Joints that get sealant, and joints that don't
A concrete slab has more than one kind of joint, and only some of them are meant to be sealed. Control joints are the shallow grooves cut into the slab itself, on a grid, to decide where shrinkage cracking happens. Most of them are left open or get nothing more than a light fill.
Expansion joints and isolation joints are different. They sit where a slab meets something else, a garage floor meeting the foundation, a driveway meeting a sidewalk, a patio meeting the house, and they get an actual flexible sealant because that gap has to move without letting water straight down to the base underneath.
What failed sealant looks like
Good sealant is still soft enough to give a little under a thumbnail, and it is still stuck to both sides of the joint. Failed sealant tells on itself. It cracks straight through instead of stretching, it pulls away from one side and leaves a gap you can see daylight or dirt in, or chunks of it go missing altogether and the joint sits open.
- Cracked straight through instead of flexing
- Pulled away from one side of the joint
- Missing in sections, leaving the gap open
- Hard and brittle instead of soft under a thumbnail
What an open joint lets through
A joint doing its job keeps water from running straight down into the base under the slab. Once the sealant fails and the joint opens up, that water has a direct path, and the same freeze and thaw cycle that works on exposed concrete at this elevation works on that water once it is underneath the slab instead of on top of it. It is a slow problem, not an urgent one, which is exactly why it gets ignored until the joint edges start to spall or a section next to it starts to settle.
What sun, water and salt do differently
Not every joint on a property ages the same way. A joint that sits in full sun all afternoon breaks down faster than one that is shaded most of the day, because UV is hard on most sealants. A joint at the low point of a slope, where water collects before it drains off, works harder than one on a flat, well drained stretch. Where de-icing salt gets used through the winter, that joint takes an additional beating on top of the sun and the water.
None of that means every joint on a property needs recaulking on the same schedule. It means the low, sunny, salted joint is the one worth checking first, and a shaded joint on flat ground can often wait years longer.