There is a constant need for new ways of thinking and building in a more sustainable way, one such way is to build with timber. KL wood stands for cross-laminated timber and is a relatively new material with a wide range of applications, from small residential buildings to large multi-storey buildings. This exciting material is both a more environmentally friendly and sustainable alternative to traditional building materials. As the name suggests, KL wood is made up of cross-laminated layers of wood. The manufacturing process is simple and incredibly efficient - from board to finished building element takes just 60 minutes. Building with KL wood can have up to 30% lower carbon footprint than building with materials like steel and concrete. There is a lot of interesting research going on in the heart of Småland. In Michael Dorn from Linnaeus University's text below, you can delve into everything from the manufacturing process to ongoing research in the field!

KL-TRÄ

The word KL-wood is an abbreviation of the word cross laminated wood, which denotes a building material consisting of a number of spruce boards glued together crosswise in several layers. Normally an odd number of layers is used - 3 or 5 being the most common. Sometimes the thickness of the layers varies. KL wood is mainly used in multi-storey residential or office buildings for walls, beams and roof structures. In addition, KL wood panels can be used in stairwells and elevator shafts. 

KL wood is a relatively new material that was developed about 25 years ago and has made a big impact around the world. It is produced in excess of 2 million cubic metres per year, mainly in Europe, North America and Japan. In Sweden there are four producers: Martinssons, Stora Enso, Södra and Setra. Together, they are expected to have an annual capacity of more than 300,000 cbm as new plants open in 2022. 

Together with glulam, among other things, KL wood belongs to the group of wood-based construction elements. The crosswise gluing process creates timber elements that are longer, wider and thicker than would be possible using only sawn timber directly from the logs. Quality control is also improved. KL wood is produced in the form of solid boards up to 300 mm thick. The boards can be up to 16 metres long and 3.5 metres wide. There is a main load-bearing direction running along the outer lamellae that carry the bulk of the load, but the transverse direction can also be used to carry loads. 

KL wood consists mainly of glue and untreated wood, with the glue representing less than 1% by weight. The wood sequesters carbon dioxide, which is stored in the elements until the building is demolished. KL wood can then be reused as board, the material can also be used for the production of, for example, particle board or burnt for energy recovery. Like other wood-based products, KL wood is therefore considered an environmentally friendly alternative to conventional products. 

In recent years, many buildings have been constructed with load-bearing timber frames. This is mainly residential, but also schools, offices and factories have been built or are planned to be built. The Limnologen project in Växjö, four eight-storey buildings, was the tallest wooden building in the Nordic countries when it was built in 2006-2010. The world's tallest wooden building, using KL wood among other materials, is currently the Mjøstårnet in Norway, with 18 floors and a height of 85.4 metres. Sweden's tallest wooden building, also with a high proportion of KL wood, is Skellefteå's Sara Kulturhus, which is not far behind at 80 metres in height and 20 storeys.

How is CLT produced?

The production of KL wood takes place in fully automated processes, i.e. machine production in the factory with very little manual work. Essentially, the same materials are used in the production of KL wood as in the production of the wooden frames of a house. The individual boards, also known as slats, are quality checked and dried before being sawn and planed to the right size. The press is the heart of the factory, where all the lamellae in each layer are laid close together. On top of each layer, a wood glue is spread before the next layer is applied - crosswise, of course! The glue hardens when it comes into contact with moisture in the air, so it has to be quick when all the layers are put together. The glue and the board are then subjected to high pressure while the glue cures. It's a fully automated process that flows smoothly and easily.

The manufacturing process takes about 60 minutes from the time the first slat is inserted until the entire element can be removed. A KL wood element can be up to 3.5 metres wide and 16 metres long and weigh up to 6.5 tonnes, depending on the thickness of the element. The next step is the sawing and milling where all the holes and grooves for windows, doors and switches that will be in the final wall or beam element are created. The finished elements are then wrapped in thick plastic to protect them from moisture and other damage when delivered to the construction site. Done! 

Long before the elements are manufactured, however, the house to be built has already been planned down to the last detail. Architects, engineers and consultants are involved in determining the length, thickness and properties of the various KL wood elements. Everything has to be determined before the elements can be ordered and manufactured. The manufacturer is usually involved in this process with his specialised knowledge of his material and his broad experience from previous projects. In this way, ready-made solutions can be provided for, for example, different types of sound classes and fire safety requirements.

