HYBRID SHORT FIBRES IN FINE GRAINED CONCRETE

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1 1 st International Conference Textile Reinforced Concrete (ICTRC) 3 HYBRID SHORT FIBRES IN FINE GRAINED CONCRETE M. Hinzen, W. Brameshuber, Institute of Building Materials Research, RWTH Aachen University, Germany ABSTRACT: In the past, the tensile strength of textile reinforced concrete was increased by the number of textile layers thus enabling the application in load-bearing structures. To improve the serviceability and to further minimise crack widths ductile matrices are used. Therefore, short fibres of different materials (glass, carbon, steel, synthetics) are added to the mix in different amounts. The fibres complement each other and improve the post-cracking behaviour of the fine grained concrete bridging cracks. Furthermore, they increase the tensile strength. As the addition of short fibres affects the flowability of the concrete, which must, however, be ensured, fine grained concrete mixes without fibres are modified to guarantee workability when fibres are added. In this paper, the influence of fibre content and fibre length as well as the post-crack behaviour in bending tests are described. 1 INTRODUCTION Textile Reinforced Concrete (TRC) is a new composite material which is reinforced by technical fabrics made of glass or carbon. In many cases, the high tensile strength of glass and carbon is suited to carry tensile loads in concrete and to substitute steel reinforcement. However, a good bond between the fabrics and the surrounding concrete is required. Therefore, fine grained concretes have been developed within the scope of the Collaborative Research Centre 32 Textile Reinforced Concrete (TRC) at RWTH Aachen University [Bro6]. These flowable concretes with a relatively high binder content and a maximum grain size of only.6 mm can penetrate a certain number of textile layers in a conventional casting process. Due to the high binder content and low w/b ratio, these matrices have a relatively high compressive strength and show a brittle fracture behaviour as do the applied glass or carbon textiles. Hence, the combination of these two materials leads to a behaviour as shown in Figure 1 where a stress strain curve resulting from an uniaxial tensile test with a textile reinforced specimen is given as an example. The concrete matrix abruptly fails in point A and the load is transferred to the textile in a crack formation process (point A-B) after which the load can be further increased until the textile fails. The aim of future research activities will be to increase the first crack stress and the serviceability of TRC in terms of smaller crack widths and more ductile behaviour as indicated in Figure 1. This shall be achieved by the use of ductile matrices which contain hybrid short fibres. In the past, steel fibre concrete has been described in many papers, but there is a restricted knowledge concerning the combination of and the interaction between different types of fibres. However, some important studies in this field have already been conducted by [Ban4, Mar3, Mie, Kaw3]. As the cracking process takes place gradually, the combination of fibres of different materials, sizes and shapes with different functions seems advantageous. In this paper, the results of bending tests of fine grained concrete with single and hybrid fibres are presented. It is described how the different fibres increase the strength and how they improve the ductility and the first crack stress of the matrix.

