French beech – a new opportunity in wood housing
LANVIN Jean-Denis*, REULING Didier, LEGRAND Guillaume
FCBA
Allée de Boutaut BP 227 Bordeaux, 33028, FRANCE
ABSTRACT
Hardwood housing especially from beech is one item of possible added value products based on solid wood and / or wood reconstituted in Europe. However, design of structures requires structural product with a certified strength according to regulatory framework of CE marking. French beech forest occupies about 1.4 Mha lead mainly in regular high forests and coppices with standards with an annual volume of harvested wood (1 million m3) for a sawn timber estimated at 400000 m3. FCBA has launched since in 2011 a study to characterize the
French beech as a raw material and its structural bonding. To qualify French beech species as solid wood, a national representative sampling was performed to collect 2400 lumbers (6 French areas, 21 stands, 99 trees, 3 cross-sections) and to establish visual grading rule (NF B 52 001-1 2017) for assessment in D40-D24 or D35-D18. A new table of strength classes of European hardwood (EN 338) will be proposed according to experimental results. Grading machine could be done for D50 with a good yield (38-47%). A general work plan has been developed to reach requirements for beech glulam in two steps. Firstly, the compatibility of different type I adhesives technology was studied by the way of lab tests usually performed in the field of common adhesives approval (EN 302-2). On the other hand, full scale tests according to EN 14080 have been done to characterize glulam beams (58 beams - GL32h) with verification of finger joint performance. French beech data could be merged with other hardwood data to develop a GLT CE marking standard as EN 14080 part 2 specifically devoted to hardwoods. The construction market in the future could provide an important alternative for French beech, particularly in the form of reconstituted products.
1.INTRODUCTION
For twenty years, production of hardwood sawn timber fell sharply in France. Today, industrials are rediscovering the immense potential of these species in the construction, especially in structure. Traditionally used for carpentry, furniture, stud, packaging, French hardwoods are used to new techniques such as finger jointing, panelling or by high-temperature treatment, enabling them to meet requirements and open to new markets.
In France, French Beech species occupies about 1.4 Mha [MEMENTO FCBA 2018] conducted mainly in regular high forests and coppices with standards for a volume of harvested wood estimated at 1 million m3 for 0.4 Mm3 in sawn timber. French beech occupies the second forest surface after oaks and some softwood like spruce and pine.
The essential characteristics of the beech are its hardness and facility of impregnation. It's easy to prevent stain if drying done as quickly as possible after sawing and this species lends itself well to gluing and is easy to transform (bending). Its qualities have made it a popular wood for the interior design and furniture, but these sectors have seriously declined over the past two decades.
The construction market could provide an important alternative for beech, particularly in the form of reconstituted products. Some recent initiatives have also demonstrated the importance of this species in building in France as the example of school of Tendon (88) using widely beech in structural part of the building. Other examples of beech species in structure make up a “showroom” for architects (Tisserand 2016).
Whatever technical solution used, development of structural beech requires lumber (solid or laminated wood ) with a certified strength grading required in the regulatory framework of CE marking. Therefore, FCBA has launched in
2011 a study to characterize the French beech and its structural bonding in the perspective of CE marking into the following phases:
• Identify the mechanical behaviour of French beech lumber as solid wood, • Validate the structural bonding ability of beech through laboratory tests,
• Conduct a mechanical characterization and classification of beech GluLam Timber (GLT).
2.BEECH SOLID WOOD CHARACTERIZATION
The knowledge of the capability of hardwood in structure has been based for two decades on the realization of experimental campaigns respectively for oak (Lanvin & Al 2007) and chestnut (Lanvin & Al 2015) on full size specimens coming from forest as opposed to a direct sampling in sawmill what was commonly practiced in Europe. This approach allowed:
• To know the mechanical properties of solid woods, as well as the influence of defects,
• To establish relationships between mechanical properties and silviculture (France is the only European country to perform this kind of sampling),
• To propose optimized classification rules by visual and/or machine grading.
