Classified Fly Ash contributes to world-class Gautrain Concrete (Part 2)
Read Part 1
Part 2 of an article that was published in “Concrete Trends” (Official Journal of the Cement and Concrete Institute) VOL 12 No4 November 2009 edition. We thank Concrete Trends and the Cement and Concrete Institute for allowing us to republish it.
A Look Along The Track
Sandton Station
After setting the trend for concrete enclosed shopping malls almost three decades ago, Sandton will soon be adding the title of the deepest underground station in the world to its repertoire of prestigious features.
At 48 m down from road level to the lowest platform, the station is approximately twice the depth of Rosebank (25 m) and Park (23 m) stations. Sandton Station is itself a massive undertaking: it is the hub of the Pretoria, OR Tambo International Airport and Johannesburg Park Station network. The underground station is beneath a rock dome and will have a three-level parkade, a large people/transport interface area for bus and car drop-offs and collections, together with a 15 000-m² concourse level – the same size as Sandton’s Nelson Mandela Square.“We are progressing well by working 20-hour days, 6 days a week,” says BCJV’s Peter Bredahl, senior contracts manager for Sandton Station. “We intend maintaining this pace to achieve the completion date of 9 May 2010. The most challenging aspect of this job is the fact that all material, shuttering, equipment and construction teams have to go down two shafts, after they have fought their way through the traffic in the middle of Sandton. Then all the shuttering has to be hauled out again!” There are no precast elements in the station, everything has had to done on site with 200-250-m³/day of readymix, using Cyril Attwell’s fly ash ‘synergy’ mixes, delivered from the Bombela batch plant at Marlboro and pumped into place.
The completed station, including the diaphragm walls to the various shafts will require 65 000 m³ of concrete. The slabs have been poured using 40/50-MPa mixes and the diaphragm walls were built by a subcontractor using specialised German machinery. Bredahl’s team poured the foundations, piers, decks, columns, and, in spite of the difficult supply logistics, constructed one shaft by sliding. “The underground slide was an interesting first for me,” says Bredahl. “With its link to OR Tambo International Airport, Sandton will become a gateway for overseas visitors to South Africa and the aesthetics of our concrete work is particularly important,” comments Bredahl. “We are using wooden shuttering and the results from the fly ash concrete mixes have been exceptional.”
Centurion
“The Centurion section between Jean Avenue and John Vorster is within the dolomitic zone and includes the above-ground station,” says Ian Connellan, a senior BCJV contracts manager. “Work is on schedule and completion of the main civils is planned for the end of 2009.” The precast viaducts and associated items such as parapets, precast station platforms and ballast walls come from the Allandale precast yard, while the majority of the rest of the civils have been handled on site: piling, in-situ casting and slabs, which included some 60-MPa sections. “The most challenging task in the dolomitic area has been the foundations,” adds Connellan. “We have had to put down close to 100 000 m³ of grouting using a CEM I cement and fly ash mix for good pumpability, together with bentonite. Fanie van Tonder is precast yard manager for Bombela Civils Joint Venture. Previously a Group Five director, he was involved in building Sasol 2 and 3 at Secunda; mines for Iscor and platinum companies, and has worked in Iraq, Ghana, Angola (petrochemical project in Luanda) and Kabinda, DRC (a major oil and gas separation project). While he has experience with
most building materials, he loves concrete and has a keen eye for quality.
Park and Rosebank stations
The Park and Rosebank underground stations are very similar in design and size, except that Rosebank has a two-storey parkade, whereas Park has six storeys. “The underground works should be finished by the end of November and the stations completed around June 2010, but we will still be busy with the parkades,” says BCJV senior contracts manager Robert Petit. “Overall the programme for this south section is on track.” In each case, excavation was done to about 25 m, then for Rosebank being a pole design, the piling was done up front, followed by the base slab, concourse slab, walls, roof and the parkade. Construction at Park Station was very similar, but started with the base slab, followed by the concourse and roof slabs and then the piling for the parkade. At Park Station most of the concrete is being pumped direct from the batch plant. At Rosebank, the smaller pours are done through a normal concrete chute and only the large pours are pumped. “The crucial aspect of the concrete was the workability provided by the fly ash mix,” added Petit. “We are skilled at concreting, but for good pumping and placement it was essential to have good workability. We have also been pleased with the off-shutter finishes that the mixes achieved. Gautrain is an exciting and challenging project. My construction teams realise that it is a privilege and probably a once-in-a-lifetime opportunity to work on a project like this. We are all committed to achieving the extremely tight construction timeline.”
The green benefits
“Although the huge scale of the increasingly visible concrete structures of Gautrain is impressive, behind the scenes the achievements on reducing the project’s carbon footprint have been equally impressive,” says Attwell. “I am speaking as a committed environmentalist who is very aware that cement is the second biggest contributor to greenhouse gases in the world. Creative management of three materials in particular on Gautrain has achieved enormous environmental benefits. “Unusually for a project of this size and pressured timeline, more than 90% of the spoil from tunnelling and blasting is crushed at BCJV’s Marlboro site and reused as aggregate. Wash water recycling has not only saved water cost and the environment but the suspended materials in the water gain us about 4 MPa and a corresponding saving in cement. Thirdly, the use of fly ash throughout our concrete mixes has made a major contribution, being a recycled waste product from power stations and an excellent cement extender. The overall effect of the various measures we have put in place is that we have reduced our cement usage by approximately 30% for a concrete requirement of over 800 000 m³. We are especially proud of that achievement.”
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