LOCALLY ADAPTED BIAXIAL WARP KNITTED TEXTILES AS REINFORCEMENT OF FOLDED CONCRETE ELEMENTS

Transcription

1 LOCALLY ADAPTED BIAXIAL WARP KNITTED TEXTILES AS REINFORCEMENT OF FOLDED CONCRETE ELEMENTS A. Koch1, C. Kerschl2, T. Gries1 and W. Brameshuber2, 1Institut für Textiltechnik (ITA) of RWTH Aachen University (Germany), 2Institut für Bauforschung (ibac) of RWTH Aachen University (Germany) Abstract: The use of textile reinforcements for building applications has been on the rise since some years now (e.g. as plaster or floor reinforcement). One of the most sustainable developments is textile reinforced concrete (TRC). TRC can save up to 80 % of concrete as compared to a steel reinforced structure for similar applications [1]. A biaxial warp knitted structure, made with AR-glass or carbon fibres, increases the mechanical properties of concrete components significantly. On the basis of its non-corrosive property, building components with TRC can be realized with very thin cross-sections and with a high bearing capacity. A significant advantage of TRC is the formability of the textile reinforcement. Curved TRC-elements of high quality finish are possible due to the inherent drapability of the textile in comparison to steel reinforcement. The minimum radius of the curve decides the required level of drapability of the textile. The drapability of a biaxial warp knitted textile is mainly depending on the stitching pattern (for e.g. pillar, tricot or plain). Textiles which use the pillar stitching pattern have a higher stiffness compared to a plain knitted textile. In the course of a public funded project we want to realize locally adapted textiles which combine two different levels of drapability in one single fabric. The overarching aim is generating the possibility to manufacture adapted textiles for special folded elements. A major challenge is to classify the possible combinations of stitching patterns to determine their influence on the drapability of the textiles on basis of a cantilever test. The public funded project ConcreteFold started in June, 2014 and has a 2 years research period. This paper will present the project scope as well as the first results. INTRODUCTION Textile reinforced concrete is a growing research topic since the last 20 years in Germany (especially in Aachen and Dresden). The basic research was done during two collaborative research projects funded by the DFG (German Research Foundation) from 1999 until 2011 at the RWTH Aachen University and TU Dresden. The technical University of Dresden had the leading subject Strengthening steel reinforced concrete with textile reinforced concrete and the RWTH Aachen University conducted research on analyzing the behavior of new precast TRC-elements (especially for façade applications). TRC has many advantages in terms of ecology, economy, durability and new possibilities of design compared to steel reinforced concrete. The most important advantages of TRC are [2, 3]: Corrosion free (especially the textile reinforcement) High drapability of the textile reinforcement compared to steel reinforcement (new scope for design) 419

2 Realization of very thin and structurally strong elements (5-50 mm) High surface quality and high durability due to using a fine grain concrete matrix Since 1999 many building applications are realized by using TRC components. These demonstrator buildings allows realization of using TRC as a building material and provide the opportunity to get a long-term evaluation. RWTH University emphasized this philosophy by constructing their own buildings (e.g. facades of institute buildings and auditorium buildings) as the first TRC-demonstrator buildings. The following two pictures show the TRC façade of the ITA machine hall (Fig. 1, left) and a free-formed TRC deck chair (Fig. 1, right). Fig. 1: Façade of the ITA machine hall (left) and a TRC deck chair (right) The drapability of biaxial warp knitted textiles is one of the advantages and enables a new freedom of concrete designs with an integrated reinforcement. Therefore, the textile design is the key between the textile structure and its draping behavior. The textile design can be adapted by changing the following factors: Yarn material (warp and weft direction, e.g. carbon or AR-glass) Yarn titer (warp and weft direction, e.g tex) Mesh size (warp and weft direction, e.g mm) Stitching pattern (pillar, tricot, plain) In this case the drapability is mainly influenced by the stitching pattern. The standard stitching pattern regarding biaxial warp knitted textiles are pillar, tricot and plain. The following overview shows stitching pattern in a technical sketch and a photograph (Fig. 2, left) and shows the connection to the mechanical properties of the textile (Fig. 2, right). 420

