We viewed your photos. In general mature trees will flake or lose bark for various reasons. If you have squirrels, then this is a possibility. As long as your foliage looks healthy and the tree is putting out new growth this should not be a reason for concern.
For more information you can also contact several certified arborists for an onsite diagnosis regarding the health of the tree and the best way to proceed. http://www.treesaregood.org/