Concrete Planet

1. Lime's Discovery Predates Agriculture and the Wheel

In a way, the story of concrete is also the story of civilization: its roots reach back to prehistoric times and even predate agriculture and the wheel.

Early human societies. The discovery of lime, a key ingredient in concrete, occurred towards the end of the Paleolithic Age, approximately twelve thousand years ago. This predates the Neolithic Revolution, which saw the rise of agriculture and the wheel.

Campfire scenario. The most popular theory suggests that lime was discovered when hunter-gatherers built a fire on a limestone outcrop. The heat transformed the calcium carbonate in the limestone into calcium oxide, or quicklime. When water was added to the quicklime, it produced a chemical reaction that generated heat and formed calcium hydroxide, which then hardened into a rock-like substance.

Technological and societal revolutions. The discovery of lime may have played a significant role in the technological and societal revolutions that led to the Neolithic Age. It provided early humans with a versatile building material and may have fostered the development of intertribal communities based on shared belief systems and rituals.

2. Ancient Concrete Reveals Advanced Neolithic Chemistry

Indeed, it is possible that the revolutions that led to the Neolithic Age may have begun with the invention of concrete, and that lime's discovery was far stranger and more interesting than anyone had previously supposed.

Göbekli Tepe. Archaeological evidence from sites like Göbekli Tepe in southeastern Turkey suggests that lime concrete was used in the construction of monumental structures as early as 11,600 years ago. This challenges the traditional view that agriculture was the primary driver of Neolithic societal changes.

Pyrotechnologies. The discovery of lime required the development of high-temperature kilns, representing humankind's first use of complex chemistry and industrial processes. These pyrotechnologies paved the way for other key innovations, such as fired ceramics and metallurgy.

Lightning strikes. One theory suggests that the discovery of lime was inspired by the observation of lightning strikes on limestone outcrops. The intense heat of the lightning would have transformed the calcium carbonate into lime, which could then be experimented with by early humans.

3. Romans Mastered Hydraulic Concrete for Enduring Structures

The Romans were the first people to realize concrete's potential, and they used it to erect dazzling buildings that stand to this day.

Roman concrete. The Romans rediscovered hydraulic concrete, a type of concrete that can set underwater, and used it extensively in their construction projects. This allowed them to build durable structures like aqueducts, bridges, and harbors.

Pozzolana. The key ingredient in Roman hydraulic concrete was pozzolana, a volcanic ash that reacted with lime to create a strong and water-resistant material. The Romans obtained pozzolana from the region around Mount Vesuvius in Italy.

Engineering skills. The Romans were skilled engineers who understood the properties of concrete and how to use it effectively. They systematized its production and application, and they were the first people to utilize concrete as we do today: putting it into large molds to create a strong monolithic architectural unit.

4. Concrete's Role in Roman Expansion and Innovation

In a way, the story of concrete is also the story of civilization...

Practical applications. The Romans used concrete for a wide range of practical purposes, including building roads, aqueducts, and public baths. These infrastructure projects improved the quality of life for people throughout the empire.

Military success. Some historians believe that Rome's lime-based technologies contributed to its success in conquering a large part of Western Europe. Roman forts and fortifications were more durable and resistant to attack than those of their enemies.

Legal system. The Romans held a transcendental respect for their legal system, which was not only accessible to all Roman citizens but to noncitizens as well. This love of stability and sensible nature contributed to their engineering skills, which they exercised toward practical ends.

5. The Great Pyramid Controversy: Concrete or Limestone?

If this is true, the pyramids represent the greatest volume of concrete manufactured and applied to a single engineering project until the construction of the Panama Canal some twenty-four centuries later.

Cast-concrete-block hypothesis. Some researchers, like Michel W. Barsoum and Joseph Davidovits, have proposed that portions of the Egyptian pyramids, particularly the Great Pyramid of Giza, were constructed using cast concrete blocks rather than quarried limestone.

Geopolymer concrete. Davidovits suggests that the Egyptians used a type of eco-friendly concrete called geopolymer concrete, which is made from clay, lime, and natron. He believes that this concrete was cast in place, eliminating the need for elaborate hoisting equipment.

Egyptologists' counterarguments. Egyptologists dispute the cast-concrete-block hypothesis, citing a lack of historical evidence and the discovery of quarry markings on many of the pyramid blocks. They also point out that the amount of stone removed from nearby quarries is roughly equivalent to that used for all monuments in the area.

6. Herod's Harbor: A Testament to Roman Concrete Engineering

The first large-scale use of Roman concrete did not take place in Rome, or even in Italy, but 2,300 km (1,400 miles) to the east, in Judea.

Herod the Great. Herod the Great, king of Judea, used Roman concrete extensively in the construction of the city and harbor of Caesarea. This was the largest application of hydraulic concrete in a single construction project until the early twentieth century.

