BEARING BEHAVIOR OF IMPREGNATED TEXTILE REINFORCEMENT

Transcription

1 BEARING BEHAVIOR OF IMPREGNATED TEXTILE REINFORCEMENT Sergej Rempel (1), Christian Kulas (2), Josef Hegger (1), (1) RWTH Aachen University, Institute of Structural Concrete, Germany, (2) solidian GmbH, Germany Abstract:The current trend in the building industry shows a growing need for highperformance material with high tension and pressure strength. An innovative building material, which satisfies the wishes of the construction designer, is textile-reinforced concrete (TRC). The combination of high pressure strength concrete and corrosionresistant reinforcement, which furthermore has high tension strength, enables slender concrete structures. The realized applications prove the existing potential. The further development of the textile reinforcement offers new possibilities for bearing structures. One main progress was the impregnation of the textiles with styrol-butadiene rubber or epoxy-resin. This process enables extensive higher failure stresses. Additionally, the durability and the strength improve. Consequently the efficiency rises. Within the scope of this paper, the bending behavior of slabs and I-beams are presented. Furthermore, a calculation approach for bending moments is introduced. INTRODUCTION Current trends in the building industry are increasingly heading towards high-performance materials with high tensile or compressive strengths. One of those innovative materials is textile-reinforced concrete (TRC), which uses mesh-like, non-corrodible reinforcements in combination with a fine-grained concrete [1], [2]. Since the reinforcements do not corrode, it is possible to significantly reduce the concrete cover compared to ordinary steelreinforced constructions. This leads to thin-walled and slender concrete members with high-quality concrete surfaces meeting the needs of modern architecture Figure 1. For complex constructions, like T-beams, shaped reinforcement structures are necessary, which cannot be realized with non-impregnated textiles due to a lack of form stability (Figure 1 right). Therefore, by impregnating the textiles with, for example, epoxy-resin or styrenebutadiene, inherently stable and manageable reinforcement structures (prepreg structures) can be fabricated. Another important advantage of those structures is the improved bondbehavior of the inner filaments of a roving leading to tensile stresses that are two or three times higher compared to non-impregnated textiles [3]. At last, the resin improves the durability of the reinforcement, which is especially important for reinforcements made of glass-filaments [4]. 71

2 Figure 1: View of the Textile- Reinforced Façade in Nimwegen (picture: Ben Vulkers, LIAG Architecten) and a cross-section of the TRC-bridge with shaped textiles (picture: solidian) Textile-reinforced concrete is often used as construction material for façade structures with concrete covers of about 10 mm to 15 mm (Figure 1, left). Hegger et al. give detailed information on the applicability of TRC for new constructions, especially on ventilated façade structures and sandwich panels [5], [6]. Furthermore, TRC is also applied for strengthening issues, e.g. strengthening of concrete shell structures [7]. Another field of application is in the construction impact from chlorides, e.g. due to de-icing salt, like bridges or maritime constructions. An actual example can be seen in a pedestrian bridge made of TRC located in Albstadt, Germany (Figure 1, right). The bridge has an overall length of 97 m and is subdivided in six parts, with a maximum length of m. Since the superstructure is made of TRC, an extreme slenderness ratio of H:L = 1:35 (height to length) is realized [8]. This article deals with the bending load-bearing behavior of impregnated textiles and presents test-results along with a calculation approach. While the bending moment of TRC members with non-impregnated textiles can only be calculated under consideration of empirical reduction factors, it is shown that those factors are not necessary for impregnated textiles. MATERIALS Textile reinforcement Actually alkali-resistant glass (AR-glass) or carbon are the main materials used for current TRC applications. The basic materials are hair-thin filaments with diameters of 14 μm for AR-glass and about 7 μm for carbon. A bundle of hundreds, up to ten thousands, of these filaments shape a roving, which will be processed to make flat textile-reinforced grids. The textile grids can be used directly without impregnation or impregnation with Styrene- Butadiene or Epoxy-Resin as reinforcement elements for concrete structures. The impregnation is responsible for higher failure stresses and consequently for an increase in the effectivity of the textile materials. Also the loss of strength of the AR-Glass, because of the alkalinity, has to be considered. The impregnation counteracts it and reduces the losses. 72

