INFLUENCE OF HYBRID FIBRES ON THE FLEXURAL PERFORMA...

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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 4, April 2017, pp. 1189–1195, Article ID: IJCIET_08_04_133 Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=4 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed

INFLUENCE OF HYBRID FIBRES ON THE FLEXURAL PERFORMANCE OF

SELF-COMPACTING CONCRETE Binita J Priyalal

Student, M-Tech Structural Engineering, SCALE, VIT University, Vellore, Tamil Nadu, India

Rama Mohan Rao.Panem

Associate Professor, Centre for Disaster Mitigation and Management, VIT University, Vellore, Tamil Nadu, India

ABSTRACT This paper discusses the experimental results of tests carried out on the flexural

properties of hybrid fibre-reinforced concretes at low volume fractions of fibres upto 1%. Hybrid fibrous concrete was prepared by using both steel and polypropylene fibres. Steel fibres were replaced with various dosages of polypropylene fibres, compared to a reference concrete with 1% steel fibres. The flexural properties such as flexural strength, toughness, and ultimate strength, were measured using four-point bending tests on beam specimens. The best flexural performance was obtained at the hybrid combination of 0.9% steel and 0.1% polypropylene dosage. At this volume fraction, flexural toughness of hybrid fibre concretes were comparable to the mix containing only steel fibre. Optimum fibre availability in the hybrid fibre mixes and the ability of non-metallic fibres of bridging smaller micro cracks, are suggested as the reasons for the enhancement in flexural properties Key words: Micro-cracks, optimum fibre, flexural properties.

Cite this Article: Binita J Priyalal and Rama Mohan Rao.Panem, Influence of Hybrid Fibres on the Flexural Performance of Self-Compacting Concrete. International Journal of Civil Engineering and Technology, 8(4), 2017, pp. 1189–1195. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=4

1. INTRODUCTION The term hybrid fibre reinforced concrete itself indicates the use of two or more types of fibres rationally combined in a common matrix to produce a composite that possess the benefits from each of the individual’s fibres and exhibits a synergetic behaviour. Addition of short discontinuous fibres plays an important role in the improvement of mechanical properties of concrete. It increases elastic modulus; decreases brittleness, controls cracks initiation and its further growth and propagation. Pull out of the fibre require more energy of

Influence of Hybrid Fibres on the Flexural Performance of Self-Compacting Concrete


absorption, resulting in increased toughness and fracture resistance of materials to both cyclic and dynamic loads. Some of the advantages of using hybrid fibre reinforced concrete are

If one of the fibre is stronger and stiffer, it helps in improving first cracks stress and ultimate strength, and the other fibre is more flexible and ductile which helps in improving toughness and strain in the post cracking zone.

If one fibre is smaller, it can bridge the micro cracks. This leads to higher tensile strength of the composite. The other fibre being larger can arrest the further propagating micro cracks and can substantially improve the toughness of the composite.

If the durability of both the fibres are different, it helps to improve the durability properties as a whole. The presence of the durable fibre can increase the strength and toughness relation after age while the other fibre can guarantee short-term performance during transportation, placement and installation of the composite elements.

Hybrid fibre reinforced concrete can be used in any kind of construction because of its unique benefits. It can very easily obtain high range of strength values. Some of its applications are bridges, tunnel linings, building components like column, sandwich structure like steel concrete structure, industrial flooring, machine foundation etc.

This paper deals with self-compacting concrete reinforced with hybrid fibres, thereby aiming to contribute the combined effects of both. Use of self-compacting concrete that has high workability and flow properties helped the high-volume mix to flow through heavily reinforced sections under its own weight. The fibres I have chosen is steel and polypropylene. The main aim of this project work is to prepare a hybrid fibre reinforced concrete with 1% of steel fibre by volume of concrete as control mix and replace polypropylene fibre dosages by 0.05% up to a limit of 0.15% by volume of concrete. The flow properties of hybrid fibre reinforced concrete are compared with the control mix with 1% steel fibres. Then the structural performance of the hybrid fibre reinforced self-compacting concrete in flexure was studied by two-point flexural loading. The type steel fibre used is straight end steel fibre with aspect ratio 50 and polypropylene fibre with aspect ratio 857.

