Industrial Requirements for Cross-Laminated Timber Manufacturing

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Industrial Requirements for

Cross-Laminated Timber

Manufacturing

Author: Matilda Torneport Supervisor: Reza Hosseinpourpia Examiner: Jimmy Johansson Date: 01-06-2021

Course code: 2TS90E, 15 credits Subject: Forest and Wood Engineering Level: Bachelor of Science in Engineering Department of Forestry and Wood

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Abstract

Wood is a valuable sustainable material that meets the requirements for structural application. Cross-laminated timber (CLT) is a wood-based product that is mainly used in the building industry. Due to the rapid global market increase, a number of new CLT plants are emerging worldwide and thereby a need for standardisation is more than ever. There is no existing harmonised standard for CLT and it means a diversity between manufacturers, CLT products and its layup, which may in turn affects the properties of available CLT in the market. Therefore, this study was performed through literature study and internet-based interviews from five manufacturer in Sweden and Central Europe, to provide more information regarding the industrial requirements for CLT production. Three specific

objectives of this study were: (1) wood and adhesive types in CLT production, (2) wood strength classes for CLT production, (3) important requirements for CLT producers and existing standards.

Literature review and interviews showed that spruce (Picea abies L. Karst.) in combination with polyurethane (PUR) adhesive is the most commonly used materials in Europe for CLT production, which are approved by EN 16351

(2021). Other wood species, e.g., pine, poplar and birch can be used or are already used in a minor extent. Strength classes for lamellas in CLT are often C24, but timbers with lower strength grades are possible. Some manufacturer use combinations of different strength graded timber and in this small scale study different strength graded timber was in generally the biggest diversity between manufactures. Only a few material properties such as modulus of elasticity, modulus of rupture, compression and shear strength are listed in EN 16351 (2021) and EN 338 (2016), as the factors for quality measurements of the CLT products. This study, however, showed that the critical material properties for the most interviewed CLT producers are merely modulus of elasticity and rarely modulus of rupture.

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Sammanfattning

Medvetenheten kring hållbara byggkonstruktioner ökar runt om i världen och trä är ett värdefullt material som uppfyller kraven för strukturell tillämpning i olika aspekter inklusive ur miljösynpunkt. Korslimmat trä (KL-trä) är en träbaserad produkt som huvudsakligen används inom byggindustrin och i jämförelse med andra byggmaterial som t.ex. betong en liknande hållfasthet. Efterfrågan på produkter av KL-trä beror starkt på byggnationer av bostäder, samt kommunala- och industriella byggnationer. Under de senaste åren har den globala marknaden, liksom den europeiska marknaden för KL-trä ökat snabbt. På grund av den snabba globala marknadsökningen växer ett antal nya KL-trä anläggningar över hela världen och därmed behovet av standardisering mer än någonsin. Det finns ingen befintlig harmoniserad standard för KL-trä vilket innebär en mångfald mellan tillverkare, produkter och dess upplägg, som i sin tur kan påverka egenskaperna hos KL-trä som finns tillgängligt på marknaden. En litteraturstudie utfördes för att få översikt över det nuvarande kunskapsläget och internetbaserade intervjuer från fem tillverkare i Sverige och Central Europa för att erhålla mer information om de industriella kraven för KL-trä produktion. Tre huvudämnen valdes för att uppnå syftet med rapporten; (1) trä- och limtyper för KL-trä produktion, (2)

hållfasthetsklasser för KL-trä produktion, (3) viktiga krav för KL-trä tillverkare och befintliga standarder.

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Preface

This thesis in Bachelor Engineering was performed for ten weeks at Linnaeus University Växjö, Sweden, at Department of Forestry and Wood Technology. Cross-laminated timber was chosen as the main subject due to an interest in learning more about this engineering building material. The method was modified and adapted to prevailing circumstances due to the pandemic.

I would like to thank my supervisor, Dr. Reza Hosseinpourpia for given me support throughout the work. Thanks to the companies who participant, also thanks to Swedish Wood for approving the use of illustrations and tables from “The CLT handbook” and Christina Brandin, Swedish Wood, for the use of picture.

June 2021

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Terms and definitions

Adhesive

Product who bonds materials together - glue.

Bond lines

Lines of adhesive between different layers.

Bonding strength

Adhesion strength between adhered substrates.

Characteristic value

Value that estimate a tolerance limit, 5 %-fractile, for strength properties.

Coniferous wood

A tree that have seed-producing cones, i.e. reproduces by gymnosperms– softwood species.

Deciduous wood

A tree that sheds its leaves at the end of the growing season and then go into resting state – hardwood species.

Delamination

Mode of failure/separation of layers in a laminate.

Edge bond

Lamellas in the same timber layers are connected with each other by adhesive.

Finger joint

Machine-made symmetrical fingers at end joints of structural solid timber that are bonded together.

Formaldehyde emissions

Emissions from wood that contain and emit volatile compounds like

formaldehyde. Emission increase when wood is processed, like for engineering wood products.

In-plane load

Load acting parallel to a standing CLT panel.

Lamination

Process that bond together lamellas and form a layer.

Layup

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Lower grade wood species

Wood species that naturally have lower strength and stiffness properties and therefore graded lower than other common used structural timber.

Narrow bond

See edge bond.

Out-of-grade timber

Timber that have low mechanical properties and cannot be graded into any of the strength classes.

Out-of-plane load

Load acting perpendicular to a laying CLT panel.

Strength grading

Process that measure the timber characteristics value of strength, stiffness and density. The timber are then divided into different strength classes.

Wood-based panels

Glued panels made of wood in different forms, for example by timber, wood chips, fibres, or veneers.

x-axis

Axis parallel to grain of the outer layer.

y-axis

Axis orthogonal to the grain of the outer layer.

z-axis

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1. Introduction

1.1 Background

Awareness of raw material from unrenewable resources for constructional application is increasing, and wood is an important alternative in this aspect as it is an environmentally friendly material and comes from renewable resources. During the entire life cycle of a building, wood-based products have enormous potential in carbon-efficient construction due to the fact that wood stores nearly one ton of CO2 per cubic meter (Fröbel and Bergkvist 2020). For the last decade,

timber constructions have increased in market shares, partly because of the development of timber products like cross-laminated timber, CLT (Brandner 2016).

