No cutbacks here

The architect, Stanton Williams, has designed an exposed concrete structure with large cantilevers upon which sit French sandstone fins and huge glazed areas. With the concrete structure standing bare with no cladding, specification and tolerances were demanding to say the least.

It’s been a year since the numerous and painstaking concrete pours began to reveal the new laboratory facility, funded with £82m from the supermarket mogul’s Gatsby Charitable Trust (see Construction Manager June 2010). Now that the 11,000m² building has opened, CM was keen to investigate what kind of envelope the facility’s £4,975/m² cost had actually procured. Here, in the 200-year-old Botanical Gardens, what you see is what you get — and then some. The tolerances on this building are nuclear power station to-the-millimetre.

Gavin Henderson, project architect, describes the design intent thus: “We wanted geological solidity, inspiring the choice of materials, but also wanted to open it out to the gardens, in a collegiate-style way, giving us the idea of having the huge walkway facing south and west and allowing researchers to meet and talk ideas.”

This academic cloister is where the building concentrates its cantilever gymnastics. At first floor level, its signature 600mm thick exposed concrete nosing runs around the building, thickening to 1.4m internally, to deal with the vibration criteria of the labs and their sensitive monitoring equipment.

“We really wanted to give the impression of a simple flat slab defining the building, with minimal joints,” says Henderson. And while they looked at pre-cast options, this would have meant compromises on jointing. The “ductile” approach eventually taken, which allowed gradual thermal expansion of the monolithic slab, meant that the quality of the concrete and the casting regime had to be almost flawless.

Ensuring construction quality made the choice of a two-stage develop and construct contract under the NEC3 EEC Option A, basically a souped up D&B contract, a strange one on the face of it. But Steve Toon, associate director with engineer Adams Kara Taylor, explains that while this was the standard form of procurement for the University estate, the intent from the client down was only for the highest quality, and so it worked.

Toon says selecting the concrete sub-contractor was one of the project’s big decisions, and driven by the whole team. Both architect and engineer were explicit about the level of finish that would be demanded, with the contract eventually awarded to local firm Whelan & Grant. Toon explains that over time numerous samples were looked at to for colour and consistency of mix. The more sustainable GGBS was looked at, but rejected.

Toon admits that this decision probably came down to him. “Standard practice is that you’d strike poured concrete at two-thirds strength after seven to eight days, but I was nervous,” he recalls. “Previous experience on another project had proved that pre-cambered slabs could be ‘pushed out’, and because we were making the slab as thin as we could we had to make sure that we were getting every ounce of tensile strength out of it. The large span of the cantilevered nosing meant that, as much as possible, deflection had to be accounted for and designed out, otherwise we were looking at deflections of 70mm.”

To stop micro-cracks occurring in the slab, which can occur when concrete is struck before being fully mature, AKT insisted on props and formwork remaining in place for 28 days to achieve the required maximum deflection criteria of +/-10mm. “We couldn’t afford to be lackadaisical about the deflections, as absolutely all of the concrete was exposed,” recalls Toon.

Whelan & Grant were seriously concerned that the 28-day curing period would result in staining of the GGBS. The team decided, therefore, to opt for less sustainable, but less risky 100% Portland cement with aggregate, secured from the same Leicestershire quarry for consistency and with a titanium dioxide admixture to lighten it. Even the concrete’s plywood formwork for the voidformers to reduce the concrete slab’s self-weight was highly specified, with the contractor stating at one point that “it felt like they were asking him to build a bathtub out of plywood”.

Toon says that the concern for the finish was such that formwork was used a maximum of three times, and the first in “non-precious” areas (see Nick Mann box, right) to ensure there was no chance of release agent staining its surface.

Tolerances on the concrete faces also accounted for stone contractor Szerelmey’s decision to build the building’s yellowy Jaumont stone fins on site, rather than prefabricating them. Concrete faces could not be more than 3mm out relative to each other, and misalignments were eradicated by hand-building the fins to ensure they aligned perfectly with the concrete faces. At the top of each fin, barely visible, is a 10mm movement joint to the soffit of the slab — one of the few joints that can be seen.

