FACTORS AFFECTING DURABILITY OF TIMBER
On exposure to sunlight the coloration of the heartwood of most timbers – e.g. mahogany, afrormosia and oak – will lighten, though a few timbers will actually darken, e.g. Rhodesian teak. Indoors the action of sunlight will be slow and the process will take several years, but outdoors the change in colour is very rapid, taking place in a matter of months, and is generally regarded as an initial and very transient stage in the whole process of weathering.
In weathering the action of light energy (photochemical degradation), rain and wind results in a complex degrading mechanism that renders the timber silvery-grey in appearance. More important is the loss of surface integrity, a process that has been quantified in terms of the residual tensile strength of thin strips of wood (Derbyshire and Miller, 1981; Derbyshire et al., 1995).
The loss in integrity embraces the degradation of both lignin, primarily by the action of ultraviolet light, and cellulose, by shortening of the chain length, mainly by the action of energy from the visible part of the spectrum.
Degradation results in erosion of the cell wall and in particular the pit aperture and torus. Fractography using scanning electron microscopy has revealed that the progression of degradation involves initially the development of brittleness and the reduction in stress transfer capabilities through lignin degradation, followed by reductions in microfibril strength resulting from cellulose degradation (Derbyshire et al., 1995).
However, the same cell walls that are attacked act as an efficient filter for those cells below, and here the rate of erosion from the combined effects of UV, light and rain is very slow indeed; in the absence of fungi and insects the rate of removal of the surface by weathering is of the order of only 1 mm every 20 years.
Nevertheless, because of the continual threat of biological attack, it is unwise to leave most timbers completely unprotected from the weather, and it should be appreciated that during weathering, the integrity of the surface layers is markedly reduced, thereby adversely affecting the performance of an applied surface coating. In order to effect good adhesion the weathered layers must first be removed.
As a general rule, timber is highly resistant to a large number of chemicals and its continued use for various types of tanks and containers, even in the face of competition from stainless steel, indicates that its resistance is very good; it is certainly cost-effective.
Timber is far superior to cast iron and ordinary steel in its resistance to mild acids and for very many years timber was used as separators in lead-acid batteries. However, in its resistance to alkalis timber is inferior to iron and steel. Dissolution of both the lignin and the hemicelluloses occurs under the action of even mild alkalis. Iron salts are frequently very acidic and in the presence of moisture result in hydrolytic degradation of timber; the softening and darkish-blue discoloration of timber in the vicinity of iron nails and bolts is due to this effect.
Timber used in boats is often subjected to the effects of chemical decay associated with the corrosion of metallic fastenings, a condition frequently referred to as nail sickness. This is basically an electrochemical effect, the rate of activity being controlled by the availability of oxygen.
Prolonged exposure of timber to elevated temperatures results in a reduction in strength and a very marked loss in toughness (impact resistance). Thus, timber heated at 120oC for one month loses 10% of its strength, while at 140oC the same loss in strength occurs after only one week (Shafizadeh and Chin, 1977). Tests on three softwood timbers subjected to daily cycles of 20 to 90oC for a period of three years resulted in a reduction in toughness to only 44% of the value of samples exposed for only one day (Moore, 1984).
It has been suggested that degradation can occur at temperatures as low as 65oC when exposed for many years. Thermal degradation results in a characteristic browning of timber with associated caramel-like odour, indicative of burnt sugar. Initially this is the result of degradation of the hemicelluloses, but with time the cellulose is also, affected with a reduction in chain length through scission of the b-1–4 linkage. Commensurate degradation occurs in the lignin, but usually at a slower rate.
The most common type of mechanical degradation is that which occurs in timber when stressed under load for long periods of time. There is a loss in strength with time under load, such that after being loaded for 50 years the strength of timber is reduced by approximately 50%. Similarly, there is a marked reduction in elasticity that manifests itself as an increase in extension or deformation with time under load. The structural engineer, in designing his timber structure, has to take into account the loss with time of both strength and modulus of elasticity by applying two time modification factors.
A second and less common form of mechanical degradation is the induction of compression failure within the cell walls of timber, which can arise either in the standing tree first in the form of a natural compression failure due to high localised compressive stress, or second, as brittleheart due to the occurrence of high growth stresses in the centre of the trunk, or under service conditions where the timber is over-stressed in longitudinal compression with the production of kinks in the cell wall.
Loss in tensile strength due to the induction of compression damage is about 10–15%, but the loss in toughness can be as high as 50%.
NATURAL DURABILITY AND ATTACK BY FUNGI AND INSECTS
Generally when the durability of timber is discussed reference is being made explicitly to the resistance of the timber to both fungal and insect attack; this resistance is termed natural durability. Recalling that timber is an organic product it is surprising at first to find that it can withstand attack from fungi and insects for very long periods of time, certainly much longer than its herbaceous counterparts. This resistance can be explained in part by the basic constituents of the cell wall, and in part by the deposition of extractives.
The presence of lignin – which surrounds and protects the crystalline cellulose – appears to offer a slight degree of resistance to fungal attack. Certainly the resistance of sapwood is higher than that of herbaceous plants. Fungal attack can commence only in the presence of moisture, and the threshold value of 20% for timber is about twice as high as the corresponding value for non-lignified plants. Timber has a low nitrogen content, of the order of 0.03–0.1% by mass and, since this element is a prerequisite for fungal growth, its presence in only such small quantities contributes to the natural resistance of timber.
However, the principal factor conferring resistance to biological attack is undoubtedly the presence of extractives in the heartwood. The far higher durability of the heartwood of certain species compared with the sapwood is attributable primarily to the presence in the former of toxic substances, many of which are phenolic in origin. Other factors such as a decreased moisture content, reduced rate of diffusion, moderate density and deposition of gums and resins also play a role in determining the higher durability of the heartwood.
