Set in Concrete? Conservation of Howard Taylor’s The Black Stump
Vanessa Roth Wiggin* and Seth Wiggin
Abstract
Howard Taylor’s The black stump (1975) is a reinforced concrete, exposed aggregate and
glass tile sculpture which was originally constructed for St George’s Terrace in Perth and
relocated to the University of Western Australia.
Corrosion of the steel reinforcement due to a combination of high chloride levels and
inherent vice had resulted in concrete cracking and tile loss. A major source of chlorides
was exposure to bore water, which had also largely obscured the decorative elements.
Another contributing factor was the ingress of ground water due to the buildup of soil and
debris in the large gap between the sculpture and the foundations.
A previous sacrificial anti-graffiti coating was removed using a hot water pressure
washer, greatly reducing the bore water staining. Further iron staining was chemically
removed and missing tiles replaced using commercial products. Losses in the terrazzo
finish were replicated by adding black marble chips to a cementitious repair compound
and polishing with a diamond disc. A silane sealer was applied to enable the concrete to
breathe and dry out, while preventing further ingress of water. Galvanised angle iron
collars were installed to prevent soil building up between the sculptures and foundations.
The sculpture was also coated with a sacrificial anti-graffiti coating. The area around the
sculpture is to be paved, making bore watering unnecessary.
Description
Howard Taylor’s The black stump was originally constructed for the AMP forecourt in St
George’s Terrace, Perth in 1975 and was relocated to a section of lawn at the University
of Western Australia in 1991. The black stump is part of the University of Western
Australia Art Collection, which is managed by the Lawrence Wilson Art Gallery.
The sculpture is constructed from reinforced concrete, glass mosaic tiles and exposed
aggregate. The sculpture consists of four components (Fig 1):
Element 1 is designed as a seat in the shape of a quadrant of a circle with bevelled edges.
It has an approximate weight of 3.5 tonnes. The exposed aggregate on the top surface has
a tiger stripe pattern formed by alternate polishing and abrasive blasting. This element is
hollow underneath.
Elements 2 and 3 are large complex prisms, approximately 3 metres tall and each weighs
approximately 12 tonnes. They mirror each other in shape. These elements were
constructed from precast panels, though no information exists on how these were fitted
together. A large amount of concrete was poured into these elements prior to their
removal from AMP, presumably to stabilise them for transportation.1 Each element has
two tiled faces, and four ‘tiger striped’ exposed aggregate faces. Polished exposed
aggregate also forms a frame around the tiled faces. It appears that the exposed aggregate
sections were all applied as a topping mix while the concrete was still wet.
Element 4 is a polished exposed aggregate ring set into the ground. This element is a
replacement for the original, which was not able to be removed from AMP.
Initial Condition Assessment
The first named author was first asked to assess the sculpture in 2004. Major problems
included extensive bore water staining which largely obscured the colours of the mosaic.
Tiles had been cracked, chipped and lost due cracking in the underlying concrete and
skateboarders riding on the sculpture. There were numerous losses and cracks in the
exposed aggregate, thought to be mainly caused during the relocation of the sculpture. In
addition, previous infills in the exposed aggregate were visually disfiguring (Fig 2). One
of the lower corners of element 3 appeared to have been broken off and re-adhered, with
the edges poor As the Gallery was concerned that some of the cracks in the sculpture
were structural and could lead to further deterioration, it was suggested that they be fully
assessed by a concrete engineer.
Treatment of the sculpture was delayed until June 2007 when the Lawrence Wilson
Gallery was able to secure funding to conserve the sculpture and the University agreed to
pave the site so that further bore watering would be unnecessary.
Assessment of Concrete Deterioration
Following advice from Professor Andrew Deeks and Dr Andrew Grime from UWA Civil
and Resource Engineering, a firm specialising in non destructive testing of concrete was
engaged by the Gallery to assess the condition of the reinforced concrete in 2006.
Elements 1 and 4 were found to be in good condition as expected, while elements 2 and 3
showed evidence of isolated corrosion cracking (Barnes & Papworth 2006).
A major cause of corrosion was inherent vice, as the steel reinforcement underneath the
tiled faces was only millimetres below the concrete surface (Fig 3). The other main cause
of corrosion was the high level of chlorides in the concrete (Barnes & Papworth 2006).
Steel embedded in concrete is protected from corrosion due to the formation of a passive
layer. This passive layer breaks down in the presence of chlorides. The resulting
corrosion products occupy a larger volume than the original steel, creating internal
pressures which lead to cracking and spalling of the concrete (Barnes & Papworth 2006).
