytbehandling finns idag många tillämpningar där nanotekniken utnyttjas för att skapa mervärde hos en yta jämfört med vad dagens mer traditionella produkter kan erbjuda. Nanobaserade ytbehandlingsprodukter marknadsförs redan idag och där uppges de göra ytor smuts- och vattenavvisande, förhindra påväxt av alger, svamp och mossa, förbättra UV- och temperaturresistensen och kulörbeständigheten, förbättra rep- och nötningståligheten, samt ha antigraffiti egenskaper etc. De flesta produkterna är dock nya och för en del finns därför frågetecken vad gäller t.ex. långtidsprestanda och teknisk livslängd, underhållbarhet och därmed sammanhängande ekonomi sett ur ett livscykelperspektiv för den produkt eller system där ytbehandlingen utgör bara en del.

En kostnadsanalys som genomförts i studien gör bedömningen att nya nanoteknikbaserade ytbehandlingssystem skulle kunna ge som mest en reduktion av underhållskostnaderna med 15 %. Antagandet är då att fasadrengöring behöver göras vart femte eller sjunde år då traditionella målningssystem används.

Återvunnet trä från träfasader definieras enligt Svensk standard som trädbränsle och benämns generellt för returträ eller när materialet är i finfördelad form för returflis.

Ett stort problem med att återvinna energin från returträ är att en del av materialet är behandlat på något sätt, t.ex. impregnerat med träskyddsmedel, ytbehandlat eller innehåller andra konstruktionsdelar av t.ex. plast eller metall. Returflis är ett utmärkt bränsle för energiåtervinning förutsatt att anläggningen har tillräcklig rökgasrening och att askan hanteras på ett korrekt sätt. Ett problem idag är vad som ska ske med förorenad askan då den klassas som farligt avfall och därmed inte kan återföras till skogen. Om halterna av tungmetaller inte är för höga kan askan användas som täck- och fyllmaterial annars måste askan gå till deponi.

En bättre källsortering och översyn av regelverk skulle dessutom kunna leda till att det renare returträet skulle kunna eldas i konventionella biobränslepannor medan den förorenade andelen då skulle eldas separat.

The external façade must give expression to a building through both design and colour, and it must also protect the insulating layers in the wall from external influences. These functions can be fulfilled by almost all materials. If wood is to be competitive in this context, the wood material, the façade design and the surface treatment system must be chosen and interact in such a way that the façade is given a long life with little need for maintenance. A wooden façade will then in a broad sense be both economically and aesthetically attractive for the user.

This study illustrates the state of knowledge regarding the outdoor use of pine (Pinus sylvestris L.) and spruce (Picea abies L. Karst.) facings above ground. Specifically, it deals with the use of wooden facings with regard to the choice of material, façade design, surface treatment and recycling. The market demands wooden facing systems, and the requirements emphasized by the actors on the market, e.g. the builders, real estate administrators, architects, designers, frame suppliers, contractors and representatives for the single-family timber housing industry can be summed up as follows:

• There must be a specified life-time and given time intervals for maintenance of the wooden facings. (Shall be similar to those of competitive materials)
• The supplier of the facing system should shoulder the long-term responsibility for its maintenance.
• Flexibility, the supplier shall be able to replace or renovate the facings when necessary.
• Building requirements, the wooden facing materials must be able to interact with other, specially fire-classified, materials.
• The facing system shall have an attractive appearance.

The primary market for the new facing systems should be multi-family houses but not necessarily multi-family houses of wood. The focus shall lie in the flexibility of the facing system in architectural expression, and in relation to other materials and systems. New building is important, but the million program, renovation and additions (ROT) and greater energy efficiency are also important spheres.

The Swedish market is small (currently ca. 70 000 m³ wood for façades), but it should nevertheless be given priority before the Nordic countries, and thereafter Switzerland, Austria and Germany. The literature describes more or less well-founded recommendations for prolonging the life of wooden facing materials and extending their maintenance intervals, although some of the recommendations are directly conflicting.

Many details relating to materials choice, façade design and surface treatment are important for the durability of wooden facings. It is difficult to separate the most important factors, but without taking into consideration aspects such as costs, availability and other factors of a practical nature, the following key factors can be identified as important for an environmentally correct and durable façade of pinewood or spruce:

Choice of material

• The wood shall have a high proportion of heartwood, preferably exclusively heartwood
• The wood shall have vertical annual rings.

Handling from forest to the façade

• The wood shall be handled so that it does not suffer mechanical damage or microbial attack, or become wet or soiled, i.e. a rapid and correct handling with good packaging.

Design

• The façade shall start at least 30 cm above the ground level.
• The façade shall be ventilated so that moisture can rapidly dry out. Ventilation of the space behind the facing is an easy way of achieving this.
• Water run-off – no horizontal surfaces.
• Flexibility –both in the construction and in the architectural design. There is a demand for facing systems which can be simply “hung onto” existing buildings.

Surface treatment

• Sealed end-grain sections – sealing of the end-grain surface to prevent moisture absorption into the wood is decisive for the life-time of the wood material. Nailing can open up new end-grain surfaces and should thus be carried out carefully and only after due consideration.
• Rounded edges – increase the covering ability of paint and reduce the risk of mechanical damage to the facing boards.
• Choice of surface treatment – vital for the performance of the facings. The wooden facings shall be delivered as part of the complete maintenance package.

Nowadays there are many types of surface treatment where nano-technology is used to create an added value in a surface compared with what the more traditional products can offer. Nano-based surface-treatment products are already on the market, and they are said to make the surfaces dirt- and water-repellent, to prevent the growth of algae, fungi and moss, to improve UV- and temperature-resistance and colour permanence, to improve scratch- and abrasion-resistance, and to have anti-graffiti qualities etc. However, most of these products are new and for some of them there are still question marks with regard to their long-term performance and technical life-time, as well as their serviceability and thereto related economy seen from a life-cycle perspective for the product or system for which the surface treatment constitutes only a part.

A cost analysis carried out as a part of the study makes the assessment that the new nano-technology-based surface-treatment systems could lead at most to a reduction of 15 %. in maintenance costs. The assumption is then that the façade needs to be cleaned every fifth or seventh year when a traditional painting system is used.

According to the Swedish Standard, recovered wood from a wooden façade is defined as tree fuel and is generally designated recycled wood or, when the material is in a disintegrated form, as recycled chips,

There is a major problem in recovering energy from recycled wood when a part of the material has been treated in some way, e.g. impregnated with a wood-protection agent or surface-treated, or when it contains other design components of e.g. plastic or metal. Recycled chips are a very good fuel for energy recovery provided the plant has adequate flue-gas cleaning and the ash is handled in a correct manner. Contaminated ash constitutes a problem, since it is classified as dangerous waste and cannot therefore be returned to the forest. If the content of heavy metals is not too high, the ash can be used as a covering and filling material. Otherwise, the ash must be deposited as landfill. A better sorting of household waste and an overhaul of the regulations would mean that the cleaned recycled wood could be burned in conventional biofuel boilers and that the contaminated portion could be burned separately.

All applications of wood involve drying the material from the green state. The cell wall may be viewed as a laminate consisting of different layers. The layers have different orientations and therefore different moisture expansion characteristics. As a result, stresses will develop in the layers due to drying. Micromechanical models for fibre composite materials were used in combination with a laminate analogy in order to calculate these drying stresses in the cell wall layers S1, S2 and S3. Resulting stresses were very high. In reality viscoelastic effects will significantly reduce stresses at high moisture content. However, at lower moisture content irreversible cell wall damage is likely to form as a result of the stresses computed by the model.