investigation of adhesive properties

LAMINATED VENEER LUMBER POLES FOR USE IN TEMPORARY SOIL NAILING- INVESTIGATION OF ADHESIVE PROPERTIES

HIRSCHMÜLLER SEBASTIAN M.Eng. Forschung und Entwicklung Fachhochschule Rosenheim [email protected]

PRAVIDA JOHANN Prof. Dr.-Ing. Fakultät Holztechnik und Bau Fachhochschule Rosenheim [email protected]

Sebastian Hirschmüller, geboren 1980, studierte Holzbau sowie Holzechnik an der Fachhochschule Rosenheim. Er ist seit 2012 in der Abteilung Forschung und Entwicklung tätig und befasst sich dort allgemein mit der Holzart Buche und im Speziellen mit der Entwicklung von Furnierschichtholzrohren zur Verwendung in der Geotechnik.

MARTE ROMAN Univ.-Prof. DI Dr.techn. Inst. f. Bodenmechanik und Grundbau Technische Universität Graz [email protected]

Roman Marte, geb. 1966, studierte Bauingenieurwesen an der TU Graz und promovierte dort am Institut für Bodenmechanik und Grundbau. Nach 12 Jahren Tätigkeit als Mitarbeiter und später Partner und Geschäftsführer im geotechnischen Büro GDP ZT GmbH erfolgte 2012 die Berufung als Universtitätsprofessor für Bodenmechanik und Grundbau der TU Graz.

studierte Johann Pravida, geb. 1968, Bauingenieurwesen an der TU München und promovierte dort im Bereich der Baustatik. Neben seiner Tätigkeit als Tragwerksplaner und Prüfingenieur für Standsicherheit ist er seit 2003 Professor für Statik und Festigkeitslehre an der FH Rosenheim.

FLACH MICHAEL Univ.-Prof. DDI Inst. f. Konstruktion und Materialwissenschaften Universität Innsbruck [email protected]

Michael Flach, geb. 1954 in München studierte Bauingenieurwesen an der TU München mit Vertiefung Holzbau (Abschluß 1978), sowie am Centre des Hautes Etudes de la Construction in Paris/F mit dem Schwerpunkt Stahl- und Spannbeton (Abschluß 1979). Nach einer 25 jährigen Tätigkeit als Tragwerksplaner in Frankreich folgte er 2002 dem Ruf zum Universitätsprofessor an das Institut für Konstruktion und Materialwissenschaften der Universität Innsbruck und gründete den Lehrstuhl für Holzbau.

Abstract Within a current research-project the possibility of using laminated veneer lumber beech wood poles as temporary soil anchors in foundation engineering is investigated. Within this project adhesive properties of laminated veneer lumber beech wood poles are determined. For use in temporary ground applications the influence of veneer bending, veneer thickness and intensive cement contact is concerned. As standard version two different adhesive systemsmelamine formaldehyde resin (MUF) and a 1-component polyurethane adhesive (PUR) were investigated and compared. Melamine formaldehyde resin glued samples with 3 mm thick veneers and 1.0�N/mm² press force show a higher tensile shear strength but also a higher adhesive failure percentage than polyurethane glued samples. In case of melamine formaldehyde resins the closed time has a great influence on bond line quality. A long closed time reveals a low penetration of adhesive in the cells and a low embedding on the wood surface. For curved structures best results were achieved with a bonding pressure of 1.0 N/mm². A significant difference in strength between the concerned MUF and PUR adhesive system was not recognized. 1.1

General aspect

Currently in foundation engineering a common way of slope stabilisation is the use of steel anchors drilled into the soil in combination with a reinforced shotcrete layer for surface protection- the so called “soil nailing system”. The connection between soil and anchor (steel rod) is ensured by filling the gap between soil and anchor with cement grout. The grout transfers shear forces between anchor and soil, similar to the composite behaviour of reinforced concrete. Temporary soil nailing typically is used for the stabilisation of excavation pit slopes and walls. Therefore the stability of the excavation slopes has to be ensured for a short period of time with a maximum of two years. After filling the working space between building and surrounding excavation surface the lateral earth pressure normally is taken by the building itself. Depending on soil properties, the supporting height of the excavation or slope, surface inclination and load on the upper side the horizontal and vertical distance of the anchor rods varies in general between about 1.50 m and 2.00 m with most typical anchor length between 4 to 10 m and an anchor rod diameter of about 30 mm to 60 mm. After filling the working space between building and excavation surface the anchors have no structural utility but remain in the ground as it is generally not possible to remove them. With the actual research project the possibility of substituting the temporary steel anchors with LVL-poles manufactured from beechwood is investigated. The reason behind this idea is to find a sustainable way of soil nailing for temporary purposes in combination with an new way of using beechwood. Positive and negative material properties of beech are high strength and low durability. The poles should have a service life of maximum two years and should rod nearly residue-free in the soil. Depending on the boring machine, the boring procedere and the bore pits normally bore holes with diameter of about 60 to 140mm (or more if necessary) are drilled in the soil. Under consideration of a necessary thickness of the sourounding cement grout the outer diamter of the poles is limited to 100mm. The wall thickness of the poles is given by the occouring tensile force in the anchor as well as the maximum possible creeping of venners in the inner diameter. For all investigations a wall thickness of aproximatelly 20mm is chosen.

