Page 13 - Historic Churches 2013

BCD Special Report on Historic Churches 20th annual edition 13 CATHEDRALC O M M U N I C A T I O N S
ST JOHN THE BAPTIST CHURCH,
STOWFORD, DEVON
Stowford Church, situated near the Cornish
border, had long suffered from the effects
of driving rain (figure 2). Indeed, Gilbert
Scott suggested in 1872 that the core should
be grouted with liquid cement, the inside
wall plastered with cement and that the
external mortar joints be painted with a mix
based on oil and litharge*. The 15th-century
tower is 230 metres above sea level and
is very exposed to the prevailing weather,
receiving about 1,100mm of rain annually.
In addition to grant-aiding the repair
work, English Heritage funded the research
required for monitoring the moisture levels
in the walls and commissioned Colin Burns
to work closely with the architect, Simon
Cartlidge, and the contractors, Carrek
Conservation, to develop the specification and
also establish some best practice principles
for carrying out grouting. The first stage
of the work was to carry out invasive tests
to establish the extent of voiding in the
core. These initial surveys clearly indicated
substantial voids up to 30mm deep. This work,
however, was not comprehensive enough
to enable a specification to be prepared,
so more detailed surveying was needed.
Finding the voids
Determining the condition of the core and the
position and depth of voids is very challenging.
The best approach is to establish a grid that
can be used to plot the voids and how they
link. The voids are then found by carefully
drilling holes in the wall and ‘feeling’ for them
with the drill: when a void is reached a sudden
lurch will be felt. With experience the operator
can quickly gauge the depth of the void, and
measuring the drill-bit will provide its location
within the wall. That information can then
be recorded on the grid. However, there is
an art to the technique: if there is moisture
in the wall and the drilling is too violent,
slurry will adhere to the drill-bit preventing
the operator from feeling the voids as they
are encountered. The aim is to drill gently.
When the voids are flushed with water,
those that interconnect can be recorded on
the grid, so gradually a three-dimensional
image can be built up (figure 3).
At Stowford, a scaffold was erected
inside the church tower to enable a series
of drill tests into the core of the wall. This
consisted of 16mm diameter holes drilled
750mm deep, on a 1m-staggered grid. The
tests showed that there was a run of 30–50mm
deep voids at 250–300mm into the core of
the wall with a further zone of 25mm deep
voids at 500–600mm into the depth of the
wall. This suggested voids immediately behind
the outer facing stones and a second area
of voiding towards the centre of the wall.
Because of the thickness of the walls and
the difficulty of the grout becoming de-watered
before it filled the voids, it was agreed that a
pumped pressure system would be used rather
than the more commonly used gravity system.
A hand pump would be used, limiting the
pressure and allowing the pump to wet the
core of the wall thoroughly before grouting.
The specification attempted to ensure a
systematic approach to the scope of repairs and
to define a clear package of works, as follows:
• removal of the existing cement mortar
pointing unless it was sound enough to
contain the grout
• masonry repair and repointing in a
hydraulic lime mortar of one part NHL3.5
to 2.5 parts aggregate (as there was virtually
no historic mortar left, the mix was
designed to be compatible with the granite
and rubble masonry)
• drilling a staggered grid of 650–750mm
deep 20mm diameter holes on each face
of the tower at 1m horizontal and vertical
centres, to clarify the number of insertion/
proving holes
• grout delivery to be limited to a pressure of
1.5 atmospheres and 1m vertical lifts every
other day
• CMS Pozzament 1–5 Heritage Grout to
be used; the specification was written
to include detailed instructions for the
grouting operations, based on previous
experience, in order to obtain prices which
reflected the work involved.
Preparing the walls
In order to pre-wet the core and to see which of
the holes were linked to interconnected voids,
water tanks with feed pipes were set up on the
scaffold at the top of the tower, with connections
at every level so that each level of the core
could be flushed and flooded, starting at the
bottom. A small electric motor pumped water
to the top and kept the tanks full at all times.
Where the water escaped at the bottom
point, the grid plan was updated to record the
interconnecting void, and small plugs were
inserted. These plugs marked the positions
where the grout was to be introduced, at
the bottom of the void in each case.
The condition of the cement pointing
was generally not sound enough to allow
it to remain as a barrier to escaping grout.
So after the walls were tested for voids and
flushed with water, the joints were raked out
and repointed in advance of the grouting.
Unusually, the pointing had to be done from the
ground upwards. Grouting is never carried out
downwards because air pockets could develop
or loose debris in the core could block a narrow
passage leaving a void below. When grouting
upwards, any loose material is collected by the
grout but falls out of suspension quickly so it
does not impede the progress of the grout.
During the pointing, lengths of water hose
were inserted to provide grouting points and to
act as proving points during the grouting process
(figure 4). Before grouting, water is again pumped
in. The core cannot be over-wetted because when
grouting starts, water will float on top of it. The
water which precedes the grout pre-wets the
surfaces within the core, ensuring a good flow.
The groutingoperation
Grouting commences immediately after the
channels have been flushed through. The grout
is mixed with water in dustbins with a powered
paddle. First a very runny grout is used. The
manufacturer’s recommendation is 20 litres/25kg
bag, but at Stowford a mix of 40–50 litres of
water per bag was used for the first pass to make
it even thinner. This is then followed by a heavier
grout, but only experience can determine what
this should be. The heavier grout will push the
lighter grout out from larger holes and force it
into those finer fissures that can satisfactorily
accommodate the runnier mix. The excess
water will appear to bleed through the external
mortar joints, which of course, is their function.
The grouting operation was carried out using
a diaphragm pump. It is simple to use; merely
pulling the handle pumps the grout (figure 5).
It is relatively delicate so any resistance or
blockage is quickly felt. It is rarely necessary to
apply more than 25psi of pressure. It is essential
to introduce the grout gently, because many
of the voids are interconnected, so that the
grout is travelling back and along, not just up.
Pumping too violently will mean excessive loss
of grout when leaks appear. A big advantage
Figure 3 A typical grid which can be used for plotting voids: trial holes (in red) locate the voids (in blue) and their
depth and extent are recorded on the grid, highlighting those that interconnect