https://doi.org/10.14311/APP.2022.33.0417 Acta Polytechnica CTU Proceedings 33:417–423, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague GLUED TIMBER CONCRETE COMPOSITE WALLS USING ULTRA-HIGH-PERFORMANCE CONCRETE Thomas Oberndorfera, ∗, Frank Hungerb, Oliver Fischera a Technical University of Munich, Department of Civil, Geo and Environmental Engineering, Chair of Concrete Structures, Theresienstraße 90, 80333 Munich, Germany b Technical University of Munich, Department of Civil, Geo and Environmental Engineering, Chair of Timber Structures and Building Construction, Arcisstraße 21, 80333 Munich, Germany ∗ corresponding author: thomas.oberndorfer@tum.de Abstract. This paper gives a short introduction of the development of cross laminated timber (CLT) concrete composite walls with a glued connection. In the composite walls, prefabricated ultra-high- performance concrete (UHPC) lamellas replace timber lamellas at the core layer of the CLT elements. To check the feasibility of gluing timber to UHPC small-scale shear, delamination and bonding tests were performed and showed promising results. The load bearing behaviour was analysed with cen- trically and eccentrically loaded tests on wall segments. Analytic modelling of the wall experiments using an effective bending stiffness, based on shear analogy method, showed a good correlation to the experimental results. Keywords: Adhesive bond, cross laminated timber, timber concrete composite, ultra-high- performance concrete. 1. Introduction Cities are forced to grow, due to an increasing urban- ization and population growth in general [1]. Growth occurs vertically, horizontally or in both directions [2]. Vertical growing cities need more high-rise build- ings to accommodate the rising number of people. Because of sustainability aspects and numerous tech- nical innovations at the turn of the millennium, tim- ber multi-storey buildings came into focus [3]. Cur- rently, there are visionary timber high rise projects with building height up to 300 m [4]. The increasing building height leads to high forces in vertically load bearing members in the lower floors. This causes a mass consumption of material and space in timber-only high-rise buildings, due to a rela- tive low compression strength and youngs modulus of timber compared to ultra-high-performance concrete. Compared to the sustainable construction material timber, ultra-high-performance concrete may be re- garded as not-sustainable, because of its high content on non-renewable material and its energy consump- tion for processing. Aim is the sustainable use of ultra-high- performance concrete (UHPC) through its targeted use for vertically load bearing members. Therefore, timber and ultra-high-performance concrete are combined in walls. The wall cross section is com- prised of a slender UHPC core, encapsulated within timber, as shown in figure 1. Applied normal forces concentrate on the concrete core. The surrounding timber prevents the slender concrete core from buckling and bears compression and tensile forces, caused by bending moments due to eccentricities or Figure 1. Wall with hybrid cross section comprised of timber and UHPC [5] external loads. The walls are economically produced in the indus- trialized environment of the cross laminated timber (CLT) production. Three steps are necessary. First, a precast concrete plant prefabricates lamellas. In the regular production process of a CLT plant, the man- ufacturer inserts the prefabricated concrete lamellas into the raw CLT panels at previously defined places. After pressing the panels, wall elements are formed out with joinery machines, automatic cutting ma- chines or machining centres. To ensure statically monolithic acting walls and to enable this production process, a glued connection between timber and concrete using standard glues in 417 https://doi.org/10.14311/APP.2022.33.0417 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en T. Oberndorfer, F. Hunger, O. Fischer Acta Polytechnica CTU Proceedings � Figure 2. Typical stress-strain relationship of differ- ent types of concrete according to [7] CLT production is inevitable. 2. Fundamentals 2.1. Ultra-high-performance Concrete Ultra-high-performance concrete is characterized by its, for concrete, high compressive strength, up to 200 N/mm2, and a young’s modulus between 45.000 and 55.000 N/mm2, which is shown in figure 2. The high compressive strength comes along with a brittle fail- ure when no steel fibres are added to the concrete. A certain degree of ductility can be achieved by adding steel fibres [6]. The high compressive strength of UHPC is achieved through its optimised packing density of the aggregate. Therefore, almost no pores exist in UHPC. This leads to a very dense surface of UHPC compared to normal strength concrete and the possibility to eas- ily glue UHPC. [8] 2.2. Cross Laminated Timber Cross Laminated Timber is a quasi-rigid composite, plate-like engineered timber product, which is com- monly composed of an uneven number of layers, each made of boards placed side-by-side, which are ar- ranged crosswise to each other at an angle of 90◦C [9]. The layers are normally glued together using polyurethane (PU) or melamine-urea-formaldehyde resin (MUF) adhesives [10]. The thickness of CLT elements is commonly between 51 mm and 300 mm, whereas the single layer thickness generally varies be- tween 20 mm and 40 mm. Common lengths of CLT elements is around 18 m with a width up to 3,0 m [9]. Figure 3 gives an overview of the CLT production process and figure 4 displays a simplified technical drawing of a CLT element. CLT bears in-plane and out-of-plane loads through its crosswise layering. The load bearing behaviour is influenced by the cross wise layering and for stability and deformation calculations the shear deformation must be considered [11]. � Figure 3. Production process of cross laminated tim- ber according to [9]. Figure 4. Simplified technical drawing of a CLT element according to [9]. 