First, breaking concrete is not the same as striking an opponent. Breaking is a training tool (or at least that was its original intention; now it is just entertainment); just as lifting weights is a training tool. As training tools, they add to your striking power, but there is little correlation between any training tool and the actual action that they train, which in this case is striking another human. Lifting weights will add to your strength and mass, which will increase your striking force, but it does not training your actual striking technique. Breaking trains certain aspects of striking technique, for example, in punching, it trains you in maintaining proper hand shape, locking your wrist straight, and transferring the force of your mass into the target. However, breaking only trains you to break inanimate objects, which is not the same as hurting or injuring an animate object.
Answer to Question 1: Assuming all the concrete slabs are identical in size, consistency, and density, and there are spacers between the slabs (which means you are actually breaking each slab separately) then the same amount of force will be required to break each slab. This means that, in the breaking motion, you must maintain the amount of force needed to break the first slab as you hit each succeeding slab. When you are no longer able to maintain this force at a slab, it will not break.