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Wood Properties, Deterioration & Preservation

Wood Properties, Deterioration & Preservation

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Wood Properties, Deterioration & Preservation

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  1. Wood Properties, Deterioration & Preservation Penn State Unversity - CE 336 Tikalsky & Tepke

  2. Introduction to Wood • 600 Different Species of trees exist in U.S. • 15-20 are commercially used • Most are forested • Growing quantities are farmed Penn State Unversity - CE 336 Tikalsky & Tepke

  3. Fir Hemlock Pine Poplar Redwood Spruce Cedar Hickory Maple Oak Ash Birch Cypress Walnut Gum Common Species of Wood Penn State Unversity - CE 336 Tikalsky & Tepke

  4. Anatomy of a Tree • Root - absorbs moisture • Trunk - mechanical strength • rigidity • height • two way transfer • moisture UP • sap DOWN Penn State Unversity - CE 336 Tikalsky & Tepke

  5. Anatomy of a Tree • Crown - Area of chlorophyll for photosynthesis • O2 given off • CO2 absorbed • Bark - protective layer Penn State Unversity - CE 336 Tikalsky & Tepke

  6. Anatomy of a Tree • Cambium Layer - Growth layer for cell division. • 1 ring for each growing season, • Inside Layer (springwood) is hollow tube(large in softwood) • Outside Layer (summerwood) is from slower growth and has thicker walls Penn State Unversity - CE 336 Tikalsky & Tepke

  7. Anatomy of a Tree • Heartwood: storage for food. • comprised of dead cells solid filled cells • mechanically strong • No starches • Dense (nearly impermeable) Penn State Unversity - CE 336 Tikalsky & Tepke

  8. Anatomy of a Tree • Sapwood: living part of tree • fed by medullar ray cells which run perpendicular to the cambium cells. • full of moisture (green) • contains starches • very permeable (can be impregnated) Penn State Unversity - CE 336 Tikalsky & Tepke

  9. Anatomy of a Tree • Aged Sapwood (moisture removed) • same strength as heartwood • same density as heartwood • contains starches • permeable Penn State Unversity - CE 336 Tikalsky & Tepke

  10. Tree Growth • Exogenous Trees • Grow Diametrically with growth rings • Fir, Pine, Maple, Oak • Endogenous Trees • Grow by intermingling cell structures in a fibrous structure. • Bamboo, Palm, Hemp Penn State Unversity - CE 336 Tikalsky & Tepke

  11. Hard vs. Soft? • Hardwood and Softwood have nothing to do with density • Balsa is hardwood • Yew is softwood (6 times as dense as Balsa) • Hardwoods are Angiosperms (fruit or nut) • Softwoods are Gymnosperms (cones) Penn State Unversity - CE 336 Tikalsky & Tepke

  12. Hardwoods • Broad Leaves • Deciduous • Grow Slowly • Expensive • Cross-Section has vessels • Spring and Summerwood Penn State Unversity - CE 336 Tikalsky & Tepke

  13. Angiosperm Cells • Cell Structure • cavity or Lumen in the center • fiber around the cavity • wall around fibers Penn State Unversity - CE 336 Tikalsky & Tepke

  14. Softwoods • Needles • Conifer • Grow Quickly • Cheaper • Small Summerwood pores Penn State Unversity - CE 336 Tikalsky & Tepke

  15. Gymnosperm Cells • Cells are individual tubes for transport • Cells have pits • which act as permeable vents between cells Penn State Unversity - CE 336 Tikalsky & Tepke

  16. Cell Walls and Connectivity • Cellulose • Long chains of fiber • strong in tension • Lignin • glue between cellulose chains • shear resistance and durability Penn State Unversity - CE 336 Tikalsky & Tepke

  17. Density and Strength of Wood • Density and Strength are related • high density ----- thicker walls • 6 to 15 rings per inch is good strength • 6 (porous hardwoods) • 15 (softwoods) • Part of Rating System Penn State Unversity - CE 336 Tikalsky & Tepke

  18. Moisture Content • living tree may have 200% M.C. • free water • water contained in cell cavities • removed without volumetric changes • bound water • water contained within the cell walls • removal of this water caused volumetric changes Penn State Unversity - CE 336 Tikalsky & Tepke

  19. Moisture Content • “shrinkage” is from the removal of bound water • “swelling” will occur if water is reintroduced Penn State Unversity - CE 336 Tikalsky & Tepke

