Segmental concrete bridge construction offers excellent economics as well as many different construction techniques that can be used. The multitude of choices available to contractors allows them to tailor each project to their manpower and equipment to maximize efficiency. All the casting methods to be discussed utilize the concept of match casting. The idea is to cast the segments so their relative erected position is identical to their relative casting position. This requires a perfect fit between the ends of the segments and is accomplished by casting each segment directly against the face of the preceding one using a bond breaker to prevent bonding of the concrete. The segments are then erected in the same sequence in which they were cast. Casting methods discussed include: short and long line systems and cast-in-place segments.
There are four major methods of erecting segmental box girder bridges: balanced cantilever, progressive placing, incremental launching, and span-by-span erection. Balanced cantilever erection is quickly becoming the classic technique for segmental concrete construction. This method solves problems of environmental restrictions, traffic interference, inaccessible terrain and many others. The general concept is to attach the segments alternately at opposite ends of cantilevers supported by piers. It is important to remember that the top of the concrete section is essentially operating at capacity during the entire erection sequence. Therefore, the construction loads must not increase significantly over what has been assumed in the design.
As segments are attached to the cantilever ends one at a time, an overturning moment is created and must be resisted. This moment may be resisted by post-tensioning the pier segment down to the pier stem, providing temporary supports on either side of the pier or stabilizing the cantilevers with the erection equipment. The final choice belongs to the contractor but the designer must assume and detail a method of stress evaluation and parameters for the contractor.