Preservation was originally introduced as a means to avoid the deterioration that occurs when untreated wood is used in various service conditions, but the introduction of reliably preserved wood effectively introduced an entirely new structural material which can be used in new situations for which untreated wood was never previously considered—
such as for durable wood house foundations. The major use of preserved wood is certainly in ground contact conditions. In many respects poles, posts and piles present similar technical problems as they all involve ground contact conditions and they vary only in their dimensions and in the fact that poles have most of their length above the ground in contrast to piles which usually have most of their length below ground.
Poles, piles and posts
Round wood transmission poles (Fig. 5.5) are used throughout the world, the principal advantages of wood being excellent strength-to-weight properties and elasticity under load.
Naturally durable wood is rarely used and most poles are vacuum/pressure treated with creosote or water-borne salt preservatives.
In relatively permeable woods such as Southern
FIGURE 5.5 Eucalyptus transmission poles in Australia impregnated with Tanalith C (CCA) preservative. (Hickson’s Timber Products Limited)
Uses of preserved wood
pine complete impregnation is achieved and almost any fixed preservative is suitable; these are the Class A conditions shown in Table A.2 in Appendix A.
Where the sapwood alone is permeable but the heartwood is moderately durable, as in European redwood, resistance to movement is desirable to reduce the tendency for checks or splits to develop and expose the untreated heartwood. Water-borne preservatives are less effective than creosote, which is particularly efficient in reducing movement, although water-borne systems are efficient on species possessing low movement where there is little likelihood of the development of checks. Checking is often ignored, yet in tropical areas a wood with a large movement can fail structurally, simply due to the physical damage that results. Whilst European redwood is normally considered to possess permeable and non-durable sapwood, impermeable and moderately durable heartwood and only medium movement, these properties apply only to wood that has grown relatively slowly. With fast-grown wood the heartwood may be non-durable but it is also more permeable so that reasonable penetration can be achieved with normal vacuum/pressure processes, although it should be appreciated that where this fast-grown wood is included in a treatment charge the absorption of preservative must be increased to ensure reliable protection.
Douglas fir also possesses non-durable sapwood and durable heartwood but even the sapwood is resistant to impregnation and poles can be reliably preserved only if they are incised.
Spruce possesses sapwood that is resistant to impregnation but incising has only limited advantages as the heartwood is also non-durable. However, as the wood is relatively impermeable a deposit of unfixed preservative within the heartwood possesses excellent resistance to leaching—rapidly fixed copper-chromium-arsenic (CCA) preservatives will treat the sapwood only to the depth of incising but with copper-chromium-boron (CCB)
preservatives the boron component will continue to diffuse, significantly improving the durability of the heartwood. The slow fixation preservatives which fix by ammonia or acetic acid loss are more efficient as they can combine this protracted diffusion with ultimate fixation.
However, it must be appreciated that higher concentrations are required if such diffusion occurs. The wrapping of the treated wood to prevent drying and to allow for diffusion is probably best avoided for poles so that higher loadings can be achieved through relatively rapid fixation in the external sapwood zone, where there is the greatest decay risk, but slow air drying ensures that the inner moisture content changes only slowly and significant diffusion is still able to occur.
The increasing scarcity of suitable sizes and species of wood for transmission poles means that there is increasing interest in the use of more readily available species, such as relatively impermeable spruce, and in the manufacture of poles by laminating smaller sections. In laminated poles complete impregnation is essential and it is important to appreciate that a species such as European redwood with an impermeable but moderately durable heartwood is entirely unsuitable—the lamination ensures that the heartwood is exposed to ground contact which represents the greatest decay risk, whereas in round poles the heartwood is protected by reliably preserved sapwood. It may seem strange but spruce is likely to be more reliable than European redwood laminated poles because laminated spruce poles can be incised after manufacture to give a treated zone to a fixed depth and the low movement of spruce avoids the danger of the development of shakes that may penetrate through this relatively superficial treatment.
