KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Content • Drying and utility • Moisture in production • Moisture in service • Measuring moisture content Wood-water relationships MC in production and use, and management www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Wood in the tree or in service contains water Moisture and shrinkage • Wood in the tree or in service contains water • Wood changes in size as it loses or gaining moisture • Wood is hygroscopic 80 70 – It loses or gains moisture with fluctuations in environmental humidity moisture content 60 Free water in the cell Bound water in the cell wall loss of moisture with little change in size low shrinkage ! free moisture 50 40 Fibre saturation point range over which shrinkage commences 30 fibre saturation point straight line portion of graph where shrinkage is proportional to moisture content 20 Dry timber Service EMC zone bound moisture 10 high shrinkage 0.0 title While detailed, this lecture’s content is important for production and use example of service conditions and Equilibrium Moisture Content Shrinkage Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Key definitions Relative humidity The amount of moisture in air as a percentage of its maximum moisture carrying capacity Fibre saturation point (FSP) The point when the cell walls are saturated with bound water but the cell cavities are free of water, about ~ 25 - 30% MC Bound water The water weakly chemically bound in the cell walls of wood Free water The water within cavities or lumens in the cells of wood Drying and utility Drying improves timber functionality as a product Moisture content (%) The weight of water in a piece at a specific time as a percentage of oven dry wood fiber Equilibrium moisture content (%) Wood-water relationships The moisture content where timber neither gains nor loses moisture from the surrounding atmosphere www.csaw.utas.edu.au 0.0 title ! www.csaw.utas.edu.au 1! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Material changes with drying Drying and utility • Drying improves stability – Unseasoned timber is subject to unpredictable shrinkage and deformation • Drying improves mechanical properties – The properties of the same graded piece improves ~ 30% • Drying improves durability • Timber cut from the tree will inevitably dry to EMC • As timber dries below FSP, the cell walls shrink and harden • As EMC approaches, the potential for shrinkage and other moisture dependant processes decreases Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics – Fungal growth and some insect attack requires ~25% MC • Drying improves general utility – Dry timber is easier to cut, drill, shape, glue, join, lift and move Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Drying improves stability • Unseasoned timber is subject to unpredictable shrinkage and deformation as it dries • In building, this can result in – Cracks and splits in appearance and structural elements – Twisting and distortion in wall and other elements – Unevenness in floor levels – Jamming in door and window – Gapping in floors, decking and many junctions Deformation in an beam used unseasoned Shrinkage induced unevenness in the top floor of a story green hardwood building Wood-water relationships www.csaw.utas.edu.au Wood-water relationships KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Drying improves mechanical properties Drying improves durability • Strength and stiffness www.csaw.utas.edu.au • Sustained fungal growth and some insect attack requires ~25% MC – The properties of the same graded piece improves ~ 30% – Timber with a lower MC either can not sustain or is less attractive to decay mechanisms • for Messmate from F11 green to F17 seasoned; • Creep • The service life of protective coatings is higher when applied to dry, stable timber – Increased deformation following prolonged loading – Wood with a higher MC creeps more while changes in moisture content accelerate it – They adhere better initially – They are not subject to: • Jointing • Stress due to unpredictable changes in size or • Failure due to moisture movement under the surface – The joint capacity of the same graded piece improves ~ 30% Checking the moisture content Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au 2! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Drying improves general utility Drying approves appearance Dry, stable timber: • retains its shape, • will keep a smooth surface, and • can accept protective coating • Dry timber is easier to mould, cut, drill, shape, glue, and join • As it is lighter, it also easier to lift and move, and more economical to transport Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics – These adhere better initially and are not subject to unpredictable size changes Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood production process • Each stage in the wood production process has specific moisture management requirements MC management in production 0.0 title ! www.csaw.utas.edu.au Wood-water relationships KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Timber drying process The drying rate • Drying the surface induces movement of moisture from the core to the surface • The cell structure limits the rate of moisture movement: • Drying timber requires generation of a moisture gradient between the surface and centre of piece • Water evaporates from the surface – Energy is required to change liquid water on the surface into vapour • Water moves from the woods interiors to the surface through bulk flow or diffusion Wood-water relationships www.csaw.utas.edu.au – Open cell structures allows bulk flow – Closed cell structures allows diffusion between cells only • If the rate of drying exceeds the rate of moisture flow, the surface layers can dry below FSP Fabrication Distribution and storage Dry milling assembly Drying Green milling breakdown Log handling To capture this improved product functionality, any wood production or distribution process requires active drying and moisture management Moisture control www.csaw.utas.edu.au Softwood Hardwood – They then start to shrink around the unshrinking core Drying and moisture control www.csaw.utas.edu.au 3! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Shrinkage effects during drying Controlled timber drying requires: • Physical constraint of the piece • Water can only leave the woods surface – As it does, the surface begin to shrinks – Rack, stack or chain • Application of a staged sequence of controlled conditions • As the core remain wet, tension is induced on the pieces surface • To avoid degrade, the drying rate has to controlled to balance: – Conditions include temperature, humidity and air flow – Stages include air drying, predrying, kiln drying and conditioning – Moisture movement from the core to the surface – The timber’s shrinkage rate and tensile strength Wood-water relationships www.csaw.utas.edu.au Wood-water relationships KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Racking Softwood racks www.csaw.utas.edu.au • Racks are units of timber where the boards are spaced apart with rack sticks • This configuration allows even air flow & restraint against drying distortion • The weight of timber above them or rack weights provide further restrain • Racks are assembled in stacks, with one rack above another Wood-water relationships www.csaw.utas.edu.au Wood-water relationships KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Production requirement by species group Air drying Softwood Hardwoods Collapse prone Non-collapse prone Air-dry Pre-dry www.csaw.utas.edu.au • Air drying is drying timber racks in natural conditions to a suitable moisture content (often FSP) with minimal unnecessary degrade • While uncontrolled, the natural conditions experienced can be moderated by the orientation and placement of racks in the open, or by their placement in buildings or shelters • Air drying is low-energy, inexpensive but slow and a large drying yard is required Recondition Kiln-dry Condition Standard practice Drying and moisture control Occasional Unnecessary www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au 4! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Predrying Kiln drying • Kilns dry: • Predryers are a form of kiln used to dry hardwood timber from green to FSP • Kilns and predryers are enclosed chambers where: – Hardwood timber at ~ FSP or – Softwoods • Conditions applied to the timber are varied to a kiln-drying schedules – Temperature, air flow, humidity varied to give optimum drying for the charge – Schedules developed for different species and grades – Typically 20-40 hours to dry to ~12% – temperature, – humidity and – air circulation (direction and speed) • Energy is provided to drive drying can be controlled to control the rate of drying process Wood-water relationships – Heated air and steam introduced to charged kiln – baffles and fans control air circulation Source: USDA Handbook 188 www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Conditioning • Application of short steaming of cycle to: – Recover cell collapse in hardwood – Repair damaged surface cells – Equalize moisture gradients - even out MC – Relieve residual stress • The saturated steam soften the cell walls • It can be applied: – before kiln drying in collapse-prone hardwood – After kiln drying in softwoods and other hardwood Softwood processing Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Target moisture contents in production Product Required MC Parquet, and Sawn or dressed furniture components to 13 % Strip flooring, Overlay strip flooring, Lining Boards, Dressed boards, Joinery Stock, Mouldings and sawn boards for Feedstock to 14 % Light decking, cladding, fascia and barge boards 10 to 18% MC management in service Target moisture content in AS 2796 for the major hardwood product groups Product Required MC To maintain this improved product functionality, design and maintenance processes for wood have to actively consider drying and moisture Strip flooring, Overlay strip flooring, Lining Boards, Dressed to 14 % boards, Joinery Stock, Mouldings and sawn boards for Feedstock Light decking, cladding, fascia and barge boards 10 to 18% Target moisture content in AS 4785 for the major softwood product groups Wood-water relationships www.csaw.utas.edu.au 0.0 title ! www.csaw.utas.edu.au 5! