Here's a picture of some good ol' local carpenter ant 'saw' work I ran into, showing how they strongly prefer the soft parts of wood. Maybe they need to sharpen their cutters.
There is an exhaustive study being done on the Tree Care & Climbers forum-Do Ants Make Trees Unsafe? Your pic and input could really help them to further their understanding of ants.
Neat picture, I remember as a kid hearing them chew away at rotten wood close to a friends cabin out at the lake. On a quiet night you can hear them from a fair distance.