There isn't exactly that, but building=ruins or building=yes + ruins=yes might work. Possibly building:levels=0 and height=0 might be better than using layer=-1 (which probably won't do anything). You could also add them as inner members of the surrounding natural=wood polygon, assuming that the concrete bases create small clearings in the woodland.

https://wiki.openstreetmap.org/wiki/Tag:building%3Druins
https://wiki.openstreetmap.org/wiki/Key:ruins

It might be worth asking for advice on the community forums, as hopefully other mappers have come across a similar problem before.

https://community.openstreetmap.org/