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Water in timber in service environments Moisture content in service • In service, timber will expand & contract as its moisture content changes to be in equilibrium with local conditions • This is a natural and unavoidable aspect of timber • Design and construction practice must accommodate this movement • Consider timber flooring as an example MC: 12-14% Sheltered unconditioned space MC: 8-9% Air-conditioned space Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Moisture content at laying Acclimatization can balance moisture content • Boards need to be laid at the correct moisture content • A moisture content to AS 2796 is generally suitable (9-14% MC) • Strip flooring will always move slightly between the boards Wood-water relationships www.csaw.utas.edu.au Timber Service Environment Response and Required Action Flooring supplied at a moisture content between 10% and 12 % Moist Conditions Average MC ~ 12.5–16% • Cool & damp or hot & very humid Timber expands • Provide extra expansion joints • Acclimatise Normal Conditions Average MC ~10–12.5% Timber remains relatively stable Dry Conditions Average MC ~ 8–10% • Air conditioned, centrally heated, or with large northern windows Timber contracts • Acclimatise • Consider alternatives to polyurethane finishes Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Protection from moisture Provide expansion gaps to AS 1684 • Ideally, the flooring should be stored inside where it is to be laid, or in a similar environment • The floor should only be installed in a fully weatherproofed building • Slabs are dry or sealed • Flooring will expand & contract with moisture changes; • Install expansion joints: – 10 mm gap at every wall plate; & – intermediate gaps in floors over m wide • Use narrower boards for large areas of floor Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au 6! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Substrates must be dry Problem for moisture and heaters • Supporting slabs should have a moisture content of no more than 5.5%; • All other substrates must be dry; • Sealing the slab with a waterproofing compound or membrane may be necessary This can be a: – Sealer coating; or – Plastic membrane Wood-water relationships www.csaw.utas.edu.au Wood-water relationships KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Problem for moisture, heaters and windows Key points www.csaw.utas.edu.au • Specify timber MC to match the service environment – Acclimatize if necessary • Preserve that MC during construction • Minimize potential MC variation during service • Expect for movement and detail for it Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Water in wood • Timbers moisture content is expressed as: & weight of waterin the wood # $$ !! x100 = MC % % dry weight of the wood " Measuring MC • Means of measuring timbers moisture content, defined in AS 1080.1, include: Methods with differing degrees of accuracy 0.0 title ! www.csaw.utas.edu.au – Oven drying – Resistance meter – Capacitance meter The available reading provides detail on each Wood-water relationships www.csaw.utas.edu.au 7! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Accuracy in moisture measurement Summary • Single measurements with non-destructive resistance and capacitance meters not accurately represent the MC of an individual board or group of boards • Establish an acceptable range of moisture contents and take multiple measurements Intended No of product usage boards tested Acceptable no outside range No of extra boards tested Total acceptable no outside range Material to store in rack 5 Material for immediate use 10 Number of moisture meter checks for packs entering the joiners Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics • Drying improves stability, mechanical properties, durability and general utility • Drying timber from green to a MC suitable for use is a critical stage of production • Controlling MC and movement is a critical part of design and construction • There are several means of measuring moisture with varying accuracy – All need to used the right way Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Things can be built unseasoned Thanks to Patrick Cook Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au 8! ... collapse-prone hardwood – After kiln drying in softwoods and other hardwood Softwood processing Wood- water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood- water. .. the moisture content Wood- water relationships www.csaw.utas.edu.au Wood- water relationships www.csaw.utas.edu.au 2! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin... strength Wood- water relationships www.csaw.utas.edu.au Wood- water relationships KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Racking Softwood