[kamray23]

Putting stress on the concrete requires force, and causes the concrete to deflect. Force over displacement is work, and energy can't be lost to nothingness by just breaking the concrete. Thus, the concrete releases the stored up energy as kinetic energy of its fragments.

Stronger concrete requires more stress to cause it to fail, and as such it takes more force to break it. There is logically more energy because of the higher force, so more energy gets released.