The works to the entrance and shop were substantially complete on programme and both buildings were opened in time for Easter 2014, to much acclaim. The Zoo now knew how much money was left for the works to the Bear Ravine and kiosk. More importantly the design team had tested the process of repair and could develop the best strategy to tackle the extensive works required at the Bear Ravine.
Unfortunately, the performance and general level of management by the contractor who had carried out the concrete repairs to the entrance and shop had deteriorated during the works. One consequence of this was that during the works one of their foremen applied for the position of clerk of works. The expertise in concrete and stone repair held by this person and obvious enthusiasm for the project presented a new possibility of employing someone who could be much more than just clerk of works. Subsequently hired by the Zoo to lead the concrete repairs, with one assistant and two apprentices, the Zoo now had direct control over the costs and quality of work. The same general contractor who had worked on the Station Cafe and Entrance was used to provide the site mobilisation and scaffold. The concrete cleaning, painting and waterproofing were procured as a separate package through the main contractor.
In contrast to the previous structures, much of the Bear Ravine’s concrete surface is covered with a delicate corrugated finish. When it came to removing the various finishes that were present a number of different techniques were trialed on a damaged area of concrete. It was decided that a two-stage approach was needed. Firstly a ThermaTech super-heated steam system would be used to remove all organic contaminants and modern paints without affecting the surface finish or texture. This process cleaned the majority of the structure back to concrete but left some mineral paints and slurry coats. These more stubborn areas were inspected and further assessments carried out. Some areas were treated with chemical strippers and removed by hand. The curved wall of the kiosk reacted very well to this treatment. The top of the kiosk roof had a dense layer of moss which was completely cleared revealing a surface that was in a remarkably good condition. These chemicals were carefully chosen due to the nature of the Zoo environment. Areas that did not react to this treatment were cleaned using a very light blasting program utilising both wet and dry blasting.
The initial feasibility-stage review of the Bear Ravine had identified significant structural issues with the cantilevered viewing platform (Fig 10). Based on this we had made an allowance in the cost plan to remove all the concrete to the parapet and place additional reinforcement alongside the existing bars before the new concrete was placed. We had also made an allowance for stiffening the cantilevered slab with a layer of mesh reinforcement within a high strength cementitious render on the top and bottom faces.
With the slab propped by a support scaffold, the layers of paint were removed by the steam cleaning process along with loose and damaged areas of concrete. This showed that the underlying concrete was in a much better condition than we had previously anticipated. As a result, we now looked to repair rather than rebuild the parapet and explored an alternative to the addition of mesh reinforcement, which would have increased the dead load and changed the appearance of the platform (Fig 11).
Where the reinforcement was not already visible we specified local opening up to confirm the size and spacing of the bars. These were transverse layers of 9.5mm (3/8″) diameter bars at 100mm centres near the bottom of the slab. The downstand beam had six Isteg bars at the bottom and 9.5 mm links at 200mm centres. There are no records of the original calculations and there was, unsurprisingly, no specific guide for the imposed loads to be used in the design of a structure like this. The 1933 HMSO publication has a category for “churches, schools, reading rooms, art galleries and the like” that gives an imposed load of 80lbs per sq.ft (4kN/m2). The Zoo does plan to allow controlled use of the viewing platform for visitors but 4kN/m2 seemed excessive. A notional imposed load of 3kN/m2 seemed more reasonable on the basis that visitors could congregate along the prow of the platform. We also wanted to provide a structure that was stiffer than before to reduce the risk of deflection-induced cracks and provide a platform that felt more secure so as not to cause undue concern to visitors.
We used this value to calculate the bending moments, shear forces and span:depth ratios for the slab and beam. This found that the slab would, theoretically, be able to support this imposed load but the beam would, again theoretically, fail in bending and shear. From our review of the calculations and the signs of damage seen on site we concluded that remedial work should be targeted to address the general lack of stiffness, the lack of top reinforcement as indicated by the crack above the downstand beam and the poor connection between the ends of the downstand beam and the spandrel panels.
