7. DURABILITY Figure 7.4. Pressure-impregnated LVL-C beams. resistant to strong acids, but its resistance to bases is good. Lignin, on the other hand, is easily dissolved in bases, whereas it is resistant to most strong acids. For these reasons, wood is quite resistant to moderate chemical effects. LVL has good resistance to mild acids and acid salt solutions. Alkalis, however, cause softening of the wood. Direct contact with oxidizing agents such as chlorine, hypochlorites and nitrates should be avoided. Wood is generally quite resistant to organic substances. However, organic solvents such as acetone, benzene, alcohol etc. dissolve resins, fats and waxes, causing similar effects to water, i.e., producing swelling and a slight reduction in strength properties. Petroleum oils have no effect on strength properties but cause discolouration. Chemical resistance can be improved with various types of coatings 42. 7.4 CHEMICAL WOOD PROTECTION 7.4.1 Surface treatment LVL can be protected against temporary weather exposure by surface treatments that repel rainwater, and therefore reduce the amount of moisture absorbed by the product, but allow water vapour movement to and from the product. This improves dimensional stability and reduces swelling of the treated LVL product during the construction time. Use in high relative humidity conditions may result in mould growth on the surface of structural LVL products. If the products are exposed to outdoor air humidity conditions (e.g. structures in unheated spaces) or wetting during the logistic chain and construction time, a brushable or sprayable treatment should be applied on the surface of the LVL components to reduce the risk of mould growth. In some cases LVL components are treated at the factory, but the default practice is for treatment to be done during offsite production of timber elements or on the construction site. If there is mould growth on the surface of LVL products, the mould must be removed, e.g. by sanding, before closing the structure. The surface treatments do not affect the product’s strength properties, but their compatibility with end finishes, such as paints, shall be verified separately 28, 43. 7.4.2 Pressure impregnation Pressure impregnation of spruce softwood LVL is possible when the impregnation process is tailored for it. Due to dimensional changes due to moisture and to achieve the required retention level of the water-based impregnation agent, it is recommended to use only LVL-C for impregnation. For beam-size products with a width of max 600 mm, a retention level similar to the AB class used in the Nordic countries can be achieved. It is also possible to impregnate panel-size products, although the retention level in the core area is lower. Applications for pressure-impregnated LVL-C include terrace and balcony structures, balustrades, stairs and piers where the products are not in direct ground contact (= Use class 3.1 conditions). AB class impregnation agents are aggressively corrosive and therefore only stainless steel connectors should be used in structures made of impregnated LVL. The surface of pressure-impregnated LVL is rougher than regular LVL. The impregnation and drying processes cause swelling and shrinkage, which open some of the peeling cracks in the surface veneers. Local cracking around knots and minor separation of scarf joints in small areas of the surface veneer might occur. Impregnated LVL products swell in the process and their equilibrium moisture content and specific weight are a couple of per cent higher than untreated products. The increased weight shall be taken into account, but otherwise structural design shall be made with the nominal dimensions of the product with the strength and stiffness reduction factors according to service class 3 of Eurocode 5. In some countries additional reduction factors given by their national requirements shall be taken into account 44. LVL Handbook Europe 171
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