Civil Engineering Reference
In-Depth Information
With this type of corrosion, also known as galvanic corrosion, the reinforce-
ment steel becomes depassivated due to chloride in the saturated side,
whereby simultaneously on the side which is not saturated with water - the
aerated side - adequate oxygen can enter.
Attention for this factor is of direct importance to the WesterscheldeTunnel: the
reinforcement on the outside of the tunnel could corrode due to an oxygen
supply on the inside of the wall. This could occur in the short term at places
where the reinforcement on the outside has little or no covering, for example
due to (unknown) damages which occurred during the building-in process.
Investigation into monitoring possibilities
As mentioned, the client undertook investigations into the possibilities to
monitor aspects in the Westerschelde Tunnel which are relevant for the life
span and the management of the tunnel.To this end, a pilot monitoring sys-
tem was built-in during the construction and fitting out of the tunnel. The
most important limiting condition for the investigation was that the moni-
toring on the life span aspects had to be geared towards the determining of
the exceeding of the service limit states. This provides the operator of the
tunnel with the possibility of intervening, prior to the chance of an ultimate
service limit being exceeded becoming unacceptably big.
Prior to the design and building-in of the pilot monitoring system, it was
firstly determined on the basis of a so-called 'potential investigation', what
the most suitable locations in the tunnel were to undertake the measure-
ments. Low potentials could indicate corrosion and thus possible damage
to the outside of the lining. It is precisely these 'critical' locations which pro-
vide the most relevant and informative data for a monitoring system, cer-
tainly where it concerns warning detection sensors. Besides, the locations
in the tunnel preferably had to be well accessible and hopefully would pro-
vide altered data in the short term. Particularly the places where repairs also
had been carried out on the inside of the tunnel, pre-eminently made these
locations suitable for monitoring.
Monitoring repairs
The spots of damage which arose during the construction of the rings were,
where possible, repaired with a cement mortar. The possibility exists that
these repair spots would loosen from the lining in the course of time. The
direct consequences of this may not be serious, because the stainless steel
reinforcement net in the fire-resistant cladding acts as a 'safety net'. The
chances that pieces of rubble end up on the road surface are therefore
extremely small: the loose repairs are more or less kept in place. At the
same time this means that the loosening of repaired spots would generally
not be visible, certainly not at the bottom of the tunnel, where the lining is
covered with the sand-cement stabilization.
However, it is of extreme importance for the management and maintenance
of the tunnel, to have an insight into the behaviour of repairs. Not only has
the protective function of a loosened repair been lost, it also means an
improper load of the reinforcement network of the heat-resistant cladding.
In order to determine the loosening of repairs, there was a possible choice
of various types of sensors: resistance strain gauges, displacement trans-
ducers (with a wider range) and the use of a breaking strings with which
cracking can be determined. The choice was made for the use of distortion
sensors based on the resistance strain gauge principle. This provided the
best chance to observe an altered signal in the short term - and thus distor-
tion.The resistance strain gauges were especially developed for application
in concrete and positioned as such, that the loosening of a repaired spot
could be determined in two directions.
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