Steel Fibre Reinforced Concrete in Australia, 2013

Leigh D Appleyard

Principal Consultant – Civil and Structural

ACOR Consultants Group, Australia

Abstract: The fundamental concept of integrating steel fibres in concrete has been known to Australian engineers since the early 1980s. Extensive literature on the subject has evolved over the past thirty (30) years, including both supplier promotional material and a range of conference papers and publications. However, the term “steel fibre reinforced concrete” (SFRC) does not appear anywhere in AS 3600 – 2009, Concrete Structures and NZS 3101 : Part 2 : 2006 makes only brief reference. The lack of code guidance for the use of SFRC has led to a quite unsatisfactory gap in knowledge on the part of consulting engineers, building contractors, concreters and clients. Marketers of SFRC and manufacturers and distributors have been not unaware of this gap in knowledge. The result has been that whilst SFRC is a meritorious product in certain clearly defined applications, its use and claimed benefits are frequently misunderstood and, of more concern, misrepresented and inappropriately promoted. The paper identifies and describes common misconceptions about SFRC in the context of the Australian design and construction fields. It also explains – by reference to Eurocodes, ACI Codes, and the NZ Code - why it remains technically challenging to provide codified design guidance for a range of dissimilar steel fibre products from disparate manufacturers. It also provides guidelines – based on a state of the art literature review – for realistic expectations of the performance of SFRC in a range of applications. Keywords: Steel fibre reinforced concrete, steel fibres, reinforcement.

1. Established Properties of SFRC

A comprehensive history of claimed benefits of the inclusion of fibres in concrete matrices is available from a number of sources (1, 2, 3). Currently, a steel fibre (and there are many forms in the market) is the most commonly used material due to its high stiffness. It is universally accepted that the addition of steel fibres to concrete enhances the post cracking properties both in terms of more ductile behaviour and reduced crack widths (4).

Fibre – reinforced concrete materials may be classified either as

(a) Strain softening materials – for which the fibre content is typically < 2% by volume, or (b) Strain hardening materials – for which the fibre content is typically 2% by volume or greater.

Fibre dosage rates are identified both in terms of mass per unit volume (kg/m

3) or by a volume ratio

expressed as a percentage. For steel, at (say) 7860 kg/m3, a comparison between these reference

systems is as set out in Appendix A. A clear understanding of fibre dosage rates which are achievable in “real world” batch production of SFRC is essential to an understanding of many of the “design solutions” offered by various commercial steel fibre distributors. This aspect will be discussed in further detail at 4.0 below. A more comprehensive load/deformation representation is that shown below based on recent work by Löfgren (5). At fibre dosage rates commonly specified in the construction industry in Australia, the resultant SFRC must be regarded as a strain softening material with properties – in a general context – not greatly dissimilar to the properties of a plain unreinforced concrete with comparable characteristic compressive and tensile strengths.

2. “State of the Art” knowledge – 1993 – 2013 It is appropriate to pose the rhetorical question – what have we learned over the past 20 years? This paper suggests that the answer is “not much!” This response will undoubtedly draw challenge. However, consider and compare the following citations: First, the following statement dates from the mid 1990’s (6) “Fibrous concrete technology still lacks a single comprehensive and meaningful test to

describe the distinctive dynamic property of the material over a range of varying combinations of different matrix strengths and different fibre combinations.”

And second, this more recent status assessment from 2007 (7):

“Although steel-fibre-reinforced concrete (SFRC) has been used in the UK and elsewhere for a number of years, there are no agreed design approaches for many of the applications.

This differs from conventional reinforced concrete using steel bars or welded fabric, which has

been covered by national or international design codes for many years.” That there has been considerable effort expended in research in the field of SFRC is beyond question. However, one of the underlying premises of this paper is that – notwithstanding such extensive research and development – designers seeking guidance for the appropriate use of SFRC remain almost as poorly served in 2013 as they were in 1993. More importantly, the considerable range of advisory material available from steel fibre manufacturers and promoters has led to “fuzzy” and often technically unsound perceptions as to the role of SFRC in Australia.

