Roman concrete was a ceramic material used in construction during the Roman Empire. Roman concrete was based on a hydraulic-setting cement with many material qualities similar to modern Portland cement. By the middle of the first century AD, the material was used frequently as brick-faced concrete, although variations in aggregate allowed different arrangements of materials. Further developments in the material contributed to structurally complicated forms, such as the dome at the Pantheon.

Historic references

Vitruvius, writing around 25 BC in his Ten Books on Architecture, distinguished between the types of aggregate appropriate for the preparation of lime mortars. For structural mortars, he recommended pozzolana, which were volcanic sands from the sandlike beds of Puteoli brownish-yellow-gray in color near Naples and reddish-brown at Rome. Vitruvius specifies a ratio of 1 part lime to 3 parts pozzolana for cements used in buildings and a 1:2 ratio of lime to pulvis Puteolanus for underwater work, essentially the same ratio mixed today for concrete used at sea.

By the middle of the first century AD, the principles of underwater construction in concrete were well known to Roman builders. The City of Caesarea was the earliest known example to have made use of underwater Roman concrete technology on such a large scale.

Rebuilding Rome after the fire in 64 AD, which destroyed large portions of the city, the new building code by Nero consisted of largely brick-faced concrete. This appears to have encouraged the development of the brick and concrete industries.

Material properties

Roman concrete, like any concrete, consisted of a mortar and an aggregate. The mortar—a hydraulic cement—was a mixture of lime and a special kind of volcanic deposit, called Pozzolana, or "pit sand." The aggregate varied and included pieces of rock, ceramic tile or brick, or stone and brick rubble from the remains of previously demolished buildings. The pozzolanic mortar used had a high content of alumina and silica.

Concrete, and in particular, the hydraulic mortar responsible for its cohesion, was a type of structural ceramic whose utility derived largely from its rheological plasticity in the paste state. The setting and hardening of hydraulic cements derived from hydration of materials and the subsequent chemical and physical interaction of these hydration products. This differed from the setting of slaked lime mortars, the most common cements of the pre-Roman world. Once set, Roman concrete exhibited little plasticity, although it retained some resistance to tensile stresses.

The setting of pozzolanic cements has much in common with setting of their modern counterpart, Portland cement. The high silica composition of Roman pozzolana cements is very close to that of modern cement to which blast furnace slag, fly ash, or silica fume have been added.

Compressive strengths for modern Portland cements are typically at the 50 MPa level and have improved almost ten-fold since 1860. There are no comparable mechanical data for ancient mortars, although some information about tensile strength may be inferred from the cracking of Roman concrete domes. These tensile strengths vary substantially from the water/cement ratio used in the initial mix. At present, there is no way of ascertaining what water/cement ratios the Romans used, nor are there extensive data for the effects of this ratio on the strengths of pozzolanic cements.

Seismic technology

For an environment as prone to earthquakes as the Italian peninsula, interruptions and internal constructions within walls and domes created discontinuities in the concrete mass. Portions of the building could then shift slightly when there was movement of the earth to accommodate such stresses, enhancing the overall strength of the structure. It was in this sense that bricks and concrete were flexible. It may have been precisely for this reason that, although many buildings sustained serious cracking from a variety of causes, they continue to stand to this day.

Another technology used to improve the strength and stability of concrete was its gradation in domes. One example included the Pantheon, where the aggregate of the upper dome region consisted of alternating layers of light tuff and pumice, giving the concrete a density of 1350 kg/m3. The foundation of the structure used travertine as an aggregate, having a much higher density of 2200 kg/m3.

See also

• Geopolymer
• Roman brick

Literature

• Heather N. Lechtman & Linn W. Hobbs, “Roman Concrete and the Roman Architectural Revolution,” Ceramics and Civilization Volume 3: High Technology Ceramics: Past, Present, Future, edited by W.D. Kingery and published by the American Ceramics Society, 1986
• W. L. MacDonald, The Architecture of the Roman Empire, rev. ed. Yale University Press, New Haven, 1982

References