As a child, you probably heard of the story The Three Little Pigs. This story taught
us as a child that
having a house made out of sticks or hay was not a suitable option for a home.
However, it did teach that bricks made with concrete was a viable option to
endure external forces such as the “huff and puff” from the big bad wolf. Even though concrete is an appropriate
building material, the variability of the strength in the concrete may not be
proper for different forces such as earthquakes to strong winds equating to a
tornado. To determine what is proper for different sizes and shapes of
building, the strength of the concrete is essential.
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Testing a 150x300-mm concrete cylinder in a compression machine |
Normally, the tensile strength is approximately 8%
to 12% of the compression strength. It is often estimated as 0.4 to 0.7 times
the square root of the compressive strength. Similarly, 0.7 or 0.8 times the
square root of the compressive strength defines the flexural strength. Moreover, the sheer
strength can vary from 35% to 80% of the
compressive strength. Tam explains that there is a “correlation between
compressive strength and flexural, tensile, and shear strength. It varies from
the concrete ingredients.”
As
Tam described, ingredients are the most influential factor that gives
variability to the strengths of the concrete. Concrete is essentially a mixture
of two components called aggregates and paste. The paste is usually comprised
of cement and water, which binds to the aggregate. Generally, the aggregate is
sand and gravel or crushed stone. Once mixed, the paste will harden because of
the chemical reaction that will occur from the cement and water.
Aggregates
are divided into two different groups called fine and coarse. The fine
aggregate “consists of natural or manufactured sand.” Compared to the coarse aggregate,
which are particles that are 1.25 mm to 56 mm. Tam further states that, “the
most commonly used maximum aggregate size is 20 mm”
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Ken Tam testing and recording the compression strength of the cylindrical concrete
The strength for the concrete varies for the
intended use. Some concrete with compression strengths of less than one MPa can
be used for insulation of underground steam lines. Others such as roof fills
are between 0.7 and 1.5 MPa. For a foundation of an average household is near
25 MPa. However, factors such as the environment could potentially change the
compression strength acceptable for the building.
With
a force of two on the Beaufort Wind Scale, which is four to six knots, the
higher buildings will have no trouble withstanding these external forces. However,
when the wind becomes stronger and has a force of nine classified as a strong
gale, which is 41 to 47 knots, the building could potentially crumble to the
earth. This is where the compression strengths of concrete play a major role in
the building. With higher compression strengths for the foundation of the building,
the more pressure it can tolerate from the external force.
The
main strengths that are essential for the concrete is the flexural, tensile,
and compressive strength. The flexural and tensile strength is necessary for
the swaying motion the building achieves from being pushed by the strong gale.
If the building does not achieve the correct flexural and tensile strength, the
concrete will slowly start to crack from the pressure and eventually lead to
the building to tumble down. Moreover, the compression strength is highly
important because of the stress the swaying motion causes to the overall
building. Tam states, “As the wind acts upon the building, the weight shifts
from side to side. The compression strength needs to be able to withstand these
shifts in weight.”
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A broken concrete cylinder with a compression strength of 22.4 MPa |
A
level five earthquake on the Richter scale can be enough to completely demolish
a household. To avoid serious damage to a house, compression, tensile, and
flexural strength is key. The tremors caused by the earthquake will violently
shake the house causing the building to vibrate back and forth at a high
frequency. This will cause the concrete to crack under pressure, but with a
high enough flexural and compression strength, the building will be able to
endure these movements because the concrete will fluctuate with the vibrations
and the compressive strength will aid in the shifting weight. The tensile
strength will prevent the concrete from cracking under the massive pressure
exerted from the earthquake.
A video was taken of a building swaying in the aftershocks of an earthquake, http://www.youtube.com/watch?v=uGYyZxKy4PI.
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A seismograph and the Richter scale on the earthquake |
Overall, the strength of concrete is vital in
buildings because it gives the foundation towards the building and prevents
large buildings from breaking apart. However, even though the strength of
concrete is important, against earthquakes the concrete will not be able
withstand high levels on the Richter scale because of how powerful they are,
but to wind it is crucial. Large buildings in particular need specific
strengths of concrete for them to withstand the external forces that are
exerted on them every day. Even households need certain strengths of concrete
for the external forces that affect them on a daily basis. With the proper
strength of the concrete, these external forces such as the “huff and puff” of
the big bad wolf will keep your house standing.