Trends in Timber Preservation––A Global Perspective
Timber is one of man’s oldest building materials. Few of any of its competitors can match it for versatility, beauty, cost effectiveness and positive environmental credentials. It can be described as strong, beautiful and sustainable – basically, wood is good! .Its easy to work with, light but strong, available in a wide range of species sizes and finishes. Timber is an excellent insulator and uses less energy in its production than any other building material. It can be finished in a number of different ways, adding both warmth and character to a whole variety of environments.
All the above outstanding elements of timber can be watered down by wood-rotting fungi and the wood-boring larvae of certain insects also known as woodworms. They feed by breaking down wood cells and, if left unchecked, ultimately destroy timber and consequently threaten the structural integrity of it, leading to a substantial financial cost and a potential danger to inhabitants.
Timber has been used as a structural material for millennia and is in many ways ideal for the job. A new-build house should be constructed with timber preservation in mind. However, most of us live in houses that we have not built and inherit timber problems along with the property. The most common problems likely to affect timber are woodworm, dry rot, wet rot, termites, and borers. Construction Review looks at timber preservation at a global perspective.
Dry rot refers to a particular species of fungi - serpula lacrymans while wet rot, is a more general term, encompassing a range of fungi. Dry rot breaks down wood cellulose and causes timber to become dry and powdery. Wet rot is more easily identified than dry rot, quite simply because affected timber will feel damp to the touch. With painted timber, the area may look and feel unaffected but the underlying timber may be rotten – try pushing a knife into the timber, you should feel resistance after a few millimetres.
Unsustainable timber harvesting,climate changes, conflict timber—conflict financed or sustained through the harvest and sale of timber- or conflict emerging as a result of competition over timber or other forest resources and Africa’s personal favourite;termites, are the major threats.Termites in tropical Africa can reduce a structure to rubble in three months. They can grow up to 15mm long and will devour wood, cow dung, paper and corn for their cellulose content. They are known to attack lead-coated cables, make holes in plastic water pipes and even attack billiard balls. They are normally first noticed in low density woods like skirting’s, architraves, floorboards and framing timbers, but can extend to denser hardwood timbers if not eradicated.
There are several types of timber borers in Africa. Borers are actually the larvae form of different families of beetles. Anoboid borers are more serious and attack softwoods, especially varieties of pine which are found on the Northern and Southern temperate zones in Africa. The Anobium punctatum species especially loves baltic wood occasionally used for floorboards and weatherboards.
Termite control on the African continent is varied. Some practices markedly differ from those in the Americas and Europe. In northern Africa, measures range from commercial services to physical removal of nests by hand. Yet some control procedures are similar to those reported in the Americas and Europe, including soil applications (topical and injection) with the usual range of termiticides, as well as baiting.
For dry wood termites, fumigation with methyl bromide, topical and subsurface chemical injections are the standard practice. Other control measures include improved building practices, physical barriers, and use of baits and safe chemicals. Countries in western Africa, use alternative methods for control such us the flooding of termite nests with water.
Preservative manufacturers have a long tradition of being at the forefront of research and development. The fact that new preservatives are available in the markets reflects an industry ahead of, not driven by, regulations and environmental pressures.
National and international standards are playing a major part in the assurance of quality and fitness for purposes of treated timber. Sustainability and carbon footprint issues are driving a new approach to the prediction of service life of building components that includes replacing or at least complementing generic standards. This is also influencing the direction of timber preservation research.
Global trends are certainly discernible but it is important to understand how and at what speed they may influence timber preservation regionally and locally.
Characterisation of the performance of preservatives
The natural durability of timber and its durability must be improved to match the customers’ requirements. A common framework for assessment of performance then becomes the next aim. Test methods alone, however, are insufficient in determining performance. The results of treated timber have to be interpreted and used to indicate the ability of a preservative to protect timber in different Use Classes (standard framework applicable to all regions). For most Use Classes, tests with decay fungi and wood-destroying insects have to be carried out. An example of a regional agreement on a common basis for performance assessment is found in the European Standard EN 599. This allocates a range of standard test methods to each Use Class and includes for each the rules for interpreting the results. For each test the performance relative to a known preservative is established .The loading of a preservative required to achieve the necessary performance is the ‘biological reference value’ (brv). Thus for any preservative, a range of brv's will be established from the tests carried out for each Use Class. EN 599 defines the highest brv in each Use Class as the ‘Critical Value’ (CV).
