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446 Barry’s Advanced Construction of Buildings
heavy and cumbersome wall units. Brick walling and stone facings may be more economically
applied on site to a structural frame.
Natural and reconstructed stone facing slabs are used as a decorative finish to precast
concrete panels. Any of the natural or reconstructed stones used for stone facework to solid
backgrounds may be used for facings to precast concrete panels. For ease of placing the
stone facing slabs in the bed of the mould, it is usual to limit the size of the panels to not
more than 1.5 m in any one dimension. Granite and hard limestone slabs not less than
30 mm thick and limestone, sandstone and reconstructed stone slabs not less than 50 mm
thick are used. The facing slabs are secured to and supported by the precast panel through
stainless steel corbel dowels at least 4.7 mm in diameter, which are set into holes in the
back of the slabs and cast into the concrete panels at the rate of at least 11 per m 2 of panel
and inclined at 45° or 60° to the face of the panel. Normal practice is that about half of the
dowels are inclined up and half down, relative to the vertical position of the slab when in
position on site. The dowels are set in epoxy resin in holes drilled in the back of the slabs.
Flexible grommets are fitted around the dowels where they protrude from the back of the
slab. These grommets, which are cast into the concrete of the panel, together with the epoxy
resin bond of the dowel in the stone slab, provide a degree of flexibility to accommodate
thermal and moisture movement of the slab relative to that of the supporting precast concrete
cladding panel.
All joints between the stone facing slabs are packed with closed cell foam backing or dry
sand, and all joints in the back of the stone slabs are sealed with plastic tape to prevent
cement grout from running in. When the precast panel is taken from the mould, the jointing
material is removed for mortar or sealant jointing. To prevent the concrete of the
precast panel from bonding to the back of the stone slabs, either polythene sheeting or a
brushed on coating of clear silicone waterproofing liquid is applied to the whole of the back
of the slabs. The purpose of this debonding layer is to allow the facing slabs free movement
relative to the precast panel due to differential movements of the facing and the backing.
The necessary joints between precast concrete cladding panels faced with stone facing slabs
are usually sealed with a sealant to match those between the facing slabs.
Joints between precast concrete cladding panels
The joints between cladding panels must be sufficiently wide to allow for inaccuracies in
both the structural frame and the cladding units, to allow unrestrained movements due to
shortening of the frame and thermal and moisture movements, and at the same time to
exclude rain. The two systems of making joints between units are the face sealed joint and
the open drained and rebated joint. Sealed joints are made watertight with a sealant that
is formed inside the joint over a backing strip of closed cell polyethylene, at or close to the
face of the units, as illustrated in Figure 7.27. The purpose of the backing strip is to ensure
a correct depth of sealant. Too great a depth or width of sealant will cause the plastic material
of the sealant to move gradually out of the joint due to its own weight.
Sealant material is applied by gun. The disadvantages of sealant joints are that there is a
limitation to the width of joint in which the sealant material can successfully be retained,
and that the useful life of the material is from 15 to 20 years, as it oxidises and hardens
with exposure to sunlight and has to be raked out and renewed. Sealed joints are used in
the main for the smaller cladding units. The sealants most used for joints between precast
concrete cladding panels are two parts polysulphide, one part polyurethane, epoxy modi-