While concrete is the natural choice of primary construction material for high-rise buildings, meeting the demands of Africa’s highest building has called for some innovative concrete engineering.
This is according to Francois Pienaar, Aveng Grinaker-LTA contracts director responsible for the multi-billion-rand Leonardo mixed-use tower being constructed in Sandton for 75 on Maude (Pty) Ltd, which is a partnership between Legacy and Nedbank.
The structure will rise 55 floors above its four-level basement when completed during 2019 and construction materials leader AfriSam was among the specialist sub-contractors chosen to supply this landmark project.
Pienaar says that among the key challenges on the project was the vertical concrete pumping height of more than 225 metres. To overcome this challenge, as well as to avoid any blockages in the pipeline which was over 200 metres long, a super-plasticised concrete mix with a slump of 180 mm was used.
To achieve the level of pumpability required to reach these heights, the hybrid plasticiser/super-plasticiser Omega 140AFR was used in conjunction with the super-plasticiser Optima100. Supplied to AfriSam by CHRYSO Southern Africa, the role of these concrete admixtures became increasingly important as pumping height progressed.
The addition of the Optima 100 super-plasticiser allowed for concrete pumping 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.
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 sub-contractor. 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.
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. The company needed to schedule its trucks and truck-drivers for a night shift, while ensuring that enough trucks and drivers were available during the day to supply other projects.
“A special feature of the building is the ‘key-wall’ columns, where each of these columns had to be cast in one single pour of 55 m3 of concrete, with 19 tons of steel reinforcing placed inside these 11 metre-high walls,” Pienaar explains.
“A special feature of the building is the ‘key-wall’ columns, where each of these columns had to be cast in one single pour of 55 m3 of concrete, with 19 tons of steel reinforcing placed inside these 11 metre-high walls,”
Hi-tech sonic vibrators were successfully used in the columns for optimal compaction; congestion of the rebar did not present any particular challenge.
“For the construction of the main lift core of the building, hydraulic climbing formwork from PERI was used,” he continues. “This was only the second time that this specialised formwork has been used in South Africa, and the experience gained at such heights enabled the development of a new skill set for future projects.”
The vertical alignment of any tall building is always critical. So, to control the plumb of the Leonardo, state-of-the-art GPS technology was used – similar to that used on the 828-metre Burj Khalifa in Dubai. Traditionally, laser beams have been used in tall buildings, but there was a concern that in a building as tall as the Leonardo, inaccuracies could occur due to wind loads, crane loads, construction sequence or other factors.
The specialised GPS technology is based on a series of automated sensors employed to deliver precise and reliable coordinates on demand, not influenced by building movements. These coordinates are used to control the position of the hydraulic climbing and slab edge prior to each concrete pour. The technology had not been used on building projects in South Africa before.
Keim, a specialist German-manufactured concrete waterproofing and protective coating, was applied to off-shutter concrete elements. Initially sold as a phased development, the 43 floor Phase 2 of the development was boosted to 55 floors when the owners decided to increase the building height for a future five/six star luxury hotel. It will offer luxury residential units in the form of apartments and penthouse suites, as well as over 7,500 m2 of premium office space. The ‘crown’ of the building is The Leonardo Suite, a 3,200 m2 apartment with 360° views of the surrounding city. The development will also house a business and conference centre and other amenities like a gym, spa, restaurants and recreation zone. It will be the most technologically advanced, environmentally-friendly building on the continent, according to the developers.
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