I can hardly believe it but I have stood beside the Berlin Wall. It seemed such an impregnable structure when I was growing up. But now much of what is left is crumbling concrete. Concrete is a wonderful material but better used for speed and cost rather than lasting significance. The mixture of sand and small pebbles is held together by cement. It is really, really hard to crush. It was one of my jobs to test the ultimate compressive strength of our concrete samples during my degree. A huge force was needed. But the cement doesn't hold it together well if the forces pull or tear the concrete. That's where the concrete reinforcing rods come in. They are supposed to be buried inside the concrete to stop them rusting. In this case they would make the concrete stronger against sideways blows.
