Toughness Characteristics Of Steel Fibre Reinforced Concrete

“Toughness Characteristics of Steel Fibre Reinforced Concrete”

Biography: Arumugam E is a professor of Civil Engineering, College of Engineering, Anna University, India. He has obtained his B.E from Regional Engineering College, Tamilnadu; M.E from P.S.G College of Technology, Tamilnadu and PhD from College of Engineering, Anna University. His research interests include Stress Concentration, Fly ash concrete, Polymer Concrete. Nanda kumar S and Deviprasadh A are first year graduate students (M.E Construction Engineering and Management) studying in College of Engineering, Anna University. They both did their under-graduate in Hindustan College of Engineering, Chennai, and did their project work in the above topic in Larsen & Toubro ltd, Chennai.

ABSTRACT The objective of this investigation was to study the behaviour of Steel Fibre Reinforced Concrete (SFRC). Hooked end fibres and corrugated (crimped) fibres with aspect ratio of 55 were used. Specimens were cast without fibres and with fibres of 0.5% and 1% volume fraction (Vf). Tests were conducted for studying the compressive, tensile, flexural strength and energy absorption. Compressive and split tensile tests were conducted on cubes and cylinders respectively. 15 Beams were cast and tested under two point loading to find flexural strength, toughness and stiffness. An empirical equation for finding the toughness index was developed based on fibre percentage. 30 panels were cast and tested under static point load to calculate the energy absorption and ductility index. Keywords: Steel Fibre Reinforced Concrete, Static load, Panels, Beams, Toughness, Energy Absorption.

1

INTRODUCTION The advantages of using concrete include high compressive strength, good fire resistance, high water resistance, low maintenance, and long service life. The disadvantages of using concrete include poor tensile strength, low strain of fracture and formwork requirement. Hence fibres are added to concrete to over come these disadvantages. The addition of fibres in the matrix has many important effects. Most notable among the improved mechanical characteristics of Fibre Reinforced Concrete (FRC) are its superior fracture strength, toughness, impact resistance, flextural strength, resistance to fatigue etc. Improving fatigue performance is one of the primary reasons for the extensive use of Steel Fibres in concrete. RESEARCH SIGNIFICANCE Although many tests were carried out on FRC materials, they were mainly based on flexure test on beam specimens. Very few literatures were available on testing of panels or slabs which is similar to most practical cases. Hence an attempt was made in this work to study the behaviour of square FRC panels simply supported on all sides and subjected to concentrated load on its center, which is the severe loading in most practical cases. EXPERIMENTAL INVESTIGATION In order to study the interaction of steel fibres with concrete under compression, split tension, flexure and static load, 45 cubes, 45 cylinders, 15 beams, 30 panels was casted respectively. The experimental program was divided into five groups. Each group consists of 9 cubes, 9 cylinders, and 3 beams, 3 panels of 50mm (1.97in) thickness and 3 panels of 100 mm (3.94in.) thickness. i.The first group is the control (Plain) concrete with 0% fibre (PCC) ii.The second group consisted of hooked end steel fibre of Vf 0.5% (HSFRC 0.5)

iii.The third group consisted of hooked end steel fibre of Vf 1.0% (HSFRC 1.0) iv.The fourth group consisted of corrugated steel fibre of Vf 0.5% (CSFRC 0.5) v.The fifth group consisted of corrugated steel fibre of Vf 1.0% (CSFRC 1.0) SFRC beams of size 150x150x700mm (5.9 inch.x5.91inch.x27.56 inch.) were tested using a servo controlled Universal Testing Machine (MTS) as per the procedure given in ASTM C-78 and the load was applied at a rate of 0.1mm/min, load and displacement was recorded constantly (Figure 5). Toughness was calculated as the energy equivalent to the area under the load deflection curve as per the procedure given in the ASTM C-1018. Stiffness of the beam specimen was found as the slope of the load-deflection curve upto the elastic region of the curve. The panel specimen of dimension was placed on a simply supported condition on all four sides and a concentrated load was applied over an area of 9.46sq.inch. (61sq.cm).The actuator as operated at a rate of 1.5 mm/min (0.06inch/min) and the corresponding load & deflection was measured as per the European Specification for Sprayed Concrete (EFNARC). The bottom deflection was also monitored using a Linearly Variable Differential Transducer (LVDT) (Figure 3 and Figure 4). The testing was continued till a deflection of 25mm (0.98inch) or failure which ever occurred earlier. The energy absorption upto the deflection of25mm (0.98inch) was calculated as area under load deflection curve for that deflection, with an increment of2mm (0.08inch). Ductility index was calculated as the ratio of the deflection upto the ultimate load to the deflection upto the first crack load. The ultimate deformation has been considered as the deformation corresponding to 15% load drop i.e. 85% of the ultimate load. The ductility so calculated is called the displacement ductility. Ductility (μd ) = Ultimate deflection (δu ) / Yield deflection (δy)