3. Effects of Inclusion of Steel Fibres in Concrete Matrices We do, in fact, know a great deal about the properties of concrete to which varying quantities of steel fibres have been added. However, even in 2013, much of this knowledge must still be regarded as empirical in the sense that it is knowledge gained from observation and experimentation. Such a process, of course, underlies the accumulation of all scientific knowledge. In the normal course of events, empirical knowledge is tested by repetition of experiments, the setting of boundary conditions and limits, and – ultimately – codification in order to provide guidance to others. This last step (i.e. codification) has not yet been achieved in terms of providing codified design guidance for the use of steel fibre reinforced concrete as has been noted previously. In broad terms, we know that the addition of steel fibres to a concrete matrix (given certain assumptions as are dealt with at 4 below) leads to an improvement in the ductility of the matrix. In more specific terms, ACI Committee 544 (4) provides the following overview of the claimed beneficial properties of SFRC:

3.1 Compression Slight gain, with the ultimate strength increased by 0-15% for up to 1.5% by volume of fibres. Note: 1.5% ≡ 115 - 120 kg/m

3

3.2 Direct Tension

Significant gain, with increases of the order of 30% - 40% for the addition of 1.5% by volume of fibres. Note: 1.5% ≡ 115 - 120 kg/m

3

However, as with conventional reinforced concrete, the measurement of “tension” in a concrete matrix is not straight forward.

3.3 Shear and Torsion For 1% by volume of fibres, the increases range from negligible to 30%. Note: 1% ≡ 80 kg/m

3

3.4 Flexure This is a complex area and it is difficult to provide a simple summary.

A limiting practical consideration is the maximum fibre volume which can be achieved – currently accepted to be 1.5% – 2% in laboratory conditions. At these values (more of which will be discussed in more detail below) the flexural strength of SFRC is about 50% - 70% greater than that of the unreinforced concrete matrix when tested by the conventional third point bending test. Note: 1.5% - 2% ≡ 115 - 150 kg/m

3

3.5 Fatigue For a given type of fibre, flexural fatigue strength increases significantly with increasing fibre percentage.

3.6 Creep and Shrinkage At fibre volumes less than 1% (80 kg/m

3) there appears to be no significant effect on the creep

and free shrinkage behaviour of concrete.

3.7 Toughness Toughness enhancement is, without question, the characteristic which most clearly distinguishes SFRC from concrete not containing steel fibres. At its simplest, toughness may be equated to deformation energy which may be absorbed by the SFRC. This is most conveniently measured by slow flexural loading techniques, of which two more common are the Japanese JSCE SF.4 method, and the US ASTM C1018 method (now replaced by ASTM C 1550). Each of these is illustrated below (4). In the European context, this “toughness” property is taken into account when determining the equivalent flexural strength of SFRC, thus (8)

for which based on Belgian Standard NBN B15-238 “Tests on fibre-reinforced concrete – Bending test on prismatic samples”. The terms ffct,eq,300 and ffct,eq,150 relate to average and equivalent flexural strengths and span/300 and span/150 values respectively. It is not the purpose of this paper to present an in depth analysis of the properties of various SRFC matrices. For those interested in pursuing this path, a reasonably comprehensive bibliography has been provided, including recent individual research papers which may be of interest. This paper focusses on the manner in which SFRC is promoted, and the claimed benefits of various fibre types are disseminated, in the Australian design and construction industries.

4. Achievable Dosage Rates

At present, it is generally accepted that around 2% fibre volume (approx. 160 kg/m

3 for steel

fibres) is the maximum which can be achieved even under laboratory conditions (9). However, this figure is considerably in excess of that which is achievable on Australian construction sites for which an upper limit of around 1% or 80 kg/m

3 would appear to be

achievable, albeit not without difficulty. Common dosage rates for industrial floors and pavements are generally accepted to be as follows (10):

(a) Floors with joints - 20 – 30 kg/m3

(b) “Jointless” floors - 35 – 45 kg/m

3

However, it needs to be borne in mind that SFRC with dosage rates of 20 – 45 kg/m

3 will not

differ significantly from plain concrete in terms of flexural tensile stress at first crack (11).

5. Lack of Codification Practice Note 35 was published by the Concrete Institute of Australia in June 2003 (12). Whilst somewhat dated, the Practice Note remains valuable. An important caveat is provided as follows:

“Design This current Practice Note is not a design manual. It does not critically assess design methods nor does it make recommendations as to appropriate design procedures or specific fibre selection for particular applications. The reader is advised to rationally assess available design procedures and commercial literature, including test results, when choosing from the many available options.”

