STRUCTURAL BEHAVIOUR OF HYBRID CONCRETE BEAMS WITH FIBRE REINFORCED LIGHTWEIGHT CONCRETE

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1 BEFIB212 Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 212 STRUCTURAL BEHAVIOUR OF HYBRID CONCRETE BEAMS WITH FIBRE REINFORCED LIGHTWEIGHT CONCRETE Linn G. Nes *, Jan A. Øverli 1 * Norwegian University of Science and Technology, Department of Structural Engineering, Rich. Birkelandsvei 1A, N-7491 Trondheim, Norway. linn.g.nes@ntnu.no, web page: 1 Norwegian University of Science and Technology, Department of Structural Engineering, Rich. Birkelandsvei 1A, N-7491 Trondheim, Norway. jan.overli@ntnu.no, web page: Keywords: steel fibres, lightweight concrete, small-scale tests, hybrid concrete beams. Summary: An experimental program has been conducted at the laboratories at NTNU including smallscale tests on fibre reinforced lightweight concrete (FRLWC) and 4-point beam tests on larger hybrid concrete beams. The hybrid beams consisted of layers of FRLWC and normal concrete (NC). The small-scale tests included uni-axial tension tests (UATT) and 3-point bending tests (3PBT). Fibre counting has also been carried out. Material parameters influenced by the fibre content obtained in the small-scale tests constitute the basis for design of hybrid concrete beams. 1 INTRODUCTION The motivation for investigating the performance of hybrid concrete beams is to design a structural element which utilises the most beneficial properties of different materials and combine them in one cross-section. The hybrid beams may represent one-way slab elements of which the bottom layer(-s) constitutes a precast formwork, resulting in a cost-effective product with respect to manufacturing, transport/assembling of the element and load capacity. Use of light-weight concrete minimizes the self-weight and the structural performance is taken care of by the conventional longitudinal tensile reinforcement and the 5cm top layer of normal concrete. In order to improve the performance and material properties of the lightweight concrete steel fibre reinforcement was used (Dramix 65/35 Steel Fiber). Another motivation for the experimental program was then to study different effects of fibre reinforcement. The testing on small-scale specimens consists of two series of UATT and 3PBT. The first series, denoted Small-scale Program I, was conducted in spring 21 as a preliminary study of the material properties and performance of FRLWC. The second series, Small-scale Program II, was carried out in autumn 21 and included more FRLWC specimens. Fibre counting was also carried out in order to relate the performance of FRLWC to the current number of fibres. Small-scale Program I constituted the basis for design of the hybrid beams in the first test series on larger hybrid beams, identified as Program 1. Four beams were designed for flexural failure and the FRLWC contained 1.% steel fibres. The second series on hybrid beams, Program 2, consisted of eight beams designed for shear failure, containing either %,.5% or 1.% fibre reinforcement. The last series of larger beam testing was conducted on eight beams originally designed for moment failure. Four of these beams contained.5% steel fibres and the remaining beams contained 1.% fibres. In order to study the effect of fibre reinforcement two reinforcement designs were used. Figure 1 shows the cross-sections of the two types of hybrid concrete beams with layers of FRLWC and NC. The cross-section to the left was tested in Program 1 and the cross-section to the right was

2 BEFIB212: Linn G. Nes and Jan A. Øverli. used in Program 2 and 3. Figure 1: Cross-sections of the two types of hybrid concrete beams 2 SMALL-SCALE TESTS Even though the fibre reinforced lightweight concrete was only used as filling material, it was important to investigate the performance of this type of concrete. The small-scale testing was carried out on FRLWC specimen containing either.5% or 1.% steel fibres. Table 1 provides an overview of the experimental program on small-scale specimens. In order to obtain the low density of about 125kg/m 3, both foam and aggregate of expanded clay were used. The casting of the 3PBT specimens was conducted in accordance with NS-EN [1] and casting of the uni-axial tension test specimens was carried out in a similar way. After demoulding all specimens were stored either in water/tanks or covered with wet burlap sacks and plastic foil, providing 95-1% humidity. Table 1 : Overview of small-scale tests Small-scale Program I Small-scale Program II Type of test Number of specimen Fibre content Type of test Number of specimen Fibre content 3PBT 3 1% 3PBT 6.5% UATT 3 1% 3PBT 6 1.% UATT 6.5% The uni-axial tension test was conducted as a modification of both the RILEM Recommendation [2] and a method described by SINTEF [3]. The specimen size was 1x1x6 [mm] and a notch was located at the middle of the specimen. The notch had a width of 4mm and a depth of 1mm on each side of the specimen. The displacement was measured using transducers located on two opposite sides of the specimen over a distance of 1mm. Figure 1 shows a picture of the test. 2

