Bed capacity may be an issue for very high prestressing forces.

Consider the heat of hydration if you are looking at a very rich mixture. It may be appropriate to extend the release and stripping time to allow for slower strength development and less heat evolution. It is important to monitor concrete temperatures because they can be higher than those achieved when steam curing is used.

Finally, don't forget that very long girders will be difficult to transport to the project site and to erect once there. Look into this aspect before designing the structure.