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Name: Kaylee M.
Status: Student
Age: 8
Location: N/A
Country: N/A
Date: 2002


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.



Replies:
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


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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