3 Pillar Tricot Plain Stitching pattern Pillar Tricot Plain Cross-section geometry of the warp rovings Bond properties with respect to the concrete matrix low medium good Bending stiffness high medium low 1 cm 1 cm 1 cm Resistance to displacement high medium low Fig. 2: Standard stitching pattern and its properties The high influence of the stitching patterns is the result of the compaction of rovings under different stitching patterns. The pillar stitching pattern extremely compacts the warp yarn and generates a circular cross-section geometry of the yarn. In comparison, the plain stitching pattern only keeps the yarn together and doesn`t compact it. The degree of compacting is directly proportional to the bending stiffness of the textile. The textile design enables a desired textile drapability behavior on destined textile parts due to the influence of the above mentioned factors. This knowledge led to start a research project in June 2014 to study the possibility of locally adapted textiles for folded concrete elements. The focus is on the manufacturing of folded concrete elements with the help of a new folding tool to fold TRC plates whilst the concrete is in a green stage. PROJECT SCOPE The research project ConcreteFold funded by the federal ministry of economics and energy (BMWE) started in June 2014 in cooperation with RWTH Aachen University (ITA, ibac, LfP), Florack Bauunternehmung GmbH, Germany, ingema t+h ingenieurgesellschaft mbh, Germany and W+S bau-instandsetzung GmbH, Germany. Different partners with different expertise work on a manufacturing process for precast folded TRC-elements. The difficulty lies in creating proper edges of the folded TRC element. The textile reinforcement should stay in a defined position in the inside of the edges and not shift to the concrete surface. Two solutions are proposed by the project consortium to answer this research question. One solution works with an overlap of the textile reinforcement to enable a displacement during the folding process. The second and more difficult solution is to develop a locally adapted textile which enables a defined deformation of the textile during the folding process. The following two pictures show the mentioned solutions as schematic sketch. 421

4 Betonmatrix Concrete matrix Normalbewehrung Standard reinforcement Konfektionierte Locally adapted Bewehrung reinforcement Betonmatrix Concrete matrix Normalbewehrung Standard reinforcement Konfektionierte Locally adapted Bewehrung reinforcement Fig. 3: Two different project approaches PRODUCTION OF LOCALLY ADAPTED BIAXIAL WARP KNITTED TEXTILE Biaxial textiles are based on two fibre systems (warp and weft). The warp yarns will be placed on top of the weft yarns and bonded by using a thin knitting yarn (usually PES 167 dtex at ITA). The knitting yarn can be inserted in different ways (stitching pattern) by using the appropriate steel disk guidance mechanism or an electronic control system. These components control the guide bar motion of the machine, which are used for the knitting fibre placement. ITA uses a Karl Mayer Malimo machine to produce the biaxial warp knitted textiles with a working width of up to 50 inches. During the preliminary tests, ITA stopped the textiles production of a pillar bond textile and changed the stitching pattern to a plain stitching pattern. On the basis of a much higher knitting yarn consumption, the knitting yarn supply was also configured. After a textile production length of approx. 4-5 cm, the stitching pattern was changed again (from plain to pillar). The following picture (Fig. 4) shows the first textile demonstrator and illustrates the main project idea. Weft direction Section A (stiff) Warp direction Section B (flexible) Section A (stiff) Fig. 4: First demonstrator of a locally adapted biaxial warp knitted textile A first manual handling test easily showed the difference of the textile bending stiffness only by changing the stitching pattern on a local section. Fig. 5 shows the textile structure with the pillar stitching pattern (standard, left) and the locally adapted textile (right). The bending stiffness is very low on the adapted local section. 422

5 60 mm 60 mm Fig. 5: First manual handling test (Comparison of the two textile structures) In a second step, ITA placed a locally adapted textile on top of a wooden folded form to perceive the bending behavior of the textile. The developed textile nicely adapts to the shape of the wooden form. Fig. 6 shows the innovative locally adapted textile in an inner and outer formed edge. 100 mm 100 mm Fig. 6: The locally adapted textile in an inner and outer form During the research project ITA will measure this change of the bending stiffness and test further options for local adaption. The bending stiffness can be measured by doing a cantilever test with the standard DIN Testing of plastics films and textile fabrics (excluding nonwovens), coated or not coated fabrics - Determination of stiffness in bending - Method according to Cantilever. The results of the cantilever test can lead to an overview of locally adapted textiles and their bending properties. This overview can be a formal tool to choose the best textile for a set geometry of a folded TRC element. FOLDING PROCESS OF THE TRC-ELEMENTS After the development of a locally adapted textile, a flexible formwork is required to manufacture folded TRC elements in a green stage of the concrete matrix. Complex shaped concrete elements can be realized by using three dimensional formworks and with the use of a short fibre reinforced concrete matrix. But the manufacturing process of these design elements is complex and extremely time-consuming. Therefore, the project consortium will develop a flexible folding tool, which allows folding a flat concrete plate in a green stage. Fig. 7 shows a drawing of the first construction concept (left) and a three dimensional picture (right). 423

