A Study on the Effect of Alternate Wetting and Drying on the Strength Properties of SIFCON Produced from Waste Coiled Steel Fibres

Dr Ashish K Gurav, Director, Dhananjay Mahadik Group of Institutions, Vikaswadi, Kagal, Kolhapur

Dr. K. B. Prakash, Department of Civil Engineering K.L.E. Society's College of Engineering and Technology Belgaum, Karnataka

Slurry Infiltrated Fibre Reinforced Concrete (SIFCON), is special form of Fibre Reinforced Concrete (FRC) containing high percentage of fibres in which coarse aggregates are absent.

In production of SIFCON steel, fibres are Prepacked in the form or in the mould to its full capacity, rather than being mixed and then cast or sprayed along with concrete. After placement of fibres, fine-grained cement based slurry is poured or pumped into the fibre network, infiltrating the air space between the fibres while conforming to the shape of the form or mould. External vibrations can also be used to aid infiltration of the slurry. SIFCON utilizes the fibres in the range of 6-20% by volume fraction as against usual range of 1-3 % for fibre reinforced concrete. Due to such a high percentage of fibres tremendous improvement in strength properties can be expected.

In this paper, effect of alternate wetting and drying on the properties of SIFCON is reported. SIFCON is made from waste coiled steel fibres obtained from lathe machine shop. In this study, fibres having aspect ratios like 80, 90, 100, 110 and 120 are used. Specimens are subjected to five cycles of alternate wetting and drying. The strength characteristics like compressive strength, tensile strength, flexural strength, and impact strength are evaluated.

Introduction

Nowadays, we talk much about high performance concrete. It has higher strength, better durability and elasticity1. Although high strength concrete is often considered as relatively new material, its development has been gradually taking place over many years2.

In the discussion of high performance concrete role played by FRC is vital. FRC is defined as a composite material which consists of conventional concrete reinforced by randomly dispersed short length fibres of specific geometry, made up of steel, synthetic material or natural fibres.3 The fibres are distributed evenly throughout the mix without balling or clustering4. The randomly oriented fibres help to bridge and arrest the cracks. As such, crack widening is gradual as compared to plain concrete5. This leads to better performance of concrete. Fibres have reported to be superior to wire mesh, for shortcrete. Also they overcome a difficultly in placing the mesh, especially on irregular surfaces6.

The concept of steel fibre reinforcement is very old. Steel fibres have been used since early 1900s7. Presently, steel fibres are considered as structural fibres as they enhance strength of the structure to a great extent3. The addition of steel fibres into concrete mass can dramatically increase the strength properties like compressive strength, tensile strength, and flexural strength and impact strength of concrete8. The strength properties of FRC can be increased by increasing the percentage of fibres in the concrete. But as the percentage of fibres increases, there are certain practical problems which have to be faced. The higher percentage i.e. higher volume content of fibres may cause balling effect in which the fibres cling together to form balls. Thus uniform distribution of fibres cannot be guaranteed, if percentage of fibres is more. Also longer fibres interfere with the aggregates during compaction thus hindering the proper orientation of fibres9. This fact limits the fibre content form 1 to 3 percent by volume.

The limitations of FRC and continuous ongoing demand for high performance material has led to the invention of SIFCON by Lankard in 1979. SIFCON is high strength, high performance material containing relatively high volume percentage of fibres as compared to FRC. SIFCON is also sometimes termed as 'High volume fibrous concrete'. In conventional FRC, the fibre content usually varies from 1 to 3 percent, while in SIFCON it varies from 6 to 20% by volume depending on the geometry of fibres and type of application.8. The material SIFCON has no coarse aggregates but has a high cementious content.

Research Significance

SIFCON, which is considered as a high performance concrete, can also be produced by using waste coiled steel fibres obtained from the lathe machine shops. Since these fibres are available locally, they can be easily used in the production of SIFCON. Due to their coiled nature they may offer more resistance to loads. The study of effect of alternate wetting and drying on SIFCON produced from such waste coiled fibres is important from the point of view of structures like water tanks, marine structures, bridge piers etc. which are subjected to alternate wetting and drying.

Experimental Programme

The main aim of this experimental programme is to find out effect of alternate wetting and drying on the strength properties of SIFCON produced from waste coiled steel fibres.

Ordinary Portland cement of 53-grade and locally available sand with a specific gravity 2.65 and fineness modulus of 2.92 was used in the experimentation. To impart additional workability, super- plastisizer (1% by weight of cement) was used. The waste coiled steel fibres were procured from local lathe machine shops. The fibres were of chrome steel having density 6.8 gm/cm3. The average thickness of fibres was taken into consideration in fixing the aspect ratios. The percentage of fibres used in the experimentation is 6%. The average thickness was 0.5 mm, with average coil diameter of 3 mm. The different aspect ratios adopted in the experimentation were 80, 90, 100, 110 and 120 giving fibre lengths 40mm, 45mm, 50mm, 55mm and 60mm respectively.