2. LITERATURE REVIEW Nehdi and landanchuk1 used steel and various synthetic fibres for their research work. They investigated the synergistic effects of both micro and macro fibres in SCC. They confirmed that the use of different steel fibres with other synthetic fibres increased the first crack loads and also toughness values. It was also observed that the replacement of steel by other synthetic fibres decreased the compressive, split tensile and flexural strength values but from observations it was confirmed that hybrid fibres controlled the development and propagation of micro-macro cracks and also provided higher stiffness.

Sivakumar2 used metallic fibres and non-metallic fibres at low volume fractions in his research. He confirmed the use of hybrid fibres namely steel-polypropylene, steel-glass, steel-polyester enhanced the ductility, toughness and flexural strength when compared to concrete without fibres. Also, he confirmed that addition of non-metallic fibres bridged the small microcracks in concrete

Tarun and Sanjay3 discussed the effect of mixed fibres on the properties of fibre reinforced SCC. It was seen that the mixes showed a decrease in the fresh properties with increase in the percentage of fibre content

Nan Su, Kung-Chung Hsu and His-Wen Chai4 proposed a simple mix design procedure for SCC and their main focus was on lowering the binder content.

Binita J Priyalal and Rama Mohan Rao.Panem


Rudzinski et al.5 used steel fibres along with PP fibres of similar dimensions and tensile strength but with lower modulus of elasticity. They showed that steel fibres increased strength whereas PP fibres enhanced post cracking behaviour.

Trtik and Vodicka6 used combinations of hooked and straight steel and PP fibres. Their results show that PP fibres increased strength and toughness when used in combination with straight steel fibres, but decreased the overall mechanical properties when used along with hooked steel fibres.

Kobayashi and Cho7 observed similar effects when they used PP and steel hybrid fibre reinforcement, which offset the shortcomings of each fibre, while the individual benefits of both types of fibres were added.

3. MATERIALS USED In this experiment 53 grade ordinary Portland cement (OPC) with brand name Zuari was used for all concrete mixes. Silica fume and GGBS were used as pozzolanic materials. The fine aggregate used for the study was river sand with size passing through 2.36mm sieve. The coarse aggregates used for the work was 12.5mm pass and 4.75mm size retained which is free from deleterious materials like clay, silt content and chloride contamination. Clean potable tap water available in the laboratory, which satisfies drinking standards, was used for the preparation of specimens and for the curing of specimens. Super plasticizers used was Auramix 350, is a high performance super plasticizer suitable for low and high grade concrete and intended for applications where high water reduction and long workability retentions are required. This was obtained from FORSOC Chemicals (India) Pvt. Ltd. Straight steel fibres of length 25mm were used and of equivalent diameter 0.5mm leading to an aspect ratio of 50. The young’s modulus of steel fibres was found to be 2×105MPa. The density of steel fibre was found to be 78500 N/m3. The macro synthetic polypropylene fibres having a length of 24mm and their average thickness was found to be 0.028mm leading to an aspect ratio of 857. The density of polypropylene fibres was found to be 9460 N/m3.

4. EXPERIMENTAL PROGRAM

4.1. Mix Proportion As per the guidelines of EFNARC and using Nan-Su method of mix design trial mixes were prepared and initial tests were conducted to prepare SCC of estimated strength 30 MPa at w/c ratio of 0.35. The following mix proportion was arrived based on the results.

Table 1 Mix Proportion

CEMENT SILICA-FUME GGBS CA FA Water SP Kg/m3 353.5 28.57 257.17 669.06 914.36 248.50 3.19 Ratio 1 0.3 0.7 1.89 2.58 0.7 0.08

4.2. Mixing Procedure Tilting drum mixer was used in preparing SCC. Cement and fly ash was added into the mixer and dry mixed for 60s. Then sand and coarse aggregates were added and dry mixed for another 60s. Water and superplasticizers were added and mixed for 3min. In case of hybrid mixes, at first steel fibres were sprayed into the mixer in small amounts to+ prevent balling after homogenization of the dry ingredients and then polypropylene fibres were added.