CLT is a building engineered timber product that is used for load-bearing

structures and can measure against other building material like concrete and steel. CLT is composed of an uneven number of wood layers, e.g., three, five, seven or more, that are connected crosswise at an angle of 90° by adhesive bonding (Figure 1) and can be used in many various applications. CLT panels are mostly used in walls and floors structures. The raw material in CLT are usually wood from the coniferous tree. However, other types of wood species may also be used, both from coniferous or deciduous tree, e.g., softwood or hardwood (Gustafsson 2019). Raw material for CLT production is often local wood species which may include wood species with low mechanical properties (Brandner 2016). For use of local wood species in CLT production it is important to increase knowledge about the mechanical properties of these wood species (Ehrhart and Brandner 2018).

Figure 1. Five-layered CLT (Christina Brandin, Swedish Wood 2021).

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are under construction in Europe, for example, in Finland, France, Sweden and the UK (Wilded 2018; Jauk 2019). Also in Northern Central Europe, where the most CLT plants exist, the number are also increasing. Currently, one of the largest producers of CLT in the world is Stora Enso, which is located in Austria, Sweden, and small production line in the Czech Republic. Besides Stora Enso, KLH

Massivholz in Austria, and Binderholz in Germany and Austria are large producers of CLT (Jauk 2020). Austria is the largest producers of CLT in the world due to the allocation of big CLT companies (Holzkurier 2020). Austria is also the biggest exporter of CLT, as they export for instance to Italy, Germany and France but also countries outside of EU, like Switzerland and Japan (Ebner 2020). The production capacity of CLT in Northern Central Europe, including Italy and the Czech Republic, is about one million m3_{(Jauk 2020).}

With increasing interest for CLT in the global market, a need for standardisation that included information about the product, testing and design parameters is also increased tremendously (Brandner 2016). The most referred standard is the one from the International Organization for Standardization (ISO) 16696-1:2019. In Europe, the design of CLT is not included in current version of Eurocode 5 (EN 1995-1-1:2004) standard for design in timber structures. It means that the European standard is not harmonised for CLT. As the importance of CLT in the construction sector is increasing (Fink 2018), work is ongoing to update Eurocode 5. When there is no harmonised standard, the CLT manufacturers follow local standards, such as EN 16351 (2021), or for companies declared values to European Technical Assessment (ETA) (Gustafsson 2019). This means that the requirements from different CLT manufacturer might be misinterpreted. In addition, different manufacturers have, for example, their own standard thickness and strength class timbers for their CLT products (Gustafsson 2019), and thus this may increase the differences in the final CLT product at the market. To the best of our knowledge, there is no comprehensive study on the criteria requirements for manufacturing CLT.

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1.2 Aim

This study aims to provide basic information about CLT to increase knowledge about the building material but also considering the industry requirements. The wood technical properties of CLT will be highlighted in this study and the aim will be achieved by the following research questions:

RQ1: What wood types and adhesive are used in CLT production?

And to what extent they can be changed to other types? RQ2: What strength graded timber can be used in CLT production? RQ3: What classifications exist for CLT in terms of quality and

application?

1.3 Limitations

Some limitations in this thesis that investigated the wood technical properties for CLT are an detailed production process for CLT and how the methods for

classifying the timber used for CLT production. The procedure of finger jointing will not be declared. Furthermore, sound- or fire properties of CLT are not mentioned or moisture content of timber.

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2. Methodology

To answers the research questions, the research method is divided into two parts, literature review and interviews of manufacturers. Data for this study were mainly collected from scientific research articles and reports, related standards, books and websites. To enhance the knowledge from practical aspect several CLT

manufacturer was contacted in Sweden and Central Europe.

2.1 Literature review

A general literature review is intended to create an overview of a limited area of knowledge and critically examine this area (Friberg 2012). The background information of CLT was written as an overview of the current state of knowledge. The literature review in this study has aimed to find essential information that fulfilled the purpose of this study. Scientific research articles or reports, as well as, existed standards were selected, reviewed and analysed based on a critical approach and to provide answers to objectives.

Sources for the basic information selected for this review research were substantially collected from “The CLT handbook” (Gustafsson 2019) from Swedish Wood and current existing European standards for the subject. Some information was collected from “Canadian CLT handbook” (Gagnon and Popouski 2019). Other sources like website from authorities, companies that declared ETA or Declaration of Performance and newspaper articles, from Holzkurier and Swedish Wood Newspaper Trä occurred. For searching scientific research articles, One Search were primarily used and thereafter Google Scholar. All articles selected for this study were peer-reviewed and limited to English. Articles published all over the world were allowed and no annual range selected since CLT is a relatively new building material. When specific information about CLT were conducted a Boolean search technology (Friberg 2012) with search operation and were used, for example, “cross-laminated timber” and “ strength class”. Further into the thesis secondary searches occurred more often, as some of the scientific research articles referred to relevant sources for this study. When secondary sources were used this substantiated by other references. Sources from books and websites that appeared in Swedish was translated into English by the author of this study.

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2.2 Interviews

Interviews is a type of data collection where the source for the data is from

peoples answers to the researcher´s questions. The interview method for this study was internet-based. This type of interview method is based on email

correspondence (without direct visual contact) or by videocast in real time. When email interviews are implemented, it is positive to have the data as written

document but it can also be negative when the respondent does not provide as much information in writing as in speech (Denscombe 2014). A semi-structured interview was selected as a useful tool because the manufacturers had the opportunity to answer freely, no fixed answer options were use (Patel and Davidson 2019). The interview process was conducted by a standard method for all participants through previously prepared questions. All companies obtain the same questions in English and in the same sequence regardless of country or language. However, when further contact was established with the manufacturers they answered different complementary questions. The questionnaire was

developed in the first week of the thesis. The manufacturer in Sweden offered to answer in Swedish. The questionnaire consisted of seven questions covering the purpose of the thesis (Appendix I). Information about the aim of the study and the interview´s questions were sent to all selected companies by email. All

respondents and companies are anonymous in this thesis. The data for this study is qualitative and content analysis was used. The outcomes of the interviews were categorised in participant countries and merged into individual headings. The material was quantified and described in the ongoing text under suitable heading (Denscombe 2014).