While Henderson says the fins mitigate solar gain, when asked what they are doing on the north side, he concedes they stem from a desire to unify this “operations” side of the building, which comprises administration offices, prep rooms and support spaces that would generate an uncoordinated facade in a building that is otherwise all about pared-down minimalism.

Instead, the team decided that the fins in front could act as a visual foil to the problematic north face, and it’s for this reason that the cladding behind them was specified as a stick-toggle system. “It’s deliberately simple and removable so that, with the fins creating an overarching order to the facade, the cladding behind to the support rooms can be changed from solid to glazed and back according to future requirements and space planning changes,” adds Henderson, alluding to the building’s ability to evolve over time.

He also concedes that on the south and especially the west glazed facades, lining the internal “street” and where the envelope is more prone to more solar gain, the stone fins are dispensed with altogether to maximise views onto the courtyard and the gardens beyond.

While it provides nice views for users, it was more of a concern for services engineer Arup and Arup Facades to provide a workable solution. “Sun patching might look fine on the ‘street’, but we had to look at it in terms of overall thermal performance and daylight penetration,” recalls Arup associate Jennifer Dimambro. “We realised after conducting studies that the fins alone at the centres specified wouldn’t be enough to deal with it, as these are big expanses of glass.” She explains that the highly serviced environment of the labs was obviously distinct from the ‘street’, but that mechanical servicing was still required.

The main line of defence was the glazed facade itself, installed by German subcontractor Feldhaus and specified with high-performance glazing with a high g-value to mitigate solar gain. In some areas this rises to 4.2m in height, requiring discreet steel fins to be fixed behind it below the concrete soffit to stop deflections. Any vertical flexing of the slab, however minimal, is taken up with a concrete soffit rubber shoe detail that holds the glass in place, but allows for incremental movement without putting load on the glazed panel itself.

In addition to the glazing, external roller blinds controlled by a BMS system were specified. “The blinds are pre-set to deploy in different positions according to amount of sun exposure and seasonal conditions,” explains Dimambro. Despite the cost, the same glass coatings were used throughout to ensure that not even minor chromatic changes in glass colour were evident from one elevation to another. 

On the east block’s west elevation, the 6.5m cantilever is propped by slender 1.2m aluminium-clad steel struts. The spaces between them are cell-like, oak-lined monastic study spaces that double-up as a kind of solar buffer for the building from strong west light, with oak composite panels facing the courtyard. Insulated soffits to the concrete are clearly seen here, which appear to be electrical service runs, but which in fact act as a thermal break for the slab.

At the south side of the block, the concrete cantilever extends to 7.5m, and Toon says achieving this was another challenge. “Pre-cambering is not an exact science, and we new when we cast it that it could not slump at all, as that would affect the final strength. On the day when Kier came to remove the props they insisted that we were there on site to ‘buy-in’ to the cantilever,” he recalls. “They were amazed that on removal, and with workers stepping out to its edge to test it, there was absolutely no flexing at all.”

Toon adds that even the concrete finishing was raised to the level of art, with tie holes filled with concrete plugs individually shaded by workers to ensure that they colour matched exactly. The whole surface was then hand rubbed with hessian. And it is such a fact that perhaps characterises the building itself, one whose envelope is high performance for its scientific purpose, but which is, in its own way, crafted and hand-made.

“It looks like a very simple building, but I don’t think people appreciate the lengths everyone went through to deliver it,” says Toon. “I have never specified tolerances that tight on any other building.”

High performance yet ultimately hand-made. It’s like the conundrum of “evolution versus intellectual design” and is one you can’t help but feel would please Darwin enormously. 