  20. Free Water F.S.P. Bound Water E.M.C. 0% M.C. Wt. of Solid Wood Moisture Content • "fiber saturation point" • the moisture content of wood with 100% bound water and 0% free water. • Typical value for f.s.p. is about 30 percent • Decreases below the FSP will increase strength properties. Penn State Unversity - CE 336 Tikalsky & Tepke

  21. Free Water F.S.P. Bound Water E.M.C. 0% M.C. Wt. of Solid Wood Moisture Content • Equilibrium Moisture Content (E.M.C.) • the average moisture content the wood will assume under service conditions • typically less than the fiber saturation point and greater than 5 % Penn State Unversity - CE 336 Tikalsky & Tepke

  22. Moisture Content • there is an optimum M.C. for each species (typically between 7-14 percent m.c.) • specification allow 19 percent maximum (some say 15%) • High M.C.--- promotes warping, fungus, insects • Low M.C. --- brittleness Penn State Unversity - CE 336 Tikalsky & Tepke

  23. Stress Grading Methods • Allowable Stress • including an avg. factor of safety » 2.5 • 99% of the time the f.s. will be > 1.25 • Permissible Bending Stress • Permissible Shear Stress • Permissible Compressive Stress (perp.) • Permissible Compressive Stress (// ) • Modulus of Elasticity Penn State Unversity - CE 336 Tikalsky & Tepke

  24. Stress Grading Methods • I) Visual Grading • species (6 classes) • number of rings per inch • proportion of summerwood to springwood (summerwood is denser and stronger) • heartwood vs. aged sapwood (decay resistance) • II) Mechanical Grading Penn State Unversity - CE 336 Tikalsky & Tepke

  25. Seasoning • Air Seasoning • open sheds • Inexpensive • No loss in quality of lumber • No loss in the quality of the timber • Takes up a lot of space for a long time Penn State Unversity - CE 336 Tikalsky & Tepke

  26. Seasoning • Kiln Seasoning • vented humidified oven is used to gradually reduce M.C. below 17 percent to prevent cracking. • Air circulation and air interchange • Faster than air seasoning • energy required Penn State Unversity - CE 336 Tikalsky & Tepke

  27. Lumber Grade Stamp • Lumber Grading Association • Mill number • Lumber grade • Species • Moisture condition (at time of surfacing) Penn State Unversity - CE 336 Tikalsky & Tepke

  28. Grade Stamps – Dimensional Lumber Penn State Unversity - CE 336 Tikalsky & Tepke

  29. Timber Defects • Cracks - "shakes, checks, and splits" • Heart shakes • Radial shakes, "check" • Seasoning problem: tangential shrinkage is greater than radial shrinkage • Ring shake • these are caused by resin pockets in the living tree. Penn State Unversity - CE 336 Tikalsky & Tepke

  30. Timber Defects Penn State Unversity - CE 336 Tikalsky & Tepke

  31. Timber Defects Penn State Unversity - CE 336 Tikalsky & Tepke

  32. Penn State Unversity - CE 336 Tikalsky & Tepke

  33. Lumber Sizes • - Dressed lumber: sides of members are planed or surfaced. • 2 x 4 is really 1 1/2" x 3 1/2" dressed size, "nominal" • Rough Sawn lumber: ussually 1/8" greater in size than the dressed lumber. • Full Sawn lumber: not available commercially Penn State Unversity - CE 336 Tikalsky & Tepke

  34. Sawing Patterns / Timber Defects Penn State Unversity - CE 336 Tikalsky & Tepke

  35. Lumber • Timbers (posts, beams, girders and stringers) • Dimension lumber (light framing) • Boards (siding, flooring, planking) Penn State Unversity - CE 336 Tikalsky & Tepke

  36. Timber for Marine structures • close grained, dense wood, • natural resistance to: • impact (from sea vessels) • infestation (sea creatures, borers) • fungal attack (wet rot, other fungi) • salt or wave erosion • temperature variations. • Oak, Larch, and Teak • natural oils for preservatives Penn State Unversity - CE 336 Tikalsky & Tepke

  37. Heavy Construction Work (Timber Construction) • Bridges, piles, shoring, abutments and transfer girders • Close grained • high density • impact resistant (deliberate and accidental loadings) • Chemical resistance • Oak, Larch, Douglas Fir, Southern Pine Penn State Unversity - CE 336 Tikalsky & Tepke

