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T W E N T Y S E C O N D 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 5
5 3
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BUI LDING CONTRACTORS
CONCRETE REPAIRS
Traditional methods and
like-for-like materials
DAVID FARRELL AND CHRIS WOOD
W
HERE HISTORIC
fabric requires
conservation, damaged material
is usually repaired in situ, and
any new material and details required are
expected to match the existing on a like-for
like basis wherever possible. However, where
concrete is concerned the European standard
Products and Systems for the Protection and
Repair of Concrete Structures
(EN1504-5:2013)
recommends that all repair mixes should
come ready-bagged from certified factories.
Almost all materials certified under this
standard are modern ‘concrete repair mortars’
not concrete. While their use on historic
buildings and structures may be justified in
some instances, like-for-like repairs will often
perform at least as well and any decision to
vary the materials must be based on a sound
understanding of the options.
BACKGROUND
Historic concrete includes reinforced concrete
(RC) and bulk (non-reinforced) concrete (BC);
a material which has been widely used for
military facilities in the past. RC generally
suffers from deterioration (mainly carbonation
and chloride attack) of the concrete which
subsequently allows the steel reinforcements
to corrode; this results in cracking and
delamination of the concrete ‘cover’. BC
tends to suffer more from ground movement
and differential expansion and contraction
which causes cracking and slippage of larger
sections of concrete. Different pours of
BC (laid down at different times) sometimes
become detached from each other resulting in
movement. Both types of concrete suffer from
weathering by pollutants in combination with
rain and mould/algal growth which mar their
appearance.
Like-for-like patch repairs to concrete
have been carried out in the past but many of
these are now failing for a variety of reasons.
Some repairs were too shallow and did not
adequately address the underlying corrosion
of the reinforcements, or they were not
‘mechanically bonded’ to the host concrete
(feather edging, for example, may result in
cracking and shrinkage). There have also been
difficulties matching the colour and texture
of the original material. These problems,
along with the desire for easy and quick repair
methods suitable for unskilled labour, led to
the development of modern repair mortars
which are now widely used for repairing
Figure 1 A staircase at the National Theatre, London (Photo: NAES, iStock.com)
concrete. Many of these contain polymer
modified components which can be applied as
thin layers to delaminated surfaces and adhere
strongly to the host material. These repair
mortars are not popular with the conservation
industry as they introduce different materials
and do not weather-in to match. Their
longevity is also uncertain when used in
combination with the parent concrete.
The adaptation of traditional types of
repair in England has been growing since
2001 when the church of St John and St Mary