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Construction Problems Questions


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#1 owiewave

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Posted 27 January 2011 - 04:32 AM

I got a set of problems from a co-worker for the construction section of the PE exam, and have a couple of questions.

1. when solving concrete mix problems, once you've gone through all of the steps listed in the CERM and determine the volume of your batch, you can go in and determine the quantities of each element (fines, coarse, water, etc.) for a specific quantity of concrete (say 25 yards). In the CERM, if you found that you would need 3.3 bags of your concrete to do a job that required 25 yards of concrete (made up numbers here), you assume, as you would in the real world, that you need 4 bags of concrete (you can't buy .3 bags of concrete). I have done a couple of example problems from other sources where to get the correct answer, you use the decimal. What is the standard? I thinmk the CERM method makes more sense, and it drastically changes your answer if you do one way or the other.

2. mean compressive strength. given a set of data from a 28 day sample, and asked what the mean compressive strenght was, I figured that you took the mean of the break values (in pounds) and divide it by the area of a cylinder's end. doing this I not only got nothing close to the supplied answers, but when i even reverse engineered from the answers, did not get an answer that made sense. Am I way off here?

TIA

#2 Walker D

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Posted 31 January 2011 - 03:56 PM

Give me a day or two and I will try to get back to you on these questions.

#3 Walker D

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Posted 01 February 2011 - 09:20 PM

QUOTE (owiewave @ Jan 26 2011, 11:32 PM) <{POST_SNAPBACK}>
I got a set of problems from a co-worker for the construction section of the PE exam, and have a couple of questions.

1. when solving concrete mix problems, once you've gone through all of the steps listed in the CERM and determine the volume of your batch, you can go in and determine the quantities of each element (fines, coarse, water, etc.) for a specific quantity of concrete (say 25 yards). In the CERM, if you found that you would need 3.3 bags of your concrete to do a job that required 25 yards of concrete (made up numbers here), you assume, as you would in the real world, that you need 4 bags of concrete (you can't buy .3 bags of concrete). I have done a couple of example problems from other sources where to get the correct answer, you use the decimal. What is the standard? I thinmk the CERM method makes more sense, and it drastically changes your answer if you do one way or the other.


There can be slight nuances to a problem like this one. Do they ask how many bags are needed to create the appropriate mix or are they asking how many bags will need to purchase?

To create the appropriate mix you will, in fact, need 3.3 bags to mix the batch of 25 yards. But just like you said, you can't buy 3.3 bags. You will have to buy 4. You don't have to use all 4 bags and you shouldn't. If you were mixing concrete at a job site and you knew you only needed 3.3 bags for the specific batch you are trying to mix, you would not add all of the 4th bag. If you did use 4 bags you would automatically change to proportioning of the mixture and therefore, change to properties of the concrete.

Make sure you look closely at the wording of the questions. They always try to throw these 'curve balls' at you in very subtle ways.

Hope this helps! I will get to your other question tomorrow.

#4 owiewave

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Posted 02 February 2011 - 02:18 PM

Thanks, that helps, I will just try to make sure that I read the problems carefully, probably not a bad tip on ANY problem on the exam...

I believe my question about the force/area was an error in the question. I found several other identical problems with different numbers, and they just did a straight calculation of mean compressive strength/the area of the cylinder face.




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