CLT creates new opportunities for hardwoods

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UNECE/FAO/ITTO/FORDAQ
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The latest UNECE Forest Products Annual Market Review highlights the rapid growth in the market for crosslaminated lumber (CLT) and the new opportunities the product creates for wood, including hardwood, to compete in high-end structural applications. Although the first CLT production facilities were constructed in the DACH countries (Germany, Austria and Switzerland) in 1994, the full potential is only now being realised following a long period of technical and market development.

CLT first entered the building market during the 2005 to 2010 period, transforming from a small-scale niche product into large-scale industrial production.

CLT panels consist of several layers of structural lumber boards stacked crosswise (typically at 90 degrees) and glued together on their wide faces and, sometimes, on the narrow faces as well.

In special configurations, consecutive layers may be placed in the same direction to obtain specific structural characteristics. CLT products are usually fabricated with three to seven layers, but with additional layers in some cases.

Thickness of individual lumber pieces typically varies from 16 mm to 51 mm and width varies from 60 mm to 240 mm. Boards are finger-jointed using structural adhesive. Lumber is visually-graded or machine stressrated and is kiln dried. Panel sizes vary by manufacturer; typical widths are 0.6m, 1.2m, and3 m, while length can be up to 18m and thickness up to 508 mm.

The dimensions and lay-up of CLT production are now internationally standardized and recognised, and production techniques are optimized in modern manufacturing facilities. CLT is designed to maximise yield, utilise lower grades of lumber, and it can be made in a high volume of very large sections.

The result is a light but very strong panel product that can be made off-site and erected quickly to form structural walls, floors and ceilings. CLT is used in a wide range of applications in single-family houses, multi-storey towers, public buildings and specialty construction.

CLT offers new opportunities for wood to compete in large-scale structural applications dominated for many years by concrete and steel. In addition to delivering comparable technical performance, CLT panels can readily out-perform these alternatives on environmental issues. Wood's renewability, low embodied energy, and potential as a carbon store during use are all considerable benefits.

Global production of CLT was about 625,000 cu.m in 2014, and this figure is forecast to increase to about 700000 m3 in 2015. About 90% (560,000 cu.m) of global cross-laminated timber (CLT) was produced in Europe in 2014, and this is forecast to increase to 630,000 cu.m in 2015. The DACH countries have continued to be the driving force in CLT development, not only as the originators of CLT products but also as the leading CLT producers. Austria has seven CLT production facilities, Germany three and Switzerland two.

Minor production sites exist in Finland, Italy, Norway, Spain and Sweden, and more CLT factories are under construction in Finland, France, Sweden and the UK. The central European CLT industry is strongly export oriented, supplying other parts of Europe as well as overseas markets.

The use of CLT is making possible the construction of tall wooden buildings. The current record-holder is a 14-storey residential high-rise in Bergen, Norway, and an 18-storey wooden building is planned in Vancouver, Canada. Earthquake-prone countries such as Japan have shown keen interest in increasing the use of CLT, and Japan has published a roadmap to pave the way for CLT in the Japanese building market.

To date commercial production of CLT has been dominated by softwood, notably Norway spruce and Sitka spruce, for reasons of price and wood consistency. However the American Hardwood Export Council (AHEC) is also now promoting development of hardwood CLT, noting that some hardwood species are underutilised and readily available at competitive prices while also offering up to twice the inherent strength of softwoods.

This last factor presents an opportunity to significantly reduce the amount of fibre, or mass, required to achieve the same strength performance. Hardwood species also provide opportunities to improve the appearance of CLT panels.

AHEC is particularly promoting the potential for CLT in tulipwood (Liriodendron tulipifera), an abundant U.S. hardwood species. Other temperate hardwood species with potential include poplar and birch.

The potential for tropical hardwood CLT has yet to be seriously explored, but there may be specific opportunities for some faster growing plantation species or the pioneer species found in large quantities in secondary tropical forests.

Eucalyptus is generally considered too difficult to machine to be a viable option for CLT, but recently progress is being made in Australia to develop a related product under the brand name “Cross Laminated Strand Timber” by Melbourne-based Lignor.

The international CLT product and design standards, and open-source software packages such as the CLTdesigner have supported the international trade of CLT. Standardization in Europe comprises product standard EN 16351 (currently a draft version, with legal validity envisaged by the end of 2015) and design standard EN 1995-1-1 (currently in revision).

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