One bad piece of stone, not a bad slab
A pop-out starts with a single piece of aggregate sitting close to the surface, one that happens to be porous or otherwise able to soak up water. When that water freezes, it expands inside the stone, and the pressure has nowhere to go but out, taking a small cone of the surrounding concrete with it. What is left is a shallow pit, sometimes with the offending piece of stone still visible at the bottom, sometimes gone entirely, and a faint ring where the cone used to sit.
It is a slow process. A pop-out rarely shows up the first year. It tends to appear gradually over a slab's first few winters, as different individual pieces of aggregate near the surface each reach their own freeze thaw breaking point, which is why they show up scattered instead of in one place. A driveway or a walkway sees this more than an indoor floor simply because it is the surface actually exposed to winter weather year after year.
- A small, roughly round or cone shaped pit, usually under an inch across
- A single piece of stone visible in the pit, or a small chip missing where one used to be
- Scattered, isolated spots rather than one continuous damaged area
- New ones appearing gradually over a slab's first few winters, not right after the pour
Pop-out versus honeycombing
These two get mixed up mostly because both leave a hole in the concrete, but they do not share a cause or a timeline. A pop-out is a small pit that shows up months or years after a slab is poured and cured, driven by one piece of aggregate reacting to freezing water. Honeycombing is a void left in a wall or column the day the forms come off, from concrete that never fully surrounded the reinforcing steel while it was being placed. One is weather working on the aggregate over time. The other is a placement gap on pour day. A pop-out also stays small and shallow, an inch or so across at most, where honeycombing can run the length of a form and reach several inches deep.