Considerable variation in durability can occur within the heartwood zone. In a number of timbers the outer band of the heartwood has a higher resistance than the inner region, owing, it is thought, to the progressive degradation of toxic substances by enzymatic or microbial action.
Durability of the heartwood varies considerably among different species, and is related to the type and quantity of extractives present; the heartwood of timbers devoid of extractives has a very low durability. The sapwood of all timbers is susceptible to attack owing not only to the absence of extractives, but also to the presence in the ray cells of stored starch, which constitutes a ready source of food for the invading fungus.
BS EN 350-1 defines natural durability as ‘the inherent resistance of wood to attack by wood destroying organisms’ and specifies the techniques to be used in assessing the durability of wood against wood-destroying fungi, beetles, termites and marine borers. The normal test method, using heartwood stakes in ground contact, is described in EN 252 and BS 7282.
However, provision is made in BS EN 350-1 for the use of laboratory tests to provide an initial indicator of the potential durability of a new species of wood. The results of these tests lead to the placement of a species of wood in a five-band classification of natural durability against each of four biological agents, namely wood-destroying fungi, Hylotrupes bajulus, Anobium punctatum and termites. The durability classes for each agent are:
- Class 1 – very durable.
- Class 2 – durable.
- Class 3 – moderately durable.
- Class 4 – slightly durable.
- Class 5 – not durable.
This classification is presented in BS EN 350-2 and illustrated for a selection of British-grown timbers in Table 1.
The relationship between service environment and risk of attack by wood-destroying organisms is defined in BS EN 335-1 by employing a set of five ‘use classes’ (formerly ‘hazard classes’) as shown in Table 2, while BS EN 335-2 relates to the application of these use classes to solid wood and BS EN 335-3 relates to their application to wood based panels.
Lastly, BS EN 460 sets out the durability requirement of the wood to be used in each use class. It should be appreciated that to meet the requirements of some of the use classes the wood may have to have its inherent durability enhanced by the introduction of preservatives, by thermally modifying the wood, or by changing the chemistry of the wood.
Nature of Fungal Decay
Some fungi, e.g. the moulds, are present only on the surface of timer and although they may cause staining they have no effect on the strength properties. A second group of fungi, the sapstain fungi, live on the sugars present in the ray cells, and the presence of their hyphae in the sapwood imparts a distinctive coloration to that region of the timber, which is often referred to as ‘blue-stain’.
One of the best examples of sapstain is that found in recently felled Scots pine logs. In temperate countries the presence of this type of fungus results in only inappreciable losses in bending strength, though several staining fungi in the tropical countries cause considerable reductions in strength. By far the most important fungi are those that cause decay of timber by chemical decomposition; this is achieved by the digesting action of enzymes secreted by the fungal hyphae. Two main groups of timber-rotting fungi can be distinguished:
- The brown rots, which consume the cellulose and hemicelluloses, but attack the lignin only slightly. During attack the wood usually darkens and in an advanced stage of attack tends to break up into cubes and crumble under pressure. One of the best known fungi of this group is Serpula lacrymans, which causes dry rot. Contrary to what its name suggests, the fungus requires an adequate supply of moisture for its development.
- The white rots, which attack all the chemical constituents of the cell wall. Although the timber may darken initially, it becomes very much lighter than normal at advanced stages of attack. Unlike timber under attack from brown rot, timber with white rot does not crumble under pressure, but separates into a fibrous mass.
In very general terms, the brown rots are usually to be found in constructional timbers, whereas the white rots are frequently responsible for the decay of exterior joinery. Decay, of course, results in a loss of strength, but it is important to note that considerable strength reductions may arise in the very early stages of attack, toughness being particularly sensitive to the presence of fungal attack.
Loss in mass of the timber is also characteristic of attack, and decayed timber can lose up to 80% of its air-dry mass. The principal types of fungal attack of wood in the standing tree, of timber in felled logs and of timber in service are set out in Table 3. More information on fungal attack of timber is to be found in Desch and Dinwoodie (1996) and Bravery et al. (1987).
Nature of Insect Attack
Although all timbers are susceptible to attack by at least one species of insect, in practice only a small proportion of the timber in service actually becomes infested. Some timbers are more susceptible to attack than others and generally the heartwood is much more resistant than the sapwood; nevertheless, the heartwood can be attacked by certain species particularly when decay is also present. Timber is consumed by the adult form of certain insects, the best-known example being termites whose adult, but sexually immature, workers cause most damage.
Few timbers are immune to attack by these voracious eaters and it is indeed fortunate that these insects generally cannot survive the cooler weather of the UK. They are to be found principally in the tropics, but certain species are present in the Mediterranean region, including southern France. In the UK insect attack is mainly by the grub or larval stage of certain beetles.
The adult beetle lays its eggs on the surface of the timber, frequently in surface cracks, or in the cut ends of cells; these eggs hatch to produce grubs, which tunnel their way into the timber, remaining there for periods of up to three years or longer. The size and shape of the tunnel, the type of detritus left behind in the tunnel (frass) and the exit holes made by the emerging adults are all characteristic of the species of beetle.
Timber used in salt water is subjected to attack by marine-boring animals such as the shipworm (Teredo spp.) and the gribble (Limnoria spp.). Marine borers are particularly active in tropical waters, nevertheless, around the coast of Great Britain Limnoria is fairly active and Teredo, though spasmodic, has still to be considered a potential hazard.
The degree of hazard will vary considerably with local conditions, and relatively few timbers are recognised as having heartwood resistant under all conditions. The list of resistant timbers includes ekki, greenheart, okan, opepe and pyinkado.