Concrete is a mixture of cement, aggregate and water. The discussion can get complicated as so many people interchange the words cement and concrete. Think of the cement as one of the dry ingredients in a cake. Cement is a combination of crushed rock and burnt lime (ground limestone heated above 825 degrees centigrade). To this cement, a binder is added. The binder is called aggregate (rocks of various sizes and sand.) The last ingredient is water. The final product, our cake, is concrete.

A second term that gets confused in the discussion is mortar. Mortar is a combination of cement, sand and water. While concrete can stand alone, mortar is used as a glue to hold bricks and stones together.

Concrete is one of the oldest forms of building material used by man. The oldest known surviving concrete is the floor of a cabin in Lepenski Vir (a Mesolithic archaeological site located in Serbia) thought to have been laid in 5,600 BCE.  The Humayama Excavation Project of the 1980s found the use of a rudimentary form of concrete in water collection systems built by the Nabataean societies that date to the 300s BCE.

Concrete as we understand it in the modern age, however, stems from Roman times. The Colosseum (82 CE) and the Pantheon (circa 128 CE) utilize what is called opus caementicium or Roman concrete. The Romans used very little water in their mixture making what would be called today a thick mortar. The binder used was gypsum and lime; Pozzolana (volcanic dust from Mt. Vesuvius) was preferred when it could be found. The addition of this volcanic dust allowed the concrete to harden underwater. The Romans also knew that adding horse hair made cement less likely to crack and that adding animal blood produced minute air bubbles and made it more frost resistant. The Romans would mix the mortar and then carry it in baskets from the mixing area to the construction site. After laying down a layer of aggregate, they pounded the mortar to remove all of the air. The combination of this low-water mixture and the close compaction is why Roman concrete has survived into this century.

The Roman use of concrete freed them from the use of brick and stone, allowing for new designs and ways of building. These new designs included arches, vaults and domes. The Roman concrete arch allowed buildings to support more weight than earlier buildings, enabling the Romans to build larger and more elaborate structures. The Roman arch (along with the vault and dome), began the Roman Architectural Revolution and has had a lasting impact on architecture throughout the world.

With the fall of the Roman Empire, the art of concrete was lost and stone once again became the favored form of building material. In 1414, Poggio Bracciolini “rediscovered”  Pollio Vitruvius’s 15th-century BC De Architectura, or The Ten Books of Architecture, in the Abbey St. Gallen in Switzerland. De Architectura contained information about Pozzolana cement, reviving an interest in the use of concrete as a building material. Pozzolana cement was used in 1499 to build the concrete piers of Pont de Notre Dame in Paris.

In 1756, repeated structural failure of the Eddystone Lighthouse off the coast of Cornwall, England, led John Smeaton, a British engineer, to conduct experiments with mortars using both fresh and salt water. In 1756, these tests led to the discovery of a hydraulic cement that did not require volcanic ash. His combination was limestone and a high concentration of clay. Hydraulic cement is cement that creates a chemical reaction when it comes in contact with water (hydration) and will cure under water. This type of cement is also used in the construction of concrete bridges and dams.

In 1824, English inventor Joseph Aspdin refined Smeaton’s formula producing Portland Cement, now the most commonly used cement. He mixed finely ground limestone with clay into a powder and then heated it to create hydraulic cement. By burning the limestone and clay together the chemical reaction created a stronger product than using simple limestone. He named the product Portland Cement because it resembled a stone quarried on the Isle of Portland off the British Coast.

Modern precast concrete brought the use of concrete to a new level in both the fields of art and architecture. Come back next week to learn how concrete has been used as an art medium here in San Francisco.

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