2 36 M. HINZEN, W. BRAMESHUBER: Short Fibre Cocktails in Fine Grained Concrete stress in N/mm aspired stress-strain curve 1 Figure 1: A B 2 layers of carbon textile (178 mm 2 /m) Concrete: FGC strain in mm/m Stress-strain diagram of textile reinforced concrete with carbon textiles 2 EXPERIMENTAL PROGRAMME 2.1 Fine grained concrete mixes In this study two fine grained concrete mixes were examined. These binder systems typically show a highly flowable consistency achieved by a small maximum grain size of.6 mm, high binder contents, and different pozzolanic additives and plasticisers. Table 1 shows the mix proportions of the mix FGC1, featuring a very low w/b ratio of., a high binder content as well as a high content of silica fume. This mix was chosen due to its good bond between matrix and fibres. Alternatively, a second mix FGC2 with a reduced binder content was developed for practical reasons. The compressive and flexural strength were determined and are shown in Table 1. All mixes were mixed in a mortar mixer for minutes. All test specimens were cured at a temperature of 2 C and a relative humidity of 9 % for 24 hours. Afterwards, they were water stored for 6 days until the test. 2.2 Fibres Several types of fibres were investigated. As a future application of hybrid fibres in combination with textiles is aimed at, it seemed reasonable not to exceed 1 mm in fibre length to ensure a good workability and penetration of the textiles. The only exception is the aramid fibre with a length of 2 mm. The fibres were made of glass, carbon, steel and synthetics. If available, a short (~-6 mm) and a long version (~12-1 mm) of each fibre were used to investigate the influence of the fibre length. In case of the steel fibres, three different shapes were chosen: plain, circular and rectangular fibres with a high thinness and corrugated fibres. The glass, carbon and synthetic fibres differ only in the dispersibility. All fibres used are listed in Table 2:

3 1 st International Conference Textile Reinforced Concrete (ICTRC) 37 Table 1: Mix proportions and mechanical properties Mix components Unit FGC1 FGC2 Cement CEM I Fly ash 21 1 Silica fume kg/m Binder content 14 1 Water 3 4 Superplasticiser % by mass of binder content w/b ratio -..4 Quartz powder kg/m 3 Sand Mechanical Properties (7 d) Compressive strength f c 99 3 N/mm² Flexural strength f ct,fl Table 2: Fibre types and properties Dimensions Tensile E Fibre Type L D Geometry Strength Density mm µm N/mm 2 g/cm 3 S1-S Steel 1, short 6 S1-L Steel 1, long 12,7 17 Straight ~ S2-S Steel 2, short h=24 S2-L Steel 2, long 1 w=1 Straight ~ S3 Steel 3 16 Corrugated G1-S Glass, short 6 Straight, 14 2 G1-L Glass, long 12 4 tex strands C1-XS Carbon, short 3 Straight, C1-S Carbon, medium 6 7 water dispersible C1-L Carbon, long P1 Aramid 2 12 Straight, strands P2 PVA* Straight P3 PVA* Straight P4 Polypropylene 1 n/a Straight * Polyvinyl Alcohol 2.3 Bending tests It was not intended to increase only the strain capacity which very often results in a loss of strength, e.g., Engineered Cementitious Composites (ECC). Therefore, the different fibres were first investigated in a parametric study with varying fibre material, fibre length and fibre content with respect to their contribution to an increased tensile strength. The tests presented here were carried out with prisms (4 mm/4 mm/16 mm) in a 3-point bending test with a span of 1 mm according to the German Standard DIN EN 196-1:199- [DIN]. The load was increased at a constant rate of. kn/s. Of each fibre type a short fibre length of - 6 mm and a longer fibre length of 1-1 mm was examined. A fibre content of 1 % and 3 % by volume was chosen. All bending tests were carried out at a concrete age of 7 days due to

4 38 M. HINZEN, W. BRAMESHUBER: Short Fibre Cocktails in Fine Grained Concrete the rapid strength development of the cement. Based on these results, fibre mixes that seemed promising were composed. In most cases 3 % by volume of a steel fibre acting as macrofibre were combined with 1 % by volume of a microfibre. Selecting these fibre volumes, the results of the hybrid fibre concretes could be compared to the results of the single fibre concretes and possible synergistic effects could be pointed out. In a next step, the post-peak behaviour of concretes with selected microfibres, macrofibres and fibre mix was determined. The tests were carried out on flatter prisms with a height of only 2 mm instead of 4 mm to better visualise the bending behaviour. The test machine was displacement controlled. As there are no guidelines concerning the loading rate, a reasonable