Materials The materials used and their specifications are as follows: CEMENT Ordinary Portland cement was used and its specific gravity is 3.15*. The brand used was “UltraTech” with P53 grade. FINE AGGREGATE Specific gravity of fine aggregate is 2.65 with water absorption 0.99%. Dry loose bulk density was calculated as1502 Kg/m3 (93.76lbm/cubic foot). COARSE AGGREGATE A crushed granite stone aggregate of maximum size of 20 mm was used. Specific gravity of coarse aggregate is 2.73 with water absorption 0.25% and dry loose bulk density 1500 Kg/m3 (93.63lbm/cubic foot). STEEL FIBRES HOOKED END STEEL FIBRES Hooked end steel fibres commercially called as Dramix steel fibres manufactured by Bekaert Corporation were used which had a length of 30 mm (1.18inch) and a diameter of 0.55 mm (0.022inch) resulting in an aspect ratio of about 55 and conforms to ASTM A820 and Belgium standard 1857*.The tensile strength of fibre is in the range of 1100 N/mm2* (156,456.78 lbf/square inch) CORRUGATED STEEL FIBRES Corrugated steel fibres from Stewols & Co - India were used which had a length of 25 mm and a diameter of 0.45 mm resulting in an aspect ratio of about 55 and conforms to ASTM A820*.The tensile strength of fibre is in the range of1200 N/mm2* (1,706,801.27lbf/square inch) Note: * as per the manufacturers report

EXPERIMENTAL RESULTS AND DISCUSSION Compressive Strength The Compressive strength of concrete mixed with steel fibres was found to vary marginally. 50% of the 28 days strength of corrugated fibres was obtained in 3 days itself.

The

compressive strength of ordinary concrete and fibre reinforced concrete are tabulated in Table 1. Split tensile strength The split tensile strength was found to be increased as the percentage of fibre was increased. For the hooked fibre with volume fraction of 0.5% and 1.0% the increase in tensile strength was 8 % and 32.4%respectively. The increase was about 30% for corrugated fibres with volume fraction of 1.0% and there was no increase in case of CSFRC (Corrugated Steel Fibre Reinforced Concrete) of volume fraction 0.5%. The 28 days strength of 0.5% volume fraction of HSFRC (Hooked Steel Fibre Reinforced Concrete) was 7% greater than that of CSFRC of same volume fraction. The results are tabulated in table 1. Flexure strength The flexure strength was found to decrease marginally. The failure was brittle in case of plain concrete and failure was ductile in case of steel fibre reinforced concrete. The addition of steel fibre resulted in a consistent increase in ductility of the beams. The toughness index for all the control beams was found to be 1. For all the SFRC beams the I5 and I10 values are greater than 2.75 and 4 respectively. The toughness indices were calculated for all the specimens and are tabulated in Table 2. Empirical equation The empirical equations for finding the toughness indices were found using the I5 and I10 values from the experimental results using Microsoft’s Excel office program which can be seen in the Figure 1 and Figure 2.

Energy absorption The maximum load and energy absorbed are tabulated in Table 3. The peak load obtained with steel fibre reinforced concrete was found to increase more than 2 times when compared to control (plain) concrete of same thickness. 50mm (1.97inch) panels For HSFRC with 0.5% and 1% volume fraction the energy absorbed was 27.5 and 32.4 times that of control concrete. For CSFRC with 0.5% and 1% volume fraction the energy absorbed was 19.4 and 32.8 times that of control concrete. The energy absorbed by 0.5% volume fraction of HSFRC was 42% more than that of 0.5% volume fraction of CSFRC. The energy absorbed by 1% volume fraction of HSFRC and CSFRC was almost equal. The energy absorbed for 1% volume fraction of HSFRC was 17% more than that of 0.5% volume fraction of HSFRC .The energy absorbed for 1% volume fraction of CSFRC was 69% more than that of 0.5% volume fraction of CSFRC. 100mm (3.94inch) panels For HSFRC with 0.5% and 1% volume fraction the energy absorbed was 18.6 and 15.6 times that of control concrete. For CSFRC with 0.5% and 1% volume fraction the energy absorbed was 10.5 and 13.7 times that of control concrete. The energy absorbed by 0.5% volume fraction of HSFRC was 73% more than that of 0.5% volume fraction of CSFRC. The energy absorbed by 1.0% volume fraction of HSFRC was 7.7% more than that of 1.05% volume fraction of CSFRC. The energy absorbed for 0.5% volume fraction of HSFRC was 20% more than that of 1.0% volume fraction of HSFRC. The energy absorbed for 1% volume fraction of CSFRC was 33% more than that of 0.5% volume fraction of CSFRC.

