Civil Engineering Reference
In-Depth Information
Week 1
1 segment
Week 2
2 segments
Week 3
3 segments
Week 4
4 segments
Week 5
5 segments
Week 6
5 segments
Week 7 onwards
6 segments.
14.3.12 Diffi culties in casting the segment
The principal diffi culties in casting segments are in ensuring that concrete has fl owed
around the bottom corner of the web into the bottom fl ange and that the concrete
is well compacted at the bottom of the webs, in particular when there are heavily
reinforced anchorage blisters at the junction of web and bottom fl ange, Figure 9.25
and Figure 14.12. The fl uidity of the concrete and the management of the shutter
vibrators are both critical to success. Generally the slump of concrete poured into the
webs needs to be of the order of 150 mm, while the concrete for the top and bottom
slabs typically has a slump of around 100 mm.
The concrete is usually fi rst poured onto the bottom slab soffi t shutter through
trunking that passes through a trapdoor in the top slab shutter. The concrete is spread
by hand, and kept short of the bottom of the web. Concrete is then poured down
the webs, aided by poker vibrators and short bursts from the shutter vibrators, until
it can be seen fl owing out of the bottom of the webs and joining up with the bottom
slab concrete, demonstrating that the webs are full. If the bottom slab has haunches,
these have a top shutter, which is suffi cient to ensure that concrete will not well up in
the bottom slab. However, if the bottom slab has no haunches, it may be necessary to
place temporary top shutters over part of its width. If the shutter vibrators are used too
intensively, they may also cause the concrete to fl ow uncontrollably out of the bottom
of the webs. The learning curve serves in part to tune the details of the construction
process, and in particular to learn how to use the shutter vibrators.
The top shutters of the bottom slab haunches and of bottom corner anchorage
blisters should have removable inspection panels. Once the correct casting procedures
and concrete consistency have been established these panels may never need to be
opened again, but in the learning stages they can avoid much remedial work and even
discarded segments.
14.3.13 Lifting the segment
Typical segments need to be lifted three or four times before they are assembled with
the bridge deck. As segments are unique in their position in the bridge deck, as well as
for reasons of safety, the lifting system has to be absolutely reliable.
Lifting systems may be active or passive. Active systems involve stressing a lifting
beam to the top slab of the segment with short prestressing bars. The residual force in
the bars should be equal to the weight of the segment plus 20 per cent, to cover any
dynamic effects during lifting. When the crane applies tension to the lifting beam in
order to lift the segment, the contact pressure between beam and segment is reduced,
but the tension in the bars does not change. The bars have been fully tested when the
lifting beam was stressed on, and any failure due to damaged bars or anchor nuts will
