Concrete is not found in nature
the way we would find aluminium, nickel or iron. Concrete is produced from
combining water, a special cement and rock:
PORTLAND
CEMENT + WATER + ROCK = HARDENED CONCRETE + ENERGY(HEAT)
Heat? Yes, and lots of it if your
concrete structure is big. The heat, and temperature variations in general, can cause cracking problems.
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A common mistake people make is to use the words cement and concrete interchangeably. It is important to remember that cement is one of the
components of concrete and concrete is the structural material. The cement
used in concrete is not used as a building material because it would be too
exorbitant and not as strong as concrete. So when you see a parking garage, a
driveway, a sidewalk or a road remember it is made of concrete, not cement.
And, by the way, that funny looking truck is a concrete mixer, not a cement
mixer!
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Cement is a general name for a material that binds other
materials together such as course aggregate, fine aggregate and more. Yes,
some people named it as glue. There are many materials which we would
classify as cements and they are usually identified with certain uses, and
can produce different types of "concrete". The type of cement used
to make the riding surface of some of our roads (blacktop!) is called asphalt
cement. It is a petroleum bi-product, and it binds rock into the road
material we call asphaltic concrete.
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Portland cement is
used for concrete construction in bridge and dam. This type of cement binds the rock (also called
aggregate) together to form concrete. Portland cement is a mixture of
processed limestone, shales, and clays which contain the following compounds:
CaO (lime), Al2O3 (Alumina),SiO2 (silica)
and iron oxides. The amount of these compound will affect properties of the
concrete.
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Hydration happen when adding water
to the dry cement starts a chemical reaction. While the mixture of cement,
water, and rock is fluid, it can be poured into molds (called formwork) of
arbitrary shape. This is a valuable property of concrete which allows us to
build dams with the many different shapes which you saw in the history of
dams. The compound gradually hardens into the desired final shape. And also
the strength of the concrete gradually increase as the time longer.
The water/cement ratio (w/c) of
the mixture has the most control over the final properties of the concrete.
The water/cement ratio is the relative weight of the water to the cement in
the mixture. Engineers are responsible in design the water/cement (w/c) ratio.
Selection of a w/c ratio gives the engineer control over two opposing, yet
desirable properties: strength and workability. A mixture with a high w/c
will be more workable than a mixture with a low w/c: it will flow easier.
However, the low the workability the mixture, the more strength the concrete
is. The engineer have to decide what ratio will give the best result for the
given situation. This is not an entirely free choice because the water/cement
ratio needs to be about 0.25 to complete the hydration reaction. Typical
values of w/c are 0.35-0.40 because they give a good amount of workability
without lossing a lot of strength.
 The other important component for strength is the aggregate, the rock that is being combined with the cement. Aggregate is what makes the difference between hardened cement and the structual material, concrete. Aggregate increases the strength of concrete and is an economical way because it takes up a large volume of the concrete and is much less expensive than an equivlant volume of cement. To make very strong concrete requires a low w/c ratio and strong aggregate. | |||||||||
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The video below shows how to pour a concrete slab.
nice informative video..^^~V
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