Concrete has evolved from lime mortar in ancient Rome to modern, high-tech carbon fiber concrete, used in a variety of structures including homes, roads, and buildings. The invention of cement and innovations such as rebar reinforcement and prestressing techniques have made concrete an even stronger and more versatile building material.
Today, it’s hard to find a place on Earth that hasn’t been touched by concrete. You can almost judge the arrival of modern civilization by its presence or absence. You can find it in houses, bridges, roads, buildings, dams, and more. We see this ubiquitous substance as a gray, hardened finished product, but it’s not always easy to understand the ingredients and processes that go into making it. If we take a look at the history of concrete, where it came from, and how it evolved to become what it is today, we can see that it has been a part of human history all over the world, from ancient to modern times. But first, let’s take a look at what concrete actually is.
Concrete is a mixture of cement, sand, gravel, aggregates, and other materials mixed with water. In other words, to know concrete, you need to know cement. In a broader sense, glue, bonds, and even solder are essentially cement, but in general, cement is a mineral bonding agent used in construction and engineering. Cement binds materials together, so its role in concrete is to bind aggregate and aggregate together into a single mass. The answer to cement’s ability to act as a binder lies in the hydration reaction. The hydration reaction is a chemical reaction between cement and water that initially results in condensation that develops into new mineralized structures, followed by hardening that gives it strength, or in other words, when cement and water mix and harden into a new substance. The heat of hydration is what causes concrete to crack.
But have concrete and cement been together since the beginning? What did concrete look like in the beginning? The origins of concrete can be traced back to ancient Rome, where the word comes from the Latin word concretus. Lime mortar, a mixture of lime, sand, and water, is what the Romans used to make concrete. However, lime mortar crumbles easily when moisture is released. Later, they used pozzolana, a mixture of volcanic ash and lime. This was used for roads, walls, aqueducts, houses, palaces, and many other structures during the Roman era because of its ability to harden in water. In particular, the dome of the Pantheon has traces of formwork and aggregate inside, which is the original concrete. Modern concrete began to develop in earnest in the 18th century when John Smeaton, a British architectural engineer, invented hydroponic lime, which is made by heating limestone with clay.
Later, in 1824, British architect Joseph Aspdin created the first artificial cement by baking ground clay and limestone together. Each time he made this cement, the mixture overheated and formed a hard lump, and he initially discarded it as useless. But in 1845, Isaac Johnson realized that grinding the lumps would produce a better quality cement. This is the Portland cement we use today. Its simple manufacturing process, high quality, and easy-to-find raw materials quickly spread the cement around the world, and with it, concrete, the main construction material.
Concrete has a fatal flaw: it’s resistant to pressure but weak in tension. To compensate for this, iron is used. Iron is strong in both compression and tension. Imagine a long concrete block used as a beam, curving downward in the center. The top part of the concrete beam will be under pressure and will be able to withstand it, but the bottom part will crack due to tensile forces. To compensate for this, an iron core is driven into the bottom. In 1867, French gardener Joseph Monnier patented a concrete planter reinforced with wire mesh, and reinforced concrete continued to be used in bridges, staircases, and railroad viaducts. Reinforced concrete became a major player in the world of architecture when Könen and Weiss of Germany formalized the theory of reinforced concrete construction in 1887, and it became the driving force behind the rise of the skyscraper.
Even after the invention of reinforced concrete, buildings were still prone to cracking due to the weak tensile strength of concrete. This is further accelerated by the building’s load, and even rainwater can enter through the cracks, allowing moisture and salts to come into contact with the rebar, causing corrosion. Prestressed concrete was created to address these shortcomings. It literally means to apply stress, or force, in advance. It solves the underlying problem of cracking concrete by creating stress during the manufacturing process before the building is loaded. It was heavily utilized and spread during the post-war recovery of World War II. It also led to the invention of ready-mixed concrete, which greatly increased its efficiency and usage. This cement, called ready-mixed concrete for short, literally means “pre-mixed” homogeneous concrete delivered uncured, which greatly reduced time. This was made possible thanks to the invention of the truck mixer.
The evolution of concrete hasn’t stopped in modern times. In addition to rebar, special concrete with high-tech carbon fibers is used to pave roads and runways, and Japan has developed super-flowable concrete for LNG tanks. There is even translucent concrete, such as the one installed at the National Building Museum in Washington, D.C., or concrete that is 30% poreless, allowing water to be absorbed and plants to grow. Its water-absorbing properties make it a great material for flood prevention, and it can also be used as a soundproofing and insulation material using its own air layer. Nowadays, concrete is being used to build skyscrapers that can withstand earthquakes with concrete that is dense enough to float on water, but strong enough to withstand high temperatures.
