Advanced Anatomy

For the Royal College of Pathologists, Bennetts Associates has created an HQ with an elegant and highly efficient skeleton – and given it every chance of a long and healthy life. By Tony Whitehead

The Royal College of Pathologists is not the sort of organisation to up sticks every five or six years. It is very much an institution, and for the past 50 years it has inhabited a grand Georgian terrace near Buckingham Palace. So when the College finally decided to move on and get a place of its own (Carlton Terrace was rented) – it was with a view to staying put for the next century or so. “This was a key reason we went for concrete at the RCP’s new building in Aldgate,” says Hannah Fothergill, project architect with Bennetts Associates. “It looks and feels long-lasting – and concrete has a weight, a gravitas, which we felt was appropriate for our client.” Early interviews with college members revealed a liking for the solidity of their old John-Nash-designed building.

“What came across was an affection for the sense of permanence inspired by exposed finishes, like the brickwork in the vaults, and this strongly influenced our approach to the design of the new building.” There are no dead bodies in the RCP’s new place. It has no clinical functions, but rather is a 3,100m2 headquarters, with office space, a lecture theatre, conference facilities and accommodation for senior members. Including the basement, it is seven storeys in all, and comprises an in-situ concrete frame all the way up to the top storey, which is set back from the rest of the building and built around a steel frame. Entering the college from North Tenter Street, the sought-after air of built-to-last sturdiness is immediately evident.

One passes through a facade of hefty, in-situ concrete wall-columns which are angled to control solar gain, and clad on the exterior with brick. Inside, the concrete walls have a highly textured, board-marked finish, which aligns neatly with the courses of exterior brickwork. Above, dramatic ceilings feature deep concrete coffers.

This is clearly not a building that is going anywhere any time soon – which is ironic given the fate of the site’s previous occupant. As Fothergill explains, this too was a concrete-framed office building. “Our first instinct was to refurbish rather than rebuild, as the building was only built in 1987. For that reason we really didn’t want to demolish.

The problem was, it had an absolute horror of a column grid. We needed to fit in atrium space and a column-free lecture theatre and it couldn’t be done without adding substantial transfer structures.” Bennetts was determined that its building would not suffer the same fate as its predecessors. “So we also wanted minimum columns to help our building stay flexible long-term,” says Fothergill. “The new building has a hierarchy if you like: an enduring concrete skeleton which cannot easily be changed, floors and staircases which can be, and then easily movable partitions.” The moral of the story is that while concrete has the robustness to last and the structural strength to allow for future alterations, it is still absolutely necessary to design with this in mind.

But, seeking to minimise waste, the team did manage to keep the original building’s 1.2m-deep basement slab, together with the basement’s retaining walls. This saved hugely on cost and programme time, but did raise issues. “Obviously we didn’t want to overload it – so we did consider a steel frame, which would have been lighter,” says Fothergill. “But we realised we really wanted concrete for its aesthetics and also to use its thermal mass as part of the building’s heating and cooling strategy. So we did our research and made it work.”

This involved looking at original documentation in Building Control archives, and carrying out tests on the slab to check that it could support the weight of a new concrete frame. Once the thumbs-up was received from the structural engineer, the challenge was to create thermal mass together with column-free space, but without using heavy beams and slabs that could overload the original foundations. “It was this that drove the integrated solution of the coffered ceiling,” explains Fothergill. “It both minimises weight, which is good for carbon and easier on the original slab, and makes full use of the concrete’s thermal mass by maximising the surface area in contact with air.”

The result is a ceiling featuring coffers that are 425mm deep, 950mm wide and set at intervals of 1.2m. The ribs between are 250mm wide and span the whole 13m depth of the building – from Alie Street on the north facade to North Tenter Street on the south. “The slab above the coffers is just 125mm deep,” says Fothergill. This slab depth performs well for thermal mass and “was very light and an efficient use of concrete. To go any thinner would risk exposing reinforcement.”

She adds that the early designs for the coffers were rectilinear, but the eventual result has a 10º angle to the side and slightly rounded corners. “The angle makes it easier to demould. We analysed angles between 5º and 35º but found that 10º was the sweet spot in terms of volume and thermal mass. The rounded corners help to prevent weird shadows forming and result in a pleasing, jelly-mould- like appearance.”

Because the coffers produce such a large area of exposed concrete, most of the building can be passively cooled most of the time. Air is pumped into the building via large 400mm voids beneath raised floors, and extracted at ceiling level having been naturally cooled by the coffered soffit. “In some highly populated areas we have added chilled beams suspended within the coffers,” says Fothergill. “These areas, like the lecture theatre, can quickly go from zero population to hundreds and the thermal mass cannot respond quickly enough.

