I did some reading on delamination & it doesn't feel like a bubble per se, and I don't see any visible bulging. It's more like a hollow give feeling like in a bathtub. You don't feel it when walking on the surface, but if you kick it with the ball of your foot, it's a squishy thump. Not what I would expect to feel with Pebble plaster over concrete. Very hard to describe online.