While concrete is used in a wide variety of today’s applications, many people don’t know the full history behind the material. Our patio paver installation company came across an interesting and in-depth article by Popular Mechanics that explains how the material came into existence and takes a look at its future:
“The story of concrete is so ancient that we don’t even know when and where it begins. It is a story of discovery, experimentation, and mystery. Emperors and kings became legends for erecting great concrete structures, some of which are still a mystery to engineers today. Many of history’s most skilled architects found inspiration in slabs of the gray building material. Common bricklayers advanced the technology, and a con man played a crucial role in the development of concrete recipes.
Today, the world is literally filled with concrete, from roads and sidewalks to bridges and dams. The word itself has become a synonym for something that is real and tangible. Press your handprints into the sidewalk and sign your name to history. This is the story of concrete.
The First Cement—and Maybe Concrete?
Let’s get this out of the way right here: cement and concrete are not the same thing. Cement, a mixture of powdered limestone and clay, is an ingredient in concrete along with water, sand, and gravel. Concrete’s invention was made possible by the development of cement, and to trace the history of cement, we must trace the use of its components.
The earliest known use of limestone in a structure has been dated back about 12,000 years. It was found in the Göbekli Tepe temple in modern-day Turkey. The historic temple suggests that perhaps humanity’s transition from nomadism to civilization was sparked not by agriculture, but by a desire to gather and worship in a great construction. Limestone made up the carved, T-shaped pillars of the Göbekli Tepe.
In the millennia that passed between that structure and the amazing concrete of Roman times, cultures around the world developed better building materials, some of which you might see as a kind of proto-concrete. Recently, for example, archaeologists have questioned whether an early form of concrete can be found in the Egyptian pyramids. The hypothesis holds that the Egyptians may not have hauled every building block to the pyramids, but that the blocks toward the top of the pyramids could have been cast in a mold just as we pour concrete into a mold today to give it its shape. However, most archaeologists believe there is no evidence that any blocks are made of an artificial material like concrete. Instead, it is widely believed that they are made of limestone, which may have naturally contained clay as well.
There is also no evidence that the Greeks used concrete. However, the Minoans of Crete did use an artificial building material for floors, foundations, and sewers, according to Robert Courland’s book Concrete Planet: The Strange and Fascinating Story of the World’s Most Common Man-Made Material. This Minoan material may not have been the concrete we know today, but it was a mixture of a similar sort. Clay was a major component, and a volcanic ash, today called pozzolana, was also used.
Pozzolana is derived from Pozzuoli, Italy, which is the site of Mount Vesuvius, whose eruption destroyed the Roman city of Pompeii in 79 AD. The same volcanic ash that covered that ancient city and froze its citizens in time also helped the Romans create the first known concrete in the world—and the strongest concrete humanity has ever seen.
The connection between Rome and concrete is so strong that we even take the name “concrete” from them. It’s derived from the Latin term concretus, meaning “to grow together,” just the way the components of concrete mix to form a solid building block. But the Romans didn’t refer to their concrete as “concretus.” In fact, they misleadingly called their concrete caementis, meaning “rocky stuff.” Caementis is of course the word that gave us “cement.”
Ancient Romans made concrete in much the same way we do today. They made cement by mixing kilned limestone with water. To thicken the mixture, they added the volcanic pozzolana, ground-up rocks, and sand. In a semi-liquefied state, the mixture was then poured into carved wooden molds to create smooth, sturdy pieces of concrete.
The Romans used concrete to build ramps, terraces, and roads. Pouring the mixture into molds allowed the Romans to build vaults, domes, and the arches of the empire’s great aqueducts. By the second century BC, the Romans began making walls out of concrete and coating it with brick masonry, which they did for two reasons. First, the ancient Romans preferred the aesthetics of brick to the gray slab of unadorned concrete. Second, after the Great Fire of Rome in 64 AD that destroyed 10 of the city’s 14 districts, concrete was revealed to be fire-resistant—though not fireproof. The outer brick helped in that regard.
