The construction of the Trevenna Super Basement in Pretoria, awarded to Concor by the Public Investment Corporation, was a mega project in concrete terms. The mammoth five level basement extends 20 m below ground level and covers almost 70,000 m2 of parking space and other amenities. Over 30,300 m3 of concrete was poured in just 12 months to accomplish this remarkable structure – along with 2,950 t of reinforcing steel.

 

This meant the manufacture, delivery and placement of some 650 m3 of concrete each week, in a busy and confined urban space in which existing tenants and their vehicles had to be accommodated within adjacent basements. The project is an extension of the existing basements at the Trevenna Campus, and links them together. This added to the complexity of the construction process, especially as the vehicular traffic of existing tenants had to be accommodated throughout the project – as the previously used access points formed part of the new build.

 

The Trevenna Super Basement is in a busy urban area, surrounded by high density residential accommodation. Community engagement was therefore key to the project’s success, as was careful planning, effective communication and a flexible approach from the contractors.

 

The sheer volume of concrete used in the Trevenna Super Basement makes this a remarkable project within the context of South Africa’s construction sector; as a basement structure with lift columns, concrete was the core aspect of the design. At the same time, the project incorporated innovative elements which were applied alongside a keen focus by the contractor and their readymix partner on world class standards in concrete design, manufacture, delivery and placement.

 

Concrete mega project

The supply of more than 30,300 m3 of concrete within only a year demanded high levels of operational excellence and quality control. A key achievement was to maintain a streamlined and consistent delivery of concrete, complying to stringent standards to ensure first-time success on all aspects of the project. This demanded detailed and constant attention to every link in the supply chain from specialised admixtures and the mineralogy of the aggregate, to the proximity of the readymix plants and the meticulous on-site testing of the delivered concrete.

 

Five mobile cranes were employed to place concrete for the vertical elements, which comprised the 24 lift shafts and numerous columns. The pouring of concrete for bases, surface beds and slabs was facilitated by mobile and static pumps – whose combined use could meet the substantial volume demands of the project.  

 

Post-tensioned slabs and shrinkage

In addition to the large volume of concrete prepared, transported and consumed in this project, the use of post-tensioned designs with demanding shrinkage requirements from the engineers made this project challenging and innovative.

 

Below water table

Among the site challenges was the shallow water table. With the depth of the excavation reaching 20 m below street level, this meant that the floor of the basement was 13 m below the water table. This required the construction of sumps and underground reservoirs, and the continuous pumping of water from site during the project. Pumping was monitored in compliance with the site’s water use licence, which required the measurement of water volumes leaving the site and entering the city’s stormwater system. 

 

The depth below the water table also affected those concrete mixes for the basement’s lift pit bases. These mixes included the waterproofing additive Penetron, a concrete additive that lowers permeability and makes concrete waterproof with crack self-healing abilities.

 

How the owner’s requirement was met

The key requirement of this project for the Public Investment Corporation was to achieve a ‘future-proofed’ basement as a precursor for A-grade office buildings in the Trevenna Office Campus. This meant that the structure included all the elements that would pave the way for the future evolution of the upper structures. The scope of work therefore involved extensive underground services such as sewer and stormwater infrastructure, electrical reticulation, lifts to the ground floor and access control. The surface slab was also designed for higher load-bearing capacity.

 

Fit for future

The future proofing of the super basement meant that all foundations and vertical structures were designed to accommodate the top structures, which at the time of the basement’s construction were not yet designed. This gave the design team a wide variety of possible designs for the top structure; it could accommodate a standalone structure or a combined building with one smaller and one larger top structure which could range from 15,000 to 35,000 m2 depending on the needs of the prospective tenants. Among the innovative design strategies was a combination of post-tensioned and conventionally reinforced slabs, shear anchors and special concrete mixes.

 

As part of this future facing design, the Trevenna Super Basement contains key services and installations for the top structures. These include:

  • fire protection tanks and associated plant room;
  • domestic and landscape water harvesting tanks and associated plant room;
  • sewer pumping plant room;
  • rainwater harvesting attenuation tanks; and
  • a plant room to deal with rainwater and underground water.

 

Designed to bear the load of various possible external elements, the conventional surface slab has a high loading capacity that can accommodate structures, walkways or soil for landscaping. Shrinkage panels around the perimeter were also an integral part of the design because of the movement of post-tensioned slabs below the water table. These panels allow for the movement that is expected from the designed elements.