(a) Standard soil nailing system

(b) Slope stabilization with bamboo rods

Source: Hinteregger Foundation Engineering

Source: Hinteregger Foundation Engineering

Figure 1: Actual Soil nailing system (a) and intended temporary method (b) 1.2

Wood bonding of curved structures in temporary use

1.2.1 Standardized requirements on bond line behaviour Laminated Veneer Lumber (LVL) has its origin in aircraft constructions, whereby in Germany one of the first investigations of laminated veneers at the research institute for aviation in the early 1930´s were done [1]. Offset mounting joints in combination with a strong lamination lead to a high modification with lower scattering of mechanical characteristics and a better form stability [2]. Glued joints for load bearing elements -especially for laminated veneers- have to fulfil several technical standards which should ensure reliability of the bond line during service life of the product. In some cases those requirements are ruled in specific technical approvals [3] or in European standards. Requirements for the bonding performance of LVL in load bearing elements are given in [4], for LVL in general in [5]. Both standards show a different method to determine the glue line performance, once in a clearly defined shear test [5] and once with puncturing method [4]. The pre-treatment of the testing samples is almost the same. Test samples are kept 4 h in boiling water, dried for 16 h, kept another 4 h in boiling water and finally conditioned for 2 hours in water at room temperature (cycling boiling). Afterwards, depending on the standard, puncturing or shear tests are done. The requirements concern two characteristics- strength and wood failure percentage (WFP) depending on the way of pretreatment. Table 1 gives a small overview of the existing testing methods for LVL bond lines. Nevertheless in all standards bond line requirements for curved structures are missing. Directive Z 9.1100 EN 14374 EN 14279

Bond line strength ---

Pre-treatment Cycling boiling

---

Wood failure Puncturing - min. 70 % WFP acc. to [6] Puncturing - min. 70 % WFP

Shear test acc. to EN 314-1 [7]

WFP depending on shear strength acc. to EN 314-2

Cycling boiling depending on service class acc. to EN 314-1

Cycling boiling

Table 1: Overview of bond line testing methods for LVL

1.2.2 Bending and compression moulding of layered structures The furniture industry early recognized the possibility of producing formed structures by using laminated veneers. Detailed information about requirements on the raw material, minimum bending radius in - and rectangular to fibre direction, influence of veneer moisture and type of wood on minimum bending radius as well as maximum veneer thickness and manufacturing parameters are given in [1]. Based on investigations of the U.S. Forest Products Research Laboratory for 3.2 mm thick and in water plasticized beech wood veneers a minimal bending radius 90° to fibre direction of 30 mm is possible. To avoid wrong adhesions a constant veneer thickness is important. The manufacturing of the layered structures by pressing with one hard and as other part with an elastic form is to prefer the pressing process with two hard forms (male and female system). The male-female pressing system with two hard parts is very sensitive on veneer thickness variations but as biggest disadvantage it is not possible to get pressing forces on the vertical line tangential on the half pole. In Figure 2a the gap between press plunger and veneer in the vertical line is obvious. To manufacture the half poles within the actual research project a system with an outer hard form and inner elastic form is used (Figure 2b).

(a) male-female pressing system

(b) Pressing system with elastic fire hose

Figure 2: Production systems for half-poles The outer hard form is made of cast resin, the inner elastic form consists a 75 mm diameter fire hose with caps on both ends and connectors for compressed air. The fire hose can be filled with hot water and the outer cast resin form can be warmed before gluing. So it is possible to glue with higher temperatures, at least 30 °C. Before gluing the veneers are preformed by putting them for 4 hours into tempered water and re-drying them in normal climate within a drainage pipe with 100 mm diameter. Without wet preforming it is not possible to form 3 mm thick veneers in a radius of 30 mm rectangular to fibre direction.

