Feature

Second Life

There is a growing awareness that our built environment could and should be valued as a resource for future development. This influences not only how we care for and refurbish existing structures but also how we design new buildings to facilitate adaptability and therefore longer lives.

A recent report by the Ellen MacArthur Foundation, Completing the Picture: How the Circular Economy Tackles Climate Change, identifies keeping products and materials in use as one of the three principles of circular thinking, making a clear link to a zero-carbon economy.

For buildings and structures, this can be achieved through demountability and reuse elsewhere, or reuse of astructure in place – the latter being arguably a less energy-intensive activity. Both require some long-term thinking on the part of designers in the early decision stages.

Reuse of existing structures

Demountability for reassembly is largely reliant upon the junctions and jointing of the original structure being designed to facilitate disassembly, so that the forms retain their value without downcycling. It is also predicated on materials having durability beyond the life of the original building. This concept is most commonly associated with temporary buildings, but there is growing interest for more permanent structures. The precast concrete top tiers of the London 2012 Olympic stadium were designed to be fully demountable for reuse elsewhere though, for now, they remain in their original places. By contrast, there are many examples of existing concrete structures that have been appropriated or adapted for new uses, saving cost, carbon and materials in the process. Centrepoint (CQ 261) and Camden’s town hall annexe (The Standard Hotel, featured in this issue, see 'Inspiration') were converted from offices to flats and a hotel respectively; while at Elizabeth II Court in Winchester (CQ 255) the thermal mass of previously covered concrete ceilings was key to a low-energy remodelling.

When planning to reuse an existing concrete frame, it is important to conduct a thorough survey, both to review the condition of the structure and also to understand the structural principles employed – such as which elements are integrated and loadbearing. Low floor-to-ceiling heights and close column layouts can pose challenges for spatial and servicing designs, requiring innovative solutions. The challenges of contemporary conversions provide useful lessons for embedding longevity in the buildings we are designing today.

Life After Life: The Many Uses of Recycled Concrete

When a concrete structure finally reaches the end of its life, it provides a different resource for construction in the form of crushed concrete aggregate (CCA). Formerly known as recycled concrete aggregate, this material can be used in various ways in construction, including being recycled back into fresh concrete. British Standard BS 8500 permits CCA to comprise up to 20% of the coarse aggregate in most designated concrete, without declaration – in other words, unless the concrete is explicitly required to exclude it.

According to the standard, it is possible to use higher percentages. In general concrete designated GEN 0 to GEN 3 – that is, non-reinforced concrete used for footings and floor slabs in domestic applications – up to 100% is allowable. Exposure class may be a limiting factor – in other applications, CCA is permitted provided it can be demonstrated to be suitable for the exposure class conditions. In practical terms, this would mean testing to meet quality protocols.

If concrete is not separated from other recycled construction materials, such as brickwork, the result is a more general recycled aggregate (RA). This has more limited uses within concrete itself, typically limited to mass concrete, but it plays a useful function in new construction by reducing the use of primary resources for hardcore, fill or landscaping and brown roofs.

Crushing concrete for reuse as aggregate substantially increases its surface area, allowing carbon dioxide to be more readily absorbed – a process called carbonation. Exposure to rain has been shown to significantly increase the rate of carbonation during this stage. Carbonation occurs during demolition and while the aggregate is stored on site, but continues during the concrete’s secondary life if used in groundworks, hardcore and landscaping. The amount of CO2 absorbed can be significant, reducing the initial cradle-to-gate embodied carbon of concrete by around a third.

ABOVE At Foster + Partners’ Bloomberg London, winner of 2018’s Stirling Prize and one of the world’s highest-scoring BREEAM rated office buildings, the concrete in the piles contained 100% recycled granite aggregate. This is classified as a secondary aggregate and is a by-product of the china clay industry in Devon and Cornwall.

Photos Alan Williams Photography, Make; Philippe Ruault, Bloomberg & Paul Riddle

Designing for a long life

Design for long life, loose fit and low energy is not a new concept – it is widely attributed to 1970s RIBA president Alex Gordon – but this mantra is even more relevant today, given the potential carbon savings associated with reusing existing structures. Flexible space planning, careful location of cores and generous floor-to-ceiling heights are all important considerations to allow for potential reuse with minimal adaptation. So too is the choice of structural frame. Long spans and regular arrangements are beneficial, as are design loads to suit a range of uses. At Glenn Howells’ Selly Oak student housing development, for example, the precast-concrete floors provide a clear span between separating, structural walls, offering future flexibility of room layouts. A long-life loose-fit approach was also adopted by Lifschutz Davidson Sandilands for the Victory Plaza apartment towers (see below).

Less well understood in the 1970s was the need to futureproof new developments against the effects of climate change, such as a greater risk of overheating, flooding and storm damage. The thermal mass of a concrete structure holds the potential for inbuilt passive cooling, for use now or in the future. With the transition to electric vehicles, there is a real and very appealing possibility that cities could become quieter and less polluted, allowing for greater use of natural ventilation and passively cooled buildings.

Another important factor is the durability and robustness of the structure itself. The predicted life of contemporary reinforced or prestressed concrete structural elements in an internal environment is 100 years, with no need to change the mix design and cover recommendations compared to the more typically requested 50 years (see British Standard BS 8500, Annex A, Table A.4 and A.5). Concrete is also fire and rot resistant, which means it needs little or no maintenance over a structure’s lifespan.

