2021-07-21 7 min read

A Symbiotic Truce

A Symbiotic Truce

On composites, crayfish, and reinforced concrete's tentative alkalinity.

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.

Spalling in a reinforced concrete balcony at the Champlain Towers North, as noted by Morabito Consultants in a 2018 report.

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.

The Coignet Stone Company Building, in Brooklyn's Gowanus neighborhood, was built by one of the inventors of reinforced concrete and is the one of the oldest concrete buildings in the country.

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.

US consumption and production of portland cement, as compiled by USGS.

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. Frank Lloyd Wright’s Unity Temple was started in 1905, just 25 years after reinforced concrete was in widespread use and five years before the American Concrete Institute published its first code on construction techniques. The Holland Tunnel was started in 1920; the Hoover Dam in 1931. Genoa’s Ponte Morandi bridge was started in 1963, when the oldest reinforced concrete structures in the world were only 90 years old. The Champlain Towers North, a mid-rise condo in Surfside, Florida, was completed in 1981 - just a hundred years into the modern concrete age, during a period in which there was a ton of ongoing research into the material. While much can be said about each of these projects, the reality is that their engineers had incomplete information about how long reinforced concrete could be counted on to last.

Concrete spalling. Credit: iStock/Photawa

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.

The Ponte Morandi after its 2018 collapse; the reinforced stays can be seen on the right. Credit: iStock/ Luca Reina Mafaraci

So what of the iconic reinforced concrete structures that stand today? Many factors affect the carbonatation process, and chloride attack, and the various other failure modes that concrete exhibits. But the reality is that we don’t know how long any given symbiotic truce will last - and we can only learn more by observing new failures. The exterior of Brooklyn’s Coignet building was refurbished as part of the development of the Gowanus Whole Foods; today it can be had for around $4M with a gut-reno-ready interior. Frank Lloyd Wright’s Unity Temple, whose inflation-adjusted construction cost was just over $1M, underwent a $25M restoration starting in 2015. Other architectural gems have undergone similar work, including Ponte Morandi - which helped to define the Genoese skyline and a generation of Italian engineering. In the 1990s, the easternmost reinforced concrete stays on the bridge were retrofitted with additional steel cables. But the westernmost stays weren’t, and in 2018 they collapsed, killing 43 people. Champlain Towers North, a more quotidian building, wasn’t nearly so lucky. Its 2018 inspection report states plainly that “most of the concrete deterioration needs to be repaired in a timely fashion,” a recommendation that (depending on your perspective) was either not severe enough, or simply ignored, or both.

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.

For anyone interested in learning more about concrete, I recommend Robert Courland’s 2011 book Concrete Planet; thanks to Felix for recommending it to me. I also loved the NYTimes’ investigation on the Ponte Morandi’s collapse, and enjoyed browsing the Federal Highway Administration’s list of bridges by construction material and date, and highly recommend David Owen’s 2018 piece on the global shortage of sand. Big hat tips also to everyone in the Members’ Slack, the WITI Slack, and on both Twitter and Instagram, who answered my requests for human-engineered mutually beneficial relationships between two other species. I especially liked thinking of aquaponic systems, invasive species, plant grafts, kombucha, and synthetic lichens.

Spencer Wright
Spencer Wright
Spencer Wright is the (mostly accidental) founder of Scope of Work, which he started writing (as The Prepared) in 2013. Today he serves as its editor-in-chief and chief dilettante.
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