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A concrete bridge that is “curing” usually has “falsework” below to support it.
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You don’t that to erect a post tensioned structure. The smaller concrete beams are transported on the highway with the beam being supported only at the ends. If you support it anywhere else, the beam will crack. During transport the beams become the trailer with a connection only to the semi at one end and to a set of axles at the other end.
This type of construction has been around at least since the 60’s when I was in school.