Previously cast in-situ concrete is often used as part of the formwork for subsequent concrete pours and the older and newer concretes may be required to act compositely. In this case the surface of the older concrete will have to be prepared (e.g. the surface laitance removed) so that the new concrete bonds adequately to it.
In addition, depending on the loads to be carried and their orientation relative to the joint between the old and the new concrete, some reinforcement will be required across the interface.
Guidance may be found in structural design codes (see Standards, codes and regulations/Structural/Design).
In other situations, such as a concrete ground slab cast on a concrete blinding layer, there will be no composite action between the two. There will usually be a damp proof membrane that will act as a slip layer between the two layers, allowing relative horizontal movement.