For a small scale research, the explorative selection is often used. The explorative selection gives insight and information for the researcher despite a small

population and an exact cross-section of the population are not necessary.The selection for this study was a non-probability sampling as the author had an individual opportunity to choose the selection (Denscombe 2014). This was beneficial because of the short time frame of the thesis and also difficulties related to finding the CLT producers in countries other than North Central Europe, which have the highest number of CLT plants in Europe (Muszynski 2017; Jauk 2020). A subjective selection was used to select the CLT plants for so many different countries in Central Europe (Peck 2000). All contacted manufacturers were found by searching on the internet, e.g., google and also newspapers. In the case of branches companies, which have plants in multiple countries, one plant was contacted.

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3. Results

3.1 Literature review

The results from the literature review are based on scientific research articles and reports, related standards, books and websites. Initially, a short background is performed in strength of timber to enhance knowledge of wood properties and strength graded timber for structural application. Further, the literature review process cross-laminated timber for basic information and process in particular parts related to the objectives for this study.

3.1.1 Strength of structural timber

Strength, density and stiffness of wood are the properties that are often considered for wood quality. As anisotropic material, wood has different properties in

different direction. The strength of wood is generally higher in the longitudinal direction (in fibre direction) (Gagnon and Popouski 2019). The properties of timber vary between wood species and also vary in the same wood species depending on growth place. The strength in timber varies in different directions depending on, e.g., fibre direction, the density of timber, time for load and moisture content (Fröbel and Bergkvist 2020).

Most of the existing standards grade the timbers based on their bending strength, e.g., modulus of elasticity (MOE), and density, while there are other important values that need to be considered. The MOE value provides information about internal abnormality in wood like knots, spiral grain angle and compression wood, which reduces the bending performances of timber. However, values can differ to a great extent and there are therefore important to grade the properties into

different strength classes. This classification helps to provide a common classification in the market, to improve the control the timber´s properties like strength and stiffness, and also to help optimise the selection of raw material for specific application (Johansson 2016). Strength of timber are tested with

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The strength characteristic values obtained from different grading methods can be classified into strength classes specified in standard EN 338. Strength classes for softwood between C14 - C50 are specified for edgewise bending tests and T8 - T30 for edgewise tension tests. For hardwood species, it is between D18 – D80 (EN 338:2016). The most common strength classes used for timber construction are C16, C18, C24 and C30. Strength class C14 (with bending strength 14 MPa) are often used for load-bearing internal- and external walls where strength and deformation of the timber are allowed to a great extent. Compared with strength class C18, moderate strength and deformation of the strength graded timber are allowed. When a higher strength is required in load-bearing structures, e.g., trusses, strength class C24 are used and only allowed deformation to a low extent (Fröbel and Bergkvist 2020).

3.1.2 Cross-laminated timber

Wood for building structures can reduce the climate impact and with the

development of products with high pre-fabrication, for example, CLT the market share for wooden buildings increase (Fröbel and Bergkvist 2020), and thereby CLT can in long term play an important role when it comes to sustainable buildings (Jauk 2020). CLT, or X-lam, is an engineered wood product used for construction application. Muszynski et al. (2017) declared that most CLT products are custom-made for specific projects and are produced mainly for small to

medium size multi-family housing, medium-size public and industrial structures. CLT panels can also be used in buildings rise with multiple floors (Gustafsson

2019), for example, MjØstårnet in Brumunddal Norway with eighteen floors,

which currently are the tallest timber building in the world (Bianchini 2020).

CLT is produced with finger jointing connection of timbers and industrially with far-reaching manufacturing as sheets, panels, posts and beams. Panels are the most common product of CLT and can be manufactured before delivery with cut out of windows and doors, and without any extra reinforcement (Gustafsson 2019). This advantage can reduce construction costs and noise at job sites (Wang 2018). CLT can be produced as a straight or curved surface component.

According to EN 16531(2021) three types of CLT are mentioned for use in buildings and bridges (Figure 2).

Figure 2. Three types of CLT according to EN 16531 (2021) for use in buildings and bridges. For all types of CLT, some requirements are needed to meet regarding the

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temperature of the lamellas, finger joints and bonds between layers. As defined in EN 16351 (2021), the CLTs that are used in service class 1 or 2 according to EN 1995-1-1 (1994), should have edge bond and a maximum thickness of 500 mm. For type two and three some extra requirements needed.

The production process of CLT are often similar between manufacturers and it is according to the relevant standards, e.g. EN 16351 (2021) or declarations for companies, e.g., Declarations of Performance (DoP) according to ETA. When the timber is strength graded they are often finger jointed to create long lamellas. Adhesive is applied to create a layup and pressing with vacuum or hydraulic method is needed (Gustafsson 2019). Adhesive is placed on the side-faces of the lamellas, and sometimes also at the lamellas narrow sides (Fink 2018). After adhesive is cured under press, the CLT panels are cut and packed (Gustafsson 2019). CLT panels are produced with a thickness between 60 – 500 mm (Figure 3) and the panels up to 4.8 m tall and 30 m long (Table 1). Often CLT is made of layers with the same thickness. However, some manufactures produce CLT with different thickness of the inner layer (Gustafsson 2019). The range of CLT component depends on the manufacture product variety, the manufacturers capacity of the machine but also limitations in transport (Gagnon and Popouski 2019).

Figure 3. Schematic illustration of CLT panel (Gustafsson 2019).

Table 1. Dimensions and number of layers for CLT panels (Gustafsson 2019).

