Timbers exposed

Figure 2 P2 in 1974. Biological degradation has severely eroded timber exposed above the mud, but the migration of iron from the survey nail has inhibited deterioration in the iron rich zone around the nail...

Soft-rot fungi are fully capable of attacking wood without any additional nutrients. However, the addition of nitrogen greatly increased the rate of attack (54). This finding may explain why soft rot is often observed in timber exposed in contact with soil from which nitrogen can be extracted. Several observations suggest that the rate of decay is faster in more fertile soils. 

Although soft rot occurs in above-ground situations, it is not so extensive, probably because of the low amounts of available nitrogen. Soft rot is thus mainly apparent in timber exposed in soil or in aquatic environments. The... 

Bacteria are commonly found in timber exposed in very diverse environments. The bacteria may be divided into degrading and nondegrading species. The former may be further divided into two groups bacteria that degrade only pit membranes and bacteria that actively attack the lignocellulose structure of the wood cell wall (i.e., true wood-degrading bacteria). 

End grain n. The surface of timber exposed when a tree is felled or when timber is cross-cut in any other way. 

Table 2 Biological hazard classes for timber exposed in service as defined by the European Standard EN 335-1 1992...

Friend, J. N., I8lh Reporl of Ihe Commillee of Ihe Inslilulion of Civii Engineers on Ihe Delerioralion of Siruclures of Timber, Melai and Concrele Exposed lo Ihe Aclion of Sea Waler, London (1940)... 

Friend, J. N., Deterioration of Structures of Timber, Metal and Concrete Exposed to the Action of Sea Heater, London, 18th Report of the Committee of the Institution of Civil Engineers (1940)... 

Lead is relatively easily corroded where acetic acid fumes are present and under such conditions it either should not be used or should be efficiently protected. Generally, any contact between lead and organic material containing or developing acids will cause corrosion for instance, unseasoned wood may be detrimental. Trouble from this cause may be prevented by using well-seasoned timber, by maintaining dry conditions, or by separating the lead from the timber by bitumen felt or paint. Lead is also subject to attack by lime and particularly by Portland cement, mortar and concrete, but can be protected by a heavy coat of bitumen. A lead damp-proof course laid without protection in the mortar joint of a brick wall may become severely corroded, especially where the brickwork is in an exposed condition and is excessively damp. 

Nail sickness Nail sickness is chemical decay associated with corroded metals in marine situations. Chemical degradation of wood by the products of metal corrosion is brought about by bad workmanship or maintenance, or unsuitable (permeable) timber species, all of which permit electrolyte and oxygen access which promotes corrosion. Chemical decay of wood by alkali occurs in cathodic areas (metal exposed oxygen present). Softening and embrittlement of wood occurs in anodic areas (metal embedded oxygen absent) caused by mineral acid from hydrolysis of soluble iron corrosion products. 

Shore, R.F., D.G. Myhill, M.C. French, D.V. Leach, and R.E. Stebbings. 1991. Toxicity and tissue distribution of pentachlorophenol and permethrin in pipistrelle bats experimentally exposed to treated timber. Environ. Pollut. 73 101-118. 

Riiping Also known as the Empty Cell process. A method for treating timber with a creosote preservative. The wood is first exposed to compressed air and then impregnated at a higher pressure. See also Bethell. 

When the EPA considered exposures to insecticide residues in the home they identified at least six possible sources and routes these are given in Table 2.6. Their original approach apportioned the acceptable daily intake (ADI) between the various routes but it soon became clear that this was unrealistic because an individual was unlikely to be exposed via all routes on any one day. The EPA s present strategy is to develop an approach called micro-exposure event modelling. Micro-exposure event modelling is based on statistical data on the frequencies and levels of contamination of food, water, etc. and on behavioural information about the frequency of use of lawn/pet/timber treatments, etc. The combined data are assembled in a probabilistic model called LIFELINE which is able to predict the frequency and level of exposure to a group of hypothetical individuals over their lifetime.12 The model is also able to take account of the relative proportions of different types of accommodation, the incidence of pet ownership or any other data that will affect real levels of exposure. The output from the LIFELINE model allows the exposures of individuals in a population to be modelled over any interval from a single occasion to a lifetime. 

As against this view, I should have thought that Schrodinger asserted with great force the real existence of atoms or particles, even though he tried to explain them as wave structures. I did not know that, by saying that a table or chair is made of timber, I should expose myself to the criticism that I have denied the existence of discrete tables and chairs, and that I have done so because I am in love with timber (as Born says of Schrodinger s attitude towards waves). 

Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

JL HE POLYMERIZATION OF VINYL MONOMERS in the void spaces of bulk wood results in wood—polymer composites of increased strength properties and dimensional stability see Chapter 6). Because the different environmental conditions expose in-service timber to attack by numerous wood-deteriorating microorganisms, it is desirable to enhance the biodegradation resistance of wood, with simultaneous improvements in mechanical behavior. This chapter summarizes the formation of bioactive wood-polymer composites (1-4). The basic approach is still in situ polymerization of vinyl monomers in wood, with the appropriate choice of a bioactive, toxic, functional group incorporated in the monomer, and with other modifications based on wood-polymer reactions. 

Woods with very large wide rays extending several millimetres, e.g. Quercus, Knightia, can have considerable decorative appeal especially if the timber or veneer is eut radially so as to expose the rays running in and out of the cut surface. In these woods the rays can comprise as much as 50% of the wood volume. More typically, the ray volume of hardwoods is around 15%. 

The polyphenols and tannins in hardwoods largely contribute to wood colour, particularly heartwood colour, and in earlier times some hardwoods were sought after for dyestuffs. Colour change is undesirable with darker timbers that are rich in extractives, where they are to be used for joinery or furniture. With light-coloured woods such as pines the effect of ultraviolet light is complex (Hon, 1991). Some pines can have very pale sapwood. Unfortunately the attractive white surface can yellow where directly exposed to sunlight (Sinclair and Vincent, 1964). 

The traditional way of limiting the movement of timber has been to apply 2-3 coats of paint (c. 0.1 mm film). Paint coatings prevent rapid surface absorption of water and the development of steep, superficial moisture gradients in exposed wood. For example, the average seasonal variation in moisture content of exterior timbers can be reduced by a factor of three or more where using three coats of a traditional linseed oil house paint (Orman, 1955). 

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