Concrete has been in use as a primary building material since Roman times. As it is strong in compression but weak in tension, it was used in arches, vaults and walls where it is stressed principally in compression. In the mid-nineteenth century, it was discovered that iron and later steel bars could be embedded in the concrete, effectively giving it tensile strength. This allowed it to be used in beams and slabs, where it worked in bending. Buildings, bridges, retaining walls and many other structures were made in this reinforced concrete. However, although it is one of the principal building materials in the world, it has shortcomings