2.

Materials and Methods

2.1

Materials

2.1.1 Wood All investigations were made with 2 mm and 3 mm thick veneers. About 1 m³ 2 mm and 1 m³ 3 mm veneer sheets of European beech wood (Fagus sylvatica L.) in dimension 75 cm x 105

cm quality class C acc. to [8] was delieverd from `K+W Formholztechnik GmbH` in Plüderhausen-Germany. Wood origin, log pre-treatement (boiling or steaming) and peeling modalities are unknwon. About 0.5 m³ of 3 mm thick veneer sheets of dimensions 80 cm x 140 cm were provided free of charge from `Sperrholzwerk Schweitzer GmbH`in St. Marienkirchen / Polsenz (Austria). The domestic logged beech wood was steamed before peeling. 2.1.2 Adhesive For bonding two common adhesives- a one component polyurethane system ( PURBOND HBS 309 with Primer PR 3105) and a melamin formaldehyde resin (BASF Kauramin 683 with hardener 688) were used. In order to determine microscopical characteristics of the different bondlines (bond line thickness and cell penetration) it was necessesary to add fluorescent pigments to the adhesive system. In case of the polyurethane system it was applied by the manufacturer, for melamin formaldehyde resin 0.3 % natrium fluorescin (C20H10Na2O5 , 376.3 g/mol) has been added. An influence of natrium fluorescin on adhesive properties of MUF could be excluded by several pre-tests based on [9]. 2.2

Sample manufacturing and test procedure

2.2.1 Sample manufacturing All veneers were wet pre-bended and afterwards stored in normal climate at 20 °C and 65 % relative Humidity (RH) until equilibrium moisture content was reached. Before manufactoring the boards were randomly mixed to avoid an influence of wood inhomogenitiy. Raw density was taken of each relevant sheet (Table 3). Consequently after pressing all batches were stored again in normal climate for 7 days to enable a adhesive hardening. For each adhesive system and each veneer thickness flat boards and curved half poles were produced and afterwards cutted in tensile shear test samples according to DIN EN 302-1. An overview about the different bonding parameters is given in Table 2. Bonding parameters MUF main test flat boards Parameter Value Comment Adhesive System MUF 683+688 Türmer open time max 10 min closed time 25 min 3 min Serie E_F_MUF Pressure 0,8 N/mm² Room temperature 20° C Relative Humidity 50% Pressing machine Bürkle Veneer Beech peeled Veneer thickness 4x3 mm 6x2 mm Veneer moisture 20°/65% Pressing time 105 min Pressing temp. mind. 30°C Hardening time 7 Tage Amount of glue 380 g/m² by weighing Application spatula Resin part 100 Hardener part 100 Treatment ­­­

Bonding parameters PUR main test flat boards Parameter Value Comment Adhesive System PUR HBS 309 Purbond open time max 10 min closed time ­ Pressure 0,8 N/mm² Room temperature 20° C Relative Humidity 50% Pressing machine Bürkle Veneer Beech peeled Veneer thickness 4x3 mm 6x2 mm Veneer moisture 20°/65% Pressing time 150 min Pressing temp. Room temp. Hardening time 7 Tage Amount of glue 180 g/m² by weighing Application spatula Primer 10 % solution PR 3105 Purbond Amount of primer 30 g/m² Time of flash off 25 min

Bonding parameters MUF main test curved poles Parameter Value Comment Adhesive System MUF 683+688 Türmer open time max 10 min closed time 3­5 min Pressure 0,8 N/mm² 1,0 N/mm² Serie G MUF Room temperature 23 °C Relative Humidity 60% Pressing machine Bürkle Veneer Beech peeled Veneer thickness 4x3 mm 6x2 mm Veneer moisture 20°/65% Pressing time 105 min Pressing temp. mind. 30°C Hardening time 7 Tage Amount of glue 380 g/m² by weighing Application spatula Resin part 100 Hardener part 100 Treatment ­­­

Bonding parameters PUR main test curved poles Parameter Value Comment Adhesive System PUR HBS 309 Purbond open time max 10 min closed time ­ Pressure 0,8 N/mm² 0,95 N/mm² Seri e H PUR Room temperature 23 °C Relative Humidity 60% Pressing machine Bürkle Veneer Beech peeled Veneer thickness 4x3 mm 6x2 mm Veneer moisture 20°/65% Pressing time 150 min Pressing temp. Room temp. Hardening time 7 Tage Amount of glue 180 g/m² by weighing Application spatula Primer 10 % solution PR 3105 Purbond Amount of primer 30 g/m² Time of flash off 25 min

