Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

Timber

VIEWS: 0 PAGES: 40

									Timber Degradation &
Preservatives


   Week 10
Durability of Timber
   Timber comes under attack from a number
    of sources:
   Biological
       Fungi
       Insects
   Fire
Longevity
   If protected from fire, insect & fungal
    attack, timber structures can survive for
    extremely long periods (in excess of 2000
    years)
Natural Durability
   Good resistance to atmospheric exposure
   Virtually unaffected by rain, frost etc.
   Sunlight can degrade timber if exposure is
    prolonged – breaks down lignin adhesive –
    appears bleached & fibrous
   Mainly attacked for food – fungi & insects
    – enzymes ‘digest’ the cellulose fibres
    and/or the lignin adhesive
Photochemical Degradation
   Exposure to sunlight causes a change in
    colouration - heartwood lightens
    (mahogany, oak) – some darken (teak)
   Exterior expose is the most severe & in a
    few months ‘weathering’ will take place
   Light, rain & wind all contribute to the
    weathering process – silver grey
    appearance
Cont’d
   Produces loss of surface integrity due to the
    breakdown of lignin under the action of
    ultraviolet light
   Further exposure will cause the shortening of the
    chain length in the cellulose – erosion of the cell
    wall results
   Timber becomes brittle & resistance to load is
    reduced as the damaged lignin cannot fully
    transfer the stress
Cont’d
   The surface damage builds to protect &
    filters the UV light – slows the effects of
    weathering – slow process – 1mm/20 years
   The application of surface protection is
    recommended – weathered surface must be
    cleaned prior to treatment
Chemical Degradation
   Generally timber is highly resistant to
    various chemicals
   Timber is more resistant to mild acids than
    cast iron or mild steel
   Timber has lower resistance to alkalis –
    dissolves lignin & hemicelluloses
Cont’d
   Iron salts are acidic in the presence of
    moisture & leads to hydrolytic degradation
    of the timber – softening & discolouration
    in the area of iron fastenings
   Corrosion of metal fittings on boats causes
    chemical decay of the timber – ‘nail
    sickness’ – electrochemical effect
    controlled by the availability of oxygen
Thermal Degradation
   Timber heated to 120°C for a period of about one
    month will experience a loss in strength of about
    10% - small increases in temperature above this
    value will accelerate the process
   Browning of the timber takes place indicating the
    thermal damage & a caramel like odour can be
    detected – degradation of the hemicelluloses –
    continued exposure will affect the cellulose
Mechanical Degradation
   Timber is stressed under load for long
    periods – creep
   Duration of load, creep & the associated
    loss of strength with time – 50 years of
    loading – strength approximately 50%
   Designers apply time modification factors
Cont’d
   Compression failure – can occur naturally due to
    the formation of ‘kinks’ in the cell walls under
    high compressive stress or as brittle heart due to
    growth stresses in the centre of the trunk
   Service conditions can induce over stressing of
    the cell walls due to longitudinal compression
   Results in reduced tensile strength & a major loss
    of toughness
Fungal Attack
   Directly linked to the natural durability of
    the timber
   The resistance can be explained by the
    make-up of the cell wall & the deposition
    of extractives
   The lignin offers some degree of protection
    against fungal attack
   Requires a moisture content of at least 20%
Cont’d
   Durability of heartwood varies according to
    species & is dependant on the type &
    quantity of extractives
   Sapwood of all timbers is susceptible to
    attack due to the absence of extractives &
    the presence of rays which store starch –
    this acts as food for the potential attacking
    fungus
Dry Rot
   Dry rot – Serpula lacrymans – a brown
    rot
   Leaves wood in a dry friable condition
   Affects areas without good ventilation
   Rust red spores come into contact with
    damp timber
Cont’d
   Fungus develops as branching white
    strands (hyphae)
   Form cotton-wool like patches
    (mycelium)
   Finally forms soft-fleshy spore producing
    fruiting bodies (sporophore)
Cont’d
   Grow rapidly once established
   Modify into vein like structures
    (rhizomorphs) 2 to 3mm in diameter
   Killed at temperatures above 40°C
   Can lie dormant at temperatures
    approaching freezing or if timber dries out
   Prolonged dry periods over 1 year may
    cause it to die off
Wet Rot
   Wet rot – Require higher moisture contents
    in order to develop
   Optimum value 50% - various types of wet
    rot – the following types are common in
    UK:
Cont’d
   Coniophora puteana – cellular fungus
   very damp situations – basements
   brown – cube formation
   large timber sections – skin of sound timber may
    be present – surface undulations
   Cut out & burn affected areas – remove affected
    plaster – use preservatives – rectify dampness
Cont’d
   Phellinus contiguus – common in window
    joinery
   white rot – wood breaks into soft strands –
    inadequate glue, poor design etc.