 

Live environment

The Trevenna Campus is already a busy working environment, and the owner required Concor to conduct the construction of the Trevenna Super Basement in a live and confined environment. Surrounded by residential and commercial buildings, the site had to accommodate numerous operational constraints as well as maintain constant and interactive contact with local stakeholders. Working after hours was limited, as noise had to be carefully controlled.

 

Linking basements

An important requirement from the owner was that tenants in the Trevenna Campus had to have ongoing access to the existing parking basements which abutted onto the new excavation. The new basement had to link together the existing basements, which previously had separate entrances. With the completion of this project, all the basements were linked.

 

To facilitate the vehicular movement of tenants to their parking areas during construction, Concor had to create temporary access to existing basements through the newly constructed basement sections. The basement slab levels had to tie up with existing adjacent basements, requiring ramps to be designed and constructed. Where access portals were necessary, these were supported with structural steel while concrete cutting or stitch coring was conducted.

 

Fast track

To facilitate the fast pace of construction, Concor deployed five tower cranes on the site for expediting movement of steelwork, formwork and other construction materials. Space was at a premium due to the busy urban location with the footprint of the basement extending across almost the entire site.

 

Concrete as the solution

Given the size and core function of the Trevenna Super Basement as a parking facility, concrete was a central aspect of the construction methodology. Concor conducted regular concrete testing including the use of an independent laboratory while readymix provider, AfriSam, conducted its own internal testing. Tests were conducted on site for every 50 m3 poured or cast, while AfriSam’s roving mobile laboratory aimed to test for every 100 m3 dispatched. Split testing further reduced risk by having different samples for the same pour to compare directly.  

 

Nature of concrete

The concrete for the Trevenna Super Basement had to comply to a maximum shrinkage requirement as per SANS CC1 of 400 microstrain for conventional concrete and 440 microstrain for pumped concrete. Most of the concrete was pumped due to the difficulties in gaining access to various parts of the site, so the 440 microstrain was the benchmark most commonly applied. The mix design was formulated and extensively tested to demonstrate its compliance, before being signed off by the engineer.

 

The concrete used on the project was also required to meet the Green Star sustainability standards from the Green Building Council South Africa (GBCSA). The lower carbon footprint of the concrete mix contributed to reducing the carbon footprint of the project as a whole. The AfriSam standard for green concrete is less than 300 kg of carbon per cubic metre.

 

To suit the concrete mix for pumping, the design had to ensure that the aggregate blend was optimal while not compromising the flexural strengths, especially in the surface beds. Extra effort was invested in optimising the stone content to the paste to deliver this optimal result. The aggregate used was a metamorphic quartzite (or metaquartzite) material sourced from AfriSam’s Ferro Quarry in Pretoria.

 

A combination of washed and unwashed crusher sand was initially used, with the blend being optimised for best results. This was later changed, in consultation with the engineer, to 100% washed sand. The admixture was a specially formulated CHRYSO water reducer, developed as a proprietary product for AfriSam. Designed to complement AfriSam’s high strength cement, the admixture helps deliver better early and late strengths, thereby boosting the performance of the cement. Other benefits of the admixture included activation of extenders such as fly ash and slagment.

 

Innovation in construction

Post-tensioned beams

During construction, a key innovation was the use of post-tensioned slabs throughout the super basement. This allowed for thinner slabs and reduced the concrete and rebar requirement – adding to the overall project’s Green Star strategy. Instead of having transfer beams on the ground floor, there are shafts that stiffen the structure so the loads are directly on the columns. The entire structure is stiffened up by the sheer walls and the lift shafts and stairs.

 

The post-tensioned designs provided flexibility for the design engineers in terms of loading and constructability within the large super basement structure. With less conventional reinforcing required, this mitigated the cost and risk of procuring steel reinforcing as steel markets have been volatile. It was also quicker to construct assisting with the implementation of the fast track schedule.

 

Hard rock

Even before the concrete work began, the project presented challenges that were met with innovative solutions from the contractor. In preparing the ground for the basement, Concor was required to work in rock with a hardness up to 250 MPa. Controlled blasting was applied, as the site is closely bounded by commercial and residential buildings with a high density of population.