Corrosion was found to be more extensive around the base of elements 2 and 3. This was
attributed to the ingress of chlorides and aggressive chemicals from the ground by
capillary action. However, the major source of chlorides was the evaporation of
chemically laden bore water from the surface of the sculpture. Infiltration of bore water
into the cracks further accelerated deterioration (Barnes & Papworth 2006).
The engineers’ report recommended that the sculpture either be stored inside a building
or cathodic protection installed (Barnes & Papworth 2006). Neither option was practical,
as cathodic protection is extremely expensive and destructive, and the sculpture was not
designed for an interior site.
Cleaning and Removal of Coatings
A sacrificial anti-graffiti coating had been applied during a previous conservation
treatment in order to allow bore water staining to be more easily removed. It is unclear
from the treatment report whether this coating was PSS 20 or Graffiti Pruf, but both
products are starch-based and removable in hot water (Gabrieli 1997).
A hot water pressure washer was used to remove the anti-graffiti coating. The cleaning
had a dramatic effect, with the majority of bore water staining, along with minor
biological growth and graffiti, removed (Fig 4). No etching of the concrete or grout was
observed during cleaning.
Cleaning revealed the presence of a deteriorated whitish coating on the non tiled faces.
This coating was previously identified as an acrylic polymer (Gabrieli 1997), and appears
to be a polyvinyl acetate based product such as Bondcrete. The coating was very
unevenly applied and did not extend to the upper surfaces, indicating that it was not
applied during manufacture. The coating was flaky in many areas after hot water
cleaning, and was further reduced using a red heavy duty scourer.
Reduction of Rust Staining
Some rust staining and dirt remained after hot water cleaning, dulling the colours of the
mosaic and staining the grout. Some of this rust staining originated from internal causes,
rather than bore water.
The decision was made to chemically clean the sculpture. An aqueous solution
containing 5% oxalic acid, 2% sodium fluoride and 1.5% citric acid had been
recommended for this purpose by a concrete engineer.2 However, the authors were
concerned about the extreme toxicity of sodium fluoride, and the large volume of solution
required. Initial tests were carried out using 5% oxalic acid and 1.5% citric acid in water,
but this solution had limited effect.
Following discussions with a tiling technician, tests were carried out using Grout King
phosphoric acid based cleaner.3 This product proved highly effective when applied neat
to dry surfaces and combined with vigorous scrubbing. As phosphoric acid based
cleaners may cause some etching of concrete, this product was applied to the tiled faces
only. Minor etching of the grout was not regarded as a problem as much of the original
grout had previously been replaced or covered over.
Grout King was applied to an entire face of the sculpture using a red scourer on a
DoodlebugTM swivel head holder. The sculpture was then scrubbed while still wet using
the red scourer and rinsed with clean water. The tiles were further cleaned using an
electric polishing machine, first with a 3M grey radial bristle disc, then a fine blue
Scotch-BriteTM disc, followed by rinsing. Spot cleaning was carried out where necessary
using Grout King and a red scourer. Some minor staining remained following treatment,
particularly in the areas surrounding cracks.
Some reduction of rust staining on the exposed aggregate surfaces was achieved by
scrubbing with a red scourer and 5% oxalic acid and 1.5% citric acid in water, followed
by rinsing with water.
Tile Repairs
The tiled faces of the sculptures had been constructed by applying a cement render to the
concrete formwork, then using an off white grout to both adhere and grout the tiles. The
grout contained a large amount of white sand, making it coarser than most commercial
products. Cleaning revealed several patches where different grouts had been used during
repairs. A grout wash containing coarse yellow sand had been applied over a large area.
Badly cracked, chipped and loose tiles and excess grout were removed with a hammer
and pencil chisel. During tile removal on the north face of element 3, it was found that
the cement render had largely separated from the sculpture, causing the mosaic to be
particularly fragile in this area (Fig 3). Drilling for sampling and boroscope inspection of
the northern faces of both elements in 2006 also appeared to have loosened a large
number of tiles. Two sections of tiles (measuring approximately 5 x 5 cm and 10 x 5 cm)
at the top of element 2 were sitting proud of the sculpture due to underlying corrosion of
the reinforcement. The tiles were still firmly attached together with grout, so the sections
were removed in entirety so that they could be adhered back into position after treatment
of the reinforcement.
A large number of spare tiles had been donated by the artist and stored at the Lawrence
Wilson Gallery for the purpose of repairs. Cleaning and sorting the tiles revealed that
more than twenty colours had been used on the mosaic, and there were not sufficient
quantities of two of those colours. Almost identical colours were found in the Vixel
range, although they were larger than the originals. The original tiles had been 2 cm x 2
cm, and were hand cut to approximately 2 cm x 1 cm. Having two cut edges rather than
one in the replacement tiles would not be obvious as there were many minor chips on the
uncut edges.