What makes Roman concrete so impressive is its ability to endure substantial weathering, survive earthquakes, and withstand crashing waves in the sea. Consider one of the first great Roman projects.
Concrete’s rise to prominence within the Empire began with the daring engineering feat of Sebastos Harbor, in Caesarea, Israel. The year was 23 BC, a time when concrete was still a largely unproven material. King Herod of Judea, whose land was a territory of the Roman Empire, wanted to improve his kingdom’s economy. What better way than to build a port on the shores of the Mediterranean Sea? It was the perfect test of concrete’s resilience.
Construction of the harbor took eight years. The result was one of the largest harbors in the world, second only to that of Alexandria in Egypt. The jetties and seawalls were made of pure concrete, likely lowered into the water with a crane. Divers—holding their breath—went into the Mediterranean to make adjustments to the structures’ positioning. Once properly aligned, each heavy piece of concrete was tamped down. The city of Caesarea finished construction five years after the harbor was completed, and the thriving port earned King Herod the title “Herod the Great.”
More than 2,000 years later, the concrete harbor is still intact. You just can’t see it from the land. Sebastos Harbor was built directly atop a fault. Earthquakes struck every few centuries, causing the jetties and seawalls to slowly submerge under the Mediterranean. But Sebastos Harbor was only the beginning. The Romans would go on to erect some of the most famous concrete structures in the world.
The Peak of Roman Concrete
After the fire of 64 AD and the death of Emperor Nero four years later, civil war came to Rome. The victor was general Flavius Vespasianus, better known as Vespasian. After becoming emperor, he set out to build the largest theater in the world. He would call it the Flavian Ampitheater, and it would hold more than 50,000 spectators and provide a full view of the events from every seat. It was the world’s first stadium. Today we call it the Colosseum.
The Roman Colosseum is an elliptical structure measuring 615 feet long and 157 feet high, with a base area of about 6 acres. It has 80 entrances, four of which were for VIPs, and one for the emperor. The Colosseum was completed 1,937 years ago, and it stands today as one of the enduring symbols of the Roman Empire—and more literally as a testament to the endurance of Roman concrete.
The Colosseum is not made entirely of concrete, however. Disproportionate quantities of brick and concrete can be found throughout the arena. Estimates of the amount of concrete have ranged widely, from 6,000 metric tons to 653,000 metric tons, according to Concrete Planet. However, about 80 percent of the concrete was used for the foundations, so it stands to reason that 6,000 metric tons is lowballing the estimate significantly. But it’s difficult to say for sure. After all the bumps and bruises and earthquakes and lightning strikes that the structure has endured over the course of two millennia, what we have left today is only about a third of the original construction.
The most pristine ancient concrete structure in Rome, however, was not built for the people, but for the gods. After 1800 years, the Pantheon is as sturdy as ever. The engineers who constructed the great temple of Rome were far ahead of their time—perhaps even ahead of our time.
The Pantheon was Emperor Hadrian’s brainchild. Hadrian was always intrigued by architecture, and when he became emperor in 117 AD he wanted to build the Empire’s grandest structure as a testament to the gods. He would do so with the largest dome the world had ever seen.
It was a risky enterprise. The Pantheon’s dome would span 143 feet. It was twice as wide and high as any dome ever created. The concrete was poured into a curved wooden mold, a perfect half sphere, propped up on scaffolding. Once the scaffolding was removed, the walls alone had to endure the pressure of the gargantuan concrete roof, which was immense even with the famed oculus in the dome’s center relieving some of the load.
Roman engineers built those concrete walls incredibly thick and covered them with brick on the interior and exterior. On the interior, the bricks were laid to construct relieving arches to take stress away from the walls. Eight barrel vaults also relieve stress, creating inset galleries for the faithful to stand before statues of the gods. An extra layer of brick was placed on the ground along the exterior perimeter of the building.
