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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.
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