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Volume of concrete from one bag of Portland cement

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Squirl:

--- Quote from: n74tg on October 24, 2008, 06:21:33 AM ---
So, to answer your question, 1 cubic foot of 3,000 psi concrete would require 285/20 = 14.25 lbs of Portland (0.15 bags).  Or, to put it another way, 1 bag of Portland will yield "about" 6-2/3 cubic feet of 3,000 psi concrete.


--- End quote ---

I may be a little off, but if 1 bag of portland equals 6.66 cubic feet of 3000 psi concrete then it would be 4 bags to equal 1 cubic yard.  (1 cubic yard = 27 cubic feet.  27/6.66 = 4.05)

To reverse math the 5 bag or 6 bag mixes. 
6 bags per yard would equal 4.5 cu feet per bag (27/6) Wich would be .22 bags of concrete per cubic foot (1/4.5) and 20.4 lbs of concrete per cubic foot (.22*92)
5 bags per yard would equal 5.4 cu feet per bag (27/5) Which would be .18 bags of concret per cubic foot (1/5.4) and 17lb per cubic foot (.18*92)

I have never been the one doing the measurments for concrete, so my question is is the 3:2:1 ratio by weight or by volume.

glenn kangiser:
Volume - shovel, bucket, cup, bag, as long as the same tool is used to measure.

Rule of thumb - 5 sack is 2500 - six sack is 3000 psi.  In reality testing show variations but near all should be above the test requirement in 28 days if well mixed and not overly watered down.  Excess water weakens concrete.  Drying - not keeping damp during the cure weakens concrete.  Most of the time it does not cause problems but it could be better if done correctly.  A sealer such as concrete cure and seal sprayed over it could maintain moisture in the concrete also.

I don't usually worry about it a whole lot if not on critical applications.

Squirl:
It seems like a lot of trouble.  I just priced out the difference in price for 20 cu ft in my area.  An 80 lb bag of 4000 psi quikrete is 3.24 at lowes.  It takes 1.6 bags of 80lb per cu/ft (spec data quikrete.com).  So for 20 cu ft it would cost $103.  On the other hand portland cement costs 9.34 per 92lb bag.  If you need 4 bags then that would be $37 for the cement.  That is before you have to add in the sand, gravel, and equipment.  You also have to add in the gas to get the extra supplies.  But, if you already have the supplies and equipment on site, save yourself the $66.  It would gost me more than $66 to drive around and pick up the extra stuff.

MountainDon:
The bagged mix prices vary locally. That 80# bag is 3.85 here. Frequently it's cheaper at HD, but today it's the same. You have to take everything into account.

Squirl:

--- Quote from: n74tg on October 23, 2008, 04:39:44 PM ---Thanks Mike, but that website gives me the info I already have; ie volume of concrete needed.  The number of 40 or 80 lb bags is for premix concrete (already contains sand and gravel).  I mix mine from raw ingredients because it is substantially cheaper that way. 

--- End quote ---

If you can, would you mind giving a break down of the costs for the project.  I am used to doing small jobs and only going to home stores and buying premix.  When I add in the cost of the cement + sand + gravel it comes out much higher so I never bothered.  I never considered myself a contractor with enough know how to go to a place that specializes in concrete.  My general assumption was that they deal in volume orders and my small order wouldn't be cost effective.

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