3 The failure stresses is dependent on the material, cross-section of the roving, the bindingtype and impregnation and can be seen in Table 1. Table 1: Properties of the selected textiles Properties Units Textile 1 Textile 2 Textile 3 Material - Carbon AR-Glass AR-Glass Penetration - Epoxy-Resin Epoxy-Resin Styrene-Butadiene Titer tex Roving distance 90 mm 23 15/ 2 X 7, mm 21 15/ 5 21 Cross section 0 mm 0,92 1,34 0,9 90 mm 0,92 1,34 0,9 0 N/mm² Tensile stress 1) N/mm² 1) determined in tensile tests on rovings, which have been cut out of the textile The grids Textile 1 and Textile 2 are impregnated with epoxy resin and differ only in the distance of the rovings and the filament material. Unlike Textile 1 and Textile 2, the Textile 3 is impregnated with styrene-butadiene. Every impregnated textile is available as planar reinforcement and as a U-shaped reinforcement. Concrete The use of a typical concrete with its standard grain size was not possible because of the slight openings in the textile reinforcement grids. By using grains with diameters that are too large, the concrete would not flow through the meshes properly and the textile would act as a sieve, resulting in a separation layer within the member. For this reason, the Institute of Building Material Research of the RWTH Aachen University (ibac) developed during the SFB 532 (Collaborative Research Center) a recipe for a concrete with a maximum grain size of 0.6 mm [2]. Due to the high cement content, this concrete tends to shrink and creep more than a concrete with a larger grain-size. Thus, the investigations presented in this paper have been done with a concrete with a maximum grain-size of 5.0 mm (Concrete 1, Table 2), developed by Hering Bau, a construction company specializing in architectural concrete. 73

4 Table 2: Properties of the applied concrete Properties Units Concrete 1 Type of cement - CEM II A-LL 42,5/R CEM I 52,5 R Cement content kg/m³ 410 Maximum grain size mm 5,0 Aggregate content kg/m³ 1540 Compression strength - cubes 40x40x40 mm³ MPa cylinder Ø150 mm/h=300 mm MPa 87 Flexural strength MPa 10,6 YOUNG s Modulus MPa The concrete offer high compression strengths of about 100 MPa and can be classified as high-strength concretes according to Eurocode 2 [9]. Since TRC members are often designed as fair-faced concrete structures, a high flexural strength is an important property. Concrete 1, which is used within this paper, has a flexural strength of 10.6 MPa. The main mechanical properties of Concrete 1 are shown in Table 2. The stress-strain behavior of the fine-grained concrete was determined in compression tests on cylindrical specimens with a diameter of 150 mm and a height of 300 mm. According to Eurocode 2 the stress-strain curve can be calculated with formula (1), which gives an adequate match with the test results shown in Figure 2. The Young s Modulus is determined at a stress of 0,4 fcm, where linearity reaches its limit. compression stress σ c [MPa] Figure 2: f cm = 87 MPa 0,4 f cm = 35 MPa ε c1 = 2,4 Eurocode 2 [13] test results (n=6) 0 0, strain ε c [ ] 2 ε 1,3 c ε c σ = c ( εc ) 87 (1) ε c 2.4 Stress-strain behavior under compression of concrete C1: test-results and calculation according to Eurocode 2 [9]. 74

5 BENDING BEHAVIOR Experimental investigations Typical applications for TRC are façade panels with common slab thicknesses of about 30 mm [10], thus, slab specimens with rectangular cross-section were considered for investigating the bending behavior. Additionally, I-beams with an overall height of 120 mm, as depicted in Figure 3, according to Hegger and Voss [11] were tested, since with this crosssection it is possible to study the load-bearing behavior of shaped reinforcement structures like U-shaped textiles. In both members the reinforcement ratio in the tensile zone was varied by adding planar textile layers. The specimens were tested in four-point bending tests with a span of 600 mm (slab) or 1,300 mm (I-beam) respectively. The members were tested displacement-controlled with a loading-rate of 3 mm/min. a) b) Figure 3: Specimen for investigating the bending-behavior: left) slab; right) I-beam Prediction of the ultimate bending moment The procedure for calculating the bending moment is shown in Figure 4 on the example of an I-beam and follows the calculation methods of steel-reinforced concrete. Figure 4: Distribution of strain, stress and forces in a bending member The basic assumption is a linear strain distribution with the concrete compression εc and the strain εt,max of the roving with the inner level zi. The stresses and forces can be calculated under consideration of the material laws of textile and concrete. Therefore, not only the rovings in the lower flange but also the rovings in the web have to be considered. To recalculate a test, the specimen was cut after the test and the real effective depths di of each roving were measured. The compressive force can be calculated by integrating the stress 75

