I Can’t Sleep - Concrete | Can’t Sleep? Learn About the Foundation of Modern Cities

Episode Date: June 29, 2026

Concrete has shaped the modern world in ways most people rarely notice. This episode explores the history of concrete, how it is made, why it became one of the most widely used building materials on Earth, and how engineers continue to use it to create everything from roads and bridges to dams and skyscrapers. Along the way, you’ll hear about ancient Roman construction, cement production, reinforced concrete, and the science behind structures designed to last for generations. It’s steady and consistent, with no whispering and no sudden changes, just enough to give your mind something to follow as you wind down. Happy sleeping! Read with permission from Concrete, Wikipedia (https://en.wikipedia.org/wiki/Concrete), licensed under CC BY-SA 4.0. — Ad-free episodes: ⁠icantsleep.supportingcast.fm⁠Have a topic in mind? ⁠Request a topic⁠ Learn more about your ad choices. Visit ⁠megaphone.fm/adchoices Learn more about your ad choices. Visit megaphone.fm/adchoices

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Wayfair, every style, every home. Welcome to the I Can't Sleep podcast, where I help you drift off one fact at a time. I'm your host, Benjamin Boster. In today's episode is about concrete. Concrete is a composite material, composed of aggregate, bound together with a fluid cement, that cures to a solid. It is the second most used substance after water, the most widely used building material,

and the most manufactured material in the world. When aggregate is mixed with dry port, Portland cement and water, the mixture forms a fluid slurry that can be poured and molded into shape. The cement reacts with the water through a process called hydration, which hardens it after several hours to form a solid matrix that binds the materials together into a durable stone-like material with various uses. This time allows concrete to not only be cast in forms, but also to have a variety of tooled processes performed.

The hydration process is exothermic, which means that ambient temperature plays a significant role in how long it takes concrete to set. Often additives are included in the mixture to improve the physical properties of the wet mix. or accelerate the curing time, or otherwise modify the finished material. Most structural concrete is poured with reinforcing materials, such as steel rebar, embedded to provide tensile strength, yielding reinforced concrete. Before the invention of Portland cement in the early 1800s, lime-based cement binders, such as lime putty were often used. The overwhelming majority of concretes are produced using Portland cement, but sometimes

with other hydraulic cements, such as calcium-luminate cement. Many other non-sementitious types of concrete exist, with other methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces and polymer concretes that use polymers as a binder. Concrete is distinct from mortar, whereas concrete is itself a building material and contains both coarse, large, and fine, small aggregate particles, mortar contains only fine aggregates and is mainly used as a bonding agent to hold bricks, tiles, and other masonry units together. Grout is another material associated with concrete and cement. It also does not contain coarse aggregates and is usually either

pourable or a sixotropic and is used to fill gaps between masonry components or coarse aggregate, which has already been put in place. Some methods of concrete manufacture and repair involve pumping grout into the gaps to make up a solid mass in situ. The word concrete comes from the Latin word concretus, meaning compact or condensed. The perfect passive participle of concrecerre from cone together and cresere to grow. Concrete floors were found in the royal palace of Tyren's Greece, which dates roughly to 1400 to 1200 BC. Lime mortars were used in Greece, such as in Crete and Cyprus in 800 BC. The Assyrian Jirwan Aqueduct, 688 BC,

made use of waterproof concrete. Concrete was used for construction in many ancient structures. Small-scale production of concrete-like materials was pioneered by the Nabatian traders who occupied and controlled a series of Oasis and developed a small empire in the regions of southern Syria and northern Jordan from the 4th century BC. They discovered the advantages of hydraulic lime, with some self-sementing properties by 700 BC. They built kilns to supply mortar for the construction of rubble masonry houses,

concrete floors, and underground waterproof cisterns. They kept the cisterns secret as these enabled in Abateans to thrive in the desert. Some of these structures survive to this day. The Romans used concrete extensively from 300 BC to AD 476. During the Roman Empire, Roman concrete was made from quicklime, pozzalana, and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture terms of the Roman architectural revolution, freed Roman construction from the restrictions of stone and brake materials. It enabled

revolutionary new ideas in terms of both structural complexity and dimension. The call Coliseum in Rome was built largely of concrete, and the Pantheon was the world's largest unreinforced concrete dome. Concrete, as the Romans knew it, was a new and revolutionary material, laid in the shape of arches, vaults, and domes. It quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick. Modern tests show that opus cementicium had a similar compressive strength to modern Portland cement concrete. However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application also