2.3. Timber Concrete Composite A well-known timber concrete composite (TCC) member is the TCC slab. In this composite element the concrete is positioned in the compressive zone, whereas the timber is placed in the tensile zone. The two elements are connected with special connectors to form one statically acting cross section. The effec- tiveness of the TCC element is mainly characterized by its connection. The usage of systems with high composite action can reduce beam depth and allow for longer span length [12]. Adhesives are a very effective connector for TCC elements and lead to a quasi-rigid connection which increases the member stiffness and strength. In ad- dition, the shear forces are distributed uniformly over the entire surface and local concentrations are avoided. [13] In the past, lots of research on glued connections has been performed, e.g. [14–17], most of these research projects have the use of epoxy resin as adhesive in common. 3. Small scale experiments 3.1. General In the beginning of the project numerous small-scale experiments were performed to determine the possi- bility to glue timber to UHPC using standard glues for CLT production. This was an iterative pro- cess, as the information obtained served as basis for 418 vol. 33/2022 Glued Timber Concrete Composite Walls Figure 5. Experimental test setup for shear experi- ments [5]. the following experiments. These small-scale exper- iments were shear, delamination and bond experi- ments, which will be discussed in this section. 3.2. Shear First, small-scale shear experiments on bonded tim- ber concrete composite specimen were performed to find combinations of concrete, concrete surface treat- ments and adhesives with a promising behaviour. The three-layered specimen consisted of a concrete layer, placed in the middle, and two layers of spruce, which were free of any defects. Per specimen two shear joints existed and shearing off took place par- allel to the grain. Three different types of concrete (C1, C2 and C3) were tested with and without steel- fibres. C1 and C2 were commercially available com- pounds and C3 was a laboratory mixture. The used concrete surfaces were a) smooth, directly out of the formwork, b) ground and c) sandblasted. Five adhe- sives were used: a one-component polyurethane ad- hesive, a melamine-urea-formaldehyde resin, a phe- nol resorcinol resin, a two-component polyurethane resin and an epoxy resin. The adhesive application was 400 g/m2 and the pressure at bonding was about 0, 75 ± 0, 1 N/mm2 for all specimen. The test setup is displayed in figure 5. A combination of concrete, concrete surface treatment and adhesive was charac- terized as promising if it achieved high shear strength and timber fracture in the experiment. Considering only standard CLT production ad- hesives, suitable results could be achieved inde- pendently from the used concrete type, with a ground concrete surface in combination with the one- Figure 6. Technical drawing of experimental test setup of inclined bond tests [5]. component polyurethane adhesive. Specimen glued using the melamine-urea-formaldehyde resin failed in the glue line, which was characterized through a low shear strength and a maximum timber fracture of 20%. 3.3. Delamination Although this research project concentrated on the short-term load bearing behaviour of CLT walls with glued in lamellas from UHPC, durability keystroke tests with glued specimen were performed. The test procedure included storing the specimen in boiling water for six hours, the storage in 20 ◦C cold wa- ter for one hour and afterwards drying in an oven for approximately 16 hours. This induces shear- and tensile stresses on the glue line due to the swelling and shrinkage behaviour of the timber. If the glued connection is insufficient there will be a gap opening of the glue line. According to DIN EN 16351 [18] the maximum tolerated failure of a single glue line is 40% of its length and 10% of glue line length may be opened for the sum of all glue line lengths. Based on the shear experiments, delamination tests with promising combinations were performed. Here, both steel-fibre reinforced concrete types C1 and C2, a smooth and ground surface, the one-component polyurethane adhesive, the two- component polyurethane resin, and the epoxy resin were used to form the specimens. The manufactur- ing procedure of the specimen was identical as for the shear tests. For each series two specimen were produced. 3.4. Bond To verify the assumption of a rigid connection in- clined shear tests with three different bond lengths, 419 T. Oberndorfer, F. Hunger, O. Fischer Acta Polytechnica CTU Proceedings Figure 7. Schematic (left) and real (right) experimental test setup for full scale tests [5]. 14 cm, 28 cm and 45 cm, were performed. A tech- nical drawing of the experimental test setup is dis- played in figure 6. The specimens were comprised of concrete type C1 lamellas with smooth and ground surface, glued to spruce glulam GL24C using the one- component polyurethane adhesive. Bonding param- eters were identical to the shear and delamination tests. All experiments showed a brittle failure when reaching the maximum force. With increasing bond length, the average shear force leading to failure in- creased. The average failure shear force for the 14 cm bond length with the ground concrete surface was 77,1 kN and increased up to 217,1 kN for the 45 cm long bond length. No difference could be seen in the behaviour of the smooth or ground concrete surface. The maximum measured relative slip between con- crete and timber, recorded with an optical measuring system, in the middle of the bond length was approx. 