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of the timber (Gustafsson 2019). The CLT panel classification is generally based on solid wood species, timber strength grade and the layers structure (Gagnon and Popouski 2019). Most important properties for CLT product is strength, stiffness and durability. In contrast to other solid wood products, the CLT panel has less shrinkage and swelling across the fibre direction, because of the crosswise layer. Its dimensional changes is determined by the number of layer and thickness of each layer (Gustafsson 2019). For designing a CLT panel, a good understanding of the layup and selected material are needed to process high performance end-product (Gagnon and Popouski 2019).

Figure 4. The direction of load (F) to out-of-plane (left) and in-plane (right) of CLT panel. CLT can be used in many different applications, both in load-bearing and non-load-bearing construction. For example, in walls, floors structure and roof elements. Load-bearing structures in walls and floors are the most common area of use for CLT. The CLT panels are characterised by their high cross-sectional area. CLT can also be used, for example, in apartment balconies, lift shafts and stairwells. CLT provides good fire safety and multi-layered structure design is used to achieve good sound insulation. A CLT panel that placed on two support are the most common structure as the support can be placed along the whole length of the panel or placed individually. The outer layer direction is important depending on which application the CLT panel is intended to be used for

(Gustafsson 2019).

The wall elements of CLT is primary loaded in-plane direction (Fink 2018) and admit a bracing structure. CLT for the wall can be applicated in external, internal and partition locations, and acts as a load-bearing or non-load-bearing component (Gustafsson 2019). For CLT panels used in walls normally, the lamellas are oriented in the outer layers parallel to loads. This increase vertical load capacity of walls. When design wall panels important mechanical characteristics are

considered, like load-bearing capacity, in-plane shear and out-of-plane bending strength (Gagnon and Popouski 2019). A wall panel with a thickness of 80 mm can be designed to take loads over 100 kN/mm in the vertical direction, i.e., thin walls can take high loads thanks to the nature of CLT panel. Even if large

openings are made on the panels, like windows or doors, no extra support is often needed (Gustafsson 2019).

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supports. For horizontal loads, the floor structures must be designed to take wind loads. The direction of lamellas in the floors system, as for walls, are generally placed longitudinal (parallel to the major load direction) (Gustafsson 2019).A critical factor for designing the CLT panels for floors is deflections, vibrations, and also in-plane and out-of-plane bending strength, shear strength, and stiffness

(Gagnon and Popouski 2019).When the floor element is exposed to out-of-plane

bending a rolling shear stress would develop (Fink 2018).

3.1.2.1 Materials

The CLT products are made from solid wood that has been strength graded, mechanically or visually. ISO 16696-1 (2019) declare that solid sawn timber shall be graded according to national standards, e.g., EN 14081-1. If the strength grade for solid sawn timber differs from national standards there shall be a document that confirms the quality and characteristic value of the various layers. The lamellas can also be selected according to their visual appearance. Where an exposed finish is required the higher quality timber can be selected, and in contrast the lower quality timber can be used in the hidden part of CLT (Wang 2018).

The thickness of the base material can also differ between CLT manufacturers (Table 2). Lamellas with a thickness between 20 – 45 mm and a width from 80 – 200 mm are generally used in CLT products (Gustafsson 2019). EN 16351 (2021) defines the lamellas width between 40 – 300 mm and the thickness between 6 – 47 mm. ISO 16696-1 (2019) describes the width-to-thickness ratio of less than 1.75% for solid wood timber in the parallel layers whit edge bonding. For the solid sawn timber placed perpendicular, the width-to-thickness ratio shall be at least 3.5%. This is for decreasing CLT creep deflection when edge bonding is not performed.

Table 2. Common dimensions and strength classes of lamellas used in CLT (Gustafsson 2019).

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Today, the adhesives that are used for glulam are mainly applied on CLT products, as there is no adhesive standard specially for CLT products (Wang 2017; Brandner 2016). The manufacturer of adhesive have in general guidelines and requirements for their adhesive and some producers have also adapted their adhesive for CLT, especially about bonding pressure and applied quantity

(Brandner 2019). EN 16351 (2021) interpret “the adhesive shall be applicable for

the species intended to be used and shall be used in accordance with the instructions of the adhesive manufacturer”.

Product standards EN 16351 (2021) defined three types of adhesives that are allowed for load-bearing timber structures:

- phenolic and amino-based thermosetting adhesives (e.g. MUF, MF, PRF) (EN 301)

- moisture curing one-component polyurethane adhesives (PUR) (EN 15425)

- emulsion polymer isocyanate adhesives (EPI) (EN 16254).

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Wood species and timber material for CLT production

For global CLT production, softwood is the most common wood species. This depends partly of where the production site of CLT is located. The biggest producers are located in countries where coniferous trees are dominant ( Foglar-Deinhardstein 2015). A survey study carried out by Muszynski et al. (2017) showed that of nineteen CLT plants in the world use spruce (Picea abies L. Karst) and only a small number of manufacturers use pine and fir. However, pine (Pinus

sylvestris L.) is becoming more important in Europe for CLT manufacturing

(Ehrhart and Brandner 2018). Gustafsson (2019) declared that other types of wood species may also be used for CLT production, like hardwood species, Liao et al. (2017) studied the use of fast-grown small diameter eucalyptus (Eucalyptus

urophylla, Eucalyptus grandis) for CLT production and revealed that this wood

species had equivalent mechanical results as compared with commercially available softwood in CLT product, and can be used for structural applications. Poplar is another wood species that has been investigated. Hematabadi et al. (2020) prepared CLT products from Iranian poplar (Populus alba L.) in order to explore the use of local wood species for building application and use of

predictive modelling. Results showed lower MOE value of three-layered poplar CLT compared with larch and pine CLT, and also showed a change in the strength property related to span-to-depth ratio. A low-density wood species like poplar can be used when designing CLT with the knowledge that there is variability between different analysis method and the performance of poplar CLT. Black spruce (Picea mariana) and Hem-fir (Abies grandis) were also used for CLT production and the results shown that these wood species can be used for

manufacturing CLT (He 2020; Wang 2018). Corpataux et al. (2020) investigated the use of three tropical fast-growing timber species for CLT panels and plates. Three-layered CLT panels with different layups were studied and compared with European standards. The results showed that red jabon (Anthocephalaus

macrophyllus) has the potential to achieve C24 strength class. The other two

investigated wood species, sengon (Falcataria moluccana) and acacia hybrid (Acacia mangium, A. auriculiformis) did not achieve strength class C24, but combinations some of these three tropical wood species alternatively with Norway spruce (Picea abies L. Karst) made it achievable to reach higher strength classes than C24. Ehrhart and Brandner (2018) studied the rolling shear strength

