The Panthenon. Built 118-128 AD.

The Pantheon's dome was the largest until 1881. It remains the world's largest unreinforced concrete dome.

Concrete is a heavy, rough building material made from sand or gravel_ (aggregate), cement_, and water, that can be spread or poured into molds and that forms a stone-like mass on hardening.

Concrete is strong in compression but weak in tension. It's tensile strength is roughly 10% of its compressive strength. This limits the use of pure concrete in construction because concrete beams will fail from tensile stresses at their bottoms.


1   Classification

Reinforced concrete has a deformed steel reinforcing bar ("rebar") placed through the lower half of a beam, which adds tensile strength.

Failure happens slower and causes crack to appear before the beam fails. Failure changes from a brittle failure to a ductile failure. This gives maintainers a chance to address the failure before it occurs. [5]

One disadvantage of reinforced concrete is that is is a passive reinforcement. Steel lengthens with stress, so rebar can't start working to help resist tension unti it's had a chance stretch. Often that means concrete has to crack before the rebar can take up any of the tensile stress. Crack of concrete isn't necessarily bad since the purpose of the concrete is to resist compressive forces which it can do with crack. But there are some cases where you want to avoid cracks. [5]

Reinforced concrete can be made into an active reinforcement by applying a put tension on the rebar before the concrete cures. Once the concrete cures, the tension will remain inside. Most concrete bridge beams are prestressed in this way. Another way to pre-stress reinforcement is called poste-tensioning. In this method, the stress in the reinforcement is developed after the concrete is cured. For example, by tightening nuts on the rods to tension them. [5]

2   Production

2.1   Environmental impact

Cement takes a lot of energy to produce and gives off carbon dioxide. It requires less energy than steel production, but world needs more concrete. Concrete per capita is 5 tons. Emits as much as greenhouse gas as aviation. (An argument could be made that without concrete, we'd spend as much if not more resources on alternative buildings.)

Operational energy requirements typically represent 85% of the total energy a building uses over its service life. Concrete provides one of the most efficient and cost-effective means of constructing energy-efficient structures.

Concrete is safe, secure and healthy for building occupants. Being an inert construction material, concrete does not burn. It also does not feed rot and mildew. It does not off-gas any volatile organic compounds and provides excellent indoor air quality. Superior quality of construction helps prevent the entry of pollen, dust and other airborne pollutants.

Produced from locally available, abundant materials, concrete's long lifespan helps make it the most responsible choice for a sustainable future.

Many wastes and industrial byproducts that would end up in landfills are used in the cement kiln or can be added to concrete mixes to provide desirable characteristics. Used concrete is recyclable and serves as aggregate in roadbeds or as granular material in new concrete.

While production of a cubic metre of conventional concrete generates 210 kilograms of CO2, this effect is mitigated by the inclusion of supplementary cementing materials, and by the use of waste products to heat the cement kilns.

The Pantheon survived in part because unlike Roman brick buildings which could be deconstructed and the bricks reused, concrete can only be reduced to rubble.

Concrete didn't remerge until the modernity. Modern concrete was born in the early nineteenth century, with the discovery of Portland cement, the key ingredient used in concretes today. [4] The process of roasting, and then grinding to a powder, limestone and clay to make 'artificial stone' was patented in 1824 by Joseph Aspidin of Leeds, UK, and later refined by his son William into a material very close to the cement used today. [4]

The main reaction occurring in Aspidin’s kiln was the formation of calcium silicates, from calcium carbonate (limestone) and silicates that make up clay. At temperatures approaching 1000oC, the two raw materials break down into their component oxides – and as the temperature rises further, then combine into di- and tri-calcium silicate. The lesser quantities of iron and aluminium in the clay also react with calcium, giving the minor components of Portland cement. Finally, this mixture, called clinker, is ground to a powder, and gypsum is added. [4]

3   History

Concrete has been used for millenia. There is a theory that the first settlement in southern Turkey was because someone had figured out how to make cement. Was by used by ancient desert traders to make underground tunnels 8000 years ago. Still exists in Jordan and Syria.

Rome used concrete to build aqueducts and bathhouses. They gathered naturally-occurring cement from volcanic ash deposits. The Romans used concrete to build long-stand landmarks such as the Colosseum and the Pantheon.

In 1824, the English inventor Joseph Aspdin invented Portland Cement, which has remained the dominant form of cement used in concrete production.

