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What is Roman Concrete?
Roman concrete was a type of concrete used with hydraulic cements that are very similar to today’s Portland cement. This type of concrete hardens because of chemical reactions that occur independently of water elsewhere in the environment. In other words, they can harden even in wet weather. When the anhydrous cement powder mixes with water, it produces non-water-soluble hydrates. For the first time in ancient Rome, concrete structures could be built for use underwater (as for aqueducts).
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What Materials were used in Ancient Roman Concrete?
The ingredients in Roman concrete binder were Pozzoulani sand, lime, and water.
In about the year 25 BCE, the Roman writer Vitruvius gave the following “recipe" for Roman concrete:
1 part chalk: 2 to 3 parts sand. Mix this with 15 to 20% water
For structures exposed to water, such as aqueducts, baths, and harbor structures, the sand used was Pouzzolanic sand (named for the city of Pouzzolan), to make the structures more durable. Modern chemical analysis has confirmed that the composition of the Pouzzolanic sand was the key to the permanence of some Roman structures.
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Why did Roman Concrete Last so Long?
What the Romans taught us about concrete was that Pouzzolanic sand’s particular atomic structure was perfectly suited for making concrete that would last for centuries. It has an amorphous silica structure that has many holes in the molecular structure. When mixed with wet lime, calcium hydroxide is formed and enters the atomic holes, creating a gel that expands and strongly bonds the pieces of aggregate rock together. Because the Pozzulanic sand was a fine powder, it gave plenty of surface area for maximizing the chemical reaction. This ancient gel matches the chemical formula of today’s bonding gel for concrete. Clearly the Romans knew they were building for the long term.
Also, what the Romans taught us about concrete was that the water to cement ratio is critical. The less water, the greater the strength of the cured concrete. The Romans mixed their cement with very little water. In fact, it was nearly dry. Once this mixture was made, it was taken to the building site and placed over a prepared rock layer. The mixture was then tamped into the rock layer with special tamping tools. The low water content and the tamping were both important. For one thing, the tamping cut down on the need for water. For another, the close packing produced more bonding gel that one would normally expect. Tightly compacting the materials gave them long life spans.
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Roman Concrete and Modern Concrete
Modern concrete builders eventually figured out the lessons of low water content and tight compaction independently, however. In the late 1980s the United States Bureau of Reclamation refined the technique of roller compacted concrete for use in building the Upper Stillwater Dam in Utah. The concrete contained 40% Portland cement and 60% fly ash. The fly ash, coincidentally, happened to contain the same silica compounds found in the volcanic ash used in Rome. At the same time, the hydrated Portland cement released calcium hydroxide just like the calcium hydroxide that resulted from the lime used in the ancient Roman concrete.
Perhaps the main difference between ancient and modern techniques is that today we reinforce concrete with steel, and the Romans didn’t. With the very low water techniques they used, they made virtually “no slump" concrete - concrete that pretty much didn’t spread. It was hard to work with, but it allowed them to make amazing structures like the Pantheon without reinforcement. This structure still stands and is a marvel of ancient engineering.
Another great feat of ancient engineering was the way the concrete masses were put together to withstand the seismic activity that was so common on the Italian peninsula. Specially designed interruptions within domes and walls created discontinuities in the concrete that allowed parts of the building to shift a little when the earth shook, allowing the buildings to accommodate the stresses without crumbling. This “flexible" building structure was ingenious, and it was used in building the dome of the Pantheon. Additionally, the aggregate in the upper part of the dome was done in alternating layers: a layer of light tuff (volcanic rock of compacted ash) followed by a layer of pumice, giving the dome a lower density than the foundation.