Civil Engineering Reference
In-Depth Information
Resin is also likely to squeeze into the prestress ducts, and if allowed to harden
would impede the threading of the tendons. A suitable tool should be pushed down
each duct to fl atten the liquid resin against the walls of the duct.
If the match-cast faces are damaged prior to the erection of the segments, concrete
or mortar repairs must remain two or three millimetres low so that they do not
compromise the accuracy of the match casting. When the segments are erected, an
extra thickness of resin may be applied to this area, in the hope that the defi ciency in
concrete will be made up by the resin. However, the cured resin has a lower Young's
modulus than concrete, and also creeps under sustained load. As a result, the damaged
area is likely to have a lower compressive stress in service, raising the average stress on
the surrounding concrete.
14.3.16 Dry joints
When the deck is prestressed with external tendons, it is no longer necessary to
waterproof the joints between segments with resin, and they may be erected dry. The
lack of lubrication of the joint during erection requires that the plant used for bringing
the segments into contact must be capable of fi ne adjustment of the segment position
if damage to the matching faces is to be avoided. It has been reported that some minor
spalling due to hard contact has taken place on such dry joints.
The system is less tolerant of damage to the matching faces. Damage that does not
reach the edges of the concrete cannot be repaired, and will result in a loss of effective
section, unless the complete joint is grouted up after erection. Damage that appears at
the edge may be repaired by dry packing or similar techniques after erection.
It is necessary to create a waterproof seal across the width of the top slab to avoid
rainwater dripping through. This may consist of a resin fi lled groove, a grouted duct
within the thickness of the slab or other methods that may be devised.
One of the principal benefi ts of omitting resin is that temporary prestress is not
necessary, simplifying and speeding up the erection process. Dry joints are normally
used on relatively short segment runs, such as statically determinate spans. However,
some authorities refuse the omission of the resin, particularly in areas subject to
freeze-thaw cycles.
14.3.17 Steering during erection
If the calculation of the alignment and the casting of the segments have been correctly
carried out, it should not be necessary to steer the segments. However when things
go wrong, such steering may avoid the need for more expensive and time-consuming
remedial works. It consists of making small adjustments to the thickness of the epoxy
joint, and of course is only possible on resin-fi lled joints.
The fi rst requirement is to survey the deck during erection to a high degree of
accuracy. If errors in the alignment are to be remedied, they have to be detected very
early in the erection of a span so that the small angular corrections possible have a
signifi cant effect.
The most trouble-free method is to vary the viscosity of the resin across the joint
so that the same assembly pressure gives rise to a different thickness of glue line.
However, only very small adjustments may be made by this method. For instance, one
deck known to the author was being erected in balanced cantilever. It was discovered
