Spalling is the flaking or peeling away of small particles of cement paste from the near-surface portion of the finished slab. This generally does not affect the life of the concrete if treated correctly
Finished concrete is porous and can water from rains or snows. Spalling occurs on exterior slabs when water that is trapped in the concrete and goes through multiple freezes and thaws. When water freezes it expands by about 9%, which creates tremendous pressure inside the slab. Putting salt onto your concrete during the winter months to thaw ice will expedite this process.