Just realized that my second (luxury modern concrete rather than old corrugated iron) privvy likely had this concrete in the roof. I hadn't realized but it had been leaking and a large section fell out - without warning
leaving rusted rebar.
I've locked the stable door and bitumened the roof then added flashing tape then bitumened it on the top again. It's a bugger to repair inside though - cement keeps falling off.
Are we saying that normal reinforced concrete doesn't fail if leaks get to it by the way?