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
Aspen, alder, ash
birch, beech, elm
basswood, maple
spruce, fir
** These
Moderate durable
Douglas-fir, larch, pines
hemlock, tamarock
red oak, honeylocst
differences are significant
Durable
bold cypress, redwood
western redcedar
junipers, Pacific yew
catalpa, sassafras
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
Brown Rot
Wood Exposed to brown-rot fungus for 6
weeks, showing decomposing cell walls.
Wood exposed to brown-rot fungus for 12 wks,
leaving behind the middle lamella (lignin)
White Rot
Sequential white rot: Some species
consume lignin first leaving behind
delignified fibers. Some of these species
have been used to produce pulp (biopulping) and to bleach pulps (biobleaching)
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 penetrate
Into the cell walls and carve out diamond-shape cavities
Wood-Destroying Insects
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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.
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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.
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Carpenter Bees: They also do not feed on wood; usually nest in dead branches.
Termites and Ants
• Recognizing termites and ants
Termites
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.
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 roundHeaded 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.
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.
Managements of Decay and Insects
Measures against Decay
Measures against Insects
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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.
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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
Thermal Degradation of Wood
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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.
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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.
Performance of Wood Beam Under Fire
Wood Preservatives
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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.
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Penta (pentachlorophenol): Penta is soluble in organic solvents and different grades of oil (oilborne preservative); often prepared as 5% solution to treat wood. Due to it toxicity,
interior use of penta is prohibited.
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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.
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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.
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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.
Fire Retardants
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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 waterinsoluble 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.
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.
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.
Preservative and Fire Retardant Treatments
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Empty-Cell Process: When done the cell walls are coated with treatments;
usually used for preservative treatments (~ 0.2 to 0.5 lbs/ft3).