Which one is correct?
I was walking on crumbling or crumbled sidewalk.
I think "crumbled" is more correct, because the side walk *crumbled by people or by cars.
It can make no sense by saying "sidewalk is crumpling"
What do you say about this?
Both can be correct.
The usual way to express this idea is crumbling sidewalk. It is in a state of falling apart. But you could say "The sidewalk was completely crumbled" or "The sidewalk had completely crumbled".
To crumble is both transitive and intransitive.
You can crumble a cookie. transitive
A brick wall can crumble with many years of freeze and thaw, if there are small cracks in the mortar which allow water to enter. intransitive
crumbling
because it is an ongoing process - even if the sidewalk is already pretty bad, it's still wearing down day by day.Crumbled
would really imply that the sidewalk's disrepair is no longer continuing to worsen - perhaps because it completely fell apart to the point it can no longer be called a sidewalk at all.