Sustainability honours for concrete at UCL

With a varying combination of exposed precast and in-situ concrete, matching all the different mixes posed considerable aesthetic challenges.

“Concrete was key to our ‘fabric first’ environmental strategy,” explains David Tompson, associate and project architect with Nicholas Hare Architects. “Thermal mass is very important to the building, which has to operate 24/7, every day of the year.”

“But, as more than half of the concrete in the building is visual, our approach to specification was crucial.”

The Student Centre is part of UCL’s transformation of its Bloomsbury estate, and forms a focal point for student life, with 1,000 study places. Mace was appointed from Stage 4 design to act as the main contractor and designer.

The centre was built between listed buildings, connecting Gordon Street – the east elevation – with UCL’s recently opened Japanese garden to the west.

“It is a challenging site with adjacent buildings that were in use throughout construction,” says Simon Allen, project director for Mace. “Early in the design phase, we used 3D modelling to investigate construction techniques ideal for working within a constrained site and reducing disruption. We worked alongside UCL Estates and stakeholders to schedule activity so that it had minimal impact on the neighbourhood.”

Hybrid structural frame

The Student Centre is spread across eight floors, six above ground, and centred around an atrium, which is dominated by exposed concrete columns and soffits, while the stair stringer beams and balustrades span 10m across this space.

The structural frame is a hybrid. “An early design decision was to switch the columns from in situ to structural precast,” explains structural engineer Jeffrey Blaylock from Curtins. “3D modelling was used to plan the junctions between precast and in situ. The heads of the columns interface with the top of the slab to a depth of about 10mm.”

The concrete used splits into roughly equal thirds: the piled foundations, the precast elements and the floor slabs.

Internal exposed concrete was key to the project’s “fabric first” environmental strategy

Broadly, the precast structural elements are vertical, including circular and blade columns – some double-height in the atrium – plus there are twin wall panels, stairs and sandwich panels where the Student Centre meets neighbouring buildings to north and south. The “blade” columns in the atrium have been aligned east-west, “to provide a sense of direction through the building”, says Tompson. There are two in-situ stair cores, one in the north and one to the south.

To achieve the desired aesthetic outcome, the architect, engineer, Mace, plus concrete contractor J Coffey and precast specialist Cornish Concrete met to agree on a consistent approach.

“We discussed the materials to use, the formwork, trial panels, and our approach to ‘making good’,” explains Tompson. “The challenge was deciding the mix and finish to achieve consistency and colour match. In total, four different concrete mixes are visible in the central atrium space, although the untrained eye would find it hard to tell them apart.

“Coffey created two large sample panels and Cornish Concrete developed the same mix and brought samples to site. Full credit to Cornish and Coffey for the colour match.”

The upper three storeys of the Gordon Street elevation feature 12.5m high brick-faced columns, cast in Cornish Concrete’s factory

Cornish Concrete director David Moses explains the process: “I took a sample of the in-situ concrete away from site – prior to the main pour – and then created four precast patch samples in our factory. I returned to the site with these, and the designers picked the one they liked most. But it was a gloomy day. So we waited for a sunny day and then returned with the samples and the results were different. Light is everything with colour matching.”

The concrete mixes also had to consider the BREEAM Outstanding requirements, which meant replacing 50% of the cement with GGBS (ground granulated blast-furnace slag). The exception was the 10-tonne stringer beams. “It was such a vast exposed area that we didn’t think we could achieve a good enough finish,” says Moses.

Cornish Concrete could source china clay waste locally in the Duchy and achieve 100% recycled aggregate in the structural precast. “The 50% GGBS content coupled with 100% secondary content aggregates meant each cubic metre of precast was 90% secondary sourced material,” says Moses.

The finishes to the exposed concrete are “plain”, with some areas trowelled. Cornish Concrete used a mixture of formwork: steel was used as the primary form, with edges in plywood held in place with magnetic falsework. The joints between the plywood sheets were filled and sanded down, before being coated in polyurethane so that the joint lines were no longer visible. “The precast columns were poured flat as the finish isn’t as good when poured vertically,” explains Moses.

The design team decided on a “light touch” approach to making good, which mostly involved just a light rub down with sandpaper, says Tompson.

Lifting “eyes” on the precast stairs and stringer beams were filled with preformed, factory-made concrete discs, then grouted in. “The columns were propped by J Coffey using ‘soft collars’, thus negating the need for cast-in propping sockets,” adds Moses.

An aesthetic issue was the appearance of an orange stain on the soffits. “This was caused by rain, with rust dripping off the rebar and sitting on the shuttering,” explains Tompson. Coffey used a light sander to remove that surface stain.

Careful attention to the mixes and finishes of both in-situ and precast concrete create a consistent visual impact

The environmental considerations meant the services design had to be coordinated with the structural design from an early stage, says Blaylock. “Most of the services are exposed but there are cast-in cooling pipes which circulate water and go down 120m via boreholes into the aquifer,” he explains. “These sit within the 300mm thick floor slabs and required their own mesh to sit on.”

The stairs include concealed lighting in handrails, so conduits were cast in to the balustrades at Cornish Concrete’s factory before transporting to site.

Concrete also provides an important acoustic role on the north and south ends of the Student Centre, which are effectively party walls though detached from the neighbouring structures. “The building interfaces with the Bloomsbury Theatre and Georgian townhouses on the other side, so we fitted precast sandwich panels on both sides,” says Blaylock. “This also provided advantages of no formwork and reduced site labour.”

Architectural precast cladding sections, along with brick-faced panels, have been used on both the front Gordon Street elevation and at the rear.

The front facade comprises white acid-etched precast columns and spandrel panels, along with 12.5m, three-storey precast brick-faced columns. This required creation of a bespoke mould in Cornish Concrete’s factory, with the bricks set out according to the desired mortar beds and perpends.

For the colonnaded top floor of the rear elevation, four precast fins, one cill and one head form a single piece. Cornish Concrete created a full-scale trial at its factory

Handmade Petersen Kolumba bricks were picked to fit in with the conservation area requirements. “The bricks at the edges of the building were laid in situ, and Swift Brickwork flush-pointed all brickwork to achieve consistency across the facade,” adds Tompson.

The acid-etch finishing took place in the factory, using what Moses describes as “a giant pressure washer”. The level of etching – to a depth of 0.5mm in the case of the Student Centre – is regulated by the ratio of acid to water in the wash.

The Japanese Garden elevation also features acid-etched precast columns and spandrel panels, with a colonnade effect on the top floor, which also provides solar shading. “Given the complexities of the temporary works required to hold the fins in place, we carried out a full-scale trial panel,” explains Moses. “The result was four fins made with one cill and one head in one piece. This meant a quicker, simpler and safer installation on site.”

The centre opened in February, and UCL hopes the sustainability features will keep its operating costs and carbon footprint low. As well as the borehole cooling system, the building includes 400 sq m of photovoltaics panels, windows which open automatically and a green roof.

“We worked with UCL Estates to ensure the building would work at its most efficient with minimal impact on the environment,” says Allen. “This includes a projected 35% reduction in building carbon emissions compared to Building Regulations requirements and 50% reduction in water use compared to equivalent buildings. Use of durable materials should also minimise maintenance costs.”