Wood modification - why tinker with nature?
Peter Wilson, architect and Director of the Wood Studio research centre at Edinburgh Napier University’s Institute for Sustainable Construction, considers the future of wood as a building material.
The mantra ‘if it ain’t broke, don’t fix it’ is heard often these days, but all too frequently used as a justification for doing nothing rather than recognising a problem for which a process of R&D, testing and implementation is required. And so it is with wood – we build with it in its rawest of states as green timber, we use it after processing has cut, planed and dried it, and we can manufacture new products from it that endow it with properties not found in its natural state. But the world of construction is in constant evolution and, just as new building types continue to emerge, so too does the need to respond to new technical challenges with innovative products and technologies.
Many traditional uses of timber or, more specifically, time-honoured applications of particular species of wood, are no longer so easily available to us, either because we have found better solutions or because contemporary environmental concerns have moved the goalposts. Think of bridges, ship decks and canal lock gates – all constructions that required materials of high durability to resist the worst of the elements. In times past, the species found to best meet these conditions invariably came from tropical locations and considerable volumes of hardwood timber from Africa, the Far East and South America were imported into Europe and the UK.
However, global deforestation and its detrimental effect on the atmosphere has caused widespread rethinking on this matter, with international certification schemes such as the Forest Stewardship Council and Programme for the Endorsement of Forest Certification emerging over the past few decades, with the laudable aim of ensuring responsible sourcing of material from well-managed forests.
Closer to home, the UK Government’s Central Point of Expertise in Timber offers support to public sector bodies and their suppliers, who must comply with strict government rules on the sustainable sourcing of timber.
But it is not only alternative forms of durable timber that are required – for modern forms of building construction where ever-more-stringent regulations apply, we also need dimensionally stable, precision components that emanate from renewable sources. Door and window frames might traditionally have been made from softwoods that were then painted to protect them from the weather, but a cultural history of poor maintenance ensured that many of these components suffered from rot and required wholesale replacement rather than repair. This was an expensive solution that saw the widespread market penetration of products manufactured from unsustainable materials, such as aluminium and PVC. Yes, they can be recycled, but we can’t grow more aluminium, just as we can’t replace the oilfields that underpin the world’s petrochemical industries.
Meeting the challenge
A significant challenge for the timber sector has been to find new ways to transform the properties of readily available and renewable species so that they perform differently to the wood in its natural state, and so successfully substitute for uncertified materials. Therefore, modified wood has emerged – the transformation of the cell structure of certain species through either chemical or thermal modification.
To be clear, this is not another way of describing the many coating or preservative treatments that aim to protect wood for extensive periods, but a fundamental alteration in the material itself sufficient to give it a completely different range of characteristics and properties.
Thermowood is often used as the the generic term for all forms of thermally modified timber, although this registered trade name refers specifically to a process patented in Finland in the 1990s. The technology has been around for some time, with scientific studies of heat treatment of wood appearing in Germany in the 1930s and continuing there and in the USA through the 20th Century.
There are four other thermal modification processes apart from Thermowood – retification (Retiwood), Le-Bois Perdure in France, the Plato process in the Netherlands and the oil-heat treatment (OHT) process in Germany. Three of these are one-step processes using oil (OTH), nitrogen (Retiwood) or steam (Le-Bois Perdure). The Thermowood process, on the other hand, consists of drying, heat treatment and, finally, cooling/conditioning and takes up to 72 hours. The Plato process consists of hydro-thermolysis, dry curing and conditioning and can take up to seven days. The important point in all of these modification processes is that lower durability softwood and hardwood species such as Scots pine, Norway spruce, Douglas fir, birch and poplar can be transformed into high-value wood products of dimensionally stable and durable quality.
Chemical modification has also been around for a long time, but until recently the scale of investment required to build manufacturing facilities made it commercially unviable. Rising prices for sustainable imported timbers and the reduced availability of certain species have to some extent changed the economics, and two processes are currently dominant in the market – Accoya is produced by Accsys Technologies in the Netherlands using acetic anhydride (derived from acetic acid – vinegar when in its dilute form), while Kebony is manufactured in Norway using furfuryl alcohol, a waste product from sugar cane processing. In both instances, the properties of the raw material employed are radically changed to create consistent, dimensionally stable and durable products.
In the case of Kebony, the wood used (usually Norway spruce or Southern Yellow pine, but Maple, Scots pine and Ash are also suitable) becomes darker in colour throughout, a feature used on boat decking and other marine projects. Conversely, Accoya (made mainly from Radiata pine) retains its light colour and has been used in a variety of applications, such as door and window frames, external timber cladding and decking.
The Dutch model
Until recently, the durable nature of these premium products was most often commercially highlighted, resulting in the bulk of sales being for external cladding and decking. However, things are rapidly changing, with architects and engineers beginning to explore other opportunities that make full creative use of these outstanding materials. In the Netherlands, tests using Accoya as a canal lining material have been ongoing since 1995, with remarkable results. Almost two decades on, the acetylated wood shows no signs of rot, decay or fungal damage, highlighting its Class 1 durability potential (BS8417 indicates a 30-year service life for Class 1 durability in fresh water). This attribute has been taken to a new level by RO&AD Architects, first in the extraordinary wood sheet piling of the Moses Bridge at Halsteren and, more recently, with its floating bridge in Bergen op Zoom. These projects were preceded in the Netherlands by Onix Architects’ heavy traffic road bridge at Sneek, a major step forward in the structural application of chemically-modified wood products. In this instance, the material was laminated into 1,080x1,400mm sections that were engineered into the striking curved lattice structure (pictured, right) visible today.
The work carried out by Edinburgh Napier University’s Centre for Offsite Construction and Innovative Structures and Neil Sutherland Architects to develop GluLam Accoya ground beams for domestic house construction has considerable market potential. The objective was to develop an external foundation support detail for an innovative low-carbon affordable home in the Scottish Highlands. Dunsmore house is a two-storey platform timber-frame superstructure, supported on beams raised off the ground on short concrete posts sprouting from shallow pad foundations. While well protected from rain and, therefore, less susceptible to permanent wetting and resultant decay, the use of solid larch beams in Service Class 2 or 3 conditions would still have been subject to seasonal shrinking and swelling, with consequent adverse effects on the level of the house.
Accoya GluLam was selected for its increased stability and durability, as well as the added benefits of improved tolerances, shortened build time and reduced carbon emissions. To assess the long-term effects on the structural performance of GluLam, a number of the beams were monitored over an 18-month period using post-completion measurement techniques (including vibrating wire strain gauges). The results were outstanding – barely any deflection from the moment of beam installation to completion of the build and the fitting out of the house.
So, in the end, why go to all the effort and expense of tinkering with nature in this way? While not rocket technology, the frontier of wood science is leading timber construction into new and highly innovative directions. As architects and engineers begin to understand more about the potential of these materials, so too is it likely that this relatively nascent industry will move beyond the manufacture of premium cladding and decking products into the development of elements uniquely suited to its technology.