Polyurea joint sealant
The joint has to keep moving, and the floor above it has to let it
A movement joint is not damage. It is the one place in the bay where the engineer decided, before the concrete was poured, that the slab would be allowed to move, and concrete moves whatever anybody decides, shrinking for months as it cures and breathing with the temperature for the rest of its life.
Most resin floors that fail with a long straight crack across them have not cracked at all. They have been poured over a joint that was still working, and the joint has simply carried on doing its job through two millimetres of epoxy that was never asked whether it minded.
Firm enough for a wheel, soft enough to stretch
Shore A 80 is a compromise and it is worth understanding which way it cuts. Softer than that and the sealant carries a loaded pallet-truck wheel badly: the edge rolls, the material extrudes above the floor line, and the joint starts collecting what the floor is swept of. Harder, and it can no longer absorb the movement it is there for, so it tears away from one side of the joint and you are back where you started.
Polyurea rather than an epoxy filler, because it stays elastic across that range and because it can be cut in within the hour. On a two-day programme a sealant that needs overnight is a sealant that costs a day.
Carried up, not buried
The joint is reinstated through the finished floor rather than sealed under it. The resin goes over the bay, and then the joint line is cut back through the coating and sealed again, so the movement has somewhere to go that is not the middle of your floor.
That is why the joints are still visible as lines when we hand over, and why we point them out on the walk-round before somebody decides they are a defect. They are the opposite of a defect. They are the reason there is no crack.
The one we will not do
We are asked, fairly often, for a seamless floor across a slab that has movement joints in it, usually because the joints are ugly and the room is going to be photographed. The honest answer is that we can pour it, and it will crack, and it will crack along the joints in roughly eight months.
So either the joints are reinstated, or the expected crack goes on the quote in writing as something you have chosen and we have priced accordingly. What we will not do is pour it seamless, say nothing, and let the warranty argument happen a year later.
How to tellLook for the joints. They should still be visible as lines on the finished floor. A floor poured straight over its movement joints hides them for about eight months, and then cracks along exactly where they were.
What to ask
Ask to stand on a jointed floor they laid more than a year ago. If the movement joints are still visible as clean lines, they reinstated them. If the floor is seamless, look along the line where the joints were.