Natural Durability of WoodNatural Durability of Wood

• Sapwood of all species is not durable and heartwood of those species containing certain extractives Sapwood of all species is not durable and heartwood of those species containing certain extractives are more or less durableare more or less durable

• Phenolic extractives, which are usually dark is color and give off fragrance are responsible for wood Phenolic extractives, which are usually dark is color and give off fragrance are responsible for wood durabilitydurability

Non-durableNon-durable Moderate durable Moderate durable DurableDurable

Aspen, alder, ash Douglas-fir, larch, pines bold cypress, redwoodAspen, alder, ash Douglas-fir, larch, pines bold cypress, redwood

birch, beech, elm hemlock, tamarock western redcedar birch, beech, elm hemlock, tamarock western redcedar

basswood, maple red oak, honeylocst junipers, Pacific yewbasswood, maple red oak, honeylocst junipers, Pacific yew

spruce, fir catalpa, sassafrasspruce, fir catalpa, sassafras

** ** These differences are significantThese differences are significant

Chemical composition of crop residues similar to hardwoodsChemical composition of crop residues similar to hardwoods

Protection of Wood and Wood products

• Biological Deterioration of woodBiological Deterioration of wood

-- Woods containing fungistatic extractives, such as western redcedar and redwood, are-- Woods containing fungistatic extractives, such as western redcedar and redwood, are

durable against fungal and insect attacks. durable against fungal and insect attacks. • Fungal Decay Fungal Decay

--Brown Rot: Brown-rot fungi decompose carbohydrates (cellulose and hemicelluloses)--Brown Rot: Brown-rot fungi decompose carbohydrates (cellulose and hemicelluloses)

and use them as foods and leave lignin behind; wood may loss as much and use them as foods and leave lignin behind; wood may loss as much as 70% of its weight and all of its strength as 70% of its weight and all of its strength

--White Rot: White-rot fungi may decompose carbohydrates and lignin simultaneously --White Rot: White-rot fungi may decompose carbohydrates and lignin simultaneously

or sequentially (lignin first), and infested wood appears to be bleached.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 --Soft Rot: Soft-rot fungi attacks moist wood slowly resulting in a spongy wood surface surface

Brow rotBrow rot White rotWhite rot Soft rotSoft rot

Brown Rot

Wood Exposed to brown-rot fungus for 6 Wood Exposed to brown-rot fungus for 6 weeks, showing decomposing cell walls.weeks, showing decomposing cell walls.

Wood exposed to brown-rot fungus for 12 wks,Wood exposed to brown-rot fungus for 12 wks,leaving behind the middle lamella (lignin)leaving behind the middle lamella (lignin)

White Rot

Sequential white rot: Some species consume lignin first leaving behinddelignified fibers. Some of these specieshave 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

Soft Rot

Soft-rot fungi typically have very fine hyphae which can penetrateSoft-rot fungi typically have very fine hyphae which can penetrateInto the cell walls and carve out diamond-shape cavitiesInto the cell walls and carve out diamond-shape cavities

Wood-Destroying Insects

• Termites: This group of insects is responsible for destruction of wood in use Termites: This group of insects is responsible for destruction of wood in use because they feed on wood; the major types are subterranean because they feed on wood; the major types are subterranean termites and drywood termites termites and drywood termites

• Beetles: This group is next in economic importance; insects in this group Beetles: This group is next in economic importance; insects in this group mainly attack stressed or recently felled trees; only few of them mainly attack stressed or recently felled trees; only few of them

seriously attack wood in use, such as powder-post beetles. Bark seriously attack wood in use, such as powder-post beetles. Bark beetles attack and kill stressed trees by girdling the inner bark; they beetles attack and kill stressed trees by girdling the inner bark; they

bring in staining fungi and cause blue stain of the wood. 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 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. 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.Carpenter Bees: They also do not feed on wood; usually nest in dead branches.

Termites and Ants

• Recognizing termites and antsRecognizing termites and ants

Termites

Subterranean TermitesSubterranean TermitesCan only stay alive in humid environment; Can only stay alive in humid environment; nest in soil and build tunnels to reach wood nest in soil and build tunnels to reach wood in houses; presence of active tunnels in houses; presence of active tunnels around the foundation of buildings around the foundation of buildings indicates their attack. They consume only indicates their attack. They consume only the softer earlywood, leaving harder the softer earlywood, leaving harder latewood behind.latewood behind.

