Concrete Diffusion

Here is pattern I have seen often on concrete sidewalks. Wear that often begins in a crack in the sidewalk spreads out (horizontally in this picture) in a pronounced bell-shaped pattern. Here is another example.

I was  told by one colleague that this is simply the basic result of a diffusion process, as wear tears up the opening and propagates through with walk. I'm not fully clear on the physics involved, but it is quite reminiscent of my very first posting on this blog back in 2007.