The joints
What happens to the control joints and the saw cuts?
A control joint is where the slab is meant to crack, so it either gets filled and coated over or left as a visible line in the finished floor.
The short version
- A control joint is a saw cut that decides where the slab cracks.
- The joint keeps moving after the floor is coated, because the concrete on both sides of it keeps moving.
- Filled and coated over gives you a flat floor with no line for dirt to collect in.
- Left as a line keeps the movement where the slab already put it.
- Either way a joint can show through later, so ask which one you are getting before the work starts.
What a control joint is for
Concrete cracks. It shrinks as it cures and it moves with the temperature, and a slab big enough to park on is going to relieve that somewhere. A control joint is a saw cut put in on purpose so the slab has a weak line to crack along, which is why they run in a grid.
So a joint is not a scratch in the floor. It is a moving part of the slab, and it keeps moving after there is a coating over the top of it. Whatever gets done with it has to account for both sides of it shifting with the seasons.
The two normal choices
The first is to fill the joint and coat straight over the top. A flexible filler goes into the cut, the surface comes level with the rest of the slab, and the finished floor is flat with no line running through it. There is nothing sitting open for dirt and salt to pack into.
The second is to leave the joint as a joint. The coating follows the cut down rather than bridging over it, and the finished floor has a visible line where the saw cut is. It reads as part of the floor, and it keeps the movement in the place the slab picked for it.
Both are normal and both go into garage floors around here. Which one you want is worth saying out loud before anybody starts, because it is a choice about how the floor looks as much as how it behaves, and it is easier settled before the grinder comes out.
What each one looks like a few winters later
A filled and coated joint can telegraph. The slab moves, the filler moves with it, and a faint line can come up through the coating along the cut. It usually reads as a shadow rather than as a crack, but nobody can tell you a joint will stay invisible.
A joint left open stays a line, and a line stays where you put it. It wants a pass with the vacuum because it collects a little grit, and when the slab moves the movement shows up in the joint instead of out in the middle of the floor. Neither choice is wrong.