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Concrete and CLT combo cuts CO2 and opens space

How a demanding university scheme prompted Ramboll to discover the major benefits of combining concrete with cross-laminated timer.

Ramboll knew it was in for a challenge when it was invited to provide engineering and design expertise for Building 57 at the University of East Anglia.

UEA is well known for its low-carbon ethos and is already home to some of the country’s most pioneering low-carbon buildings.

The brief for this latest addition to the campus demanded outstanding carbon results.

The project also required open-plan spaces that were functional and flexible to effectively accommodate the changing requirements of the university.

In addition, it had to be developed in a tight timeframe with as minimal disruption to university life as possible.

Even as a cross-laminated timber firm, Ramboll knew these particular needs were not something that could be met through timber alone.

Concrete and CLT partnership proved

The UEA is recognised for delivering low-carbon buildings that push the boundaries of construction methods and Building 57 was no different.

Ramboll undertook an in-depth study looking at the construction material hybrids that would deliver optimum carbon results.

“The study rapidly made it clear that concrete offered by far the best ally for the CLT frame”

Using data provided by the building services engineer, Ramboll modelled the embodied carbon and operational carbon implications of various methods of construction to derive the optimal material combination for the four above-ground storeys.

Concrete is well known for its strength, durability and its versatility, as well as its energy efficiency and excellent thermal mass. The study rapidly made it clear that concrete offered by far the best ally for the CLT frame.

Unique hybrid solution

By developing a never-before-used combination of precast concrete floor planks and CLT walls, we exploited the most advantageous qualities of both materials.

This particular combination of building construction types had yet to be attempted, mainly because uniting reinforced concrete, precast concrete, steel and CLT into one harmonious solution is highly challenging.

To connect these materials together, often at one interface, requires inventive and bespoke connection detailing for both the permanent and the construction phases.

To deliver this, the team developed a connection using a Dywidag bar, a plate tightened to specific requirements and a shelf angle that connects the CLT to the precast concrete planks.

Fixings cast into the concrete basement secured the connection between the concrete basement and the CLT walls, while allowing both ease of construction and building tolerance.

Both these developments were crucial in leveraging concrete’s advantages alongside those of a CLT framework.

In terms of carbon performance, although carbon is sequestered in the timber, the role of the concrete floor planks was crucial. Their thermal mass acts to warm or cool the air passing through their hollow cores, reducing the emissions from heating and cooling.

Major carbon savings

By adopting concrete into the timber CLT design, this approach will save about 2,500 more tonnes of CO2 than a standard CLT frame over the building’s 50-year life.

The CLT offers a very airtight façade, helping create a building with an air leakage rate nearing Passivhaus standards, which is far more airtight than Building Regulations require.

All U-values also met the high levels specified, producing an extremely energy-efficient construction.

Building 57 is already on target to be twice as efficient as the Zicer Hub, UEA’s next most efficient building and one that is already recognised for its sustainable credentials.

“Building 57 has become the first BREEAM-assessed building on its campus to achieve an Excellent rating”

In looking to allow for the large open spaces desired by UEA, with an absence of supporting structures as far as possible, complex 3D modelling was undertaken to demonstrate the overall stability of Building 57.

The precast concrete planks enabled an unobstructed internal space of 13 m, significantly longer than what can typically be achieved with CLT alone.

In addition, these areas also crucially provide future flexibility for UEA, as walls have the potential to be relocated and shifted in according with the university’s changing needs.

Using this combination of prefabricated systems provided a modern method of construction that also optimised safety, quality and speed of assembly.

Constraints of a restricted site area and a busy campus - open around the clock - dictated a fast- track build programme.

With CLT and precast hollowcore planks there was minimal onsite material production and the entire project took only 27 months from start to finish.

Method for the future

The use of concrete and CLT is an excellent hybrid, with great future potential for schemes that require large, open internal spaces - such as office developments or public buildings.

Building 57 has continued the UEA’s legacy of pioneering low-carbon buildings, becoming the first BREEAM-assessed building on its campus to achieve an Excellent rating.

Likely now holding one of the smallest carbon footprints of any commercial or educational UK building, the long-known opportunities that concrete offers have been exploited in a unique and entirely fresh form.

Oliver Neve is an associate at Ramboll

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