A Detailed History of Concrete
Burnt Lime and Civilization [All historical dates are annotated BCE – before the common era – or CE – common era – rather than the Christocentric BC – before Christ – and AD – anno domini (year of our lord).]
While the Roman Empire was known for its magnificent and widespread use of lime cement mortar for floors, walls and aqueducts, there is now archeological evidence that the earliest uses of burnt lime predates civilization and began in Paleolithic times among hunter-gatherer peoples – not the sedentary agriculturalists of the Tigris-Euphrates and Nile deltas as commonly thought.
Göbekli Tepe (“Potbelly Hill”) is an archaeological site at the top of a mountain ridge in the Southeastern Anatolia Region of Turkey, near the headwaters of the Euphrates River. It was excavated by a German archaeological team under the direction of Klaus Schmidt from 1996 until his death in 2014.
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Classical Era Concrete History
In the Roman era, it was re-discovered that adding volcanic ash to the mix allowed it to set underwater. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened, and adding blood made it more frost-resistant. Crystallization of strätlingite and the introduction of pyroclastic clays created further fracture resistance.
German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400–1200 BC. Lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of waterproof concrete. Concrete was used for construction in many ancient structures.
The Romans used concrete extensively from 300 BC to 476 AD, a span of more than seven hundred years. During the Roman Empire, Roman concrete (or opus caementicium) was made from quicklime, pozzolana (volcanic ash), and an aggregate of pumice. Its widespread use in many Roman structures was a key event in the history of architecture, termed the Roman Architectural Revolution. This 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.
Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick.
Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement concrete. However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application was also different:
Modern structural concrete differs from Roman concrete in two important details. First, its 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 structures greater tensile strength, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.
The Pantheon was one of the wonders of the ancient world: it has exterior foundation walls that are 26 feet wide and 15 feet deep and made of pozzolana cement (lime, reactive volcanic sand and water) tamped down over a layer of dense stone aggregate. That the dome still exists is something of a miracle. Settling and movement over almost 2,000 years, along with occasional earthquakes, have created cracks that would normally have weakened the structure enough that, by now, it should have fallen. The exterior walls that support the dome contain seven evenly spaced niches with chambers between them that extend to the outside. These niches and chambers, originally designed only to minimize the weight of the structure, are thinner than the main portions of the walls and act as control joints that control crack locations. Stresses caused by movement are relieved by cracking in the niches and chambers. This means that the dome is essentially supported by 16 thick, structurally sound concrete pillars formed by the portions of the exterior walls between the niches and chambers. Another method to save weight was the use of very heavy aggregates low in the structure, and the use of lighter, less dense aggregates, such as pumice, high in the walls and in the dome. The walls also taper in thickness to reduce the weight higher up.
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