Civil Engineering Reference
In-Depth Information
Crack-forming
Cracks come about due to force actions between the segments which didn't
take place as desired and for example, local force concentrations occur.That
would be the case when the segments are not entirely flat at the points where
they connect to the other segments. Later it appeared (see also
Chapter 9 -
'Tunnel Lining') that the longitudinal joint indeed was not straight but slightly
curved. That provided force concentrations which caused crack-forming.
Triplex instead of kaubit: less forces on the dowels
Between the rings themselves, there is a coupling by means of the so-called
'Nocke-Topf' connection (recess-dowel).This connection is meant to take up
radial forces: when the rings move mutually (as a major over-exaggeration:
being pushed out of the tube shape), the associated transfer of forces take
place via the dowels. A ring which deforms, is thus supported by the adjacent
ring.The dowels however, hardly play a role in the transfer of the tremendous
jack pressures in the axial (longitudinal) direction. Here the reinforcement is
of essential importance, whereas the place where the force transfer take place
is 'fixed'.
Initially such fixation occurred by means of kaubit, a bituminous material.
Besides, the kaubit serves to smooth out possible unevenness of the con-
crete. What appeared to be the case however, was that when kaubit is
pressed together into a layer of negligible thickness, it acts as a lubricant.
That also meant that in respect of each other, the rings could deform easier.
When that happens, the dowels make contact sooner, they become (too)
heavily loaded and the chances of shearing increases.
Therefore the choice was soon made for the application of triplex panels
instead of kaubit, in order to affix the force transfer in the longitudinal direc-
tion.This method had previously been used in the construction of the Second
Fig. 11.20
Triplex panels instead
of kaubit
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