In other words, the walls were tremendously reinforced, and incredibly, the dome was not. Today’s engineers wouldn’t dare build an unreinforced dome of that size. Such a structure with today’s concrete would be in constant danger of collapsing.
How, then, did Hadrian and his engineers pull it off? They tinkered with the concrete recipes. The dome contained a bit more volcanic ash than rock to make it slightly lighter, while the walls contained much more rock aggregate to make them heavy and strong.
But to this day we still don’t know all the secrets of the Pantheon. The most comprehensive surviving text on Roman concrete is Vitruvius’s On Architecture. However, that volume predates the construction of the Pantheon by about 150 years. When the Western Roman Empire officially fell in 476 AD, the recipe for the Pantheon’s concrete was lost to history.
It took about a thousand years for concrete to make a comeback. Europe went through the Dark Ages, and ancient Roman texts were not rediscovered until the Renaissance. Renaissance engineers studied Vitruvius’s On Architecture, but with no knowledge of the mysterious gray building material, scholars had a tough time deciphering Vitruvius’ terminology. Only an Italian friar named Giovanni Giocondo was able to crack the code.
Giocondo was trained in archaeology and architecture, and he noticed something impressive about caementis. Its resistance to weathering suggested it must be hydraulic, meaning it hardens under water. Concrete, Giocondo thought, must be replicated.
And so Giocondo built structures that mixed lime and pozzolana, as Vitruvius instructed. His first attempt was the original Pont Notre-Dame Bridge. Houses were built atop the bridge, but about 250 years after the structure was completed, the entire thing was demolished. The houses put too much stress on this primitive version of concrete, and Giocondo’s efforts would go down in history as the only attempt to create concrete during the Renaissance. But bigger breakthroughs were on the horizon.
In the 16th century, trass—a volcanic ash similar to pozzolana—was discovered as a useful material for making tools in Andernach, Germany. A bricklayer tried using the ash in lime mortar, a mixture quite similar to concrete, and learned that the resulting material was stronger and water resistant. The result was a chain reaction that led to the creation of modern cement.
In the 17th century, the Dutch began selling trass to France and Britain. The trass was used for buildings that required hydraulic properties. In constant conflict and competition, France and Britain began efforts to create their own hydraulic building materials. The British had the advantage, though. They had John Smeaton.
In the mid-1750s, Smeaton was commissioned to build a lighthouse on a troublesome perch on the Eddystone Rocks, just off the southern coast of England. Three lighthouses on the site had all been destroyed. One couldn’t survive the winter. The second collapsed during a hurricane. The last, which had a wooden interior, caught fire from the light and burned to the ground. Smeaton, up to the challenge, was determined to build the strongest lighthouse in the world.
The English civil engineer experimented with known hydraulic materials. He rolled up balls of lime (the cooked version of limestone) and trass and dropped them into boiling water. The lime on its own dissolved, but the lime that came into contact with trass endured. Smeaton then tested limestone from a town called Aberthaw, dropping it into water and a nitric acid solution used to separate minerals. The experiment revealed that about a tenth of the limestone from Aberthaw contained clay. Smeaton took note of the high strength of this limestone-clay conglomerate. Today we call the same material natural cement.
The lighthouse was constructed between 1756 and 1759 using Smeaton’s hydraulic cement-filled mortar. It stood on the Eddystone Rocks for more than a century before the rocks began to erode. In 1882, the lighthouse was disassembled and rebuilt in Plymouth, England.
Every businessman in Britain wanted to capitalize on the new building material. For marketing purposes, manufacturers started referring to their natural cement as “Roman cement.” Deception in the concrete business would follow, and in a stroke of luck, lead to even sturdier materials.
The Con Man
Joseph Aspdin was a bricklayer from Leeds, England. In the 1820s, he would walk to the paved roads of town and steal bricks of limestone. He was fined twice, but that didn’t stop the limestone thief from making off with the bricks for his materials science tests.
The historical record is a little spotty, but we know that Aspdin managed to invent his own cement mixture. He named it “Portland cement” after the limestone-clad Isle of Portland. Like the term “Roman cement,” the name “Portland cement” became a marketing scheme. But Joseph Aspdin was not the con man—his son William was.