value of. mm/min. was chosen. Due to the relatively high deflection of several millimetres at the mid-section of the beam, the settlement of the supports was neglected. For the same reason the cross-head displacement of the testing machine was recorded instead of measuring the deflection of the prism. In these preliminary examinations on the post-peak behaviour, the specimens were loaded in a 3-point bending test due to the much easier test execution and in order to maintain the comparability to the previous parametric study. Further investigations will be carried out in 4-point bending and uniaxial tensile tests for a better assessment of the crack development. The results of these tests are stress-deflection curves that give a first impression of how the different fibre types improve the first crack stress and the post-cracking behaviour. They are presented and interpreted in the following section. 3 RESULTS AND DISCUSSION 3.1 Parametric study The fine-grained concrete mix FGC1 with a high binder content was chosen to have an optimum bond between fibre and matrix. The flexural strength of this concrete without fibre reinforcement is 13.1 N/mm². Figure 2 shows the flexural strengths of the fibre-reinforced concretes depending on two different volume fractions for each fibre type. In most cases, a higher fibre content leads to an increase in flexural strength. An opposed behaviour was observed for one fibre type only. The flexural strength of the concrete with polypropylene fibres decreases with increasing fibre content. The reason therefore is the low tensile strength of the polypropylene fibres which is not sufficient to bridge the cracks. The increasing fibre content reduces the workability and compactibility and the cross-section of the specimens is more and more weakened. In case of the short steel fibre S2-S and both glass fibres G1-S and G1-L, a fibre content of only 1 % by volume turned out to be insufficient to absorb the energy released by the crack. The fractured surfaces of the specimens show that the fibres of S2-S and G1-S are pulled out of the matrix, whereas the longer G1-L fibres break. Hence, the flexural strength of the plain concrete cannot be exceeded in these cases. However, the use of a glass fibre dosage of 3 % by volume leads to an improvement of the flexural strength. The carbon fibres show a contrary behaviour. Already small amounts of carbon fibres (1 % by volume) increase the flexural strength significantly. As very short carbon fibres of only 3 mm also break in bending tests, it can be assumed that the critical length of the fibre is very short. Thus, the high tensile strength of the carbon fibres can be fully activated. In case of a fibre content of 3 % by volume, however, only a small further increase in flexural strengths can be achieved. One possible reason is the high specific surface of the carbon fibres and the

5 1 st International Conference Textile Reinforced Concrete (ICTRC) 39 resulting poor workability of the concrete. Hence, a good compaction is difficult. A substantial improvement of the flexural strength was achieved by aramid fibres. A dosage of 1 % by volume led to a flexural strength of 18.2 N/mm 2 and with a fibre content of 3 % a flexural strength of 36.2 N/mm 2 was reached. This can be explained by the high tensile strength and the length of the fibre combined with a good bond to the matrix. As expected, the steel fibres showed good results and lead to a higher flexural strength with rising fibre content. Due to their low specific surface, high volume fractions are possible. Only the corrugated steel fibre turned out to be unsuited as it fractures the embedded matrix during pull-out. 4 3 in N/mm 2 1 % fibre volume 3 % fibre volume Figure 2: plain S1-S S1-L S2-S S2-L S3 G1-S G1-L C1-S C1-L concrete Influence of the fibre content on the flexural strength P1 P2 P3 P4 For four different fibre types, the influence of the fibre length at a constant fibre content is shown in Figure 3. With increasing fibre length, the steel fibres S1 and S2 lead to a higher flexural strength provided that the fibres do not exceed the critical fibre length. The fibre S1 shows better results possibly due to a higher tensile strength and better bond to the matrix. In case of the glass fibres, the situation is different. No significant increase in the flexural strength resulted from the use of longer fibres. The fractured surface shows that most of the longer fibres are broken. Hence, as far as this mix is concerned, the longer fibres exceed the critical length of the glass fibres and cannot activate their larger