6. Entrance and Shop Repairs

The contract for the entrance and shop started in September 2013. When work began the strip-out and cleaning processes happened concurrently. When the trial cleaning was carried out on the underside of the soffit a blue colour was visible. When the entire soffit was cleaned it was seen that this blue was on the entirety of the underside. The edges however, showed no sign of colour. The microscopic analysis backed this up by showing a layer of dirt on the concrete edges before the first layer of paint was applied, indicating it had been left untreated for some time. The stripped soffits revealed extensive areas of exposed, corroded reinforcement and spalling concrete. It was clear that our tender allowance to treat all the soffits was indeed what was required.

Another tender-stage allowance was to break out the concrete around a number of the column heads, install additional bars and re-cast with new concrete. A trial investigation was carried out to the column head which had the greatest amount of damage to the concrete.  After the slab had been temporarily supported and the concrete locally removed we saw that the reinforcement around the column was in a poor condition. We also saw that the column had a 180mm x 180mm x 20mm thick welded head plate which showed no significant signs of damage. Our tender detail for this area had been to clean all the steelwork, provide additional steel bars around the steel post and install a cementitious grout, poured from above through holes drilled through the concrete slab. For the trial location, where a hole had been formed through the slab a new section of concrete was cast. Elsewhere the contractor was able to repair these areas from below, either with a traditional concrete repair placed by a letterbox shutter, or with a render repair mix.

As mentioned earlier, there were cracks on the top surface of each canopy which were on the hogging curve of the slab. There were no obvious structural reasons for this and the most likely cause was small scale folding of the slab from thermal movements. As it is not possible to stop these movements our initial suggestion was to fill these cracks with a soft lime mortar and accept that this was an area that would need a higher level of ongoing maintenance. However, it was decided that water ingress into the slab had to be prevented across its entirety to prevent future damage to the newly repaired soffit and paint finishes.

The canopies had never been waterproofed on the top surface and instead relied on a shallow fall to a small drain that ran down through the kiosks below, with the majority of rainwater running over the edge of the roof slabs. The concrete was, unsurprisingly, damp and a temporary covering was placed over each section to allow the structure to dry out before the new finishes were applied. We had planned to treat the top surface with a silane to stop any water from penetrating through the concrete. However, through further discussions with various specialists we believed that any ponding water would eventually find its way through. The criteria for a new covering included the need to be reversible and to be able to accommodate thermal movements. It also needed to be visually acceptable as the top of the Entrance can be seen from the top of Castle Hill and when journeying down the chair lift. No flashings or mechanical fixings could be evident as this would destroy the slender edge detail of the wave forms. With the exposed location of the Entrance the new covering would also be subject to uplift pressures. Through various discussions with the local conservation officer and English Heritage, it was agreed that a permanent solution was best to safeguard the future maintenance of the structure whilst providing little visual impact. The solution was a liquid applied membrane that, when dry, formed a single covering fully adhered to the concrete surface. This system allowed the waterproofing to extend right up to the edge of the slab, leaving only a 3mm thick visible profile. The resulting surface will accommodate also any thermal movement experienced by the curved concrete surfaces. To match the surface colour and texture with the existing concrete a resin-set quartz was applied to the top. The aggregate for the quartz was sourced from a local quarry to best match the tone and consistency of the concrete. The resulting view from the chair lift is five seemingly untreated concrete canopies (Fig 8).

At the shop, the initial work involved the stripping out of all the internal fixtures and fittings and the removal of the external roof finishes. Once stripped, the concrete roof structure was in a better-than-expected condition and only required a few local patch repairs. A single-ply membrane was installed over new insulation. The existing roof lights were opened up and combined with the removal of the high-level brick infill this completely changed the feeling of the internal space, flooding it with natural light as was originally intended.

We had seen a part of the underside of the roof slab within the storeroom and once all the finishes and the black paint – another reminder of the nightclub – had been removed we were able to complete our investigations and finalise the scope of repairs. On the underside of the slab the main issue was the poor concrete cover to the bars and a number of areas with exposed reinforcing bars needed to be repaired. As most of the soffit was to be hidden behind a new suspended ceiling a different approach was adopted for isolated exposed bars and hairline cracks. To make best use of the available funds it was agreed that if the current damage was not structural and was not likely to deteriorate in the future it was left untouched.