3 Load [kn] Load [kn] BEFIB212: Linn G. Nes and Jan A. Øverli. Section A-A Figure 2: Uni-axial tension test Figure 3: 3-point bending test The 3-point bending tests were conducted according to the Norwegian Standard, NS-EN [1]. The test setup is shown in Figure 2. The notch had a width of 4mm, a depth of 25mm and the deflection was measured using a displacement transducer connected to a thin plate located across the notch mouth. 2.1 Small-scale Program I Due to a large total number of specimen tested at the same time, the number of specimen of each kind was limited to three. Only results from testing of specimen with 1.% fibre content will be presented. The achieved mean compressive cylinder strength for the FRLWC in this test series was f c = 16.4N/mm 2 and the mean density was 1246kg/m 3. Figure 4 shows the obtained results from the UATT which indicate strain hardening in the initial part of the diagram. Figure 5 shows the results from 3PBT. The responses are smooth and continuous UATT1S-1 UATT1S-2 3 UATT1S PBT1S-1 3PBT1S-2 3PBT1S Figure 4: Load-displacement diagrams from uniaxial tension tests, Small-scale Program I Figure 5: Load-deflection diagrams from 3-point bending tests, Small-scale Program I The uni-axial tensile strength varied from N/mm 2 and Young s Modulus was calculated to approximately E = 12N/mm 2. 3

4 Load [kn] BEFIB212: Linn G. Nes and Jan A. Øverli. 2.2 Small-scale Program II Based on the results from Small-scale Program I, an extended investigation was carried out in this series which included more specimens and fibre counting. The obtained mean compressive cylinder strength for the FRLWC is listed in Table 2 based on fibre content. Table 2 : Compressive strength and density FRLWC Small-scale Program II Fibre content Mean compressive strength f c [N/mm 2 ] Mean density ρ [kg/m 3 ] % % % Load-displacement diagrams from the UATT on specimen with.5% fibre content is shown in Figure 6. The obtained uni-axial tensile strengths ranged from N/mm 2 and the responses show that there is a relatively large scatter. However, none of the specimens experienced a dramatic loss of capacity after cracking. The results also show that there is a large scatter in the results after the point of cracking, i.e. when only the fibres are assumed to carry load. Varying number of fibres crossing the crack is assumed to be the reason for the large scatter UATT5A-1 UATT5A-2 UATT5A-3 UATT5A-4 UATT5A-5 UATT5A-6 Figure 6: Load-displacement diagrams from uni-axial tension tests, Small-scale Program II 4

5 Load [kn] BEFIB212: Linn G. Nes and Jan A. Øverli. Load-deflection responses from the 3-point bending tests are shown in Figure 7. In this diagram the ID 3PBT5A-i denotes specimen with.5% fibre content, and specimen 3PBT1A-i has 1.% fibres PBT5A-1 3PBT5A-2 3PBT5A-3 3PBT5A-4 3PBT5A-5 3PBT5A-6 3PBT1A-1 3PBT1A-2 3PBT1A-3 3PBT1A-4 3PBT1A-5 3PBT1A-6 Figure 7: Load-deflection diagrams from 3-point bending tests, Small-scale Program II The results show that the scatter is large and that there is almost no effect of increased fibre content. However, the curves are smooth and continuous which indicates a good interaction between fibres and concrete. 2.3 Fibre counting on small-scale specimen Due to the large scatter in the results from the small-scale tests fibre counting is carried out. On the UATT specimens two approaches were employed. Fibre counting was carried out on a section about 5mm to the side of a failure surface and directly on the failure surface, section A-A and B-B respectively in Figure 8. On Section B-B a rough counting was performed and only the fibres assumed to carry load were counted. Section A-A was sawn and all fibres crossing this section was counted. Hence, the results on this section will only provide an indirect estimate of the amount of fibres that would actually have contributed to the load carrying. Figure 8: Load-deflection diagrams from 3-point bending tests, Small-scale Program II 5

