[m4rtink]

Won't the foam deteriorate over time, possibly faster than the concrete ?

[londons_explore]

Yes, but it doesn't matter. The foam really acts as a spacer to convert a single block of concrete into two blocks separated by a few millimeters.

As soon as the concrete is set into place, the foam has done its job and can degrade all it likes.

[m4rtink]

Interesting! But what holds these blocks at the separation once the foam is no longer load bearing ? Some external bracing ?

[londons_explore]

Typically the foam is only in the middle of the concrete slab, so there are still a few inches of attachment all around all the edges with contact. There is rebar in those edges too.

[m4rtink]

Interesting, do that's how it works - thanks for the explanation! :)