bond surface. A similar behaviour can be observed in case of the carbon fibres. No pulled-out fibres are visible on the fractured surface. Thus, the critical length of the water-dispersible carbon fibres must be very short. Considering the fact that longer fibres reduce the number of fibres, an increase in the flexural strength cannot be expected. It is known from literature [Mar3, Ban4] that the combination of microfibres and macrofibres is advantageous. Both fibre types provide reinforcement at different fracture levels and may complement each other. Based on the previous results it seems reasonable to use the long versions of both steel fibres as well as the aramid fibre as macrofibre. However, the workability of 3 % by volume of the aramid fibre was problematic and therefore the steel

6 4 M. HINZEN, W. BRAMESHUBER: Short Fibre Cocktails in Fine Grained Concrete fibres were chosen in this investigation. Carbon fibres were chosen as microfibres due to their mechanical properties and the good results yielded with small dosages in N/mm short fibre version long fibre version plain concrete S1 1 % S1 3 % S2 1 % S2 3 % G1 1 % G1 3 % C1 1 % C1 3 % Figure 3: Influence of the fibre length on the flexural strength in N/mm % S1-L 1 % C1-S 3 % S2-L 1 % C1-S Steel 1 Carbon Steel 2 Carbon % S2-L 1 % G1-S Steel 2 Glass % S2-L 1 % S1-S Steel 2 Steel % S2-L 1 % P4 Steel 2 PP % S2-L 1 % P2 Steel 2 PVA in N/mm steel carbon %.39 combination 3% steel 1mm (S2-L) 1% carbon 6mm (C1-S) steel % PP 17.9 combination 3% steel 1mm (S2-L) 1% PP 1mm (P3) Figure 4: Flexural strength of hybrid fibre reinforced concrete Figure : Sample of synergistic effects of hybrid fibre reinforced concrete In case of the steel fibre S1 a combination of 3 % by vol. steel and 1 % by vol. carbon led to a flexural strength of 31.8 N/mm 2 and in case of the steel fibre S2 38. N/mm 2 were achieved (see Figure 4). Although the fibre S2 provided lower flexural strengths when acting alone, it seems very efficient in combination with a microfibre. Therefore, also glass, steel S1, polypropylene and PVA fibres were examined as microfibres. The flexural strengths, however, remained

7 1 st International Conference Textile Reinforced Concrete (ICTRC) 41 below the fibre mixes with carbon. All fibre mixes were additionally investigated in terms of synergistic effects. In some cases, the combination of fibres turned out to be ineffective as the flexural strength was lower than the sum of the single fibres. But also significant synergistic effects were found, two interesting examples of which are indicated in Figure. The increase in flexural strength resulting from two single fibres in comparison to the increase that is achieved by the hybrid fibres is demonstrated. In the first case, 3 % by volume of the steel fibre improves the flexural strength of the concrete by 14.1 N/mm 2 while 1 % by volume of the carbon fibre leads to an increase of 6.34 N/mm 2 when acting alone. However, the combined use of both fibres increases the flexural strength by.3 N/mm 2 which is 24 % more than the sum of the individual improvements. This gain in strength can be ascribed to a more effective work of the steel fibres resulting from the carbon fibres that reinforce the matrix around the steel fibre. This behaviour is also described in detail in [Kaw3]. In the second case, polypropylene is combined with steel. Although polypropylene on its own does not improve the flexural strength of the concrete, the improvement by the hybrid fibres is 4 % higher than the sum of the individual improvements. 3.2 Post-cracking behaviour When combining short fibres, normally microfibres and macrofibres are used. Microfibres are capable of increasing the first crack stress of the concrete by reducing and delaying the microcrack formation. After the formation of a first macrocrack the macrofibres allow a further increase in flexural stress bridging the cracks. Hence, the interaction between microfibres and macrofibres leads to a significant improvement of flexural strength. Subsequently, the macrofibres either break or are pulled out. For a high ductility a fibre pullout under high loads is aimed at. To investigate these properties, stress-deflection curves of fibre concrete with several microfibres and macrofibres have been determined using the mix FGC2 and a fibre content of 2 % by volume. Figure 6 compares stress-deflection curves of microfibre reinforced concretes. flexural stress in N/mm % by vol. Carbon (6mm) 2 % by vol. PVA 1 (8mm) 2 % by vol. Polypropylene (1mm) flexural strength of plain concrete 2 % by vol. Glass (6mm) Figure 6: deflection in mm Stress-deflection curves of fibre reinforced concrete with several microfibres

