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Jul
02
2009
 
PPC plays its part in the design and safety of the R25 billion Gautrain project
 

When South Africa’s R25 billion Gautrain speeds along the 80km route linking Johannesburg, Tshwane and O.R. Tambo International Airport, commuters can relax in the knowledge that PPC Cement has played an integral part in its design and safety.

Over 200 000 tons of PPC’s CEM 1 cement is at the heart of the Gautrain’s infrastructure, having been utilised for the shotcrete in the tunnelling, as well as in the precast yard for both the viaduct and tunnel lining segments. PPC’s product strength and durability means that the Gautrain has a project life of 100 years – itself a South African first.

A fleet of bulk tankers delivers the PPC cement seven days a week to six different batching plants located at the station sites along the Gautrain route: O.R. Tambo International Airport, Pretoria, Midrand, Sandton, Rosebank and Park Station.

“We are immensely proud of the role played by PPC in the success of the Gautrain project,” says PPC’s Chief Operating Officer Dr Orrie Fenn. “PPC is justifiably proud of its 116-year history in building South Africa’s infrastructure. What we are doing on this project is proof of our commitment to enhancing South Africa's heritage for the benefit of future generations”.

Cyril Attwell, Bombela Civils Joint Venture (BCJV) Senior Concrete Technologist agrees: “The combination of high- early strength and long-term durability are essential properties required for all the concrete used in the Gautrain. The CEM 1 cement that PPC supplies us is pivotal to the eventual engineering and commercial success of this mega-project.”

PPC cement is used in the BCJV concrete plant near Midrand which is the largest precast yard in the southern hemisphere. It produces a number of precast products which are assembled on site using a purpose-made gantry, including the sleek, box-girder elements that comprise the elevated viaducts used when the Gautrain project crosses a road or a river. These impressive structures are becoming visible along a number of stretches of the Gautrain route.

Another application which makes use of PPC’s CEM 1 cement is in the 3km tunnel section being built by a Tunnel Boring Machine (TBM) which lines the tunnel it has bored using precast concrete segments installed within the protection of its tail shield. The inner diameter of this tunnel is 5.86m and the outer diameter 6.46m, giving a tunnel lining segment thickness of 300mm.

The tunnel’s precast concrete segments are manufactured by a specialist company using concrete produced at the factory-owned 18 m³ per hour batching plant . Slump tests are performed on every batch before casting of segments. Overhead skips pour concrete into steel formwork supplied by French tunnel engineering group CBE, a world leader in the highly specialised market of steel moulds for concrete tunnel segments. Each mould is equipped with pneumatic vibrators and is required to produce concrete segments to the very tight tolerances of +/-1.0 mm.

To achieve the production of one segment per mould per 24 hour cycle the segments require early-age strength of at least 15 MPa after 20 hours. The segments require a compressive cube strength (150mmx150mm) of at least 50 MPa at 28 days. Before erection in the tunnel, the concrete segment strength requirement is 55 MPa. Therefore, three extra test cubes are cast for each batch of segments and tested at 56 days to check that the compressive strength has reached 55 MPa.

The concrete mix consists of CEM I 42.5 N (350kg/m3), Blastfurnace Slag (100kg/m3), Fly Ash (100kg/m3), aggregates (19.0mm max size), water and plasticiser. Workability calls for 60mm slump concrete and the maximum water/cement ratio is set at 0.45.

“Our CEM 1 product provides BCJV with the ability to develop and produce a consistent concrete mix. It is a strong and durable cement, often used in massive projects such as dam walls because of its ability to withstand extreme pressure and loading,” said Fenn.

Technical Information
Total cement supplied to date 166 000 tons
Total cement requirement for the project 248 000 tons
Concrete design life 100 years
Concrete strength 85MPa
*Tunnel Boring Machine (TBM) information from a paper by Ian Thoms and Oliver Gertsmann, Bombela Turnkey Contractor

 
 
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