TWE 411 Lecture 1 & 2 2024 PDF - Biodeterioration of Wood

TWE 411 Lecture 1 Biodeterioration of Wood Under proper conditions, wood will give centuries of satisfactory service. However, under conditions that permit the development of wood-de...

TWE 411 Lecture 1 Biodeterioration of Wood Under proper conditions, wood will give centuries of satisfactory service. However, under conditions that permit the development of wood-degrading organisms, protection must be provided during processing, trading, and use. The organisms that can degrade wood are principally i. fungi, ii. insects, iii. bacteria, and iv, marine borers. Moulds, most sapwood stains and decay are caused by fungi, which are microscopic, thread-like microorganisms that must have organic material to live. For some of them, wood offers the required food supply. The growth of fungi depends on suitably mild temperatures, moisture, and air (oxygen). Chemical stains, although they are not caused by organisms, are discussed because they resemble stains caused by fungi. Insects also may damage wood and in many situations must be considered in protective measures. Termites are the major insect enemy of wood, but on a national scale, they are a less serious threat than fungi. Bacteria in wood ordinarily are of little consequence, but some may make the wood excessively absorptive. In addition, some may cause strength losses over long periods of exposure, particularly in forest soils. Marine borers can attack susceptible wood rapidly in salt water harbours, where they are the principal cause of damage to piles and other wood marine structures. Wood degradation by organisms has been studied extensively, and many preventive measures are well known and widely practiced. By taking ordinary precautions with the finished product, the user can contribute substantially to ensuring a long service life. Fungus Damage and Control Fungus damage in wood may be traced to three general causes: (a) lack of suitable protective measures when storing logs or bolts; (b) improper seasoning, storing, or handling of the raw material produced from the log; and (c) failure to take ordinary simple precautions in using the final product. The incidence and development of moulds, decay and stains caused by fungi depend heavily on temperature and moisture conditions. Moulds and Fungal Stains Moulds and fungal stains are confined to a great extent to sapwood and are of various colours. The principal fungal stains are usually referred to as sap stain or blue stain. The distinction between moulding and staining is made primarily on the basis of the depth of discoloration. With some moulds and the lesser fungal stains, there is no clear-cut differentiation. Typical sap stain or blue stain penetrates into the sapwood and cannot be removed by surfacing. Also, the discoloration as seen on a cross section of the wood often appears as pie-shaped wedges oriented radially, corresponding to the direction of the wood rays (Fig. 1). Fig. 1: Typical radial penetration of log by stain. The pattern is a result of more rapid penetration by the fungus radially (through the ray) than tangentially. The discoloration may completely cover the sapwood or may occur as specks, spots, streaks, or patches of various intensities of colour. The so-called blue stains, which vary from bluish to bluish black and gray to brown, are the most common, although various shades of yellow, orange, purple, and red are sometimes encountered. The exact colour of the stain depends on the infecting organisms and the species and moisture condition of the wood. Mould discolorations usually become noticeable as fuzzy or powdery surface growths, with colours ranging from light shades to black. Among the brighter colours, green and yellowish colorations are common. On softwoods, though the fungus may penetrate deeply, the discolouring surface growth often can easily be brushed or surfaced off. However, on large-pored hardwoods (for example, oaks), the wood beneath the surface growth is commonly stained too deeply to be surfaced off. The staining tends to occur in spots of various concentration and size, depending on the kind and pattern of the superficial growth. Under favourable moisture and temperature conditions, staining and moulding fungi may become established and develop rapidly in the sapwood of logs shortly after they are cut. In addition, wood and such products as veneer, furniture stock, and millwork may become infected at any stage of manufacture or use if they become sufficiently moist. Freshly cut or unseasoned stock that is piled during warm, humid weather may be noticeably discoloured within 5 or 6 days. Ordinarily, stain and mould fungi affect the strength of the wood only slightly. Their greatest effect is usually confined to strength properties that determine shock resistance or toughness. They increase the absorbency of wood, and this can cause over-absorption of glue, paint, or wood preservative during subsequent processing. Increased porosity also makes wood more wettable, which can lead to subsequent colonization by typical wood-decay fungi. Stain- and mould-infected stock is practically unimpaired for many uses in which appearance is not a limiting factor, and a small amount of stain may be permitted by standard grading rules. Stock with stain and mould may not be entirely satisfactory for siding, trim, and other exterior millwork because of its greater water absorbency. Also, incipient decay may be present, though inconspicuous, in the discoloured areas. Both of these factors increase the possibility of decay in wood that is rain-wetted unless the wood has been treated with a suitable preservative. Chemical Stains Non-microbial or chemical stains are difficult to control and represent substantial loss in wood quality. These stains, which should not be confused with fungal brown stain, include a variety of discolorations in wood that are often promoted by slow drying of the wood and warm to hot temperatures. Such conditions allow naturally occurring chemicals in wood to react with air (enzymatic oxidation) to form a new chemical that is typically dark in colour. Common chemical stains include: (a) interior sapwood graying, (b) brown stain in softwoods, and (c) pinking and browning in the interior of light-coloured woods (e.g. …). Another common discoloration, iron stain, is caused by the interaction of iron with tannins in wood. Iron stain is more prevalent in hardwoods (for example, oak and many tropical hardwoods) and in some softwoods such as Douglas-fir. Control is achieved by eliminating the source of iron. Decay Decay-producing fungi may, under conditions that favour their growth, attack either heartwood or sapwood in most