All Day and All of the Night

Nicholas Hare’s UCL Student Centre shows how exposed concrete’s thermal mass can be harnessed even in a hardworking 24-hour environment, writes Tony Whitehead

Using a building’s thermal mass to regulate its internal temperature is now a well-established technique, and one for which concrete is ideally suited. Exposed concrete surfaces absorb heat from sunshine, occupants and computers during the day, which is usefully given up later in the day, helping minimise heating requirements during the winter months. In summer, the heat is purged overnight by opening vents so that by morning the concrete is ready to absorb more heat and reduce the need for mechanical air conditioning.

This works because most non-residential premises are occupied only during the day. But what if they are intensely used 24 hours a day, seven days a week? Can thermal mass still play a part in reducing energy needs?

The new seven-storey Student Centre for University College, London (UCL), is proof that it can – albeit with a little help from two bore-holes sunk more than 100m below the building. “The Student Centre is continually used,” explains David Tompson, project leader with architect Nicholas Hare. “You have lots of students using loads of IT all through the night – so in summer there is little opportunity for it to lose any heat naturally.

“Our solution has been to supercharge the thermal mass effect by circulating water through 10km of pipes cast into the concrete slabs – water that has been cooled by sending it deep beneath the building into the chalk aquifer below, where the temperature is around 12°C.”

If this seems like a lot of trouble to go to, Tompson is quick to explain that concrete was chosen for more than its thermal qualities alone. “We took a fabric-first approach and decided on concrete early on,” he says. “This is a highly populated building, designed to last, and we selected materials we thought could last around 200 years. There is a lot of oak, and exposed concrete walls and soffits which are robust enough to withstand heavy traffic over time.”

Tompson points out that the fairly regular concrete frame and flat slab construction means the Student Centre will be straightforward to reconfigure should the building’s use change, adding that the exposed soffits help to maximise floor-to-ceiling heights, which “lets us use tall windows to allow natural light deep into the building”. Concrete’s fire-resistant and acoustic properties are also important on this tight city-centre site neighbouring a busy theatre.

Finally, says Tompson, the concrete solved a local geological issue by providing weight as well as mass: “We needed a fairly heavy building to resist hydrostatic pressure from below.”

UCL’s new Student Centre, which cost £38.5m to construct, is situated on a former bomb site that had never been fully redeveloped. It provides 5,750m2 of space, mainly comfortable, IT-friendly environments for up to 1,000 students to work – a little like a library without books. It also carves a new route through the campus, connecting Gordon Street to Gower Street via the new building.

Inside, concrete is very much the dominant material. Smooth pale concrete defines the soffits, columns and dramatic flights of stairs. Much less obvious is quite how many different mixes and types of concrete are on display (see box, below).

Most notably, while the foundations and floor slabs are made from in-situ concrete, virtually all the vertical elements, including columns, stair stringers and facade panels, are precast. This, says Tompson, offered practical and programming advantages: “It was a very tight site, so it helped to be able to crane columns and other elements into place. The in-situ slabs had a lot of cast-in services, including electrics and sprinklers.
“There were also the cooling pipes, which sit on a wire grid between the two layers of reinforcement, so setting that out before pouring the slab was a major undertaking. It made sense for the contractor to be able to work on the slabs without the complication of the formwork and propping that would have been needed for the columns. The precast elements kept it clean and simple.”

The Campus Has Two Faces

The pale concrete elements framing the doors and windows of the front facade are all precast, though they are supported off the foundations and do not form part of the structural frame. The tall brickwork piers are also precast – the bricks having been sawn in half and set into 12m-high pieces. “If the brickwork had been laid by hand we would have needed long-term scaffolding on Gordon Street,” says David Tompson at Nicholas Hare Architects. This way the brickwork was simply craned into place.”

For the rear facade the technique was completely different, with the windows formed from rows of large L-shaped precast elements which then had brickwork laid around them by hand. Like all the exterior precast, the colonnade of concrete pillars around the rear entrance has been acid-etched to expose the fines and provide texture. “It also makes it even harder to pick which of the four surfaces is the unformed, trowelled surface,” says David Moses at Cornish Concrete.

Finally, at the top of the rear facade is a row of vertical concrete fins, only 100mm wide and chamfered to 60mm at the front. These provide design interest and solar shading to the cafe at the rear of the top storey.