In tropical areas problems are frequently encountered in the treatment of hardwoods, particularly Eucalypts treated with water-borne preservatives. Soft rot frequently develops at the ground line, apparently because this fungus is able
to invade the cell walls which have not been penetrated by the toxic components in the preservative—this is a problem of micro-distribution of the preservative within the wood and is currently the subject of extensive investigation. Class AS preservatives in Denmark are considered to be those that are particularly suitable for use in situations where there is a danger of soft rot; this generally means continuous immersion in water in the Danish context.
In the Poulain process, described in detail in Chapter 3, a relatively light overall treatment of a pole is followed by a second and more thorough treatment of the butt. This originally involved a Rüping empty-cell treatment of the entire pole with a light creosote followed by a further treatment of the butt with a heavier oil. In more recent years creosote has been used following a water-borne and sometimes unfixed treatment to give the desired additional protection at the ground line. In the Dessemond process in France, poles were first treated with copper sulphate but mercuric chloride Kyanising or zinc chloride Burnettising treatments were also used. The Card, Tetraset and other similar processes are described in detail in Chapter 4. In recent years it has often been argued that creosote butt treatment should be applied to all poles treated with water-borne preservatives but this achieves little advantage—a fixed water-borne treatment such as CCA will give reliable protection at the ground line in normal conditions and it is the exposed part of the pole that suffers from the development of checks. Butt treatments with creosote or non-toxic bitumen have an advantage only when the pole is treated with a poorly fixed preservative which is unable to withstand the ground contact conditions.
Creosote treatment (Figs 5.6 to 5.8), has the distinct disadvantage that it is dirty and likely to bleed, perhaps causing serious damage to clothing where poles are erected in areas with heavy pedestrian traffic; the problems of bleeding with empty-cell and particularly Rüping treatments has been discussed in detail in Chapter 3. Preventing bleeding only reduces the
prob-FIGURE 5.6 Peeling transmission poles prior to creosote treatment. (Industri- og Byggnadsaktiebolaget Suecia, Sweden)
FIGURE 5.7 Poles on bogies being loaded into the treatment cylinder. Creosote is heated electrically to reduce viscocity and improve penetration. (Industri-og Byggnadsaktiebolaget Suecia, Sweden)
FIGURE 5.8 The stock yard with modern handling equipment. (Industri- og Byggnadsaktiebolaget Suecia, Sweden)
Uses of preserved wood
lem as the poles still remain dirty. Water-borne treatments are clean and attractive but shakes or splits may develop and many of these systems contain arsenic which is a danger to livestock, even if it is proved to be reliably fixed. Fence posts can be a source of arsenic poisoning just as much as transmission poles; the easiest way to avoid arsenic toxicity is to use only arsenic-free preservatives such as the copper-chromium (ACC) types are completely reliable, except in situations where there is a serious hazard from termites or copper resistant fungi such as Poria species.
Fences receive little attention yet they represent a very large volume of treated wood.
Usually, local species are used and there is thus a tendency for their value to be largely ignored as replacements can be readily obtained. However, any replacement or repair of a fence represents substantial labour costs and there is always justification for the selection of naturally durable wood or the use of a preservation process.
Round posts are always best and as they have only a small section they usually consist almost entirely of sapwood. In view of the short length of the posts even relatively impermeable species can be treated by penetration through the more permeable end-grain. In many countries standard fence posts, pressure treated with creosote or water-borne preservatives, represent a normal commodity which can be readily purchased by the agricultural community, but in other countries there is a tendency to prepare posts on each individual farm. In this case preservative treatment, if it is used at all, is normally applied by the butt hot and cold method described in Chapter 3.
Although preserved construction piles are completely reliable and widely used in America, they are not popular in Europe where tubular steel and concrete piles are preferred. However, wood piles are widely used in marine situations throughout the world. Some naturally durable woods are used such as greenheart but preserved piles, particularly incised creosoted Douglas fir, are most popular. In situations where there is a
risk of marine borers it is necessary to use either very high retentions of creosote or additives such as arsenic or contact insecticides which will prevent damage by gribble. Whilst marine defence works such as groynes must be similarly protected, the superficial and decking timbers in wharfs, jetties and marinas represent only normal decay hazards and Class A preservatives are completely satisfactory for such structures.