   water penetration – fungal growth – can occur in
    combination with insect attack (weevils)
   Cut out affected timber – replace with new timber
    – use of preservative – remove cause of dampness
Life Cycle of a Fungus
Insect Attack
   Beetles – life cycle – egg – caterpillar
    (lava) – chrysalis – adult beetle
   Beetle then fly to new timber & cycle is
    repeated
   Burrowing larvae cause most of the
    damage
   Does not require wet timber – sapwood is
    more susceptible
Identification of Insect Attack
   Insect attack does not require the timber to
    be damp – although higher moisture
    contents are preferred
   Flight holes may be visible in the timber
   Dust ejected from holes relates to the level
    of activity – a light colour dust indicating a
    recent infestation
   Advanced attack – serious loss of strength
Common Insects
   Powder post beetle – mainly found in
    timber yards – can be found inside
    seasoned timber
   Common furniture beetle – widespread –
    can take many years to become evident
Cont’d
   House longhorn beetle – rare – needs the
    right climate – found in the south of the
    country – long life cycle (6 years) – skin of
    timber left in sound condition with not
    many flight holes
   Death watch beetle – a problem in decayed
    oak timbers in old buildings – associated
    with damp conditions
Life Cycle of a Common
Furniture Beetle
Preservatives
   Preservatives should be toxic to fungi
    and/or insects
   Should be of sufficient chemical stability
    for the environment in which they are used
   Should be able to penetrate the timber & be
    non aggressive to surrounding materials
Selection Process
   Based on:
       Service life
       Natural durability of timber used
       Risk of attack
       Ease of inspection
       Risk of structural failure
       Ease of repair
Types Used
   Tar oil – creosotes – external use
   Water borne – salts based on metals –
    (sodium, zinc, arsenic, copper etc..) – cause
    swelling of timber – cause corrosion of
    metals in contact until drying is complete
Cont’d
   Organic solvent – metallic naphthanates – gives
    excellent penetration - may be brushed or
    sprayed – can be painted once dry – expensive &
    release solvents on drying
   Boron diffusion – freshly felled ‘green’ timber
    processed by dipping in to a borate solution
    which diffuses through the wood using its own
    moisture
Methods of Application
   Brushing – limited penetration – requires
    repeated treatments
   Spraying – limited penetration – requires
    repeated treatments
   Dipping – immersion for a suitable period
    of time – considerable penetration – not
    suited to hardwoods
Cont’d
   vacuum - sealed chamber - preservative
    introduced - vacuum released forcing in the
    preservative - vacuum reapplied after a
    period of soaking
   pressure impregnation - injection of water
    based & tar oil based preservatives -
    pressure used to give greater depth of
    penetration – excellent protection
Fire Resistance
   Temperatures greater than 300°C leads to the
    ignition of flammable gases – pyrolysis
   Timber surface heats up due to low thermal
    conductivity – flames easily – fire spreads –
    flames, heat & smoke – possible structural failure
   Rate of burning is slow in large sections of timber
    – due to charcoal formation – acts as an insulator
    – evaporating moisture diffuses inward
Rate of Burn
   Charring rate is predictable:
       0.64 mm/min (density < 650kg/m3)
       0.50 mm/min (density > 650kg/m3)
   Structures can be designed to include fire
    resistance – sacrificial timber thickness
    above that required for structural purposes
Fire Protection
   Fire retardant coating are available –
    increase the proportion of charcoal &
    decrease the production of combustible
    gases
   Intumescent paints, varnishes or pastes can
    be applied – protective films form under
    the action of fire – prevent oxygen from the
    process
Cont’d
   Application of such coatings do not reduce
    charring rates
   Best option to guard against structural
    failure is to provide sacrificial timber
Engineered Timber
   As conventional building materials become
    more expensive & difficult to procure,
    more house builders are using engineered
    materials.
   Engineered wood products are becoming
    widely used with mill floor waste forming
    part of a new age of building materials
    rather than contributing to landfill.
Better Performance
   Glulam engineered
    timber used for roof
    trusses, beams & floors.
   Improved durability &
    high levels of insulation.
   Transforms the natural
    orthotropic product into
    one with more
    homogenous properties.
The Engineered Timber Home
   Timber members &
    special metal
    fasteners safely bear
    the loads of the
    building.
   Engineered finger
    jointed studs are
    strong, straight and
    less likely to warp.
Rapid Construction
   Close centre joist & rafter
    systems particularly
    suited to floors.
   Stable ,uniform sections
    in greater lengths and
    depths.
   Can save on intermediate
    walls & foundation costs.
   Faster & more accurate
    installation.

								
To top