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Review of FRC brittle failure considerations Mar 2015

Charles Allen, Director, OtB Concrete Ltd, London
As a member of the International Tunnelling Association’s ITAtech Committee Activity Group on support and lining, Charles Allen of the UK offers comments on references in recent technical articles and papers on the suggested long-term loss of ductility of steel fibre reinforced concrete (SFRC) arguing that there are inappropriate conclusions being promoted about brittle failure concerns and that it is the selection of the wrong fibre (steel or synthetic) for the different reinforced concrete applications that cause a loss of ductility, not the concrete itself.

The design criteria and testing procedures for production of fibre reinforced concrete segmental linings has become confused with the same processes for sprayed concrete or cast concrete linings. Segmental linings are specific structures and their design criteria and testing procedures should be considered specifically and not in conjunction with fibre reinforced shotcrete or in situ concrete linings.

In several papers published recently and presented at various international conferences, there has been, in my view, flawed consideration of the embrittlement phenomenon of fibre reinforced concrete.

In a paper published and presented in 2008 at the Shotcrete conference in Norway entitled Embrittlement of Fibre Reinforced Shotrete, author Stephan Bernard tested outdoor aged FRC panels in the round plate test (ASTM C1550) to deflections of 5,10,20 and 40mm.

Copy of the paper’s Fig 2. Load-deflection curves for specimens reinforced with Dramix RC65/35 in 7252 psi (50 MPa) shotcrete tested at various ages after spraying, showing an increase in load resistance at small deflections with aging but a fall in load resistance at large deformations
Copy of the paper’s Fig 2. Load-deflection curves for specimens reinforced with Dramix RC65/35 in 7252 psi (50 MPa) shotcrete tested at various ages after spraying, showing an increase in load resistance at small deflections with aging but a fall in load resistance at large deformations

This test and the deflections noted are not applicable to FRC for the design and quality control testing of concrete for segmental linings. The paper is more relevant to shotcrete linings in mines, where large deflections occur and are acceptable.

Fig 2 of the paper illustrates improved ductility of the shotcrete containing Dramix RC65/35 steel fibres at deflections of less than 5mm, at all ages up to two years. In fact, Bernard acknowledges this in the paper that “All the steel FRS mixtures exhibited an increase in energy absorption at small deformations with age, while concurrently exhibiting a fall in energy absorption at large deformations.”

Bearing in mind the fact that we do not design FRC segmental linings to accommodate large deformations, reference to this paper should not be considered as a source of technical information for designers of FRC segmental linings as it has no relevance to the design of FRC segments.

In another paper by Bernard and presented at the Australasian Tunneling Conference in 2014 entitled Age-dependent changes in postcracking performance of fibre reinforced concrete for tunnel segments, the author again promotes the phenomenon of “embrittlement” of steel fibre reinforced concrete tunnel lining segments.

In this paper, Bernard tests samples of laboratory beams, cured at 230C to the ASTM C1609 beam test method. The beams are tested to deflections up to 4mm, which is substantially more relevant to segment design than the round plate test.

The steel fibres selected for the tests were Dramix RC65/60BN. This type of fibre is definitely one that I and many other design engineers would not consider for segmental linings nowadays.

There appears to be a lack of understanding of the concept ofconcrete maturity and consequent strength development in this paper. Similarly to the shotcrete embrittlement paper of 2008, the concrete mixtures tested contained a combination of Portland cement, PFA and GGBFS. The latter materials will contribute latent pozzolanic and cementitious hardening to concrete at ages up to one year or so – when cured in standard laboratory conditions of 20 or 23oC.

Tunnel segments are more often manufactured using a heat curing chamber, in which the concrete temperature will peak at about 50 to 60oC. This imparts a high level of maturity into the concrete, thereby enabling a demoulding strength to be achieved in a matter of hours. Due to this requirement for very early age strength developments, almost all concrete for segmental linings will have a characteristic strength of 60 MPa or even greater.

In my experiences with heat-cured concretes, there is very little strength gain between seven and 28 days and virtually none after 28 days. The comparison, therefore, to laboratory-cured tests of concrete for in-situ tunnel linings is not relevant. All major design standards acknowledge the fact that in-situ concrete has a lower strength than laboratory cured samples.

In addition, I would suggest that any references to long-term ‘embrittlement’ (loss of ductility) of steel fibre concrete is addressed professionally and scientifically with selection of the correct type of fibre for high strength concrete. Selection would include consideration of fibre geometry and anchorage, fibre tensile strength, etc. Pre-production testing of mixtures for the specific purpose of producing tunnel lining segments is vital to prove the performance of the SFRC.

For a current major tunnel project in London, on which I have been involved with for several years, segmental linings were produced with C50/60 concrete and steel fibres. In two years of weekly beam sample production testing there has not been a single incidence of brittle failure.

Similarly, the same project has been using the latest development in steel fibres in C50/60 concrete for an in-situ tunnel lining. Concrete cube strengths have been in the order of 90MPa at 28 days and again, there has been no single case of brittle failure.

I have searched for work by other researchers to corroborate Bernard’s claims of the embrittlement phenomenon of SFRC but have failed to find anything.

Brittle (fibre tensile strength) failure of steel fibre reinforced concrete (SFRC) can occur at any age of the concrete, from one day to 10 years, and is a consequence of selecting an incorrect fibre type for the concrete, not the age of the concrete.

References

           

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