Civil Engineering Reference
In-Depth Information
broken areas of the shotcrete layer. This means that damaged areas in the vault may not be
noticed and are then sprayed over with the application of the next layer.
When the layer consists of steel fibre shotcrete, damaged areas are more easily recognised
or come loose independently after blasting and can be repaired with the application of the
next layer.
As a conclusion, it can be maintained that tunnel outer linings of steel fibre shotcrete have
a more homogeneous structure, less defects in the form of spraying shadows, a better bond
between the individual layers of shotcrete and show altogether better load-bearing capac-
ity and permeability than mesh-reinforced shotcrete.
Comparison of the load‑bearing capacity in bending.
In order to compare the load-
bearing capacity in bending of mesh-reinforced and steel fibre shotcrete, a parameter study
was carried out for a 25 cm thick shotcrete cross-section of concrete grade B25 to develop
moment-curvature curves (
M-
k curves) for various levels of the normal compression force
N. In order to describe the relevant level of normal force, the term load factor is used be-
low to describe the relationship between the compression force under consideration and
the permissible compression force under axial loading.
For the mesh-reinforced shotcrete, two layers of reinforcement (each Q188) and the pa-
rabola-rectangle diagram from DIN 1045 [53] were assumed for the production of the
M-
k curves.
For steel fibre shotcrete A, a ratio of strength parameters of
f
c
/
f
t1
/
f
t2
/
f
t3
= 1/0.1/0.03/0.015
was assumed. In deviation from DIN 1045, the compression strain parameters were set at
e
c1
= -2.2 ‰ and e
cu
= -4.5 ‰. In addition, a 5 % higher compression strength was assumed
for steel fibre shotcrete B in order to quantitatively consider the qualitative influential fac-
tors listed in Section 4.2 (Fig. 3-25).
Fig. 3-25 shows an example of the
M-
k
curves determined for load factors of 25 % and
50 % normal compression force. It can be clearly recognised that much higher curvatures
of the cross-section are reached for steel fibre shotcrete at a load factor of 25 % than with
mesh-reinforced shotcrete. In the latter case, the bending moment that can be resisted is
about 7 % higher. Due to the small height of the compression zone when the load factor
of the normal force is low, scarcely any difference can be determined between steel fibre
shotcretes A and B.
At a load factor of the normal force of 50 %, the bending moment that can be resisted by
the mesh-reinforced shotcrete is still about 2 % higher than that of steel fibre shotcrete A.
Compared to steel fibre shotcrete B, however, the mesh-reinforced shotcrete shows about
2 % less bending moment. Analogously to the load factor of 25 %, the steel fibre shotcretes
reach greater curvatures.
Fig. 3-26 shows as a summary of the parameter study the relationship of the bending
moment capacity between the mesh-reinforced shotcrete and the steel fibre shotcrete B
depending on the load factor of the permissible compression force. While at lower normal
force load factors, the mesh-reinforced cross-section can resist higher bending moments,
the steel fibre shotcrete has the advantage at higher load factors (Fig. 3-26).
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