6 Number of fibres Number of fibres at section A-A Number of fibres at section B-B BEFIB212: Linn G. Nes and Jan A. Øverli. The results from the fibre counting are presented as a relationship between number of fibres and tensile strength in diagrams, see Figure 9 and Tensile strength [N/mm 2 ] UATT5A-1 UATT5A-2 UATT5A-3 UATT5A-4 UATT5A-5 UATT5A Tensile strength [N/mm 2 ] UATT1S-1 UATT1S-2 UATT1S-3 UATT5A-1 UATT5A-2 UATT5A-3 UATT5A-4 UATT5A-5 Figure 9: Number of fibres at section A-A vs tensile strength Figure 1: Number of fibres at section B-B vs tensile strength The results indicate that the tensile strength depends on the number of fibres. This relationship is strongest in Figure 1 which provides the most direct method as the fibres are counted at the same section of which the tensile strength is measured. Fibre counting on the 3PBT specimens is carried out on a section 5mm to the side of the notch. The results do not show the same strong relationship between load capacity and number of fibres, see Figure 11. When comparing only specimens with.5% fibres there is an increase in load capacity with increasing number of fibres. However, when these results are compared with specimens with 1.% fibres the effect of increased fibre content vanishes. A possible explanation for the lack of increase might be the orientation of fibres which seems to be more uni-directed in the specimens with.5% fibres. In the diagram the residual flexural tensile strength is calculated at a CMOD = 2.5mm according to the Proposal for Norwegian guidelines for design, execution and control of fibre reinforced concrete [4]. 65 1A A-2 1A-3 1A A-5 1A A-1 5A A Res. flexural tensile strength f R,3 [N/mm 2 ] Figure 11: Number of fibres at section A-A vs residual flexural tensile strength For comparison the number of fibres is related to the cross-section area of the section 5mm to the side of the notch, see Table 4. For the specimens with.5% fibre content the number of fibres per mm 2 ranges from For specimens with 1.% fibres this ratio ranges from

7 BEFIB212: Linn G. Nes and Jan A. Øverli. Table 3 : Number of fibres per mm 2 at section A-A UATT Number of fibres per mm 2 3PBT - Number of fibres per mm 2 3PBT - Number of fibres per mm 2 5A A A A A A A A A A A A A A A A A A HYBRID BEAM TESTS In total 2 hybrid concrete beams have been subjected to the 4-point bending test designed for either flexural or shear failure. The intention was to investigate how the FRLWC and NC interacted with special focus on the interface between the layers. Another motivation was to study the performance of the FRLWC due to its low density and relatively poor material properties. In order to strengthen the FRLWC different amounts of steel fibres were used. The performance of the different beams is also compared with the calculated capacity. 3.1 Program 1 Beams designed for moment failure Based on the preliminary investigation from Small-scale Program I, four hybrid beams were designed for flexural failure. Figure 12 and 13 show the geometry and reinforcement of two of these beams, identified as HB3 and HB4. Two similar beams were also tested but without the top layer, identified as HB1 and HB2. These beams were intended to represent a slab in the construction phase. Figure 12: Geometry, bar reinforcement and loading for the sandwich beam element (measures in mm) Figure 13: Cross-section of the beam (measures in mm) By employing guidelines for design of fibre reinforced structures, the design failure load of the beams with top layer was 36.5kN and 24.4kN for the beams without top layer. The theoretical compression zone at failure was limited to the top layer and yielding of the reinforcement was designed to initiate failure. The surface of the FRLWC layer was relatively rough due to the aggregate (pellets). The load-displacement diagrams for the beams are given in Figure 14. 7

