Page 53 - The Building Conservation Directory 2013

t w e n t i e t h a n n i v e r s a r y e d i t i o n t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 5 3
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Professional services
the whole wall or, more likely, ‘face separation’
where the outer skin parts company with the
core, often known as delamination.
➌ Hammer inspection
Tapping (not hitting) the face of the structure
with a lump hammer or similar tool may
reveal evidence of deterioration. If the wall is
sound there should be a good ‘ring’ but a dull
thud can indicate delamination. However,
like so many things in conservation this sort
of test relies on experience. The sound of
a hammer striking a wall can also depend
on the state of the mortar and the size of
the individual brick or stone elements. This
particularly affects the sound of a flint wall,
where there is a much higher ratio of mortar
to stone than in a brick or stone wall. Being
a very hard material set in soft mortar it is
sometimes possible to inadvertently drive
flints into the wall. The wall shown in figure 5
looked very sound and a hammer inspection
confirmed this but experience and the
presence of a bulge suggested otherwise.
➍ Monitoring
Where there are fractures and signs of
movement it is vital to ascertain whether
these are active and ongoing movements
or merely indications of historic problems
that settled down long ago and only need
some local pointing. The structure may
need to be monitored by a structural
engineer experienced in conservation to
finally establish whether or not it is moving.
Figure 4 shows a classic example of historic
movement. This brick gable wall belongs to a
Georgian house but the major crack does not
pass through the Victorian plaster cornice,
indicating that the crack existed before the
ceiling was installed and has not moved since.
➎ Excavation
Removal of small elements of the wall allows
the assessor to properly consider the condition
of the core of the wall. This may just involve
cutting out a few bed joints here and there
(using a pointing chisel or quirk) or perhaps
carefully removing the occasional brick or
stone. It is important to remember that this
sort of intrusive inspection may well require
listed building consent.
➏ Borescope inspection
Consider using a borescope (endoscope)
to inspect the core of the wall. These
instruments, which come in a variety of
lengths and may be rigid or flexible, can be
inserted into existing cracks in the wall or it
may be necessary to purpose-drill small holes.
More sophisticated,
non-destructive methods
There is a range of more sophisticated
(and therefore more expensive and time-
consuming) methods of investigating masonry
structures. These methods are not examined
in detail here as they are highly technical
and the outcome of any survey using them
can depend on the type of structure being
investigated. It is important to seek further
information, initially by searching the internet
(see Further Information below) and then,
most importantly, by discussing the work with
experienced practitioners before embarking
on an expensive and possibly futile procedure.
Will the method proposed work on the type
of structure concerned? Are there better
methods available? What access is needed?
How much will it cost? These questions will
need to be answered and there may be other
concerns to be addressed that are specific to
each technique.
Nevertheless, the results of good non-
destructive testing can be extremely useful.
For example, It is often useful to ascertain
whether there is any metal in the structure
either in the form of cramps (these will usually
give themselves away by breaking the corners
off stones due to the expansion of rust) or
buried metal ties (see figure 6). The latter
may be present as wrought iron rings around
towers and domes to resist outward thrust or
as hoop iron used as bed joint reinforcement.
Options
• Radar An echo-sounding method which
uses radar (radio waves) to detect voids or
buried metals, ground penetrating radar
(GPR) offers greater penetration than
metal detectors but is more expensive.
GPR can also determine something of
the corrosion level, whereas a metal
detector will not respond successfully
to a pile of corrosion product which is
perhaps all that remains of buried metal.
• Ultrasonics An echo-sounding method
which uses ultrasound, this technology is
also used for seeking voids in masonry.
• Thermal imaging The use of thermal
imaging cameras can highlight
delamination and moisture penetration due
to fine variations in surface temperature,
and may be carried out remotely.
• Video surveys A wide range of video
capture services are available for remote
inspections and detailed surveys, from
drain surveys to aerial inspections.
Video survey cameras have been
mounted on dirigibles and helicopters,
wheeled robots and telescopic masts,
and hand-held cameras can be used
Figure 2 A decayed stone (left), as yet causing no structural problems and (right) these stones were undermining the courses above
Figure 3 A column threatened by the corrosion
expansion of iron cramps: no serious masonry decay
but more than enough to threaten the stability of the
column and, in turn, that of much of the structure