Decay of timber

Wet rot Decay of timber caused by fungi which flourish in alternate wet and dry conditions. 

Leightley, L. E. and Eaton, R. A., 1977. Mechanisms of decay of timber by aquatic micro-organisms. Rec. Brit. Wood Pres. Assoc. Ann. Conv. 1-26. 

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. 

Possible approaches to deal with this problem include partial reconstruction of timber structural members by replacing the degraded timber with epoxy grout cast on-site into a permanent timber formwork, or, preferably, replacing decayed material with a new timber splice in both cases, the load is transferred to the remaining sound timber by FRP or steel rods or plates (Fig. 5). 

Decay resistance of timber varies greatly depending on the type of tree and the type of wood. Heartwood is more resistant than softwood because of the lower permeability and presence of toxic substances. Table 12.1 shows the relative resistance to decay of the heartwood of various tree species. Table 12.2 provides a more comprehensive listing. Those not shown have little or no resistance without being treated. 

Wood is painted in order to preserve from decay, to minimize changes in moisture content that lead to distortion, and for decoration. It is a waste of time and effort to apply expensive paints or stains to exterior wood not previously treated with preservative. Preservatives are designed to penetrate the timber to protect against microorganisms and insect attack. Modern preservatives can be over-coated with paint without problems. 

Most wood species are subject to some form of biological attack, a hazard not encountered by most other constructional materials. Fungal decay and attack by termites and other insect borers and marine borers are the main problems. The heartwood of many timber species and the sapwood of most species can be impregnated with wood preservatives to prevent these problems. 

For the remedial treatment of fungal decay and insect attack on timber in buildings, clear solvent-type preservatives applied by spraying all available surfaces are generally used, supplemented with wood preservative pastes to obtain deep penetration. Water-based emulsions are also used for woodworm control. There has been a general move towards the use of those... 

When an organism dies, it stops acquiring new carbon, and the amount of radiocarbon it contains begins to decline through radioactive decay. Wood from a tree that died (when felled for timber, say) 5,730 years ago has only half as much radiocarbon as that from a similar tree felled recently. Wood that is 11,460 years old... 

The greatest care in. manipulation, and the most scientific methods in the treatment of wine, have been tried in France, and the result has been the production of the finest winos in the world. The removal of the wine from the vat to the cask is there as carefully regarded as the work of fermentation. The casks are all made ready at the same time as the vats. If the wood of which they are made is new, they will not fail to communicate an astringeuey and bitterness to the wine. To prevent this, the casks are repeatedly washed with cold water, and then with hot, in which peach leaves and salt are infused. They are repeatedly shaken while the fiald is hot, and then are left so that the wood may absorb them, and they may penetrate into the pores of the timber. The casks are now emptied, and boiling hot must, to the extent of two or three quarts, is introduced. The bungs are put in, the casks shaken, and then left to cool. Some use only hot wine in place of the above substances. If the casks ore old, they scrape off the tartar that lines the staves, and wash them vrtth hot water and must, or wine. If they have contracted an ill smell, or show the least mark of decay, they are burned at once, because, in... 

The timber used in this type of cooling tower is commonly pressure-treated redwood, Douglas fir, and other wood species, the preservative being chromated copper arsenate or, more usually today, acid copper chromate. Despite the wood preservative, it is not uncommon to find wood decay and biological fouling of the plenum area and drift eliminators. 

A most effective way to extend the Nation s timber supply is to use wood so that its service life is increased. The service life of wood in hazardous use conditions can be increased severalfold by the proper use of wood preservatives. It is estimated that the preservative treatment of railway ties results in an annual savings of 2.4 billion board feet of lumber and that, if utility poles were not treated, an additional 20 million mature trees of pole-stock quality softwoods would be needed each year simply as replacements for those destroyed by decay and termites. 

All of the commercial wood preservatives presently used in the United States are effective in preventing attack by microorganisms because of their toxic nature. Most of these preservatives are classified as broad spectrum preservatives, that is, effective against several different types of living systems. Because of the toxic hazards and environmental concerns and because prevention of wood decay is needed if we are to extend our timber resources by increasing its service life, we have investigated alternative methods of wood preservation not based on toxicity for their effectiveness. 

Taylor and Velbel point out that biomass storage may also decrease—i.e., the biomass does not always cause a net uptake of nutrients. If organic decay returns more nutrients to the soil solution than are taken up by plants, then a balance does not require mineral weathering, and nutrients are being lost to streams. Biomass decay can exceed biomass uptake when there is, e.g., insect defoliation of trees, storm losses of trees, timber cutting, or fire (Taylor and Velbel, 1991). 

Historieally, heartwood of certain timbers would be used where decay was foreseen, a practice that declined with the rise in wood preservation. Today environmentally friendly solutions to decay relying on naturally durable wood are increasingly sought. 

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