Building for Eternity: The History and Technology of Roman Concrete Engineering in the Sea

The longevity of Roman construction on land is well known, and some notable concrete structures that still stand today are justifiably celebrated. The survival of Roman concrete harbor works is perhaps even more astonishing, having resisted for so long the force of the sea—modern concrete structures are commonly demolished after but a small fraction of the time. So what is it about Roman concrete that explains its extraordinary resilience?

Here an interdisciplinary team answers this and other questions concerning engineering, technology, and trade on the basis of the activities of the Roman Maritime Concrete Survey (ROMACONS) between 2002 and 2009. This ambitious project was directed by Brandon, Hohlfelder, and Oleson, who coauthor the volume along with Jackson and other collaborators. The project benefited from international funding as well as technical and logistical support provided by the Italian CTG Italcementi Group. Fieldwork and sampling was undertaken at numerous sites around the Mediterranean, followed by laboratory analysis at the University of California, Berkeley, and other institutions. 

The volume is impressive for its scale and scope. It marshals a range of expertise: in the archaeology of maritime structures (Hohlfelder and Oleson), in architecture and infrastructure (Brandon), and in chemical and mineralogical analysis (Jackson and six other contributors). The acknowledgments give a sense of the complexities of the enterprise, and all involved are to be congratulated on their energy and perseverance.

The first chapter (Oleson and Jackson) introduces the technology of Roman maritime concrete and related research questions, which are usefully summarized at the end, along with the main emergent principles and conclusions, thus orientating the reader to the chapters that follow.

The main discussion dives directly into volcanic products and pozzolanic material. However, perhaps a more natural progression would have been to begin with the ultimate basis for ancient mortar and concrete, the lime cycle, before moving on to the effect of adding quartz sand to make mortars, the limitations of such mortars (not being hydraulic and deteriorating when saturated), then to additives such as crushed brick that contribute hydraulic properties, and finally to volcanic material and the superior performance it yields. Volcanic ash produced by the Santorini eruption of ca. 1600 B.C.E. was used before Roman times, but the critical advance came in Campania in the third or second century B.C.E. with the exploitation of Puteolanus pulvis from the craters near Puteoli and Baiae, a kind of pumiceous ash pozzolan. This publication stresses the need to distinguish between this preferred generic term for siliceous and/or aluminous material as opposed to pozzolana (likewise named after Pozzuoli, the modern name for ancient Puteoli), which is often used in looser fashion to describe volcanic ash whether from around the Gulf of Naples or elsewhere (including Rome and its environs). Using pozzolan primarily from the Campi Flegrei, Roman builders went on to develop terrestrial and maritime concrete, the success of which relied on empirical observation, skilled workmanship, and highly organized logistical support (4). Emblematic of the value placed on Puteolanus pulvis—which allowed concrete to cure underwater without exposure to air as was otherwise normal—was its shipping in large quantities for Herod’s sea defenses at Caesarea Maritima.

Chapter 2 (Oleson) reviews the relevant ancient literary sources relating to maritime concrete and its material components. Besides familiar passages by Vitruvius and Pliny the Elder, 16 authors are covered, beginning with Theophrastus and ending with Procopius of Caesarea. The original Greek or Latin is accompanied by English translations. Although published translations exist for most of these passages, misunderstandings arise from the varying English equivalents used by different translators, leading Oleson to supply his own new translations that combine with his commentaries to make this a most welcome resource.

The next chapters focus on particular aspects. Chapter 3 sets out the history and procedures of the ROMACONS project and the logic behind it. Chapter 4, being a descriptive “Narrative of the ROMACONS Fieldwork,” will interest anyone wishing to conduct comparable investigations. Chapter 6 presents a catalogue of the 36 sites investigated (a mixture of harbor defenses and installations, fishponds, and other maritime structures), which, together with the substantial appendices, contains the primary results and data.

The project included some inspired experimental archaeology, especially the creation of a single pila of approximately 8 m³ at the inner harbor of Brindisi, as documented in chapter 5. This was the occasion not only to experience various logistical and practical challenges but also to test the effectiveness of Vitruvius’ recommendations (De arch. 5.12.2). This was evaluated by analyzing cores taken at intervals, thus revealing objective increases in performance over time and so substantiating significant aspects of the ancient writer’s account. An important question raised is whether the Romans would have used dry quicklime or slaked lime (lime putty, or grassello di calce in Italian). There is a little awkwardness in presenting the case (109), which gives the impression that slaked lime could be disputed even if its actual use in the ROMACONS pila (106) is amply justified by ancient testimony and other evidence. This leaves one wondering why deliberations (161–62) revolved around quicklime. In any event, the Brindisi episode is fascinating, and it is instructive to see how the practicalities involved relate to the lessons learned from Brandon’s detailed interpretation of the traces of Roman formwork that survive at some sites (ch. 8).

Jackson and others, in chapter 7, tackle the material and chemical analysis of Roman seawater concretes using microscopic, macroscopic, and mechanical testing, with the results being interpreted in the light of the ancient sources covered in chapter 2. Important findings are reached in several respects: identification of the sources of the materials used and their diffusion; composition (mortar relative to aggregate); and consistency, or lack of it (some from Caesarea Maritima being notably heterogeneous). Particularly revealing is the ancients’ appreciation of the virtues of the pozzolanic material from the Campi Flegrei; this was used even at Chersonesos on Crete despite the availability of volcanic ash from nearby Santorini.

The main text of the volume, excluding the extensive appendices, concludes with chapter 9, a review of maritime concrete technology in context, addressing transport, trade, and knowledge transfer, while reflecting on chronological and regional patterns. It is here that the general reader will find issues of wider interest, although the same readers will regret the relative absence of a synthesis of the results from the valuable but disparate and compartmentalized chapters.

Maritime concrete developed out of terrestrial concrete, but anyone who wishes to probe their similarities and differences may be disappointed, given the authors’ admittedly understandable wish to stay on target and not provide a general study of Roman concrete engineering. While it is true that other publications do this, some are outdated and most do not benefit from the depth of technical knowledge that this team possesses. Might the understanding of Roman concrete on land become a future team project?

To conclude, this is an impressive enterprise with considerable strengths. Technical expertise has intersected with detailed knowledge of ancient sources and practices to produce a substantial advance with regard to coastal archaeology as well as concrete engineering. The high-quality illustrations are informative, especially Brandon’s color reconstructions that embrace allied processes and infrastructure and so contextualize the Romans’ signal achievements. Delivered with authority, this book illuminates what lay behind the Romans’ success in “building for eternity” and will surely define the field going forward. 

Mark Wilson Jones
Department of Architecture and Civil Engineering
University of Bath