6 formwork abgestufte Formgrößen TRC-element formwork durchgesteckte connection Stangen rod joint perforated plate Lochblech Fig. 7: Flexible formwork for different folded TRC elements The size of the folding tool is set on 0.5 to 1.5 m. The numbers of edges and the maximum angle have to be defined for the first prototype of the formwork. The engineering office ingema t+h ingenieurgesellschaft mbh will do the first construction details in December RESULTS OF THE FOLDING TESTS In order to find a suitable concrete mixture for the folding process the fresh concrete behavior was studied at the Institute of Building Materials Research (ibac) using a folding tool and different mixture designs. The mixtures should have a high green strength to be able to remove the first formwork after a few minutes and a high ductility to bear the movement of the flexible formwork. CEM II/A-LL 42.5 R was used with metakaolin as binder with a water to binder ratio varying between 0.40 and The maximum grain size was 1 mm. As an additive a PCE based super-plasticiser was used. A simple folding tool allowing an angle between 0 and ± 90 (Fig. 8) was built to test the different mixtures in regard to their tendency of cracking during the folding process. Fig. 8: Simple folding tool for angles between +0 and +90 The concrete was cast in two layers into a formwork with dimensions of 250 x 1000 x 15 mm³. The locally adapted textile was placed between the two layers during the casting process. After a certain setting time the specimen was moved with the help of the Polyethylene foil that had been placed in the formwork prior to casting into the folding tool. One side of the folding tool is tilted up (0 to +90 ) or down (0 to -90 ). Afterwards the specimen hardens on the folding tool for 24 hours. 424

7 The experiments have shown that angles between -15 and +90 can be achieved without the development of cracks in the stretched side of the specimen. Angles between -15 and - 90 cannot be achieved without cracks. It has been shown that with the application of an additional Polyethylene foil on the surface the crack formation can be avoided. The adhesion forces between Polyethylene foil and fresh concrete apparently bridge the cracks successfully. A further advantage of the Polyethylene foil is improved exposed concrete quality and reduced evaporation. In order to check the position of the textile the specimen was cut perpendicular to the folding edge. In Fig. 9 the position of the locally adapted textile is shown in a specimen folded with an angle of +90. Fig. 9: Position of the locally adapted textile in the concrete specimen So far, only single folded edges have been investigated. In this case it can be seen that the folding process did not lead to a displacement of the textile. In order to prove this fact for specimens with more than one edge a new folding tool will be built in association with ingema t+h ingenieurgesellschaft mbh. CONCLUSIONS AND FURTHER OUTLOOK The research project ConcreteFold offers the possibility to develop a new and simple manufacturing process for folded TRC elements. Besides the development of a flexible formwork to enable this folding process, the development of a locally adapted textile plays a major role of the project. A new type of textiles can be produced with a local adaption regarding their bending stiffness. This adaption allows the use of these textiles for folded TRC elements, because of their adaptability on set folded edges. The project will do the folding tests with the help of a new developed folding tool in The first demonstrators of the folded TRC elements will be analyzed regarding the defined textile placement. The difference between a standard textile and the locally adapted textiles should demonstrate a better textile placement in the inside of a folded TRC element. At the end of the project a wall will be built to demonstrator a combination of many folded TRC elements. The research plans to take the first steps towards realizing the true design potential of TRC as a material. However, the material and formwork research is only the first step towards institutionalizing the use of TRC in complex folded component manufacturing. The performance limits of such folded elements require to be tested according to the norms. A quality testing procedure to ensure proper placement of textiles within the advanced form- 425

8 work ensures higher level of replicability. In addition, non-destructive testing (NDT) to see the exact positioning of textiles within the hardened concrete component will help illustrate the efficiency of the newly developed textile material as well as the formwork. ACKNOWLEDGEMENT good cooperation. The authors would like to acknowledge the financial support from the ConcreteFold -project funded by the BMWE for carrying out the project. The authors would like to thank Florack Bauunternehmung GmbH, Germany, ingema t+h ingenieurgesellschaft mbh, Germay, W+S bau-instandsetzung GmbH, Germany and Lehrstuhl für Plastik (LfP) der RWTH Aachen University for the PROJECT CONSORTIUM REFERENCES [1] Gries, T., Tomoscheit, S., EU Life06 Insu-Shell: Environmentally Friendly Facade Elements made of thermal insulated Textile reinforced concrete, Bund Deutscher Architekten, Ortsverein Aachen, Aachen [2] Janetzko, S., Methodik zur Gestaltung von Bewehrungssystemen für textilbewehrten Beton, Ph. D. Thesis, Institut für Textiltechnik (ITA) der RWTH Aachen University, Germany, [3] Arbeits- und Ergebnisbericht / Sonderforschungsbereich 532 "Textilbewehrter Beton - Grundlagen für die Entwicklung einer neuartigen Technologie", Aachen, [4] DIN 53362, Bestimmung der Biegesteifigkeit - Verfahren nach Cantilever, Oktober