2.1.METHODOLOGY AND DESCRIPTION OF TREES SAMPLING
Based on current volume provided by French Ministry, eight areas were selected to provide a representative sample of trees. The sample selected must indeed have at least 65% of the country's diversity to ensure the representativeness of national resource. Four trees are chosen by stand around mean DbH of the stand. An additional tree was selected in case of log splits during felling. The table 1 summarizes the sampling made by areas and the total number of collected trees. All the harvesting was made over the year 2013 and 2014. The whole logs were regrouped in log yard of sawmiller’s partners for analysis (EN 1316-1) by FCBA before sawing.
Table 1: description of trees by sub-samples
Area sub sample Stand number Number of Trees Mean height (m) of tree and CoV Mean Age of tree and CoV Mean DbH (cm) of tree and CoV Alsace 1 9 ;18 ;19 ;20 20 27,4 - 5% 95,2 - 12% 61,7 - 18% Bourgogne 2 7 ;8 10 18,8 - 5% 126,4 - 13% 61,6 - 8% Champagne-Ardenne 1 ;2 7 NA 112 - 17% 70,5 - 14% Franche-Comté 3 15 ;16 ;17 15 30,6 - 8% 110,2 - 21% 65,6 - 17% Lorraine 4 3 ;4 ;5 ;6 20 34,1 - 15% 132,8 - 7% 56,3 - 17% Midi-Pyrénées 5 11 5 36,2 - 4% 124,9 - 20% 53,3 - 8% Normandie 6 12,13,14 12 36,8 - 11% 114,2 - 20% 65,1 - 13% Picardie 10 ;21 10 32,5 - 10% 95,2 - 12% 62,4 - 13% Total 21 99 31,9 - 20% 126,4 - 13% 61,1 - 17%
2.2.DESCRIPTION OF BENDING TESTS
Logs were sawed in 6 sawmills following the geographical localization of the sampled stands. Logs were transformed into sawing in three different cross section (48*112 ; 58*162 ; 78*220 mm²) and were dried in 14 % +/-2 % in sawmill. The difference between 2400 collected boards and the 1872 mechanical results correspond to the boards moved away because they break out of the centre part of the bending tests (122 pieces) or lake of traceability, broken piece, too distorted during drying which represent 406 pieces. On each lumber, defect measurements have been made as following to establish the visual grading criteria tables:
• Measurement of the projection of maximum knots on edge and face of lumber, • Measurement of the other singularities (slope of grain, decay, wane, distortions), • Annual growth rings.
All pieces were tested in bending test according to EN 408. The tests of modulus of elasticity were done according to clause number 10 in EN 408 and be calculated from the EN 384 equation, which includes an adjustment to a pure bending modulus of elasticity. The bending strength values were adjusted to a width of 150 mm (kh-factor) and the modulus of elasticity to moisture content (U) of 12%. The following Table summarizes the mean value and the coefficient of variation of bending strength (fm) modulus of elasticity in bending test (Em) and density (ρ12) for each sub-sample.
Table 3: Mechanical properties of French beech per sub-samples.
