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concrete

37

FIBRES

S

prayed concrete linings require a

rotation capacity in order to follow

ground movements while the

arching effect in the surrounding

rock mass develops. The fibre reinforcement

overcomes the brittleness of concrete

and provides the necessary ductility after

the sprayed concrete cracks, to prevent a

sudden failure. Due to the load-bearing

capacity available after cracking, the

failure mechanism is transformed from a

brittle mode characteristic of unreinforced

concrete to a nearly elasto plastic mode.

Therefore, energy absorption or residual

strength performance criteria (post-crack

performance) are generally required to specify

and rate performance.

When evaluating fibre-reinforced sprayed

concrete, it is very important to use a test

methodology that is well representative of

the loading mechanism and reflects the full

capabilities of the fibre. Since primary sprayed

concrete linings in hard rock tunnelling and

mining are typically relatively thin, and the

lining follows the naturally irregular rock

surface, the lining on an excavation cross-

section scale does not provide a structural

bearing arch. As such, rather than failing

due to a combined thrust/bending load as in

thick-shell lining, any failure mode is most

likely governed by bending induced by a more

or less concentrated load, sometimes referred

to as a ‘punching load’. Based on this failure

mode, structural tests to evaluate the post-

crack performance of the lining are typically

conducted on different types of panels rather

than on classical beams.

Best practice for testing fibre-

reinforced sprayed concrete

Fibre-reinforced sprayed concrete is used inmany underground applications

from low-stress friable

ground to deep-level hard rockmines. It can be subjected to harsh conditions, including very high static

stresses, high temperatures, dynamic loading and a very corrosive environment. When designing a

surface support system, such as a fibre-reinforced sprayed concrete lining, it is important to understand

how the systembehaves and themechanismof loading towhich that lining supportmay be subjected.

RalfWinterberg

of

ElastoPlastic Concrete

reports.

EN 14651 simply supported

beam test set-up.

Design

Since the mid-1990s,multiple bodies have

developed guidelines or Standards for the use

of fibre-reinforced sprayed concrete. These

documents provide guidance that is generally

independent of the fibre material, be it steel

or synthetic.

The design of fibre-reinforced sprayed

concrete linings herein is largely based on

energy absorption capacity in ultimate limit

state obtained from panel test results. The

panel tests were specifically developed to

assess the performance in larger deflections

that need to occur to enable the ground to

stabilise and take the stresses by creating

the arching effect in the surrounding rock

mass itself, typically for NATM tunnelling

methods (see Nitschke andWinterberg

(1)

).

Testmethods

Simply supportedbeamtests

The simply supported beam test

(EFNARC

(2)

, EN 14488-3

(3)

, ASTM

C1609

(4)

,EN 14651

(5)

) has serious limitations.

A beam test is not representative of a sprayed

concrete lining, as it does not correctly

reflect the behaviour and performance