We've been building with it for several millennia. Following the discovery of rebar, our imaginations soared. Only recently have we begun understanding its environmental cost.
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When you see the name Thomas Alva Edison, what’s the first thing to come to mind?
Lightbulb? Phonograph? Battery? Telegraph? Or maybe motion picture camera?
All are correct. He either invented or worked on all these technological wonders. An indefatigable inventor, Edison had 1,091 patents to his name, not counting the hundreds of incomplete or rejected applications lying around.
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Edison invented or improved quite a few gadgets. Some successes, some flops, and some too ahead of their time. And that might explain why one of his passions is not widely known.
Among those patents are 49 that concern a particular building material: concrete. The second-most used material on the earth (water handily comes in first place), it’s no wonder why he fancied it. And if Edison were alive today, there’s a good chance that he’d be amazed not only by how far concrete has come but also by how far it still has to go.
The basics
First, cement and concrete are not the same thing. Well, they are if sand and glass are the same thing. Which they aren’t.
Second, there are only three essential ingredients to make concrete:
Cement + Aggregates + Water = Concrete
A quick history lesson
This is probably why ancient peoples in the Middle East used concrete 12,000 years ago. Not widespread by any means, but builders used it in floors, cisterns, and walls. By around 3000 BC, the Egyptians were mixing lime and gypsum to make mortar—the Great Pyramid of Giza used about 500,000 tons of the stuff.
Even so, concrete came into fashion during the Roman Empire. By mixing volcanic ash and rocks, lime, and water, the Romans produced a kind of concrete that was strong enough to build bathhouses, aqueducts, the Colosseum, and even the Pantheon, the largest nonreinforced dome in the world, created 19 centuries ago.
Romans tweaked the recipe using seawater instead of freshwater to construct seawalls and docks. This seemingly simple substitution made the concrete super hard, as described by the Roman author and naturalist Pliny the Elder in Naturalis Historia (c. AD 79): “[A]s soon as [the concrete] comes into contact with the waves of the sea and is submerged, [it] becomes a single stone mass, impregnable to the waves and every day stronger.” He wasn’t hyperbolizing. Many of those seawalls and docks are still standing. Only within the last two decades have scientists discovered the secret to their resiliency: over the previous two millennia, the volcanic mix has been chemically reacting with the seawater, producing concrete that’s as nearly hard as granite.
With the development of reinforced concrete, builders scaled heights that were unreachable by wood and stone.
The Romans themselves weren’t as resilient as their concrete; when their empire fell in the fifth century, so did concrete’s popularity. However, that changed in the 19th century, following two significant developments.
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The first was in the 1820s, when Joseph Aspdin received a patent for modern cement. He mixed finely ground limestone with clay, fired it in a kiln to produce clinker, and ground it into a powder. The result was a modern cement that, when mixed with water and aggregates, hardened into durable concrete nearly impermeable to water. The second came about 30 years later, when French builders reinforced concrete with metal rods (rebar). With the development of reinforced concrete, builders scaled heights that were unreachable by wood and stone.
After that, concrete came into its own—in homes, small buildings, streets, and even bridges. The first skyscraper—the 16-story Ingalls Building in Cincinnati, Ohio—appeared in 1903. The Empire State Building followed in 1930. Six years later, the Hoover Dam began operating. Concrete helped rebuild Europe after World War II, pave the US during the 1960s, and expand cities worldwide.
Rebar makes the impossible possible. But, because steel contains iron, it’s prone to rust. So, it also makes the possible corrodible and collapsible.
All that glitters …
Despite all its durability, modern concrete lasts for at best a century, sometimes half that, depending on the structure. Less than a century. Twenty times less than Roman concrete.
Rebar makes the impossible possible. But … it’s prone to rust. So, it also makes the possible corrodible and collapsible.
Concrete doesn’t last because the very thing that gives it tensile strength—rebar—is also its greatest weakness. Made of steel, rebar can be bent and laid out into grid-like patterns, welded, and set into molds. Because of this, we can build gravity-defying structures that twist and arc as if carved out of marble.
As concrete ages, tiny fractures form within it. These fractures combine, creating microcracks that web between the aggregate materials. When moisture pushes its way inward and encounters the rebar, it produces an electrochemical reaction resulting in iron oxide (aka rust). This leads to oxide jacking, in which the rust expands, stressing the surrounding concrete. Weakened by the microcracks, concrete begins spalling—flaking off bit by bit, exposing more of its surface area (and the underlying rebar) to the elements. If left untreated—sometimes despite treatment—this damage can spread quickly, further impairing the structure.
There’s another side to concrete, which comes with a hefty environmental price tag. About 4.4 billion metric tons of concrete are produced every year. Globally, its production accounts for up to 8 percent of total carbon dioxide emissions, primarily because of the energy-intensive clinker process. We gobble up 40 to 50 billion metric tons of sand annually to produce this much concrete. And now the unthinkable is happening: some parts of the world are running out of sand.
When concrete reaches its end of life, other issues crop up. Every year, it accounts for two-thirds of the total construction and demolition waste produced in the US and nearly half of all landfill waste. Other countries, like Brazil and China, face a similar issue.
Concrete certainly comes with some hefty, climate-inducing baggage for such a fabulous building material.
Edison's single molding operation
In the months following the devastating 1906 San Francisco earthquake, Edison began looking for a solution to construct tremor- and fireproof buildings and homes. He also wanted to make them easier to build and more affordable to the masses. To him, at least, the answer was obvious: concrete. Having studied it for a quarter-century and founding three different companies to exploit it, the most recent the Edison Portland Cement Company in 1899, he was knowledgeable about its properties.
In 1908 he filed a patent for a “single molding operation” to form “all [a home’s] parts, including the sides, roof, partitions, bath tubs, floors, etc., being formed of an integral mass of cement mixture.” By using a double-walled cast-iron mold, builders poured an entire house “in the same molding operation,” working continuously from start to finish.
Concrete homes were nothing new. Once rebar came into the picture, concrete homes appeared in France and England and then elsewhere. However, those homes were custom-built and expensive. The only way to make them more affordable was to build them on an industrial scale, and Edison’s process did just that.
On paper, at least. The mold for a single home required more than 2,000 pieces at the cost of around $175,000. This cost didn’t include the time spent setting up and taking down the mold, much less pouring the concrete. So, to sell a home for $1,200, as Edison envisioned, a builder would have to construct lots and lots of homes to turn a profit, if one was even achievable.
Nuts and bolts
Although Edison’s process built close to 90 buildings, it failed to take off, despite the ever-growing popularity of concrete. Even so, his fascination with concrete is understandable. Its pliability pushes us to design ever more fantastical architecture. Its resiliency allows us to build anywhere we desire. And its versatility permits us to tweak its simple list of ingredients to produce concrete for multitudes of applications.
But there’s no way around it. Concrete is dirty, energy-intensive, and a significant emitter of greenhouse gas emissions. We shouldn’t ignore or overlook these facts. However, just as Edison recognized more than a century ago, concrete also has lots of potential. I explore some of that potential in Self-Healing Concrete. What Is It, and Will It Help Fight Climate Change?
Suggested reading
Keulemans, Guy. “The Problem with Reinforced Concrete.” The Conversation, June 17, 2016.
Kiger, Patrick J. “6 Key Inventions by Thomas Edison.” History.com, March 6, 2020.
“Life of Thomas Alva Edison.” Library on Congress, no date.
Van Mead, Nick. “A Brief History of Concrete: From 10,000BC to 3D Printed Houses.” Guardian (London), February 25, 2019.
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