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concrete

39

FIBRES

testing against approved project performance

specifications.

Panel test methods have the advantage of

low variability and due to the fact that the

test specimens are the sprayed panels, the

specimens do not have to be prepared (sawn)

from it, which eliminates additional labour,

costs and potential influences on the test

results.

Continuously supportedpanel tests

Toughness, or energy absorption capacity,

is the first and most basic requirement for

fibre-reinforced sprayed concrete in order to

provide sufficient performance data at larger

deformations. For sprayed concrete linings,

a panel test that measures energy absorption

is the more relevant test method to represent

a lining working and failure mechanism.

Panel tests (EFNARC

(2)

, EN 14488-5

(7)

)

are statically indeterminate (hyperstatic),

just as the lining itself is, allowing stress

redistribution and multiple cracking and

thus, represent the structural behaviour of

the lining significantly better than a simply

supported beam.

These panel tests involve the application

of a central load to a square or round panel,

which is continuously supported along all

four edges or along the perimeter. This

models the typical punching effect of a

rock bolt on the sprayed concrete lining.

The hyperstatic test set-up allows for stress

redistribution so that, depending on the

performance of the fibres,multiple cracks can

develop. Furthermore, the panel test measures

significantly larger deformation (out to

25mm), and thus provides a much more

accurate model of a real sprayed concrete

lining behaviour than a beam test.

Panel tests have a significantly lower

Formation of multiple crack patterns, which more effectively

simulate the hyperstatic nature of a sprayed concrete lining.

variability than beam tests because a much

larger (cumulative) crack length develops

due to multiple cracking,meaning COVs of

less than 15% can be achieved in these panel

tests. Thus, the meaningfulness and the

reliability of panel testing are much higher.

Consequently, panel testing requires fewer

specimens per set than do beams in order to

yield stable average numbers – that is, project

costs can be reduced using fewer panel tests

than beam tests.

It must be noted that panel tests such as

EN 14488-5

(7)

suffer a limitation due to

friction effects. By supporting the panels

continuously, a high level of friction is

induced between the panel and the support

frame in the test set-up. This occurs because

as the deflection increases, the panel tends to

move over and subsequently rotate around

the support. The free movement is impeded,

however, as a result of friction at the support

surface, which induces an axial stress in the

panel. This effect can considerably increase

the apparent performance of the panel (see

Bjøntegaard and Myren

(8)

). This makes it

more difficult to assess the effective fibre

performance for comparison.

Rounddeterminatepanel test

The round determinate panel test (RDP test,

ASTMC1550

(9)

) was developed in Australia

in the early 2000s, as a means of more

accurately determining the performance

criteria for fibre-reinforced sprayed

concrete for underground mining and

tunnelling. In contrast to the continuously

supported panels, this test set-up is statically

determinate, as the round panel sits on three

pivoted support points. Thus, a predefined

crack pattern with three radial cracks between

ASTM C1550 round determinate

panel test set-up.