Sustainable construction with CLT

Sweden intends to become climate neutral by 2045 and the construction sector plays a major role in this transition. In January 2022, national rules on climate declarations for buildings came into force with the aim of reducing the carbon footprint of new buildings. Increasing the use of wood as a building material is seen as an important part of achieving the target. Already today, the majority of small houses are built with wood and KL wood will increase the possibilities to use wood in larger structures as well.

In multi-storey buildings, building materials account for most of the greenhouse gas emissions. Steel, concrete and insulation are non-renewable materials whose production emits large amounts of greenhouse gases. Research results show that multi-storey houses built with KL wood have up to 30% less_{CO2 footprint} compared to an equivalent house built with a concrete frame. Choosing timber frame, instead of using non-renewable materials, means that significantly less greenhouse gases are produced during the manufacturing process.

In addition, the carbon is stored in the wood material throughout the life of the building, preventing the release of greenhouse gases into the atmosphere. When dismantling a house with a wooden frame, it is possible to reuse the wood as a construction product or in the form of another wood product, such as particle board. This in turn means that less fresh raw material is needed to make new construction products. The last residues that cannot be reused after demolition are burnt to produce energy that can be used for heating or electricity generation. 

In addition to the environmental aspects of using timber, there are also social and economic aspects. Living in a wooden house is perceived by many as a pleasant and natural experience. That is why houses are designed with the outer layers of KL wood visible inside the house, these visible layers create a special expression.

Buildings with optimised KL timber structures contribute to achieving the UN's global sustainability goals under Agenda 2030. The main goals addressed are: goal number 9 on inclusive and sustainable infrastructure; goal number 11 on sustainable cities and communities that are inclusive, safe, resilient and sustainable; and goal number 13 on combating climate change and its impacts.

Research on CLT at the Department of Building Technology

The Department of Building Technology at Linnaeus University has been conducting research and development work in the field of timber for several years. Areas of research include: improved use of the raw material, development of joints and fasteners, long-term studies and risk analysis, as well as sustainability and life cycle analysis. These areas are briefly presented below.

Improved use of the raw material

Wood is the starting material, which is then glued together to form large KL wood elements. It is important that the product meets the requirements as a construction material and that profitability is achieved for the forest owner, the sawmills and the producer. Research at Linnaeus University includes investigating the different qualities used in different wood elements. Layers can consist of lower quality timber and be used in those parts of a KL timber element that have lower loads. Research is also being carried out in the area of hybrid products, where, for example, the possibilities of combining softwood and hardwood are being studied.

Development of dressings and fasteners

The elements of a building are assembled on site with nails and screws, mostly to transfer loads between KL elements but also between KL timber and, for example, concrete. The behaviour of wood when single fasteners transfer loads is primarily studied. Also whole groups of fasteners are investigated as the different lamellae do not have the same properties. In addition, it is important to gain more knowledge about how the behaviour of KL-wood structures changes over time, how the elements are affected by recurring loads and how the connections can be designed to simplify the dismantling of a KL-wood structure.

Long-term studies and risk analysis

Wood is a natural material that is affected by its environment. In particular, the amount of water in the wood material is of particular interest. For example, the amount of moisture in wood affects its strength and stiffness; if the wood becomes sufficiently moist, there is also a risk of mould and decay. In order to study real-life conditions, buildings have been equipped with measuring instruments that measure moisture conditions, settlement and the dynamic properties of buildings. A weather station can also be used to measure moisture, rain, wind and ambient temperature simultaneously. In order to prevent damage, researchers are trying to find correlations between changes in the environment and the condition of KL timber structures, and using such correlations to predict harmful conditions.

Sustainability and life cycle assessment

To get a good picture of a building's environmental impact, it is necessary to look at the whole picture and not just at the choice of materials. One way to do this is through life cycle assessments, which include the design, construction, operation and maintenance and dismantling of a building. Life cycle assessments can be used to compare different building systems and to broaden the picture by including the origin of materials, for example from forests, as well as to find opportunities for reuse and recycling.

MICHAEL DORN, RESEARCHER LINNÉUNVERSITETET

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