Wood Properties and Classification

Posted on May 4, 2024 in Geology

WOOD

Wood is a fibrous material composed of cells firmly united, consisting of water, cellulose (40-50%), hemicellulose (20-25%), tannin, lignin (25-30%), alkaloids, and other substances. Essential elements include carbon (46-50%), oxygen (38-42%), hydrogen (6%), nitrogen (1%), and other elements like sodium, potassium, phosphorus, sulfur, and calcium (10%). Wood is commonly used in construction due to its elasticity, lightweight nature, insulating capacity, and resistance.

Components of a Tree Trunk

  • Bark: The outer layer, forming a waterproof fabric.
  • Liber: A less tough membrane with small filaments, not ideal for construction (e.g., cork).
  • Cambium: Located between the bark and sapwood, responsible for tree growth.
  • Sapwood or Alburnum: The external part of the trunk containing young tissues, more water, and organic substances. It is susceptible to parasitic organisms and represents the living area of the tree.
  • Heartwood: The internal wood with strong lignin bonds, considered good quality wood.
  • Pith: The central part of the trunk with porous and loose tissue, not suitable for construction.

Physical Properties

Water Content

Three types of water exist in wood:

  1. Constitutional Water: Integral to the trunk and evaporates upon drying.
  2. Saturation Water: Retained between the membranes of the woody material.
  3. Free Water: Disappears when the tree is cut.

Sapwood generally contains more water than heartwood. Dry wood typically has a water content of 13-18%.

Thermal Conductivity

Wood has a low thermal conductivity value (0.12 kcal/hm°C), making it a better thermal insulator than aluminum.

Mechanical Properties

Hardness

Older woods, particularly heartwood, are generally harder than younger woods or sapwood. Fast-growing woods tend to be less hard. Wood hardness can be classified as follows:

  • Very Hard/Hard: Ebony, server, oak, yew
  • Medium Hard/Average Hardness: Chestnut, beech, walnut, Galician pine, white pine
  • Soft: Spruce, willow, pinaster pine, larch
  • Very Soft: Poplar, lime

Workability

Wood is generally easy to cut in the longitudinal direction, such as chestnut.

Mechanical Resistance

  • Bending: Wood exhibits very good resistance to bending, making it suitable for structural applications.
  • Tensile Strength: Very high when parallel to the fibers.
  • Compressive Strength: High when parallel to the fibers (160-200), but reduced when perpendicular (43-50). Suitable for beams and pillars.
  • Shear Strength: Dependent on the degree of humidity.
  • Fatigue Resistance (Toughness): Resistance to repeated loading and unloading cycles. Humidity plays a significant role in fatigue resistance.

Wood responds well to fire and exhibits good resistance, even when subjected to fire-retardant treatments.

Wood Classification

Softwood or Coniferous Wood

These are the oldest species, found in cold and temperate zones. They possess good mechanical properties, making them suitable for construction. Examples include:

  • Fir (very resistant)
  • Cedar (easy to work with)
  • Pine (good elasticity and toughness)
  • Galician Pine, White Pine/Carrasco Pine (auxiliary pieces)
  • Larch (platforms)
  • Red Pine (large beams and trusses)
  • Yew
  • Cypress
  • Juniper (protected)

Hardwood

Hard, compact, and resistant woods found in temperate or tropical areas. They can be further classified as:

  • Hard: Acacia, alder (furniture), southern alder, elm (boards), oak (flooring), ash, maple, eucalyptus
  • Soft: Pear, cherry, walnut, linden (boards), birch (boards), olive

Tropical and Exotic Woods

Found in the tropics and often used for decorative elements, parquet, and high-quality, dense wood. Examples include:

  • Bubinga
  • Sapele
  • Okoume
  • Bolondo (flooring)
  • Mongoy
  • Limoncillo (furniture)
  • Teak
  • Ebony
  • Mahogany
  • Sucupira
  • Ipe
  • Jatoba
  • Merbau

Commercial Morphology of Wood

Solid Wood

  • Log/Bole: Solid wood without bark or specific structure.