This led us to propose remedial works that used carbon fibre bonded to the concrete. Once the principle of this approach was agreed with all parties we discussed the works with Fibrwrap, a carbon fibre specialist, and visited one of their projects in London to see how carbon fibre sheets were fixed onto reinforced concrete. We then provided them with values of the existing and proposed bending moments and shear forces which they then used to develop their design. Two layers of 1mm thick carbon fibre sheets were applied locally to the slab to enhance the hogging and sagging bending moments. A single layer of sheeting was applied to the downstand beam and carbon fibre cables were installed through two small holes in each spandrel panel and then splayed out to link both sides of the beam with the outside face of the spandrels. Another unscientific heel-drop test found significantly less bounce in the platform than before the repairs.
The kiosk in comparison needed minor structural repairs. The hollow steel columns were corroded just above ground level and a low-key repair of new steel sleeves that were welded to sound metal was used.
The concrete repairs were on a much larger scale than those previously carried out but followed the same philosophy, cement – sand mixes for all vertical and horizontal surfaces and a proprietary blended repair mortar for the underside of the walkways. The quality of the repairs to the corrugated finish of the parapet walls was a significant achievement by the on-site team. A series of tools were developed for this that ranged from a float profiled by a latex mould taken from an original section of corrugation to a bespoke timber profiled float. However, the most useful tool turned out to be a long round timber dowel that was used to line the profiles through. When breaking out the necessary areas of concrete any sound pieces of concrete were left no matter how small. These small areas ensured that the rhythm of the corrugation was retained in the repairs in relation to the existing. The finished work is truly impressive and a credit to the site team (Figs 12, 13).
Another aspect of the Bear Ravine was the steelwork present in the stair balustrade and the remains of the infill bars to protect the viewers against the bears. The later was in extremely poor condition with only a few pieces remaining. These were removed and a blacksmith used the few pieces left to recreate the pattern and reinstate this element. This was important to the overall composition as it tells the story of the dangerous former residents. The balustrades were also removed, stripped of all paint finishes and elements rebuilt where necessary making sure to retain as much of the existing fabric as possible. These were re-galvanised, finished in bright silver and reinstalled.
The paint research here again found a thin layer of pollution between the face of the concrete and the first layer of blue paint. This indicated that the Bear Ravine was also unpainted when first opened to the public. However, the layer of dirt present was much smaller than others found on unfinished areas of the Station cafe leading to the conclusion that it was not left unfinished for long. We decided to put the structure back to its form following completion and an unfinished appearance by using the Keim Lasur coating. When the concrete surfaces were being blasted some very stubborn finishes remained. It was agreed that to remove these would need such an aggressive medium that it would damage the surface of the concrete. Therefore, these small flecks were left and instead the dilution ratio for the Lasur was lowered to 1:2. As elsewhere all external surfaces were first treated with Silane.
The kiosk was, like the Station Cafe, coloured with bright reds and blues. As with the Entrance the underside of the canopy was blue, with the lower wall wrapping around the counter being red. These colours were reinstated using the Keim Soldalit with the rear of the kiosk using the Lasur.
The original timber screen within the kiosk had been removed and so the original drawings and pictures a pattern was developed and a new screen installed. The paint analysis on the Entrance and Station Cafe revealed that all timber was painted creme and the steel columns were finished in a bright silver. The surfaces of the walkways and viewing platform and the top of the kiosk canopy were treated with the liquid membrane and quartz layer to match the underlying surface.
The works were completed early in 2015. Following this the on-site team has worked on the sea lion pool and is currently repairing the former reptillary, now home to meerkats. This will prove a demanding challenge as this is the only enclosure never to be painted. The work will also need to be carried out in stages so that the meerkats remain in place as moving a colony is known to disrupt breeding by up to five years.