In 1877, Joseph Monier developed reinforced concrete by reinforcing concrete with steel mesh to make flowerpots. Reinforced concrete is much stronger than regular concrete, but if cheaply made it will rot from the inside. Monier exhibited his invention at the Paris Exposition of 1867.

Used to construct the Hoover dam. Part of brutalism.


U.S. Geological Survey.

Construction dips during the Great Depression, World War II, and the recession of the early 1980s.

Shangai 1987 versus Shanghai 2013. Reuters / Carlos Barria.

China has used more concrete between 2011 and 2013 (6.4 - 6.6 gigatons, according to the International Cement Review and U.S. Geological Survey) than the United States did in the entire 20th century (during which the U.S. built almost all of it roads and bridges, the Interstate system, the Hoover Dam, and many of the world's tallest skyscrapers). Yet China and the United States are roughly the same in terms of geographic area, ranking third and fourth in the world, respectively. [6]

Gates plucked the statistic from the historian Vaclav Smil, who calls cement “the most important material in terms of sheer mass in our civilization.”

So how did China use so much cement? First, the country is urbanizing at a historic rate, much faster than the U.S. did in the 20th Century. More than 20 million Chinese relocate to cities each year, which is more people than live in downtown New York City, Los Angeles and Chicago combined. This massive change has taken place in less than 50 years. In 1978, less than a fifth of China’s population lived in cities. By 2020, that proportion will be 60 percent. [6]

China's cities have been transformed to make room for this influx of people. By some estimates, half of China’s infrastructure has been built since 2000, with new rail networks, interstates, dams, airports and high-rise apartment buildings springing up across the country.

What’s almost more impressive than China’s biggest cities is the incredible number of “small” cities that no one has ever heard of. In 2009, China had 221 cities with more than a million people in them, compared with only 35 in Europe. Even relatively minor cities like Zhengzhou and Jinan are more populous than Los Angeles or Chicago.

As Goldman Sachs pointed out in a note, China’s population today is only about four times as large as the U.S., but it is 15 times as large as the U.S. was in the early 20th Century, and nine times the size of the U.S. in 1950.

The world also experienced a shift in building materials over the 20th Century. In 1950, the world manufactured roughly as much steel as cement; by 2010, steel production had grown by a factor of eight, but cement had gone up by a factor of 25. And where many houses in the U.S. are made of wood, China suffers from a relative lack of lumber. Unlike in the U.S., many people in China live in high- or low-rise buildings made out of cement.

This massive cement industry also takes a heavy toll on the environment. Scientists estimate that the global cement industry accounts for around 5 percent of the world's carbon emissions, and more than half of the world's cement production capacity is based in China.

What's more, low standards for construction quality mean some of China's concrete buildings may have to be knocked down and replaced in as little as 20 or 30 years. According to Goldman Sachs, about a third of the cement that China uses is low-grade stuff that wouldn't be used in other countries.

When Bill Gates wrote in his blog about China's stunning cement consumption, he pointed out that the issue of materials is key to helping the world's poorest people improve their lives. Replacing mud floors with concrete improves sanitation; paving roads with concrete allows vegetables to get to market, kids to get to school, and the economy to flourish. In China, the building boom has spurred economic growth that has lifted hundreds of millions of people out of poverty.

4   Functions

Concrete is fireproof, waterproof, storm-proof, strong, and cheap. It is flexible when building and inflexible once made. Steel and wood will decay before concrete.

4.1   Construction


Jørn Utzon. October 20, 1973. Sydney Opera House.

Concrete is the most widely used construction material in the world.

It can be used to make roads and pavements. However, it cannot be laid out as continuous units; space must divide it every so often so that it can expand in the heat. (On concrete roads cars will repeatedly make noise "dum-dum".) Concrete isn't often used for roads because, though it has low maintenance costs, it has a higher upfront cost.

