A hairline crack that stays level on both sides
If you can barely catch a fingernail in it, both sides of the crack sit at the same height, and it runs along or near a joint, that is ordinary shrinkage. It is the kind of crack a bottle of concrete crack filler from the hardware store is made for, and doing it yourself is a reasonable weekend job.
Clean the crack out with a wire brush or a shop vac first. Filler that goes over dirt or old sealant does not bond well, and it tends to pop back out within a season.
A crack where one side sits higher than the other
This is the crack a tube of filler cannot fix, and it is worth knowing the difference before you spend an afternoon on it. A step between the two sides means the ground under one side of the slab has moved, settled, washed out, or frozen and lifted, and the concrete is only reporting what happened underneath it.
Filling that crack closes the gap on the surface and does nothing about the movement causing it. The crack reopens, usually within the same year, because whatever is happening below the slab is still happening.
If that is the crack you are looking at, the honest fix is addressing what is under the slab, which is what our tear out and replacement work covers in full, not a caulk gun.
What is actually in the tube
Most store bought crack fillers are a flexible sealant or an epoxy, and they are doing two different jobs. A sealant flexes with the slab and keeps water out of the crack, which matters because water sitting in a crack is what a freeze and thaw winter turns into a bigger crack. An epoxy bonds harder and can add some strength back to a crack that is not moving, but it is less forgiving of a crack that still shifts a little season to season.
Neither one restores the concrete to what it was before it cracked. Both are keeping water and dirt out of a crack that a slab has already decided to have.
Cracks along a joint versus cracks across open concrete
A crack that follows a control joint is doing exactly what the joint was cut there to do, and filling it is optional, mostly about keeping water and weeds out rather than fixing a problem. A crack that runs across open concrete, away from any joint, is a different animal. It means the slab found its own place to crack instead of the place we planned for it, and that is worth a second look even if both sides are still level.
If you are not sure which one you are looking at, a photo sent to us or a five minute look in person answers it faster than guessing.