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Concrete In Australia : June 2013
Concrete in Australia Vol 39 No 2 51 • Protection from sharp reinforcement bars being placed on and around it. • Tensioning systems that can adapt to changing temperature environment. • Safe handling onsite without compromising the integrity of the liner system. e project team addressed these challenges with the help of RMD, the formwork company, by designing a special formwork to support and form the pipe shape and hold the liner in position. On removal of the invert formwork, the base of the newly cast invert was levelled to roll the obvert formwork over the section. e levelling was achieved by filling the invert to a depth of 300 mm with Rapid A, a liquid fill containing cement, fly ash and sand. e fill was self-levelling and provided a suitable flat base to prop the obvert formwork, as shown in Figure 6. Hardwood sleepers were placed over the top of fill, directly underneath the legs of the internal obvert formwork and bolted into the parallel flange channel (PFC) rails that allowed the formwork to roll into position. e Rapidfill base remained in the aqueduct to allow internal welding of the HDPE joints after the bridge launch was completed. EPTEC developed the installation system for placing the HDPE liner into the formwork. After the crown pour of the first segment, there was a longitudinal bubble or vein in the obvert caused by the expansion of the liner, due to the heat of hydration of the concrete during the pour, as shown in Figure 2. As a result, the liner expanded and deflected upwards into the concrete creating the vein. A robust tensioning system was developed to counteract the above thermal movement during the concrete pours and during exposure to the elements, as shown in Figure 7. e system incorporated heavy angle steel clamped to the liner and tensioned with Reid bar. e remaining segments were poured successfully without recurrence of the vein. e internal joints of the HDPE liner were welded after the 70 m aqueduct was fully launched to allow the aqueduct to creep as soon as it was stressed. Leaving the welding until the end of the launching was considered to be the best option as it provided time for the structure to sag. 4.3 Precast concrete trough A non-composite precast concrete trough section of 6.6 m width was bedded onto the top surface of the aqueduct forming a single lane carriageway for maintenance trucks, as shown in Figure 8. e 6.6 m wide trough was considered to provide access for the utility services of water main and drainage pipes which were to be underslung from the cantilever length of the trough units. e trough section was architecturally detailed to provide function and aesthetics. Each 3 m long precast section was tied together using continuous grouted strand. e trough was designed for a T44 maintenance truck, with the upstand acting as a low performance barrier. e height of the trough was raised further by casting 3 m long and 400 mm high sections of precast concrete barrier units dowelled into the top of the trough unit making the overall height to be 1300 mm above the deck surface. e barrier provided additional safety to pedestrians to comply with the AS 5100.1 requirements for pedestrian barrier height. e trafficable surface was sealed and Figure 5. Reinforcement near anchorage zone. Figure 6. Rapid A liquid fill at invert. Figure 7. Obvert tensioning system.