A major new development in central London is demonstrating how stacked precast wall cladding panels can make concrete work easier, faster and cheaper. Jo Stimpson reports.
On a plot in a densely populated corner of the City of London, a wall of unobtrusive scaffolding belies the hive of construction activity behind it.
The upcoming 14000m2 office and retail development taking shape at 40 Gracechurch Street is remarkably self-contained. Workmen guide lorries into a gateway through the scaffolding, and bus passengers have found their route through the street disrupted − but otherwise the project causes very little disturbance in the surrounding area.
And when the work is done, and the scaffolding is stripped his month, the results of the work may not immediately register with passers-by, as the façade of the nine-storey development will look rather similar to before construction began.
The work has seen an existing building demolished to basement level and then rebuilt − with its period façade preserved upright throughout.
“The concept is to take all vertical load down to a ground beam and provide the lateral structure”
“The front of the building has been retained and we’ve built our structure up and built the façade back into our structure,” explains Costain senior site manager Matthew Cook. “We’ve restored [the façade], cleaned it up and repaired some damage.”
The structure has also been extended northwards meaning a section of a new, modern façade has had to be built to the right of the original.
Meanwhile, the top two floors of the original façade have been demolished and rebuilt in reconstituted stone and a timber framed mansard roof tops the building.
Work on site began in mid- 2008 and is due to finish in June 2010. The work has progressed “impeccably” to date, says Marble Mosaic Company marketing consultant Mariott Irons.
The project is notable for its use of stacked precast wall cladding panels, produced by the Marble Mosaic Company under a £1.3M contract. In the right circumstances, these precast panels can be stacked and supported at ground level instead of at each floor level as is typical.
A reduced and simplified structural frame can then be used, as it will need only provide for the lateral restraint of the cladding. “The concept is to take all vertical load down to a ground beam and provide the lateral structure,” says Irons.
Building with these panels brings undeniable advantages. “You minimise the number of operations you have on site,” says Irons.
With hand-fixed stone, there would need to be more people on site and the work would be subject to regular inspection by the design team. Also, much more space is needed for offloading and storage of materials and for support structures.
“You’re building without scaffolding,” Irons says. “Less scaffolding is better as this reduces staining and the potential for exterior damage,” agrees Cook.
“We can only make noise two hours on, then two hours off. We’ve got noise and vibration monitors around the partition wall.”
This all means less manpower and less time is needed. “It also saves waste,” says Cook. As the units are precast off site, site managers don’t have to deal with skips full of site debris sitting on the road − which is a boon in such a densely urban location as this.
Off site prefabrication also ensures consistency of tonality, avoiding the prospect of “letterboxing” where one piece stands out as being a different shade to its neighbours.
The erection of precast panels is also less weather-dependent, and makes for relatively low noise levels. But all the advantages of this type of cladding depend on appropriate and communicative design. “You’ve got to have early involvement with both trades − steel and concrete,” says Irons.
This is fundamental to maximising the benefits of the precast technique and avoiding the need for retrofitting, he says. “I’ve always said the key to good buildings is getting the specialist trades involved early on.”
The location of the project in the bustling, traffic-heavy centre of the City − while ideal for the finished development − has caused some serious logistical headaches during construction.
“Like all the city jobs, access is a real nightmare,” says Irons. “There is a bus stop directly outside the site. Loading could only happen between 10am and 4pm.
A solution was found that helped to ease the problem. The arches in the original façade are large enough for a lorry to pass through, so a temporary steel platform was built just inside the right-most arch. Here, lorries can park while they are unloaded. The platform was also used as a site for the concrete pumps.
“We used that to pump all our concrete from,” says Cook. “It’s robust enough to take a static concrete pump. It’s been invaluable in getting things off the road.”
Vehicles have also been kept off-road as much as possible through careful timing of deliveries so that materials arrive at a time when they can be used straight away, says Irons. “You deliver just on time. There’s no room for storage, so you deliver right to its position.”
Cranes also had to be squeezed in, with their placement meticulously planned to ensure their reach covered all the necessary space.
The site’s location also means the project is surrounded by close neighbours, a fact that must be taken into account. “In the City of London we’ve got noise limits we’ve got to abide by,” says Cook. “We can only make noise two hours on, then two hours off. We’ve got noise and vibration monitors around the partition wall.”
The commitment to monitoring is part of an agreement with the nearby buildings, he says. “The vibrations weren’t really a problem. It’s just making sure you’ve got a good relationship with your community.”
A regular newsletter about the project’s progress is sent out to neighbours and Transport for London, and copies of the document are posted on the site’s perimeter where it can be read by passers-by. “A harmonious relationship makes life easier for everyone,” says Irons.