Today when we say that something is “concrete”, we mean that it exists in reality, something capable of being perceived by the senses, as opposed to abstractions. I had a first hand impression of something concrete last Sunday, when I visited an ancient Roman quarry together with my son. I was struck by how immense it was, and what the Romans achieved with the tools of their age. Above is a picture of the quarry.
The rock mined at this place was used to build the Trophée des Alpes, or in English the Triumph of the Alpes, a monument built in 6 BC by Emperor Augustus, to celebrate his victory over the ancient tribes who populated the Alps. The monument’s remains are in the village La Turbie in France, a few kilometres from the Principality of Monaco. Below is a picture of the village and the moment.
The quarry made me think about the creativity and innovations of the Romans. Both their civil and military innovations were unique and in many dimensions. Not only products and tools, but also organisation, processes and early concepts of brands. Some examples of Roman innovations are aqueducts, paved road networks, newspapers, bound books, taxation systems, large scale, government and governance structures and the Julian calendar.
Another of the unique innovations the Romans created was the usage of concrete, the construction material. The Romans did not invent concrete, but they made it into the common building material we know today, as they used concrete extensively from about 300 BC to 476 AD, a span of more than seven hundred years.
The use of concrete freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs in terms of both structural complexity and dimension and it ensured that many Roman structures survive to the present day. The Baths of Caracalla in Rome, which I visited for the first time one warm day in June 1985, is one example. Many Roman aqueducts and bridges such as the magnificent Pont du Gard in Provence have masonry cladding on a concrete core, as does the dome of the Pantheon, the temple of all Gods.
Built in three years 27-25 BC, the Pantheon is the best-preserved of all historic Roman buildings and the oldest important building in the world with its original concrete roof intact.
The composition of the Roman concrete used in the dome remains a mystery. An unreinforced dome in these proportions made of modern concrete would hardly stand the load of its own weight, since unreinforced concrete has very low tensile strength, yet the Pantheon has stood for more than 2,000 years.
It is known from Roman sources that their concrete is made up of a pasty hydrate lime, ash from a nearby volcano and fist-sized pieces of rock. The high tensile strength appears to come from the way the concrete was applied in very small amounts and then was tamped down to remove excess water at all stages. This appears to have prevented the air bubbles that normally form in concrete as the material dries, thus increasing its strength enormously.
Other famous concrete structures include the Hoover Dam, the Panama Canal and the Coliseum. However my personal favourite is the Pantheon. When in Rome, one of my favourite evening pastimes is to visit the cool interior at dawn, and then relax with a meal or drink in one of the restaurants across the Piazza della Rotonda.
The Pantheon was enormously influential on European and American architects from the Renaissance to the 19th century. Numerous city halls, universities and public libraries echo its portico-and-dome structure.
Through incremental innovation, modern structural concrete differs from Roman concrete in two important ways:
First, modern concrete mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble.
Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.
Concrete´s recent history dates back to 1824, when Joseph Aspdin invented Portland cement, and since then the industry has embraced a culture of continuous innovation in the production and use of cement and concrete in order to make an even greater contribution to sustainable development.
In 1909, the genius innovator Thomas Edison was issued a patent for the first rotary kiln, which revolutionised the production of Portland cement. Prior to the rotary kiln, cement was produced by heating large caldrons filled with limestone and clay. Upon cooling, a hardened mass would form, which was then pulverized to produce Portland cement. Production was slow and costly. Imagine the creative genius of Edison, to span from technology innovations like the light-bulb to mechanical production facilities like the kiln. One wonders what energy and curiosity Edison possessed, to be able to cover as broad range of subjects as he did.
The rotary kiln is nothing more than a long cylinder-like tube, about 50 meter long and about four meter in diameter, lined with firebrick. It is inclined 15 to 20 degrees from the horizontal. Raw materials are loaded in at the upper-end of the rotary kiln and heated to about 1300° C. As the rotary kiln turns, the raw materials flow “downhill.” Along the way, they are converted into thousands of golf ball-sized, spherical cinders called “clinkers.” These clinkers are then mixed with gypsum and pulverized. The end product is Portland cement.
The rotary kiln made it possible for Portland cement to be produced as a continual flow-through process. Using the rotary kiln, large amounts of Portland cement could be produced and cement became the low-cost common construction material it is today.