Major South African construction company Concor is currently building a super-basement in Sunnyside, Pretoria as the first phase of the Trevenna Office Campus. It is part of a larger venture driven by the Public Investment Corporation (PIC). This expansive 68 000 m2 basement will service the future three A-grade office buildings of this campus. AfriSam, a leading supplier of superior quality construction materials and technical solutions, is supplying the readymix concrete for the second half of this project. Construction Worldrecently visited the bustling urban site where readymix deliveries must be timed meticulously to suit the construction programme.
Martin Muller, Senior Contracts Manager at Concor, says that the basement is 20 metres below ground level. “The scope of the project is to build a 68 000 m2 super-basement structure of five levels of reinforced concrete slabs which will form parking for the overall Trevenna campus. It ties into the existing campus which already has three buildings.”
“The super-basement that we are building in this multi-phase development has the capacity to facilitate another three office buildings on top of it. Concor is responsible for the extensive underground services such as sewer and stormwater infrastructure, electrical reticulation, lifts to the ground floor and access control,” says Muller.
Trevenna’s challenges
Muller says that apart from meticulous planning and management of logistics because of the site’s urban location, the ingress from surface and groundwater and rock conditions posed challenges. “The site is below the water table and we therefore had to contend with large volumes of groundwater when we started with the bases. “Water had to be pumped out of the worksite and into the stormwater system,” says Muller. “The client has a water usage licence and on average, since bulk earthworks started, some 40 million litres have already been pumped out.”
The blasting was done by the bulk earthworks contractor and once this portion of the contract had been done, Concor moved onto site in February 2022. “This was a phased handover of the basement because of the amount of work and the challenges experienced by the bulk earthworks contractor with the hard rock – up to 250 MPa - and groundwater,” Muller says.
The basement is constructed off conventional bases on this hard rock as it is sufficient to support the structure with no piling required,” says Muller.
“The way in which we have managed these potential risks showcase Concor’s experience and expertise on functioning on complicated projects to be able to meet the client’s deliverables,” adds Muller.
The limited working space necessitates that the rebar and formwork are stacked on the deck itself that has already been completed. The footprint of the basement extends across almost the entire site. “Logistics, material handling and deliveries are critical and require careful and meticulous project planning to ensure that materials arrive in line with the construction programme, which relates directly to AfriSam’s - the readymix supplier - deliveries,” says Muller.
In-situ construction of post-tensioned beams
All the beams used for the super-basement are post-tensioned. This enables the use of thinner slabs that use less 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 structure below so there is no need for transfer beams as the loads are directly on the columns. 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, 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 m3 of readymix concrete per hour. “We have 14 base trucks at Ferro. For scheduling purposes, Concor supplies AfriSam with their concrete requirements for the week in advance. We either confirm, cancel or move orders as required which in turn enables the resourcing of the plant from a trucking, material and staff complement perspective. ”
He explains that the batching is computerised. “The plant is fully automated and uses Command Batch software, a US based company that services readymix companies worldwide.
“Each load that leaves the plant is accompanied by a batch printout which highlights if there are any deviations from the target mix design. This is expressed as a percentage in relation to the limits set by SANS 878. This standard sets out percentages for every material type in terms of allowed deviations. AfriSam has tightened these percentages from e.g. 2% for cement down to 1%,” says Pillay.
He adds that the supply of readymix to this urban mega basement project has logistical challenges as readymix has to be transported to the construction site and requires efficient planning to ensure timely delivery.
An added benefit is that AfriSam is able to produce readymix during loadshedding as all its plants have been equipped with backup generators. This allows for uninterrupted supply of readymix concrete.
Batching and testing
Pillay explains that the plant loads in three batches as this provides the system the opportunity to correct itself in relation to over- or under batching.
“Everything is automated,” explains Pillay. “The batcher provides oversight and support for the system and will check moisture content of the fine material as it will affect the strength and workability of the concrete. The moisture content check is done every hour to ensure that consistent product is supplied from the plant,” says Pillay.
AfriSam conducts its own internal testing. “We have roving mobile laboratory assistants and aim to test every 125 m3 that is dispatched. The site’s own testing regime is more comprehensive and tests after every 50 m3 that is poured or cast. AfriSam conducts split testing with the site laboratory to ensure that there is directly comparative results between two independent sources ”
This process control ensures that there is consistency in terms of product that leaves the plant and delivered to site.
Pillay adds the readymix concrete for a project with tight timelines needs to tick various boxes. “Speed, consistency, flexibility, cost-effectiveness and durability are some crucial aspects. In addition, it is important that the readymix is delivered in line with AfriSam’s aim of environmental sustainability where, aside from the product that reduces the carbon footprint, it must be delivered in a responsible manner and reduce waste and emissions.”
Pumped concrete
As construction progressed, all concrete now has to be pumped for the super-basement. Pillay says that pumped concrete is different in design to directly discharged concrete, the design methodology for pumped concrete involves reducing coarse aggregate and increasing fine aggregate to enable improved flow of the concrete via the pipe line delivery system.
Safety
Concor has a commitment to safety and zero harm. This has earned it the highest safety rating on the Trevenna. The Master Builders Association (MBSA) awarded the project a Five Star Safety Grading. Concor also achieved first place in the 2022 MBA National Safety Competition. The project had not had LTIs at the time when this interview was done.
