Building on the edge: Mace’s latest project in London Bridge

In its latest project on London Bridge, Mace adheres to strict environmental criteria

Mace’s Seal is located on a small stretch of SE1, east of London Bridge and south of the River Thames. The firm’s billboards still stand along St Thomas Street more than a decade after the tier one contractor completed the Shard. The firm’s other works in the area include luxury apartments completing the Shard block and a 39-storey student tower opposite London Bridge station.

Attentive commuters may have spotted another Mace-branded concrete rod that exploded in recent months next to the student tower. Although his shields look a bit different. “Greetings! My name is Edge London Bridge, nice to meet you,” they say. “I have 275,000 square feet of workspace ready for tomorrow.”

On a sunny summer day Construction news inspected the initial concrete works. Designed by architecture firm Pilbrow & Partners, the 27-story tower designs certainly have several hallmarks of post-Covid workplace aspirations. They include a flexible exoskeleton
allowing tenants to divide the space as they see fit, and an open cross-laminated timber staircase leads to the public auditorium. There is also landscaping that seeps in from the street, blurring the lines between inside and outside.

But the most important aspect – at least as far as Edge Technologies customers are concerned – is the building’s climate impact. A Dutch developer has set 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 normal conversations about program schedule and cost, the client was asking about the carbon impact of every decision we made,” says Mace project manager Stewart McDonald. “There were some things he didn’t want to negotiate.”

One of the non-negotiables was the carbon cap that was sealed in the construction contract. Mace was already on the same page. The contractor has its own tough carbon targets, including a commitment 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 its way to complying with the alphabet of environmental credentials. It aims to achieve BREEAM Outstanding, NABERS 5*, a Paris Standard energy consumption target of 55 kWh/m², 100 per cent FSC certification for wood used on site, 98 per cent minimum diversion from landfill and 35 percentage reduction in carbon emissions compared to the RIBA Phase 3 project. All the data needed to prove that the building is meeting its objectives is stored in Mace Optimise’s proprietary sustainability software and provided to the client on a quarterly basis.

Underground, above ground

The 25,550 sq m site is conveniently located next to London Bridge station – the fourth busiest in the country – and the bustling high street adjacent to foodie paradise Borough Market. Becket House, a seven-story concrete block from the 1980s that was used as a government immigration center, was demolished before Morrisroe broke ground in October 2023.

Subcontractor Cementation Skanska drove 150 existing piles to install 335 new ones. “Even now, we’re still digging piles out of the old building, even though we’ve been here for over a year,” McDonald says. Some of the existing piles went deeper than the four-level basement Mace is currently excavating, even though the old building is four times lower than Edge’s 108-meter height. Cementing used a combination of the continuous auger method, the rotary pile driving method, and the deeper support piles were installed using polymer as the support fluid. Mace also found defects in the sidewalk around the perimeter of the site, which he filled with Cemfree concrete, a mixture that does not contain carbon Portland cement.

The top-down approach to building the basement has limited destruction due to underground unknowns, a necessity since the program is now in its 169th week, Macdonald says. Subcontractor Morrisroe required six pours to cast the first floor slab before beginning excavation for the 16.6m deep foundation. The four basement floors are supported by 40 columns, the largest of which is 21 meters long and weighs 27 tons, allowing Morrisroe to drop below. The basement floor slab overhanging the basement also supports a jump-shaped core that rises to the east of the site, while a steel mega-truss rises nearby.

Concrete jungle

Mace addressed the challenge of reducing the carbon footprint of concrete in a number of ways, working with concrete subcontractor Morrisroe to develop custom mixes and rationalize the design of the concrete elements so that less was needed overall. When CN Mace is still excavating the basement and has poured only eight of the 27 floors. However, the team has already reduced carbon emissions by 21 per cent compared to RIBA Stage Three projects, achieving a target of 35 per cent.

This hybrid approach to reducing the carbon footprint of concrete is best demonstrated in building piles. The concrete mix replaces 70 percent of the high-carbon Portland cement with ground granulated blast furnace slag (GGBS), a byproduct of steel production. GGBS mixes work like regular concrete, but take much longer to set to full strength. “You can have high GGBS in piles because you’re not looking for rapid strengthening,” says Mace engineering manager Tom Webster. “You pour them in and then you can wait almost a year before you’re going to get anywhere near the projected load on them.”

The piles were designed to be smaller so that they cut not only concrete but also rebar and polycarbonate. Fiber optic cables are located in some piles, loosely tied to rebar, descending all the way from the first floor to the bottom of the pile about 55 meters below. The pile monitoring system should remain in the building for 10 years to help estimate loading over time. Mace hopes the data gathered will help him rationalize pile design in future projects and determine what temporary work is needed to support the next phase of ongoing construction. “We want the building to work really hard for us because we’re also building it,” McDonald says.

The ground floor slab is more cementitious, 40 percent GGBS. “When you’re pouring slabs, curing time is critical because you want to be able to load them back up as quickly as possible so you can build on top,” says Webster.

40 percent GGBS concrete is also used in the jump form core, a method chosen over slip form in part because it allows the concrete to cure longer before it gains weight. The additional tension slabs will make up approximately 80 percent of the floor slab of the upper levels, further minimizing materials. Each upper floor slab will have an area of ​​1,080 square meters: 805 of prestressed concrete, 107 of reinforced concrete and 168 of cross-laminated timber and steel.

The material world

While concrete inevitably makes up a significant portion of the project’s environmental impact, Mace has gone further in its quest for carbon savings. One of the success stories was the armature. Mace had already decided on a rebar supplier when Morrisroe came forward with an alternative that uses half the carbon. “It had a huge impact,” says Webster. “We have approximately three and a half thousand tons of fittings on the project. That’s a big number.”

The reuse of materials also plays a significant role, not only because the project aims for a minimum diversion from landfills of 98 percent. Mace has partnered with structural steel firm Severfield NI to procure reusable temporary steel structures that will either be transported to other projects or remain in place, encased in concrete or exposed.

During excavation of the foundation, the skip descends from the trestle, which rests on the mortise columns and the sheathing beam. Once the London clay is dug out, a system of gantries transports it up onto the back of a lorry, which takes it to be processed 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 neighbor to the Shard and its other Mace-branded siblings. “We’re looking at all these functional projects north of the river,” McDonald says. “Soon north of the river we’ll be looking at all functional projects south of the river.”