Building on the Edge: Mace’s latest London Bridge project

Mace is sticking to strict sustainability criteria on its latest project in London Bridge

Mace has its stamp all over a small patch of SE1, east of London Bridge and south of the River Thames. The firm’s hoardings still line St Thomas Street more than a decade after the tier one contractor completed the Shard. The firm’s other jobs in the area include the luxury apartments rounding off Shard Quarter and the 39-storey student accommodation tower opposite London Bridge Station.

Eagle-eyed commuters might have noticed another Mace-branded concrete core shooting up in recent months next door to the student tower. Its hoardings look a bit different though. “Hi! I’m Edge London Bridge, nice to meet you,” they say. “I am 275,000 sq ft of workspace shaped for tomorrow.”

On a sunny summer’s day, Construction News took a look at the early-stage concrete works. Architecture firm Pilbrow & Partners’ designs for the 27-storey tower certainly have several hallmarks of the aspirational post-Covid workplace. They include a flexible exoskeleton
that allows tenants to divide the space how they see fit, and an exposed cross-laminated timber feature staircase leads to a community auditorium. There’s also landscaping that filters through from the street, blurring the boundaries between inside and outside.

But the most important aspect – at least where client Edge Technologies is concerned – is the building’s climate impact. The Dutch developer has set itself tough environmental standards for its first UK project, which it claims will be London’s most sustainable office tower. “Even before we got to the usual conversations about programme timetables and cost, the client was asking about the carbon impact of each decision we were making,” says Mace project director Stuart McDonald. “There were some things it wouldn’t negotiate on.”

One of those non-negotiables was a cap on embodied carbon, which was sealed into the construction contract. Mace was already on the same page. The contractor has its own strict carbon targets, including a pledge to help its clients save 10 million tonnes of carbon by 2026, the same year it plans to complete Edge London Bridge.

The building is on track to meet an alphabet soup of environmental credentials. It is aiming for BREEAM Outstanding, NABERS 5*, a Paris Proof energy use target of 55 kWh/m², 100 per cent FSC certification for timber used on site, 98 per cent minimum diversion from landfill and a 35 per cent embodied carbon reduction in comparison with its RIBA stage three design. All the data needed to prove the building meets its targets is stored in Mace’s proprietary sustainability software, Optimise, and presented to the client quarterly.

Underground, overground

The 25,550 square metre site is conveniently located next to London Bridge Station – the country’s fourth busiest – and a vibrant high street adjacent to the gastronomical paradise of Borough Market. Becket House, a seven-storey 1980s concrete block used as a government immigration reporting centre, was demolished before Morrisroe commenced the groundworks in October 2023.

Subcontractor Cementation Skanska cored through 150 existing piles to put in 335 new ones. “Even now, we’re still digging out the piles from the old building, even though we’ve been here for well over a year,” says McDonald. Some of the existing piles went deeper than the four-level basement Mace is currently excavating, despite the old building being four times shorter than Edge’s 108 metre height. Cementation used a combination of continuous flight auger method, rotary piling method and the deeper bearing piles were installed using polymer as a support fluid. Mace also discovered pavement defects around the site perimeter, which it backfilled with Cemfree concrete, a mix free from carbon-intensive Portland cement.

A top-down construction approach to the basement limited disruption from underground unknowns – necessary as the programme is already on the long side at 169 weeks, McDonald says. It took six pours for basement subcontractor Morrisroe to cast the ground floor slab before it began to excavate the 16.6 metre-deep basement. The four basement floors are supported by 40 plunge columns, the biggest of which is 21 metres long and weighs 27 tonnes, allowing Morrisroe to cave below. The ground floor slab capping the basement also supports a jumpform core that is rising up at the east of the site, while a steel mega-truss goes up alongside.

Concrete jungle

Mace tackled the concrete carbon-reduction challenge in a number of ways, working with concrete frame subcontractor Morrisroe to develop bespoke mixes and rationalising the design of concrete elements so less was needed overall. When CN visits, Mace is still excavating the basement and has only cast eight of the 27 storeys. Yet the team has already reduced carbon by 21 per cent compared with RIBA stage three designs – cruising towards its 35 per cent target.

This blended approach to reducing concrete’s carbon impact is best demonstrated within the building’s piles. The concrete mix replaces 70 per cent of carbon-intensive Portland cement with ground granulated blast furnace slag (GGBS), a byproduct of steelmaking. GGBS mixes perform like regular concrete but take much longer to harden to full strength. “You can have high GGBS in piles because you’re not after rapid strengthening,” says Mace engineering lead Tom Webster. “You pour them and then you can wait almost a year before you’re going to put anywhere near the amount of designed stress on them.”

The piles have been designed to be smaller, cutting down not only on concrete, but also on rebar and polycarbonate. Fibreoptic cables sit within some of the piles, loosely tied to the rebar, going down all the way from the ground floor to the bottom of the pile about 55 metres below. The pile monitoring system should stay in the building for 10 years, to help assess loads over time. Mace hopes the data collected will help it rationalise pile design on future projects, and work out what temporary works are needed to support the next stage of the current build. “We want the building to work really hard for us as we’re constructing it as well,” says McDonald.

The ground floor slab is more cementitious, at 40 per cent GGBS. “When you pour slabs, hardening time is critical, because you want to be able to load them again as soon as possible so you can build on top,” says Webster.

A 40 per cent GGBS concrete is also being used in the jumpform core, a method chosen over slipform in part because it allows longer for the concrete to harden before it takes weight. Post-tensioned slabs will make up about 80 per cent of the floorplate of the upper levels, further minimising materials. Each upper floor plate will be 1,080 metres square: 805 of post-tensioned concrete, 107 of reinforced concrete and 168 of cross-laminated timber and steelwork.

Material world

Although concrete inevitably makes up a big proportion of a project’s environmental footprint, Mace has gone further in its quest for carbon savings. One success story was the rebar. Mace had already decided on a rebar supplier when Morrisroe came forward with an alternative that uses half the embodied carbon. “It had a huge impact,” says Webster. “We’ve got roughly three and a half thousand tonnes of rebar on the project. That’s a big figure.”

Material reuse is also playing a significant role, not only because the project is targeting a minimum diversion from landfill of 98 per cent. Mace worked with structural steel firm Severfield NI to procure reusable temporary steelwork, which will either be taken to other projects or remain in place, encased in concrete or exposed.

During the basement excavation, a skip descends from a gantry supported by plunge columns and the capping beam. Once the London clay is dug out, the gantry system transports it upwards onto the back of a lorry, which will take it away to be remodelled into signs and nameplates installed outside and in the reception area.

It’s an example of the attention to detail that makes Edge London Bridge a worthy neighbour of the Shard and its other Mace-branded bedfellows. “We look across at all these feature projects north of the river,” says McDonald. “It won’t be long until north of the river is looking at all the feature projects south of the river.”