Concrete - May 2017

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Mǘǰ ǻǹǺȀ 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.