Natural Durability of Wood

1. Natural Durability of Wood • Sapwood of all species is not durable and heartwood of those species containing certain extractives are more or less durable • Phenolic extractives, which are usually dark is color and give off fragrance are responsible for wood durability Non-durable Moderate durable Durable Aspen, alder, ash Douglas-fir, larch, pines bold cypress, redwood birch, beech, elm hemlock, tamarock western redcedar basswood, maple red oak, honeylocst junipers, Pacific yew spruce, fir catalpa, sassafras ** These differences are significant Chemical composition of crop residues similar to hardwoods

2. Protection of Wood and Wood products • Biological Deterioration of wood -- Woods containing fungistatic extractives, such as western redcedar and redwood, are durable against fungal and insect attacks. • Fungal Decay --Brown Rot: Brown-rot fungi decompose carbohydrates (cellulose and hemicelluloses) and use them as foods and leave lignin behind; wood may loss as much as 70% of its weight and all of its strength --White Rot: White-rot fungi may decompose carbohydrates and lignin simultaneously or sequentially (lignin first), and infested wood appears to be bleached. --Soft Rot: Soft-rot fungi attacks moist wood slowly resulting in a spongy wood surface Brow rot White rot Soft rot

3. Brown Rot Wood exposed to brown-rot fungus for 12 wks, leaving behind the middle lamella (lignin) Wood Exposed to brown-rot fungus for 6 weeks, showing decomposing cell walls.

4. White Rot Sequential white rot: Some species consume lignin first leaving behind delignified fibers. Some of these species have been used to produce pulp (bio- pulping) and to bleach pulps (bio-bleaching) Simultaneous white rot: species that Consume carbohydrate and lignin simultaneously by carving out cell walls

5. Soft Rot Soft-rot fungi typically have very fine hyphae which can penetrate Into the cell walls and carve out diamond-shape cavities

6. Wood-Destroying Insects • Termites: This group of insects is responsible for destruction of wood in use because they feed on wood; the major types are subterranean termites and drywood termites • Beetles: This group is next in economic importance; insects in this group mainly attack stressed or recently felled trees; only few of them seriously attack wood in use, such as powder-post beetles. Bark beetles attack and kill stressed trees by girdling the inner bark; they bring in staining fungi and cause blue stain of the wood. • Carpenter Ants: They do not feed on wood, only nest in wood. They make nests by carving out decayed or partially decay wood to build the colonies. • Carpenter Bees: They also do not feed on wood; usually nest in dead branches.

7. Termites and Ants • Recognizing termites and ants

8. Subterranean Termites Can only stay alive in humid environment; nest in soil and build tunnels to reach wood in houses; presence of active tunnels around the foundation of buildings indicates their attack. They consume only the softer earlywood, leaving harder latewood behind. Drywood Termites Do not depend on liquid water, they get water from digesting wood; nest in dry wood; very difficult to detect their presence but sometimes may find fecal materials in corners; must find professional help once active colonies are found. They consume both earlywood and latewood, leaving only an empty shell. Termites

9. Wood Damages by Beetles Left: Oak sapwood damaged by powder-post beetles, Inactive. Adults 2 mm in size, damages mostly done by Larvae. Right: Powder-post beetles Attacking bamboo LVL. Powder-post beetles feed on Starch; they do not attack wood void of starch grains. Left: Wood damaged by round- Headed borers (Larvae of longhorn beetles). Right: Galleries of bark beetles; girdling of the inner bark kills trees. Beetles bring fungi spores in and cause blue stain of the sapwood.

10. Damages by Carpenter Ants and Bees Carpenter ants carve out decayed or partially decayed wood and make a clean “home.” Carpenter bees make galleries in dead branches or rotten wood and pack honey pollens in the galleries for larvae. They re-use the galleries.

