Ask A Scientist© Environmental Earth Science Archive

name         Kaylee M.
status       student
age          8

Question -   I need help for a science project and am allowed to ask
an expert. I am making 2 bricks with two sizes of sand particles mixed
with glue to see which is stronger. Can you tell me how to put weights or
pressure on the bricks so I can measure how much strength they hold
before they break. My mom said to use books or vice grips or clamps, but
how would you measure how much pressure to break the bricks. also do you
know how much weight or pressure a real brick can take or half a brick
since we are making our bricks half as tall as a real house
brick.
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Dear Kaylee,

Sounds like a neat experiment.  What you are doing is called compression
testing.  The same things are done to bricks and concrete, just to
mention a few, to see how much load or force they can handle.  In the
lab, we use machines that measure the force (in your case, weight) that
causes the material to fail or fall apart.  We take this force measured
in pounds and we divide it by the area over which this force was
distributed to give us a value of pounds per square inch.  Pounds per
square inch is abbreviated by psi. This value of pressure is know as
stress.  Different materials have different values for this stress
(which we call yield stress) which makes different materials better for
different applications.  The concrete used on a patio in your back yard
will be different than the concrete used for building bridges because
the bridges will need to carry more weight.
Now with that out of the way, let's talk about your experiment.  Using
vice grips and clamps will be tough in your case because you don't
exactly know how much force you can create by using them.  The key to
this experiment is knowing what force (or weight) it takes to cause the
brick to fail, and there is no direct way of you measuring that force
with clamps.  We could do some calculations that would determine that,
but there is an easier way.  I suggest putting the brick on a flat
surface and then putting weights on top of the brick until it falls
apart.  Once you know that weight, you need to determine the area of the
face of the brick on which you placed the weight.  Divide the area into
the weight and you will have determined the yield stress in psi of your
brick.  It is important that you use the area of the face of the brick
that you placed the weight on.  I hope it goes without mentioning, but
measure the area first before you start putting the weights on it.  The
weights can come in just about any form as long as you know the values
of the weights.  You might could use the kind weights that are in a gym
for instance because they are marked with the weight values.  You could
use books, but you'll have to weight them on a scale and then mark how
much they weigh.  Put the weights on the brick in increments so that you
can hopefully determine reasonably close how much weight the bricks can
actually take.
A real brick can take around 2,200 to 2,500 psi before it starts to fall
apart, but understand that these are rough numbers.  The reason is
because there are many things that can cause the brick to have different
yield stress like the material used to make the brick, the firing
process to "bake" the brick, and the shape of the brick just to name a
few.
I hope this helped and good luck on the experiment.

Chris Murphy, PE
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Hi Kaylee!
What a nice project!
Beeing  an elementary school project I think I can help you even though I am 
not an engineer.  See first why make your bricks half as tall as a real brick 
house?  I would say if you could have all 3 the same size that will make
your experiment  more correct because as much similar your experimental
are, more correct your results would be.
Another thing that matters also besides the sand particles size, it is
the full weight of your samples. In order to compare them you must begin
with bricks of the same weight, size and form.
The idea Kaylee, is to eliminate all the factors that could influence
the strength but for the size of the sand particles.
If you have all the conditions equal: bricks size, bricks weight,
bricks  form ( height , length and width), using also the same kind
of glue,  and using also the same instrument or object to evaluate
the strength of both bricks , then you could determine which is the stronger.
That is a must for any experiment or comparison, that is  all the characteristics,
with the exception  of the one you are measuring or evaluating, must be equal.
Now for your measurement:
you must make some samples that will act as weights, i suggest  iron nails that
are enough heavy. Take a  bag (cloth, paper or plastic, but must be sturdy)
put over the first brick  and begun adding nails until the brick breaks.
I do not know but it will need a lot of nails.
Do the same for the second brick, taking care that the nails bag to repose over
the same position at both bricks.
After determining the number of nails needed to break either brick, you can
have a more precise measurement weighting them.
And you can do the same experiment with the house brick.
If you can, repeat the measurement with more than one sample of bricks, so you
can have an average measurement.
I am sure if you work properly you will have quite a good project and results!
An thanks for asking NEWTON! Tell  your friends about us!
Mabel
(Dr. Mabel Rodrigues)
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