Concrete is composed of aggregates, which can be any or a combination of sand, gravel or rocks that is held together by cement. The cement itself, when mixed with water, serves as a paste that holds all the components of the concrete together once the cement sets and hardens.
One of the earliest evidence of manmade concrete were the floors of royal palace of Tiryns in Greece, which was constructed around 1400 to 1200 B.C. The ancient Romans were one of the most prolific users of concrete, using the material for their architectural and other construction works.
During a 700-year period between 300 B.C. to 476 A.D., the Romans used a concrete material called opus caementicium that was made from pumice as an aggregate with pozzolana and quicklime as cement. Modern-day analysis show that opus caementicium had similar compressive strength to modern Portland cement. However, since the Romans did not use any reinforcements, opus caementicium had significantly lower tensile strength compared to modern reinforced cement.
The proportions of the materials used for making modern-day concrete varies according to its application. For example, the proportions used on concrete for floors, driveways and sidewalks is ideally one part cement, two and half parts sand and three parts gravel. On the other hand, the recommended concrete mix proportions for high strength floors and columns is one part cement, one part sand and two parts gravel.