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CHAPTER 4: THE CONSTRUCTION PROCESS OF SEGMENTAL BRIDGES - page 8 / 47

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Chapter 4: The Construction Process of Segmental Bridges

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the previously cast segment being moved into position for match-casting on a mobile carriage. Short-line casting can be carried out in the horizontal position or with the segments tilted facing upward (Podolny and Muller 1982), however, the normal horizontal position facilitates match- casting. The overall bridge alignment requires careful adjustment of the formwork prior to each concrete placement. Short-line casting does not take much workspace.

Long-line casting on the other hand means erection of formwork for about a complete bridge span. According to Levintov (1995) the formwork can be erected stationary for the superstructure soffit only, with smaller movable forms for web sides and interior formwork. This formwork will be cheaper than the flexibly adjustable formwork for short-line casting, but will require much more workspace. Levintov cautions that the long-line casting is feasible for straight superstructures or superstructures with constant curvature. Segments are match-cast progressively on the long-line formwork by step-by-step advancement of the movable formwork units and a movable bulkhead.

Phipps and Spruill (1990) describe the precasting cycle that was used in construction of the Biloxi Interstate I-110 viaduct. According to them, the freshly cast segments were steam cured in a movable shed covering the casting bed of the short-line formwork. The pretensioning strands were released by cutting them, quality control and testing of concrete samples was performed, and internal formwork units were removed from the new segment. After lifting the previously cast segment from its position for match-casting into the storage area, the new segment was rolled out of the formwork. It was positioned for match-casting according to the required overall alignment. Cleaning of the joint face and the bulkhead was done prior to casting the next segment. Reinforcement bars were preassembled in reinforcement cages to speed up placement. Pre-tensioning strands were used in the box girder segment, being stressed prior to concrete placement. After concrete placement and consolidation with vibrators the segment was screeded and given a surface finish before the curing shed was set up over the casting bed. With the sequence described a casting cycle of one superstructure segment per day could be achieved. In the final superstructure post-tensioning cables were installed to stress the precast segments together.

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