WHY DOES CONCRETE AND MASONRY NEED TO BE REINFORCED
Concrete and most other cement based products are very strong in compression, which means they have a high capacity to resist compressive forces. Concrete has a very high compressive strength. It can range anywhere from 2,500 psi, in most residential type foundations, to 4,000 psi in suspended slabs and walls in buildings, and even higher strengths. On the other hand, concrete is comparatively weak in tension, which means it doesn't resist tensile, or pulling forces very well.
On the contrary, steel is very strong in tension. Steel is very good at resisting the forces that pull apart or bend it. Thus, when you combine reinforcing steel with concrete or masonry results in a product that can resist both compressive forces and tensile forces. Also, reinforcing steel can be used when there is a need to draw two or more concrete sections together or place them in compression. Compressing the concrete increases itís tensile strength. By increasing the tensile strength of the concrete itself, it makes the concrete slab or masonry wall stiffer. Using this technique, a designer can achieve longer spans with thinner concrete sections.
Placing the concrete into compression also adds to the resistivity of the development of shrinkage cracks. Shrinkage cracks, while typically not harmful to the performance of the structure, can allow the transmittal of moisture or even termites. Shrinkage cracks will develop in most cement based products as the water combines with the cement and the concrete cures, and when the concrete go through freeze/thaw cycles. The more the concrete is compressed, the less likely it is that shrinkage cracks will develop or open.