Table 2: Pressing parameters of batches Boards with dimension of 40 cm x 70 cm were pressed and tensile shear samples were formed according to DIN EN 302-1. In case of curved samples half poles with a length of 70 cm were manufactured and afterwards cut into stripes. The upper and under cut followed the radius of bond line (Figure 3). In general all cuts were performed wit flat tooth to avoid local notch stress. In case of curved veneers the cut was manufactured with a grooving cutter and a variable thrust ring. Otherwise it was not possible to mill the groove on the inside of the pole. The possible depth precision was about 0.5 mm.

(a) side view shear cuts 6 x 2 mm curved

(b) cross section 6 x 2 mm curved

Figure 3: Tensile shear sample 6 x 2 mm curved structure For reference also samples for testing wood properties were produced. Therefore one more veneer layer was applied and upper and under cut was led into the middle of the mid veneer. Table 3 gives an overview about the manufactured samples.

Batch

[-]

Layers

[-]

Quantity

[-] Flat boards 64 49 64 49 49

Adhesive

[-]

A C B D AB

5 x 3 mm 7 x 2 mm 4 x 3 mm 6 x 2 mm 6 x 2 mm

Shear in veneer Shear in veneer PUR PUR MUF

CD

4 x 3 mm

44

MUF

E

4 x 3 mm

31

MUF

F

6 x 2 mm

26

MUF

AB AC

5 x 3 mm 6 x 2 mm

E

4 x 3 mm

12

MUF

G

4 x 3 mm

15

MUF

CE

4 x 3 mm

24

PUR

DF

6 x 2 mm

18

PUR

H

4 x 3 mm

10

PUR

Curved half poles 30 Shear in veneer 16 MUF

Comment

[-] Planed 12 mm Planed 12 mm

Closed time 2025 min Closed time 2025 min Closed time 3-5 min Closed time 3-5 min Planed 12 mm Pressure 0,8 N/mm² Pressure 0,8 N/mm² Pressure 1,0 N/mm² Pressure 0,8 N/mm² Pressure 0,8 N/mm² Pressure 1,0 N/mm²

WoodDensity (Mean) [kg/m³] 673 664 686 662 644 655 647 691

628 684 605 716 660 694 721

Table 3: Overview of batches

2.2.2 Cyclic hot cement water treatment DIN EN 14374 (2005) requires for bonding tests a cyclic pre-treatment with following puncturing of the samples. Therefore test pieces have to be boiled in water for 4 hours, afterwards dried in an oven at about 60 °C for 16 hours, boiled in water for 4 hours again and cooled in water at room temperature. Afterwards the samples have to be punctured and wood failure must be determined. For the intended use the influence of a high alkaline cement milieu has to be included in the investigations. Three cement pastes with cement CEM II 42.5 N and different water-cement ratios were mixed and pH-value was measured by a potentiometric method (Voltcraft PH-100ATC). A high viscose cement paste with a watercement ratio (w/c) of 2.0 delivers a satisfying pH-value of 12.46 (Table 4). Therefore for all further investigations sample pre-treatment was done in cement water with w/c = 2.0. Water-cement ratio

pH-value

0.5

12.80

1.0

12.52

2.0

12.46

Table 4: pH-value of cement pastes with different water-cement ratios

2.2.3 Tensile shear test Immediately after cyclic cement-water treatment tensile shear tests according to DIN EN 3021 [9] were performed in wet stage on a hydraulic tensile testing machine `Schenk Trebel UPM T`. The load was applied position controlled with 0.9 mm/min and the specimens failed within 30-90 s. The applied boiling cement-water treatment is comparable to class A4 according to DIN EN 15425 [10] and DIN EN 301 [11] with a threshold value of 6.0 N/mm² for tensile shear strength. 2.2.4 Determination of adhesive failure percentage (afp) Corresponding to DIN EN 14374 (2005) adhesive failure percentage (afp) of punctured samples determined according to DIN EN 314-1 (2005) must be lower than 30 %. This procedure delivers very subjective results depending on the experience of the executing person. In case of laminated curved structures puncturing is not a possible way of testing, afp was determined after tensile shear tests in a combination of daylight images and UV-light pictures. In order to avoid a subjective determination of afp, all fracture surfaces were analysed with an image analysis software `Scientific Colour` - DatInf GmbH Tübingen. The separation of different fracture types is based on [12] and illustrated in Figure 4. Except the cohesion failure of wood, all types are seen as adhesive failure. a) b) c) d) e)