Sub
sample N
Moisture content fm Em ρ12
Mean
(%) COV (%) (MPa) Mean COV (%) (GPa) Mean COV (%) (kg/m³) Mean COV (%)
1 372 10,8 11,2 79,6 32,1 14,7 15,8 723 4,9 2 275 11,6 13,5 70,0 30,6 11,7 20,1 681 6,2 3 251 9,4 17,4 69,2 37,0 12,2 24,8 711 6,1 4 524 10,3 10,6 80,2 27,2 14,5 16,1 699 5,3 5 123 10,9 13,7 70,6 30,9 14,0 15,9 715 8,3 6 327 9,0 16,7 80,2 30,9 16,1 16,9 698 5,9 Mean value 1872 10,3 15,7 76,5 31,5 14,1 20,6 704 6,1
2.3.FRENCH BEECH STRENGTH GRADING
To be used for housing market, beech lumber must be CE marked (EN 14081-1), and thus a mechanical assessment of lumber must be done by sorting through visual method (see NF B 52-001) or machine using non-destructive methods (EN 14081 (parts 2 to 4) Whatever method, a representative sample of the resource lumber will be ranked from the qualification of its intrinsic defects in several categories called mechanical classes defined in EN 338. Analysis has shown that the measurement of the defect of beech allowed a classification of wood in two mechanical ranges classes with visual method as following:
• Solid wood in carpentry and stud o Combination 1 : D40 & D24 or
• Solid wood for GLT
o Combination 0 : D45
French mechanical strength classes reach German ones (DIN 4074-5). The following figure shows yields obtained by strength grading methods
Figure 2: percentage of French beech graded lumbers per methods for EN 338 strength classes.
3.VALIDATION OF STRUCTURAL BONDING OF FRENCH BEECH
The feasibility of beech GLT structural beams relies on studying the potential of adhesive systems to fulfil the requirements of the conventional approvals for timber structures. The results of this study could be transferred, in the end, to the production of GLT beams. The compatibility of different adhesives technologies was studied by way of lab tests usually performed in the field of usual adhesives approval (EN 301, EN 15425, EN 16254) for load bearing timber structures (according to EN 302-2) on Melamine Urea Formaldehyde (MUF), Polyurethane (PUR), Resorcinol Phenol Formaldehyde (RPF) and Emulsion Polymerized Isocyanates (EPI).
Delamination requirements according to EN301 (2013) must be 5% at the maximum. The tests configurations can take into account the wood “reality” and can be performed with the lamination and glulam cross-sections and sawing patterns of the laminations conforming to the maximum sizes and the lamination sawing patterns used in the respective glued product.
PRF and MUF adhesives systems fulfil the EN301 requirements regarding resistance to delamination (Konnerth & Al 2016). It is important to emphasize that these conclusions are valid only for the adhesives tested and are not necessarily indicative of the performance of similar products. As the industrial transfer (premix or separate application, pressing time, mixing ratio) should be more convenient with the MUF adhesive system, regarding working properties, it was decided with the adhesives producers to scale up the study on MUF adhesive solutions, especially since most of French GLT plants use MUF adhesives.
Figure 3: Delamination of beech EN 302-2 GLT for 4 kind of type I glues (cross section 24*150 mm²).
4.CHARACTERISATION OF BEECH GLT
4.1.PROTOTYPE GLT BEAMS MANUFACTURING STEP
The third step was to study the production of a large volume of two different cross sections of beams in one French plant usually producing structural glulam beams made from Spruce. Production settings should consider reduced lamellae thicknesses as compared to usual sawing from sawmills (20 mm thick raw lamellae) from D35 wood quality and one single lamellae width (115 mm).
The challenge for this study is to use length of lamellae between 500 to 960 mm in order to reduce price of beech GLT. Initial price has been found to 1500 €/m3 (Ressel 2005), the idea is to reduce by two final prices with small length solid wood. By the way, only one of French glulam manufacturer was able to produce GLT with smaller length of lamellae but the length of finger jointing is smallest too.
This production step was divided in 2 main parts as:
One production step of beams during 2 working days, focusing on lamellae finger jointing and face gluing to lead to 2 different cross sections of beams. Have been produced in 6 m length pieces in 2016 :
o 36 final section beams of 100 x 100 mm² in 5 lamellae o 22 final section beams of 100 x 260 mm² in 13 lamellae.
One first small production of finger jointed (20 x 105 mm) samples made for defining production parameters and mechanical properties in flat bending.
This step was also finely studied according to the requirements of EN 385 and EN 386 (replaced in 2013 by EN 14080) and validated the production process applied for beech finger jointing and faces gluing for structural glulam application as following.
