The Building Conservation Directory 2015

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BUI LDING CONTRACTORS
Figure 3 Concrete test flags using different mixes
and with retardants applied to one side only. Half
the surfaces were wire brushed after early breakout
(12 hours).
Figure 2 Sample of concrete from the National
Theatre exhibiting naturally round and broken
aggregate, with centimetre measure
Magdalene, Goldthorpe, South Yorkshire
was successfully repaired and conserved.
Additional trials and testing were carried
out at Tynemouth Priory Coastal Battery
(bulk concrete structures dating back to the
1880s) and the Listening Mirrors, Dungeness
(reinforced concrete dating from the 1930s).
Other developments and testing carried out in
this period are discussed in English Heritage’s
Practical Building Conservation: Concrete
(see Recommended Reading).
Several ongoing repair projects where
like-for-like repairs are being implemented are
referred to below. However the main focus of
this article is the National Theatre, London
where an initial investigation of the concrete
was carried out and methods and materials
were specified for traditional repairs. These
were further improved by the contractor and
many smaller scale repairs are currently being
carried out.
THE NATIONAL THEATRE
Built in 1976, mainly of steel reinforced
concrete, the National Theatre is an iconic
building on London’s south bank which is
now listed Grade II*. Its concrete is generally
in good condition but over the years it
has suffered some deterioration. In places
the steel reinforcements had insufficient
cover causing the concrete to crack or
spall, while in others cracks were caused
by thermal movement and subsidence.
Mechanical damage had also occurred where
fixtures have been added and removed.
The theatre is currently undergoing
alteration and its concrete is being repaired.
Because of its historic status the repairs are
being carried out on a like-for-like basis as
far as possible. More modern methods and
materials are only being employed where this
is not possible, or where a different approach is
sure to result in a better repair.
To replicate the original mix accurately,
documentary sources were checked first, then
samples were taken for analysis.
Documentary assessment The only
documented evidence of the original
constituents of the concrete used was the ‘Bill
of Quantities for Superstructure to National
Theatre (Phase 2)’ which was dated 1970, six
years before it was built. From this it was
concluded that the ‘white’ concrete forming
the external board-marked (wood grain),
bush hammered and fair-faced concrete was
originally intended to comprise the following:
• 80% Snowcrete (WPC) and 20%
ordinary Portland cement (OPC)
• Leighton Buzzard sand as fine aggregate
• Newmarket flint (¾” to ⅜”) as large
aggregate (angular or rounded
crushed rock) – specified at the time
to be obtained from Allen Newport
in Fordham, Cambridgeshire.
• The mix was generally described as
between 1:8 and 1:3.3 (by dry weight)
cement to total aggregate. However,
other sections in the book describe their
ratio as being between 1:7 and 1:4.
Visual assessment A view of the
aggregate from a fair-faced core sample
is shown in Figure 2. This shows the large
aggregate to be mainly rounded stones, much
of it broken. The size distribution of the large
aggregate was 11–22mm, corresponding with
the ⅜” to ¾” specified. Particles of the sand
were separated out from the concrete and
measured. Typical fine aggregate diameters of
0.3–0.8mm were found.
Geological assessment Further analysis
of aggregate from the broken sample was
carried out at Aberdeen University by Natalie
Healy, a postgraduate researcher whose report
is summarised below:
The samples of Newmarket Flint
aggregate consist of concrete and
poorly sorted quartz and feldspathic
grain (sandy) matrix derived from
TRIAL MIXES FOR THE NT TEST SLABS
Mix No. and Ratio Cement Sand Aggregate
1 (1:2:3) 80:20 Leighton Buzzard white 10mm Newmarket flint
2 (1:2:3) 80:20 Leighton Buzzard white 20mm Newmarket flint
3 (1:2:3) 80:20 Leighton Buzzard yellow 10mm Newmarket flint
4 (1:2:3) 80:20 Leighton Buzzard yellow 20mm Newmarket flint
5 (1:2:3) 100% WPC Leighton Buzzard white 10mm Newmarket flint
6 (1:2:3) 100% WPC Leighton Buzzard yellow 10mm Newmarket flint
7 (1:2:4) 80:20 Leighton Buzzard white 10mm Newmarket flint
8 (1:2:4) 80:20 Leighton Buzzard yellow 10mm Newmarket flint
9 (1:2:4) 100% WPC Leighton Buzzard white 10mm Newmarket flint
10 (1:2:4) 100% WPC Leighton Buzzard yellow 10mm Newmarket flint
an Arkosic sandstone. The sample
contains rounded clasts of various
sizes and lithologies including flint,
shelly debris and chalk and few clasts
of crystalline quartz. Clast lithology
has been identified using standard
mineral identification techniques
including Moh’s scale of hardness.
Clast types identified include:
• Predominantly orange and brown
and few grey clasts of silica rich,
cryptocrytalline flint identified by
a hardness of 7 and characteristic
conchoidal fracturing. Clast
size ranges from 3mm-20mm.
• Clasts of silicified indeterminate
Pectin shells (scallops) ranging
in size from 5mm-<10mm.
• Few clasts of iron stained
translucent crystalline quartz.
• Few clasts of fine grained, well
cemented chalk identified
by a hardness of 3.
• Rare clasts of black, lustrous,
soft organic material possibly
produced from fossilised wood.
Sandy matrix
The concrete matrix comprises poorly
sorted rounded grains of quartz,
feldspar, lithics and few green coloured
grains of the mineral glauconite.
Matrix sandstones may be sourced
from the overlying Upper Greensand
Formation which outcrops nearby
Fordham.
MATCHING THE ORIGINAL CONCRETE
The lack of availability of the original
constituents can often present problems
for the conservation of historic concrete.
Early concrete aggregates (pre-1960s) were
normally sourced from local suppliers which
were themselves supplied by local quarries,
but many have since been ‘worked out’ or
closed. So the first task was to see if these
constituents were still available:
Cements – White Portland Cement
(WPC) is ubiquitous, although it is likely to