Civil Engineering Reference
In-Depth Information
An important aspect which plays a vital role in the spalling of concrete, is the
rapid evaporation of water present in the concrete, whereby steam-forming
occurs in the concrete. Dependent on the degree of permeability for water
vapour, high compressive stresses build up in the pores of the concrete and
tensile forces appear.
In any case, the nature of the system of pores changes considerably at an
increasing temperature and it is particularly this aspect which makes the
spalling phenomenon difficult to calculate. Further, the occurring temperature
gradients also cause tensile forces as well as minor difference in coefficients of
expansion between the composite parts of the concrete at high temperatures.
If the increasing of the temperature - as a function of time - remains below
a certain threshold value at a particular place in the concrete, there will be
sufficient time for the steam to escape from the concrete without causing
major tensile forces. Aside from that, dry concrete is substantially less sen-
sitive to spalling than concrete which contains water. Although - given a certain
quality of concrete, a fire curve, the size of tensile forces present, etc. -
it is not known exactly, below which moisture content minor or no spalling
occurs.
The fire tests undertaken in view of the Westerschelde Tunnel project, were
carried out with concrete specimens with a 'natural' moisture content. This
means that the specimens were sealed with plastic foil after being demoulded,
so that no water could evaporate, but also that no water could enter from out-
side. This provided a well defined starting point regarding moisture for the
spalling test, which justified the circumstances expected in practice: the con-
crete of the lining of a water-restraining tunnel will sooner become wetter than
drier in the course of time. An exception to this is perhaps formed by a rela-
tively thin zone of 10 to 20 mm near the concrete surface on the inside of the
tunnel. However, it appears that this dry zone does not prevent the spalling of
concrete: in some of the tests carried out on behalf of the Westerschelde
Tunnel, scales measuring approximately 50 mm thick chipped off the concrete.
Intrinsic to the factor of spalling, is that important scale effects are present.
Each time, both in the laboratory and at the fire in practice, it appeared that
the spalling had advanced the most in the middle of the tunnel elements.
The damage near the edges has the tendency to lag behind a great deal.This
can be explained due to edges having a different temperature and stress
distribution than in the middle of a plate. For the carrying out of the spalling
tests this consequently meant that the specimens must be rather large.
Immersed tunnels versus bored tunnels
As already pointed out in the introduction, the function of the reinforcement
in the lining of the bored tunnels to be (or are) built in the Netherlands dif-
fer substantially from the function of the reinforcement of the concrete of
immersed tunnels. Generally this means that the perpetual functioning of
the reinforcement for immersed tunnels is essential, whereas the function
of the reinforcement for bored tunnels is limited in the final situation: the
reinforcement is particularly necessary to reduce the damage during the
building-in of the elements.
In terms of the permissible damage during and after a fire, this means that
for immersed tunnels the temperature of the reinforcement may not rise too
high and that the reinforcement must remain adherent in concrete that is
still 'healthy'. This means that for immersed tunnels almost no spalling is
permitted. This doesn't happen either because, by comparison with bored
tunnels, a lower quality of concrete is applied. It is different for a bored
tunnel - thanks to the circular shape and the rather massive normal forces
thus present. Applicable here, is particularly the criteria that the concrete
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