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.

The core wall, at a height of 118 metres and thickness of 450 mm, was constructed using an interesting method of construction – slip forming.

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 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.

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 ® As the concrete supplier to this complex project, AfriSam designed 19 project-specific and special application 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.

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CONSTRUCTION WORLD MAY 2017