4.3. Testing Procedure Fresh concrete tests done were the slump flow, J-ring, V-Funnel and L-box tests for determining the workability measurements such as slump flow diameter, time (T-50), passing ability, V-Funnel time, etc. Beams of 100mm×150mm×1100mm were tested using universal testing machine at a displacement- controlled rate of 0.05 mm/min. The mid span deflections were recorded. Also, the longitudinal strain values were measured using demountable mechanical strain gauges at various positions along the length of the beam. The load versus displacement curve foreach specimen was obtained and the toughness parameters, namely, toughness indices I5 and I10, and residual strength factors R(5, 10) were calculated based on ASTM C1018. The load-deflection plots obtained for different fibre volume fractions are given in Fig 2. The toughness indices and residual strength factors are calculated using the following equations: I5 = Area of load-deflection curve up to 3.0 times the first crack deflection / area of load-deflection curve up to first crack deflection I10 = Area of load-deflection curve up to 5.5 times the first crack deflection / area of load-deflection curve up to first crack deflection R (5, 10) = 20× (I10 – I5)

Figure 1 Experimental Test Set Up

Table 2 Mix Designation

MIX ID STEEL (%) POLYPROPYLENE (%) CONTROL MIX 1 0 HFRSCC-1 0.95 0.05 HFRSCC-2 0.90 0.1 HFRSCC-3 0.85 0.15 HFRSCC-4 0.8 0.2



5. RESULTS AND DISCUSSIONS

5.1. Fresh Properties The workability of the mix was found to reduce to a large extent on adding polypropylene fibres when compared to only steel fibres. HFRSCC-1 and HFRSCC-2 satisfied all the specifications of EFNARC.

Table 3 Fresh Properties of Hybrid SCC Mixes

MIX ID FLOW DIAMETER T-50 J-RING PASSING

ABILITY V-FUNNEL

CONTROL MIX 685 4.2 674.5 8.6 10.5 HFRSCC-1 673 4.9 664.8 8.2 11 HFRSCC-2 669.5 5.3 659.4 10.1 11.9 HFRSCC-3 653 6.1 634.5 18.5 12.3

5.2. Flexural Strength Test

5.2.1. Toughness indices, residual strength and ultimate load values

Table 4 Toughness Indices, Residual Strength and Ultimate Load of Hybrid Mixes

MIX ID FIRST CRACK DEFLECTION

(mm)

FIRST CRACK

LOAD (kn)

TOUGHNESS INDICES

RESIDUAL STRENGTH

R(5,10)

ULTIMATE LOAD (kn) I 5 I 10

CONTROL MIX 2.02 29.4 7.56 22.83 305.52 61.50

HFRSCC- 1 1.86 18.04 5.49 18.86 267.34 46 HFRSCC -2 1.24 19.40 7.46 22.25 295.84 63.80 HFRSCC -3 2.38 12 7.21 20.50 252.60 40.90

The trends observed in Table 4 shows that by replacing steel with polypropylene fibres the first-crack load, first-crack deflection, toughness indices, residual strength and ultimate load was found to decrease when compared to the control mix. The value for HFRSCC-2 was found to have least variation when compared to the control mix. The toughness index value for HFRSCC-2 and control mix was almost found to be similar, thereby showing that both have the same toughness values. The residual strength value R (5,10) gives a measure of ductility of the mixes. HFRSCC 2 showed almost similar values when compared with the control mix. This clearly shows that at a replacement beyond a limit polypropylene fibres were not able to bridge the cracks when compared with steel fibres. Thus, should be adopted as the limit for polypropylene dosage.

5.1.2. Load-Deflection Curve Fig 2 clearly shows the variation of vertical deflection with increment in load. Each curve follows the same trend in variation with load. At first it shows a linear variation up to the first crack load, after which a dip is observed and then the curve varies non-linearly. It was also observed that by increasing the percentage replacement of steel with polypropylene the first crack was formed at an earlier load compared to the mix containing only steel fibres. The ultimate load for HFRSCC 2 was found to be almost same as that of the control mix.



Figure 2 Load-Deflection Plot for Hybrid Mixes

5.2.3. Longitudinal Strain Variation along the Beam at Tension and Compression Fibres

Table 5 Longitudinal Strain Values at Tension and Compression Fibres

MIX ID MIDDLE SPAN QUARTER SPAN

COMPRESSION FIBRE TENSION FIBRE COMPRESSION

FIBRE TENSION FIBRE

CONTROL MIX 0.002375 0.002765 0.00291 0.00298 HFRSCC-1 0.00312 0.002635 0.00235 0.00245 HFRSCC-2 0.00200 0.003550 0.00228 0.00351 HFRSCC-3 0.00232 0.002515 0.00252 0.00271

The values observed in Table 5 are the average strain values obtained during each load increments. The trend observed shows that the least strain value was obtained for the mix HFRSCC-2 at the compression fibre in the middle span and quarter span when loaded up to 40% of the design ultimate load. The same mix showed the highest strain variation in the tension fibre. Least strain variation implies higher ductility value. Thus, clearly implying the ductility of the mix is not impaired by incorporating non-metallic fibre in the mix.