Around this time there was a common engineering practice called slurry mixing, in which powdered (but un-kilned) limestone was mixed with clay and water. The concoction turned into a paste. The paste was then kilned into a solid and crushed, turning it into cement powder. If the paste was kilned too long, the resulting material, called “clinker,” was generally thrown out.
William Aspdin decided he’d test the unwanted scraps of clinker. As a young man, William left his father to find his own way in London. He began taking the clinker off cement makers’ hands. He had no employees, no kiln. All he did was whack the clinker with a hammer to break it down. Once clinker was mixed with the other cement materials, the result was a new cement that, years later, an independent firm would confirm to be twice as strong as “Roman cement.”
William Aspdin had created a cement that was better than the rest, and yet he proceeded to find investors who knew nothing about the cement industry, egregiously lie about his product to the public, swindle his partners, and start all over again.
In a circular for his first cement firm, William set out to establish the validity of his unproven product. He wrote that his cement had been around for years in Northern England, fraudulently claiming his own “Portland cement” was the same recipe his father had developed.
William’s firm managed to buy out another cement factory, yet within a year the company went bankrupt. William found new inexperienced investors and began a new firm using one of his previous factories. He published more lies. This time he claimed that his father’s Portland cement had been around since 1821, and that it was used in one of England’s most grueling construction projects: the Thames Tunnel. Several men died due to flooding during the then-recent construction project. William claimed that workers used Portland cement to patch up holes when the river leaked into the tunnel. The reality behind these stories is that the holes were patched with clay, and in 1821, Joseph Aspdin was still stealing limestone from the street.
Records on how William’s second partnership ended are also scarce, but his third business enterprise is well-documented. One day, the board of directors granted William 300 GBP to invest in the factory. William was instructed to buy a steam engine—and he did, for only 80 GBP. The rest of the money went into William’s pocket.
Upon learning that he forged the receipt, the board of directors investigated William. They soon learned that he had swindled them from the beginning. He had embezzled funds allocated to the firm. He created records of fake employees and took home the salaries. William was out.
But not for long. He quickly found yet another investor and started a fourth cement business. Around this time—in the 1850s—competitive firms tried to figure out the recipe for his Portland cement. To hide his secret of over-kilning the cement mixture, William displayed different chemicals on the open floor of his factory for everyone to see. Nevertheless, William eventually stopped paying rent on the factory and was arrested for longstanding debts. His fourth partnership ended. He moved to Germany and bounced around from cement business to cement business before falling and hitting his head. He died in 1864 at age 48.
A century and a half later, we still use the con man’s Portland cement.
The Birth of Modern Concrete
In the mid-1800s, most industrialized countries were making Portland cement on their own. Around this time, the United States, Britain, and France each had the same idea to increase concrete’s tensile strength, or its ability to resist an exerted force. Concrete could be poured over iron bars to form reinforced concrete.
In the 1880s, a California-based engineer named Ernest Ransome was starting his own construction firm. Ransome noticed that reinforced concrete tended to crack, subsequently weakening significantly. He decided to experiment with the reinforcement bars, using 2-inch iron rods to see if they’d bond with the concrete. The experiment was a success. Ransome then tried twisting the iron bars in accordance with the concrete’s desired shape. It worked like a charm. The engineer called his idea the Ransome system. Today we call it reinforcing bar, or rebar, and modern engineers typically use steel.
Ransome’s first rebar concrete building was the Arctic Oil Company Works warehouse in San Francisco, completed in 1884. It was demolished around 1930. Ransome later built the Alvord Lake Bridge, the world’s oldest surviving reinforced concrete structure, also in San Francisco. In 1903, construction was completed on the world’s first concrete skyscraper, the 16-story Ingalls Building in Cincinnati. Ransome himself was not involved in the skyscraper’s construction, but it would not have been possible without his reinforcing bar method.