4.2   Defense

One of the first uses for concrete on the battlefield was in response to growing numbers of IEDs. As early as 2004, the major tactical and technical focus in Iraq was oriented at stopping these roadside bombs. One of the primary tactics used to fight the IED threat was to line every major road with twelve-foot-tall concrete T-walls. Soldiers spent days, weeks, and months lining first every major highway and then other, smaller roads with concrete barriers. At over $600 a barrier, the cost of concrete during the eight years of the Iraq War was billions of dollars. [3]

To be sure, concrete walls did not eliminate the IED threat. As with any protective obstacle, they should have been under direct observation, which was not always feasible. Consequently, the enemy adapted by placing IEDs in or on top of barriers. They also used advanced forms of IEDs from foreign sources—explosively formed penetrators, many of which US military officials believe originated in Iran—that could penetrate any concrete wall. This allowed IEDs to be placed on the opposite, non-road side of barriers. But the concrete walls did take away the ease of access for enemy forces to emplace IEDs, degrade the lethality of their homemade devices, and forced them towards specialized materials that could be interdicted at checkpoints—which themselves were most effective when concrete walls were used to canalize traffic to them. They also took away the ability of insurgents to freely transit Baghdad with large, vehicle-borne IEDs, which created mass casualties and threatened the authority of the Iraqi government. [3]

IEDs were not the only major threat to American forces. Shortly after the 2003 invasion of Iraq, US forces also began to come under direct attack by mortars and rockets in their outposts and bases. These attacks became even more dangerous when US forces moved out of large bases and into smaller outposts deep in cities and among the populations, where the ability to maintain safe standoff distances or retaliate to indirect fire was difficult for fear of causing civilian casualties. Again, the solution was concrete. Slabs were placed to form not only the walls of compounds, but also walls around and bunkers between every structure within them. This significantly reduced the effects of any enemy incoming fire. [3]

Concrete also gave soldiers freedom of maneuver in urban environments. In the early years of the war, US forces searched for suitable spaces in which to live. Commanders looked for abandoned factories, government buildings, and in some situations, schools. Existing structures surrounded by walled compounds of some type were selected because there was little in the environment to use for protection—such as dirt to fill sandbags, earthworks, and existing obstacles. As their skills in employing concrete advanced, soldiers could occupy any open ground and within weeks have a large walled compound with hardened guard towers. [3]

A major component to the well-known Iraq surge of 2007 in response to rising sectarian violence was the mission to clear and secure the neighborhoods of Baghdad. US forces found concrete to be their most effective weapon to reduce violence and protect the local population. They used concrete to reduce the complexity of the environment. They walled in neighborhoods and placed either Iraqi security forces or local members of the Sons of Iraq (SOI), a volunteer, armed neighborhood watch, on checkpoints at the limited entries into what were effectively new, smaller, cities or neighborhoods within the larger environment. The checkpoint guards of these closed neighborhoods searched vehicles, questioned outsiders, and reacted to any trouble near their posts. This reduced the ability of insurgent forces to create mass-casualty events with IEDs and disrupted their ability to move freely or resupply forces. Walling off troubled neighborhoods became the daily mission. One brigade emplaced over 30 miles of twelve-foot-tall concrete T-wall barriers to create what they dubbed “safe communities.” [3]

5   Substance

The basic composition of Roman and modern concrete is similar, but the binding agent in Roman concrete was pozzuolana, a volcanic ash that underwent a chemical action when ground and mixed with water, forming an artificial stone. In modern concrete, developed in 1824 by Joseph Aspdin, the binding cement is made of chalk and clay, carefully burned, with the resulting nodules ground to a fine powerder. When mixed with water, sand, and fine gravel, the resulting artificial stone closely resemvles the fine-grain natural limestone found in the region of Portland England, as Aspdin first note. As a result, this artifically produced cement is still called Portland Cement to this day. [1]

For both the Romans and us, cement itself is too costly to make entire buildings, sidewalks, or other constructions from it alone. Even the morar used between bricks and stone is stretched by adding sand; and in making concrete, gravel and sand are mixed in as the aggregate. In Roman concrete, the brick and tile relieving arches also served as a kind of large aggregate.

Like stone, concrete is immensely resistant to compressive forces, but relatively weak resisting tensile of stretching forces. Realizing this, the Romans added irons bars to concrete in some instances, but they prefered to use integrated relieving arches of brick and tile. Since the mid-nineteenth century, iron or steel rods have been placed in the formwork for modern concrete wherever tensile forces will occur.

Formwork is one of the cost disadvantages of concrete. As first mixed, concrete is a thick viscous material and must be contained in forms, or molds, until it has cured and dried; the formwork (called shutting in England) is like the centering used in construction. In large structures, both in Roman times and now, the means the construction of substantial and expensive wooden structures, significant in themselves, which are then destroyed once the concrete has cured sufficiently for the forms to be removed.

