MATERIALS - CONCRETE SPECIFICATIONS
|The new high-speed rail track in Taiwan runs through one of the world's most active earthquake zones. The mammoth-project, which has an investment volume of 14 billion US Dollars, will shorten the journey time on the 326 km route between Taipeh and Kaohsiung from currently 4½ hours to 90 minutes.|
A mountainous topography, highly active tectonic fault zones, highly populated areas and the extremely tight construction schedule qualify the High Speed Rail-Project as one of the world's most ambitious and challenging building projects to date. A regular routing is impossible here: 244 km of the track are elevated, it runs through 50 km of tunnels and over 32 km of bridges. The lots THSR Contract C260 and C270 were awarded to co-contractors Bilfinger + Berger Bauaktionsgesellschaft and Continental Engineering Corporation. After 27 months of high-pressure work, the workers had every reason to celebrate on October 27, 2003: The last of 2000 partial elements was being cast. Due to the extremely tight construction schedule it was necessary to pre-fabricate the superstructure from box-girders. These pre-stressed prefabricated parts, which are 30 to 35 km long and weigh between 750 and 800 tons, were produced in two prefabricated parts plants and installed using the so-called "launching-method". Pillars, abutments and pile caps are supported on foundation piles with a diameter of 2 m, which are embedded up to 75 m into the foundation soil. On the whole more than 3.3 million m³ concrete were used. 1.5 million m³ of that was used just for the piles. DEMANDS ON THE CONCRETE
Late in 2000 Bilfinger+Berger and MC-Bauchemie had already started a test series in close cooperation, to develop the optimal concrete compositions. With the help of MC it was possible to create individual high-performance concretes for each of the three track elements, foundation piles, pillars and box girders, that fulfilled the respective material requirements. The essential characteristic of the concrete for the foundation piles was its special processing properties. At ambient temperatures of up to 40°C the concrete had to have a pot life of 8 hours, while maintaining a specified consistency (10< slump/cm <20). Additional requirements were good cohesion and the prevention of bleeding and sedimentation. Decisive for the fabrication of the box-girders was the best-possible utilization of the formwork elements. The tight schedule demanded a concrete with an extremely fast strength-development in order to be able to release the parts as soon as possible from the formwork. The superior material properties made a heat treatment of the concrete at the prefab plant unnecessary. The high early strengths of the prefabricated concrete parts was achieved with high binder-contents, but even more by employing low water/cement-values. By using a suitable PCE-based superplasticizer it was possible to lower the water/cement-value to 0.34 while maintaining excellent processing properties. Polycarboxylate-ethers are polyether-modified copolymers made from methacrylic acid, maleic anhydride and styrene, or from styrene and vinyl-/allyl-derivates.To date little is known about the PCEs' exact mechanism of action. A widely accepted model postulates that cement particles receive a negative static charge from the free carboxyl groups within the PCE-molecules and that the polyether-groups reach into the aqueous phase. This leads to an electrostatic and steric repulsion between the cement particles and thus to improved flow-characteristics. Tenside chemistry has shown that polyether and water form gelatinous structures. It is therefore conceivable that the cement particles are also enclosed by a sort of "protective gel", which has a strong plasticizing effect, allowing for a prolonged workability. Polycarboxylate-based concrete admixtures offer clear advantages for high-performance concretes:
Despite difficult conditions we succeeded in developing concrete formulations, which fulfil the requirements for durability and stability of the THSR. A great number of concrete-technological problems have only been solved by the use of polycarboxylate-ether-based concrete admixtures. See also "Bendable Concrete".
- reduced water consumption
- low slump loss
- short wait times
- high early strengths
- good processing properties
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