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They Built To Last

September 5, 2022

by: Janet Aslin, BFP Staff Writer

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Western Wall excavation showing Roman concrete layer above quarried stones

The aqueduct at Caesarea

While on a Western Wall Tunnel tour, my curiosity was piqued when our tour guide pointed out a section of a wall under the arch that was constructed of various sized stones. These appeared very different from the quarried stones of the ancient arches and wall below, not only because of their varied shapes but because they were held in place by mortar, which the guide referred to as “Roman cement.” After returning home, I googled the words “Roman cement” and began a fascinating journey of discovery.

Building an Empire

The Romans were prolific builders who left behind many reminders that Israel was once part of their empire. Roman roads, amphitheaters, aqueducts, the spectacular underwater remains of Caesarea’s harbor, and so many more structures dot the Land of Israel, to the amazement of tourists and locals alike.

How did all this evidence—these buildings and roads—last until today? According to the World History Encyclopedia, “The Romans were innovators and they combined new construction techniques and materials with creative design to produce a whole range of brand new architectural structures.”

One of these new materials was concrete. The Roman use of concrete revolutionized architectural design of the day—both structurally, as exemplified by its use in constructing domed ceilings, and also in the longevity of the concrete itself. In an article for Popular Mechanics, Jay Bennett wrote, “Ancient Roman concrete was more durable than any developed before or since.”

Roman Concrete

Although the Romans did not invent concrete—the Egyptians used an early form consisting of mud, straw, gypsum and lime to construct the pyramids as early as 3000 BC—they did develop and use a hydraulic cement that enabled their concrete to set underwater. Not only did Roman concrete set underwater, when exposed to sea water it became stronger over time. Roman piers constructed 2,000 years ago remain intact to this day.

Concrete consists of three components: water, an aggregate (rock, sand or gravel) and cement, which binds the aggregate together. For their cement, the Romans used a mixture of pozzolana, volcanic ash from the Italian town of Pozzuoli, and lime. The Romans called their concrete opus caementicium. The consistency of the finished product was thicker than modern concrete and was laid rather than poured.

During the period from 300 BC to AD 476, the Romans used concrete extensively in massive building projects. After the fall of the Roman Empire, concrete became rare until the technology was revived with the invention of Portland cement in England in 1824.

Harbor at Caesarea

Herod the Great, as the Roman-appointed king of Judea, is known as an ambitious builder of colossal projects. He designed and built the desert fortresses of Masada and the Herodian as places of escape in case of insurrection. These undoubtedly included opus caementicium as it was the core of every Roman wall built after the second century BC. Thanks to the durability of this building material, much of these building projects can still be seen today.

Artistic rendition of submerged harbor

When Herod built Caesarea Maritima, he showcased the use of Roman concrete’s maritime properties in a way unparalleled before or since. There were no natural coves or bays along the Mediterranean coastline of Herod’s kingdom. Because he wanted an important seaport, he designed and constructed the largest artificial harbor built in the open sea at that time.

Construction of two jetties that would form the harbor began in 22 BC and was completed in 15 BC. The northern jetty was 275 meters (902 ft.) in length, while the southern one was more exposed and required a length of 500 meters (1,640 ft.). Herod imported over 24,000 cubic meters (847,550 cubic feet) of pozzolana from Italy which was mixed with lime and sea water for the cement, or the binding component. Kurkar, a locally-available rock formed from lithified sea sand dunes, was used as the aggregate and completed the necessary ingredients.

Wooden forms—and in the case of the southern jetty, barges—were used to put the concrete in place. Divers secured the forms to the bottom of the seabed by staking them, and while the wood eventually disintegrated, due to the chemical reaction between the pozzolana, lime and seawater, the concrete itself became almost indestructible.

The Harbor Today

After the harbor at Caesarea Maritima was finished, Josephus wrote, “Although the location was generally unfavorable, [Herod] contended with the difficulties so well that the solidity of the construction could not be overcome by the sea, and its beauty seemed finished off without impediment.” Today we know that Josephus was mistaken—although the concrete remains intact, the sea did overcome the breakwaters, and the magnificent harbor lies five meters (16.4 ft.) underwater.

Remnants of harbor above water

However, this was not a failure due to the building materials. Unbeknownst to both Herod the Great and Josephus, the harbor was constructed over a geological fault line that runs along the coast. Over time, repeated seismic activity along with a possible tsunami caused the breakwaters to tilt down and settle on the seabed.

Although the concrete that formed the jetties is underwater, it remains solid and can be viewed by divers who want to explore the area. The Caesarea Underwater Archaeological Park is the first of its kind and teaches about the techniques used by Herod’s engineers when the harbor was constructed over 2,000 years ago. Both snorkelers and advanced scuba divers are entranced by their glimpses of the magnificent harbor, built by Roman ingenuity and held together with Roman concrete.

Photo Credit: Click on photo to see photo credit

Photo License: Amphitheatre

Photo License: Caesarea Harbor

Photo License: Harbor Information

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