Stonehenge in England, the Great Pyramid of Giza, the Peruvian citadel at Machu Picchu—three wondrous examples of how stone
structures can last hundreds or even thousands of years. But though
stone is one of the oldest and most durable building materials, it isn't
exactly easy to work with. It's heavy, hard to transport, and
usually comes in giant chunks, which have to be
laboriously cut to shape. Wouldn't it be great if there were a recipe for stone—a
kind of gooey cake mixture we could throw together wherever it was needed, simply pressing it
into molds to make buildings and structures of any shape or size?
Well that kind of "liquid stone" really does exist: we call it
concrete. Although it sometimes gets a bad press, because many
people associate it with brutal urban architecture from the mid-20th
century, concrete is the great, unsung hero of the modern, material
world. From the Hoover Dam to the Sydney Opera House, you'll find
it in the world's tallest skyscrapers, biggest
highways, deepest tunnels, and quite probably even under the floor in
your own humble little home. Concrete is pretty amazing stuff, but
what is it and how exactly does it work? Let's take a closer look!
Photo: Concrete is the power behind virtually every modern building
and major structure—but it's not as ugly as many people suppose.
This is the 12-arch Calstock Viaduct that carries a railroad over the River Tamar in Cornwall, England.
Although it looks as elegant as old stone, it is in fact made from concrete
blocks that were precast on site, and was completed in 1908.
Thrown together and mixed well, these simple
ingredients make a composite, which is the name we give to a hybrid
material that's better in some important way than the materials from
which it's made. In the case of concrete, the "important" thing is that it's
strong, hard, and durable. Thinking of concrete as a
composite material, the cement hydrate is the background, binding
material (technically called the "matrix") to which the sand and gravel add
extra strength (the "reinforcement").
Photo: Concrete composite: Look closely at this concrete and you can clearly see how it's working: the lighter colored aggregate (stones of various shapes and sizes, which acts as the reinforcement) is bound together by the darker colored cement (the matrix). Not all concrete looks as rough as this, however; I had to look around quite hard to find this example in a concrete post near my home.
How does concrete form from ingredients that are nothing like the final product?
When you add water to cement, crystals of cement hydrate (technically, calcium-silica-hydrate) start to grow, which lock the sand and gravel tightly together. It's this gradual
crystal formation that gives concrete its strength, rather than the
simple fact that it's drying out. Indeed, the reason you have to keep
wetting concrete for several days, as it sets, is to "power" the
chemical reactions that hydrate the cement. The mushy slushy mixture that tumbles from your
concrete mixer gradually turns much harder than the materials from
which it's formed. "Liquid stone" becomes stone for real—well,
artificial stone, at least. And by "gradually," I really do mean
gradually: concrete hardens in hours, gets properly hard after about
a month, but continues to harden and strengthen for at least five
years after that.
An interesting fact, from
recent scientific studies of concrete, is that the "crystals" inside it aren't really crystals at all: they're not well-ordered and
perfectly regular, as crystals are supposed to be, but actually have
some of the random structure you find in materials like glass
(scientifically known as amorphous solids). Concrete contains quite a
bit of trapped air (as much as 5–10 percent), because there is some
space around the open, three-dimensional structure of cement hydrate
crystals and the sand and gravel trapped between them. And that, in
turn, explains why concrete can bend and flex, stretch and compress
(just a little bit, anyway).
Just like any recipe, you can vary the mixture for concrete somewhat (more
water, perhaps, more aggregates, or even chemicals of different
kinds) to produce concrete that flows faster, sets harder or more
quickly, weathers better, or has a particular color or appearance.