5.1 Australian Standard AS3600 – Concrete Structures SFRC is not deal with by the current (or previous) version of AS3600 (13).

5.2 New Zealand Standard NZS 3101 : Part 1 : 2006 – The design of concrete structures At Section 5.5, NZ 3101 refers to SFRC in a single paragraph and advises (14):

“5.5 Properties of steel fibre reinforced concrete

The design properties of steel fibre reinforced concrete shall be determined by means of deflection controlled bending tests with the specific fibre to be used, or with this information supplied by the fibre manufacturer. The methods of Appendix A to the Commentary on Section 5 may be used.”

The Commentary to Section 5.5 provides limited further guidance:

“C5.5 Properties of steel fibre reinforced concrete

The design properties of steel fibre reinforced concrete are dependent on the post-cracking toughness of the composite material. The properties of the fibre, such as its aspect ratio, ultimate tensile strength and end anchorage have a significant influence on the performance of the fibre reinforced concrete. Different fibre properties will result in different fibre dose rates to meet specific design properties. Designs must be based on the test data supplied by the fibre manufacturer, or confirmed by tests. The design method of Appendix A to Section 5 may be used.”

The added emphasis, in the opinion of this author, is one of the sources of confusion apparent in the market place. This will be dealt with in further detail shortly.

5.3 ACI 544 – Fiber Reinforced Concrete The ACI prepares and updates various Standards and Committee Reports which are amalgamated annually in the ACI Manual of Concrete Practice. ACI 544.1R-96 is one such report. It is not a Standard. At 1.5 (4), the Committee advises:

“A current need is to consolidate the available knowledge for SFRC and to incorporate it into applicable design codes.”

5.4 EN 1992-1-1 Eurocode 2 : Design of concrete structures – Part 1-1 : General rules and rules for buildings. SFRC is not dealt with by Eurocode 2 (15).

5.5 Concrete Society Technical Report No. 63 Guidance for the Design of Steel-Fibre-Reinforced Concrete This UK document has been identified previously in this paper (see 2.0 above). In the opinion of this author, TR No. 63 – published in 2007 – provides the most comprehensive current available guidance to consultants who wish to design elements using SFRC.

5.6 DIN EN 14889 – 1 Fibres for Concrete – Part 1 : Steel Fibres : Definitions, specifications and conformity. This document is of particular relevance (16). It is frequently cited in the promotional material provided by steel fibre distributors, albeit not always in correct context. In short, EN14889-1 relates specifically to the manufacture of steel fibre products. It is this author’s opinion that this limitation is frequently “glossed over” in promotional literature. This “glossing over” leads to an implied but quite inaccurate perception that a concrete matrix resulting from the inclusion of fibres (which themselves conform to EN 14889-1) is a “code compliant” product. At Sections 5.7 and 5.8 of EN 14889-1, reference is made to the likely effects of steel fibres on both the consistence and strength of concrete. It is necessary also to give consideration to: EN14845-1 Test methods for fibres in concrete – Reference concretes (17). EN14845-2 Test methods for fibres in concrete – Effect on concrete (18). Again, is this latter interrelated interdependence between these European Standards which appears to be poorly articulated in the industry in Australia. Put simply, absent project and/or site specific testing of SFRC reference and sample mixes, it is quite unsound to infer that the addition of any particular steel fibre type at a selected dosage rate will guarantee a particular performance outcome.

6. Project specific testing of SFRC Alan Ross dealt with this matter quite well in a recent New Zealand presentation (19). Ross’s paper suggested that it might be appropriate to develop a graduated Performance Class ranking system for SFRC grades for both ultimate and serviceability limit states design protocols. The concept has merit but much development work is required before this can be progressed. Of singular importance, however, is the need for recognition that specifying and confirming design properties for SFRC is a complex process which requires an understanding of (amongst other variables) test variability in conjunction with targeted project testing. It is quite unrealistic, and potentially misleading, for a steel fibre distributor to suggest that nominated or requested performance benchmarks will be achieved unless specific mix designs have been formulated, tested and the results analysed. Such a process must, without exception, involve the concrete supplier, be it a commercial batch plant or a site specific batch plant.

Therefore, any representations as to SFRC performance criteria based on generic fibre type and dosage levels need to be treated with considerable caution.

