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/

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