In other exposure class situations, such as external parts of a frame, durability can be provided by increasing or changing the cementitious content and/or concrete cover to the reinforcing steel, or through the use of admixtures. Increased cover is a standard prerequisite for concrete design and workmanship, and is commonly provided using proprietary spacers. Covermeters can be used to verify the cover achieved. In the event of any shortfall in cover, sealants are available for local application, reducing carbonation of the concrete and providing additional durability.

Victory Plaza, London, 2019

        

The Olympic Park is a good place to go in search of lessons on the reuse of buildings. After all, its original purpose as a grand stage of sporting spectacle lasted little more than a few weeks. Lifschutz Davidson Sandilands was well placed to absorb, and indeed guide, best practice on the 2012 programme, working on everything from athletes’ housing to drug-testing facilities before playing a major role in transforming the development into a long-term community. “The village was a good example of loose-fit architecture,” says LDS director Alex Lifschutz. “The living spaces were wide enough to be used as two bedrooms in Games mode, then swapped into a living room with a kitchen post-Games.”

LDS has now completed the first new homes on the East Village site since the Games and, appropriately, the two towers at Victory Plaza are also paragons of loose-fit, long-life design, where space is maximised so it can be adapted to future uses. This was partly driven by the fact that the towers – of 30 and 26 storeys respectively – were being built for an untested market. When Qatari Diar and Delancey Estates took over the Olympic Village in 2011, they planned to develop more than 2,000 homes for the private rental sector (PRS). “That was the novelty,” says Lifschutz. “It wasn’t just that things might change in the future. It was that nobody knew, for this scale of PRS, what the market was now. To give an example of how it foxed everybody, in the athletes’ village we built duplexes around the perimeters of the blocks, and the assumption was that these would be let to families after the Games. But they weren’t – I think well over 70% were let to sharers.”

It therefore seemed all the more essential to build flexibility into the new towers at Victory Plaza. “At the design stage, the mix of flat sizes was constantly changing,” says LDS director Douglas Inglis. Their response was to open up the floorplan as much as possible. Working with a building footprint of 26m2, LDS proposed a 7.5m-deep central core, built with twinwall structural panels. This would include four lifts and a stair, a corridor on two sides and a further core wall to provide stability. The floorplates would then span 7.5m on Omnia slabs to precast-concrete perimeter columns, which measured 600mm x 600mm on the lower floors, before narrowing further up. There were no structural party walls or blade columns, and all of the services risers hugged the core wall, which meant that any floor could house a moving number of flats – or theoretically be a single room.

It was still necessary to future-proof the design by making as much space on each floor as possible. “The first rule of loose fit is make it more generous volumetrically than it absolutely needs to be,” says Lifschutz. “Any use other than a very low-density living space requires more air.” With floor-to-floor heights of 3.3m, LDS was able to agree relatively high 2.6m ceilings with the PRS operator, Get Living. More innovatively, it managed to negotiate extra living space with Tower Hamlets council by designing out the balconies that would typically feature on an apartment tower of this type.

“There’s a huge westerly prevailing wind – it’s very low-lying and there’s basically nothing between the City and this part of the East Village,” points out Inglis. “So instead of balconies, we put 2.6m-high sliding windows in every living room, and the corner flats had them on both sides. It means you can open up the space without feeling like you’ll be blown away.” On a 70m2 two-bed flat this approach added another 7m2, but the deal with the council was that this had to all be in the living space – they couldn’t simply add an extra bedroom. “They were quite far-sighted,” says Lifschutz. “They said, ‘we get the point, as long as it’s creating better living space’.

And what’s happened since is that many of the two-bed flats are shared – again something we would not have assumed – and that extra living space has become really important.”

While Get Living has no grand ambitions to transform the use of its new towers any time soon, Lifschutz reckons its innovative “jump factory” method of precast manufacture and site assembly (see CQ 269) has also helped to make it more adaptable. ‘If you wanted a co-working floor, you could do that,” he says, pointing out that if the suspended ceilings were removed, an occupier could have a floor-to-ceiling height of 2.75m, which is 150mm better than the BCO standard. “The Omnia deck soffits are perfect, the precast columns are perfect. If you cleared it out and created a shell it would look pretty cool.”

Top from left to right Communal spaces are an important part of the PRS offering; Corner flats have 2.6m-high sliding doors on two sides, which were felt to be more appropriate than balconies in such an exposed location; The striking facade of the towers – instead of balconies, substantially more living space was provided.

Right Floor plans showing how the structural solution created 7.5m-deep spans between the core and perimeter, with no structural party walls.
This means that the space is completely adaptable, and can be used for up to eight apartments, depending on what the

Photos Alan Williams Photography, Make; Philippe Ruault, Bloomberg & Paul Riddle

Health checks

Although concrete is typically very robust, occasions do arise where it needs some maintenance or repair. Techniques are well understood and can often extend the usable life of a structure by many decades. New techniques have been developed to monitor the health of our building stock, including use of drones, digital surveys and artificial intelligence. Retrospectively applied microamp circuits can detect problems with steel reinforcement early enough to limit further damage and enable targeted interventions.

In the future, adaptation, renovation and repair should be facilitated by more easily available data, the increased use of BIM and initiatives such as material passports and e-tagging of building elements. Certainly more value should be placed on communicating product information and service manuals. But fundamentally it is about valuing building structures themselves, making the most of them now and for the future.

Photos Alan Williams Photography, Make; Philippe Ruault, Bloomberg & Paul Riddle