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ISG's lofty Cardiff speed build proves something to look up to

One of the Welsh capital’s tallest new buildings is rocketing up to provide luxury student accommodation

Project: Zenith Student Accommodation Cardiff
Client: Fusion Students
Contract value: £40m
Contract type: JCT Design and build
Region: Wales
Main contractor: ISG
Concrete frame subcontractor: Midwest Formwork
Piling subcontractor: Green Piling
Start date: April 2017
Completion date: Summer 2019

We have come a long way since the average university undergraduate might have been ploughing out a final year dissertation in a damp bedroom of a squalid shared house.

Fast-forward a few decades and students can expect to be housed in single, centrally heated en-suite bedrooms with decent wifi.

But while the quality of student digs has rocketed for most students, some developers are going even further by offering accommodation built to a specification so high that it would make urban professionals jealous.

One of these developers is Fusion Students, which offers luxurious accommodation in university cities across the country. Its latest development is in Cardiff, where main contractor ISG is working on the delivery of a 675-bedroom block based around a 26-storey tower on a tight city centre site.

Rapid riser

Its slipformed core already makes it one of the tallest buildings in Cardiff, remarkable given it took the ISG-led team less than 20 weeks to bring it up to full height.

Senior project manager Ian Packer is overseeing the scheme, which will be ready for its first intake of students and post-graduates in September 2019.

“This is a large project on a small triangular piece of land in the middle of a city centre,” he says. “We have a railway running down one side of the site, the feeder canal to Bute East Dock on another and a busy city road to the front. Most of that footprint is being built upon, there is very little space,” he says.

The scheme encompasses three separate blocks: the 26-storey North Tower, and the smaller nine-storey Central and eight-storey South blocks.

Under the project’s JCT design-and-build contract, the design team looked at the various options open to them that would bring it in on time and budget.

Concrete over steel

“We looked at steel and traditional concrete solutions with the lower blocks being constructed to a podium and then a steel-framed system jumping from there – but for us the combination of a slipformed core for the North Tower and a precast concrete frame for the lower blocks made more sense,” Mr Packer explains.

Planning considerations and the difficult site meant that the only really effective way to design the scheme was to have the tower at the back of the site with the Central and South Blocks toward the roadside.

isg cardiff dsc0317

isg cardiff dsc0317

The contractor is delivering a 675-bedroom block based around a 26-storey tower

For Mr Parker, it was important to get the outline design set as soon as possible so that the team could draw on the expertise of the supply chain to help set the final construction method.

The scale and weight of the structure on the site’s mixed ground necessitated a robust piling plan for the project. Subcontractor Green Piling installed 285 CFA piles of 600 mm diameter to depths of 26 m through the layers of made ground and silts before hitting the mudstone bedrock.

Canal protection

Alongside the feeder canal, a 95 m sheet pile wall has gone in to a depth of 11 m. With the location of the railway embankment directly alongside the site, Network Rail has been monitoring all piling work for vibration, but there was never the need to work inside track possessions.

The Central and South blocks both feature an eight-storey precast concrete frame, with the Central Block heightened a further storey by a light steel-framed extension. The North Tower itself will be stepped back at its midpoint.

The precast concrete frame cores in the South and Central blocks feature a twin-wall, double-sided precast system that enables the team to offload the precast panels and hoist them directly into position. The cavity between the twin walls is then pumped full of concrete to provide one full homogenous core wall.

isg cardiff dsc0493

isg cardiff dsc0493

Concrete is placed to create one of the floor slabs on the Central Block. The team is using a full depth cast in-situ slab to tie the precast concrete frame together

Floor slabs across the project are 225 mm thick in general. But with the ground floor being double height a 500 mm-thick concrete podium slab has been installed using standard table forms in the South and Central blocks. For the North Block, the team used a different, smaller panelled formwork system around it.

“We went with the smaller panels because they are easier to handle,” Mr Packer says. “They can be stripped and placed by a team of two. From a health and safety point of view it makes sense, but also it frees up valuable crane time and on this project, that is important,” he says.

The ground-floor 4 m slab-to-soffit height decreases after the first floor, with 525 precast 600 x 300 mm and 450 x 290 mm columns. There are some circular-sectioned columns too, all of which are fixed by Middlesex-based framework contractor Midwest Formwork, which also carried out the complicated slipforming for the main 26-storey core in the North Tower (see box).

The final structure, with its façade clad in a variety of styles including facing brickwork, terracotta rainscreen cladding, aluminium panels and curtain walling, is well on schedule to meet its handover date.

By summer 2019, Cardiff’s more discerning students will have accommodation everyone can look up to.

Slipform speeds away

There is no doubt that the main focus of the scheme is the 26-storey North Tower. The ISG team has used slipforming techniques to get the sky-scraping core up to its full height in record time.

“It has been a great system,” Mr Packer says, adding “the team has been gaining confidence as the core has risen and it has taken us 16 weeks to complete. We are very pleased with how it has worked out.”

The team has been averaging a climb rate of 1.7 m per day and is using a Hanson Concrete-supplied C50 concrete design mix with admixtures to both accelerate its rate of hardening and maintain workability. The pouring team starts at 8am and works constantly throughout the day before last delivery at 4pm. The next few hours are used up making sure everything is ready for the next morning’s start time. It means there are no cold joints – just a day joint – and the entire process is cyclical and efficient.

“The rate of climb is dictated by three influencers – the weather, available working time and the concrete itself. We’ve done well on all three, which is why the core has gone up so quickly,” Mr Packer says. “That 1.7 m per day climb rate equates to the height of a door, every day for 16 weeks.”

The slipform itself is set over three levels with the top deck being the ‘receiving’ deck where the concrete is skipped onto the structure, the middle deck is the main ‘working deck’ and then there is a trailing deck where the concrete faces are finished.

The concrete is poured in 300 mm layers – think of an ice cream man filling a cone – that effectively spiral up to final height. Each day the first three ‘layers’ are placed before there is a lull for around an hour. After that stage the first 300 mm cast has gone off sufficiently to enable the team to drive the formwork, lifting the shutter face itself by 50 mm before hydraulically driving the whole three-level slipform up the structure.

The core is constantly checked for alignment and plumb, but small deviations can be ironed out by the skill of the driving operator. A tape judiciously wrapped around the rebar marks the final level that the team is aiming for.

It seems slightly incongruous that, despite its engineering brilliance, the final height of this tower that will soar over Cardiff city centre affording amazing views across Cardiff Bay, the Bristol Channel and the coast of England beyond, should be marked by a piece of tape.

It doesn’t matter, though. Thanks to the slipform, the speed of delivery of this project is already far outstripping the more ponderous approaches of surrounding sites. This is a project that will grace the sky of Cardiff for many years and it is good to know that there are honest, solid construction techniques behind it.

Readers' comments (1)

  • The precast twin walls which are mentioned in the article are seeing more and more uptake in the UK. These particular ones are produced by Kerkstoel 2000+ at their highly automated factory in Belgium.

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