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Cement And Concrete
How is concrete
made?
made?
BY ROBERT LAMB
Few professions are
as unappreciated as that of the gangland concrete shoe cobbler.
Maybe
you've never had to send a rival mobster to the bottom of the East River and,
therefore, can't fully appreciate the craftsmanship. After all, it looks pretty
simple in "Billy Bathgate."
However,
if the movies teach us anything about the ins and outs of the criminal
underworld, then fitting a doomed gangster with concrete shoes is obviously a
far from simple undertaking.
First,
you have to make sure you have the right proportions in your concrete mixture
and then you have to convince the intended victim to keep still for a few hours
while it hardens around his or her feet.
Even
at gunpoint, that requires some serious conversational talent.
We've
all seen concrete in action. Someone drives up in a truck, pours the gritty,
wet concrete into the desired area and workers shape and smooth it into place.
After
it dries, you essentially have a custom-sized slab of rock -- only without
having to cut blocks of stone out of the Earth. The technique is nothing new.
The
ancient Egyptians used a lime and gypsum mixture very similar to modern
concrete as early as 3000 B.C., and opus caementicium was used
throughout the Roman Republic.
Making
concrete essentially resembles the common children's pastime of making mud pies
in which mud is poured into a pan and allowed to dry into solid dirt.
Of
course, no one ever built a skyscraper out of mud bricks or sealed a mafia
informer's feet in a slab of Georgia red clay.
What
is it that makes concrete special, and how do we turn sand and water into
something as solid and immobile as a concrete dam?
In
this article, we'll take a look at the recipe for modern concrete and the steps
that go into paving our world.
Grandma's Concrete Recipe
When you get right down to it, concrete is a mixture
of two components: aggregates and paste.
In
primitive brick construction, straw served as the aggregate and mud served as
the paste. Once the mud had hardened, the embedded straw strengthened the
structure of the brick.
In
modern concrete, a variety of materials help make up aggregates, including sand
and various sizes of gravel and crushed stone.
Aggregates
are divided into two categories based on particle size: fine aggregate, which
is 0.2 inches (5 millimeters) or smaller, and coarse aggregate as large as 1.5
inches (38.1 millimeters).
Concrete
makers have even found ways to reuse such items as oyster shells and even waste
ash from coal-fired power plants.
Cement serves as the paste in modern concrete. Dry cement
mixes contain a host of different materials that typically include limestone,
clay, gypsum, and various other chemicals and minerals.
After
the materials have been crushed, mixed in appropriate proportions, burned and
ground, the resulting mixture is ready to be combined with water.
When
this occurs, a chemical process called hydration takes place,
and the resulting paste dries into a solid stonelike mass.
Portland
cement is the most common variety
of cement and is typically used in concrete production.
Supplementary
cementitious materials (SCM)
such as pozzolans (such as power-plant ash) and slag (a
by-product of iron smelting) are sometimes added to the cement mixture as well.
Pozzolans
are materials that exhibit cement-like properties when combined with lime.
When
it comes to ensuring proper proportions, making concrete is a lot like baking.
Mess up the contents of the mixing bowl, and you're likely to have a disaster
of a cake on your hands.
Likewise,
cement, water and aggregate measurements have a huge influence on concrete's
strength, permeability and durability.
The
amount of water used can also influence how well concrete bonds with
reinforcement and how much shrinking and cracking take place during drying.
Additional
ingredients, called admixtures, further complicate the process. An
admixture's effect depends greatly on the other proportions in the concrete
but, if added during mixing, they can serve as a kind of power-up for the
concrete.
Some
admixtures affect color, hydration time or shrinkage rate, while others provide
such benefits as minimized corrosion for steel beams inside the concrete block.
Once
the best ingredient proportions have been determined for a given job, it's time
to mix it up and pour a few slabs.
Mixing and Pouring Concrete
After all the required dry materials have been
assembled, they typically go into one of two destinations: a bag or a mixer.
Ready-mix
concretes are premixed bags of dry concrete ingredients that only require water
and mixing.
These
are ideal for small projects, and manufacturers tend to offer mixtures that are
ideal for various specialized home projects. Users can generally add water in a
mixing pail and apply the concrete with a shovel.
If
you're looking to pour a lot of concrete though, premixed bags get expensive
really fast -- to say nothing of the packaging waste.
For
larger projects, some construction companies mix concrete on-site. Otherwise,
mixing takes place at the concrete plant, and the batch is subsequently
transferred to the job site in a rotating drum mixer.
These
devices keep the concrete from setting by constantly tumbling the mixture
around and are usually either truck or trailer mounted.
However,
this hot potato act can't go on indefinitely. Most batches of concrete need to
be discharged 90 minutes or 300 revolutions after the addition of water.
After
that point, the batch experiences slump loss, which refers to
decreased workability.
If
construction crews know in advance that time will be an issue, certain
admixtures can make it possible for concrete to travel longer in drum mixers.
Once
at the job site, it's time to pour the concrete into prearranged forms. These
wooden molds are carefully measured to meet the necessary slab specifications.
In
the case of reinforced concrete, the form will contain steel rods or mesh. Then
the concrete can be compressed and smoothed over as needed.
Temperature
and moisture are both important factors in proper concrete drying.
As
the concrete dries, it inevitably shrinks. To prevent this process from taking
place unevenly and potentially warping the finished slab, it's important to
keep the surface of the concrete damp to slow the shrinkage uniformly.
Extremely
low or high temperatures can also pose a problem. Extreme heat will cause rapid
curing, while chilly temperatures can draw the process out and produce weaker
concrete.
As
such, summer construction crews typically work during the cooler portion of the
day and use warming additives during cold weather.
Calcium
chloride, for instance, will speed up the chemical curing process, which
produces heat. A good sealant will help protect the concrete once the slab has
fully dried.
Just
as you can avoid baking mishaps by simply buying a prebaked pie, you can also
purchase various sizes and shapes of precast slabs directly from the concrete
plant. Sorry, mobsters -- they're typically not available in boot size.
About Robert Lamb
As a child, Robert Lamb dreamed of becoming a mad
scientist when he grew up. As this profession proved to be largely fictional,
however, he swallowed his heartbreak and turned his attention to the written
word instead. He earned his bachelor's degree in creative writing from the
University of Tennessee in Knoxville, which launched him on a career path
through high school English classrooms, small-town newsrooms and finally into
the offices of HowStuffWorks.
As a senior writer and podcaster for Stuff to Blow Your Mind, Robert now spends his days sifting through all the
scientific wonders that make the world so mad and amazing. He currently lives
in Atlanta with his lovely wife and their beautiful one-eyed cat. When he's not
researching the apocalypse or the miracle we call a space toilet, he enjoys
listening to electronic music, painting the odd miniature and writing fiction.
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