Good feedback in general with the integration of the beech wood in the full production chain, by limitation of defects suppression only at the board ends (minimum distance from the base of the finger joint and a knot = 3 times diameter of knot). Due to the beech properties, it would have been better to reduce the profile machining head speed for the 10 x 3.8 x 0.6 mm profile to avoid damage on the finger joint and optimize final strength. Measures performed on each lamellae to control the wood planning quality, fulfil EN 386 standard
requirements.
Pressure applied to the beam assembly has been increased for the use of beech to the maximal level allowed by the device. The probability to get thicker joints exists and could induce some local delamination during further tests on the beams.
4.2.TENSILE AND BENDING TESTS OF FINGER JOINTS ACCORDING TO EN408
Due to the good performance of solid wood, the bending test quality lamella Finger jointing remains a key element of the overall mechanical performance of GLT beams tested by way of 4-point bending tests, since the failure occurs most often at the last lamella when the finger joints are in the central third highly stressed in tension.
Finger-jointed samples were tested in 4 point bending and tensile tests at initial state. The finger jointing profile used (finger length = 10 x pitch = 3.8 mm * tip width 0.6 mm) was chosen according to standardized profiles (EN 14080) and still demonstrates a very satisfactory level of performance. The final characteristic value (EN 14358) reaches 41.4 MPa, higher than the requirement for graded wood as shown below rather than for tensile results probably due to smallest length of finger joints.
Table 3: Mechanical performance on beech finger joints samples (cross section = 20*105 mm²)
Bending Tensile
Mean fm,j or ft,j 64.5 MPa 34.0 MPa
CV% 12% 20%
fm,j,k or ft,j,k (MPa) 51.5 MPa 22.9 MPa
Number of specimens 80 27
Requirements EN 14080 41.4 MPa 25 MPa (fm,k (D35)/1.4)
4.3.CHARACTERISATION OF BEECH GLT
The last step was to characterize all beams produced, regarding bonding quality of gluing and overall strength of the beams. The quality of the bonded beams, proven by means of bending tests on finger jointed lamellae and delamination of glue lines, should compare to general requirements of the EN 14080 standard dedicated to softwood species and poplar.
Four point bending tests were performed in accordance with EN 408. Loads are applied perpendicular to the glue line. The following table presents results for nominal and adjusted dimension. If the overall height or depth of the glued laminated timber is less than 600 mm the 4 points bending strength parallel to the grain fm,g,k, determined by testing, shall be multiplied by kh. As the lamination thickness is less than 40 mm, the bending strength may be divided by k as given in formula. 05 . 1 1 . 0 40 min = k t and 9 . 0 1 . 0 600 max = h k h (1)
Compression test have been performed on prototype beams according to the methodology given by EN 408 to estimate compressive strength as shown in the following figure.
Table 4: Bending Mechanical behaviour for French beech GLT (MC%=12%)
Nominal Cross section 100*100 mm 100*260 mm² All sampling
Mean fm,g (MPA) 47.1 38.5 43.9 CV % 15.4% 16.1% 18.8% fm,g,k (MPa) 37.5 31.5 31.8 E0,g,mean 14230 15480 14700 CV % 6.2% 2.8% 5.4% E0,g,05 12300 13400 12150 ρ0.05 670 650 655 Nb of specimen 36 22 58
Table 5: Compression performance on beech GLT (cross section = 100*100 mm²) * softwood
Compression strength
parallel to grain perpendicular to grain Compression strength
Mean fc,0,g or fc,90,gt 55.8 MPa 10.0 MPa
CV% 7% 13%
fc,0,g,k or fc,90,g,k (MPa) 49.7 MPa 8.4 MPa
Requirements EN 14080* GL32c 32 2.5
Number of specimens 35 35
5.NEW TABLE OF STRENGTH CLASSES OF EUROPEAN HARDWOOD (EN338)
More and more, industrial manufacturers of glulam beams classify their woods in tension (Ehrhart & Al 2018) according to methodology given by EN 14080 for softwood. At equal visual quality, we get better yield in tensile due to coefficients between the tensile values from the classes of flexural strengths with EN 384 formulas. For instance, tensile grade T21 gives characteristic bending strength equal to 29 MPa, grade D35 allow a tensile strength equal to 21 MPa.