Ductility The ductility index for control concrete was found to be 1.00. The ductility index for all SFRC panels was found to vary between 4to 5 for all 50mm thick panels and 2to3 for 100mm panels. The energy absorbed for 1% volume fraction of CSFRC was 33% more than that of 0.5% volume fraction of CSFRC.

CONCLUSION Based on the results of this experimental investigation the following conclusions are drawn: 1. Addition of steel fibres to concrete increases the compressive strength of concrete marginally. 2. The tensile strength was found to be maximum with volume fraction of 1%. 3. The addition of fibres to concrete significantly increases its toughness and makes the concrete more ductile as observed by the modes of failure. 4. The stiffness of beams was studied and was found to be maximum for hooked end fibre with 1% volume fraction. 5. The ductility of steel fibre reinforced concrete was found to increase with increase in volume fraction of fibres and the maximum increase was observed for hooked fibres with 1% volume fraction. 6. The improvement in the energy absorption capacity of steel fibre reinforced concrete panels with increasing percentage of steel fibre was clearly shown by the results of the static load test on panels. 7. The 100mm thick panel absorbed the maximum energy of 1010Nm with Hooked end steel fibre with volume fraction 0.5% for a deflection of 20mm.

ACKNOWLEDGEMENT The authors thank Larsen & Toubro ltd. for their technical and other facilities provided at various stages of this research work. The authors express their sincere gratitude and heartfelt thanks to Dr.B.Sivarama Sarma, Head, R&D, Larsen & Toubro ltd, Chennai for his valuable guidance and supervision throughout the project work. The authors are grateful to Dr.M.Neelamegam, Deputy Director, SERC, Chennai for his esteemed suggestions and guidance for this work. The authors sincerely thank all others who have helped directly or indirectly at various stages of this work. REFERENCES 1. Basi, Z. and Kaiser, H. (April 2001) "Steel Fibres as Crack Arrestors in Concrete." The Indian Concrete Journal. 2. Craig, R., S. Mahadev, C.C. Patel, M. Viteri, and C. Kertesz. "Behaviour of Joints Using Reinforced Fibrous Concrete." Fibre Reinforced Concrete International Symposium, SP-81, American Concrete Institute, Detroit, 1984, pp. 125-167. 3. Craig, R. McConnell, J. Germann, N. Dib, and Kashani, F. (1984) "Behaviour of Reinforced Fibrous Concrete Columns." Fibre Reinforced Concrete International Symposium, SP-81, American Concrete Institute, Detroit,

pp. 69-105.

4. Gopalakrishnan, S. Krishnamoorthy, T.S. Bharatkumar,B.H. and Balasubramanian, K. (December 2003) “Performance Evaluation of Steel Fibre Reinforced Shotcrete” National seminar on advances in concrete technology and concrete structures for the future, Annamalai University 5. Kaushik S.K., Gupta.V.K., and Tarafdar.N.K., (1987) “Behaviour of fibre reinforced concrete in shear” proceedings of the international symposium on Fibre Reinforced Concrete International Symposium, volume I, chapter II, pp 1.133-1.149 6. Krishnamoorthy, T.S. Bharatkumar, B.H. Balasubramanian, K. and Gopalakrishnan,