The level of internal finish is as high as externally — effectively the structure passes inside. Concrete slabs move in, with an insulated soffit below acting as both a foil to thermal bridging and a services zone. Oak and Jaumont stone predominate

The contract administrator’s view

Margaret Winchcomb, Hannah, Reed & Associates

The NEC 3 EEC Option A is not a traditional D&B contract, but this fixed price contract has been used by the university estate for seven years, and it’s working well. We feel that it’s simpler, less adversarial, and engenders good communication between the parties.

There was close liaison between the consultants up to stage D, but the minute we hit stage E, we got the contractors on board, basically as early as we could. The consultant team all had their aspirations about what they wanted to achieve, but that had to be tempered with the contractor’s view and what it considered to be possible.

How the formwork would be constructed and slabs poured to the required tolerances obviously, but also other things. The contractor’s knowledge of how the stone lay in the quarry in France allowed the designers to optimise the stone size for the cladding early on.

At £65m build cost the labs are not cheap, but it was still a fixed price. That said, everyone was aware from the outset that we wanted a world-class building.

I wouldn’t say in contract administration terms this was a complex project, and due to the team liaison, there were relatively few variations.

The south and west elevations both have to deal with solar gain. High “g” glass deals with some, but the west elevation also requires external BMS blinds as a further measure

In the detail

Gavin Henderson, director of Stanton Williams, describes construction of three key elements

Enhanced construction tolerances were required for the 70m long continuous concrete slab edges at roof and first floor levels at the north and east facades. The columns are traditionally laid in solid limestone blocks and are set out to be flush with the concrete at head and base. Full height 4.2m glazing panels were installed early in the construction sequence to provide a weathertight building envelope and are designed for future reglazing through the spaces between the columns.

The concrete frame is the primary space-forming element of construction within which finishes are expressed as secondary insertions. Deep concrete beams span the column free spaces and provide back-spans for cantilevers within the internal street. The openings below the beams are for ductwork integrated within the GRG profile of the finished rooflight.

The external appearance of the building on the north and east elevations is characterised by horizontal strata formed by the in-situ concrete slab edges and limestone walling. This solidity is balanced by permeability suggested in cantilevers and the columns at first floor level. Behind the columns the full-height glazed screen of transparent and solid fritted panels is designed to be easily reconfigured to suit the fenestration requirements demanded by changing uses within the building.

Achieving the perfect pour

Nick Mann of Kier Construction reveals the secret behind the high-quality finish

The good thing was the two-stage NEC partnering contract that the Sainsbury Laboratory was procured under, which meant that we were engaged with the design team from an early stage. Main subcontractors for the envelope — Whelan & Grant for the concrete and Feldhaus for the glazing — had the support of the client from the outset, giving Kier the confidence that the design’s ambitious demands could be met, and given the two-stage nature of the bidding process, subcontractors had early buy-in in the assessment of its buildability.

As far as the concrete went, a number of trial panels were cast and held on site to ensure the desired quality and aesthetic was achieved. Lafarge was brought in early to ensure the consistency of the batching: we ended up with Portland cement with no aggregate but fillers and pigments.

Whelan & Grant insisted that the whole project should be considered a fair-faced concrete job, so they used the basements of the building as a test-bed for the concrete that would be made in the exposed parts of the building above. The concrete was really a major aspect of the design. For the shuttering we had to consider pour layouts, board points and bolt hole position, all the shuttering was double-boarded and much of it wasn’t used more than once or twice. Not even rebar was allowed to sit on the concrete in case it stained it.

Quality control was paramount. There was our own dedicated QA engineer, a Clerk of Works on site all the time as well as the sub-contractor’s QA. Everyone would have assessed the shuttering prior to the pour so the quality of the formwork was exceptional, in the whole project we only needed to take down one column.

The double-boarded formwork in place

Concrete quality is evident close up

Engineer AKT’s structural load model

The sense of quality passes through into the highly serviced lab spaces, 1.2m deep downstand beams are covered with curved GRG panels that reflect diffuse light down, and act as an acoustic damper