Sawn or Transformed Wood

  • Beam: Rectangular section with live edges, used for structures like arcades. Sizes range from 25×35 to 15×20 with lengths of 4-10m.
  • Joist: Smaller than beams, used for making frameworks. Sizes range from 8×8 to 15×15 with lengths less than 5m.
  • Board: Rectangular section with thin live edges, used as an auxiliary piece. Sizes range from 5×10 to 10×30.
  • Post: Width of 20-40cm, thickness of 2-5cm, and length of 5-10m. Used for thin formwork.
  • Platform: Interlocking boards with a thickness of 2-3cm, width of 20-40cm, and length of 4-5m. Used for flooring.
  • Latin: Low-quality board with a thickness of 2-3cm, width of 5-7cm, and length of 5m.
  • Veneer: Very thin piece with a thickness less than 1mm, applied to lower quality wood.
  • Strip: Small piece with a rectangular section, sizes 4×6 or 5×8.
  • Small Strip: Similar to a strip but with smaller sizes, such as 1.3×2.5cm for windows and 2x4cm for other applications.

Wood Products

These products have low porosity and are free from parasites.

  • Veneered Board: Board with a veneer sheet adhered to a more economical plywood.
  • Plywood: Overlapping sheets in a crossed position, assembled with glue for shape and dimensional stability. Used for supermarkets.
  • Laminated Board: Overlapping boards assembled with glue, with all veneers in the same direction. Used for supermarkets.

Regenerated Wood

Made from leftover raw materials from other woods. It is homogeneous, compact, hard, polished, free from parasites and strains, does not crack, and is moisture-resistant.

  • Fiberboard: Made from fibrous wood pulp and resins. Types include DM, Tablex, etc. They are sound, solid, and durable boards with resistance to moisture and bending depending on the type and manufacturing method.
  • Particle Board or Chipboard: Composed of wood wool or chips with resins (urea, melamine, and phenol derivatives) and compressed. Particles are visible.
  • Particle Board with Plastic Finish: Includes melamine, Formica, bakelite, PVC, and PS.
  • White Cement Particle Board: Excellent coating, fire-resistant, and breathable.
  • Particle Board with Binders: Uses natural binders.
  • Strand Board: Composed of strands, such as “NOVOPAN”.

Agglomerated Cork

Cork is cooked and pressed, resulting in good thermal insulation.

Glued and Laminated Wood

Obtained by joining and gluing pieces of wooden slats and small sections with high-strength adhesives. Manufacturing systems ensure perfect adhesion.

Wood Finishes

  • Oil: Provides a protective layer.
  • Varnish: Resin solutions (urea, melamine) create a protective film that increases resistance and hardness. Water-repellent.
  • Wax: Natural and artificial waxes for protection.
  • Dye: Adds color to the wood.
  • Lasur: A coating that impregnates the wood, regulating humidity, and protecting against fungi and insects.
  • Paint: Pigmented varnishes for color and protection.

Wood Defects and Alterations

Anomalies

These are linked to the fibers and tree growth.

  • Knots: Produced by branches, they can cause issues in traction when used in beams.
  • Twisted Fibers: Fibers that grow in a helix shape due to faster growth of peripheral fibers. Not suitable for construction, except in compression applications.
  • Pith Eccentricity: When the pith is not centered in the shaft, resulting in an asymmetrical trunk. Causes include wind and rocky terrain. This leads to low elasticity, low resistance, and difficulty in working, making it unsuitable for construction.

Defects

These are related to the tissues of the tree.

  • Bark Pocket: A piece of bark trapped between the rings due to poor branch welding. This results in weak and potentially diseased timber, which is rejected for construction.
  • Sapwood Pocket: A dead zone within the sapwood that eventually breaks. Rejected for construction, often caused by prolonged and intense cold.

Alterations

These occur after the tree’s growth.

  • Cracks/Fissures: Deep or shallow cracks in the longitudinal direction of the tree. Deep cracks are caused by ice, while small cracks result from drying.
  • Ring Shake: Separation of weak rings, leading to very low resistance. Caused by cold and violent winds, making the wood unsuitable for structural construction.
  • Star Shake: Fissures that reach the pith and extend outward. Caused by aging of the tree, rendering the wood useless.