S. (February 2000) “Investigation on durability characteristics of SFRC” Indian Concrete Journal page 94-98 7. Marc vandevalle, N.V. and Ganesh, P. (March 2003) Fibres in Concrete Indian Concrete Journal, pp 939-940 8. Marc vandevalle, N.V. (1998) “Tunnelling the world” Dramix reference manual 9. Sivarama Sarma, B. (1997) , “Investigations on laced reinforced concrete beams with normal and fibre reinforced concrete under monolithic and cyclic loading” Ph.D Thesis, IIT, Madras. 10. P.Srinivasalu, N.Lakshmanan, K.Muthumani, B.Sivarama Sarma (1987) “Dynamic behaviour of fibre reinforced concrete” proceedings of the international symposium on Fibre Reinforced Concrete International Symposium, volume I, chapter II, pp 2.85 11. Taylor, M.R. Laydon, F.D. and Barr, B.I.G. (October 1996) “Toughness characteristics of fibre reinforced concrete”, Indian Concrete Journal, pp.525-531 TABLES AND FIGURES List of Tables: Table 1 - Results of Compressive and Tensile strength Table 2 - Results of beam stiffness Table 3 - Results of energy absorption and ductility index List of Figures: Figure 1 - Empirical equation for CSFRC Figure 2 - Empirical equations for HSFRC Figure 3 - Panel failure in static load Figure 4 - Panel arrangement for test Figure 5 - Beam arrangement for test

Table 1 Results of Compressive and Tensile strength Specimen type

Control specimens

Average Compressive strength N/mm2(lbf/square inch)

Average Tensile Strength N/mm2 (lbf/square inch)

3 days

7 days

28 days

3 days

7 days

28 days

25.27 (3663.85)

39.59 (5740.08)

59.89 (8683.34)

2.55 (369.72)

3.54 (513.26)

4.81 (697.39)

Hooked 0.5% vf

fibre

24.50 (3552.21)

37.29 (5406.61)

58.24 (8444.11)

2.90 (420.47)

4.76 (690.14)

5.19 (752.49)

Hooked 1.0% vf

fibre

26.32 (3816.09)

38.04 (5515.35)

59.01 (8555.75)

4.01 (581.40)

5.66 (820.63)

6.37 (923.588)

Corrugated fibre 0.5% vf

27.38 (3969.78)

39.76 (5764.73)

58.43 (8471.66)

3.40 (492.96)

5.02 (727.84)

4.83 (700.29)

Corrugated fibre 1.0% vf

40.35 (5850.27)

32.17 (4664.27)

60.00 (8699.29)

3.82 (553.86)

5.29 (766.99)

6.27 (909.08)

Table 2 Results of beam stiffness Specimen ID

Load kN (kip)

Deflection mm (inch)

28 days flexural Strength N/mm2 (lbf/square inch)

Toughness indices

I5 Control specimens

Hooked fibre0.5%

Hooked fibre1.0%

34.00 (7.64) 28.50 (6.41) 30.00 (6.74) 28.50 (6.41) 27.00 (6.07) 25.50 (5.73) 33.80 (7.60) 31.50 (7.08)

1.30 (0.051) 1.13 (0.044) 1.10 (0.043) 1.00 (0.0394) 1.30 (0.051) 0.90 (0.035) 1.00 (0.0394) 1.00 (0.0394)

6.04 (875.73) 5.06 (733.64) 5.33 (772.77) 4.59 (665.50) 4.80 (695.94) 4.53 (656.80) 6.00 (869.93) 5.68 (823.53)

Stiffnes s (kN/mm ) (kip/inc h)

I10 1.00

1.00

1.00

1.00

1.00

1.00

3.26

5.00

3.44

4.67

3.18

4.86

3.79

5.63

4.16

5.88

26.15 (149.80) 25.30 (145.68) 27.28 (171.07) 28.50 (162.69) 20.77 (119.02) 28.33 (163.71) 33.80 (192.89) 31.50 (179.70)

Corrugated fibres0.5%

Corrugated fibres1.0%

32.00 (7.19) 26.00 (5.85) 27.00 (6.07) 27.20 (6.12) 26.50 (5.96) 27.50 (6.18)

1.00 (0.0394) 1.00 (0.0394) 1.10 (0.043) 1.20 (0.047) 1.30 (0.051) 1.10 (0.043)

5.69 (824.98) 4.62 (669.85) 4.80 (695.94) 4.80 (695.94) 4.71 (682.89) 4.80 (695.94)

3.81

6.23

2.51

3.16

2.70

4.18

3.12

4.08

3.1

5.02

3.71

5.92

29.00 (6.52)

1.05 (0.041)

5.16 (748.14)

2.65

6.00

32.00 (182.49) 26.00 (148.48) 24.55 (141.16) 22.67 (130.21) 20.38 (111.57) 25.00 (143.72) 27.60 (159.02)

Table 3 Results of energy absorption and ductility index Specimen ID

Control panel 50mm

Control panel 100mm

Hooked 50mm with 0.5%vf

Hooked 100mm with 0.5%vf

First crack load kN (kip)