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The popularity of CLT as an construction material is increasing in Europe but also globally (Jauk 2019), this has led to an increased number of research studies that investigate hybrid CLT compound of different wood species. For example, Aicher et al. (2016) tested three-layered CLT plates with European spruce (Picea abies L. Karst.) in the outer layers and European beech (Fagus sylvatica L.) as a cross-layer in the middle. A combination of softwood and hardwood for CLT is promising for future CLT products as a structural element, even with low-grade hardwood. Hardwood has some benefits compared with softwood, i.e., they have higher strength properties, and higher density can lead to better material

influences. However, hardwood species have in general higher shrinkage values which can affect the service performance. Hybrid CLT, with hardwood in the outer-layer and softwood in the centre can still give a positive effect on the mechanical properties and also decrease the material costs (Espinoza and Buehlmann 2018).

According to product standard EN 16351 (2021), sixteen wood species including softwood species and poplar are approved for CLT production. CLT shall be made from one of these specified wood species and layers in CLT can be made of one wood type or combination of wood species. ISO 16696-1 (2019) declare that national standards and a minimum density of 300 kg/m3_{are required for wood}

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Table 3. Wood species that are shown in the literature review with the potential for CLT production.

Softwood: Reference:

Norway spruce (Picea abies L. Karst.) Ehrhart and Brandner (2018); Corpataux et al. (2018) Pine (Pinus sylvestris L.) Ehrhart and Brandner (2018)

Black spruce (Picea mariana) He et al. (2020) Western hemlock (Tsuga heterophylla (Raf.) Sarg) Wang et al. (2018) Amabilis fir (Abies amabilis (Dougl.) Forbes) Wang et al. (2018) Irish Sitka spruce (Picea sitchensis) Sikora et al. (2016)

Hardwood: Reference:

Eucalyptus (Eucalyptus urophylla, Eucalyptus grandis) Liao et al. (2017)

Poplar (Populus alba L.); (Populus tremula L.) Hematabadi et al. (2020); Ehrhart and Brandner (2018) Birch (Betula pendula R.) Ehrhart and Brandner (2018)

Beech (Fagus sylvatica L.) Ehrhart and Brandner (2018) European ash (Fraxinus excelsior L.) Ehrhart and Brandner (2018) White ash (Fraxinus americana L.) Crovella et al. (2019) Red maple (Acer rubrum L.) Crovella et al. (2019) Sugar maple (Acer saccharum) Ma et al. (2020)

Hybrid: Reference:

European spruce (Picea abies L. Karst.) and beech (Fagus sylvatica L.) Aicher et al. (2016)

Tropical: Reference:

Sengon (Falcataria moluccana) Corpataux et al. (2020) Red jabon (Anthocephalaus macrophyllus) Corpataux et al. (2020) Acacia hybrid (Acacia mangium, A. auriculiformis) Corpataux et al. (2020)

Because of the structure of CLT with its layer nature, wood species with a lower mechanical quality than Norway spruce (Picea abies L. Karst.) can be used in CLT production (Brandner 2016). Some research study shown that wood species with low mechanical properties are used for CLT production, e.g., sugar maple (Acer saccharum) and Irish Sitka spruce (Picea sitchensis) (Ma 2021; Sikora 2016). Crovella et al. (2019) studied three low-grade material for CLT panels, two hardwood species such as white ash (Fraxinus Americana L.) and red maple (Acer

rubrum L.), and one softwood species such as white pine (Pinus strobus L.). The

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Except for low-grade wood species from both softwood and hardwood, timbers that have been graded as a out-of-grade are also in small scale investigated for CLT production. Cherry et al. (2019) examined the potential of out-of-grade sawn pine for CLT manufacturing by a literature study and they found that in the future, the out-of-grade timbers have the opportunity to be utilized more through new approaches and methods. Design of CLT with a cross-wise structure gives the product higher mechanical properties and can contribute to the uses of high volumes of out-of-grade timber. Even a timber with very low stiffness compared with a in-grade timber in CLT products can achieve spans of 80 %. The rolling shear strength is also an important value to considered when using out-of-grade timbers due to their smaller width-to-thickness ratio. Other timber material for CLT production, not mentioned in EN 16351 (2021), are re-used timber. Re-used timbers were investigated for their mechanical properties in CLT production by Munandar et al. 2019. They used wooden boxes, wooden pallets and scrap wood to create a three-layered CLT panels. Beams were cut and then tested for their bending performance and compared with the ASTM standard. The results showed a lower flexural strength than the standard values. However, they stated that the re-used timbers have the potential to be used in CLT and act as a complement to primary timber depending on the application for the product.Rose et al. (2018) used re-used timber from construction and demolition sites in CLT production and declared that it has good potential to use high-quality timber in combination with high-quality re-used timber in the cross-section for the most application for CLT production. Most of the produced CLT are manufactured custom-made so there is possible to determine if lower grade timber or re-used timber can be used for the specific application. Furthermore, Rose et al. (2018) described that a CLT product only made of re-used timber can have the possibility to use, for example, in elements where the structural demands are low as in single-storey buildings or for interior walls.

According to EN 16351 (2021), other timber material than solid sawn timber can be used for CLT production, i.e., wood-based panels. For example, plywood or oriented strand board (OSB), which are used as a substitute for single layers (Brandner 2016). A combination of solid sawn timber and different structural wood products, for example, laminated strand lumber (LSL) (Wang 2015; Gagnon and Popouski 2019) or only use LSL or LVL (laminated veneer lumber) can be used to build up a CLT component (Gagnon and Popouski 2019). However, none of the product standards EN 16351 (2021) or ANSI/APA PRG 320 certified this product type, today.

