7. DURABILITY Figure 7.3. Moisture protection treatment on LVL beams. WeatherGuard® Metsä Wood 7.2.4 Discussion of durability class definitions The biological durability classification based on EN 350 is poorly applicable to LVL or other engineered wood products. The biological durability of LVL and other wood-based panels (uncoated or overlaid) is dependent on the end use conditions, and this is where the EN 335 use classes provide useful guidance. EN 350 durability classes are defined for the heartwood of wood species in use class 4. However, there is no information on the suitability of different natural durability classes for different end uses. CEN/TS 1099 gives guidance for plywood, but does not provide information on the suitability of different natural durability classes for different end uses. LVL and other wood-based panels contain sapwood. The fact that sapwood is always regarded as non-durable argues against using the EN 350 classification as, based on this, LVL and all wood-based panels are universally classed as ‘not durable’ (class 5). EN 350 states that the durability classes are designed to indicate the expected level of performance of wood when used in contact with the ground (service conditions as described for use class 4 in EN 335) and that in other use classes the service conditions can result in wood performance that differs from that implied by the durability classification. Based on this, the standard may not be directly applicable to use class 3, 2 and 1 conditions and should therefore not be used as the biological durability criteria. The natural durability of LVL can differ from that of the wood species used in its manufacture. Additional factors contributing to durability include the thickness of the plies, the structure of the board, and the characteristics and quantity of the binder. In conclusion, it is recommended to declare the product as durable in use class 1 and 2 (or service class 1 and 2 according to EN1995-1-1) conditions. 7.3 STRUCTURAL WOOD PROTECTION The best way to manage the durability of LVL, as well as all other timber structures, is structural protection. This means, for example, long enough roof overhangs and high foundation levels and moisture isolation between the foundation and sole plates. Structural protection keeps the structures in service class 2 (use class 2) or dryer conditions, which wood products can resist well. 7.3.1 Resistance against temporary moisture exposure of structural LVL products Exposure to rain, splashing as well as water convection from other structures shall be avoided. The designer must pay attention to the details of the construction to ensure that no water pockets will be formed. The product may be exposed to weather for a short period during installation. During the erection of a building, structural LVL products and elements, which are structurally glued from LVL components, have good resistance to temporary exposure to water without damage or decay. This requires, however, that it is ensured that the products can afterwards dry to the desired moisture content before the structural envelope is closed. The integrity of the glue bonding is maintained according to the assigned service class throughout the expected life of the structure 28. LVL products swell when the moisture content increases, and shrink when the moisture content decreases. A part of the swelling is permanent and the extent of these dimensional changes depends on the grain direction. Wetting can cause permanent deformations and impair the visual appearance of surface veneers, such as colour changes due to water staining, surface cracks and falling knots due to drying shrinkage after wetting 28. Joints with mechanical connectors, such as bolted connections, may become loose due to swelling and shrinking cycles. Drying shrinkage after severe wetting may cause cracking, which often reduces the load-bearing capacity of dowel-type connections, notched beams and beams with holes. 7.3.2 Resistance against UV radiation Like all wood products, non-treated surfaces of LVL will slowly fade to grey due to the action of UV radiation from the sun. This greying does not affect the strength properties. If this natural greying is not desired, an adequately pigmented coating system or a coating containing special additives must be applied. The higher the proportion of pigments, the greater the protective effect. The coating layer must be of sufficient thickness to fulfil the minimum protection requirement across the entire surface. More information is provided in the technical data sheets of the coating manufacturers 42. 7.3.3 Chemical durability The main components of wood: cellulose and lignin, react in opposite ways to acids and alkalinity. Cellulose is not very 170 LVL Handbook Europe
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