A new French beech sample benchmarked with MTG device has been created from threes visual grades in 2019 and performed into flexion and tensile tests. The statistical analysis of curves can convert bending data to obtain tensile data using a coefficient as :
ft,0 = 0,85 * fm,0 (2)
Et,0 = 0,85 * Em,0 (3)
The following table summarized results and corresponding strength classes between bending and tensile but additional data could be merged to improve regression.
Table 6: Mechanical strength classes for French beech (* for softwood; + for hardwood)
French Beech visual strength classes Bending Tensile Test results. EN 338 graded Tensile strength class from EN 384* 0.6 fm,k Test results. EN 338 graded Bending strength class from EN 384* 3,66+1,213ft,0,k New EN 338 strength classes+ ft,k/0.85 LHO -D4 5 MOR0.05 MPa 68.7 47.9 MOE0.5 GPa 17.5 D50 TD27 15.1 TD45 D58 TD45-D50 ρ0.05 kg/m3 620 625 NB 54 54 H2 -D3 5 MOR0.05 MPa 35.0 27 MOE0.5 GPa 16.8 D35 TD21 15.7 T27 D36 TD27-D32 ρ0.05 kg/m3 660 645 NB 52 52 H4 -D1 8 MOR0.05 MPa 31.5 21.4 MOE0.5 GPa 16.3 D18 TD11 12.7 T21 D29 TD21-D24 ρ0.05 kg/m3 690 645 NB 17 17
6.DISCUSSIONS AND CONCLUSIONS
French Beech forest occupies about 1.4 Mha lead mainly in regular high forests and coppices with standards with an annual volume of harvested wood (1 million m3) for a sawn timber estimated at 400000 m3. The goal of French industry is to extend the knowledge of mechanical properties of French beech (already initiated in Germany), thus promoting its use in construction applications (sawn timbers, glulam beams).
A national representative sampling protocol was created in order to reach 1872 lumbers (6 French areas, 21 stands, 93 trees, 3 cross-sections) which were being bent until failure. French beech sawn timber could be graded 60% in D40 (95% in optimal) according to new visual rules published in NF B 52-001 (2017). However, strength grading machines as XYLOCLASS, MTG or ViSCAN allow also classification in D50 (37 - 48%).
A general work plan has been developed to reach requirements for hardwood glulam. On one hand, the compatibility of different type I adhesive technologies was studied by the way of lab tests usually performed in the field of structural adhesives approval according to EN 302-2. On the other hand, full scale tests have been performed to characterize glulam beams (more than 50 beams – 2 cross sections) in GL32h according to EN 14080 requirements on glue bonding (bending tests of finger-joints, shear tests of glue lines and autoclave delamination tests) and strength (bending and compressive). Weakest point is due to smaller length for finger joints due to short length of lamellae between 500 to 960 mm in order to reduce price of beech. It is to be noted that compression strengths are very important, this characteristic is fundamental for high-rise timber construction.
French beech data will be merged with other hardwood studies in order to create a new GLT CE making standardisation devoted to hardwoods (EN 14080-2) and to upgrade EN 338 with specific thresholds.
ACKNOWLEDGEMENTS
The authors are pleased to thank on one hand Ministry of Agriculture (MINAGRI), France Bois Forêt for financial support and ONF for trees as well their come from public forest, on the other hand, Manubois, Avivés de l’Est , Scierie
et Caisserie de Steinbourg, Akzo Nobel Industrial Finishes (Antony, France), Simonin respectively for collecting the sawn timber, glue and plant for GLT part of the study.
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