Last year New Civil Engineer brought news of Mace’s NCE100 award winning jumping factory. Twelve months on we return to see whether it has lived up to its promise.
Last year, contractor Mace made a £9M leap into the unknown.
It set itself an innovation challenge: build an onsite factory which could construct two residential towers, 30 and 26 storeys high, in just 33 weeks – that’s completing one floor on each building every 55 hours.
Full floor construction
The concept was for the two concrete residential towers in the east of London to be built floor-by-floor, fully clad and fully fitted out with all mechanical ducts, bathroom pods, critical plumbing stacks and service modules right down to the wi-fi routers.
To do this Mace devised two self-contained, self-jumping factories, housed in giant five-storey high tents erected around the perimeter of the new buildings. Inside, the towers were constructed floor by floor. When one was completed, the factory was “jumped” up 3.3m to the floor above to repeat the process, revealing the completed building below – a process that took 45 minutes (see diagram).
Inside the factory, a ring truss around the perimeter of the structure supported two 30t gantry cranes – actually four 15t cranes, tethered together in pairs, one situated over the loading bay along one side of the building and the other a working crane which ferried materials across the floorplate. Secondary trusses then supported the roof and membrane walls.
A year on, the team is now in the process of dismantling the factories after completing the two towers on schedule with the last floors built in just 39 hours.
The key, says Mace, was military precision with the logistics.
To start with the entire building process was broken down into individual construction sequences and activities. As much as possible was set to be off-site manufactured and completed on site or brought in in pods ready to plug in and play.
Rapid construction elements were used, such as 250mm thick Omnia floor slabs – 75mm thick precast slabs which act as the permanent shuttering for insitu concrete – and a 250mm thick precast twin walling system used for the vertical structure. Admixtures were added to the concrete to speed up the curing time and make sure it reached the 23KPa strength it needed so it could be built on three days later.
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Meetings were held to work out the best order for the activities and flush out clashes there might be with crane use and storage
space on the floorplates. Everything had to be planned out in minute detail.
“Each day was a recipe of what was coming and what activities had to be achieved, with timings for when it had to be completed by,” says Mace construction manager David Bones.
Bones says experience of knowing how long each activity took reassured the team that the 55 hour target was achievable.
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But the ability to carry out each one together efficiently and smoothly enough to hit the deadlines, required a significant change in mindset and buy in from the supply chain.
“I likened it to a relay race where the baton got passed on from one contractor to the next and then back to the first person again,” he says.
“The key was arranging the teams to make sure they knew they were in a very different relay race to make sure they did a floor in a week.
“That was the biggest challenge to make sure the contractors worked seamlessly together.”
Mace project director Tony Blackledge says the first stage was to impress on the supply chain that this was no ordinary construction job.
“The quality was out of this world, the accuracy of the twin wall was superb,” he says.
“The workforce could see the quality and that gets you into the mindset of ‘we’re doing something better’.”
70 hours per floor
Initially, the team was realistic. It set a goal of 70 hours to construct a floor and over a period of two weeks it worked hard to refine the process and reduce the time taken.
“With an awful lot of effort, we got down to 65 hours, but then naturally we got to a point where we all wanted to do it quicker,” says Blackledge.
Bones adds that the extensive use of offsite manufacturing meant there were only 15 people on the site instead of, say, 40 on a traditional building site. This made for a much more collaborative atmosphere.
“It was a tight knit family,” says Bones. “All the people on site, knew and understood the next stage and the next persons’ requirements intimately and that was the key to it. They all knew when the materials would be delivered for a certain activity and everyone had their stations. The materials were all cleared within a couple of hours and that wouldn’t happen on a normal operation.”
I likened it to a relay race where the baton got passed on from one contractor to the next and then back to the first person again
He says habits changed as the workers on site engaged with the process.
If materials were delivered late people changed schedules, took later or earlier breaks, rallied round to help others finish their jobs or split shifts, to make sure the task in hand was completed on time. Bones says the level of community spirit was breath-taking.