Strength classes and diversity

The CLT with three to nine layers of glued lamellas is the most common product and the layer shall be placed orthogonally to each other, unless at least one layer is orthogonally oriented to two adjacent layers (EN 16351:2021). Only one strength class shall be in each timber layer (EN 16351:2021) but different layers of

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Europe for a homogeneous layup in CLT products are C24. With a combined layup strength class usually, the C24 timber is used in the longitudinal layers and C16/18 timber(s) used in the transverse layers. Layers can also be composed of ≤ 10 % lower strength graded timber. These are often approved by ETA and the CLT product properties do not have to be declared again (Brandner 2016). According to a manufacturer in Germany, their ETA (ETA-10/0241) declare a homogeneous CLT panel with ≥ C16 are possible and also combinations of strength classes according to Table 4. Further, the companies ETA declare that without new calculation, 30% of the lamellas belong to the next lower strength class can be used in one layer. A manufacturer in Sweden that produce CLT, produces homogeneous C24, but also a combination of C24 and C14 (Martinsons 2017).

Table 4. Possible combinations of strength classes according to ETA-10/0241.

70% 30%

Strength class C24 C16

C30 C24

C35 C30

C40 C35

The application of different strength graded timber in CLT products can lead to different strength properties (Fink 2018). Diversity between the CLT

manufactures exist and more CLT plants will cause CLT with various

performance. There are, for example, differences when it comes to the scale of the production line (Muszynski 2017; Brandner 2016) and the variety of CLT

products. In addition, the number of layers and orientation of the layers can differ between different manufacturers but also in the same plant (Figure 5). These parameters can lead to different CLT products in the market (Fink 2018).

Figure 5. Diversity in strength graded timber and layup that can lead to different CLT products (Adapted from Fink et al. 2018).

3.1.2.2 Performance of CLT

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adapted with glued laminated timber (GLT), also no own strength class exist for CLT (Fink 2018; Gustafsson 2019). For a CLT panel, the stiffness and bending strength are determined by the layers material properties, the thickness and strength of the lamellas, and the composition of the cross-section. The lamellas in the outer layer are often placed in the main direction of the load as this direction often take the highest forces (Figure 6) (Gustafsson 2019). Espinoza and

Buehlmann (2018) quoted that almost all stiffness strengths are determined by the outer-layers in a CLT panel.

Figure 6. Major and minor force direction in CLT panel.

CLT creates, thanks to its design, a system effect that works as the structural timber have together with higher average strength than the timber has separately. It is only a small risk that the weakest strength of the structural timber land

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The raw material properties in a CLT product influence bending strength more than the rolling shear strength (Fink 2018). The rolling shear is specifically influence by the width-to-thickness ratio and the sawing pattern. The rolling shear is more related to system properties than a material property (Ehrhart and

Brandner 2018). O´Ceallaigh et al. (2018) stated that a width-to-thickness ratio of < 4:1 in the C16 timber layer of Irish Sitka spruce (Picea sitchensis) had a greater characteristic rolling shear strength (0.88 MPa) compared with the one defined in EN 16351 (0.7 MPa). It means that, as mentioned, the number of layers in a CLT product affects the strength properties. There is a bigger risk for shear failures when lamellas are not edge-glued and have a width-to-thickness of < 4:1 (Table 5).

Table 5. Rolling shear strength (MPa) of CLT panel made of different strength classes.

Strength class Edge-glue and ≥ 4:1 No edge-glue and <4:1 Source

C16 - 0.88 _{O´Ceallaigh (2018)}

C24 1.1 0.7 Gustafsson (2019)

C30 / C14 1.1 0.7 Gustafsson (2019)

- 1.4 0.7 EN 16351 (2021)

Rolling shear tests in six different wood species was investigated by Ehrhart and Brandner (2018). For example, Norway spruce (Picea abies L. Karst.) had a mean value of rolling shear of 1.88 MPa and for two of the hardwood species such as European beech (Fagus sylvatica L.) and European ash (Fraxinus excelsior L.) had mean values of rolling shear 5.37 MPa, respectively 5.57 MPa. All

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Table 6. Tested wood species for CLT production by Ehrhart and Brandner 2018.

Wood species Width-to-thickness ratio Rolling shear strength (MPa)

N. spruce (Picea abies L. Karst.) 2 1.16

E. ash (Fraxinus excelsior L.) 4 5.57

E. beech (Fagus sylvatica L.) 4 5.37

E. birch (Betula pendula R.) 4 3.45

Poplar (Populus tremula L.) 4 2.88

Pine (Pinus sylvestris L.) 4 2.29

Besides rolling shear strength, another important mechanical property for CLT is the modulus of rupture (MOR), where in this study is referred as bending strength. Three types of bending forces in three different directions of a CLT panel can be seen in Figure 8. Bending strength in the CLT panel is affected by its thickness but also by the number of layers, as described by O´Ceallaigh et al. 2018. When the thickness of the panel increases the mean value of bending strength decreases. However, the mean value of bending strength was not affected by the number of layers, but the characteristic of bending strength that was affected by the number of layers. EN 16351 (2021) define that when the width-to-thickness ratio is at least 4:1, bending strength characteristic of 16 N/mm2 _{(1 N/mm}2 _{= 1 MPa) or}

higher is required for all types of CLT. EN 16351 (2021) also define compression strength perpendicular to the grain as 3 MPa when tested according to the

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Figure 8. Different bending stresses in a CLT panel (Gustafsson 2019).

Table 7. Strength classes for softwood with bending tests of single boards (EN 338:2016).

