Construction World May 2017

CONSTRUCTION WORLD

MAY

2017

PROJECTS & CONTRACTS

What makes a twisted building so different?

The Council on Tall Buildings and Urban Habitat defines a twisted

building as one that progressively rotates its floor plates or its façade as

it gains height. The PwC Tower twists both its façade and floor plates as

well as the supporting structure, which is essentially a concrete frame

with post-tensioned concrete floor plates. These twisted manipulations

create a stunning variety of textures, view angles, and ripple effects.

Admixtures

AfriSam also paid particular attention to the aggregate grading

and fines content. If the grading of the aggregates was incorrect,

they could be pushed out during pumping and this would result in

blockages in the pump or pipeline.

When pumped, concrete moves in a cylinder and is separated

from the pipeline wall by a lubricating layer of water, cement and

fine aggregate or sand. This lubricating film is vital to achieve the

necessary workability of the concrete so it can be pushed through

this channel.

The CHRYSO

Superplasticiser was therefore added by AfriSam;

it is designed with the CHRYSO

Fill Free technology for a cohesive

concrete paste which is less sticky, as stickiness can increase the

resistance of the flow of the concrete in the pipeline. And will make

it difficult to maintain a constant rate of pumping.

It was important to achieve exactly the correct slump. The

sliding process may be delayed if the slump is too high and it may

be difficult to slide if the slump is too low. Since the CHRYSO

The core wall, at a height of 118 metres and thickness of 450 mm, was

constructed using an interesting method of construction – slip forming.

As the concrete supplier to this complex project, AfriSam designed 19

project-specific and special application mixes.

Plasticiser is a multi-dose admixture, it could create concrete with

the best suited slump for the project.

This admixture created a dense, cohesive concrete mix with

sufficient paste volume that enabled the correct level of workability

of the concrete. It also assisted with a good aggregate coating by

dispersing the cement grains, thereby minimising agglomeration

of the mix and increasing plasticity and therefore workability of the

concrete. The 180 mm slump was achieved without any segregation

of the mix. Any segregation of the mix would cause blockages as the

slush is pushed through the coarse aggregates causing a plug

CHRYSO

Plasticiser also helped in creating a favourable water-

cement ratio by reducing the amount of water, improving durability

by preventing shrinking, cracking and porous concrete.

The landmark PwC Tower now stands as testimony to the skill

and innovation found in South Africa’s built environment disciplines,

including the providers of cement, concrete and specialised

chemical admixtures.



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.

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

pumpability and slow down the initial setting time. A combination of

three CHRYSO additives was used including a superplasticiser and

retarder which delays the setting time of the concrete.