Why does concrete and masonry need to be reinforced?
  Concrete, masonry, and most cement based products are very strong in compression, or, in other words, they have a high capacity to resist compressive forces. Compressive forces can be described as crushing forces. Concrete has a very high compressive strength. It can be anywhere from 2,500 pounds per square inch, in most residential foundations, to 4,000 psi in suspended slabs and walls in buildings, to even higher strengths in bridges. However, concrete is relatively weak in tension, i.e. it doesn't resist tensile forces very well. Tensile forces are the forces that pull an element apart.