  38. Medium/Light Construction • Roof trusses, partition, floors, walls • resistance to insects and fungal attack, • minimum dimension changes • flame resistance • market availability • Douglas Fir, Southern Pine, Hemlock, Redwood, (other softwoods) Penn State Unversity - CE 336 Tikalsky & Tepke

  39. Falsework • Plywood, board, formwork, glue lam supports • inexpensive • available • weight • ease of handling and storage • resistance to impact and abrasion (reuse and stripping ease) • any graded wood, hemlock, pine, softwood Penn State Unversity - CE 336 Tikalsky & Tepke

  40. Glu-Laminated Timber • geometry, long spans • Engineered Properties • Shape • X-section Changes Penn State Unversity - CE 336 Tikalsky & Tepke

  41. Reaction Wood • Hard brittle wood formed on the underside of a leaning or crooked tree, usually denotes uneven wood grain. • not acceptable if identified • tension wood in hardwoods • compression wood in softwood Penn State Unversity - CE 336 Tikalsky & Tepke

  42. Fungi & bacteria deterioration Insect deterioration Marine borer deterioration Weathering & photochemical deterioration Mechanical wear Thermal deterioration Fire damage Chemical deterioration Connection deterioration Deterioration Mechanisms Penn State Unversity - CE 336 Tikalsky & Tepke

  43. Fungi & Bacteria • Bacteria • Only a problem if member is constantly submerged • Primarily a surface problem • Can increase permeability of wood structures • Much less degree of degradation than fungi…

  44. Fungi & Bacteria • Fungi • Non decay types (cosmetic problems only) • Molds • Serviceability problem • Stains

  45. Fungi Deterioration • Causes • Decay type fungi need… • A food source (wood) • Small amount of oxygen • Suitable temperatures (about 40 oF up to about 90 oF to 100 oF) • Suitable pH (best at 4-6) • Absence of poisons or toxins • Moisture Above F.S.P.

  46. Fungi Deterioration • Types of Decay Fungi • Brown-rot (soft woods – mostly) • White-rot (hard woods – mostly) • Soft-rot • Manifestations • Cubical, spongy, pocket, stringy • Depends on type

  47. Fungi Deterioration • Dry rot (A type of brown rot) • 30 to 40 percent M.C. in wood with low ventilation • Wet rot (collective term for white and brown rots) • very wet wood discolors and gets brittle • eliminate moisture, remove growths, cut out or burn adjoining timber and treat timber Penn State Unversity - CE 336 Tikalsky & Tepke

  48. Decay Fungus Typical Structure(s) Type of Rot Environ. Characteristics White cotton-like or gray felt-like substance, then red rust-like and finally cubical fracture Serpual Lacrymans (Dry-rot) Brown, cubical Buildings Humid Fibroporia Vaillantii White, fluffy substance, then white spores Brown, cubical Buildings, coal mines Humid Caniophoria Puteana (Cellular fungus) Very high humidity – water leaks, condensation Longitudinal splits or cracks, then nearly identical to dry-rot. Brittle. Brown, cubical Buildings Paxillus Panuoides Softwoods, buildings Very moist conditions Brown, cubical Fibrous, yellow, purple Oak timbers, softwoods, buildings (damp walls) White, fibrous, brown pores. In advanced stages, the wood is soft, white substance Usually near water leakage Donkioporia Expansa White Outdoor timber (telephone poles, railway sleepers, paving blocks) Lentinus Lepideus Brown, cubical Brown woody mushroom Moist Amyloporia xantha Thin plate, small yellow pores, smells like lemons Brown, cubical greenhouses --- External joints (softwood window frames) Phellinus Contiguus Fibrous strings, brown wooly Moist White Penn State Unversity - CE 336 Tikalsky & Tepke

  49. Fungi Deterioration • Mechanisms • Fungi use wood cells as food source • Cellulose & Hemicellulose (Brown rot) • Cellulose & Lignin (White rot) • Effects • Reduced strength & other properties • Degradation • Regions • Lower susceptibility to attack from SE to NW

  50. Decay Prevention • Durable, seasoned wood • Protection from water sources (vapor barriers, ventilation) • Pipes • Removal of deteriorated elements • Heartwood (extractives) • Treatments Penn State Unversity - CE 336 Tikalsky & Tepke