Chapter 4: The Construction Process of Segmental Bridges
This industrialized process allows easy quality control of segments prior to placement in the superstructure and saves money through reuse of the precasting formwork. Surface finishing works, such as texturing, sandblasting, painting, and coating can be performed on the ground level without scaffolding when the segments are still accessible from all sides prior to installation in the superstructure.
Another major advantage mentioned by Mathivat (1983, p212) is that the complete casting of the superstructure can be removed from the critical path of the overall construction schedule, since superstructure “segments can be precast during construction of the substructure.” Assembly of the bridge superstructure takes much less time than cast-in-place construction, as precast segments do not need to cure on site before being prestressed together. Through the early casting of segments material properties are also influenced positively. As segments are usually stored at the precasting yard or on site for a while the concrete will have gained more strength until installation than cast-in-place elements have when being loaded. The time-dependent effects of concrete shrinkage and creep will occur with reduced extent because of the increase age of the concrete segments (Mathivat 1983) and will cause smaller deflections of the superstructure than with cast-in-place construction.
However, cost for the precasting yard, storage, transportation, and installation of precast segments needs to be evaluated in comparison with cost for the form travelers for cast-in-place construction to achieve an economical solution.
The precasting yard requires investment in equipment. Adjustable formwork to form the bridge geometry and alignment needs to be installed. Lifting equipment is also required to put the segments into the storage area and later load them on truck to be hauled to the construction site.
It is common practice to use the match-cast method to achieve high accuracy in segment prefabrication. Match-casting means that the segments are cast in the formwork between a “bulkhead at one end and a previously cast segment at the other” (Levintov 1995, p46). Segment joint faces need to be clean of any dirt for match-casting.
Levintov (1995) distinguishes concrete segment prefabrication into short-line casting and long- line casting. Short-line casting would comprise formwork of the length of only one segment; with