Concrete is not that strong, depends on the size of the surface. Take a pencil made out of bone and one made out of concrete. Which would break? Bone would be a bitch to snap in half. Concrete wouldn't take much to snap. Now on to your story.... If one uses incorrect tech, then of course they are going to break their hand. When you have to objects hitting one another, it comes down f/m and a. Another factor is surface area. In other words a slow sloppy strike equals uselessness and pain.