The cement mortar slurry was prepared with 1:1 proportion using w/c ratio 0.42. A superplastisizer (1% by weight of cement) was added to this slurry which increased infiltration capacity of the slurry. The moulds were filled with 6% fibres, as the case may be and slurry was poured into the moulds. Vibration was given to the moulds using table vibrator. The slurry was poured until no more bubbles are seen. This ensured a thorough infiltration of slurry into the fibres. The top surface of the specimen was leveled and finished. After 24 hours, the specimens were demoulded and were transferred to curing tank where they were allowed to cure for 28 days.

The effect of alternate wetting and drying on SIFCON was studied on compressive strength, tensile strength, flexural strength and impact strength. The cube specimens of dimension 150×150×150 mm were cast, from which the compressive strength was calculated. The specimens of dimension 150 mm diameter and 300 mm length were cast for split tensile strength. The specimens of dimension 100×100×500 mm were cast for flexural strength test. Two point loading10 was adopted on these specimens with an effective span of 400 mm. The impact strength specimens consisted of plates of dimension 250×250×35 mm. Four different methods of impact test are described in the literature11. Out of these methods drop out method was used owing to its simplicity. A steel ball weighing 20 N was dropped from the height of 1 m over the specimen, which was kept on the floor. The number of blows required to cause first crack (FC) and complete failure (CF) were noted. From this number of blows the impact energy was calculated as follows.

Impact energy = w × h × N (Nm).

Where w = weight of the ball = 20 N

h = height of fall = 1 m

N = number of blows required to cause first crack or complete failure as the case may be.

After 28 days of curing, the specimens were taken out of the water. They were kept in the open atmosphere for three days and then they were immersed into the water for three days. This constituted one cycle of alternate drying and wetting. The specimens were subjected to twenty-five such cycles of alternate drying and wetting and then they were tested for their respective strengths.

Test Results

Table 1—Compressive strength test results
Different
aspect
ratios
of fibres
Compressive strength (MPa) of SIFCON without subjecting to alternate wetting and drying (ref. mix)
Compressive strength (MPa) of SIFCON subjected to alternate wetting and drying
Percentage reduction
in compressive strength as compared to ref. mix
80
30.37
29.48
3.86
90
32.44
31.11
4.10
100
33.92
32.59
3.90
110
35.55
34.22
3.75
120
37.04
35.70
3.60

Table 2—Split tensile strength test results
Different
aspect
ratios
of fibres
Tensile strength (MPa) of SIFCON without subjecting to alternate wetting and drying
( ref. mix )
Tensile strength (MPa) of SIFCON subjected to alternate wetting and drying
Percentage reduction
in tensile strength as compared to ref. mix
80
2.92
2.74
6.45
90
3.11
2.92
6.06
100
3.58
3.40
5.26
110
3.96
3.77
4.76
120
4.15
3.91
5.68

Table 3-- Flexural strength test results
Different
aspect
ratios
of fibres
Flexural strength (MPa) of SIFCON without subjecting to alternate wetting and drying ( ref. mix )
Flexural strength (MPa) of SIFCON subjected to alternate wetting and drying
Percentage reduction
in flexural strength as compared to ref. mix
80
5.84
5.49
5.96
90
6.37
5.89
7.53
100
7.03
6.64
6.38
110
7.65
7.17
6.27
120
8.32
7.76
6.73

Table 4-- Impact strength test results
Different
aspect
ratios
of fibres
Impact strength
(Nm) of SIFCON without subjecting to alternate wetting and drying ( ref. mix )
Impact strength
(Nm) of SIFCON subjected to alternate wetting and drying
Percentage reduction
in impact strength for complete failure
as compared to ref. mix
FC
CF
FC
CF
80
200
4700
100
4400
6.38
90
220
5020
120
4700
6.37
100
280
5500
140
5180
5.82
110
300
5640
180
5300
6.03
120
360
5960
220
5660
5.03
FC= first crack; CF= complete failure

Tables 1, 2, 3 and 4 give the compressive strength, tensile strength, flexural strength and impact strength test results respectively, for SIFCON with and without alternate wetting and drying. The tables also indicate the percentage decrease in the strength of SIFCON when subjected to twenty-five cycles of alternate drying and wetting.

The figures 1, 2, 3 and 4 give variation of compressive strength, tensile strength, flexural strength, and impact strength respectively, for SIFCON with and without alternate wetting and drying.

