Mixture - Concrete - Solution/Explanation

How is the problem solved ? I don't know.

It has been shown to you how the problem is solved.

You got the solution too.

I'm sorry it's beyond your current capabilities.

I have been out of school over 30 years and am trying to re-learn on my own. I guess I am too rusty. I just don't get it.

Would someone kindly give more information on ratios in reference to mixtures? I am trying to understand.

A follow-up question:
I just a related statement about another 1:2:4 mixture of concrete. The statement indicates for a cubic FOOT of concrete, .22 cu. ft. of cement, .44 cu. ft. of sand, and .88 cu ft. would be used. Would those be the correct answers ? The statement did provide the calculations. Just curious. Thanks.

The statement should be: I just read a related statement.

We cannot tell, just with those bits of statements.

The answers COULD be correct depending on the enouncement of the problem.

They do not fit your problem as stated.

Here is the statement:
"Concrete is made by mixing cement, sand, gravel, and water. A 1:2:4 mixture is made up of one volume of cement, twice that volume of sand, and 4 times that volume of stone. The volume of the mixed cement is less than the sum of the volume of the parts, because the open spaces between the stones which must be filled in making the concrete. One cubic foot of concrete contains: for 1:2:4 concrete - .22 cu. ft. cement, .44 cu. ft. sand, .88 cu. ft. gravel."

Maybe that will help.

Yes, that helps.

Now, what's the question?

I know the mixture would be 7 parts total. Would the mixture be calculated as 1/7 cu. ft. cement, 2/7 cu. ft. sand, 4/7 cu. ft. gravel to get the .22, .44, and .88 ? I am trying to work out the solution.

Those are indeed the volumes used to make a cubic foot of the mixture.

If you add them up, you'll have a total volume of 1.54 cu. ft. but you are looking for a 1 cu ft.

So where is the difference?

It's because when you mix the parts, the smaller granules fill the place taken by the air, which is expelled, diminishing the total volume.

Then you take your previous values to exclude the air:

x= .22

y= .44*.66 =.290

z= .88*.55 = .484

Then x+y+z=1 cu. ft.

Thanks.

One more question:

Concrete weighs approx. 150 lbs. per cu. ft. Would the parts for the 1:2:4 mixture be 150 x 1/7 = 21.42 = 21 lbs. of cement, 150 x 2/7 = 42.85 = 43 lbs. of sand, and 150 x 4/7 = 85.71 = 86 lbs. of gravel/rocks ? Just curious. Thanks again.

Please reply. Thanks.

No, you cannot use that ratio as the relative density of the mixture's components is different.

You can always make the calculations using the relative density of cement, sand and gravel taken from this table:

Density of building materials