We used external chilled beams as there was no room to place them within the slim slab.” The smoothness of the coffered ceiling is impressive and Fothergill explains that the choice of mix played a key role in helping to achieve the silky matt finish: “We have C40/50 concrete for the slab and C50/60 for the columns with 30% GGBS throughout,” she says. “We felt this was the right balance between reducing embodied carbon and achieving the desired visual quality.

New face of the RCP

Though essentially constructed from brick-clad in-situ concrete wall-columns, the North Tenter Street face of the Royal College of Pathologists also features some of the building’s few precastconcrete elements.

The facade is defined by a portal frame of precast concrete panels, and the in-situ structure is expressed on the outside by precast concrete spandrels at the level of each slab.

“Using precast concrete fixed back to the frame was a way to incorporate the thermal break,” says Hannah Fothergill at Bennetts Associates. The spandrels sit in the triangles created by the distinctive zigzag of the facade. Each is 220mm thick at the slab, tapering slightly to 200mm to create a slight fall to aid drainage.

The spandrels are also sealed, and feature a rebated aluminium drip to push water away from the front edge to prevent rainwater staining the concrete over time. “The zigzag is quite deep, around 1.5m,” says Fothergill, “and we are using the structure here to create solar shade on this south-facing elevation, rather than having to add things onto the building.”

We hoped to use 50% GGBS but the concrete contractor [Oliver Connell] advised that as well as longer strike times, a 50% mix could result in lumpiness, which is more challenging with deep-trough construction.” Ed Bourke, operations director with Oliver Connell, adds: “You might think you need small aggregate and a wet mix to achieve a smooth finish like this, but it isn’t always so. We used a 20mm aggregate with a 120mm slump. It actually gives a finer finish than a 10mm aggregate with 200mm slump.”

The reliability of the mix was equally important when it came to the distinctive board-marked walls. To create a highly textured finish, these were formed from 75mm-wide, rough-sawn pine boards nailed to Peri formwork. “Because of the way the wall columns on the facade swap angles, they are quite heavily reinforced to cope with the stresses that creates,” says Bourke. “But to ensure a consistent finish here you really need to use large vibrating pokers. We worked with the engineer to ensure that the reinforcement was designed in such a way that we could still get 2.5-inch pokers through it.”

The quality of finish suggests such attention to detail has paid off – testimony, says Fothergill, to a great understanding between designers and contractors. “We have learnt such a lot about concrete during this project,” she says, “I can’t wait to do another one.”

Dissecting the coffers

The coffered ceilings that are such a feature of the RCP’s headquarters are the result of a meticulous design and construction process. It was originally envisaged, for example, that they would be made with the help of polystyrene formers, but, says Hannah Fothergill at Bennetts Associates, these would not have achieved the desired quality. “It would be difficult to get the rounded corners to work in polystyrene, and we also could not source formers long enough to create 12m coffers,” she says. “We would have had to use two or more and have joins which would show up in the finished concrete.”

The solution cost more than polystyrene but, the team agree, using two floors-worth of made-to-measure glass reinforced plastic (GRP) formers was worth it. The formers were arranged on a deck of plywood formwork with reinforcement placed between each to create, in effect, 1m-wide downstand beams. Once the concrete had cured sufficiently, the supporting formwork was struck and the timber deck lowered from beneath to release the coffer moulds. “Because of the long spans we had to allow for deflection of the slab at the facades of up to 25mm,” says Fothergill.

Following cleaning, the moulds were manoeuvred through the atrium of the building and into position to follow the same process for the construction of the floor above.

Ed Bourke, operations director with concrete contractor Oliver Connell, explains that much of the smoothness of the coffers results from the use of GRP formers. “The GRP was treated to give a smooth matt finish, so we had to match that by using Tulsa-form ply for them to sit on, rather than phenolic which would have resulted in too shiny a finish.”

The connection between the formers and plywood deck was also crucial: “It was sealed with a neat bead of mastic,” says Bourke. “You have to use just enough. Too much and it can spread, rubbing onto the plywood where it would affect the water absorption of the formwork and result in dark staining of the finished concrete.”

Architect Bennetts Associates
Structural engineer Waterman Group
Contractor Gilbert-Ash
Concrete contractor Oliver Connell
Precast concrete supplier Cornish Concrete
GRP coffer supplier Cordek

Photos Peter Cook