Precast to precast connections
The joints between precast units are required to transmit loads. In simple cases, such as a floor slab resting on a beam, this will be chiefly in compression though in other situations there will be a requirement to transmit shear loads. In both cases the basic principle is that there should be sufficient overlap between the two units so that the bearing or shear stresses are sufficiently low. Packing pieces (e.g. steel shims) may be required to obtain the required levels for the units.
To avoid local stress concentrations, some form of interface layer between the two units will generally be required. Commonly this is a thin layer of mortar, though materials such as neoprene may be used that will allow some relative movement. In some applications (such as segmental construction) epoxy-based mortars or adhesives are used, that will carry a significant amount of shear as well as compression across the joint.