Dry packing
Also called Dry pack, Dry-pack mortar, Packing and grouting, Bedding
Dry packing is filling the gap beneath a bearing — a precast unit, a steel baseplate, a stair landing — with a very stiff, low-water sand and cement mortar rammed into place, so that load transfers across the whole bearing area instead of through the shims that levelled it. It is done once the element is set to level and before it is fully loaded, and the mortar is deliberately too dry to flow, which is why it has to be worked in rather than poured.
In practice
The reason it is dry is shrinkage. A wet mortar shrinks as it cures and leaves the element bearing back on its shims, which is the opposite of what packing is for; a stiff mix rammed home shrinks far less and stays in contact with both faces. That makes this a workmanship operation rather than a materials one — it is packed from one side, in layers, worked hard against the far face — and the characteristic defect is a void left in the middle of a bearing where nobody can see it and nothing will ever reveal it. Where the gap is deep, the access is one-sided, or the bearing matters too much to leave to ramming, the alternative is a proprietary non-shrink cementitious or resin grout, poured or pumped from one side with a head on it to drive the air out. That is a different product with its own data sheet, its own flow and its own cure time, and which of the two applies is a decision for the engineer rather than for the gang.
Where this comes from: Long-standing UK concrete and precast site practice. The mix, the strength it has to reach and whether the packing is structural are set by the structural engineer for the specific bearing; no mix proportion, strength or cure time appears on this page, because none of them is general.
Where it matters on site
The question that decides the detail is what happens to the shims, and it is worth settling on the drawing rather than at the bearing. If the packs stay in, they sit permanently inside the load path and the packing has to be compatible with them, which is why the compressive behaviour of an engineered shim matters more than a small plastic plate looks like it should. If the packs come out, the sequence has to allow it. Either way this is one of the few operations on a frame where the quality becomes completely invisible within the hour, so it is a watching job rather than an inspecting one. Our Stacker Packer shims are the levelling half of the same operation, and the levelling shim calculator sizes the packs from gap width and location count before anybody orders mortar.