Construction World April 2023

The basement is 20 metres below ground and covers 68 000m² over five levels.

35 MPa for the walls and 30 MPa for the slabs. For the post-tensioned concrete mix, the engineer NAKO ILLISO required shrinkage tests which is a three week process to determine how much drying shrinkage the concrete will experience over time to ensure that there is no cracking later in the structure. These tests required that the drying shrinkage not exceed 400 micro strain. AfriSam has the advantage that it has almost 90 years of experience in the industry. “We have lots of history in terms of our various quarries with regards to what deposits it contains and we understand the inherent and physical properties well, to the extent that AfriSam has published a Technical Reference Guide which is available on our website at no cost. We have knowledge of cementitious products and the extenders that we use to reduce the carbon footprint – the concrete mixes for this project have been extended with fly ash in line with the engineer’s requirement to meet green star standards. “AfriSam’s Ferro readymix plant in Pretoria North is the main supplier plant for this site – it is based 14 km from the construction site,” says Nithia Pillay, AfriSam’s Regional Product Technical Manager: Construction Materials. It is adjacent to AfriSam’s Ferro aggregate quarry and has the capacity to batch 60 m 3 of readymix concrete per hour. “We have 14 base trucks at Ferro. For scheduling

concrete and rebar and adds to the overall project’s green star strategy. “There are no transfer beams on the ground floor – normally you transfer the top load down to the columns with transfer beams. In this case we have shafts that stiffen the The entire structure is stiffened up by the sheer walls and the lift shaft,” says Muller about this design innovation. The concrete mix required for the readymix Readymix concrete is pumped for this in-situ construction of the post-tensioned slabs. The concrete mix is a green star mix (three or four stars) and is likely to be higher for subsequent phases. “At tender stage it was specified that the concrete had to have a 30% replacement of cement. AfriSam used these requirements to match them to the available materials in the area and indicated that the readymix concrete would be extended with fly ash. In addition, we had to supply the readymix concrete from a plant in close vicinity to the site location,” says Mervin Govender, Regional Sales Manager AfriSam. The engineers specified that the concrete suppliers had to design various strengths: 40 and 50 MPa for the columns, structure below so there is no need for transfer beams as the loads are directly on the columns.

Left: Concor is currently building a super-basement as the first phase of the Trevenna Office Campus. Right: The structure will contain 2 300 tons of rebar.

21 CONSTRUCTION WORLD APRIL 2023