Tensile forces pull apart the bottom of this concrete slab when it bends

  Conversely, steel is very strong in tension. It has a high capacity for resisting the forces that pull apart or bend it. Therefore, combining reinforcing steel with concrete or masonry results in a product that can resist both compressive forces and tensile forces. Additional, substantial benefits can be obtained by using the reinforcing steel to “squeeze the concrete together”, or place it in compression. Compressing the concrete increases it tensile (bending) strength. By increasing the tensile strength of the concrete itself (making the concrete slab or masonry wall stiffer), a designer can achieve longer spans with thinner concrete sections.