 

Careful planning and hands-on implementation of resources allowed Concor to break up the challenging areas into manageable blocks – to progressively work towards the upper levels. Special access strategies were devised to move large equipment around the site to deal with the hard rock. The range of options used in rock breaking included excavator-mounted peckers and hand-breakers, as well as rock cracking technology using an expansive mortar injected into drill holes in the hard rock.

 

Water ingress

With the basement floor at 13 metres below the natural water table, a number of sumps and channels had to be created to remove water continuously from the site. In addition to the structural elements, this solution also required installation and maintenance of robust pumping equipment to cope with the large volumes of water.

 

Dealing with water ingress required extensive waterproofing of the structure, so hybrid systems were used to ensure no ingress of rain or underground water. This included the use of concrete additives in shaft pits and waterproofing applications on tanks to ensure no leakage. The additive selected – Penetron – was ideal for these conditions, as the constant moisture content leads to crystallisation within the concrete, sealing the pores and preventing ingress. Use was also made of heat-applied waterproofing solutions to the podium system to ensure no ingress from above, especially during construction of the next phases.

 

Quality assurance and testing

Rigorous testing regimes were applied to the Trevenna Super Basement project by both Concor and AfriSam – in line with national standards and both companies’ established quality management systems, policies and procedures.

 

Concor testing

Tests were conducted by the contractor on site for every 50 m3 poured or cast. Applying the stringent test requirements for post-tensioned slabs, Concor conducted initial stressing of the tendons in the slab after 72 hours and reaching 18 MPa minimum concrete strength (where up to 60% design strength was stressed), and again after five to seven days when full strength was stressed after reaching minimum 25 MPa concrete strength. The concrete slabs were tested at three days to ensure that they met the required 18 MPa strengths for further stressing of tendons, before moving on to the next slab. An independent concrete testing service was used to make sure of the integrity and accuracy of the test results.

 

AfriSam testing

Quality controls applied at AfriSam ensured every load of readymix that left the plant was compliant within the deviation limits set by national standard SANS 878. Their system included a batch printout highlighting any deviations from the target mix design, with AfriSam tightening these percentages from 2% for cement down to 1%.

 

Spot samples were taken of the aggregate, cement and admixture to confirm performance. The concrete was tested by AfriSam’s readymix business unit and the cement was tested by the company’s Centre for Product Excellence (CPE). The extenders were also tested on a weekly basis.

 

AfriSam’s laboratory technicians also visited the Trevenna Super Basement site and sampled concrete from the pumps. Test cubes were made on site, and then transport back to the central testing facility at the company’s Jukskei facility in Midrand. The cubes were cured and tested at three, seven and 28 days; the monitoring of early strengths allowed the concrete’s 28 day performance to be predicted. This, in turn, allowed quicker action to be taken in the event of any issues being identified with cement or admixture performance, contamination or other factors which could deleteriously affect concrete strength.

 

Test results were entered into the AfriSam central database to allow rapid analysis and tracing of trends and deviations. Key indices measured were:

  • standard deviations – reflecting plant performance;
  • strength margin – measuring deviation of the sample’s strength compared to the target strength;
  • percentage failures – in relation to industry standards; and
  • slump non-conformance – AfriSam has tighter internal limits which are compared samples taken from the site.

 

Innovating sustainability into concrete

The partnership between Concor and AfriSam on the Trevenna Super Basement project was able to embed the philosophy of sustainable development into the concrete aspects of the project. From design stage, the project was committed to limiting its environmental impact, including the reduction of the carbon footprint through lower carbon emissions in the supply chain.

 

Green Star concrete

As a starting point, the various concrete mixes for the columns, walls and slabs were specified to have a 30% replacement of cement with fly ash to reduce its carbon footprint. This provided a Green Star mix of four stars in terms of the Green Building Council of South Africa criteria.

 

Vehicle emissions

The project plan also aimed to reduce carbon emissions related to the transport of readymix to the project site. The main supply for the readymix concrete came from AfriSam’s nearby Ferro Plant in Pretoria North – just 14 km from the site – ensuring carbon emissions from the readymix trucks were minimised.

 

This principle was taken further into the rest of the project, through the focus on local procurement. The preference given to the participation of local subcontractors and suppliers meant that most resources – in labour and materials – were from the Tshwane region. This ensured that the distances travelled to and from the site were reduced, thereby keeping carbon emissions low.

 

Less waste

AfriSam also supported the project’s environmental sustainability aims by managing the production and delivery of readymix in a responsible manner that reduced waste.

