Civil Engineering Reference
In-Depth Information
time required to fix the reinforcement. This can have a positive effect on the sequence of
construction operations, which demands particular attention in tunnelling because the site
is linear. The use of steel fibre concrete offers the opportunity to considerably shorten the
time required.
Load-bearing properties of steel fibre concrete.
Steel fibre concrete is a construction
material, which is able to resist tension only to a limited extent. Its tensile strength is deter-
mined by the tension strength of the concrete matrix. In contrast to unreinforced concrete,
the formation of macro-cracks under tension and bending tension loading do not lead to
sudden failure. The steel fibres work to prevent the formation and widening of cracks; the
influence on crack formation and crack propagation behaviour of concrete starts with the
formation of micro-cracks and results in a finer distribution of the cracks in the structure
with smaller crack widths.
Figure 3-19
Bending moments and subgrade
reactions with reduced bending stiffness.
The use of steel fibre concrete in tunnelling is limited from the structural point of view.
The conditions, with high compression force acting on a tunnel lining combined with
relatively low bending moments, are indeed basically suitable for steel fibre concrete, but
the load-bearing capacity of steel fibre concrete reduces considerably with increasing ec-
centricity of the normal compression force. If the loading on a tunnel lining leads to rein-
forcement being required for structural reasons, it is normally difficult to verify structural
stability for steel fibre concrete as an alternative.
Load‑bearing behaviour of tunnel linings.
Static systems for the structural analysis of
tunnel linings under consideration of the ground-structure interaction have a high degree
of static indeterminability. While a tunnel lining that is relatively stiff in bending com-
pared to the ground bears most of the acting load itself, a lining that is weak in bending
suffers more deformation, which activates a stronger subgrade reaction of the ground and
thus reduces the share of load-bearing on the lining. In a lining that has been deformed by
“weakening”, a resulting loading form is established, for which the geometry of the struc-
ture forms approximately the line of thrust, meaning that bending moments are reduced
and normal forces increase [28].
A construction material that is optimal for tunnel linings is thus characterised by the prop-
erty of accepting high deformation without failing and resisting high normal forces. Quali-
tatively, this applies to steel fibre concrete. When the forces and moments in the section
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