Experimental Peak load kN (kip)

Deflection upto 0.15% ultimate load drop mm (inch) 1.56 (0.06) 2.31 (0.091) 1.51 (0.06) 2.88 (0.11) 3.06 (0.12) 3.33 (0.13) 10.75 (0.42) 12.10 (0.48) 13.00 (0.51) 11.50 (0.45)

1.00

25.91 (5.82) 15.92 (3.58) 17.91 (4.03) 77.62 (17.45)

Energy absorbed for 20mm deflection Nm (lbs-foot) 12.60 (894.04) 10.30 (730.85) 5.76 (408.71) 53.55 (3799.69) 56.00 (3973.53) 58.13 (4124.66) 288.50 (20470.77) 243.87 (17304.01) 259.50 (18413.05) 936.00 (66414.69)

10.92 (2.45) 8.54 (1.92) 7.30 (1.64) 31.36 (7.05) 40.04 (9.00) 37.51 (8.43) 10.56 (2.37) 8.65 (1.94) 10.38 (2.33) 37.63 (8.46)

_

44.83 (10.08)

87.55 (19.68)

1105.80 (78462.99)

8.60 (0.34)

2.56

_ _ _ _ _

Ductility Index

1.00 1.00 1.00 1.00 1.00 4.72 5.45 4.64 3.73

Hooked 50mm with 1.0%vf

Hooked 100mm with 1.0%vf Corrugated 50mm with 0.5%vf

Corrugated 100mm with 0.5%vf

Corrugated 50mm with 1.0%vf

Corrugated 100mm with 1.0%vf

51.69 (11.62) 9.87 (2.22) 12.61 (2.83) 9.30 (2.09) 50.0 (11.24)

84.26 (18.94) 19.35 (4.35) 23.94 (5.38) 23.16 (5.21) 94.00 (21.13)

988.00 (70104.39) 327.50 (23238.05) 262.63 (18635.14) 338.25 (24000.82) 890.00 (63150.72)

11.00 (0.43) 10.15 (0.40) 11.10 (0.44) 10.00 (0.39) 7.10 (0.28)

2.46

33.43 (7.52) 8.75 (1.97)

100.00 (22.48) 13.23 (2.97)

952.70 (67599.65) 164.50 (11672.24)

10.00 (0.39) 9.00 (0.35)

2.52

8.82 (1.98) 11.4 (2.56)

18.74 (4.21) 17.97 (4.04)

180.00 (12772.06) 211.44 (15002.91)

6.60 (0.26) 10.1 (0.40)

4.47

46.58 (10.47)

90.0 (20.23)

544.00 (38599.99)

6.75 (0.27)

3.17

49.45 (11.12) 46.20 (10.39) 11.15 (2.51)

62.59 (14.07) 89.89 (20.21) 31.14 (7.00)

564.50 (40125.54) 644.25 (45713.31) 361.50 (25650.54)

5.10 (0.20) 6.80 (0.27) 9.10 (0.36)

1.93

16.37 (3.68) 9.57 (2.15) 41.06 (9.23)

21.78 (4.90) 23.51 (5.29) 88.00 (19.78)

303.25 (21517.37) 274.25 (19459.65) 791.00 (56126.09)

9.00 (0.35) 10.75 (0.42) 8.10 (0.32)

4.37

45.18 (10.16)

95.00 (21.36)

769.88 (54627.50)

8.20 (0.32)

3.00

4.77 7.87 4.27 2.08

3.26

4.04

2.31 4.95

5.54 3.12

FOR I10 y = 3.68x + 1.9667

FOR I5 y = 0.7533x + 2.4

Toughness Indices

10 I5

8

I10

6

Expon. (I5) Expon. (I10)

4 2 0 0

0.25

0.5

0.75

1

1.25

1.5

Percentage of Fibre

1.75

2

2.25

Figure 1 Empirical equation for CSFRC

FOR I10 y = 2.14x + 3.7733

FOR I5 y = 1.2533x + 2.6667

Toughness Indices

10 I5

8

I10

6

Expon . (I5) Expon . (I10)

4 2 0 0

0.25

0.5

0.75 1 1.25 1.5 Percentage of fibre

1.75

Figure 2 Empirical equations for HSFRC

2

2.25

Figure 3 Panel failure in static load

Figure 4 Panel arrangement for test

Figure 5 Beam arrangement for test