The crack it is cut to catch
Concrete shrinks a little as it cures, and the ground underneath a slab moves a little with the seasons, so there is always some stress that has to go somewhere. Left alone, that stress finds its own path and opens a crack wherever the slab happens to be weakest, which is rarely a straight line and never a spot you would have picked. A control joint is a groove tooled or saw cut partway through the slab, on purpose, in a spot chosen ahead of time, so that when the crack comes, and it will, it follows that line instead of wandering across open concrete.
Cut within the first day, not after
Timing matters as much as placement. Joints get cut early, generally within the first day after the pour, because shrinkage starts working on the slab almost right away. Cut too late, and the concrete may have already found its own path and cracked somewhere else before the saw or the tool ever gets to it. The slab has to be firm enough to cut cleanly without tearing or raveling the edge, but the window closes fast, so this is one of the parts of a pour where the schedule is not flexible.
Reading the crack once it shows up
A thin crack running right along a joint line is not a defect. It is the joint doing exactly what it was cut to do, and it shows up on almost every slab eventually. What is worth a second look is different.
Any of those points at something moving underneath the slab rather than ordinary shrinkage at a joint, and that is a call worth making rather than something to watch and wonder about.
- A crack that runs away from the joint line and out across open concrete
- A crack wide enough to catch a fingernail
- A spot where you can feel a step between the two sides of the crack
Spacing that fits the slab, not a fixed number
How far apart the joints sit is not one number applied to every job. Slab thickness, the shape of each panel, and where a driveway curves, narrows, or meets a garage apron all change where a joint belongs. A joint grid laid out to fit those things gives the shrinkage crack a short, direct line to follow in every panel. A grid that ignores them, evenly spaced without regard for shape or thickness, leaves some panels a lot more likely to crack outside the lines than others. That is why the layout gets walked and planned before the saw or the tool ever touches the slab, not measured off a tape after the fact.