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Page | 14 GUIDE TO THE SPECIFICATION, INSTALLATION AND USE OF PRESERVATIVE TREATED ENGINEERED WOOD PRODUCTS Species with high natural resistance to marine borers, as listed in AS 5604. Only Class 1 species used in EWPs could be expected to give reasonable service life in northern waters. Few wood species have high natural marine borer resistance, therefore supply of EWPs using appropriate species is unlikely. EWPs impregnated with chemical preservatives to AS/NZS1604. Chemical and or mechanical barriers. Chemical attack EWPs have a definite advantage over steel members when exposed to corrosive environments. Timber and wood products are able to withstand mild acid conditions and are more resistant to degradation. The behaviour of EWPs in chemical environments depends upon a number of factors, including timber species, pH and temperature. Wood essentially responds by either swelling (Category S), similar to moisture response, or by chemical degradation (Category D). Damage due to swelling is essentially reversible, but chemical degradation results in breakdown of the wood structure and is non-reversible. Category S agents include alcohol and other polar agents. These agents swell dry wood causing a strength (and stiffness) loss proportional to the swelling. Category D agents include acids, alkalis and salts and result in a loss of strength and stiffness directly related to the loss of member cross-section. Table 2 provides a rough guide to performance of EWPs in chemical environments. The effect of chemicals on wood will generally be worsened by increased exposure time, temperature, extremes of pH and chemical concentration. Wood generally offers considerably less resistance to alkalies than acids. Softwoods generally have better resistance to acids than hardwoods. Where there is the possibility of chemical attack on EWP members, designers should seek expert advice. Agent category Table 2: Effect of chemicals on EWPs. Chemical agent Mode of attack Damage Reversible or Neutral Non-polar liquids such as petroleum hydrocarbons S Alcohol and other polar (swelling) solvents D Permanent Severity (loss of strength and/or stiffness) None Negligible Negligible Swelling Reversible Proportional to volumetric swelling Inorganic acids Hydrolysis of Permanent Slight to moderate (degrading) cellulose D Organic acids such as: Hydrolysis of Permanent Slight (pH 3-6) Formic, acetic, propionic and lactic acid cellulose D Alkalis such as: sodium, Delignification Permanent Moderate (pH > 9.5) calcium and magnesium of wood and hydroxide dissolving of hemicellulose Severe (pH > 11) D Salts Hydrolysis of Permanent Slight (considered as weak acids) cellulose Note: Ref. Williamson, T.G. (Ed) 2002 Corrosion of fasteners The correct selection of fastener type and material is important for the long term performance of structures. Wood is slightly acidic in the range pH 3 to 6. In the presence of moisture, an acid environment is created posing a hazard for the fastener and surrounding wood material. Unprotected mild steel will corrode under these conditions and will cause a chemical reaction which also breaks down the surrounding wood.
GUIDE TO THE SPECIFICATION, INSTALLATION AND USE OF PRESERVATIVE TREATED ENGINEERED WOOD PRODUCTS The following measures may be employed to protect the fastener and the surrounding wood structure: Shield the connection from the weather and moisture. Use non-corrosive or protected metals (galvanised or protective coating) Prevent moisture traps by countersinking and plugging fasteners Don’t use dissimilar metals in contact with each other (be aware of metals used in wood preservative treatment) Grease, coat or sheath fasteners to isolate from wood surface. This is particularly important where EWP components have been treated with water-borne copper treatments such as CCA, ACQ and copper azole. ACQ and Copper azole have been found to have around twice the corrosion rate of CCA. Moisture affects on stability EWPs, like all wood products, are hygroscopic, which means they have an affinity for water. EWPs will readily take up and release moisture in response to changes in the local environment. Most water is removed from the wood structure of EWPs during the manufacture process. Effective glue bonding requires lowered moisture content in the wood components. For example at the time of manufacture OSB has a moisture content of 2-3%, plywood about 6-8%, I-beams about 8-12%, and Glulam about 10-14%. Post manufacture and during service EWPs may be exposed to environmental conditions that cause changes in their moisture content, and therefore their dimensions. EWPs will shrink and swell in proportion to changes in their moisture content between 0 and 30% fibre saturation point. The most significant moisture movement will occur across the grain (tangential and radial directions within a log). Longitudinal movement (in the grain direction) may be a factor depending upon the particular EWP and the type of structure. EWPs will generally be more stable than solid kiln dried timber because the process of gluing together multiple wood components tends to randomise and balance moisture responses. Warping, bow and spring may still result in response to differential moisture content. Dimensional change: ΔD = Di S (MCi – MCf)/ FSP where ΔD = change in dimension Di = initial dimension S = shrinkage coefficient (Plywood = 3.5, LVL = 6.0% radial, Particleboard = 1.7% typical) MCi = initial moisture content MCf = final moisture content FSP = fibre saturation point (approx. 28%) Where the possibility of wetting exists, design and detailing should allow for dimensional instability. For example, structural plywood or OSB sheathing panels should be installed with a minimum 2 mm gap around panel edges to prevent warping. Dimensional changes to EWP in response to changes to moisture content can be estimated from known shrinkage coefficients for the product and direction within the product from the equation shown above. Values of shrinkage coefficient may be obtained from the product manufacturer. EWPs such as OSB and particleboard which are densified in the manufacture process may exhibit irreversible swelling due to cyclical moisture effects. Glue durability Durability of the adhesives used to manufacture EWPs is integral to the product performance. The adhesives used need permanence and non-creep characteristics to ensure performance under long term exposure. A wide variety of adhesives are available for the manufacture of EWPs. Manufacturers select adhesives appropriate for the intended applications of their products. Type I adhesives include resorcinol, phenol-resorcinol and poly-phenolic adhesives and are generally viewed as suitable for long-term, exposed applications. Type II adhesives include melamine-urea formaldehyde (MUF) and are generally viewed as suitable for long-term interior/protected applications. At the time of publication polyurethane (isocyanate) adhesive has not formally been classified, but is generally accepted as exceeding the requirement for Type II. For example structural LVL and plywood manufactured to AS/NZS4357 and AS/NZS2269 respectively are required to use Type I adhesives (Type A bond - permanent). Other EWPs may use Type I or II or other adhesives depending upon the intended application. Type I adhesives are generally suitable for applications where there is full exposure to weather and where there may be prolonged exposure to high Page | 15
Page 1 and 2: MARKET ACCESS & DEVELOPMENT PROJECT
Page 3 and 4: Publication: Guide to the specifica
Page 5 and 6: TABLE OF CONTENTS GUIDE TO THE SPEC
Page 7 and 8: GUIDE TO THE SPECIFICATION, INSTALL
Page 9 and 10: Figure 1: Design Process APPLICATIO
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Page 25 and 26: Good design and detailing Design an
Page 27 and 28: Treated Glulam and plywood structur
Page 29 and 30: Finishing of preservative treated E
Page 31 and 32: Special considerations GUIDE TO THE

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