Railway sleepers (ties)
The first use of pressure creosoted wood was for railway sleepers (ties). Wood sleepers (Fig. 5.9) are still extensively used but the declining availability of large-section wood has progressively increased their cost and metal and particularly concrete sleepers have been adopted in several countries for economic reasons, although without taking proper account of the life of the sleeper which is almost indefinite with creosote treatment to a suitable wood. In recent years the system of mounting the rails in chairs screwed to the sleeper has been abandoned in many countries in favour of the use of flat-bottomed rails secured with spikes.
Unfortunately, the spikes do not hold so well in creosoted wood and there are several disadvantages with water-borne treated wood such as movement splits, exposing only moderately durable heartwood in European redwood sleepers, and electrical insulation problems with salt systems, where signalling systems operate through the rails. With new high-speed tracks, where spiked flat-bed rails are unsuitable, and where there are doubts about the life of concrete sleepers, there is now a tendency to return to creosoted wood sleepers.
With European redwood transmission poles and sleepers there is a danger, particularly with water-borne treatments, that untreated moderately durable heartwood will be exposed by cracking and will slowly decay. When such cracking is observed in adequate time it is possible to carry out remedial treatments using
the Cobra and similar injection processes described in Chapters 3 and 4. Diufix, a spreadable mixture of creosote, tar, pitch and filler, was developed for coating railway sleepers to fill existing cracks and reduce the tendency for further shakes to develop.
With some preservatives a further problem is slow and progressive Soft rot attack at the ground line. With softwood poles this damage generally occurs only with the old fluorine-chromium-arsenic-dinitrophenol (FCAP) treatments; with copper-chromium (ACC) and copper-chromium-arsenic (CCA) preservatives it occurs only on hardwoods. In 1928 Allgemeine Holzimpragnierung GmbH introduced AHIG, the first pole bandage for wrapping round the exposed ground line of a pole to control Soft rot damage (see Fig. 3.5). AHIG, which consists of a water-proof bandage lined with Wolman salts, has since been followed by many similar products such as the Osmose bandage and Pile
Card. In the Mayerl process a trough is fixed round the pole at the ground line and filled with creosote which is then slowly absorbed into the pole. These processes are specific remedies for progressive surface Soft rot and injection is essential if an attempt is to be made to control heart rot developing in untreated heartwood.
Road works
Wood blocks treated with creosote and tar were extensively used in the past as road paving blocks. They were laid with the end-grain upwards and gave an exceptionally durable and resilient road. Whilst they are no longer generally used for public roads they are still used in parts of Europe as flooring in heavy industrial works (Fig. 5.10). In modern road building, preserved wood is most extensively used for fencing and for crash-barrier posts—large-section wood posts can be installed directly into FIGURE 5.9 Using preservative cartridges of Wolmanit TSK to protect heartwood at risk in sleepers (ties) through checks extending through the outer preserved zone. (Dr Wolman GmbH)
Uses of preserved wood
the soil whereas steel posts must be mounted in concrete if they are to provide adequate resistance to impact damage. Both fencing and crash-barrier posts are usually treated with water-borne preservatives, particularly copper-chromium-arsenic (CCA) types at the Class A retentions shown in Table A.2 in Appendix A.
Bridges
Bridges are also sometimes constructed from wood but it is important to be aware of the dangers of exposing heartwood which is only moderately durable when, for example, sawn European redwood is employed. Incised European whitewood or spruce is more reliable, even though it is classified as non-durable and impermeable, than European redwood or Scots pine with easily penetrated sapwood and moderately durable heartwood; the low movement of spruce means that incising results in treatment to a controlled depth which is unlikely to be penetrated by the development of shakes.
