In one of the most demanding concrete applications in South Africa’s industrial history, 2,400 m3 of concrete was continuously poured to form a vault base – two metres deep, 40 metres long and 32 metres wide – at the third iron ore tippler facility being constructed at the Port of Saldanha.
This was the largest continuous pour ever done by leading contractor Group Five, who is constructing the Tippler 3. The facility is part of Transnet SOC Ltd’s construction programme to ensure that export volumes of 60 million tons of iron ore per year are sustained.
Expected to last some 72 hours, the June 2017 pour demanded a concrete supply rate of 45 m3 per hour; it was in fact completed without disruption in about 54 hours, well ahead of schedule.
Francois Rust, Group Five project manager, explains that the continuously poured slab was able to address the presence of a shallow water table on site, just eight metres below surface.
“Casting the slab without jointing reduced the future risk of water ingress through joints. This was a significant improvement on the design of the existing tipplers at the Saldanha site, were water seepage into their vaults needs to be continuously pumped out to avoid flooding,” he says.
The vault excavation, which reached 22 metres deep, effectively made the vault construction a ‘reverse swimming pool’ – keeping out any water seepage while addressing the dangers of buoyancy. In addition to providing the solution required by the client, the pour represented an important contribution of concrete technology in large industrial applications. Casting of the slab in one pour also saved time, helping the contractor keep to the project schedule.
A single continuous pour of this size has seldom been attempted; it required not just careful planning and coordination between project partners, but a range of risk mitigation measures, with particularly close collaboration between Group Five and construction materials specialist AfriSam.
Rust says concrete mixes were designed and tested to ensure that the project specifications were met. These included the need to be pumpable and to achieve 45 MPa at 28 days. To minimise the heat of hydration, 40% fly ash was combined with a CEM II A-L 52,5 N cement, resulting in a total extender content of 50%. The specification also required a crystalline waterproofing admixture (CWA), which was added at a dosage rate of four kilograms per cubic metre.
Concrete was supplied from AfriSam’s Saldanha batch plant, which was modified to accurately dose the CWA, by adding a scale to weigh and dispense the large quantities of waterproofing additive. The system recorded the exact amount of admixture on a delivery ticket, for checking by the site engineer.
Arrangements were also made in case of a breakdown. As AfriSam did not have another facility near its Saldanha plant to provide backup, a competitor’s plant was placed on standby. This meant the backup plant had to carry all the same materials as the supply plant, although it was not in the end necessary to call on its services.
Incoming deliveries of raw materials for the concrete also had to be planned meticulously to ensure no stock-outs. This included cement deliveries from AfriSam’s Ulco cement plant near Kimberley in the Northern Cape, and deliveries by bulk bags to the debagging facility in Cape Town.
“Some 50,000 tonnes of granite were blasted to reach the required level of excavation for the vault, so there was a strong rock layer of granite with strengths of up to 180 MPa underlying the vault to which the slab could be secured. After a baseline survey to attain the founding level surface, concrete blinding was cast using a concrete pump and the surface trowelled level using a straight edge,”Rust explains.
“Some 50,000 tonnes of granite were blasted to reach the required level of excavation for the vault, so there was a strong rock layer of granite with strengths of up to 180 MPa underlying the vault to which the slab could be secured. After a baseline survey to attain the founding level surface, concrete blinding was cast using a concrete pump and the surface trowelled level using a straight edge,”
A rock anchor rig drilled anchors 50 mm in diameter at marked points on the surface of the blinding, down to a depth of two metres below the underside of the blinding. Water was removed from the grout holes, and galvanised bars dowelled into the rock using a non-shrink, cementitious grout. This was injected using a tremie funnel to achieve maximum penetration and compaction.
With the base of the excavation being about 18 metres below the water table, the ingress of water posed a challenge even during the casting process. Drains were therefore created below the blinding levels to channel water away from the working area so that it would not mix with the concrete being poured.When casting began, it had to be a non-stop process. To ensure seamless handovers, each shift change had a 45-minute switchover period, so that no activity was disrupted. Concrete was delivered 2,5 km from the readymix plant to the site, and replacement drivers were used to ensure that drivers did not work extended hours. Plant staff also worked shifts to ensure continued supply. At the batching plant, millwrights and engineering staff were on standby to deal with any breakdowns should they arise.
A key challenge of the pour was the heat differential between the core and surface of the base. Measures to address this included increasing the aggregate size from the usual 19 mm stone to 26 mm, to bring down the heat of hydration and prevent thermal cracking. A mist-blower was also installed to reduce ambient temperatures, and the aggregate was physically cooled on surface before being mixed into the concrete.
When the concrete was being placed, strict systems ensured zero opportunity for error. The slump of every truck was checked – both at the batching plant and by Group Five on site – before pumping began. A full set of 150 mm cube moulds were taken from the first and last truck, with a further three cubes taken at 100 m3 intervals – for correlation testing to be checked against supplier records. So numerous were these tests that both Group Five and AfriSam had to procure dozens of extra moulds.
Having ‘pushed the edges of the envelope’ to prove the practicability of a large continuous concrete pour, the integrity of the final vault slab met all expectations and work could continue on the other aspects of the Tippler 3 project.
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