Adding a pigment called titanium dioxide, for example, is a simple
way of making concrete bright and white—a million miles from the
drab gray stuff that gives concrete car parks a bad name. Another
variant is aerated concrete, which looks a bit like a very hard
sponge with masses of tiny air pockets inside. These allow the
concrete to expand and contract in hot and cold weather without
fatally cracking and also make it an excellent heat insulating
In cities, at least, concrete is everywhere you look—and it's
not hard to understand why. It's easy to make from cheap and readily available
ingredients, easy to pour into molds and make into all kinds of
shapes (because it starts life a very viscous liquid), and it's both
fireproof and (relatively) waterproof. But the main reason it's so
widely used in buildings is that it's extremely strong in
compression: you can squeeze it or stand a great deal of weight on
it. It's widely used in walls and foundations (the vertical
supports, in other words) because it's great for resisting weight piled on top. Unfortunately,
concrete's very big drawback is that it's about 10 times weaker in tension
than in compression. It cracks or snaps easily if you bend or stretch it, unless you
reinforce it with steel inside, so it's
not much use in horizontal beams. Although concrete looks heavy and monolithic, it's
actually much lighter than you might suppose: it's about a fifth as dense as
lead, a third as
dense as steel, 10 percent less dense than aluminum, and only
fractionally more dense than glass.
Although concrete is often mixed on-site and formed into whatever
shapes are needed at the time, it can also be supplied in precast
"modules"; blocks, beams, wall sections, pavements, and cladding
can all be made this way. Giant, modern
segmental bridges, for
example, are often quickly and inexpensively assembled from identical
concrete sections that have been precast in a factory and shipped to their final
location. That makes them quicker and easier to construct than if the
entire bridge had to be cast in-situ, which is much harder to do in
the middle of a river, for example, or in adverse weather conditions.
Another option is to make concrete structures that combine some
precast sections with other sections formed on-site.
Artwork: Concrete ideas: Thomas Edison immediately understood the brilliance of concrete as a material for making "instant" buildings. In the early years of the 20th century, he devised this method for making "single-pour" concrete houses that could be mass-produced inexpensively in very large quantities. Concrete, from a pair of mixers (blue), is fed to a tank (red), agitated (green), and then carried by an auger screw (orange) to the top of a huge, three-dimensional mold. Poured through the mold, it forms the walls, floors, and roof of the building—and even some of the fittings (like bathtubs) inside! Unfortunately, the idea never caught on. Artwork from US Patent 1,219,272: Process of constructing concrete buildings by Thomas Edison, March 13, 1917, courtesy of US Patent and Trademark Office.
As we've already seen, concrete is a composite material—a cement matrix with aggregates
for reinforcement—that works well in compression, but not in
tension. We can solve that problem by casting wet concrete around strong, steel
reinforcing bars (tied together to make a cage).
When the concrete sets and hardens around the bars,
we get a new composite material, reinforced concrete
(also called reinforced cement concrete or RCC), that works well in
either tension or compression: the concrete resists squeezing
(provides the compressive strength), while the steel resists bending
and stretching (provides the tensile strength). In effect, reinforced
concrete is using one composite material inside another: concrete
becomes the matrix while steel bars or wires provide the
The steel bars (known as rebar, short for
reinforcing bar) are typically made from twisted strands with nobbles
or ridges on them that anchor them firmly inside the concrete without
any risk of slipping around inside it. Theoretically, we could use
all kinds of materials to reinforce concrete. Generally, we use steel
because it expands and contracts in the heat and cold roughly as much as
concrete itself, which means it won't crack the concrete that
surrounds it as another material might if it expanded more or less.
Sometimes other materials are used, however, including various kinds
Photo: "Liquid stone" to go—pouring concrete from a mixing truck.
These construction workers from the US Navy are spreading wet concrete
from a truck onto rebar (a grid of steel reinforcing bars).
When the concrete sets, the steel bars will give it added strength:
concrete plus steel equals reinforced concrete. Picture by Lt. Edward Miller, courtesy of
Although reinforced concrete is generally a better construction
material than the ordinary stuff, it's still brittle and liable to
crack: in tension, reinforced concrete can fail in spite of its
steel reinforcement, letting water in, which then causes the concrete
to fail and the rebar to rust. The solution is to put reinforced
concrete permanently into compression by prestressing it (also
called pretensioning). So instead of putting steel bars into wet
concrete as they are, we tension (pull on) them first. As the
concrete sets, the taut bars pull inward, compressing the concrete and making it stronger.
Alternatively, rebars in reinforced concrete can
be stressed after it starts to harden, which is known as poststressing
(posttensioning). Either way, keeping concrete in compression is a
cunning trick that helps to stop it cracking (and stops cracks from
spreading if they do form). Another advantage is that
it's possible to use less prestressed or poststressed concrete or smaller,
more slender pieces to carry the same loads, compared to ordinary,
Photo: Science runs through concrete—how it sets, why it's strong, and why we use it.