Drywood TermitesDrywood Termites

Do not depend on liquid water, they get Do not depend on liquid water, they get water from digesting wood; nest in dry water from digesting wood; nest in dry wood; very difficult to detect their presence wood; very difficult to detect their presence but sometimes may find fecal materials in but sometimes may find fecal materials in corners; must find professional help once corners; must find professional help once active colonies are found. They consume active colonies are found. They consume both earlywood and latewood, leaving only both earlywood and latewood, leaving only an empty shell. an empty shell.

Wood Damages by Beetles

Left: Left: Oak sapwood damagedOak sapwood damagedby powder-post beetles, by powder-post beetles, Inactive. Adults 2 mm in size,Inactive. Adults 2 mm in size,damages mostly done bydamages mostly done byLarvae.Larvae.

Right: Right: Powder-post beetlesPowder-post beetlesAttacking bamboo LVL.Attacking bamboo LVL.

Powder-post beetles feed onPowder-post beetles feed onStarch; they do not attack wood Starch; they do not attack wood void of starch grains.void of starch grains.

Left: Left: Wood damaged by round-Wood damaged by round-Headed borers (Larvae ofHeaded borers (Larvae oflonghorn beetles).longhorn beetles).

Right: Right: Galleries of bark beetles;Galleries of bark beetles;girdling of the inner bark kills trees.girdling of the inner bark kills trees.Beetles bring fungi spores in and Beetles bring fungi spores in and cause blue stain of the sapwood.cause blue stain of the sapwood.

Damages by Carpenter Ants and Bees

Carpenter ants carve out decayed orCarpenter ants carve out decayed orpartially decayed wood and make a partially decayed wood and make a clean “home.”clean “home.”

Carpenter bees make galleries in deadCarpenter bees make galleries in deadbranches or rotten wood and pack honeybranches or rotten wood and pack honeypollens in the galleries for larvae. They pollens in the galleries for larvae. They re-use the galleries.re-use the galleries.

Managements of Decay and Insects

Measures against Decay• Use dry and decay-free woodUse dry and decay-free wood

• Use durable or preservative-treated Use durable or preservative-treated wood for places of high hazard.wood for places of high hazard.

• Keep woodwork dry (< 20% MC)Keep woodwork dry (< 20% MC)

• Good designs for dryness and good Good designs for dryness and good

ventilation in foundation, basement ventilation in foundation, basement and attic.and attic.

• Frequent inspectionFrequent inspection..

Measures against InsectsMeasures against Insects• Use kiln-dried wood (heat kills them)Use kiln-dried wood (heat kills them)

• Keep woodwork dry.Keep woodwork dry.

• Avoid direct contact with soil; use Avoid direct contact with soil; use treated wood if contact is necessary.treated wood if contact is necessary.

• Clean rotted or insect-infested wood, Clean rotted or insect-infested wood, scraps and stumps around buildingsscraps and stumps around buildings

Thermal Degradation of Wood

• Fire destroys more wood in use than fungi and insects combined, and building fires Fire destroys more wood in use than fungi and insects combined, and building fires also cause loss of human lives.also cause loss of human lives.

• Exposure of wood to temperatures below 200 Exposure of wood to temperatures below 200 ooC for some time causes permanent C for some time causes permanent loss of wood strength as discussed.loss of wood strength as discussed.

• The following events occur when wood is exposed to high temperatures:The following events occur when wood is exposed to high temperatures:--Pyloysis: Tthermal degradation in the absence of oxygen; wood substances are degraded --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 into gases and oil, leaving a surface charcoal layer. The charcoal layer may act as a insulation to prevent further damages from external heat. 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.--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 Ignition of wood depends on surface/volume ratio, degree of confinement and

temperature (generally 200 temperature (generally 200 ooC, could be as low as 66 C, could be as low as 66 ooC)C)

--Growing: It is flameless burning of charcoal in two steps; in the first step charcoal is --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 oxidized to carbon monoxide (CO), followed by further oxidation of CO to

produce COproduce CO2 2 large amount of heat.large amount of heat.

--Smoking: Smoke is an aerosol of gases, small oil droplets, charcoal particles and water--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.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 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 combustible and always contribute to spread the flame. Treat them with fire retardants reduces flame spread.retardants reduces flame spread.

Performance of Wood Beam Under Fire

Wood Preservatives

• Creosote:Creosote: The first wood preservative for treating railroad ties; the main ingredient is coal tar; 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 may mix with wood tar or oil tar and fortified with other preservatives such as

penta penta and copper naphthanates. and copper naphthanates.