Strength class Bending strength (MPa) MOE (parallel) (MPa) _{(perpendicular) (MPa)}Compression strength

C14 14 7 000 2.0

C16 16 8 000 2.2

C24 24 11 000 2.5

C30 30 12 000 2.7

3.1.2.3 Standards and regulations

There is a difference between standard test specimens and the production

procedure of CLT. Some regulations for CLT like calibration of safety factor are necessary (Fink 2018). These safety factor are regulated in Europe by Eurocodes, which are structural design rules that are associated with load-bearing capacity, stability and durability. Eurocode 5, design for timber structurers, have some national adaptations, for example, regarding the countries climate zone, it is divided into different service classes (Gustafsson 2019). Eurocode 5 are not harmonised for CLT (Fink 2018) and instead, ETA work as an alternative for construction products that have not been harmonised. ETA document contains information on the intended use and performance of the CLT product. A general description of the product, essential characteristics, the methods and criteria for assessing the performance of the production relation to these essential

characteristics and principles for factory production control (FPC) are needed to be applied. An ETA cannot be issued if CLT is covered by a harmonised

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variability of CLT products will be rather large when the CLT production is

regulated by ETA.The product properties are specified and verified to ETA and

can therefore be CE marked as the European market requirement for construction products. This document provides information on CLT product performance assessment and contributes to the free movement of the European market

(European Commission n.d). To issuing ETA a European Assessment Document (EAD) 130005-00-0304 is used for CLT products (Figure 9). EAD can also be used to fulfil the requirements in Construction Product Regulation (CPR), which regulate CE marking. In both cases a Declaration of Performance (DoP) it needed. A DoP declare product performance and is unique for the product (European Commission n.d.)

Figure 9. Simplified process for certifying CLT products by ETA (European Organisation of Technical Assessment 2021).

European Standardisation Organisations (CEN/CENELEC) develop harmonised European standards. Harmonised standards are beneficial as they create a common technical language for; regulatory authorities in EU, manufacturers, design engineers and contractors. A harmonised standard define and verify requirements and demands, and declare product performances, which in turn reduces trade barriers and can also increase competitiveness in the construction sector (European Commission n.d). Product standard in Europe for CLT, EN 16351 (2021) are not harmonised with European standard. This standard contains requirements for CLT and is the first European standard that sets out the

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3.2 Interviews of CLT manufacturers

The results from interviews of CLT manufacturers was obtained from five manufacturers in Sweden and Central Europe to enhance the knowledge from a practical aspect. The results are listed divided into headings from each participant countries.

3.2.1 Austria

Company I

The main wood type for CLT production is spruce. Visible surface timber can also be pine, larch, stone pine, fir, oak and birch. The MUF is used as an adhesive with hardener. The manufacturer explained that due to the higher fire resistant, they use a special kind of MUF with formaldehyde blocker which is responsible for very little free formaldehyde. The company has a DoP that declares MUF according to EN 301 (2017) both for finger joints and surface bonding.

The produced CLTs are used for all type of application, especially for ceilings, walls and roofs. In special cases also as beams. The manufacturer refers to

companies ETA when answering to what extent different combinations of strength grades can be used in their CLT products. ETA states that the strength graded timber shall determine the strength classes according to EN 338 (2016). Strength graded timber used for CLT are divided into two strength classes:

- Class 1 = C24 (T14) homogeneous or for cover layer. Inner

layer:≤ 30 % C16 (T11) and ≥ 70 % C24.

- Class 2 = C40 (T26) for the outer layers and C24 for the inner layers.

As stated by the manufacturer, the most important property of CLT is MOE because of deflection and vibration. MOE is calculated according to the

companies ETA, and for Class 1 shall be 11 600 – 11 800 MPa depending on load direction (Table 8). All of these values are calculated according to the companies ETA with different design considerations.

Table 8. Values for MOE according to companies ETA.

Strength class MOE (parallel) (MPa) _{Out-of-plane load} _{In-plane load}

Class 1 11 800 √

Class 2 14 000 - 14 700＊ √

Class 1 11 600 √

Class 2 14 000 - 14 700＊ √

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Company II

Spruce, pine and fir are the main wood species used for CLT production. Also other wood species can be used depending on the request. Formaldehyde-free 1 C PUR adhesive are used for load-bearing timber components. CLTs made from the company are used for structural elements like walls, floors and roofs. Lamellas are

graded according to EN 338. Most common used strength grade of raw material: ≥

90% C24 (or T14) / ≤ 10% C16 (or T11). Other combinations are possible according to companies ETA (Table 9). Homogeneous C16 (or T11) can also be used. Combinations of the strength classes can only be in one product and are therefore not combined in the same product.

Important material properties of CLT are bending strength and stiffness (MOE), but also other properties are important. All of these values are calculated

according to the companies ETA with different design considerations (Table 10). Table 9. Combinations of strength classes according to companies ETA.

≥ 90% ≤ 10% Strength class C24 C16

C30 C24

Table 10. Bending strength and MOE according to companies ETA.

Strength class MOE (parallel) (MPa) _{Out-of-plane load} _{In-plane load}

C16 8 000 √ √

C24 12 000 √ √

C30 12 000 √ √

Strength class Bending strength (MPa) Out-of-plane load In-plane load

C16 1/ksys x 17.6 √ C24 1/ksys x 26.4 √ C30 1/ksys x 33.0 √ C16 16 √ C24 24 √ C30 30 √

3.2.2 France

Company I

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on the French market. C18 layers are introduced in CLT panels perpendicular to the load direction as it is rare to need C24 properties. This is a good opportunity to enchase this kind of wood. One important material property for CLT is MOE as this is mostly the property that design the CLT thickness.

3.2.3 Germany

Company I

Wood species used for CLT production is spruce, fir and just a little of pine. This depending on availability, costs and material properties. PUR 1 C is used, because of no formaldehyde emissions. The manufactured CLTs are used for walls, floors and roofs.

For now, only C24 are used for CLT production but in the future other strength classes will be used. It is possible to combine timbers with different strength grades, increase the yield of raw material as well as bending stiffness as a function of the density of the outer layers, i.e., the aim is to increase yield and stiffness. For the manufacturer, the outer layers will be better than C24 and the inner layers lower.

The producer stated that all material properties are important and CLT have to be produced according to companies ETA. Users erect buildings of CLT according to national building regulations in line with building permissions.