wood species (Fig. 2). The result is a condition designated as decay, rot, dote, or doze. Fresh surface growths of decay fungus may appear as fan-shaped patches (Fig. 3), strands, or root-like structures that are usually white or brown in colour. Sometimes fruiting bodies are produced that take the form of mushrooms, brackets, or crusts. The fungus, in the form of microscopic, threadlike strands called hyphae, permeates the wood and uses parts of it as food. Some fungi live largely on cellulose, whereas others use lignin and cellulose. Certain decay fungi colonize the heartwood (causing heart rot) and rarely the sapwood of living trees, whereas others confine their activities to logs or manufactured products, such as sawn wood, structural timbers, poles, and ties. Most fungi that attack trees cease their activities after the trees have been cut, as do the fungi causing brown pocket (peck) or white pocket in some softwoods. Relatively few fungi continue their destruction after the trees have been cut and worked into products and then only if conditions remain favourable for their growth. Although heartwood is more susceptible to decay than is sapwood in living trees, for many species, the sapwood of wood products is more susceptible to decay than is the heartwood. Fig. 2: The decay cycle (top to bottom). Fig. 3: Mycelial fans on a wood door Thousands of spores produced in a fungal fruiting body are distributed by wind or insects. On contacting moist, susceptible wood, spores germinate and fungal hypha create new infections in the wood cells. In time, serious decay develops that may be accompanied by formation of new fruiting bodies. Decay Resistance of Wood Generally, the heartwood of wood species has various degrees of natural resistance. However, the untreated sapwood has low resistance to decay and usually short service life under conditions favouring decay. The natural decay resistance of heartwood is greatly affected by: i. differences in the preservative qualities of the wood extractives. ii. the attacking fungus and iii. the condition of exposure. Considerable differences in service life can be obtained from pieces of wood cut from the same species, even the same tree, and used under apparently similar conditions. There are further complications because, in some species, like Obeche (Triplochiton),the heartwood and sapwood are so similar in colour that they cannot be easily distinguished. Precise ratings of decay resistance of heartwood of different species are not possible because of differences within species and the variety of service conditions to which wood is exposed. However, broad groupings of many native species, based on service records, laboratory tests, and general expertise, are helpful in choosing heartwood for use under conditions favourable to decay. Groupings by natural resistance of some domestic and imported wood species to decay fungi can be obtained, which ranks the heartwood of a grouping of species according to decay resistance. The extent of variations in decay resistance of individual trees or wood samples of a particular species is much greater for most of the more resistant species than for the slightly or non- resistant species. Natural resistance of wood to fungi is important only where conditions conducive to decay exist or may develop. Where decay hazard exists, the heartwood of a species in the resistant category generally gives satisfactory service for wood used above-ground, while those in the very resistant category generally give satisfactory performance in contact with the ground. The heartwood of species in the other two categories will usually require some form of preservative treatment. For mild decay conditions, a simple preservative treatment—such as a short soak in preservative after all cutting and boring operations are complete—may be adequate for wood low in decay resistance. For more severe decay hazards, pressure treatment is often required. Even the very decay- resistant species may require preservative treatment for important structural uses or other uses where failure would endanger life or require expensive repairs. When selecting naturally decay-resistant wood species for applications where conditions are conducive to decay, it is important to utilize heartwood. Marketable sizes of some species are primarily second growth and contain a high percentage of sapwood. Consequently, substantial quantities of heartwood lumber of these species are not available. If wood is subjected to severe decay conditions, pressure-treated wood, rather than resistant heartwood, is generally recommended. Effect of Decay on Strength of Wood Decay initially affects toughness, or the ability of wood to withstand impacts. This is generally followed by reductions in strength values related to static bending. Eventually, all strength properties are seriously reduced. Strength losses during early stages of decay can be considerable, depending to a great extent upon the fungi involved and, to a lesser extent, upon the type of wood undergoing decay. In laboratory tests, losses in toughness ranged from 6% to >50% by the time 1% weight loss had occurred in the wood as a result of fungal attack. By the time weight losses resulting from decay have reached 10%, most strength losses may be expected to exceed 50%. At such weight losses (10% or less), decay is detectable only microscopically. It may be assumed that wood with visually discernible decay has been greatly reduced in all strength values. Prevention of Mould, Stain, and Decay Logs, Poles, Piles, and Ties The wood species, geographic region, and time of year determine what precautions must be taken to avoid serious damage from fungi in logs, poles, piles, ties, and similar thick products during seasoning or storage. In dry climates, rapid surface seasoning of poles and piles will retard development of mould, stain, and decay. The bark is peeled from the pole and the peeled product is decked on high skids or piled on high, well-drained ground in the open to air-dry. In humid regions, these products often do not air-dry fast enough to avoid losses from fungi. Pre- seasoning treatments with approved preservative solutions can be helpful in these circumstances. For logs, rapid conversion into lumber or storage in water or under a water spray (Plate1) is the surest way to avoid fungal damage. Preservative sprays promptly applied to the wood will protect most timber species during storage for 2 to 3 months, except in severe decay hazard climates. For longer storage, an end coating is needed to prevent seasoning checks, through which infection can enter the log. Plate 1: Spraying logs with water (This protects them against fungal stain and decay)