(Roughly) Daily

Understanding how the materials we use work– and don’t work– together…

For most of a red swamp crayfish’s life, cambarincola barbarae are a welcome sight. Barbarae – whitish, leech-like worms, each a couple of millimeters long – eat the swamp scum off the crayfish’s shells and gills, and in most cases improve the crayfish’s health and life expectancy. Together, barbarae and crayfish form a mutualistic symbiotic relationship. Both species benefit from their cohabitation, and barbarae have evolved to the point where their entire life cycle, from egg to adult, occurs while attached to a crayfish.

But their symbiosis is contextual – a tentative truce. Young crayfish (who molt their shells more frequently and therefore accumulate less scum) don’t need much cleaning, and will take pains to remove barbarae from their shells. And even when molting has slowed and a crayfish has allowed the symbiosis to flourish, there are limits to barbarae’s loyalty: If there isn’t enough food for them to survive, they’ll turn parasitic, devouring their host’s gills and eventually killing them.

Like symbioses, composite materials can be incredibly productive: two things coming together to create something stronger. But like crayfish and barbarae, their outcomes can also be tragic. Rarely are two materials a perfect match for each other, and as the environment changes their relationship can turn destructive. And when composites turn destructive – as was evident in the reinforced concrete when the Champlain Towers North were inspected back in 2018 – the fallout can be catastrophic.

The history of what we now call composite materials goes back many thousands of years. For modern consumers, the most common composites are fiber-reinforced plastics (the colloquial “carbon fiber” and “fiberglass”), but perhaps the first composites in history were reinforced mud bricks. The Mesopotamians learned to temper their bricks by mixing straw into them at least as early as 2254 BC, increasing their tensile strength and preventing them from cracking as they dried. This method continues around the world today.

But by far the most commonly used composite material in history is steel-reinforced concrete. Roman concrete usage started as early as 200 BCE, and almost three centuries later Pliny the Elder included a note about what appears to be high quality hydraulic concrete in his Naturalis Historiae. These recipes were subsequently forgotten, and the material largely disappeared between the Pantheon and the mid nineteenth century. Modern concrete involves some legitimate process control: limestone and other materials are heated to around 900° C to create portland cement, which is then pulverized and mixed with water (and aggregate) to create an exothermic reaction resulting in a hard and durable object. The entire process consumes vast amounts of power and produces vast amounts of carbon dioxide, and the industry supporting it today is estimated to be worth about a half a trillion dollars.

But in spite of the fortunes that have been invested in the portland cement process (as well as in a wide range of concrete admixtures, which are used to tune both the wet mixture and the finished product), the true magic of contemporary concrete is the fact that it is so often reinforced with steel – dramatically increasing its tensile strength and making it suitable for a wide range of structural applications. This innovation arose in the mid-nineteenth century, when between 1848 and 1867 it was developed by three successive Frenchmen. In the late 1870s, around the time that the first reinforced concrete building was built in New York City, the American inventor Thaddeus Hyatt noted a critical quality of the material: through some fantastic luck, the coefficients of thermal expansion of steel and concrete are strikingly similar, allowing a composite steel-concrete structure to withstand warm/cool cycles without fracturing. This quality opened up the floodgates, and in the 1880s the pioneering architect-engineer Ernest Ransome built a string of reinforced concrete structures around the San Francisco Bay Area. From there it was history.

More than any other physical technology, it is reinforced concrete that defines the 20th century. Versatile, strong, and (relatively) durable, the material is critical to life and industry as we know it. Reinforced concrete was the material of choice of Albert Kahn, who with Henry Ford defined 20th century industrial architecture; reinforced concrete is a key part of  nearly every type of logistical infrastructure, from roads to bridges to container terminals; reinforced concrete makes up the literal launch pads for human space travel. It’s a critical component of power plants, dams, wind turbines, and the vast majority of mid- to late-twentieth century homes and apartment buildings. Its high compressive strength makes it ideally suited for footings and foundations; its high tensile strength lets it cantilever and span great distances easily.

But reinforced concrete is really only 140 years old – the blink of an eye, as far as the infrastructure of old is concerned. The Pantheon was built around 125 CE, by which time the Romans had been experimenting with concrete construction for well over 300 years. When we see the Pantheon, we’re seeing a mature method – a technology with full readiness, being used in an architectural style that’s tuned for its physical properties.

By contrast, even our most iconic steel-reinforced concrete buildings are prototypes…

Early on in the history of steel-reinforced concrete, it was known that the high alkalinity of concrete helped to inhibit the rebar from rusting. The steel was said to be sealed within a monolithic block, safe from the elements and passivated by its high pH surroundings; it would ostensibly last a thousand years. But atmospheric carbon dioxide inevitably penetrates concrete, reacting with lime to produce calcium carbonate – and lowering its pH. At that point, the inevitable cracks and fissures allow the rebar inside to rust, whereupon it expands dramatically, cracking the concrete further and eventually breaking the entire structure apart.

This process – carbonatation, followed by corrosion and failure – was often visible but largely ignored into the late twentieth century. Failures in reinforced concrete structures were often blamed on shoddy construction, but the reality is that like the crayfish and the barbarae, the truce between concrete and steel is tentative. What protection concrete offers steel is slowly eaten away by carbonatation, and once it’s gone the steel splits the concrete apart from the inside…

There are of course many potential innovations to come in reinforced concrete. Concrete mixtures made with fly ash and slag produce high strength and durable structures. Rebar rust can be mitigated by using sacrificial anodes or impressed current. Rebar can be made of more weather resistant materials like aluminum bronze and fiberglass. Or the entire project could be scrapped – after all, the CO2 emitted by the cement industry is nothing to thumb your nose at. Whatever we do, we should remember that the materials we work with are under no obligation to get along with one another – and that a symbiotic truce today doesn’t necessarily mean structural integrity tomorrow.

On composites, crayfish, and reinforced concrete’s tentative alkalinity: “A Symbiotic Truce,” from Spencer Wright (@pencerw), whose newsletter, “The Prepared” (@the_prepared), is always an education.

* Gunter Grass

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As we delve into durability, we might recall that it was on this date in 315 that the Arch of Constantine officially opened. A triumphal arch in Rome dedicated to the emperor Constantine the Great, it was constructed of Roman concrete, faced with brick, and reveted in marble.

Roman concrete, like any concrete, consists of an aggregate and hydraulic mortar – a binder mixed with water (often sea water) that hardens over time. The aggregate varied, and included pieces of rock, ceramic tile, and brick rubble from the remains of previously demolished buildings. Gypsum and quicklime were used as binders, but volcanic dusts, called pozzolana or “pit sand”, were favored where they could be obtained. Pozzolana makes the concrete more resistant to salt water than modern-day concrete.

The strength and longevity of Roman marine concrete is understood to benefit from a reaction of seawater with a mixture of volcanic ash and quicklime to create a rare crystal called tobermorite, which may resist fracturing. As seawater percolated within the tiny cracks in the Roman concrete, it reacted with phillipsite naturally found in the volcanic rock and created aluminous tobermorite crystals. The result is a candidate for “the most durable building material in human history.” In contrast, as Wright notes above, modern concrete exposed to saltwater deteriorates within decades.

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