Chapter 4: The Construction Process of Segmental Bridges
When the form traveler has thus been prepared the reinforcement and tendon ducts for bottom slab and webs will be installed and connected with the previous ones. In cast-in-place construction it is possible to have continuous mild reinforcement in the superstructure, whereas in precast segmental construction only the longitudinal prestressing tendons will cross the segment joints.
Reinforcement can be pre-assembled into cages that are lifted into place by crane. Prestressing tendons are already inserted into their ducts prior to placement of concrete because of better accessibility. After these preparations, concrete is placed. Accessibility of the bottom part of the box girder may require that the bottom slab is cast before internal formwork for webs and top slab is advanced and aligned. After curing sufficiently for strength and durability, the tendons in the newly cast concrete segment can be prestressed. Finally, the casting cycle starts all over again to cast the next segment.
Concrete placement can be carried out by various means, e.g. with buckets that are hoisted by crane, or by pumping. While placing the concrete in lifts care needs to be taken that no segregation of the concrete mixture occurs, and that proper consolidation will be achieved. The most common method is to vibrate the concrete in the formwork by means of internal or external vibrating devices. Most important for the quality of the concrete is curing to achieve strength and durability. Upon gaining enough strength, the tendons in the newly cast segment will be stressed to some degree and the cycle starts all over again. Based on Mathivat (1983, p201) an overview of casting steps and typical values for their duration is be given in Table 4-2. It should be noted that the sequence of steps given in this table is only a generic example and would be broken down into more steps for planning an actual construction project:
Table 4.2: Typical Duration of Casting Steps
Duration 1 day
2 days 1 day 3 days
Activities Post-tensioning tendons in previous segment Stripping formwork Advancing form traveler Placing reinforcement, ducts, and tendons Placing concrete for bottom slab, webs, and top slab Curing concrete (including Sunday)