Chapter 4: The Construction Process of Segmental Bridges
Precast segments have joints that require special attention. An epoxy agent is usually applied to the joint faces shortly before putting a segment into its location in the superstructure. Joints are usually only a few millimeters thin. Podolny and Muller (1982) explain the functions of the epoxy agent that is applied to the joint faces when placing precast segments. During segment placement the epoxy serves to lubricate the joint faces, which are cleaned by sandblasting and “compensate for minor imperfections in the match-cast surfaces” (Podolny and Muller 1982, p485). In the finished structure the hardened epoxy seals the joints against moisture and thus additionally protects the tendons in their ducts. Furthermore, the epoxy is able to transmit compressive forces and shear forces. Information on mixing, handling, and properties of the two main ingredients, the epoxy resin and the hardener, is provided by Podolny and Muller (1982). Interestingly, the epoxy agent can reach a higher final strength than the concrete itself.
In addition to the epoxy transmitting shear forces between segments the joint faces are given a special shaping to transmit shear. So-called shear keys are cast into the joint faces to lock the segments together. They transmit shear forces and also help in exact alignment of the segments during assembly. Segments of the so-called second generation facilitate many smaller shear keys that are located not only in the box girder webs, but also in top and bottom flanges (Podolny and Muller 1982).
18.104.22.168 Cast-In-Place Construction
Podolny and Muller (1982) provide an example for a typical casting cycle. As outlined in Section 22.214.171.124, any previously cast segment needs to have developed at least the specified strength to be prestressed to previous elements and support the subsequent one. After finishing all work on a segment the form traveler is detached from the previous position and moved forwards on rails that are mounted on the bridge superstructure. In order to remain balanced during advancement the form traveler may be equipped with a counterweight. Upon arrival at the new position it is adjusted and anchored to the existing superstructure at its rear to be able to withstand overturning moments that will occur from the weight of new concrete. The external formwork is cleaned and aligned to the required geometry of the next segment, also incorporating the desired camber.