2. Appraisal and Investigation Stage

From the outset the approach was the same as for any pre-20th century listed buildings. This was to gain an understanding of the form and condition of the structures and use this to target repairs based on the principles of using compatible materials with the minimum amount of intervention into the historic fabric wherever possible. A key part of this approach was not to repair unless there were clear signs of damage. For example, the poorly formed construction joints and local irregularities in the surface from poor compaction would be preserved as a record of how the structures were built.

We began with a review of the historic documents held within the Zoo’s archive. This vast amount of information allowed us to build up a detailed picture of the structures intended use, original forms and even colours. The original Tecton drawings clearly outlined the structures’ intended form. Photos taken post-completion allowed us to see materials and finishes and even though they were black and white these would help to inform later decisions on colours. These pictures also allowed us to identify alterations to elements of the buildings. For example, the ‘ZOO’ lettering on the front of the Entrance kiosks was thought to be original (Fig 2). After reviewing the historic images, we identified that the ‘Z’ had a slightly different shape on the corners, telling us that they had been replaced at a later date. Within the archive we also found Zoo programmes from every year dating back to its opening. On the programme from 1937 a hand painted image of the Entrance clearly showed red lettering with a white border.

The next stage was a visual assessment of the structures. We posed questions such as how would they have been designed, how did they build with reinforced concrete in the 1930s, what was the quality of the construction and how had they been altered or repaired? From this we were able to get a general ‘feel’ for the structures, assess if the defects were generic or specific and identify where there were particular structural issues.

The undulating entrance structure was now used only at peak times. Several of the kiosks had been shut off from use and wrapped with half-round timber cladding. Every element of the structure had been painted in a variety of colours through its history. The internal turnstiles had been removed and in one area a large industrial sliding gate had been installed. The proposed design for the entrance was to strip away all of the added items and to reanimate the original structure. All of the kiosks were to be reopened and put back to their original design in both form and material.

The neglected structures of the Bear Ravine and Kiosk were the most unchanged due to the fact they had been left unused for many years. As with the other structures both had many years of different paint finishes. The design approach for these two structures was much the same as the Entrance, to reinstate the original design intent but it was clear that the Bear Ravine would need a detailed structural appraisal due to its ambitious form and obviously poor condition.

The Station Cafe was the most changed from its original construction (Fig 3). Through the years the building had been carved-up, adapted, split up and put back together again. It had been used for a fish and chip shop, nightclub and various other purposes which would never have crossed Lubetkin’s mind. The original open, windowless pavilion style of the building had been infilled and the existing zoo shop bore very little resemble to the original station cafe (Fig 4). A large portion of the building had been closed off and used for overflow storage. The intention was to open up the building once more and to remove all of the later additions to leave only the original structure. To fit within the wider zoo masterplan and the way the majority of visitors now arrived at the zoo, the shop also would become the main entrance. It would also house an interpretation space to explain the history of the zoo and the Tecton structures.

The structural assessment found the majority of the problems were due to corrosion of the reinforcement. In many areas the cover to the reinforcement was much less than the one inch (25mm) cover for main bars and half-inch (12mm) cover for secondary reinforcement which was recommended at that time in the 1933 “Report of the Reinforced Concrete Structures Committee of the Building Research Board with recommendations for a Code of Practice of the use of reinforced concrete in buildings”.  There were however some areas where the damage appeared to be as a result of structural problems.  At the entrance building we noted there were transverse cracks in similar locations on the top face of each of the five curved slabs. We also had concerns about the integrity of the connection between the top of the solid steel columns and the slabs. At the Bear Ravine there were cracks in the side walls of the cantilevered viewing platform, a longitudinal crack in the slab above the downstand beam and a distinct “bounce” at the outside edge of the platform from a heel-drop test.

Concurrent with our appraisal was a series of investigations and tests carried out by Rowan Technologies Limited. Schmidt hammer tests which found the compressive strength to be mostly in the range of 40N/mm2 to 55N/mm2 with only a few readings between 30 and 40N/mm2.  All the readings were greater than the minimum cube strengths for the four categories of “Ordinary Grade Concrete” in the 1933 BRB report. This, combined with our own assessment, confirmed that the strength of these structures was generally not a concern.

Tests on samples of the concrete found the chloride and sulphate levels to be within acceptable levels. The main issue was the widespread inadequate concrete cover to the steel reinforcement and carbonation depths of up to 40mm from the face of the concrete. The curved slabs to the entrance generally had a cover on the top face of between 13mm and 65mm and there were only occasional signs of corroding bars. On the underside the covermeter survey was hardly required as large areas of reinforcement were clearly visible through the many layers of paint (Fig 5). So why was the concrete cover so poor?

There is little information from the 1930s of how reinforcement was to be supported during concreting. We found one contemporary reference, Cassell’s Reinforced Concrete of 1920, which mentions the use of notched timber templates which “can be removed shortly after the concreting has begun, quite a small quantity of concrete sufficing to hold the rods in place”.  If this was the method used at Dudley, then the undulating shape of the slabs meant it was almost inevitable that the bars would slump towards the bottom of the wet concrete. We can only surmise that the pressure to open the zoo to the public led to a ‘make do’ approach with a render coat and paint to hide the reinforcement.

There were also areas of exposed reinforcement and spalled concrete to the shop and kiosk but the greatest damage was to the Bear Ravine. On the cantilevered viewing platform there were large areas of the parapet where the whole of the face of the concrete had fallen away and exposed the corroded reinforcement behind. We were also sufficiently concerned about the extent of corrosion to some of the circular hollow steel columns around a stairwell to ask the zoo to immediately install temporary props.

At the end of this initial stage our report to the HLF was able to place the structural defects into two broad categories, those due to poor quality construction and subsequent deterioration, and locations where there appeared to be structural issues that required more extensive works. As the finishes were removed and our knowledge of the structures improved over the course of the project we found that some of the structural concerns we had originally identified were less significant and the number of areas in this second category gradually reduced.

Our initial appraisal also confirmed that the proposal to use ‘traditional’ concrete for the repairs and to avoid modern polymer-modified materials wherever possible was feasible. Using compatible materials reduces the risk of future differential movements due to thermal expansion and contraction or changes in the moisture content. In addition, for listed structures with exposed concrete like these, a “traditional” concrete mix will provide a closer visual match to the original fabric. Since 2009 we have been advocating this approach alongside the Twentieth Century Society at the annual course on concrete conservation at West Dean. The tendering of the project at Dudley coincided with the publication of an English Heritage book on the conservation of concrete, which proposes a similar approach.

The next step, during early 2013, was to undertake trials on part of the Entrance to ascertain the best method of cleaning the concrete to remove the layers of paint while maintaining the surface texture and board marks. It was believed that blast cleaning was the most appropriate technique. Varying types of blast medium and air pressures were trialed until the best balance of effective paint removal and damage to existing surface was found. We also specified a trial repair to an area of spalled concrete on the shop to explore methods of how best to remove the damaged concrete, how to deal with the limited concrete cover to the bars, the best proportions for the cement: aggregate repair mix and how to match the surface finish.

Concurrent with the cleaning and repair samples on the structures, a paint survey was undertaken to analyse the layers of paint present on the different surfaces. This microscopic analysis told us the colour history of the structures and also and more importantly the original finishes and colours.