Where did engineers first develop the precursors of modern concrete and what’s that have to do with plate tectonics?
Concrete is among the most widely-used engineering materials in 21st century, with enough cement produced each year to issue nearly three tons to every living individual. Our modern mixtures are the result of over five thousand years of continuous experimentation with ongoing research into ever-stronger admixtures and aggregate materials.
To appreciate recent developments in incredibly durable, environmentally friendly concrete, it’s useful to spend a few minutes catching up on its invention and early application.
Early History of Concrete
Cement composites were an early part of the civil and structural engineering toolkit; pyramid facing stones in 3000 BCE were attached via a non-hydraulic gypsum-lime mortar, the use of which spread quickly after development as encasement for rammed earth structures in Mesopotamia. Contemporaneously, the builders of the Great Wall employed a cement mixture derived from rice gluten. Within a few centuries, engineers were combing cement with various aggregate particles and admixtures to form recognizable precursors to modern concrete.
While it’s unclear exactly when this experimentation produced hydraulic cement materials, but we know they were in active use by the 800s BCE. Underground cisterns were employed by the Nabataea in the 700s, and aqueducts (such as the Jerwan aqueduct of 688 BCE) soon after. In the Mediterranean, various Greek cultures used concrete in palace construction and smaller-scale infrastructure projects, but the history of Western concrete, but large-scale deployment of concrete as a core building technology required a fortuitous combination of military might, engineering culture, and plate tectonics.
Pozzolana, Germs, and Steel
In a way, the proliferation of concrete technology – and its contribution to the development of civil engineering – is an example of environmental determinism. The term, originating as an intersection between the fields of history, biology, anthropology, and geography, stresses the primacy of environmental systems factors in the development and fate of human political or regional powers. First explored in the early 20th century by Ellsworth Huntington and Ellen Semple, in part to model the rise and fall of the Western Roman Empire, the concept first achieved mainstream recognition with the publication of Jared Diamond’s Guns, Germs, and Steel. (There is an accessible PBS documentary outlining Diamond’s work on the subject, but the book is far more comprehensive and data-driven.)
Where the theory becomes interesting for our purposes is in the mineral inputs available to Roman engineers. While concrete and cement technology were established by the time Roman civil engineers came on to the scene, the unique blend of minerals present in regional deposits of volcanic ash enabled the development of a concrete measurable superior to modern varieties.
Italy is a volcanic region, owing to its proximity to the Eurasian/African plate boundary. Local tectonic activity has always been a substantial driver in Western history; its contributions to the fall of Bronze Age civilization hubs should not be underestimated, and the volcanic ash deposits left behind at Latium and Naples were particularly suited to application as pozzolanic material in hydraulic cement.
The higher alumina and silica contents in available ash deposits produced a concrete with comparable structural strength to our modern cements and a much greater inherent resistance to seawater. Whereas previously-used cements (both hydraulic and non-hydraulic) wore relatively quickly and were entirely inappropriate as breakwater or harbor material, the Roman breakwaters in the harbor at Pozzuoli Bay remain intact and serviceable after thousands of years. Modern Portland cement, by contrast, is rated for a fifty-year service life under comparable conditions.
Roman pozzolan-lime concrete cured slowly, but was incredibly strong and resilient once fully hardened. Tinkering with the mixture lead to low-CO2, high-strength concrete blends which last for thousands of years in the field.
Further experimentation with aggregate and admixtures led to Roman engineers developing concrete with greater frost resistance (just add blood) and differing weight or tensile strengths (specifically, with different mixes for foundation versus dome-pouring applications).
Without the lucky coincidence of geology, history, and technology that lead to uniquely Roman concrete, however, it is hard to say if modern civil engineers would make such wide use of the material. Even as Europe began its recovery from the Dark Ages, the evidence of concrete’s potential for infrastructure and civic projects was simply part of the landscape. Some emerging Middle Age societies continued to make use of Roman aqueducts, for example, and many concrete religious and civil structures survived the turmoil of the Dark Ages pretty much intact. Learning how to replace the lost knowledge of the collapse became a priority for Renaissance engineers and scholars, many of whom came to age amongst Roman ruins.
The Roman Civil Engineering Revolution
The refinement of concrete freed Roman architects and civil engineers from restraints dictated by building with stone or brick. Being able to mold and pour features such as arches, domes, and vaults lead to an exploration of previously impractical designs and scale. Further, the incredible endurance of Roman concrete mixtures made large, permanent infrastructure projects far less expensive and more robust.
Many notable examples of Roman civil engineering projects survive into the present. Numerous aqueducts remain standing, even semi-functional – where not severed through enemy action or mostly clogged with silt and chalk deposits after almost two thousand years without maintenance. Evidence of mining and ore processing machinery, powered by pressurized water from Roman leats and concrete aqueducts, was discovered in Wales relatively recently.
Aside from specific applications made entirely possible through the use of high-strength Roman concrete, it is pure speculative to suggest that the coincidental presence of uniquely suited volcanic material is what allowed Romans to engineer and build at their preferred scale. What is clear, however, is that future civilizations and regional cultures wouldn’t have been able to study and learn from Roman innovations, possibly slowing the redevelopment of architecture and civil engineering disciplines in the early Modern era.
As is evidenced in the forms of 20th century buildings and infrastructure, Rome’s example remains foundational to Western civil, structural, and architectural engineering. Without the local pozzolana, the projects upon which we based our rediscovery of these disciplines would have crumbled away.
Next Time: Modern and Future Concrete
Concrete is ubiquitous, a standard component of modern infrastructure, architecture, and construction engineering. With its history in mind, it’s easy to see concrete as an example of the transformational power of simple, inexpensive technologies. This is more true today, perhaps, than it was in the ancient world; with concrete in such common use, a small change to its manufacture, composition, or deployment could yield surprising returns.
Next time, we’ll examine some of the ways modern engineers look to improve modern concrete, building on biomimicry and the hard work of ancient engineers. The next generations of concrete are greener, stronger, and more resilient than ever.
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