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T W E N T Y S E C O N D E D I T I O N
2
BUI LDING CONTRACTORS
PRACTICAL CONSIDERATIONS
AND ALTERNATIVES
There are still some areas where like-for-
like repairs are not suitable for historic
concrete and modern lightweight repair
mortar would be recommended. These
include underside surfaces such as soffits, or
where the original cover was minimal and
the steel cannot be moved deeper into the
concrete. Tests are currently under way to
see if more adhesive traditional materials
such as NHL5 (a strong natural hydraulic
lime) could be used for these areas.
These and other traditional repair
techniques more commonly associated with
the work of stonemasons are increasingly
being used for the conservation and
repair of historic concrete, as at London’s
National Theatre, Tecton’s buildings
at Dudley Zoo, and the Alexandra
Road Estate in Camden, London.
More extensive repairs may be required
where reinforced concrete has been affected
by chlorides, either because they were present
in the original mix or because they had
ingressed from a marine environment. In such
cases cathodic protection may be required.
One of the 13 Tecton buildings at Dudley
Zoo currently being repaired and conserved
is the Safari Shop (Figure 14) which has been
unused and neglected for decades. Here the
original concrete was in a very poor condition
with significant amounts of corrosion and
matching the original structure, which will
continue to weather in a similar manner,
and which will improve the longevity of the
buildings and enhance their unique character.
Recommended Reading
A Wiles, ‘St John and St Mary Magdalene,
Goldthorpe: The Conservation of an
Early Concrete Building’,
Transactions
(Association for Studies in the Conservation
of Historic Buildings)
, vol 32, pp40-50, 2009
D Farrell and K Davies, ‘Repair and
Conservation of Reinforced Concrete’,
Historic Churches
, Cathedral
Communications Limited, Tisbury, 2005
English Heritage,
Practical Building
Conservation: Concrete
, Ashgate,
Farnham, 2012
DAVID FARRELL
is the managing director
of Rowan Technologies Ltd, a company
that specialises in the development and
application of new conservation methods.
He gained his MSc in maintenance
engineering at the University of Manchester
in 1982 and went on to complete his PhD in
corrosion engineering in 1984.
ROWAN TECHNOLOGIES’
current research
and development work for English Heritage
focusses on expanding the application
of cathodic protection to conserve iron
and steel in historic buildings and on the
development of appropriate repair strategies
for the conservation of concrete structures.
CHRIS WOOD
is head of the Building
Conservation and Research Team at English
Heritage. He is a series editor of EH’s
Practical Building Conservation
books and
contributed to the volume on concrete. He
has commissioned a number of research and
monitoring projects which seek to repair
historic concrete structures in ways that
preserve their character and appearance.
Figure 14 Tecton’s Safari Shop at Dudley Zoo following extensive conservation involving like-for-like repairs below
Following extensive training, Dudley Zoo’s in-house
maintenance team were able to carry out repair
processes developed on site.
delamination of the concrete cover. All repairs
used like-for-like materials from the slab
library. The exception was the soffits, where a
modern lightweight repair mortar was used.
On close examination the recent like-for-like
repairs can be differentiated from the original
concrete although the colour and texture
match are close. However, in this case the
original concrete building had always been
coated, so a translucent silicate coating was
specified which helped to blend in the repairs.
The concrete industry’s criticisms for
specifying like-for-like repairs include:
• greater skill requirements of the operatives
• increased time to cut out and effect repairs
• the possibility of incipient anode
corrosion (increased corrosion
adjacent to the repairs).
At Dudley Zoo the skills issue was resolved
through training the zoo’s in-house team to
carry out the works. The time requirement
has also been minimised by carrying out the
various repair processes in batches.
The incipient anode problem has mainly
been associated with chloride-infected
concrete (marine environments and car
park decks). This has not been identified on
any of the traditional works carried out so
far, although monitoring for this problem
is currently being undertaken on one of the
buildings at Dudley.
In conclusion, the authors are hoping to
see broader use of concrete repairs closely