The initial strip-out process proved to be fascinating and delivered a few surprises. One such surprise was that when the existing screed was broken out the original stone flag paved floor was found underneath. In some areas it was in remarkably good condition and after trialing some cleaning methods it looked almost new. However, due to the changing use of the building through the years the paved floor had been removed in some areas and other areas were in very poor condition. The floor had been laid onto compacted ash and it would need to be removed and re-laid over a new slab, a damp proof membrane and insulation. This coupled with the large areas that would need replacing led to the decision to lay new floor finishes over the slabs and leave them for a future renovation. The new timber floor floats above the existing and allowed insulation and underfloor heating to be installed. This provided the same benefit as the new ceiling, the ability to hide services. This was especially important for the underfloor heating, which avoided other, more visible methods of heating. The flooring finish was a textured vinyl tile that mimicked the finish of the existing concrete slabs. The pattern of the original floor was recorded and used to inform the layout and detail of the new flooring above.

The non-original steel beams and columns that had replaced the concrete columns were removed and new steel columns installed onto new concrete pads placed around the original damaged foundations. Steelwork was chosen in place of re-forming the columns in concrete because of the speed and ease of installation and the significantly lower cost. It was also a concern that new casting techniques would look too good next to the existing columns. Instead, a mould was taken from one of the existing columns and used to create in GRP the new cladding which was installed in two half-sections with the joints mimicking the cast lines in the original shuttering (Fig 9).

The most significant structural alteration was the result of changes to the way visitors now arrive at the zoo. Rather than arriving by public transport at Castle Hill, the majority of visitors now arrive by car and park some distance from the original entrance. As part of a bigger site-wide development the new entrance would now come through the Station Cafe. To facilitate this a new opening had to be made through a reinforced concrete wall. A new steel angle was added to support the structure above the new opening and the mechanism for the sliding glass doors.

Throughout the strip-out process, as years of paint finishes were stripped away from the concrete vivid colours started to emerge. Colour was found externally during the test cleaning but was not expected internally within the Station Cafe. The use of colour internally transformed the proposed interpretation of the space. It was interesting when reviewing the original black and white pictures as the different tones could now be identified as different colours. Bright red and blue were used on the internal walls, those running parallel to the entrance were red and those running perpendicular in blue. This along with the continuous gap between wall and roof served to identify the walls not as connected surfaces but as a collection of planes. Different methods were trialed to expose these coloured surfaces whilst causing as little damage as possible. After several attempts it was decided the best method was to strip the surface with blast cleaning and then reinstate the colours with a mineral based paint. Samples were taken and analysed and then compared against the paint remnants on site as well as reports from interviews with Lubetkin. A sample wall was created on the back of the toilet block to test the paint types and colours on the concrete surface. After a few trials a Keim Lasur (semi-transparent) paint was chosen for external walls. A colour matched to the existing tone was diluted at a ratio is 4:1, not to cover the repairs and surface inconsistencies but to provide an evening tone so as to not detract from the overall building form. Internally a Keim Soldalit (solid paint) was used to recreate the vibrant blue and red colours. Internal surfaces originally left fair-faced concrete were treated with the Lasur. External surfaces were treated with a breathable silane prior to colour being applied, to provide a water resistant surface whilst still allowing moisture to escape from within the structure.

The most significant repair works were to the attached toilet block. A section of the cantilevered roof slab above one of the entrances was so friable that the concrete could be unpicked by hand. The rebuilt section was considered as a piece of new construction and designed for imposed loads from modern codes. This required some additional reinforcement alongside the original bars before the section was re-cast to the original profile. To the rear of this building were two high-level windows which ran the full length of the building. These six-metre long openings each had a slender 130mm x 130mm reinforced concrete mullion at mid-span. Once the damaged concrete had been removed the reinforcement to one mullion was found to be in a reasonable condition. After cleaning and coating the bars the mullion was re-cast with new concrete. The bars on the second mullion had lost a significant amount of their cross-section and we prepared two repair options: re-casting with additional bars or replacing with a steel hollow section. After discussions within the project team and English Heritage the option for a steel post was agreed and installed.

Paint analysis of the existing timber showed a creme enamel paint finish. A colour code was produced from this analysis and used to create a match finish. The same analysis showed the steel columns to the entrance to be bright silver. The colours, which appeared very extreme in isolation, come together as a nice composition.