Strategic importance. Herod built Caesarea to create a major international harbor that could rival Alexandria in Egypt. The harbor would serve as a safe haven for Roman grain ships and facilitate trade between the East and West.

Engineering challenges. The construction of Caesarea's harbor faced numerous engineering challenges, including a lack of natural protection from the elements and a sandy seabed. Roman concrete was essential for overcoming these obstacles.

7. The Pantheon: A Pinnacle of Roman Concrete Architecture

…go to Rome and try to break old Roman concrete with an axe; you will only dent the steel.

Architectural marvel. The Roman Pantheon is one of the most remarkable buildings in the world, renowned for its massive concrete dome. The dome is the largest unreinforced concrete dome in existence and has stood for nearly two millennia.

Engineering ingenuity. The Romans used a variety of techniques to reduce the weight and stress on the Pantheon's dome, including using lighter aggregates in the upper sections and incorporating coffers into the dome's interior surface.

Aesthetic and spiritual design. The Pantheon was designed to be both beautiful and awe-inspiring. The dome was meant to represent the heavens, and the oculus at its center allowed sunlight to stream into the building, creating a dramatic effect.

8. Mesoamerican Concrete: A Parallel Development

By the first centuries CE, the Mesoamerican cultures were using a tremendous amount of lime for plaster (including types that served as a mural base) and stucco, which they lavishly applied to their public buildings.

Olmec civilization. The Olmec civilization in Mesoamerica, which thrived between 1500 and 400 BCE, was one of the first to use lime for plaster. This knowledge was later adopted and expanded upon by other Mesoamerican cultures, such as the Maya and the Aztecs.

Lime plaster and stucco. The Maya and Aztecs used large quantities of lime to create plaster and stucco, which they used to decorate their buildings and create murals. The production of lime required significant deforestation, which may have contributed to the decline of some Mesoamerican civilizations.

Limited use of concrete. While the Mesoamericans used lime extensively, they did not develop concrete to the same extent as the Romans. They occasionally made concrete beams for post and lintel structures, but surviving examples are somewhat crude.

9. The Rediscovery of Concrete in the Industrial Age

As odd as it may seem, the archaeological evidence shows that the earliest ovens created by humans were not low-temperature affairs for baking or roasting food but rather high-temperature limekilns.

Fra Giocondo. The rediscovery of Vitruvius's "On Architecture" in the 15th century sparked renewed interest in Roman concrete. Fra Giocondo, a Franciscan monk, used Roman concrete to build the Pont Notre-Dame in Paris, marking the first time in many centuries that the material had been employed in a construction project.

John Smeaton. In the 18th century, British engineer John Smeaton rediscovered natural cement while building the Eddystone Lighthouse. He found that limestone containing clay produced a hydraulic mortar that was stronger and more durable than pure lime mortar.

Joseph Aspdin. In 1824, Joseph Aspdin patented Portland cement, a type of cement made by burning a mixture of limestone and clay. Portland cement would eventually become the most widely used type of cement in the world.

10. The Dark Side of Reinforced Concrete: Corrosion and Decay

As we initially rushed to use concrete to build almost everything, we were still ignorant of the limitations of this novel substance.

Limited lifespan. Despite its widespread use, reinforced concrete is not a permanent building material. The steel reinforcement bars within concrete are susceptible to corrosion, which can cause the concrete to crack and crumble.

Corrosion process. The corrosion of steel rebar is an electrochemical process that is accelerated by the presence of water, air, and chlorides. The expansion of the corroding rebar can exert tremendous pressure on the surrounding concrete, leading to structural failure.

Infrastructure crisis. The widespread use of reinforced concrete has created an infrastructure crisis, as many bridges, roads, and buildings are now in need of repair or replacement. The cost of repairing or replacing this infrastructure is estimated to be trillions of dollars.

11. Green Cements and Alternative Reinforcement Materials

An important controversy found within these pages is the idea that concrete has a far longer lifespan when not reinforced by steel rods (called rebar), and that alternative materials for producing rebar should allow the building of structures with a thousand-year lifespan instead of a single century, facts that the author writes about here convincingly.

Fly ash and slag. One solution to the environmental and durability problems of concrete is to use green cements made from fly ash and slag, byproducts of the steel and coal industries. These materials can replace a significant portion of the limestone and clay used in conventional Portland cement.

Nonferrous rebar. Another approach is to use nonferrous rebar made from materials like glass fiber reinforced polymer (GFRP) or aluminum bronze alloys. These materials are resistant to corrosion and can significantly extend the lifespan of reinforced concrete structures.

Unreinforced concrete. In some applications, such as roads and highways, it may be possible to use unreinforced concrete. While unreinforced concrete is more prone to cracking, it is also less expensive to maintain and does not suffer from the problem of rebar corrosion.