8 42 M. HINZEN, W. BRAMESHUBER: Short Fibre Cocktails in Fine Grained Concrete The used fibres are carbon fibres (6 mm), PVA fibres (8 mm), polypropylene fibres (1 mm) and glass fibres (6 mm). As microfibres are supposed to increase the first crack stress, special attention was paid to the linearity of the stress-deflection curve. Referring to this, the polypropylene fibre is not suitable due to its low stiffness. Although the PVA fibre leads to a relatively high flexural strength of the concrete, its stiffness is not much higher than that of concrete. Therefore, the first crack stress cannot be significantly increased. A small rise in the first crack stress can be reached using glass fibres. In this case, the stress-deflection curve is linear up to a flexural stress of about 8 N/mm 2, which is an increase of 36 % compared to plain concrete. However, with the use of carbon fibres the highest rise in the first crack stress can be achieved as a result of the high strength and the high Young s modulus of carbon. The stress-deflection curve is linear up to a flexural stress of 11 N/mm 2 which is an improvement of 9 %. This leads to the conclusion that for the used fine grained concretes the carbon fibre is the most effective fibre to yield an increase in the first crack stress. In Figure 7 the behaviour after the first macrocrack is primarily examined. It is important for the intended combination of short fibres and fabrics that the fibres allow a strain hardening and that they are pulled out of the matrix under high loads to allow a constant load transfer from the matrix to the fabric. However, conclusions cannot be drawn from a deflection hardening observed in bending tests to the strain hardening in a uniaxial tensile test [Naa3]. Hence, the measured curves can only give a clue to a possible strain hardening. Due to their high Young s moduli, the deflection hardening of both steel fibres is more distinct than that of the aramid and the glass fibres. In combination with the high initial bond, the highest flexural strengths are achieved by the steel fibres. Referring to the pull-out behaviour of the fibres which mainly influences the ductility, all fibre types behave similar. However, the aramid fibres and the steel fibres S1 are slightly better than the steel S2 and the glass fibres. 2 1 flexural stress in N/mm 2 2 % by vol. Steel 2 (1mm) 2 % by vol. Steel 1 (12.7mm) 2 % by vol. Aramid (2mm) 1 2 % by vol. Glass 1 (12mm) deflection in mm Figure 7: Stress-deflection curve of fibre reinforced concrete with several macrofibres

9 1 st International Conference Textile Reinforced Concrete (ICTRC) 43 To demonstrate the interaction between micro- and macrofibres regarding their stressdeflection behaviour, two fibre mixes are demonstrated in Figure 8. For both mixes the macrofibre reinforced concrete is presented as well as the concrete with an additional microfibre reinforcement. A macrofibre content of 2 % by volume and, depending on the workability of the mix, a smaller amount of carbon fibres was used as carbon showed the best results in the previous investigation. Both fibre mixes show that the positive properties of the single fibres can be added combining them. Some properties even show synergistic effects. It can be seen that the linearity of the stress-deflection curve of both concretes is improved by the addition of a small amount of carbon fibres. Furthermore, the flexural strength of the fibre mix with aramid increases significantly. This is not the case with steel fibres. However, the pull-out behaviour of the steel fibre mix is improved by the addition of carbon fibres. Within the framework of this study the fibre mix with aramid and carbon fibres showed the best result and a distinct synergistic effect. 2 flexural stress in N/mm 2 2 % by vol. Aramid (2mm). % by vol. Carbon (3mm) 2 % by vol. Aramid (2mm) 1 Figure 8: 1 2 % by vol. Steel 1 (12.7mm) 2 % by vol. Steel 1 (12.7mm).7 % by vol. Carbon (6mm) deflection in mm Stress-deflection curves of single macrofibre reinforced concretes compared to concretes reinforced by hybrid fibres 4 CONCLUSIONS Based on the results presented in this paper the following conclusions can be drawn: Except for the polypropylene fibre, the flexural strength of the concrete grows with increasing fibre content. With a volume fraction of 3 %, the aramid fibre provides the highest flexural strengths. When using steel fibres, an increase of the fibre length at a constant fibre content leads to a higher flexural strength.

