Construction World May 2017

CONSTRUCTION WORLD

MAY

2017

Due to the close proximity of the readymix plants to the PwC site,

the batch load size and loading sequences were optimised at 2 m

to make sure the mix was homogeneously mixed before it left the

plant. The concrete slump was also maintained strictly at 180 mm

from the plant.

Slip forming the core wall

The building’s core wall, at a height of 118 metres and thickness

of 450 mm, was constructed using an interesting method of

construction – slip forming. This sliding methodology starts with the

assembly of the sliding system at ground level. Concrete is poured

continuously within shutters and the system is lifted incrementally

as concrete is poured, creating a structure without joints.

Reinforcing is then fixed into position as the slide progresses so

as not to slow down the placement of concrete; over 400 tonnes of

reinforcing was used in the core wall.

Working continuously in two 12 hour shifts, the sliding team

comprised at least 62 workers on the slide shutter itself at any one

time. This allowed the core wall to be constructed in 65 days, with

an average of 90 m

of concrete placed per day.

Concrete mixes

AfriSam had to design concrete mixes that were both pumpable and

suitable for slip forming, including specialised retarded mixes that

could be continuously pumped up into the moving slide around the

clock. A static pump would pump the concrete into the slide cavities

using a spreader that was fixed to the slide.

Pumping vertically over 100 metres would mean possible high

pump pressures and the need for lubrication within the lines had to

be considered, and steps taken to avoid segregation and blockages.

If the pumping pressure was too high, excessive pressure would be

placed on the mix and this could force segregation of the concrete

(water washing out of the paste) and cause blockages in the pipe.

In addition to this, the pump would be started and stopped for

extended periods when shuttering inserts and reinforcing had to be

installed, with the pump and pipeline full of concrete. Another factor

would be the continuous stop/starting action as the hose was lifted

out of the reinforcing forest and inserted again into its next position;

the reinforcement in the lift shafts would also be very dense. This

highlighted the need for the concrete to remain workable for up to

six hours, long enough for it to pass through the pipeline and be

placed into the structure. A concrete non-return valve was used to

prevent any backflow; opening and closing as the pump worked.

AfriSam incorporated about 30% fly ash in the mixes to increase

the pumpability and slow down the initial setting time. To achieve

this, a combination of three CHRYSO additives was used including

a superplasticiser and retarder which delays the setting time of the

concrete. This provided enough time for concrete to be placed into

the form, around the core wall, and vibrated so the sliding process

could continue.

The concrete would exit the bottom of the slide 10 to 12

hours after casting. It could not be allowed to harden during this

time as this would reduce the ‘slip effect’ and make it difficult to

incrementally lift the sliding system. The concrete supported the

slide platform.

Visible from a 30 km radius, the R1,5-billion PwC Tower will comprise

45 000 m

of office space and house 3 500 employees.

Concrete is poured continuously within shutters and the system is lifted

incrementally as concrete is poured, creating a structure without joints.

Grand scale

A total of 6 057 m

of concrete was used to construct the core wall.

Concrete is normally placed using a bucket and crane; however, due to the

large quantities of concrete involved, it was decided to pump the concrete

vertically up to the height of 118 metres. When concrete was pumped to

the top of the building, there was 3,5 m

of concrete in the pipeline.

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