“Supporting and setting the fins separately on site would have been expensive and complicated so we made them in units of three fins complete with a coping and parapet all in one.”

Creating elements so slim required a special approach to reinforcement, Moses adds: “The units were cast with the fins horizontal. They were too slim to use spacer blocks or clips for the reinforcement as these might have been visible on the face of the unit. Instead we suspended the reinforcement from the top of mould and cut away the ties once the concrete had set.”

To create a stable base for the building, a secant piled concrete wall was sunk around the 30m x 30m site perimeter and supporting piles driven 120m below street level. The two boreholes for the water cooling system were also sunk, and the raft slab cast around them some 10m below the street to make space for two basement levels.

Once the slab had reached sufficient strength, precast columns were craned into place, mostly to a 7.2m grid, and connected using column shoes. The columns also feature well voids, holes into which rebar could be grouted at the top of each column. This was then tied into the reinforcement for the next slab up. This being a visual connection, the formwork for the slabs was set 100mm below the top of the columns to allow each column to “bite” neatly into the slab. Most of the columns have a 1,200mm by 300mm section, oriented with the smaller dimension facing the entrances to emphasise the new route through the campus.

Where the Student Centre abuts its neighbours – a Georgian terrace to the left and the brutalist Bloomsbury Theatre to the right – prefabricated insulated concrete sandwich panels were fixed to the adjacent buildings’ walls and supported off the slabs. “Each panel is one-storey (3.9m) high, between 2m and 2.8m wide, and 400mm thick, comprising 100mm of concrete abutting the neighbouring building, 100mm of insulation and a 200mm reinforced concrete structural layer which provided the support for the floor slabs at each side of the building,” says Tompson. “The structural layer is the one that occupants can see and has the same smooth finish and colour as the columns.”

The accommodation is ranged around a full-height atrium spanned by flights of stairs. The airiness of this atrium is vital to the feel of the building – but it did present some structural challenges. “Around the atrium you see a thin slab edge of just 300mm,” says Tompson, “but set back from that is a substantial upstand beam, hidden in the raised floor, making the slab 500mm thick.”

The staircases are also integral to the structure. “These are made from 200mm-thick precast concrete stringers or balustrades,” says Tompson. “Each stringer weighs about 10 tonnes and provides the structural span across the atrium.”

Despite their obvious strength, the stairs appear almost to float above the reception areas. It is a neat trick, and emblematic of the clever use of concrete throughout. Nicholas Hare has combined reassuring solidity with the lightest of touches.

The Illusion of Sameness

As part of the strategy to design the UCL Student Centre to BREEAM Outstanding level, the concrete mixes used include high proportions of cement replacement and recycled aggregate. Combined with the different types of concrete involved in the building and the need for them to match each other visually, this requirement led to some interesting mix choices.

All of the in-situ concrete, for example, features 50% ground granulated blast-furnace slag (GGBS) but the use of recycled aggregate was restricted to non-visual areas, mainly in the basement, as locally supplied recyclate could not easily achieve the fine finishes required.

Once the in-situ contractor, J Coffey, had completed a capping beam on top of the secant piled wall, precast supplier Cornish Concrete produced a number of samples to match its colour. Most of the precast concrete also features a 50% GGBS mix, but uses 100% recycled aggregate – Cornish Concrete’s regular supply being stent, a secondary granite aggregate that uses local china clay production waste.

“The only exceptions to this were the stair stringers,” says David Moses, Cornish Concrete’s product director. “Like the columns and sandwich panels, the stringers were cast flat on their side – so the upper surface, which forms the inner wall of each stringer, had to be trowelled smooth. This is a large and highly visible surface which we needed to get very flat. Power floating was not an option as working the fines that much would darken the concrete. Instead we used a non-GGBS mix which made trowelling it smooth easier. Because GGBS naturally results in pale concrete, we included some white cement in the stringer mix to ensure the colour matched.”

Yet another mix was required for the balustrades to the ground floor “grand staircase”, which exists to take account of the difference in level between the front and back entrances. “Unlike all the other stairs in the building, this one is made from in-situ concrete,” says Tompson. “However because its balustrades naturally have sloping top surfaces, these are made from self-compacting concrete to fill the forms more reliably.”

Architect Nicholas Hare Architects
Structural engineer Curtins
Main contractor Mace
In-situ concrete contractor J Coffey
Precast concrete provider Cornish Concrete

Photos Alan Williams Photography, Richard Chivers