Natural Durability of Wood

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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
• Chemical composition of crop residues similar
  to hardwoods

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

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Termites and Ants

• Recognizing termites and ants

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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.

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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.
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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.
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Preventive Measures Against Decay and Insects

• Measures against Decay
• Use dry and decay-free wood
• Use durable or preservative-treated wood for
  places of high hazard.
• Keep woodwork dry (lt 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

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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.

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Performance of Wood Beam Under Fire
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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.

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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.

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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.

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Preservative and Fire Retardant Treatments

• Full-Cell Process When done the wood cells are
  filled with treatments for maximum treatment (gt
  2 lbs/ft3) fire retardant treatments is done
  with this process.

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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).

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Diffusion Treatment with Borates

• In China large columns are traditionally
• finished by wrapping columns with chess
• cloth, followed by a layer of plaster to
• provide a smooth surface for painting.
• In the proposed method, the installed
• green wood columns could be tightly
• wrapped with chess cloth and followed
• with a layer of octaborate paste. Then,
• the columns would be wrapped with
• plastic sheets for up to six weeks for borate
• diffusion and to prevent drying, after which the
  plastic sheets are removed to allow air-drying
  for few more weeks before application of plaster
  and final painting. The whole process would take
  few months to complete but the treated columns
  would be resistant to decay and insect attacks
  for a long service.