Page 14 - Historic Churches 2013

14 BCD Special Report on Historic Churches 20th annual edition CATHEDRALC O M M U N I C A T I O N S
with this system is that the pump is operated
from the ground at all times avoiding the need
to lift it up the scaffold, although a converter
is needed to make sure the grout is delivered
by manageable hosepipes rather than the very
heavy delivery hoses that come with the pump.
The optimum number of operatives is
seven. Two are needed on the pump, two
mix the grout, one manages the grouting
point, and two look for leaks, one outside
and the other inside, communicating with
the pump operator by two-way radio.
A copper pipe is put into the grouting tube
and grouting starts. When the grout starts to
appear from one of the proving/grouting points
(figure 6), a wooden dowel with cotton wool
is put in to plug the hole. It is important to
maintain the grout flow at the hole: if it is left
even for short periods de-watering can occur
and voids will be left. Any escaping grout on the
wall face has to be cleaned off immediately to
avoid staining, and hopefully glistening mortar
joints should indicate de-watering taking place.
At Stowford, grouting was carried out
for a day and then left for 24 hours to allow
de-watering and reduction in volume to
take place before continuing. Non-grouting
days were spent continuing repointing
up the tower. A minimum of one month
was allowed for pointing mortars to set
sufficiently in order to prevent the escape
of grout. This was easily achieved because
the grouting process was relatively slow.
Figure 6 When grout appears at the proving hole a
wooden dowel and cotton wool are put in to plug the
hole and prevent grout leaking out.
Inspections were also carried out during
the work, when the contractor was required
to remove selected stones to ensure that the
grout had satisfactorily filled the voids behind.
When the top of the tower was reached
it was important to let the grout de-water
and reduce in volume so the voids could be
topped up adequately. In the event, however,
not enough time was left and a void was
created which led to water ingress, so a return
visit was needed to re-grout this section.
On completion, 7.5 tonnes of grout had
been injected into the core of the tower. From
this it could be calculated that voiding within the
tower had amounted to five per cent by volume.
SOME CONCLUSIONS
The moisture levels on all four elevations at
each level of the tower were monitored using
the oven-balance method (calculating moisture
loss by weighing and drying timber dowels that
have been inserted in the masonry). At the
outset the west and south walls were running
with water and therefore clearly saturated. After
the work was finished the moisture levels still
appeared to be very high (on account of the
amount of water used) but within a year these
had reduced down to less than 30 per cent.
Drying has continued and the architect’s recent
quinquennial confirmed that the tower is dry
with no reports of any moisture ingress. The
grout clearly acts as an effective sponge. After
one very wet month the moisture level in the
south wall rose dramatically but by the following
month this had evaporated to the outside and
equilibrium water content was re-established.
Grouting may be highly effective, but
it is certainly not a cheap option. At least
seven operatives are required and once the
scaffolding and all other costs are taken into
consideration, the process is likely to cost
£1,200–1,500 per square metre. In Stowford’s
case rendering was considered unacceptable
because it would have obscured the highly
decorative facades of the tower, so grouting
was the only remedial option available.
Grouting is also an invasive operation and is
essentially irreversible. We do not yet know how
effective it will remain. However, it does provide
a remedial option that retains the appearance
of the tower and one that should remain
essentially maintenance-free (unlike rendering
and pointing) for many decades to come.
GLOSSARY
bentonite a clay mineral used in grouts to
improve flow
hydraulic term describing the ability of a lime to
set by chemical reaction with water, as well as
by carbonation (a ‘non-hydraulic’ lime sets by
carbonation alone)
litharge a form of lead monoxide (PbO) once
commonly used in varnishes and glazes for its
drying properties and its ochre colour
PFA pulverised fuel ash, a fine mineral by-product
of burning pulverised coal in a power station;
when added to a lime mortar PFA promotes a
hydraulic set
CHRIS WOOD is head of the Building
Conservation and Research Team at English
Heritage. The team specialises in dealing with
deteriorating materials on historic structures and
is responsible for the revised Practical Building
Conservation handbooks. He is series editor and
sometime author. He has been involved with
the Damp Towers project for two decades.
COLIN BURNS is a stonemason who worked on
monuments for 20 years in the Department of
Ancient Monuments and Historic Buildings before
joining John Ashurst’s Research and Technical
Advisory Service at English Heritage in 1985. He
was the senior training officer at Fort Brockhurst
until 1997 when he became a consultant and
trainer in the conservation of historic masonry.
Notes
¹ The results of English Heritage’s Damp
Towers research project were presented
at a conference in Exeter in April 2013.
A summary of the papers will be published
on the English Heritage website. The full
results of the work are to be published in
the Research Transactions series, together
with a guidance note, later this year.
Figure 4 20mm holes were drilled to accommodate
lengths of hose to mark where voids were found; these
would act as grouting points
Figure 5 Architect Simon Cartlidge on the handle and Piers Denny of Carrek Conservation getting the feel of the
diaphragm pump