This concrete word is one of the details on the Onondaga County War Memorial in Syracuse, New York.
Credit: Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
Cracks are the last thing you want to see in a building or bridge,
especially a relatively new one made from concrete. But if we've got
concrete structures dating back to Roman times, how come some of the
concrete bridges, skyscrapers, and other structures built just a few
decades ago, in the late 20th century, are already falling apart?
There are several explanations. Older, Roman-type, pozzolanic
concrete, made from volcanic ash, tends to crack less than more
modern forms of concrete, and it was used mainly in compression, so
even if cracks had the opportunity to form, they were less likely to
spread. Reinforced concrete is more likely to be used in tension, which
is why it has those steel reinforcing "rebars" inside. But, as we've
already seen, it can still crack unless it's prestressed.
Modern concrete fails through what's informally known as concrete cancer
or concrete disease, which involves three interrelated problems.
First, alkalis from the cement react with silica in
the aggregates from which the concrete is made. This makes new
crystals grow very slowly inside the concrete, which take up more
room than the original "crystals," so making the
concrete crack apart from the inside out or flake away ("spall")
from the surface, letting in water from outside.
On something like a highway bridge, any water that gets in
might also be alkaline because of the salts used
to treat the road in winter. The second problem is that the water
that gets in will eventually come into contact with the steel reinforcing bars inside, causing
them to rust and decay, possibly expanding so they cause fatal
weaknesses in the structure. The dirty brown stains you see on
concrete with "cancer" are often caused by rusty water draining through
the cracks. A third issue is that water that has seeped inside
concrete through cracks can freeze in winter, which means it will
expand and cause further cracks through which even more water will
penetrate, causing a vicious circle of degeneration and decay.
Artwork: How reinforced concrete fails: (1) Alkalis from the cement react with silicas in the aggregates,
forming larger crystals that crack the concrete apart from the inside, (2). Water flows in down the cracks (3),
rusting the rebar (4), which can break apart and cause more cracking or "spalling" at the edges (5). In cold weather,
water trapped inside cracks will expand as it freezes (6), causing new cracks to appear (7). The cracks aren't
necessarily large: some are very thin capillaries, which means water can move up them by
simple capillary action as well as draining down through them due to gravity.
Environmental impacts of concrete
Photo: Some love concrete, some hate it. Opinion divides sharply on "brutalist" urban buildings like this, the Xerox Tower in Rochester, New York, which was constructed in the mid-20th century. Credit: Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
Growing concerns about the environment, and climate change in
particular, have highlighted another major problem with concrete:
after transportation and energy, cement production is the third
biggest source of carbon dioxide emissions. That's partly because the
process of making cement releases a lot of carbon dioxide but also,
very importantly, because of the enormous amount of cement and
concrete used worldwide. The carbon dioxide is released in two quite
different ways (split roughly half and half between them): first,
because of the fossil-fuel energy used during the manufacture of
cement; second, because cement is produced when calcium carbonate
turns to calcium oxide, releasing carbon dioxide in the process.
Concrete relies on cement, so it's anything but a sustainable
material, which worries architects, in particular, because they tend
to be very environmentally conscious.
Photo: An early example of greener concrete from 1953: Hungry Horse Dam on the Flathead River, Montana, USA,
was built using 120,000 metric tons of recycled fly ash from incinerators. Picture courtesy of US Bureau of Reclamation.
Since carbon dioxide is released in two ways during cement
production, it follows that there are two ways of making more
environmentally friendly concrete. Historically, since the Industrial
Revolution, much of humankind's energy has come from burning coal,
which releases more greenhouse gases than other fuels, and
traditionally cement kilns were coal-fired too. Switching them from
coal to natural gas is one solution, since gas releases less carbon
dioxide for a given amount of energy. Making cement kilns more
efficient reduces the total energy they need, which also reduces
their carbon dioxide emissions. The other solution is to reduce the
amount of cement in the concrete mixture by using recycled materials,
such as fly ash from incinerators. Another exciting prospect is the
development of concrete that doesn't use calcium carbonate at all.