7. A rough guide to fibre dosage/mesh and rebar equivalences It would be incorrect to suggest that the comparison which follows is other than a superficial treatment of possible equivalence of SFRC with conventionally reinforced concrete. None the less, it is submitted that – at an intuitive level at least – it puts into context some correlation between steel fibre dosage rates and conventional reinforcement percentages when considering crack control targets for slabs on grade based on the recommendations of Clause 9.4.3 of AS3600 – 2009 for a 200 mm thick slab. These comparisons are set out in Table 1.

Table 1 – Bar, mesh and steel fibre equivalences

Minor Crack Control

Moderate Crack Control

Strong Crack Control

Mesh or bar reinforcement

SL102 or

N12 @ 250 c/c N16 @ 250 c/c N16 @ 175 c/c

Steel fibres (generic)

30 kg/m3 60 kg/m

3 90 kg/m

3

It is interesting to note that the NSW Roads and Maritime Services (ex RTA) specification for the use of SFRC in road pavements currently stipulates (20):

“The steel fibre content must be not less than 75 kg per yielded cubic metre of concrete.”

8. Equivalent flexural strength/Characteristic flexural strength Table 2 below is reproduced with acknowledgment to N. V. Bekaert S.A. (21). It is one of several tables contained in a 1997 publication relating to Dramix® products, and the author acknowledges that these particular products may no longer be at the forefront of the Bekaert product line. None the less, the equivalent flexural strength values cited in these tables are of interest.

Table 2 – Dramix Design Guidelines (ex Table 3)

For the design of industrial floors and pavements, the current Cement Concrete & Aggregates Australia publication T48 (22) suggests that the characteristic flexural tensile strength of concrete may be determined as: f’cf = 0.7 f’c MPa (An additional 10% of the calculated value may be adopted as the 90 day value of f

’cf).

The calculated values of f’cf for a range of commonly utilised commercial grades of concrete in Australia may be shown as in Table 3: Table 3 : Correlation between f’c and f’cf (90)

f’c MPa

f’cf (90) MPa

32 4.35

40 4.87

50 5.44

It is immediately obvious that – in broad terms – there is little difference in the characteristic flexural strengths of concrete either with or without steel fibres until the fibre dosage levels become significant, say > 50 kg/m

3 or 0.65% by volume.

9. Conclusion and Recommendations Steel fibre reinforced concrete (SFRC) is an established product with wide acceptance in Australia and in many countries overseas. The technical literature about SFRC is extensive and research is continuing with several recent masters and doctoral theses being focussed on the material. Before deciding to specify SFRC, Australian design engineers should be aware that: 1. At present, there is no Australian Code recognition of SFRC.

2. There are no overseas codes which provide formal guidance for the design of SFRC

components, other than limited recent guidance in the USA for some shear applications.

3. UK Concrete Society Technical Note No. 63 provides (arguably) the best guidance for designers wishing to specify SFRC

4. At low fibre dosage rates (20 – 45 kg/m

3) the properties of SFRC must be regarded as

essentially similar to the properties of plain concrete of comparable characteristic compressive and flexural strengths.

5. There are (presently) limitations to the volume of steel fibres which may be incorporated in a

concrete mix, such limitations restricting the potential benefits which may have been demonstrated under laboratory conditions.

6. Reliable specification of SFRC for a particular project requires the formulation of a

dedicated testing regime involving the selected concrete supplier (and knowledge of component material sources etc.) and subsequent calibration of test results against specification bench marks.

10. References

1. Balaguru, P.M., and Shah S.P., “Fibre-Reinforced Cement Composites”, McGraw-Hill,

1992. 2. Lorentsen, M., “Steel Fibre Concrete for Structural Elements”, Steel Fibre Concrete U.S. –

Sweden Joint Seminar, Stockholm, 1985. 3. Batson, G.B., “Use of Steel Fibres for Shear Reinforcement and Ductility”, Steel Fibre

Concrete U.S. – Sweden Joint Seminar, Stockholm, 1985. 4. “Fiber Reinforced Concrete”, ACI Committee 544, ACI 544.IR-96. 5. Lőfgren, I., “Fibre-reinforced Concrete for Industrial Construction – a fracture mechanics

approach to material testing and structural analysis”, Ph.D. Thesis, Department of Structural Engineering, Chalmers University of Technology, Gőthenberg, Sweden (2005).

