Construction World April 2019

The Leonardo will be the tallest building in Africa.

pressures to be reduced to acceptable levels; it also ensured a five-hour workability of the concrete mix, which would allow for traffic delays delivering to site. While the super-plasticised mix increased workability, further precautions had to be taken to counteract friction stresses in the placing pipes and between the concrete-formwork interface. To this end, the normal aggregate size of 22 mm for structural concrete was reduced to 13,5 mm and 9,5 mm. The result was a reduction in heat generation during placing and ensured that the aggregates had consistent paste coating from discharge to placing, with no segregation at this extreme placing height. Pumping concrete to these extraordinary heights, often in continuous pours, necessitated a dedicated and experienced concrete team. At times, adjustments were necessary to the pumping process – based on the skilled operator’s interpretation of the sound of the pump – to avoid blockages and delays. The higher the placing, the more adjustments were required, by recognising the sounds of the machine and responding accordingly. For the conventionally placed concrete, as well as the concrete pumped to conventional pumping heights of 30-50 metres, the Omega 140AFR admixture was sufficient on its own, being specially formulated to perform optimally with concrete mix components used by AfriSam. Both the columns and the core varied from 70 MPA at the lower levels, reducing as the builder got higher. Around the clock Concrete for this project was being supplied on a 24-hour basis to achieve the very tight seven-day floor-to-floor cycle, placing extra demand on the subcontractor. The AfriSam readymix plants supplying the project worked a 24-hour shift to ensure that the concrete work was supplied and poured as per the project plan. It was also vital to ensure a continuous, uninterrupted supply of concrete, so the company activated a backup plant in case of a breakdown. Working through the night, AfriSam maintained continuous communication with the contractor to ensure that all deliveries were made as per the schedule and specific concrete mix requirements. Continuous supply is key Concrete supply had to be planned to keep traffic disruption to a minimum, so most of the pours were done after normal business hours. As a result, it was critical that all raw materials needed for the evening shift were meticulously planned, delivered and stocked at AfriSam’s Wynberg readymix plants.

Challenging vertical concrete pumping at more than 225 metres.

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CONSTRUCTION WORLD APRIL 2019