Caesarea Maritima: Pioneering Hydraulic Concrete in 22-10 BCE
Caesarea Maritima, a city constructed over 2,000 years ago (circa 22-10 BCE) on the Eastern Mediterranean coast, stands as a testament to Roman engineering and the ambitious vision of Herod the Great (r. 37-4 BCE). This metropolis, featuring a colossal harbor, became a central maritime trading hub of its time.
Remarkably, Caesarea Maritima boasted the largest artificial harbor ever built in the open sea up to that point, showcasing an engineering feat that remains impressive even by today’s standards. Named Sebastos (the Greek equivalent of Augustus), the harbor, also known as Caesarea on the Sea, was established on a shifting sand beach. This location was politically strategic but lacked natural features favorable for port construction. The shoreline faced the longest fetch in the Mediterranean and was subject to a strong long-shore current, creating significant challenges.
The key to this ancient engineering marvel was Roman hydraulic concrete, composed of lime, pozzolana (a volcanic ash rich in aluminosilicates), and water, mixed with various rubble aggregates. This mixture produced a hard, durable concrete capable of setting underwater. While still malleable, it was poured into wooden forms to create monolithic structures, known as pilae.
Investigations at Caesarea have revealed three distinct methods employed by ancient builders to set and cure the hydraulic concrete underwater within wooden formwork. The reasons for employing multiple formwork variants remain unclear, but this ingenuity is a hallmark of Herod’s project.
One method involved constructing a box with vertical planks driven into the ocean floor, supported by horizontal cross beams, or vice versa. Once in place, the box was filled with concrete.
The second method, identified by the Caesarea Ancient Harbour Excavation Project (CAHEP), used a large double-walled hollow box built on shore and towed into position. The space between the walls was filled with mortar until the formwork sank, after which it was filled with concrete.
The third method was a variation of the second, involving barges constructed with horizontal planks joined by mortise-tenon techniques. These barges were partially filled with concrete in shallow water to ease transportation. Once towed to the desired location, they were fully filled with concrete until they sank.
In conclusion, the concrete at Caesarea, made with pozzolana from the Bay of Naples, differs from contemporary Italian concretes due to the construction site’s unprotected shoreline, the distance from pozzolana sources, and possibly the workforce’s unfamiliarity with hydraulic concrete. This adaptation to local conditions exemplifies the ingenuity and resourcefulness of ancient Roman engineers.
