Wood Structure, Properties, Classification and Defects Explained

Wood Composition

Wood is composed of fibrous and healthy cellulose, solidified with water, cellulose (base is 40-50%), hemicellulose (prevents cracks, 20-25%, the fiber acts as cement), tannin (color), lignin (impermeability, 25-30%), alkaloids and mineral substances. Essential elements: carbon 46-50%, oxygen 38-42%, hydrogen 6%, nitrogen 1%, and minerals 10% (sodium, potassium, phosphorus, sulfur, calcium). It is used in construction because it is an elastic, resistant and durable material.

Parts of a Trunk

• Bark: the outer layer formed by an impermeable tissue.
• Liber: a thin, not very resistant membrane, made of filaments. Used in construction (cork).
• Cambium: located between the bark and the wood, it is responsible for the growth of the tree.
• Sapwood or alburnum: the outer part of the trunk, made of young tissues that contain much water and organic substances. It is attacked by organisms and parasites and is the living area.
• Heartwood: the inner part of the trunk, made of old tissues that are strong thanks to lignin. It is good wood.
• Pith: the central part of the trunk, very porous and loose, not used in construction.

Physical Properties

Water Content

• Constitution water: water integrated into the trunk that does not evaporate.
• Saturation water: water retained in the membranes of the cell walls.
• Free water: water that disappears when the tree is cut. Sapwood contains more water than heartwood. Timber is considered dry at 13 to 18% water.

Thermal Conductivity

The value is 0.12 kcal/m°C, which is very low. It is much lower than that of aluminum.

Mechanical Properties

Hardness

Timber varies in hardness. Heartwood is harder than sapwood. Trees that grow slowly have harder wood than those that grow fast.

• Hard: ebony, service tree, oak, yew.
• Semi-hard (medium hard): chestnut, beech, walnut, pine, white pine.
• Soft: alder, willow, Scots pine, larch.
• Very soft: poplar, lime.

Workability

Ease of working with timber, especially lengthwise. Chestnut is an example.

Resistance to Bending

Very good. Dense wood has a high resistance.

Resistance to Tension

Very high when the load is parallel to the fibers. It decreases when the load is perpendicular (to 43-50).

Resistance to Compression

Very high when the load is parallel to the fibers (160-200). Good wood for beams and pillars.

Resistance to Shear

Depends on the degree of cohesion of the fibers.

Fatigue Resistance (Toughness)

The resistance of the material to repeated loading and unloading cycles. Materials that resist fatigue are very tough, and the limit of this property is very important.

Fire Behavior

Wood reacts very poorly to fire, although it is good for fire resistance. Wood can be treated with fireproofing agents (salt baths, fireproof paint).

Classification of Wood

Conifers or Resinous Woods

Species that are old, fast-growing and treated. They are not suitable for construction because of their bonds and mechanical characteristics. Examples: fir (very resistant), cedar (easy to work), pine (flexible and hard), white pine, red pine (large beams and trusses), yew, cypress, Scots pine (protected), larch.

Hardwoods

Tough, compact and resistant. Found in temperate or tropical zones.

• Hard: acacia, alder, elm, oak (parquet), ash, maple, eucalyptus.
• Soft: pear, cherry, walnut, lime, birch, olive.

Exotic Tropical Woods

Tropical woods are decorative elements, parquet and wood that is very good and very dense. Examples: bubinga, sapele, bokapi, bolondo (parquet), mongoy, limoncillo (furniture), teak, ebony, mahogany, sucupira, ipe, jatoba, merbau.

Commercial Morphology of Wood

Wood with Bark

Log / round log: wood with the bark still on the trunk. No processing.

Sawn or Processed Timber

• Beam: rectangular cross-section, sharp edges, used in concrete structures and arches. Sizes 25x35, 15x20, with lengths of 4 to 10 m.
• Joist: thinner, lighter, forged from 8x8 to 15x15, with lengths up to 5 m.
• Plank: rectangular cross-section, thin sharp edges, used as auxiliary material. Sizes 5x10 and 10x30.
• Post: 20 to 40 cm wide, 2 to 5 cm thick and 5 to 10 m long (thin formwork).
• Platform: pieces with a thickness of 2.3 x 20 cm, 5 x 40 cm, and lengths of 4 to 5 m.
• Lath (parquet): very low-quality board, 2 to 3 cm thick, 5 to 7 cm wide and 5 m long.
• Sheet: thin and thick pieces of variable length, less than 1 mm thick. Applied over many other types of timber.
• Batten: small pieces, rectangular cross-section, 4 x 6 and 5 x 8.
• Small lath: same cross-section, medium sizes of 1.3 x 2.5 cm and 2 x 4 cm (up to 3 m long).

Derivatives of Wood

These products have no excess of minerals and are not susceptible to parasites.

• Veneer panel: thin veneer sheets of wood glued together, cheaper to produce.
• Plywood: layers of sheets placed crosswise and glued together, which gives dimensional stability, good shape and a strong surface.
• Laminated panel: layers glued together on all sides, with the grain in the same direction (large sheet surfaces).
• Reconstituted wood: made with excess minerals from raw wood. Homogeneous, compact, hard and polished, with no parasites or deformations. Does not crack and is resistant to traction.
• Fiberboard: wood pulp fibers mixed with resins. Types include soft, hard and medium (MDF). Not visible as particles. These boards are solid and durable. Their resistance depends on the type of resin and the manufacturing method. Wet process fibers are used for soft boards.
• Particleboard: visible particles, shavings or chips of wood or wood waste agglomerated with resins (urea, melamine and derivatives of formaldehyde) and compressed.
• Particleboard with plastic finish: melamine, Formica, bakelite, PVC, PP.
• Cement-bonded particleboard: very good fire resistance. It is breathable.
• Gypsum-bonded particleboard and magnesite-bonded particleboard: natural binders, with good acoustic properties.
• Wood wool board: visible particles, such as "Novopan".
• Cork agglomerate: made from cork granules, cooked, with good thermal insulation.
• Glued laminated timber: made by joining and gluing pieces of wood slats and sections, reduced and adjusted to achieve high resistance. Manufacturing systems ensure perfect adhesion.

Finishes

• Oil and varnish: resin solutions (urea, melamine) that create a protective film and provide hardness. Water-based.
• Wax: natural and artificial.
• Stain: color.
• Lasur: a coating that impregnates timber (not a surface coating), allowing the wood to breathe and protecting it from fungi and insects.
• Paint: varnishes with pigments.

Defects and Alterations of Wood

Anomalies

Anomalies appear in the fibers and in the growth of trees.

• Knots: produced by branches. They affect the beam when it works in tension.
• Twisted fiber: fibers grow in a spiral shape. Peripheral fibers grow faster than internal ones. Very problematic for construction, only when working in compression.
• Eccentricity of the pith: not located in the center, so the trunk is not symmetric. Caused by wind or rocky ground. Low elasticity, low resistance. Hard to work and not suitable for construction.

Defects

Related to the tissues of the tree.

• Bark inclusion: bark enclosed between rings, formed by welding of the branch. This gives low resistance to the beams and can cause them to break. Wood is easily rejected.
• Double sapwood: a dead zone, and over time the sapwood decomposes. It is completely rejected for construction. Very large and intense.

Alterations

Occur after growth.

• Cracks / fissures: deep or shallow fissures in the longitudinal direction of the tree. Deep cracks are caused by frost. Small cracks produce little harm.
• Shakes: separation of rings. Weak, with little resistance. Caused by intense frost and violent winds. Such wood is not used for structures.
• Chicken-coop cleft: cracks that reach the pith. Caused by the environment of the tree. Wood is useless once rejected.