3.2.4 Sweden

Company I

Spruce are now the only used wood species in their CLT production. While, they plan to use pine in the future, also, as the company cuts these two type of wood species. They started with spruce as it is market standard. For CLT production, the PUR adhesive is used because of production technical reasons; simpler and more energy-efficient pressing.

CLTs are produced for residential and non-residential building often for

commercial use, in the case of panels for walls, ceilings and roofs. The company also made CLT for stairs, balconies and beams. Only C24 (according to EN 338) is used for CLT production. It is possible to combine different strength graded timber but then new product specifications must be created. At present, it is easiest to maintain one quality to reduce the proportion of different raw materials in production. All constructive properties are important for CLT products since it is material to a load-bearing product. For visual appearance, a surface quality is important if the panel are used for this area.

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4. Discussion

This study conducted a literature review and internet-based interviews to answer the research questions. Some diversity between the manufacturers were

highlighted in this study and the differences were in the used wood species and adhesive, although the differences were small. To answer the first research

question (RQ1), this study revealed that spruce is the most common wood species used for CLT production in Europe and with PUR adhesive. These findings are accordance with the previous reports by Fink et al. (2018) and Brandner et al. (2016). Multiple manufacturers mentioned reason for this adhesive in CLT production is due to the non-formaldehyde emissions. Also, other benefits stated by the manufacturers, like energy-efficient pressing and fast curing time of PUR adhesive. Another benefit for PUR adhesive is related to all service classes. One manufacturer use MUF Type I with a formaldehyde blocker that can also be used in all service classes. All interviewed manufacturers stated that softwood species is the dominant wood type for CLT production, like mentioned spruce, expect for the one uses hardwood species in outer layer as visual appearance. This study shows that a number of different wood species that commonly used today have the ability to be used for CLT production. Many of the mentioned wood species in this study are wood species that exist worldwide, not only in Europe. However, all mentioned softwood species and adhesive has given for the manufacturers are according to EN 16351 (2021).

Many CLT manufacturers have their own variations of CLT products, with different product name and layup for different applications. Differences were found between the interviewed manufacturers, but neither between countries nor among producers in the same country. However, the survey population in this study was small. The manufacturers which exists in multiple countries have often similar materials, processes, and products in all of their CLT plants. For example, Stora Enso, has no differences between adhesive type or strength graded timber regardless of the country that their production plant is located. There is only a small difference between the utilized wood species, according to the Declaration of Performance from three Stora Enso manufacturer sites.

A major difference found in this study was between the interviewed

manufacturers when it comes to the second research question (RQ2) related to the use of strength graded timber. This was mentioned previously by Fink et al. (2018). Although C24 is the most used strength graded timber for CLT

production, other strength graded timber are possible, both homogeneous and in combinations, also stated by Brandner et al. 2016. Lower strength classes timber than C24 for homogenous CLT panel is C16 according to this literature study and the interviewed manufacturers. Strength classes between C16 - C40 are used, either as homogeneous or in combinations. Interviewed manufacturer in France declares that their CLT panels of both homogeneous C24 and C24/C18 can be used for walls, floors and roofs elements. EN 16351 (2021) defined a

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at least be used for. If only the bending strength of 16 MPa are utilized a

homogeneous C16 can be used in all CLT types, which is showed in this thesis by literature review and interviews. Company II in Austria described that C16

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5. Conclusion

This study was aimed to increase the knowledge of CLT production and product. A literature review and internet-based semi-structured interviews were conducted to answers the research questions.

For CLT production mainly Norway spruce (Picea abies L. Karst.) and PUR adhesive are used in Europe. Other (local) wood species, as listed in this thesis, are also used or have the potential to be used for CLT production. Although more studies are needed in this regard, the published researches were suggested that hardwood species have the potential for CLT production in the future. However, for now, only one hardwood species is stated in product standard EN 16351 (2021) for CLT production, which is poplar. And this brings the need for constant updating the CLT product standards.

According to the mechanical properties of CLT, timber with lower strength grade can be also used, as the structure of CLT creates system effect and also as almost all stiffness properties is determined by the outer-layer, the layup can be extent greater.For CLT products loaded both out-of-plane and in-plane, there is a possibility to use strength graded timber from homogeneous CLT panel with C16 according to European standard and ETA of interviewed companies. Other combinations from C14 with higher strength graded timber in the load direction can also be used. This means combining different types of wood with different strength grades should be reasonable to a greater extent than what is used today. Only a few material properties are listed in EN 16351 (2021), as a factor for quality measurements of CLT such as bending strength, rolling shear strength, compression strength, and also bonding strength. This study showed that all important material properties that are listed in the standards are not followed by the manufacturers. However, bending properties, e.g., MOR and MOE, are the most referred quality control test from the CLT manufacturers. As indicated in EN 16351 (2021) and EN 338 (2016), and also confirmed by this study, a bending strength of at least 16 MPa and a MOE value of at least 8 000 MPa are required for CLT.

There are only a limited number of studies that investigated the CLT products directly from the industry. Further studies, with larger population group are needed to describe the industry more uniformly, especially since there is no existing harmonised standard. There is a need to update the standards

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Wang, B.J., Wei, P., Gao, Z., Dai, C. (2018). The evaluation of panel bond quality

and durability of hem-fir cross-laminated timber (CLT). European Journal of

Wood and Wood Products 76:833-841.

Wilded, D. (2018). KL-trä har äntligen blivit ett begrepp. Swedish Wood. Newspaper; Trä number 4:2018.

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7. Appendix

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Appendix I

Q

UESTIONS

Þ What type of wood species do you use for CLT production and why this

wood species?

Þ What type of adhesive do you use for CLT production and why this type

of adhesive?

Þ For what type of application will your CLT be used for?

Þ What strength grade timber do you use for CLT production?

Þ Do you use different strength grade timbers for different CLT type?

Þ It is possible to combine timbers with different strength grades in your CLT, and if so, to what extent?

Þ What material properties of CLT are important, both for youas a

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Industrial Requirements for Cross-Laminated Timber Manufacturing