Maybe soak it in a solution of potassium nitrate, let it dry - this ought to encourage the wood
itself to burn in the presence of an oxidiser.

Liquid Oxygen would probably work as well...

Just where do you think you can get potassium nitrate from ??? bombs
and all that.

May be fill the hole up with potassium nitrate, a bit of charcoal and
maybe a bit of sulphur, mix it up a touch. Put a good heavy cap on it,
stand back and set light. That will clear the concrete and make a hole
at the same time. No need to recycle the concrete as you will not be
able to find it. The neighbours might complain thou ???