My tree is old & has a hollow spot. Should I fill it with concrete or something else?

No! You should never “fill” a tree with concrete or anything else. This will only cause more injury to the tree, leading to more problems like dead wood spreading decay to other parts of the tree.

Call an arborist to evaluate the structural stability of the tree. It could be a dead or hazardous tree and if so, it should be removed as soon as possible due to the risk they could pose to your home or people in the area.