6   Further reading

7   References

[1]Roth, 1993.
[2]Feb 8. Riddle of cement’s structure is finally solved.
[3](1, 2, 3, 4, 5) John Spencer. Nov 14, 2016. The most effective weapon on the modern battlefield is concrete.
[4](1, 2, 3) James Mitchell Crow. March 2008. The concrete conudrum. Chemistry World.
[5](1, 2, 3) Practical Engineering. Apr 25, 2018. Why Concrete Needs Reinforcement.
[6](1, 2) Ana Swanson. 2015-24-03. How China used more cement in 3 years than the U.S. did in the entire 20th Century.


The cement industry is the third largest consumer of energy and the second largest industrial emitter after the steel industry with 6% of global emissions (IEA, 2017).

IEA = International Energy Agency


Not many want to fret over cement, the world’s second-most consumed material behind water.

When accounting for its use in human-made structures, it is responsible for more than a third of the world’s carbon emissions. But unlike the transportation sector, in which a new type of fuel can dramatically decrease the sector’s pollutants, cement’s problem is, well, cemented in its formulation: Limestone is mixed with other raw materials in an immense kiln at high temperatures; as the kiln separates the limestone’s calcium carbonate structure, an extremely dirty strand of carbon is emitted by the ton.

The resulting hard substance is called a "clinker” (this onomatopoeia has its origins in Holland from the word “klinken,” which means “to ring”), and it is then pounded into the recognizable powder that is blended with material binders and water to form cement.

The average wind turbine, for example, needs about 12,400 to 17,700 cubic feet of concrete made with cement. On the conservative end, that’s 57 trucks worth of concrete.

We desperately need these infrastructure projects to transition to a carbon-neutral world, but in doing so we will have to emit a massive amount of carbon.

, so abundant that its production is one of the leading sources of greenhouse gas emissions.

Concrete forms through the solidification of a mixture of water, gravel, sand, and cement powder. Is the resulting glue material (known as cement hydrate, CSH) a continuous solid, like metal or stone, or is it an aggregate of small particles?

In a paper published this week in the Proceedings of the National Academy of Sciences, a team of researchers at MIT, Georgetown University, and France’s CNRS (together with other universities in the U.S., France, and U.K.) say they have solved that riddle and identified key factors in the structure of CSH that could help researchers work out better formulations for producing more durable concrete.

One key question was whether the solidified CSH material, which is composed of particles of many different sizes, should be considered a continuous matrix or an assembly of discrete particles. The answer turned out to be that it is a bit of both — the particle distribution is such that almost every space between grains is filled by yet smaller grains, to the point that it does approximate a continuous solid. But the grains within the CSH “are not able to get to equilibrium,” or a state of minimum energy, over length scales involving many grains, and this makes the material vulnerable to changes over time, he says. That can lead to “creep” of the solid concrete, and eventually cracking and degradation.

After the pyramids are gone and Earth is a wasteland, the Hoover Dam will be the last remnent of humanity.

Concrete doesn't "dry", it cures. The difference is like putting clay versus dough in an oven. The oven dries the clay and you are left with pottery, whereas it "cures" the dough, chemically converting it into bread. Oddly enough, concrete needs to remain wet in order to cure properly - if it gets dry then it won't reach its full strength.

Essentially what is going on is that concrete is made up of cement (a powder, like flour to bread) and sand/rocks ("aggregate"). Adding water causes a chemical reaction in the cement that causes it to stick to itself and the aggregate and solidify in that shape. If there is not enough water or the concrete dries out too quickly, the cement particles will stop curing and remain in their dust state. Even a mature, properly cured concrete has bits of cement remaining. As time goes on, these remnants slowly become cured themselves, further strengthening the concrete. That might be the source of the "the center of the Hoover Dam hasn't dried yet" misconception. However, concrete never finishes curing. It adds strength logarithmically, so it really slows down after the first few weeks, but it never stops entirely. Even Roman concrete is still curing under this principle. However, 2000 years adds as much strength as the first 200 years, first 20 years, first 2 years, etc.,* so it really isn't perceptibly stronger. Especially since modern advancements mean we can make much stronger concrete.

I just want to clarify that this strength is in fact logarithmic, not exponential. So 200->2000 years doesn't double the strength, but rather increases it by the same amount as 20->200. So it's more like the strength going from 4000psi to 4100psi to 4200psi, not 4000psi to 8000psi to 16000psi, etc.