Civil Engineering Reference
In-Depth Information
Maintenance
The following are the basic principles
of repair.
Case study
Failed stonework in an office block
BRE was asked to inspect the stonework to an
office building converted from a large mansion
and to propose methods for its restoration.
The stones which were to be repaired or
replaced could be divided into categories:
●
accommodated at the centre of the lintel by
cracks forming in the joints either side of the
new keystone, but it would not prevent
cracking occurring elsewhere in the lintel.
As to the remainder of the cases,
whether or not a block should be completely
replaced would be a matter for judgement.
For minor defects it would probably be more
economical to repair the block than to
replace it. However where the amount of
work required on a single block would be
considerable, repair would no longer be the
most economic option.
Mortar repairs to the upper surfaces of
stones are more likely to fail in the long term
than repairs to vertical faces, unless
protected from the weather. Protection
should prevent water getting to the repairs -
best achieved with a covering of lead; the
application of bitumen or silicone water
repellents was not recommended.
Plastic repair
This type of repair should be
employed only for relatively
unimportant areas which have
decayed. It is essential that the area to
be repaired is properly prepared and
that the mortar mix is matched as
closely as possible to the appearance
of the original stone. Any decayed
stone should be cut away and the
edges of the are to be repaired should
be undercut to provide a dovetail key.
Feather edges to repairs should never
be used. For larger areas a mechanical
key in the form of ragged non-ferrous
dowels or non-ferrous screws firmly
fixed to the background should be
employed.
Plastic repair should never
continue across mortar joints. Rather,
each block of stone should be treated
independently from its neighbours
and the repaired block pointed
afterwards in the normal manner
when the repairs have cured.
Mortar should be mixed to match
the colour and texture of the stone
being repaired and should never be
stronger than the stone. Two layers of
mortar will normally be used.
cracked but both pieces were still in-situ
cracked and fallen off
●
repaired with mortar
●
not previously repaired, and showing
varying degrees of decay
●
Stones in the first category were mainly lintels
over window openings. It was proposed by the
building owner's architects that, for those
lintels with one central crack, the central part
was to be cut out to allow a keystone to be
inset. This would give the lintels a satisfactory
visual appearance but would be unlikely to
further prevent the movement that initially
caused the crack. If there was a tendency for
further movement, this would be
from the back is preferred if access is
available, otherwise holes drilled in
the face will need pointing with a
mortar made of crushed stone to
match the original.
Work on site
Workmanship
For those stones with bedding planes,
probably the most important
consideration is that the correct
attitude of the stone in the wall is
adopted. Stones should be laid as
nearly as possible in the same attitude
as they were originally formed,
though bedding in tracery should be
at right angles to the line
of thrust.
See also the same section in
Chapter 4.2.
Preservation
It may be possible to treat certain
stones to reduce the rate of erosion.
However, specialist advice should
always be sought.
Repairs using new stone:
'piecing-in'
Repairs by this method can be quite
small - down to 25 mm square on the
face. The thickness of the repair will
depend on the method of fixing and
the size of the repair, but will not
normally be less than 25 mm. Large
pieces of stone can be fixed by
normal masonry techniques using
cramps and ties where appropriate.
Smaller pieces can be grouted in, or
fixed with nonferrous dowels and
resin and grouted after the resin has
set.
Cleaning
Limestones may be cleaned by the
use of copious amounts of water to
which nothing should be added.
Detergents are definitely to be
avoided, as are caustic or peroxide
solutions. Gentle brushing may be
needed once the surface deposits have
been loosened by the spray. Washing
is likely to be less effective on
siliceous sandstones.
Further information on the decay
and conservation of stone masonry is
available in BRE Digest 177
(115)
.
Inspection
The problems to look for are:
◊
narrow piers or returns overstressed
◊
spalling, contour scaling, cracking,
oversailing, bulging or leaning
◊
dampness and algal growth
◊
inadequate or blocked air bricks
◊
pointing erosion
◊
lintels deteriorating or cracking
◊
DPCs missing, damaged or deteriorated
◊
sulfate attack migrating from backing
masonry
Repairing a cracked lintel
If a stone lintel has cracked across at
an angle, and has dropped, it may be
possible to jack the soffit to level, and
to drill normal to the crack to insert
stainless steel pins cemented in with a
proprietary epoxy grout. This should
prevent further slippage. Drilling
◊
inappropriate repairs
◊
face bedding of stone