Cohesion failure of wood ( type 7.3.1) Cohesion failure of adhesive system (type 7.3.2) Adhesion failure within the interface wood-adhesive (type 7.3.3) Rupture near interface (type 7.3.4) Rupture in the fringe area of the bond line (type 7.3.5)

a)

b)

c)

d)

e)

Figure 4: Illustration of rupture surfaces based on [12]

3. Results and discussion 3.1

Tensile shear strength tss and adhesive failure afp

After tensile shear tests the cutting type was classified and split in three different groups depending on the depth of the cut in -

g (“gut”): -“good”- Separation cut was exactly in the depth of the bond line- no wood fibre of the same veneer layer passed the cut

-

-

w (“wenig”): -“poor”- Separation cut ended just before bond line- some wood fibres of the same veneer layer passed the cut and transfer tensile forces. Those normally higher values of tss were not taken into account. v (“viel”): -“much”- Separation cut severed the bond line- no wood fibres passed the cut

Concerning wood failure all batches showed an insufficient rupture combined with a high scattering of values. Even though MUF and PUR batches with a higher bonding pressure showed higher tss values, a high value of afp with a high scattering could be recognized (Figure 5).

(a) Adhesive failure

(b) Tensile shear strength

Figure 5: Tensile shear strength and adhesive failure of flat and curved samples A Shapiro–Wilk test shows that tss of all batches is normal distributed at level 0.05, but only 3 batches of curved samples have normal distribution. sample tss adhesive failure

Curved Curved Curved Curved Curved Curved Curved Flat Flat Flat Flat Flat Flat Flat Flat AB3FU AC2MU E3MU G3MUF CE3PUR DF2PUR H3PUR A3FU C2FU B3PUR D2PUR AB2MU CD3MU E3MUF F2MUF x x x x x x x x x x x x x x x --x x ----x : At 0,05 level data was significantly drawn f rom a normal distributet population

Table 5: Shapiro-Wilk test on normal distribution Therefore determination of adhesive failure in bond line tests of laminated veneers, especially for curved structures, is not a proper way of classifying. Batch E –flat and F –flat had a shorter closed time (3-5 min) which is reflected in a slightly higher tss. Nevertheless all batches of flat panels achieved the threshold of 6 N/mm² given in DIN EN 15425 and DIN EN 301. Both control batches (Flat A and Flat C) with upper and under cut in the mid veneer reach a high tss which is questioned. In [13] comparable values for solid wood samples with treatment A4 according to DIN EN 15425 were with approximately 7 N/mm² quite lower.

Pre-tests on samples cut in mid veneer also delivered comparable values to [13] with 12 N/mm² for treatment EN 15425 A1 (normal climate samples) and 8.5 N/mm² for cementwater boiled samples. This value is in better accordance with tss of control batch AB of the curved samples. The reason for those high values of the control batches can be found in the cutting depth which has a high influence on the strength. All samples of the control batch were cut “gut” and it cannot be excluded that wood fibres passed the separation cut and tensile force were transferred. Adequate tensile shear strength of curved samples was achieved with a bonding pressure of 1.0 N/mm². Samples with a pressure of 0.8 N/mm² didn’t reach the threshold of 6 N/mm². The difference of mean values in tss between curved samples pressure 0.8 N/mm² and pressure 1.0 N/mm² is at level 0.05 significant (Tukey test) which may be concerned with the way of manufacturing by inside pressure with an elastic fire hose. After spreading the veneer surfaces with adhesive the layers are inserted in the outer (hard) resin mould. Afterwards the water filled fire hose is putted in and filled with compressed air. Bonding pressure is therefore applied radial on the inside of the half pole on the whole surface at the same time. To get in contact the single layers have to slide against each other, veneers have to overcome the cohesion induced friction of the adhesive and therefore a higher pressure is necessary. 3.2

UV-light microscopic images of bond lines

Investigations of bond line were made using an incident UV-light microscope (Zeiss Smartzoom 5) on randomly selected samples. Figure 6 shows a comparison of PUR- and MUF- glued curved specimens with 3mm thick veneers.