6 distribution σc. Together with the tensile forces σt,i of each roving the bending moment can be calculated with the formula (2). M u, cal = Ft, i zi = ( At σ t ) z, i i (2) i i In Figure 5 the ultimate bending moment achieved in the tests divided by the calculated bending moment is plotted over the reinforcement ratio ρl. In the diagram results from both the four-point bending tests on I-beams and on slabs with a rectangular cross-section are considered. The predicted bending moment gives an adequate match to the experimental results. A difference of only 5% is due to test scattering. a) b) Figure 5: Bending tests: a) specimen in failure state; b) Comparison of calculation and test results (each data point is one single test) The results in Figure 5 show that for impregnated textiles the main input parameter in his calculation model is the roving strength σ t,max. 76

7 CONCLUSION Impregnated textiles have certain advantages compared to the ones that are not impregnated, like higher tensile stresses, improved durability and a sufficient dimensional stability. For calculating the bending behavior of concrete members which are reinforced with impregnated textiles, the ultimate bending moment can be calculated with a simple iteration of the strain distribution. The simple calculation model was used for structural analyses of several TRC applications. Due to the fact that the model is based on the well-known calculation methods of steel-reinforced concrete, already many engineers were able to realize TRC constructions. ACKNOWLEDGEMENT The authors acknowledge the German Research Foundation (DFG) for financing the Transfer Project T08 within the scope of the Collaborative Research Center (SFB) 532 at RWTH Aachen University. Furthermore, the authors thank the partners Groz-Beckert KG, V. Fraas Solutions in Textile, SGL Carbon, Hering Bau, the Institute of Materials Building Research and the Institute of Textile Technology at RWTH Aachen University for their support and the efficient collaborative relationship. REFERENCES [1] J. Hegger, S. Voss, Investigations on the bearing behaviour and application potential of textile rein forced concrete, Engineering Structures, Issue 30 (2008). pp [2] B. Banholzer, T. Brockmann, W. Brameshuber, Material and bonding characteristics for dimension ing and modelling of textile reinforced concrete (TRC) elements, Materials and Structures 39, Issue 8 (2006), pp [3] M. Raupach, J. Orlowsky, T. Büttner, U. Dilthey, M. Schleser, Epoxy-impregnated textiles in con crete load bearing capacity and durability. In: W. Brameshuber (Ed.): Textile Reinforced Concrete, RILEM Report 36. Bagneux, France. ISBN (2006), pp [4] T. Büttner, J. Orlowsky, M. Raupach, M. Hojczyk, O. Weichold, Enhancement of the durability of alkali-resistant glass-rovings in concrete. In: W. Brameshuber (Ed.): 2nd ICTRC Textile Reinforced Concrete. Proceedings of the International RILEM Conference on Material Science (MatSci). International RILEM Conference on Material Science (MatSci), Aachen, ISBN pp [5] J. Hegger, C. Kulas and M. Horstmann, Realization of TRC façades with impregnated AR-glass textiles, Key Engineering Materials, Issue 466 (2011), pp [6] C. Kulas, M. Schneider, N. Will and R. Grebe, Ventilated structures made of textile reinforced con crete structural behavior and construction, Bautechnik, Vol. 88, Issue 5 (2011), pp [7] D. Jesse, F. Jesse, High Performance Composite Textile Reinforced Concrete Definitions, Proper ties and Applications. In: 3rd International fib Congress, Washington, D.C., , paper no 157 on CD-ROM. [8] J. Hegger, C. Kulas, M. Raupach, T. Büttner, Load-bearing behavior and durability of a slender textile reinforced concrete bridge. Beton- und Stahlbetonbau 106, Issue 2 (2011), pp [9] Eurocode 2 (DIN EN ): Design of concrete structures - Part 1-1: General rules and rules for buildings; DIN (Deutsches Institut für Normung), Beuth Verlag GmbH, Berlin, [10] C. Kulas, M. Schneider, N. Will and R. Grebe, Ventilated structures made of textile reinforced con crete structural behavior and construction, Bautechnik, Vol. 88, Issue 5 (2011), pp [11] J. Hegger, S. Voss, Application and dimensioning of textile reinforced concrete, In: T. C. Trianta fillou (Ed.): Fibre Reinforced Polymer Reinforcement for Concrete Structures. Proceedings of the 8th International Symposium for Fiber-Reinforced Polymer Reinforcement for Concrete Structures; Patras, Greece, , pp ISBN

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