Shallow versus deep: the split that decides what happens next
Not all spalling is the same problem. Shallow spalling stays in the top skin of the concrete, the layer that took the trowel and the weather first. Deep spalling has worked further down, past the finish and into the coarse stone the mix is built on, or on older work, down to the steel that used to be buried in it. That difference decides almost everything about what a fix looks like.
- Shallow: thin flakes that lift with a fingernail or a broom, sound concrete underneath
- Shallow: isolated spots rather than the whole surface
- Deep: the coarse aggregate, the stone itself, is visible where the surface used to be
- Deep: rebar or wire mesh has been exposed by the loss of surface concrete
De-icing salt, a rushed finish, and water freezing at the surface
Salt is one of the most common causes. It drives moisture through the surface faster than a plain freeze and thaw cycle would manage on its own, and a few winters of that cycle work the top layer loose bit by bit. Troweling or sealing a surface before it was ready to take it does something similar, sealing moisture in that the slab still needed to release. Even with no salt anywhere near it, a slab at this elevation goes through enough hard freezes most winters that plain moisture trapped near the surface can start the same kind of damage on its own.
None of it stays still once it starts. Every freeze that catches moisture sitting near the surface carries off another thin layer, and each layer that goes leaves a rougher, more open surface for the next one to work on. A handful of spots you could sweep clean with a broom this year can, after enough winters left unaddressed, turn into a surface where the coarse stone and even the steel underneath are sitting exposed. At that point it has stopped being a flaking surface and become a slab that is failing.
Resurfacing over spalling that runs deeper than it looks
Shallow spalling on a slab that is otherwise sound and sitting flat on good ground can sometimes be resurfaced, and that is a real, honest option worth asking about. Once the damage has reached the stone or the steel, a resurfacer is a thin layer stuck to a surface that is already failing underneath it, and it fails the same way the original surface did, usually inside a season. Whether a given slab is a resurfacing candidate or a tear out candidate is not something to guess at from a description. It is worth a look before either gets decided.
The first winter is when it usually starts
A new slab is at its most vulnerable to this before it has finished curing, which usually means through its first winter. That is why salt gets left off a fresh pour that first season, with sand doing the job of traction instead. Put salt on a slab that has not made it through that first winter yet, and the surface takes the damage harder than it would once it has had the time to cure fully.