6. CONCLUSIONS The primary objective of this study was to obtain the effectiveness of hybrid fibre combinations at low volume fractions in improving the flexural behaviour of high strength concrete. Results from the study indicate the following:

The steel-polypropylene hybrid combinations in concrete result in comparable levels of flexural strength, toughness, and ductility to the control steel fibre concrete. HFRSCC-2 showed almost similar toughness values when compared with the control mix.

The experimental observations for toughness and ductility reveal that the best performance for steel-polypropylene hybrid combinations is obtained at an optimum combination of 0.90% steel and 0.10% polypropylene (HFRSCC-2); the reason could be that at high levels of non-metallic fibres, there is significant enhancement in the early crack formation.

Increased fibre availability in the hybrid fibre systems (due to the lower densities of non-metallic fibres), in addition to the ability of non-metallic fibres of bridging smaller micro cracks, could be the reasons for the enhancement in flexural properties.

010203040506070

0 2 4 6 8 10 12 14

LOA

D (k

N)

DEFLECTION (mm)

CONTROL MIX HFRSCC 1HFRSCC 2 HFRSCC 3



REFERENCES [1] Moncef Nehdi and Jennifer Duquette Ladanchuk (2004), “Fiber Synergy in Fiber-

Reinforced Self-Consolidating Concrete”, ACI materials journal, Title no. 101-M57.

[2] A.Sivakumar (2011), “Influence of hybrid fibres on the post crack performance of high strength concrete: Part I experimental investigations” Journal of Civil Engineering and Construction Technology Vol. 2(7), pp. 147-159.

[3] Syal Tarun, Goel Sanjay, Bhutani Manish (2013), “Workability and Compressive Strength of Steel Polypropylene Hybrid Fibre Reinforced Self-Compacting Concrete”, International Journal for Science and Emerging ISSN No. (Online):2250-3641Technologies with Latest Trends” 6(1): 7-13

[4] Nan Su, Kung-Chung Hsu, His-Wen Chai(2001), “A Simple Mix Design Method for Self-Compacting Concrete” Journal of Cement and Concrete Research 31(2001) pp 1799-1807.

[5] Rudzinski, L.; Turlej, B.; Hebda, L.; and Grochal, W (, “Mechanical Properties of Hybrid Fibre Reinforced Mortars,” Proceedings of the International Symposium on Brittle Matrix Composites,AM. Brandt, V. C.Li, and I. H. Marshall, eds., IKE and Woodhead Publishers, Warsaw, Poland, pp. 342-351

[6] Trtik, K., and Vodicka, J., “The FRCs Reinforced by Combination of Steel and Polypropylene Fibres,” Proceedings of International Symposium on Brittle Matrix Composites , A. M. Brandt, V. C. Li, and I. H. Marshall,Eds., BIGRAF and Woodhead Publishers, Warsaw, Poland, pp. 172-178.

[7] Kobayashi, K., and Cho, R (, “Flexural Characteristics of Steel Fiber and Polypropylene Fiber Hybrid Reinforced Concrete,” Composites, V. 13, pp. 164-168

[8] R.H. Mohankar, M.D. Pidurkar, P.V Thakre, S.S. Pakhare (2016), “Hybrid Fibre Reinforced Concrete”, International Journal of Science, Engineering and Technology Research, Volume 5, Issue 1,ISSN: 2278 – 7798.

[9] Dr. Jamal Abudulsamad Khudair And Rafid Saeed Atea (2015), “Flexural Behavior of self-Compacting Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer Sheets”, International Journal of Advanced Research, Volume 3, Issue 1, 714-722.

[10] G. Jeenu*, U. R. Reji,V. Syam Prakash (2007), “Flexural Behaviour Of Hybrid Fibre Reinforced Self-Compacting Concrete”, 32ndConference on Our World In Concrete & Structures, Singapore.

[11] Malgorzata Pajak (2016), “Investigation on Flexural Properties of Hybrid Fibre Reinforced Self-Compacting Concrete”, World Multidisciplinary Civil Engineering-Architecture-Urban Planning Symposium.

[12] ACI 544 R (reapproved 2002), “State-of-the-Art Reporton Fiber Reinforced Concrete”, ACI Committee 544.

[13] ASTM C - 1018-97, “Standard Test Method for Flexural Toughness and First-Crack Strength of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading)”, ASTM standards.

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