The tell that separates it from ordinary shrinkage
A hairline crack by itself does not tell you much. Plenty of them show up as concrete finishes curing and never mean a thing. What sets a settlement crack apart is not how thin it is, it is whether the two sides still match. Run a hand across it. If one side sits a little proud of the other, if a corner has dropped below the rest of the panel, or if the slab rocks or flexes when you step near the crack, something under it has given way.
Standing water is another sign worth watching for. A slab that used to shed rain and now holds a puddle after every storm has usually dipped somewhere it did not used to dip.
- One side of the crack sits higher than the other, even slightly
- A corner or an edge has visibly dropped
- A panel rocks or tips when weight lands near the crack
- Water stands on the slab where it used to run off
The ground under it, not the concrete itself
A slab is built to take weight bearing straight down on it, not to hold itself up over open space. As long as the ground under it stays put, it never has to do anything but sit there. Once a gap opens underneath, that same four inches of concrete is being asked to act like a little bridge, and a bridge that thin with nothing supporting the middle does not last.
The clay under most lots around here is the usual reason a gap opens up. It takes on water and swells, lets go of it and pulls back, and heaves when a hard freeze gets into it, sometimes moving in more than one of those ways under the same slab within a year. Loose fill or old topsoil left under a pad settles further as years of weight sit on it. And a downspout dumping close to an edge, or a grade that slopes the wrong way, can carry the fine material out from under the slab a little at a time until there is a gap nobody can see from on top.
Patching the crack without touching what moved
The instinct with any crack is to fill it and move on, and for a shrinkage crack that is often fine. For a settlement crack it treats the symptom and leaves the cause sitting right there under the slab. The ground that dropped is still dropped, the void is still open or still filling with water every time it rains, and nothing about a surface patch changes that.
That kind of fix tends to buy a season, sometimes two, before the crack reopens wider than it started or the panel drops further. It is worth knowing going in rather than finding out after a second patch fails the same way.
Calling it shrinkage when it isn't
The opposite mistake is just as common: a crack gets waved off as normal because it is easier to ignore than to look at closely. Left alone, a settling section keeps settling. A step that started as barely enough to catch a fingernail can become enough to trip on, or enough to catch the edge of a plow blade in winter.
By the time that happens, a repair that might have addressed one panel and the ground under it has often turned into a larger tear out. Diagnosing it early, honestly, is the cheaper path almost every time.