The tiles were adhered with Ardex STS 8 high strength tile adhesive mixed with Ardion
90 for improved flexibility and adhesion. Tile adhesive was also inserted into the gap
between the cement render and the sculpture on element 3. Once dry, the tiles were
grouted with Ardex FG8 flexible coloured grout mixed with fine white sand. This
mixture was chosen in order to match the original grout’s colour and textile. Thinned
grout was applied to all tiled surfaces in order to fill minor cracks and produce a more
even appearance.
Reinforced Concrete Repairs
Advice and treatment proposals provided by concrete engineers indicated that major
cracks in the sculpture would need to be routed out and repaired. Repair systems
suggested included cutting out the cracks and filling with epoxy or silicone base products
(Barnes & Papworth 2007, BCRC 2007 & French 2007). While epoxy injection systems
are considered to provide the best strengthening effect, Emaco® S88C with MCI, a
synthetic reinforced cement with migrating corrosion inhibitors was recommended for the
conservation treatment. Compared to epoxy, this product is far easier to apply, has a
much lower cost, and its thixotropic nature would make it less likely to spill over onto the
surrounding tiles.4
Removal of the tiles revealed that were no large structural cracks penetrating beyond the
grout layer into the sculpture, so no crack repairs were undertaken.
Tile loss and drilling during the engineers’ assessment had left sections of steel
reinforcing exposed on the tiled faces. Corrosion was removed with a wire brush, and the
steel painted with an epoxy-based zinc primer prior to retiling.
Repair of Exposed Aggregate
Three products were tested for repairing cracks and losses in the exposed aggregate:
Barra® Mortar HL, Emaco S88C with MCI and Ardex B12. All products were combined
with black marble chips. The Barra Mortar HL was found to be too soft and crumbly for
this application, as machining would be required to replicate the polished exposed
aggregate finish. Emaco S88C was also unsuitable because the reinforcing fibres
produce a hairy appearance. Ardex B12 was selected as the repair product as it has a
compatible texture and colour with the existing exposed aggregate and can be polished
with a grinder.
Where necessary, previous repairs were removed using a diamond cutting blade on an
angle grinder. Once cracks and losses had been filled with the Ardex B12 and marble
chip mixture, they were covered with plastic overnight to slow the drying process and
prevent cracking in the repair material. The repairs were built up proud of the sculpture
surface to allow them to be machined.
A diamond disc on an angle grinder was used to grind down the repairs, which were then
further polished with a Scotch-Brite pad on a polishing machine. Obvious stone losses
which occurred during polishing were filled with Ardex B12, and polished when dry.
Protection from Ground Water Ingress
Having been informed that the sculptures had been placed directly onto the ground, the
original treatment proposal was to lift elements 1, 2 and 3 in order to construct new
concrete footings with waterproof membranes.
Digging around the sculptures prior to the proposed lift revealed the presence of concrete
foundations approximately 15 cm below ground level under elements 2 and 3. The
foundations were bolted to the base of the sculptures with steel footings. The metal
footings were corroded but still in solid condition. Concrete packers had also been
placed between the sculpture and the foundations. Dirt and tree roots had filled in the
void between the foundations and sculptures, allowing the ingress of ground water.
Element 1 was placed on three piles of concrete packers and scrap granite blocks. This
element had subsided so that it was sitting well below element 4 and the intended paving
level. Tree roots had completely filled in the cavity underneath element 1.
Plans to lift elements 2 and 3 were abandoned as it was considered that the foundations
would be adequate with some modifications, and lifting would place the sculpture at
considerable risk. Earthworks were carried out at the site in order to fully expose the
foundations and provide a level surface for height access equipment.
Dirt and debris were mechanically removed from between the foundations and the
undersides of elements 2 and 3, and the area then cleaned with a pressure washer. The
metal footings were de-scaled with a needle gun and coated with an epoxy-based zinc
primer.
Advice was sought on the best method of waterproofing the underside of the sculptures.
As there was limited access, it was not possible to properly prepare the surface and apply
a paintable waterproof membrane. Any defects in the membrane would be problematic
due to the effects of water pressure. An alternative suggestion was to apply the same
silane based coating, Protectosil® CIT that was proposed for the rest of the sculpture.
This product could be easily applied with a garden sprayer.4
Galvanised angle iron collars were constructed to surround the bases of elements 2 and 3
and prevent the ingress of water and debris. The base of the collars was bolted and then
cemented to the existing foundations. Leaving the undersides of the sculptures open to
the air was the best option to allow them to dry out after the application of the silane
coating. The collars will also provide an edge to facilitate the installation of paving, and
will be visually inconspicuous once this has been completed. Soakwells were installed at
a distance from the sculpture, and connected to the foundations with PVC piping, to
allow drainage in the event of flooding.
