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Concrete In Australia : June 2013
Concrete in Australia Vol 39 No 2 49 of two spans on a straight alignment and is continuous over three piers, making a 56 m span between two external piers. e two end spans are cantilevering, by 12 m at the east and 2 m at the west, which then connects to the inground WTS system via precast units with cast in HDPE liner. e world s first full internal surface lining of a cast insitu, circular post-tensioned structure was incorporated in this aqueduct. e HDPE liner was cast into the 4.7 m internal diameter opening within the 5.3 m square prestressed concrete section. e substructure was constructed using precast prestressed concrete pier segments and was supported on 1500 mm diameter bored piles with insitu pile caps. Disruption to the river flow at the crossing was minimised by placing new piers in line with the existing heritage piers. e design configuration for the aqueduct structure is shown in Figure 1. 3.1 Preliminary design by others An earlier design proposed for the new aqueduct involved a single 70 m span cast insitu externally post-tensioned concrete beam. e proposed design would have required people to work 15 m above the river bed on extensive temporary works. A number of OHS issues and the complex nature of construction prompted the Pipelines Alliance review panel to provide justification for an alternate bridge design. 3.2 Value-added alternative design Pipelines Alliance developed an innovative alternative design solution to address the OHS issues. e alternative design reduced the construction time, overall project costs and the environmental and OHS issues. Benefits that arose from the alternative design are detailed in Table 1. Figure 1. Elevation and section of the aqueduct. 4.0 SUPERSTRUCTURE 4.1 Aqueduct e 5.3 m square superstructure with a 4.7 m diameter internal void was designed for construction in four 17.5 m segments, each cast in two stages. A 1.8 m high invert section was cast first followed by the remaining 3.5 m high obvert section. After casting, each segment was post-tensioned once the concrete strength reached the designed compressive strength of 50 MPa. e segment was then launched forward to allow the next section to be cast against the previous one. e post-tensioning involved 16 continuous (coupled) 6-22 internal grouted tendons arranged in groups of four tendons in each corner, as shown in Figure 2. PT Plus ducting (Polyethylene) and VSL optimised grout were used to resist the aggressive environment. e tendons were designed to provide compression in concrete during all stages of construction and in the final completed stage. A major challenge was the evolution of an economic concordant prestressing design for the aqueduct that was appropriate for both the permanent and launch conditions. e changing support conditions during construction and the launch stage resulted in a theoretically large number of force distributions within the structure. e structural complexity Constructability Launching allowed casting segments on a prepared ground slab to reduce substantial temporary works in the waterway at height. Durability Durable internal post-tensioned tendons with fully sealed "Plyducts". Environment Launching from ground significantly reduced construction impacts on the Werribee river. Quality Superstructure constructed on the ground in a controlled environment and launched progressively across the river. Time savings Fewer construction stages and a shorter construction period. Significant time savings to construct and remove temporary works. Substructure constructed while segments being cast. Material savings Significant savings on temporary works. Savings on 420 t of concrete and 55 t of prestressing. Cost savings Approximately $2 million savings compared to the preliminary design. OHS Avoidance of the OHS issues associated with erecting and dismantling temporary works supporting the superstructure. Table 1. Benefits of alternate design.