8 Load [kn] Load [kn] BEFIB212: Linn G. Nes and Jan A. Øverli HB1 - without top layer HB2 - without top layer HB3 - with top layer HB4 - with top layer Figure 14: Load-deflection response of hybrid beams Program 1 During the tests no problems were registered with the interface or the performance of the FLRWC layer. Figure 15 and 16 show the crack patterns at failure for the beams HB2 and HB4 respectively. Figure 15: Moment failure of beam without top layer, HB2 Figure 16: Moment failure of beam with top layer, HB4 3.2 Program 2 - Beams designed for shear failure Eight beams were designed for shear failure focusing on studying the influence of different fibre content. The test setup is shown in Figure Sa Sb S,5a S,5b S,5c S1,a S1,b S1,c Figure 17: Geometry and reinforcement for beams designed for shear failure (measures in mm) Figure 18: Load-displacement diagrams, shear failure 8

9 Load [kn] BEFIB212: Linn G. Nes and Jan A. Øverli. Two beams had no fibre reinforcement (Va,b), three beams contained.5% (S,5a,b,c) and the last three beams contained 1.% fibres (S1,a,b,c). The load-displacement diagrams for these beams are given in Figure 18. Despite the scatter, the results indicate that the maximum load increases with increasing fibre content which is supported by results from fibre counting. The calculated capacity employing the guidelines given in Eurocode 2 [5] show that the shear resistance for the lightweight concrete without fibres is overestimated, V Rm,c = 2.5kN ( F c = 41kN). Employing the Norwegian design rule proposal for fibre reinforced concrete [4] the characteristic shear capacity for beams with both.5% and 1.% fibre content is V Rc = 31.3kN ( F c = 62.6kN). Comparing the characteristic capacity with the achieved results, only the beams with 1.% fibre content reach higher loads than calculated. Note that the calculations on the fibre contribution are based on a similar fibre distribution and orientation of fibres as the small-scale 3-point bending test. The results show that despite the lack of increase in load capacity with increasing fibre content for the 3PBT in Small-scale Program II, the shear resistance increases with an increase in fibre content. A possible explanation is the orientation of fibres. Results from fibre counting show that the number of fibres crossing a plane parallel to the diagonal crack is higher for beams with 1.% fibre reinforcement than for beams with.5% fibres. Figure 19, 2 and 21 show the crack patterns at failure for different beams. Figure 19: Crack pattern at failure beam Sa Figure 2: Crack pattern at failure beam S,5a Figure 21: Crack pattern at failure beam S1,a 3.3 Program 3 - Beams designed for moment failure Another eight beams were originally designed for flexural failure and the tests focused on studying the influence of different fibre content (.5% and 1.%) and two different designs of reinforcement. Reinforcement design 1 Four beams were designed with conventional shear reinforcement, shown in Figure 22. Two of the beams contained.5% steel fibres and two beams contained 1.% fibres M1-,5a M1-,5b M1-1,a M1-1,b Figure 22: Geometry and reinforcement for beams designed for flexural failure, design 1 (measures in mm) Figure 23: Load-displacement diagrams, flexural failure 1 9

10 Load [kn] BEFIB212: Linn G. Nes and Jan A. Øverli. All beams experienced flexural failure and the load displacement diagrams are given in Figure 23. The maximum load was approximately the same for all the beams and ranged from 7-73kN at a displacement of mm. The calculated characteristic load capacity is 63.1kN. Figure 24: Crack pattern at failure of beams M1-,5b and M1-1,a The crack patterns at failure for beams M1-.5b with.5% fibre reinforcement (top beam) and M1-1,a with 1.% fibre reinforcement (lowest) is shown in Figure 24. The picture shows that the cracks in the beam containing.5% fibres tend to split when developing. For the beam M1-1,a the fibres seems to limit the crack development leading to a somewhat stiffer behaviour. Reinforcement design 2 The last four beams had no vertical reinforcement, see Figure 25. Two beams contained.5% steel fibres and two beams had 1.% fibres. The load-deflection responses are given in Figure M2-,5a M2-,5b M2-1,a M2-1,b Figure 25: Geometry and reinforcement for beams designed for flexural failure, design 2 (measures in mm) Figure 26: Load-displacement diagrams, flexural failure 2 All of these beams experienced shear failure although the preliminary design indicated flexural failure. Note that the maximum load for the beams containing 1.% fibres is close to the failure load for the similar beams with reinforcement design 1. The fibres are not capable to prevent the initiation of severe diagonal cracks which lead to shear failure. However, strain measures indicate that the beams post-cracking load capacity is large enough to cause yielding of the reinforcement. The calculated and achieved capacities of these beams are listed in Table 4. These results show that based on mean estimated values, moment failure would happen. Based on characteristic values - shear failure should be expected. The experience from Program 2 indicated that shear failure would 1