“On a normal site, you quite often get the mentality of ‘well that ain’t here so I’m not going to do anything’,” says Blackledge. “Whereas here, because of that sense of community they were saying: ‘what can we do?’ There was a real unwillingness to fail.”
The operation became slicker by the week. Each Monday the team would start the process again.
De-propping newly cured slab
The first task would be to partially de-prop a newly cured slab allowing it to redistribute more of the forces into the supporting walls. As it had not fully developed its strength, it would then be re-propped with the bracing between the supports removed, opening up space on the floor below for workers to start installing mechanical ducts. Twin walls could then be installed on the new slab above, with the critical plumbing stack installation following on.
Throughout the week, cladding would be installed on the lower floors of the factory, together with bathroom and service pods and plasterboard.
Jumping would take place on a Thursday to allow the floor slab on the next level up to be poured and have enough time to cure before the process could be repeated the following Monday. Redundancy was built into the programme in case something went awry.
But the factory was so efficient that delays were measured in minutes and at most in hours.
“Bones had a schedule that was minute perfect. The terminology we were using was ‘we were an hour behind’,” says Blackledge. “At no other time in my 30 years in construction have I ever worked in hours.”
One of the ways the team managed to streamline the process was to continually assess and refine the construction techniques it was using. One of the examples of this was the twin walling propping systems.
“We really honed some of the techniques we used as well,” says Bones. “Some of the initial support systems for the twin walls were a bit labour intensive and so we changed them as we went through.
“That became such an efficient process. We also improved the grout controls over the course of two or three cycles.”
The benefits of the factory were numerous, he says.
Enclosing the unclad floors in a large tent meant that the consequences of unfavourable weather conditions were minimised. Waste from electrical and mechanical trades was nearly non-existent and the tidier site meant that health and safety was improved. Even a fear of heights was eliminated.
“If there was one thing that was brilliant, it was that enclosed cover,” says Bones.
“There wasn’t one time when I woke up at 3am and listened to the rain beating down that I thought ‘oh my god has my temporary weatherproofing survived the night’?
“We had none of that because it was protected. If it rained hard
or blew a gale through the night it didn’t affect us. It was all nice and dry and it was a brilliant environment to work in.”
Blackledge says while the project team did many amazing things, it was being “very self-effacing” to ensure that action was taken on issues where it had identified that improvements could be made.
For example, Bones thinks chang ing the way deliveries are made would make a huge difference to the factory’s efficiency.
He says although the holding area for materials was in Purfleet only 24km away, deliveries could be held up by delays beyond the team’s control, if there was an accident or roadworks. This would eat into redundancy time or mean the day’s activities had to be replanned.
Blackledge also adds that the overhead gantry cranes inside the building could be improved.
“If you look at gantry cranes, they are traditionally used in factories,” he explains.
“The drop is usually 20m maximum, so they’re not made to drop 108m [the height of a tower block].” This extra distance made them very movement sensitive. “One of the things we learnt was that the software behind them is very technical and very sensitive. As much as our drivers were superb you have to be very careful in the selection process in the cranes you go for.
“We were pushing those cranes to new boundaries.”
More changes could be made to the dismantling strategy.
Blackledge says that it was originally envisaged that the factory would self-dismantle using a derrick crane placed on the roof. Steelwork was to be broken up into small sections and passed down through the building. But he says this would have taken too long and so was unfeasible.
Instead, Mace applied for a three-week road closure and hired a 1,000t mobile crane to take down steelwork in much larger chunks.
Holding the team together
Bones says his only regret is that Mace do not have another immediate job to move the factory to.
“The team and the system that we developed was amazing. Just think if we were able to move it straight to another job now what it would be capable of doing,” says Bones.
But Blackledge says even if they didn’t do another jumping factory, the lessons they learned about honing the capabilities of the supply chain and how to build more efficiently will be applied to future jobs.
“The overriding feeling is the immense pride because we have changed the way we work and even our own mentality,” says Blackledge. “That will live on irrespective of what job we do next.”