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M

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concrete

FIBRES

the support points will develop independent

of fibre type or performance. This yields

significantly reduced variability in the results,

thus COVs of 10% or lower can be achieved.

This test specimen has the same depth as

an EFNARC beam (75mm), but involves

1200mm of crack length out to 40mm

central displacement, which is far more

representative of deformations experienced

by a ground support system, especially with

regard to larger ground deformation in-situ.

The small depth of the panel further ensures

that bending is the primary mode of failure,

excluding shear influences and arching

effects, just as happens in a typical thin fibre-

reinforced sprayed concrete lining.

The introduction of a second deflection

criterion at 10mm in the RDP test accounts

for in-service considerations (SLS) for

situations where crack widths must

be limited. This was first proposed by

Papworth

(10)

in order to overcome the need

for beam testing.

The RDP test also involves a lower level

of inherent friction at the supports compared

to the EFNARC test. The contribution

of friction to energy absorption capacity

in this test is significantly lower compared

to continuously supported panel tests (see

Bernard

(11)

), so the effect of axial stress on

the panel is reduced, providing a much

more accurate comparison between the

performance of different fibre types, which is

the primary aim of these tests.

In addition, RDP panel test results can

be correlated to EFNARC panel results.

Bernard

(12)

found a correlation of:

R

2

= 0.88 for EFNARC

25mm

= 2.5 × RDP

40mm

(both in Joules).

This allows for comparison between results

that are obtained using regionally different

Standards.

Concluding remarks

When choosing a test for fibre-reinforced

sprayed concrete, it is necessary to nominate a

test methodology that reflects what is actually

occurring in the field, ie, how such linings

work under site conditions. Because the mode

of failure experienced in the specimen is more

representative of in-situ lining behaviour, a

panel test should be nominated in preference

to simply-supported beam tests for the

performance evaluation of a fibre-reinforced

sprayed concrete.

There are similarities and differences

between the panel test methods discussed

above, but if the aim of the testing is to

determine the best fibre for a project, then

the ASTMC1550

(9)

round determinate

panel test is arguably the most suitable

Bespoke support points of the RDP test lower

friction and enable a repeatable 1200mm-long

crack pattern, thus lowering variability.

methodology to use. This is because it

involves a large crack area and significantly

reduced friction, aiming at measuring the

performance of the fibre as the main outcome

of the test. The RDP test overcomes

problems related to cost, reliability and

repeatability, which are shortcomings of the

other standardised test methods.

Standard testing at 28 days does

not adequately describe the long-term

performance of a fibre-reinforced sprayed

concrete lining. It is advisable that designers

also request testing at a later age, eg, at

90 days, to better assess the long-term

performance.

References:

1. NITSCHKE, A. andWINTERBERG, R. Performance of

macro synthetic fiber reinforced tunnel linings.

ProceedingsoftheWorldTunnelCongress2016

, San

Francisco, USA, 22–28 April 2016.

2. EUROPEAN FEDERATION FOR SPECIALIST

CONSTRUCTION CHEMICALS AND CONCRETE

SYSTEMS.

EuropeanSpecificationforSprayed

Concrete

. EFNARC, Farnham, Surrey, 1996, available

at:

www.efnarc.org .

3. BRITISH STANDARDS INSTITUTION, BS EN 14488-3.

Testingsprayedconcrete.Part3–Flexuralstrengths

(firstpeak,ultimateandresidual)offibrereinforced

beamspecimens

. BSI, London, 2006.

4. AMERICAN SOCIETY OFTESTING ANDMATERIALS,

C1609/C1609M.

StandardTestMethodforFlexural

PerformanceofFiber-ReinforcedConcrete(UsingBeam

WithThird-PointLoading)

. ASTM International,West

Conshohocken, PA, USA, 2010.

5. BRITISH STANDARDS INSTITUTION, BS EN 14651.

Test

methodformetallicfibreconcrete–Measuringthe

flexuraltensilestrength(limitofproportionality(LOP),

residual)

. BSI, London, 2005.

6. PICKETT, A. andTHOMAS, A.Where are we nowwith

sprayed concrete lining in tunnels?

Shotcrete

Magazine

, Fall 2013, American Shotcrete Association,

Farmington Hills, MI, USA.

7. BRITISH STANDARDS INSTITUTION, BS EN 14488-5.

Testingsprayedconcrete.Part5–Determinationof

energyabsorptioncapacityoffibrereinforcedslab

specimens

. BSI, London, 2006.

8. BJØNTEGAARD, Ø. andMYREN, S.A.

Energy

absorptioncapacityforfibrereinforcedsprayed

concrete.Effectoffriction inroundandsquarepanel

testswithcontinuoussupport(Series4).Technology

reportNo.2534

, Norwegian Public Roads

Administration, Directorate of Public Roads,

Technology Department, 2009.

9. AMERICAN SOCIETY OFTESTING ANDMATERIALS,

C1550.

StandardTestMethodforFlexuralToughnessof

FiberReinforcedConcrete(UsingCentrallyLoaded

RoundPanel)

. ASTM International,West

Conshohocken, PA, USA, 2012.

10. PAPWORTH, F. Design guidelines for the use of fibre

reinforced shotcrete in ground support.

Proceedings

of27thConferenceonOurWorld inConcrete&

Structures

, Singapore, 29–30 August 2002.

11. BERNARD, E.S.The role of friction in post-crack

energy absorption of fibre reinforced concrete in the

round panel test.

JournalofASTM International

,Vol.2,

No.1, January 2005.

12. BERNARD, E.S. Correlations in the behaviour of fibre

reinforced shotcrete beamand panel specimens.

MaterialsandStructures

, RILEM,Vol.35, pp.156–164,

April 2002.