Discussions on Test Results

1) It has been observed that the compressive strength, tensile strength, flexural strength and impact strength of SIFCON goes on increasing as the aspect ratio of fibers in it goes on increasing. This is also true for SIFCON with and without subjecting it to alternate wetting and drying.

sifcon
sifcon
Figure 1: Variation of compressive strength of SIFCON for different aspect ratios of fibres
Figure 2: Variation of split tensile strength of SIFCON for different aspect ratios of fibres

This may be due to the fact of optimum infiltration of slurry for fibres having more aspect ratio.

Thus it can be concluded that the SIFCON produced with fibres having an aspect ratio of 120 yields the maximum strength.

2) It has been observed that percentage reduction in the compressive strength of SIFCON, produced with 6% fibres when subjected to alternate wetting and drying is 3.86%, 4.10%, 3.90%, 3.75% and 3.60% respectively for different aspect ratios of fibres like 80, 90, 100, 110 and 120.

3) It has been observed that percentage reduction in the tensile strength of SIFCON, produced with 6% fibres when subjected to alternate wetting and drying is 6.45%, 6.06%, 5.26%, 4.76% and 5.68% respectively for different aspect ratios of fibres like 80, 90, 100, 110, and 120.

4) It has been observed that percentage reduction in the flexural strength of SIFCON, produced with 6% fibres when subjected to alternate wetting and drying is 5.96%, 7.53%, 6.38%, 6.27% and 6.73% respectively for different aspect ratios of fibres like 80, 90, 100, 110, and 120.

5) It has been observed that percentage reduction in the impact energy (for complete failure) of SIFCON, produced with 6% fibres when subjected to alternate wetting and drying is 6.38%, 6.37%, 5.82%, 6.03% and 5.03% respectively for different aspect ratios of fibres like 80, 90, 100, 110, and 120.

sifcon
sifcon
Figure 3: Variation of flexural strength of SIFCON for different aspect ratios of fibres
Figure 4: Variation of impact strength of SIFCON for different aspect ratios of fibres

The reduction in the strengths of SIFCON, when subjected to alternate wetting and drying may be due to the fact of formation of hairy cracks due to expansion and contraction of concrete during the process of alternate wetting and drying.

Thus it can be concluded that, the SIFCON subjected to alternate wetting and drying do undergo some reduction in strengths.

Conclusion

1)The SIFCON produced with fibres having an aspect ratio of 120 yields the maximum strength.

2) The SIFCON subjected to alternate wetting and drying does undergo some reduction in compressive strength, tensile strength, flexural strength and impact strength.

3)The reduction in strength of SIFCON subjected to alternate wetting and drying is in the range of 3% to 7%.

Acknowledgment

The authors would like to thank and Dr. S. K. Kudari and Dr.S. C. Pilli, Principals of D. Y. Patil College of Engineering and Technology, Kolhapur and KLE Society's College of Engineering and Technology, Belgaum for their encouragement throughout the work. Authors are also indebted to management authorities of both the colleges for their wholehearted support, which boosted the moral of the authors. Thanks are also due to HODs of the Civil Engineering Department and other staff for their kind cooperation.

References

  • Ahluwalia S. C., 'Material used for high performance concrete,' New Building Material & Construction World, September 2004, pp 38-44.
  • Anand Parnade, K Tangravel et al, 'High strength concrete and its characteristics – a review', New Building Materials & Construction World, September 2004, pp 77-82.
  • Sikdar P. K., Dr, Saroj Gupta and Satander Kumar, 'Application of fibres as secondary reinforcement in concrete,' Civil Engineering & Construction Review, December 2005, pp 32-35.
  • Kieth Carr, 'Polypropylene and steel fibres combinations,' Concrete, September 2004, pp 60-61.
  • Ziad Bayasi and Henning Kaiser, 'Steel fibres as crack arresters on concrete,' The Indian concrete journal, March 2001, pp 215-219.
  • Marc Wandewalle N.V., Bekaert S.A., Choudhari G.P., 'Fibres in concrete-Dramix steel fibres for SFRS & SFRC,' The Indian concrete journal, March 2003, pp 939-940.
  • Fibre reinforced concrete, A report published by cement and concrete institute, Midrand, 2001.
  • Prakash K. B. et al, 'Performance evaluation of slurry infiltrated fibrous silica fume concrete,' Proceedings of International conference on Fibre Composites, HPC and smart materials,' Chennai, India, pp 201-211.
  • Saluja S. K. et al, 'Compressive strength of fibrous concrete,' The Indian Concrete Journal, February 1992, pp99-102.
  • I. S. 516-1959, 'Methods of tests for strength of concrete,' Bureau of Indian standards, New Delhi.
  • Balsubramanain K. et al, 'Impact resistance of steel fibre reinforced concrete,' The Indian Concrete Journal, May 1996, pp 257-262.
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