Civil Engineering Reference
In-Depth Information
The decision to refurbish using
overcladding
Overcladding is not a panacea for all
the ills of system building. On its own
it will not reinstate the structural
integrity of a building nor prevent
further decay where inherent
problems are to be found with the
manufacture or assembly of the
original components. Unless suitably
designed and installed, it will almost
certainly make it more difficult to
identify any continuing deterioration.
The decision to overclad must
therefore be taken only after
exhaustive consideration of the
condition of the building. Reinforced
concrete of the quality found in many
LPS buildings must be recognised as
a limited-life material, and provision
made to monitor the performance of
the buildings over time.
may be possible to push a closed cell
foam gasket through to the back of
the joint if the baffle is first removed.
All spalled concrete should be cut
out and repaired; there is certainly no
advantage to be gained by painting
the area with a bituminous paint
system since it forms a vapour control
layer in the middle of the cladding
system. It is not desirable to leave any
spalled areas unrepaired on the
assumption that the overcladding will
safely retain spalling which is already
in progress.
BRE Digests 263
(120)
, 264
(121)
and
265
(122)
deal with the mechanisms of
corrosion, diagnosis and assessment,
and repair of reinforced concrete.
With overcladding systems of the
rain-screen kind, joints in the original
external concrete panel wall should
be made airtight before any
overcladding work is begun, unless
the proposed solution automatically
includes an airtight barrier in itself.
This would involve:
●
removing the baffle and gunning
fresh sealant into the backs of all
joints, or as far as can be reached
into the back of horizontal joints,
making sure that the verticals and
horizontals interconnect
●
sticking a new (could be self-
adhesive) strip over the fronts of
all vertical and horizontal joints
Undercut edges to area of
spalling
Figure 2.56
Patch repairs to precast concrete window
surrounds
Maintenance
As already seen, there are normally
two basic kinds of joints in the
external envelope of LPS dwellings:
●
two-stage (or open drained) joints
●
one-stage (or face-sealed) joints
Both kinds have been found to be
repairable in the majority of cases,
though some have given rise to
problems. In the case of two stage
joints, renewal of the baffle may be
awkward, though possible, but it is
the air seal at the back of the joint
which is crucial to performance. Even
turning a two stage into a one stage by
filling the exterior of the joint may
not work. Repairing a single stage
joint depends entirely on the integrity
of the seal which, in turn, may depend
on compatibility of old with new
seals or adequacy of cleaning off the
old.
Open drained joints, to be found
for example on Taylor Woodrow
Anglian, Bison, Jesperson, Reema
and Wates, sometimes suffer from
deterioration of the baffle material.
Replacement should be
straightforward. However, some
joints have suffered breakdown of the
air seal. This is not so easy to remedy,
although it may be possible to insert
the nozzle of a sealant gun into the
rear of the joint to re-create an
effective air seal. Alternatively, it
Not less than 60 mm
total adjustment in each of three planes
injecting an expanding
polyurethane foam into the joint to
seal it.
Figure 2.57
Fixings for overcladding support rails will
need to tolerate rather large inaccuracies in
the original building. The more practical
designs are usually based on the twin
slotted angle
●
Work on site
Workmanship
Precast concrete or reconstituted
stone can be patch-repaired in situ -
the so-called plastic repair method.
The area to be repaired should be cut
back to square undercut edges,
preferably with a disc cutter (Figure
2.56). The mortar to be used should
not be too strong for the original
background. It may require a degree
of experiment to obtain a suitable
mix, but one of the order of 1:6
Portland cement:sand may well be
strong enough. The mortar should be
trowelled into place using coats of not
more than 10 mm thickness. It may be
possible to cramp shuttering into
