Casting large sections of concrete is normally done in stages, where the previous pour is left to partially set before casting the next section. The Hoover dam was cast continuously in multiple sections, with careful temperature control, but it is an exception. The joint between the new and previous pour is tricky if the aim is to produce a monolithic continuity. The top of the previous pour has relatively little force on it, while the bottom of the new pour on top of it has the weight and pressure of all the concrete above it, pushing the formwork out further than the top of the previous pour. You can see that in this example in Winnipeg, where the second pour starts to fill the gap of the pushed out formwork, and the thickness of the wall increases at the bottom of the second pour. This example is for a housing project, but this similar examples can be found all over the Prairies in the bases to grain silos, where the sort of care normally taken in architectural situations is not required. Once solidified, the forces of formation are petrified in the joint.
Thanks to Phoebe chard for the Photos.