Concrete, a compound consisting of sand, gravel, and cement has been found in its basic form in structures dating as early as 5600 BC.
This earliest form of concrete is found in the former Yugoslavia where a red lime was used as the cement. The Egyptians and the Romans both used forms of concrete widely in the construction of many projects for hundreds of years. The roof of the Pantheon was created using an advanced form of concrete with lightweight aggregates and bronze reinforcement bars.
Concrete continued to be used in relatively unchanged forms until the early 1800's when Portland cement was developed. A British bricklayer named Joseph Aspdin patented Portland cement in 1824. His cement was similar to the Roman cement patented in 1796 by James Parker but used an artificial hydraulic lime. Joseph Aspdin's son, William, added further modifications in 1843 and 1848. The name Portland cement is originally associated with a William Lockwood and possibly others, however it was first manufactured by William Aspdin in England in 1842. It was also during this time when iron bars were first used in the reinforcement of concrete. William Wilkinson of Newcastle applied for a patent for the introduction of steel reinforced concrete in 1854.
When concrete, which has a high compressive strength, is combined with rebar, which has a high tensile strength, the result dramatically increases the sum of the two parts allowing huge continuous unsupported beams of concrete to be produced. The rebar allows whatever weight is imposed on the slab to be redistributed through the length of the rebar.