The Pantheon: The Construction Of Roman Concrete

Roman concrete was based on a hydraulic-setting cement. So, before going further, let me talk briefly about Roman cement which together with aggregates make up Roman Concrete. Roman cement is made by burning cement-stones, or septaria in a kiln “with a heat stronger than that used in burning lime” as described by James Parker in 1796. It sets quickly even in the presence of water which explains why the Romans could use it to construct under-water harbors and tunnels. Moreover, because of its ability to stick better and longer to iron and steel, along with its waterproof properties, Roman cement was still in use as recent as the nineteenth century. However, it did not get the wide recognition it deserves despite it being used in constructing

Perhaps, the most striking feature of this masterpiece is its dome which was built entirely without steel reinforcing rods to resist tensile cracking a feat no engineer will dare to do today. So how did the Romans accomplish this feat? The foundation of the Pantheon like the wall and the dome is made of Roman concrete. To build a load-bearing wall, the Romans created a form using wood, stacked rock, or brick. They laid down a layer of aggregate, consisting of hand-sized rock or broken brick, inside the form and then tightly tamped pozzolan mortar into the aggregate with a special tool. They then repeated this process, layer by layer upward. The mortar was made with carefully controlled ratios of pozzolan, lime, and water. Each batch of mortar was hand-carried in baskets to the placement. The Roman builders were very mindful of their materials, and reduced the weight of the concrete by using aggregate of different weights for different zones. The foundation had an aggregate of travertine, a heavy rock. In the highest parts of the dome, the aggregate was light tufa and

Once the wooden formwork was in place, concrete was placed within the box to set and then cure.

The second method, discovered through excavations at Caesarea, employed a containment system for the concrete that featured a large double-walled hollow box (c.11x15x4m) that was constructed on shore and towed into position. Once in location, the space between the two walls was filled with mortar until the formwork sank to the bottom. Only then was it filled with concrete (Oleson, 1985; Oleson, 1988; Hohlfelder, 1987, 264-5).

The third method was a variant of the second. Barges built with horizontal planks linked by mortise-tenon joinery were constructed on or near shore and towed into position. Researchers believe that they had most likely been partially filled with concrete in shallow water to reduce their freeboard and susceptibility to the winds and waves and thus facilitate their transport to the desired location. When the barges had been towed to the desired location, they were topped off with concrete until they sank to the