(a) 3mm PUR 1,0 N/mm²

(b) 3 mm MUF 1,0 N/mm²

tss : 6,6 N/mm² afp : 60 % Type: 7.3.3

tss : 6,9 N/mm² afp : 100 % Type: 7.3.2 + 7.3.5

(c) 3 mm PUR 0,8 N/mm²

tss : 5,4 N/mm² afp : 100 % Type: 7.3.2

(d) 3 mm MUF 0,8 N/mm²

tss : 3,6 N/mm² afp : 80 % Type: 7.3.2 + 7.3.3

Figure 6: UV-light microscopic images of curved 3 mm samples Even with a bonding pressure of 1.0 N/mm² in case of PUR-samples carbon dioxide caused cavities are visible but with a smaller diameter than in specimens with lower bonding pressure. Generally, thinner bond lines are achieved with a higher pressure. All samples show a high adhesive penetration into cells, even peeling cracks are completely filled with adhesive. The high bond line thickness along with a low adhesive cell penetration of sample d) in Figure 6 is connected with the high closed time of 20 min. The polymerisation was progressed too far and therefore an adequate embedding of adhesive on the veneer surface was not possible.

4. Conclusion To reach the threshold of 6 N/mm² for tss after cyclic treatment in case of curved structures a higher bonding pressure of 1.0 N/mm² compared to flat structures with 0.8 N/mm² is necessary but even with a high pH-level possible. Adhesive failure percentage does not allow any conclusion about bond line quality. Because of a high penetration of glue into cells and peeling cracks adhesive residues remain on the shear surface and influence adhesive failure percentage.

5. References [1] Kollmann F. Technologie des Holzes und der Holzwerkstoffe: Zweiter Band: Holzschutz, Oberflächenbehandlung, Trocknung und Dämpfen, Veredelung, Holzwerkstoffe, Spanabhebende und Spanlose Holzbearbeitung Holzverbindungen. Berlin, Heidelberg, s.l.: Springer Berlin Heidelberg; 1955. [2] Krackler V, Keunecke D, Niemz P. Verarbeitung und Verwendungsmöglichkeiten von Laubholz und Laubholzresten. Zürich; 2010 [3] Deutsches Institut für Bautechnik. Furnierschichtholz "Kerto S" und "Kerto Q"(Z-9.1100); 2011 [4] Deutsches Institut für Normung e.V. Holzbauwerke- Furnierschichtholz für tragende Zwecke-Anforderungen;79.080(DIN EN 14374). Berlin: Beuth Verlag GmbH; 2005 [5] Deutsches Institut für Normung e.V. Furnierschichtholz (LVL)- Definition, Klassifizierung und Spezifikationen;79.060.20(DIN EN 14279). Berlin: Beuth Verlag GmbH; 2009 [6] Deutsches Institut für Normung e.V. SperrholzTeil 2: Stab- und Stäbchensperrholz für allgemeine Zwecke;79.060.10(DIN 68705-2). Berlin: Beuth Verlag GmbH; 2003 [7] Deutsches Institut für Normung e.V. Sperrholz- Qualität der Verklebung- Teil 1: Prüfverfahren;79.060.10(DIN EN 314-1). Berlin: Beuth Verlag GmbH; 2005. [8] Deutsches Institut für Normung e.V. Massivholzplatten: Klassifizierung nach dem Aussehen der Oberfläche; Teil 2: Laubholz;79.060.99(DIN EN 13017-2). Berlin: Beuth Verlag GmbH; 2001 [9] Deutsches Institut für Normung e.V. Klebstoffe für tragende Bauteile- Prüfverfahren- Teil 1: Bestimung der Längszugscherfestigkeit;83.180(DIN EN 302-1). Berlin: Beuth Verlag GmbH; 2013 [10] Deutsches Institut für Normung e.V. Klebstoffe - Einkomponenten-Klebstoffe auf Polyurethanbasis für tragende Holzbauteile - Klassifizierung und Leistungsanforderungen;83.180(DIN EN 15425). Berlin: Beuth Verlag GmbH; 2008 [11] Deutsches Institut für Normung e.V. Klebstoffe, Phenoplaste und Aminoplaste, für tragende Holzbauteile - Klassifizierung und Leistungsanforderungen;83.180(DIN EN 301). Berlin: Beuth Verlag GmbH; 2013 [12] Künniger T. Automatische Bestimmung des prozentualen Faserbruchanteils beider industriellen Klebfestigkeitsprüfung. Abschlussbericht. Dübendorf; 2007 [13] Kläusler O, Hass P, Amen C, Schlegel S, Niemz P. Improvement of tensile shear strength and wood failure percentage of 1C PUR bonded wooden joints at wet stage by means of DMF priming. Eur. J. Wood Prod. 2014;72(3):343–54.