0,25 mm for all experiments. In most cases the fail- ure occurred in the timber part. Only in two exper- iments, with the 14 cm long bond length, the failure occurred in the glue line. 4. Full scale tests 4.1. Methodology To analyse the load bearing behaviour of the com- posite walls, centrically and eccentrically loaded, ex- periments at scale 1:1 have been performed. Figure 7 displays the test setup. For every eccentricity, 0 mm, 10 mm and 20 mm, three wall segments were tested. The tested wall segments were 0,5 m in depth, 2,84 m high and had a thickness of 15 cm. Two roller tilting bearings were used to test a pendulum rod with a buckling length of 2,97 m. The five layered wall segments with a complete concrete core, every layer was 30 mm thick, were produced in a CLT pro- duction facility. The adhesive application of a one- component polyurethane was production based 170 g/m2 and the bonding pressure was 0,08 N/mm2, because the elements were pressed in a vacuum bed. The used timber was spruce with a strength class of C24, the concrete had a mean compressive strength of fck, cyl = 132, 5N/mm2 and a mean young’s modulus of 46.774 N/mm2. These values were determined ac- cording to DIN EN 12390-13 [19] and DIN EN 12390- 3 [20]. The tests were conducted displacement con- trolled with a speed of 0,12 mm/min. Laser distance sensors controlled the horizontal displacement of the wall segments during the experiments. Strain gauges recorded the strain on the outermost timber fibres and in the middle of the cross section on the con- crete. The strain gauges were placed in the centre, at the bottom and the top of the wall. 4.2. Results Figure 8 displays the force-horizontal deflection in the vertical wall centre and the force-piston stroke diagram for the centrically loaded experiments. All three experiments showed similar behaviour and the maximum force varied between 1331 kN for V_D_30_00_3 and 1693 kN for V_D_30_00_2. With increasing load, the horizontal deflection in- creases slowly until the critical point is reached and the walls buckle. This is indicated by a rapid in- creasement of the horizontal deflection, accompanied with a decreasing load. After the specimen were re- moved out of the testing machine material failure could visibly not be detected. 420 vol. 33/2022 Glued Timber Concrete Composite Walls Figure 8. Force-horizontal deflection and force- piston stroke diagram for centrically loaded experi- ments [5]. Figure 9. Force-horizontal deflection and force- piston stroke diagram for 10ămm eccentrically loaded experiments [5]. The force-horizontal deflection diagram of the 10 mm eccentric loaded experiments is shown in figure 9. The three specimens showed a similar behaviour upon reaching the maximum force, which varied be- tween 908 kN and 1025 kN. The horizontal deflection at the maximum load is close to 30 mm. Experi- ments V_D_30_10_1 and V_D_30_10_2 showed a ductile behaviour, whereas V_D_30_10_3 failed brittle. Material damage could clearly be seen on the compressive side of the wall segments in the outer most timber lamella. The recorded strains in the wall centre showed a similar behaviour for all experiments. Figure 10 shows a typical force-strain diagram, explicitly for ex- periment V_D_30_10_1. The concrete middle layer suffers only compressive strain, whereas the outer- most timber fibres are either under compressive- or tensile strain. 4.3. Analysis To calculate the maximum buckling force for the cen- trically loaded experiments, Euler’s formula using an effective bending stiffness, accounting for the addi- tional shear deformation, according to [11], was used. The eccentric experiments were analytically modelled Figure 10. Development of strains at the vertical wall centre for timber and concrete for experiment V_D_30_10_1. [5]. Figure 11. Development of strains at the vertical wall centre for timber and concrete calculated (gray) and experiment (black). [5]. with the differential equation for second order prob- lems, also using an effective bending stiffness. Due to the scattering of the elastic timber proper- ties the mean stiffness values and the characteristic material strength of nominal timber strength class C24 according to DIN EN 338 [21], and for con- crete the determined young’s modulus and compres- sive strength were used for the calculations. The calculated buckling forces were lower than the ones achieved in the experiments. Figures 11 and 12 compare the calculated strain distribution and hori- zontal deflection for different timber strength grades in the vertical wall centre with ones obtained in an experiment. The curves fit well. Because a linear elastic material model was used to model the experi- ments, the declining branch in the experiment cannot be considered. 421 T. Oberndorfer, F. Hunger, O. Fischer Acta Polytechnica CTU Proceedings Figure 12. Force-horizontal deflection calculated (gray) and experiment (black). [5]. 5. Conclusions The completed test program showed that it is pos- sible to bond timber to UHPC using standard glues for CLT production regarding short-term behaviour. The tests on wall segments verified, that the cross sec- tion can be considered as monolithic and the elements can be calculated using an effective bending stiffness. The maximum load capacity depends on the slender- ness of the walls and the eccentricities. Compared to homogenous CLT walls, the hybrid walls tested with a 10 mm eccentricity, show a 25% higher load bear- ing capacity, calculated with characteristic values for material strength. Acknowledgements The authors want to thank the Forschungsinitiative ZukunftBau for funding this project. This paper is only a short introduction in the project, the described experi- ments, and results in section three, four and five are de- tailly described in [5]. References [1] United Nations, Department of Economic and Social Affairs, Population Division. 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