The approach and implementation of repairs to historic concrete is still at an early stage. To help develop the understanding of best practice a programme of monitoring and testing has been established in conjunction with English Heritage (now Historic England) to gauge the effectiveness of the recent works. This will include a photographic assessment at 1, 3, 5, and 10 year intervals and corrosion rate mapping pre- and post-repair to assess changes in the corrosion rates of the adjacent reinforcement. Concrete hardness tests using a Schmidt hammer will be undertaken on repaired and surrounding concrete, and samples will be tested to assess changes in the alkalinity at various depths across the depth of the concrete and at distances away from the repair. Historic England is hopeful that the tests will confirm that the high alkalinity of the new concrete repairs will migrate into the adjacent original concrete.
These repairs form an important part of the history of these buildings. To explain and promote this and the rich history of their inception a portion of the Station Cafe now houses an interpretive display telling the story of the zoo and its Tecton buildings. The display uses historic images of the zoo, presents information on its architectural significance and describes the works that have been undertaken. The surviving section of the original concrete counter to the cafe forms part of the exhibit. This has been finished to its original form with a polished concrete top, timber edging and a white tiled face. A portion of the counter was re-cast, with a portion of the previously exposed reinforcement retained to show visitors how these buildings were formed. A large section of the counter has been rebuilt in timber and finished completely in white so as not to detract from the existing section but to better show the original sinuous form.
A key part of the success of the project has been the very close collaboration between the client, architect, engineer and contractor, alongside regular reviews with English Heritage and the Twentieth Century Society. This is especially important where the ‘light touch’ approach for listed buildings is applied. The project has been fortunate to have a client who has a long-term commitment to the buildings and understands that it is not possible to ‘solve once and for all’ the issues with these buildings and structures. The evolution of the relationship with the contractor moving into the works on the Bear Ravine and Kiosk allowed for an evaluative approach without cost-pressure from a main contractor. This avoided the “tender and leave us alone” approach under which a main contractor would have looked to penalise later changes. This was especially beneficial to the Bear Ravine where the approach changed significantly from tender stage. It is hoped that the approach taken here can act as an exemplar to other historic reinforced concrete structures, in the UK and elsewhere.