Modified wood
Although one may argue that preservation of wood by impregnation with preservative chemicals is indeed perhaps the original form of ‘wood modification’, this term according to Mr. Mai of the Institute of Wood Biology and Technology is increasingly being applied to technologies that mostly aim to achieve improvements in properties of wood.
Several processes that are based on heating wood under carefully controlled conditions to alter its chemistry are at the forefront of the modified wood programme in terms of commercial exploitation. Taking one commercial example, ThermoWood® production has reached 30 000 m3 per annum in Finland and the majority is exported to other EU countries. Wood is heated to either 190 or 212 oC for around three hours. A higher temperature is required for greater durability.
The most obvious characteristic of such wood is its darker colour (that may fade when exposed to sun and rain) but the manufacturers also claim improved durability and stability among other improvements. Another method of ensuring the durability of otherwise non-durable species is to impregnate the wood with chemicals that are not biocides but which react with the wood substance to render it more durable and improve properties such as dimensional stability. Examples are acetylating, which involves pressure impregnating wood with acetic anhydride, produced from acetic acid, and furfurylation using furfurylalcohol. A modified wood using the acetylating process has been launched in the market in Europe under the brand name Accoya®.
An advantage of these techniques is that cheap and plentiful, but not naturally durable species can be used without using traditional chemical preservatives. Other advantages are that the protection extends right through the wood, unlike preservative treatment which often penetrates to a limited depth and therefore requires the addition of site-applied preservative to cut ends, notches, holes, etc. The wood also becomes more resistant to moisture absorption, resulting in less moisture movement than is typical of untreated wood.
The industry faces a challenge in characterising modified wood so that informed decisions can be taken by specifiers and users. How for example can the durability of modified wood be compared with that of naturally-durable timber or preservative-treated timber? How can its service life be predicted? What is the cost compared with preservative-treated timber? Are there environmental questions such as the carbon dioxide ‘footprint’ of heat treatment compared with preservative treatment?
Modified wood then already plays a part in the overall market for wood where improved properties are required. Europe seems to be the hub of this activity but we can expect it to make a global impact in the future.(www.woodprotectionassociation.org).
Conclusion
The timber preservation industry is facing challenges on a scale never seen before. These according to Angus Currie of South African Wood Preservers Association include adaptation of new approaches to service life prediction and standardisation, ensuring a level playing field for comparisons with modified wood and other competing technologies, managing a transition to regulation at a new level and a higher cost and finally in ensuring strategies for disposal of treated wood at the end of its service life that are acceptable to regulators and society.
The industry has an obligation to manage these issues successfully so that the benefits of treatment are not lost. These benefits include long life for lower durability timbers in situations where they would otherwise have an unacceptably short life; the prevention of fungal decay and insect attack keeping carbon locked up in the timber and not contributing to global warming; preserving confidence in the performance of timber and reducing the risk of timber being replaced by less sustainable materials like steel, concrete and plastic.
The purpose for which the timber is being purchased defines the treatment required. A piece of timber to be used in the roof need not be treated with the same amount of chemical as a piece of timber being used for a pier. The treatment changes with different applications. The chemicals need not penetrate to the same depth, nor need the solution be the same. These two factors are called penetration and retention levels respectively.
LIST OF CONTRIBUTORS
1.The timber preservers association of Australia
+61 3 9596 8155
3 Wright street
Brighton VIC 3186
tpaa@tpaa.com.au
2.Institute of Wood Biology and Technology
Contact person:Mr.C Mai
cmai@gwdg.de
+49551392051
3. SOUTH AFRICAN WOOD PRESERVERS ASSOCIATION
Contact person: Angus Currie
TEL: +27 11 974 1061FAX: +27 11 392 1995
E-MAIL: sawpa@global.co.za
