5. Tender Information

Within the tender pack was a full strip out specification. Informed by our research into historic documents and site investigations we prepared detailed drawings that identified areas to be removed and more importantly areas and items to remain untouched. This was a key document and one that would be referred back to throughout the project. There were many unknowns at this stage due to the fact the structures had not been fully stripped out or cleaned. These were identified in the tender together with an approach to cover the investigations and appraisal needed to prepare final details.

The core of the tender documents was made up of detailed specifications that outlined the treatment and finish of the materials. Items which we knew were to be replaced, such as one of the shop front glazed timber screens, were detailed to match the existing profile. The middle screen was covered and assumed to be in a salvageable condition, although it turned out that this too needed replacing. The end screens were to be repaired in situ with well-seasoned timber to match the existing. The client’s desire was to introduce double glazed panes but this would have been detrimental to the aesthetics of the building and it was agreed to use single glazed panes. The front screens were originally open and without glass but had been glazed for a long time. To maintain their delicate look and to emphasise the slender section size of the timber the single glazed panel had to be maintained, as the double glazed panel would have appeared far too heavy.

The opening below the roof slab had been filled with masonry that was removed to expose the true form of the building. These areas were filled with bespoke, single gazed panels set into a slim aluminium profile.

As noted above we were not entirely sure what we would find on the internal concrete surfaces, or indeed to some extent the external ones either. Reading through previously carried out reports, it was noted that vivid colours were discussed. None were evident but a specification for a painted finish was prepared at tender stage in the hope that cleaning would give some clue to colour and its location. A mineral paint was specified to enable the structure to breathe. Reports were received to the effect that when an impervious silane was used moisture was becoming trapped inside the structure causing deterioration. We wanted to stop water ingress but allow the concrete to breath naturally.

The findings from the various tests, trials and investigations were incorporated into the tender package. For the structural works this included drawings of the existing structure which recorded the areas of damage we had seen and those areas which were still covered with finishes where we expected there to be defects. This included most of the inside of the shop, which continued to trade until the start of the works. Plans and details showing the proposed works noted the known areas of defects and indicated allowances for repairs which would be confirmed once the structure was exposed. The repairs were cross-referenced to our specification which gave details and methodologies for a number of types of repair. These were referenced to locations (on the top, side or soffit) and to the anticipated depth of the repair. This allowed the contractor to cost the scope of works and identify which repair to use for each location.

The standard repair for use on the top of level surfaces and vertical faces was a 1:2:4 cement:sand:aggregate mix to match the original concrete. Shallower repairs used a 1:4 mix of cement and graded aggregate up to 5 mm in size. Up to 20mm aggregate was used in the mix for larger areas of work where the concrete was re-cast. The procedure was to cut out the edges of the damaged concrete using a small disc cutter, taking care not to cut the reinforcement. The arrises needed to be undercut slightly to improve the mechanical adhesion of the repair material. Once all the loose and damaged concrete was removed to expose all of the surface of the reinforcement the bars were cleaned back to a bright, shiny finish.  They were then given an anti-corrosion coating and covered with a cement slurry immediately before placing the concrete.  In general the concrete had a plain finish but the shuttering did need to match the existing finishes including joints between boards (Fig 6).

There were some locations, mostly to the soffit of the slabs and beams, where a repair using a traditional concrete mix was not possible without significant implications for the historic fabric. Either the concrete would need to be entirely removed to recast around retained reinforcement, thus losing significant amounts of historic fabric, or the thickness of the slab would have to increase to place concrete from below that would provide an acceptable cover to the bars.  Both of these options were rejected and a 4mm thick proprietary render was applied to the cleaned and primed surface, which was then manually worked on the surface to match the surrounding board marks. For the entrance slabs our tender drawings showed an allowance to provide this render coating to all of the soffits (Fig 7).

The soffit within the shop was hidden by a suspended ceiling. The area within the store room showed a considerable amount of exposed reinforcement. It was believed that the rest of the soffit would be in similar condition and it was agreed that instead of repairing this, which would have considerably stretched the project, the contractor would carry out minor repairs and install a new ceiling below. This would leave the repairs for future refurbishment. This approach also gave the advantage being able to hide the new services such as cabling to new lighting and safety systems. A design was developed to stop this new ceiling finish inside the new glazing around the perimeter of the shop to help identify the ceiling as a new element.