differed. Modern structural concrete differs from Roman concrete in two important. important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand layering together with the placement of aggregate, which in Roman practice often consists of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength and tension. Whereas, Roman concrete could depend only upon the strength of the concrete bonding to resist tension. The long-term durability of Roman concrete structures was found to be due to the presence of pyroclastic volcanic rock and ash in the concrete mix. The crystallization of Stretlingot during the formation

of the concrete and its merging with similar calcium-aluminum-aluminum-silicate hydrate structures, helped give the Roman concrete a greater degree of fracture resistance compared to modern concrete. In addition, Roman concrete is significantly more resistant to erosion by sea water than modern concrete. The aforementioned pyroclastic materials react with seawater to form aluminum-to-barmerite crystals over time. The use of hot mixing in preparation of concrete, leading to the formation of lime class and the final product, has been proposed to give the Roman concrete a self-healing ability. The widespread use of concrete in many Roman structures ensured that many survive to the present day. The baths of Caracalla and Rome are just one example.

Many Roman aqueducts and bridges, such as the magnificent Pondugard in southern France, have masonry cladding on a concrete core, as does the dome and the pantheon. In Britannia, after the Roman Empire, there is evidence of the continued use of burned lime, and remains of an eighth-century mortar mill have been found in North Amptonshire. but it is thought that low kiln temperatures in the burning of lime lack of potzolana poor mixing and an anglo-saxon culture of timber construction all contributed to a decline in the quality of concrete and mortar there but it did not become a lost art. From the 11th century in England, the increased use of stone and church and castle construction

led to an increased demand for mortar. Quality began to improve in the 12th century through better grinding and seaving. Medieval lime mortars and concretes were non-hydraulic and were used for binding masonry, Harding, binding rubble masonry cores, and foundations. Bartolomeus Anglicus, in his de-Proprietadibus re-room, 1240, describes the making of mortar.

In an English translation from 1397, it reads, Lime is a stone brunt. By meddling thereof with sand and water, cement is made. from the fourteenth century the quality of mortar was again excellent but only from the seventeenth century was pozzolana commonly added myon concrete at the ruins of uschmal a d eight fifty to nine twenty five is referenced in incidents of travel in the yucatan by john l stevens the roof is flat and had been covered with cement the floors were cement in some places hard but by long exposure broken and now crumbling under the feet but throughout the wall was solid and consisting of large stones embedded in mortar almost as hard as rock Medieval hydraulic mortars were made from the addition of potcelons, such as crushed ceramics, volcanic soils, or metamorphosed soils from specific deposits,

with known bozzalonic properties, like trass or non-crystalline opal. Hydraulic mortar was used in medieval crete. The use of specific sands containing volcanic ash and peasant, 10th and 11th centuries South Italian mortars may indicate a knowledge of its properties. The Roman architectural treatise de architectura by Vitruvius contained a description of Roman cement, and a copy at the Scriptorium of Charlemagne produced many of the medieval copies that subsequently survived. A copy at St. Kyle Abbey was found by Poghobie. Braccholini in 1414. The first printed edition was published in 1486 by Fra Giovanni Sopiches,

and versions in Italian, German, French, Spanish, and English were published between 1520 and 1692. The 16th century dockyard in Venice used hydraulic lime mortars in foundations. Hydraulic mortar was also used in Ottoman baths and Budapest from the same period. The canal du Mide was built using concrete in 1670. Tras from the Rhineland was added to lime in England in the 17th century, including for the mole of English Tangier, which also used Italian Potsolana, recommended and supplied, by the Genoese engineers. At the port of Toulon, concrete foundations were built using

Potsilana or Tras, Quiclime, sand, pebbles, and slag or cinder. The 1752 reconstruction of the foundations of Essex Bridge by George Semple included a mixture of small stones, sand, and powdered lime. Perhaps the greatest step, forward in the modern use of concrete was Smedon's tower, built by British engineer John Smeaton in Devon, England, between 1756 and 1759. This third Eddystone lighthouse pioneered the use of hydraulic lime and concrete, using pebbles and powdered brick as aggregate. A method for producing Portland cement was developed in England and patented by Joseph Aspton in 1824. Aspton chose the name for its similarity to Portland Stone, which was quarried on the aisle of

Portland in Dorset, England. His son William continued developments into the 1840s, earning him the recognition for the development of modern Portland cement, reinforced common Concrete was invented in 1849 by Joseph Monnier, and the first reinforced concrete house was built by Francois Quagnie in 1853. The first concrete reinforced bridge was designed and built by Joseph Monnier in 1875. Pre-stressed concrete and post-tensioned concrete were pioneered by Eugene Fresne, a French structural. and civil engineer. Concrete components or structures are compressed by tendon cables during or after their fabrication in order to strengthen them against tensile forces developing when put in service. Presene patented the technique on October 2nd, 1928. Concrete is an artificial composite material

comprising a matrix of cementitious binder, typically Portland cement paste or asphalt, and a dispersed phase or filler of aggregate, typically a rocky material, loose stones, and sand. The binder glues the filler together to form a synthetic conglomerate. Many types of concrete are available, determined by the formulations of binders and the types of aggregate used to suit the application of the engineered material.