10 44 M. HINZEN, W. BRAMESHUBER: Short Fibre Cocktails in Fine Grained Concrete When using glass fibres, the influence of the fibre length is small. For concretes with a good bond between fibre and matrix, the critical fibre length may lie below the fibre length. In this case, most fibres break. For the fine grained concretes used in this investigation the critical length of the carbon fibres is very short because they always break. The flexural strength decreases with increasing fibre length. The combination of fibres is not advantageous in all cases. However, the addition of carbon fibres mostly leads to high flexural strengths and synergistic effects. For a significant increase in the first crack stress carbon fibres are most suitable. Due to their good initial and friction bond, the steel fibres show the most distinct deflection hardening. Altogether the best result was achieved with the combination of aramid and carbon which generates great synergistic effects. The investigations in this paper provide a basis for upcoming uniaxial tensile tests with a combined use of fabrics and short fibres. Information on the influence of fibre content and fibre length as well as on the combination of short fibres has been obtained. The measured stress-deflection relations furnish information on first crack stress, deflection hardening and pull-out behaviour of several micro- and macrofibres. REFERENCES [Ban4] [Bra1] [Bro6] [DIN] [Kaw3] [Mar3] [Mie] [Naa3] Banthia, N. ; Gupta, R.: Hybrid Fiber Reinforced Concrete (HyFRC): Fiber Synergy in High Strength Matrices. In: Materials and Structures (RILEM) 37 (24), Nr. 274, S Brameshuber, W.; Brockmann, T.: Development and Optimisation of Cementitious Matrices for Textile Reinforced Elements. London: Concrete Society, In: Proceedings of the 12th International Congress of the International Glassfibre Reinforced Concrete Association, Dublin, May 21, S Brockmann, T.: Mechanical and fracture mechanical properties of fine grained concrete for textile reinforced composites. Ph.D. thesis - In: Schriftenreihe Aachener Beiträge zur Bauforschung, Institut für Bauforschung der RWTH Aachen (26), Nr. 13 DIN EN 196-1:- Prüfverfahren für Zement Teil 1: Bestimmung der Festigkeit; Deutsche Fassung EN 196-1: Kawamata, A.; Mihashi, H. ; Fukuyama, H.: Properties of Hybrid Fiber Reinforced Cement - Based Composites. In: Journal of Advanced Technology 1 (23), Nr. 3, S Markovic, I. ; Walraven, J.C. ; Mier van, J.G.M.: Development of High Performance Hybrid Fibre Concrete. Bagneux : RILEM, In: Fourth International Workshop on High Performance Fiber Reinforced Cement Composites (HPFRCC4), Ann Arbor, USA, June, 1-18, 23, (Naaman, A.E. ; Reinhardt, H.W. (Ed.)), S Mier van, J.G.M. ; Stähli, P.: Development of Hybrid Fibre Concrete: Manufacturing, Material Fibre Concrete and Mechanical Properties, Stuttgart : ibidem,. - In: Hochduktile Betone mit Kurzfaserbewehrung : Entwicklung - Prüfung - Anwendung, Tagungsband, Kaiserslautern, Oktober, (Mechtcherine, V. (Ed.)), S. -68 Naaman, A.E.: Strain Hardening and Deflection Hardening Fiber Reinforced Cement Composites. Bagneux : RILEM, In: Fourth International Workshop on High Performance Fiber Reinforced Cement Composites (HPFRCC4), Ann Arbor, USA, June, 1-18, 23, (Naaman, A.E. ; Reinhardt, H.W. (Ed.)), S