Buildings
Many exposed structures represent problems similar to those for bridges but in buildings (Figs 5.11 to 5.14) there is usually less risk as they should be designed to ensure that wood remains dry. For this reason a special Class B is shown in Table A.1 in Appendix A for buildings, including cladding and structural elements exposed to the weather, as above-ground conditions are generally less severe than ground contact. Wood preservation is required in flat roofs, swimming pool roof linings and industrial buildings where a decay risk may arise through condensation, but in other circumstances the main reason for a preservative treatment may simply be the desire to guard against future damage by accidental leaks or by House Longhorn beetle in temperate areas and Dry Wood termites in the tropics. Where insecticidal protection is required, preservatives meeting the Class I requirements in Table A.1 are required.
Generally, water-borne preservatives containing FIGURE 5.10 End-grain wood blocks impregnated with creosote and used as a heavy industrial floor. (Orben Bois SA)
FIGURE 5.11 Pine, pressure impregnated with an organic solvent preservative BP Hylosan, used as cladding for a prize-winning housing project in Sweden. (Svenska BP Aktiebolag)
FIGURE 5.12 Preservative-treated framing and Douglas fir plywood used in Canada for construction of durable house foundations, a system widely used in North America. (Council for Forest Industries of British Columbia)
FIGURE 5.13 Pine, impregnated with BP Hylosan, used as ceiling and wall lining to a swimming pool in Sweden, (Svenska BP Aktiebolag)
FIGURE 5.14 Pine, impregnated with Boliden K33 preservative used as a durable cladding for a factory building in Sweden. (Anticimexbolagen)
Uses of preserved wood
arsenic or boron are most economic but these are stomach poisons and tasting damage may be significant, particularly in the case of exposure to termites. The use of contact insecticides such as Lindane, Dieldrin or Permethrin is desirable to avoid the damage but their effective life is considerably less. With non-flying termites, damage is normally avoided by poisoning the soil around the building or by using shields which will prevent the termites from gaining access; these are illustrated in Fig. 5.15 and are rather reminiscent of the staddle stones used in old English barns to prevent rats from gaining access to the stored grain.
Fencing
Whilst wood used above ground level in buildings generally represents a considerably reduced risk compared with that in ground contact conditions, there are situations where wood is exposed to the weather and may decay where rainwater can penetrate into joints or cracks. It is perhaps worth mentioning that one example of this risk occurs in fencing where the mortices used in the construction of gates and
for fixing rails to posts represent water traps and are therefore invariably the areas where decay progresses most rapidly, even if naturally durable wood such as oak is employed.
Joinery (millwork)
In buildings, the external cladding does not usually present any serious problems and the main danger is associated with the joinery (millwork) such as the window and door frames.
Decay develops under the paint or varnish coating when water is trapped following absorption through splits at the joints. The natural reaction is to introduce a preservative to prevent decay but splits then continue to occur at the joints and water is absorbed which, even if it is unable to cause decay, results in preferential wetting failure of the paint coating.
The use of water-repellent organic-solvent preservatives tends to prevent the water from being absorbed, although cracks at the joints still occur so that water repellents tend to delay rather than completely prevent failure. The best way to reduce cracks is to use only wood with low movement and natural durability; suitable
FIGURE 5.15 Protecting buildings from non-flying termites. Shields (or caps) on supporting walls and all pipes and cables passing from the oversite to the floor will give some protection but regular inspection of the crawl space is necessary and any mounds or tunnels found, which tend to by-pass the shields, must be destroyed. Soil poisoning is more reliable and now more commonly used. Trenches are dug, then the oversite is levelled and the site sprayed. Treatment is also applied to all fill returned to the trenches. In this way the building is isolated by a poisonous zone.
tropical hardwoods are available. Alternatively, preservative treatment can be used but it is still essential to use wood with low movement to avoid the splitting of the coating system at the joints so that a longer decorative life can be achieved.
Remedial treatments
Remedial treatments in buildings (Fig. 5.16) are described briefly in Chapters 3 and 4, but these are extremely complex processes which are also related to dampness problems and masonry deterioration and they are best considered entirely separately, as in the book Remedial
Remedial treatments in buildings (Fig. 5.16) are described briefly in Chapters 3 and 4, but these are extremely complex processes which are also related to dampness problems and masonry deterioration and they are best considered entirely separately, as in the book Remedial