6. “Fibresteel Technical Manual”, Bosfa Dramix® Fibresteel, 1996. 7. “Guidance for the Design of Steel-Fibre-Reinforced Concrete”, Technical Note No, 63,

Concrete-Society Working Group, March 2007. 8. “Design guidelines for Dramix® steel wire fibre reinforced concrete”, Bakaert, 1997. 9. Bentur, A., and Mindess, S., “Fibre reinforced cementitious composites”, Taylor & Francis,

2006. 10. Concrete Society TR34, “Concrete industrial ground floors”, 2003. 11. Davis, A.J., and Appleyard, L.D., “SFRC Pavements – A rational design approach for

thickness design”, Concrete Institute of Australia, Perth, 2001. 12. Current Practice Note 35, “Fibres in Concrete”, Concrete Institute of Australia, 2003. 13. Australian Standard AS3600 – 2009 : Concrete Structures. 14. New Zealand Standard NZS3101 : Part 1 : 2006 – The design of concrete structures. 15. EN1992-1-1 Eurocode 2 : Design of concrete structures – Part 1-1 : General rules and

rules for buildings. 16. DIN EN 14889-1 Fibres for Concrete – Part 1 : Steel Fibres, Definitions, specifications and

conformity, 2006.

17. DIN EN 14845-1 Test methods for fibres in concrete – Reference concrete.

18. DIN EN 14845-2 Test methods for fibres in concrete – Effect on concrete.

19. Ross, A., “Steel Fibre reinforced concrete (SFRC) – Quality, performance and specifications”, 2010.

20. Annexure R83/8 – Steel Fibre Reinforced Concrete – NSW Roads and Maritime Services,

2013.

21. Dramix® “Design Guidelines for Dramix® steel wire fibre reinforced concrete”, N.V. Bekaert, S.A., 1997.

22. Guide to Industrial Floors and Pavements – design, construction and specification”.

Publication T48, 2009.

APPENDIX A

Comparison Table for Steel Fibre Dosage

Reference Steel Density – 7860 kg/m

3

kg/m3

Volume Fraction

10 0.13%

15 0.19%

20 0.25%

25 0.32%

30 0.38%

35 0.45%

39.3 0.50%

40 0.51%

45 0.57%

50 0.65%

55 0.70%

60 0.76%

70 0.89%

78.6 1.00%

80 1.02%

117 1.50%

157.2 2.00%

Highlighted boxes are the principal reference values of

0.5%, 1%, 1.5% and 2% equivalences

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Vitt, G., Crack control with combined reinforcement: from theory into practice, Concrete Engineering International, Vol. 9, No. 4, Winter 2005. American Society For Testing And Materials, ASTM C 1609, Standard test method for flexural performance of fiber-reinforced concrete (using beam with third-point loading), ASTM, West Conshohocken, Pennsylvania, USA , 2005. Jones, A.E., and Cather, R., Ultra-high performance fibre-reinforced concrete, Concrete Engineering International, Vol. 9, No. 1, Spring 2005. “Guide for the Design and Construction of Fibre-Reinforced Concrete Structures”, Italian National Research Council CNR-DT 204/2006, November 2007. Foster, S.J., “The Application of Steel Fibres as concrete reinforcement in Australia : from material to structure”, Materials and Structures, 42, 1209 – 1220, 2009. Deluce, J.R., and Vecchio F.J., “Cracking Behaviour of Steel Fibre-Reinforced Concrete Members containing conventional reinforcement”, ACI Structural Journal/ May – June 2013. Minelli, F., and Plizzari, G.A., “On the effectiveness of Steel Fibres as Shear Reinforcement”, ACI Structural Journal/ May – June 2013. B. Recent Masters and PhD Theses of relevance Jayakumar, P., “A Study on the Cracking Behaviour of Steel Fibre Reinforced Concrete Beams”, Department of Civil Engineering, Trivandrum, India, 2010. Abid, A., and Franzen K., “Design of Fibre Reinforced Concrete Beams and Slabs”, Department of Civil and Environmental Engineering, Chalmers University of Technology, Gothenburg, Sweden, 2011. Karki, N.B., “Flexural Behaviour of Steel Fibre Reinforced Prestressed Concrete Beams and Double Punch Test for Fibre Reinforced Concrete”, University of Texas, Arlington, USA, 2011. Jansson, A., “Effects of Steel Fibres on Cracking in Reinforced Concrete”, Department of Civil and Environmental Engineering, Chalmers University of Technology, Gothenburg, Sweden, 2011.