11 BEFIB212: Linn G. Nes and Jan A. Øverli. occur, at least for the beams with.5% fibre content. However, the achieved failure loads for the beams containing.5% fibres show that both the characteristic and mean estimated capacities are overestimations. For the beams with 1.% fibres the calculated characteristic shear and moment capacities are both lower than the achieved failure load. Again it should be noted that the calculations on the fibre contribution are based on a similar fibre distribution and orientation of fibres as the smallscale 3-point bending test. Table 4 : Calculated and achieved load capacities for M2 beams M2-.5a/b 1.a/b Serious diagonal crack development [kn] Failure load [kn] / /65 Char. calculated load capacity, shear [kn] Mean calculated load capacity, shear [kn] Char. calculated load capacity, moment [kn] Mean calculated load capacity, moment [kn] Figure 27 and 28 show the crack patterns at failure for the beams M2-.5b and M2-1.a. Figure 27: Crack pattern at failure beam M2-.5b Figure 28: Crack pattern at failure beam M2-1.a 5 SUMMARY Based on experiments and fibre counting, the performance of this type of FRLWC is very dependent on the fibre content. The uni-axial tensile strength increases with increasing number of fibres crossing the crack. However, results from the small-scale 3PBT do not show the same tendency. The beams containing 1.% fibres achieved approximately the same load capacity as the beams with.5% fibre reinforcement and the scatter was large. A possible explanation is the fibre orientation which seems to be more uni-directed in the longitudinal direction of the beams with.5% fibre content. This explanation is supported by the results from the shear tests in Program 1 on hybrid beams. Considering the number of fibres normal to a plane parallel to the diagonal crack, this number is higher for the beams with 1.% fibres than for beams with half the fibre content. Hence, the results support the assumption of increase in shear resistance with increasing number of fibres. For the beams designed for moment failure containing both fibre reinforcement and conventional shear reinforcement, the fibres do not provide a large contribution to the load capacity. However, the fibres are able to limit the crack development in the FRLWC layer of these beams. The beams without vertical reinforcement in Program 3 experienced shear failure, but the results show that the longitudinal reinforcement in the beams with 1.% fibre reinforcement yields at failure. 11

12 BEFIB212: Linn G. Nes and Jan A. Øverli. 5 CONCLUSION For the type of lightweight concrete used in this study, the performance is considerably improved by use of fibre reinforcement. Both the tensile strength and the capability of transferring stresses after cracking increase with increasing fibre content. Results from fibre counting also indicate that specimen with 1.% fibre content have a more three dimensional orientation than specimen with.5% fibres. Based on pictures of the crack development at shear failure, increasing fibre content also leads to improved bond between longitudinal bar reinforcement and lightweight concrete. The research presented in this paper is based on work performed in COIN - Concrete Innovation Centre - which is a Centre for Research based Innovation, initiated by the Research Council of Norway (RCN) in 26 [6]. REFERENCES [1] NS-EN 14651:25+A1: Test method for metallic fibre concrete. [2] Vandewalle, L. et al, RILEM TC 162-TDF: Test and design methods for steel fibre reinforced concrete. Uni-axial tension test for steel fibre reinforced concrete, Materials and Structures, Vol 34, Jan-Feb 21, pp 3-6 [3] Skjølsvold, O.: KS Tensile test on concrete Determination of Youngs modulus and tensile strength by uni-axial testing, SINTEF Byggforsk 27 (In Norwegian) [4] Terje Kanstad et al., Proposal for Norwegian guidelines for design, execution and control of fibre reinforced concrete, Technical report COIN 211 [5] NS-EN :24+NA:28: Eurocode 2; Design of concrete structures. Part 1-1; General rules and rules for buildings. [6] COIN. 12