• Penta (pentachlorophenol): Penta (pentachlorophenol): Penta is soluble in organic solvents and different grades of oil Penta is soluble in organic solvents and different grades of oil (oil-(oil- borne preservative); often prepared as 5% solution to treat wood. Due to it borne preservative); often prepared as 5% solution to treat wood. Due to it toxicity, toxicity, interior use of penta is prohibited. interior use of penta is prohibited.

• CCA (chromated copper arsenate): CCA (chromated copper arsenate): This is the most important water-borne preservative; use This is the most important water-borne preservative; use 2% aqueous solution to treat wood; its main ingredient dichromate and arsenic 2% aqueous solution to treat wood; its main ingredient dichromate and arsenic

oxide oxide are acute poisons; Since 2004 CCA-treated wood is not allowed to be used in are acute poisons; Since 2004 CCA-treated wood is not allowed to be used in places where there are often human contacts. places where there are often human contacts.

• ACQ (ammoniacal copper quat): ACQ (ammoniacal copper quat): aqueous solution containing 50% copper sulfate and 50% aqueous solution containing 50% copper sulfate and 50% quaternary ammonium compounds in ammonium hydroxide. This preservative quaternary ammonium compounds in ammonium hydroxide. This preservative

is is much more benign than CCA; has replaced CCA since 2004 to treat lumber for 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 decks an playground structures, etc.; its long term performance has yet to be

seen;seen;

Its current problem is its metal corrosiveness. Its current problem is its metal corrosiveness.

• Borates: Borates: Borates, such as borax (NaBorates, such as borax (Na22BB44OO77) and boric acid (H) and boric acid (H33BOBO33), are colorless, odorless and ), are colorless, odorless and

benign chemicals very effective to protect wood from fungi and insect attacks; 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 because of water solubility they can be leached out when treated lumber is

used in used in outdoor exposure. outdoor exposure.

Fire Retardants

• Treating wood with fire retardants can not protect wood from being destroyed by fire. Treating wood with fire retardants can not protect wood from being destroyed by fire. Under elevated temperatures, effective fire retardants accelerate wood decomposition, Under elevated temperatures, effective fire retardants accelerate wood decomposition, increase charcoal formation and reduce production of flammable gases. increase charcoal formation and reduce production of flammable gases.

• Effective fire retardants are those contain at least one of the elements phosphor (P), 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 (NHnitrogen (N), boron (B) and chlorine (Cl), such as ammonium phosphates (NH44HH22POPO4 4

and (NHand (NH44) ) 22HPOHPO44), borax (), borax (NaNa22BB44OO77) and zinc chloride (ZnCl) and zinc chloride (ZnCl22).).

• To be effective, Wood must be treated to a high loading of fire retardants, more than 2 To be effective, Wood must be treated to a high loading of fire retardants, more than 2 pounds/ftpounds/ft33. Most inorganic fire retardants can cause chemical degradation of wood . Most inorganic fire retardants can cause chemical degradation of wood when the treated wood is used in warm and humid conditions.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-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 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 water-insoluble organic fire retardants would not harm the wood under warm and humid conditions, but under very high temperatures will breakdown into components humid conditions, but under very high temperatures will breakdown into components to perform the tasks of wood decomposition, charcoal formation and reduction of to perform the tasks of wood decomposition, charcoal formation and reduction of flammable gas evolution.flammable gas evolution.

Preservative and Fire Retardant Treatments

• Pre-treatmentsPre-treatments

----Poles, pilings and lumber must be dried (water removed) to accept Poles, pilings and lumber must be dried (water removed) to accept treatments.treatments.

--All machining done before treating: There is a limit how deep the 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 can penetrate into the wood but after treating the treatments form a protective shell. If machining is done after treating the protective protective shell. If machining is done after treating the protective

envelope envelope would be broken, also wastes the treatments and creates a problem of would be broken, also wastes the treatments and creates a problem of disposing the wastes. disposing the wastes.

--Some species of are very difficult to treating, therefore the surfaces of large --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 wood members such as poles, pilings and railroad ties are incised to facilitate penetration.facilitate penetration.

Preservative and Fire Retardant Treatments

• Full-Cell Process: When done the wood cells are filled with Full-Cell Process: When done the wood cells are filled with treatments; for maximum treatment (> 2 lbs/fttreatments; for maximum treatment (> 2 lbs/ft33); fire retardant ); fire retardant treatments is done with this process.treatments is done with this process.

Preservative and Fire Retardant Treatments

• Empty-Cell Process: When done the cell walls are coated with treatments; Empty-Cell Process: When done the cell walls are coated with treatments; usually used for preservative treatments (~ 0.2 to 0.5 lbs/ftusually used for preservative treatments (~ 0.2 to 0.5 lbs/ft33).).