Chapter 4: The Construction Process of Segmental Bridges
are installed in tendon ducts that are made from steel or polyethylene. The ducts need to be fixed to the normal reinforcement to prevent misalignment during casting. After post-tensioning the ducts are filled with cement grout under pressure for and protection against corrosion of the tendons. Grouting the ducts will introduce bond between the steel and the surrounding grout. Unbonded post-tensioning is less common. Very similar to prestressing tendons are the techniques used for protection of stay cables of cable-stayed bridges against corrosion, as described e.g. by Funahashi (1995).
Two different ways of construction exist for post-tensioning. The prestressing tendons can be located either inside the concrete or outside of it. External post-tensioning has the advantage of easy accessibility for inspection, maintenance works and replacement. Nevertheless problems with corrosion protection are the reason for use of interior post-tensioning in most projects.
Post-tensioned tendons need special anchorages that are cast into the concrete structure. Anchorages have the shape of cones that are sitting on the end of the duct for better accessibility to single tendon strands with the prestressing jack. Anchorages are mostly surrounded by spiral reinforcement, which serves to distribute the compressive stresses into the concrete member. Small wedges around each strand or nuts (Menn 1990) fix the strands to the front plate of the anchorage. Special anchor blocks, so-called blisters are cast into the structure to provide enough space for the anchorages, e.g. on the inside of box girder segments of the second generation (Podolny and Muller 1982). Previously, tendon anchorages were also found in the joint faces, where problems with accessibility occurred. Textbooks on prestressed concrete structures provide more information on the layout and calculation of prestressing systems.
CONCRETE BRIDGE ERECTION TECHNIQUES
Concrete segmental bridges have already been introduced in Section 3.6.1. The following sections will present the important methods that are used in erecting concrete segmental bridges nowadays and the equipment employed. Special focus is put on constructability issues, pertaining