As the planned paving of the site would have made the subsidence of element 1 more
obvious and visually disturbing, this element was lifted using soft slings on a 10 tonne
Franna crane and temporarily placed upside down at a distance from the site. This
enabled the removal of tree roots and the application of silane on the underside. The
element will be lifted back into position on top of the pavers once siteworks have been
completed.
Application of Protective Coatings
Three coats of Protectosil CIT, a silane based corrosion inhibitor, were applied to the
entire sculpture using a garden sprayer. This product will enable the concrete to breathe
and slowly dry out while preventing further ingress of water and chlorides (Barnes &
Papworth 2007).
Following silane coating, APP ‘S’ sacrificial anti-graffiti coating (an improved version of
PSS 20) was applied using an airless sprayer. This product provides protection from
soiling as well as graffiti.
Conclusion
While reinforced concrete cannot be expected to last indefinitely, the conservation
treatment of The black stump will considerably extend its lifespan. It remains to be seen,
however, whether paving the site to avoid bore watering will have the unwelcome side
effect of encouraging more skateboarders to use the site. It is hoped that improving the
aesthetic qualities of the sculpture will increase the public’s appreciation of it as an
artwork and discourage deliberate and accidental vandalism.
Acknowledgements
We would like to thank Davide Degli Esposti from Ardex, David Dixon from BASF and
Artjom for being so generous with their technical expertise. Thanks also to Kate
Hamersley and Matthew Hunt from the Lawrence Wilson Art Gallery for their patience
and good humour during this project; and to John Barrett-Lennard for useful suggestions
on the article. Wendy Brown did a fantastic job of proof reading and correcting the text.
Artedomus provided two sheets of glass tiles to replace the missing colours in the
mosaics.
Suppliers
Protectosil® CIT
Barra® Mortar HL
Emaco® S88C
BASF Construction Chemicals Australia Pty Ltd
17 Valentine St,
Kewdale WA 6105
APP ‘S’
All Remove (Australia) Pty Ltd
45-47 Keysborough Avenue,
Keysborough Vic 3173
Grout King
Klenz Pty Ltd
Unit 4/ 7 Collingwood St,
Osborne Park WA 6017
Ardex STS 8
Ardion 90
Ardex B12
Ardex FG8
Ardex Australia Pty Ltd
69 Vulcan Rd,
Canning Vale WA 6155
Vixel glass tiles
Artedomus (Perth) Pty Ltd
837 Wellington St,
West Perth WA 6005
Endnotes
1. Holding, D. 2007. WA Universal Rigging, personal communication.
2. Papworth, F. 2007. Building & Construction – Research & Consultancy, personal
communication.
3. Degli Esposti, D. 2007. Key Account Representative, Ardex, personal
communication.
4. Dixon, D. 2007. Technical Sales Representative Building Systems, BASF,
personal communication.
References
Barnes, R and Papworth F. 2006. “Black Stump” Assessment for Repair and
Protection. Perth: BCRC.
Building & Construction – Research & Consultancy. 2007. Concrete Repair
Specification Black Stump Art Works at UWA. Perth: BCRC.
Cement & Concrete Association of Australia. 2000. Exposed-Aggregate Finishes for
Flatwork. Sydney: C & CAA.
French, P. 2007. Quotation 90107: Black Stump Repair. Perth, Asset Systems.
Gabrieli, S. 1997. Condition Report: Howard Taylor The Black Stump.
Biographies
Vanessa Roth Wiggin graduated from the University of Canberra in 1995 with a Bachelor
of Applied Science in Conservation of Cultural Materials, specialising in objects and
metals. She has worked in a number of government and private conservation laboratories
around Australia, as well as spending a year at the Belau National Museum in the
Republic of Palau. In 2002, she was awarded a Bachelor of Cultural Studies (Hons) in
Marine Archaeological Conservation from the Evtek Institute of Art and Design in
Finland. Vanessa is the Director of artWorks conservation, the business she established
along with her husband, Seth.
Contact Details:
7 Wrasse Glade, Warnbro WA 6169.
vanessa@artworksconservation.com
Seth Wiggin
Seth is a qualified boilermaker who has also worked as a hardwall plasterer. Seth first
encountered conservation as a volunteer at Samson House in Fremantle and has since
gone on to attend the Conservation: Metals unit of the Certificate in Museum Studies at
Edith Cowan University. Seth is now the Director of Works at artWorks conservation.
Seth specialises in logistics, occupational health and safety, corrosion repair and custom
made metal fittings.