These variables determine strength and density, as well as chemical and thermal resistance of the finished product. Construction aggregates consist of large chunks of material in a concrete mix, generally a coarse gravel or crushed rocks, such as limestone or granite, along with finer materials such as sand. Cement paste most commonly made of Portland cement is a most prevalent kind of concrete binder.

For cementitious binders, water is mixed with the dry cement powder and aggregate, which produces a semi-liquid slurry that can be shaped, typically by pouring it into a form. The concrete solidifies, and hardens through a chemical process called hydration. The water reacts with the cement, which bonds the other components together,

creating a robust stone-like material. Other cementitious materials such as fly ash and slag cement, are sometimes added, either pre-blended with the cement, or directly as a concrete component, and become a part of the binder for the aggregate. Fly ash and slag can enhance some properties of concrete, such as fresh properties and durability. Alternatively, other materials can also be used as a concrete binder. The most

prevalent substitute is asphalt, which is used as the binder and asphalt concrete. Admixtures are added to modify the cure rate or properties of the material. Mineral admixtures use recycled materials as concrete ingredients. Conspicuous materials include fly ash, a byproduct of coal-fired power plants, ground granulated blast furnace slag, a byproduct of steelmaking, and silica fume, a byproduct of industrial electric arc furnaces. Structures employing Portland cement, concrete, usually include steel reinforcement because this type of concrete can be formulated with high compressive strength, but always has lower tensile strength. Therefore, it is usually reinforced

with materials that are strong in tension, typically steel rebar. The mixed design depends on the type of structure being built, how the concrete is mixed and delivered, and how it has place to form the structure. Portland cement is a most common type of cement in general usage. It is a basic ingredient of concrete, mortar, and many plasters. It consists of a mixture of calcium silicates, illuminates, compounds which will react with water. Portland cement and similar materials are made by heating limestone, a source of calcium with clay or shale, a source of silicon, aluminum, and iron, and grinding this product, called clinker, with a source of sulfate, most commonly gypsum. Cement kilns are extremely large, complex, and inherently dusty industrial installations.

Of the various ingredients used to produce a given quantity of concrete, the cement is the most energetically expensive. Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely. As stated by Abrams Law, A lower water to cement ratio yields a stronger, more durable concrete,

whereas more water gives a freer flowing concrete with a higher slump. The hydration of cement involves many concurrent reactions. The process involves polymerization, the interlinking of the silicates and aluminum components, as well as their bonding to sand and gravel particles to form a solid mass. Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates from construction, demolition, and excavation waste

are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash, are also permitted. The size distribution of the aggregate determines how much binder is required. Aggregate was a very even size distribution has the biggest gaps, whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate,

as well as paste the surfaces of the aggregate together. It is typically the most expensive component. Thus, variation in sizes of the aggregate reduces the cost of concrete. The aggregate is nearly always stronger than the binder. so its use does not negatively affect the strength of the concrete. Redistribution of aggregates after compaction often creates non-homogeneity due to the influence of vibration. This can lead to strength gradients.

Decorative stones, such as quartzite, small river stones, or crushed glass, are sometimes added to the surface of concrete for a decorative exposed. aggregate finish, popular among landscape designers. Admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. Admixtures are defined as additions made as the concrete mix is being prepared. In normal use, admixture dosages are less than five. 5% by mass of cement, and are added to the concrete at the time of batch mixing.

Inorganic materials that have bozzalanic or latent hydraulic properties, these very fine-grained materials are added to the concrete mix to improve the properties of concrete, or as a replacement for Portland cement. products which incorporate limestone, fly ash, blast furnace slag, and other useful materials with pozzellanic properties into the mix are being tested and used. These developments are ever-growing in relevance to minimize the impacts caused by cement use. The use of alternative materials also is capable of lowering costs and recycling wastes. the latest being relevant for circular economy aspects of the construction industry whose demand is ever growing with greater impacts on raw material extraction waste generation and landfill practices