 

Water management on site

Management of water on the project site was done in compliance with the client’s water use licence. Working below the water table meant continuous pumping of water from the site, which required the measurement of water volumes leaving the site and entering the city’s stormwater system. 

 

Two large underground tanks – one collecting from subsoil sources and the other from rainwater runoff – optimised water resource efficiency for use in the future buildings and the super basement. Stilling chambers on the upper level of the basement also improve the water quality by allowing sediment to settle out before water is returned to the city’s underground network.

 

Water management in concrete production

Water for the concrete mix was sourced from a borehole at AfriSam’s Ferro Facility and comprehensively tested to ensure strict compliance to requirements for concrete manufacture. The AfriSam Ferro Plant recycles the water used for washing its equipment and rinsing out the readymix trucks. The water leaves the washbays and is channeled to settlement ponds, where solids are settled out before the recycled water is used again in washing processes. Borehole water and recycled water are kept in separate silos to ensure that no contamination occurs.

 

Investing in skills and communities

Supplier development

Local subcontractors and suppliers were used extensively on this project, to optimise its positive social impact on the community. It also provided employment for local labour, with 500 to 600 people working on site. Among the key subcontractors were specialists in excavation, formwork, concrete, post-tensioning and reinforcing. Selection of subcontractors and suppliers was made firstly from those operating within the immediate ward, and then from those operating in the greater Tshwane area. 

 

Building future skills base

The Trevenna Super Basement provided another opportunity for Concor to proactively develop the skills and experience of graduates within the building and construction sector. The company’s Graduate Development Programme allowed young graduates to be employed on the site to gain valuable experiential learning while receiving mentoring and valuable site exposure.

 

Students in courses such as construction management and quantity surveying were also given the opportunity to do vacation work on this project, which is a pre-requisite for completing their courses. They are mentored and coached by Concor’s experienced staff allowing them to safely apply their theoretical training.

 

Doing good in communities

The project made financial contributions to the Bramley Children’s Home in Pretoria, a residential care facility for traumatised, abused and neglected children. 

 

Safety and risk

Keeping it safe

The highest standard of safety compliance was achieved at the Trevenna Super Basement project, earning the project a Five Star Safety Grading from the Master Builders South Africa (MBSA) as well as winning first place in the MBSA 2022 National Safety Competition in Category G for projects between R250 million and R450 million.

 

The MBSA audit included a detailed physical inspection of site activities, housekeeping, cranes and equipment. Aspects such as temporary formwork were also required to be signed off by the responsible engineer. All documentation was checked for compliance with legal requirements including inspections by a qualified Lifting Machinery Inspector (LMI).

 

No Lost Time injuries

The project had no Lost Time Injuries (LTIs), safely executing at a fast track pace with five tower cranes in a busy urban setting with little space for laydown areas. In accordance with its corporate policy, Concor conducted its own internal safety audits in line with the Occupational Health and Safety Act and other related regulations during the life of the project.

 

Underpinning Concor’s safety culture is its awareness philosophy – Stop.Think.Act! – which was applied through Visible Felt Leadership to ensure that safety was prioritised among staff and subcontractors alike.

 

Safety around water

Among the added risk elements on this project were the large sumps to deal with water ingress. Demarcation, barricades and signage prevented access to these sumps, while lifebuoys were also provided as part of the safety protocol.

 

Managing risk

In a project that demands such large volumes of concrete – delivered reliably on precise schedules and to exacting quality standards – AfriSam effectively mitigated the risk of delivery disruptions. This was through creating a standby option for readymix to be manufactured at, and delivered from, the company’s Olifantsfontein facility in Midrand – just 25 km south of the Pretoria site.

 

Not only was the production capacity available at Olifantsfontein, but a special arrangement was made to stock construction materials from the company’s Ferro Quarry which was stockpiled at the Midrand facility. This ensured the consistency of the readymix, as the metaquartzite material from the Ferro Quarry was an important aspect of the project’s mix designs.

Rock breaking, earthworks and foundations underway.

Construction the sumps for water pumping, amongst other ongoing activities.

Construction of the foundation underway.

Decking being constructed for the first slabs.

Decking progressing on the project.

Construction of the parking decking with post-tensioned cables.

An overview showing the phased construction approach applied on the project.

Rebar being installed on the ramp slab.

Decking formwork being constructed.

The basement area with finishes.