11. Measures against Decay Use dry and decay-free wood Use durable or preservative-treated wood for places of high hazard. Keep woodwork dry (< 20% MC) Good designs for dryness and good ventilation in foundation, basement and attic. Frequent inspection. Measures against Insects Use kiln-dried wood (heat kills them) Keep woodwork dry. Avoid direct contact with soil; use treated wood if contact is necessary. Clean rotted or insect-infested wood, scraps and stumps around buildings Managements of Decay and Insects

12. Thermal Degradation of Wood • Fire destroys more wood in use than fungi and insects combined, and building fires also cause loss of human lives. • Exposure of wood to temperatures below 200 oC for some time causes permanent loss of wood strength as discussed. • The following events occur when wood is exposed to high temperatures: --Pyloysis: Tthermal degradation in the absence of oxygen; wood substances are degraded into gases and oil, leaving a surface charcoal layer. The charcoal layer may act as a insulation to prevent further damages from external heat. --Combustion: It is burning of flammable gases evolved from pyrolysis on the wood surface. Ignition of wood depends on surface/volume ratio, degree of confinement and temperature (generally 200 oC, could be as low as 66 oC) --Growing: It is flameless burning of charcoal in two steps; in the first step charcoal is oxidized to carbon monoxide (CO), followed by further oxidation of CO to produce CO2 large amount of heat. --Smoking: Smoke is an aerosol of gases, small oil droplets, charcoal particles and water vapor. It is the most deadly part of a building fire. • Wood & wood products often are not the culprit to start a building fire, but they are combustible and always contribute to spread the flame. Treat them with fire retardants reduces flame spread.

15. Performance of Wood Beam Under Fire

16. Wood Preservatives • Creosote:The first wood preservative for treating railroad ties; the main ingredient is coal tar; may mix with wood tar or oil tar and fortified with other preservatives such as penta and copper naphthanates. • Penta (pentachlorophenol): Penta is soluble in organic solvents and different grades of oil (oil- borne preservative); often prepared as 5% solution to treat wood. Due to it toxicity, interior use of penta is prohibited. • CCA (chromated copper arsenate): This is the most important water-borne preservative; use 2% aqueous solution to treat wood; its main ingredient dichromate and arsenic oxide are acute poisons; Since 2004 CCA-treated wood is not allowed to be used in places where there are often human contacts. • ACQ (ammoniacal copper quat): aqueous solution containing 50% copper sulfate and 50% quaternary ammonium compounds in ammonium hydroxide. This preservative is much more benign than CCA; has replaced CCA since 2004 to treat lumber for decks an playground structures, etc.; its long term performance has yet to be seen; Its current problem is its metal corrosiveness. • Borates: Borates, such as borax (Na2B4O7) and boric acid (H3BO3), are colorless, odorless and benign chemicals very effective to protect wood from fungi and insect attacks; because of water solubility they can be leached out when treated lumber is used in outdoor exposure.

17. Fire Retardants • Treating wood with fire retardants can not protect wood from being destroyed by fire. Under elevated temperatures, effective fire retardants accelerate wood decomposition, increase charcoal formation and reduce production of flammable gases. • Effective fire retardants are those contain at least one of the elements phosphor (P), nitrogen (N), boron (B) and chlorine (Cl), such as ammonium phosphates (NH4H2PO4 and (NH4) 2HPO4), borax (Na2B4O7) and zinc chloride (ZnCl2). • To be effective, Wood must be treated to a high loading of fire retardants, more than 2 pounds/ft3. Most inorganic fire retardants can cause chemical degradation of wood when the treated wood is used in warm and humid conditions. • It is more desirable to treat wood with combinations of chemicals so that water-insoluble organic compounds containing P, N, B or Cl are formed in wood. These water-insoluble organic fire retardants would not harm the wood under warm and humid conditions, but under very high temperatures will breakdown into components to perform the tasks of wood decomposition, charcoal formation and reduction of flammable gas evolution.

18. Preservative and Fire Retardant Treatments • Pre-treatments --Poles, pilings and lumber must be dried (water removed) to accept treatments. --All machining done before treating: There is a limit how deep the treatments can penetrate into the wood but after treating the treatments form a protective shell. If machining is done after treating the protective envelope would be broken, also wastes the treatments and creates a problem of disposing the wastes. --Some species of are very difficult to treating, therefore the surfaces of large wood members such as poles, pilings and railroad ties are incised to facilitate penetration.

19. Preservative and Fire Retardant Treatments • Full-Cell Process: When done the wood cells are filled with treatments; for maximum treatment (> 2 lbs/ft3); fire retardant treatments is done with this process.

20. Preservative and Fire Retardant Treatments • Empty-Cell Process: When done the cell walls are coated with treatments; usually used for preservative treatments (~ 0.2 to 0.5 lbs/ft3).