Instead, the carbonate is made by bubbling carbon dioxide from a
power plant through seawater. This has an overall environmental
benefit, since it takes the harmful waste CO2 emissions from power
plants and turns them into very useful concrete instead. It's a kind
of carbon capture and storage (CCS).
Another environmental drawback of concrete comes from its use of
aggregates, which have to be quarried, often from environmentally
sensitive areas such as river valleys. Using recycled aggregates
(including recycled concrete from old demolished buildings) is a
possible solution here.
A brief history of concrete
~7000 BCE: A Neolithic settlement at
Yiftahel in Galilee, Israel has a crude "concrete" floor made using burnt lime plaster.
~5600BCE: A concrete-like material is used in the floors of
Mesolithic (Middle Stone Age) Serbian dwellings at
Lepenski Vir, in Serbia,
on the banks of the Danube River.
~3000BCE: Egyptians use crude forms of cement and concrete in the
~200BCE: Romans use a type of concrete called pozzolana (sometimes
referred to as pozzolanic cement), based on volcanic ash sourced from
Pozzuoli, Naples. It's used in iconic Roman structures such as the
Colosseum and Pantheon in Rome.
400AD–~1750CE: Effectively, the concrete Dark Ages: knowledge
of concrete is entirely lost after the fall of the Roman Empire.
John Smeaton, an English engineer, rediscovers the art of
making "hydraulic" cement (that hardens with water) using Blue
Lias stone, clay, and pozzolana, originally for the
Eddystone Lighthouse off Plymouth, England.
1824: Englishman Joseph Aspidin develops Portland cement, which
resembles a natural stone extracted from Portland in Dorset, England.
Portland cement is destined to become the key ingredient in concrete.
1832–1834: William Ranger patents precast concrete.
patents reinforced concrete for use
in garden flower pots, showing them off at the Paris Exposition the
~1850s: French builder François Coignet begins widespread use of
concrete in buildings, including the first iron-reinforced house in
1884: English-born, American-based architect
Ernest Leslie Ransome
patents twisted rebars, which give better grip inside concrete, so
making it stronger.
1870: Frenchman François Hennebique develops an efficient new
process for constructing buildings with reinforced concrete, leading
to its widespread uptake.
1880s: Prestressed concrete is invented in Germany, though not
Photo: A memorable modern use of reinforced concrete. This is the famous Great Workroom of architect Frank Lloyd Wright's Johnson Wax Headquarters in
Racine, Wisconsin. The roof is supported by astonishingly slender reinforced concrete columns
that taper down from 5.5m (18ft) at the top to just 23cm (9in) at the bottom. According to
Jonathan Lipman's book about the building, Wright
got the idea after seeing a waiter carrying a tray on his arm.
Picture courtesy of the Carol M. Highsmith Archive,
Library of Congress, Prints and Photographs Division.
1891: The first street in the United States with a concrete pavement
is laid at Bellefontaine, Ohio. A section of it remains in place to
1913: The first load of ready-mixed concrete is delivered by truck
to a site in Baltimore, Maryland.
1915: Colored concrete is invented by Chicago-based engineer Lynn
1920s: Frenchman Eugene Freysinnet turns prestressed concrete into
a commercially successful building material.
1936: Concrete is used to complete the mighty Hoover dam, the
biggest concrete structure ever attempted up to that point.
1956-1959: American architect Frank Lloyd Wright builds the iconic
Guggenheim Museum, in New York City, from concrete.
1962: Finnish architect Eero Sarinen constructs
the famously bird-like concrete roof of the Trans World Airlines (TWA) Flight Center at New York's John F. Kennedy International Airport.
Three years later, he designs New York City's iconic concrete skyscraper, the CBS Building.
1970s: Reinforced concrete based on plastic fibers is invented.
2010s-: The environmental impact of concrete becomes a growing concern.
Scientists and engineers begin to draw attention to how climate change could dramatically
shorten the lives of concrete buildings.
Eero Saarinen: Shaping the Future by Eero Saarinen et al. Yale University Press, 2006. A photo guide to structures and buildings by one of the pioneers of 20th-century reinforced concrete architecture.
Concrete Architecture by Catherine Croft. Gibbs Smith, 2004. A coffee-table "celebration of concrete," including a history of the material and a photo guide to iconic concrete buildings and structures.
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