Author Topic: Volume of concrete from one bag of Portland cement  (Read 16946 times)

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Offline n74tg

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Volume of concrete from one bag of Portland cement
« on: October 22, 2008, 07:08:01 PM »
I'm trying to figure how many bags of Portland to buy to mix the concrete for my front porch. 

I need right at 14 cft of concrete.  I'm using a 3:2:1 mix (rock, sand, Portland).  As 3+2+1 = 6 and I need roughly twice that in volume, I'm figuring about 2.5 cft of cement. 

the question is "how many cubic feet" of Portland (Type 1) in a 92 lb bag. 

I found two different numbers on the internet.  One says Portland weighs 1506 kg/m3 (cubic meter), which converts to 93.85 lbs/cft, so a 92 lb bag is right at 1 cft of Portland. 

The other number says Portland is 3150 kg/m3, which is about twice the first number. 

Anybody got any experience with this. 

Maybe another way to ask this is one bag of Portland, if using a 3:2:1 mix yields how many cft of concrete. 

thanks
« Last Edit: October 23, 2008, 09:48:06 AM by n74tg »
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Offline MikeT

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Re: Volume of concrete from one bag of Portland cement
« Reply #1 on: October 23, 2008, 04:31:46 PM »
If you go to Decks.com and click on the calculator tabs at the top, you can do some figuring there.  It gives calcs for 60 and 80 pound bags.  But it also gives volume.

I plugged in and a cubic yard will require just over 40 bags of 80 lb bags.  Then if you plug in a cubic foot, it will require 1.5 bags of the 80 lb concrete.  The volume is .04 cu yard.

I do not know if this helps or not.

mt

Offline n74tg

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Re: Volume of concrete from one bag of Portland cement
« Reply #2 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. 
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Offline Redoverfarm

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Re: Volume of concrete from one bag of Portland cement
« Reply #3 on: October 23, 2008, 06:29:23 PM »
I remember hearing the concrete plant owner try to explain to me what the different mixes were.  Something to the effect that a 4 bag mix was 2,000 PSI, 5 bag @ 2500 and each bag added ups the PSI by 500.  That was for a Cu yd.  I think gravel or aggregate constitues about 60 % mix with water adding about 15% of the mix.  Give your local redy mix plant a call and maybe get some answers.  Ours is real helpful well at least to me anyway as I have probably bouight 150-200 yards in the last 10 years or so.  Small stuff I usually mix my own.  Don't really have the figures as I just have the three major components and mix what I need.  More of a guestimate really.  But nothing really goes to waste.

I am like you . I Googled some for the answer by basickly they were sponsered by pre-mix and not cement plus other portions to make.

Offline n74tg

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Re: Volume of concrete from one bag of Portland cement
« Reply #4 on: October 24, 2008, 05:24:56 AM »
Thanks "Red":

I called the batch plant, and for 3/4 yard of 3,000 psi mix, here is the ingredient list.
1489 lbs of pea gravel, which weighs 2500 lbs per yard
1010 lbs of cement sand, which weighs 2500 lbs per yard if dry, 2700 lbs if a "little" wet
285 lbs of Portland, which is 3.03 bags (92.4 lbs per bag)
134 lbs of water (16 gallons). 

The 134 lbs of water divided by 285 lbs of Portland, yields a water-cement ratio of 0.47, which is about right for 3,000 psi concrete.

So, my question is answered.  I need to go buy one more bag of Portland.

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alcowboy

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Re: Volume of concrete from one bag of Portland cement
« Reply #5 on: October 24, 2008, 05:54:06 AM »
Pardon my ignorance but how many bags of portland total does one need for 1cu ft?  I could not find where you told how many bags you had to begin with.

Offline n74tg

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Re: Volume of concrete from one bag of Portland cement
« Reply #6 on: October 24, 2008, 06:21:33 AM »
Okay, a few things have changed in my requirements, so let's bring you (and everybody else) up to date.

The load is now 3/4 yard (20 cubic feet), not the 14 cft originally mentioned.

Cement requirements will be 3.03 bags, of which I already have 2 bags. 

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.

===========================
Now, let's confuse the issue completely.

I called in all 5 batch plants and asked the same question, what ingredients would you put in 1 yard of 3,000 psi concrete (using pea gravel)
And, as expected, the answers were kinda all over the map.  So, here goes.

                                                Plant A          Plant B           Plant C             Plant D              Plant E
Pea Gravel                                  1985             1703              1600                1973                 1850
Sand                                          1346             1512             1440                 1296                 1400
Cement, Portland                          380               470                400                 517                   460
Water                                         178               300                50                   250                   233

Comments:
The plant E guy, quoted 3,000 psi concrete as "5 bag mix", ie 5 bags of Portland per yard (92.4lb bags).  He kinda, sorta disagreed with my contention that 3,000 psi concrete was typically 6 bag (per yard) mix. 
The Plant B guy was an idiot, I had to talk him through every step of what I wanted.
The plant A and plant E guy's asked the most intelligient questions about my requirements; ie what would the concrete be used for, was it to be trucked in and/or "pumped" into place.
There is a small possibility that the Plant C guy quoted me 50 gallons of water, not 50 lbs.  But 50 gallons of water would be 417 lbs, much higher than anybody's estimate --AND-- would equal a water cement ratio of 1.00 (exceedingly soupy concrete).

So guys, I guess it's pay your money and roll the dice.  I buy all my aggregate and sand from Plant A, so I will use their numbers.   
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alcowboy

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Re: Volume of concrete from one bag of Portland cement
« Reply #7 on: October 24, 2008, 06:41:19 AM »
Uh ok. Now it is as clear as mu..er cement. No actually I do understand it better, I think. To be sure appx 6 - 92lb bags will yield appx 1 yard. Right, wrong, indifferent?
Scott Gleaton

Offline glenn kangiser

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Re: Volume of concrete from one bag of Portland cement
« Reply #8 on: October 24, 2008, 08:00:07 AM »
I agree with John - Redover - in general.  5 sack around 2500 six around 3000.

Keep in mind that smaller aggregate such as pea gravel pump mix requires more cement than inch rock from a mixer truck to attain the same sterength.  Many more pieces to glue together. 

My concrete supplier said water curing the test cyls is the way only to assure good tests and the last tests at Harry's place we did went over 5000 psi o some of the tests.  A six sack - pea gravel pump mix with an onsite 10 yard computerized mixer truck.
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Offline glenn kangiser

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Re: Volume of concrete from one bag of Portland cement
« Reply #9 on: October 24, 2008, 08:02:23 AM »
Uh ok. Now it is as clear as mu..er cement. No actually I do understand it better, I think. To be sure appx 6 - 92lb bags will yield appx 1 yard. Right, wrong, indifferent?
Scott Gleaton

With normal aggregate that would be a high strength mix - 5 sack mix would do for most applications.  I poured about 26 yards of about 6 sack pump mix yesterday for about a 1500 sf floor.
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Offline Squirl

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Re: Volume of concrete from one bag of Portland cement
« Reply #10 on: October 24, 2008, 08:54:14 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.


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.

Offline glenn kangiser

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Re: Volume of concrete from one bag of Portland cement
« Reply #11 on: October 24, 2008, 09:14:56 AM »
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.
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Offline Squirl

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Re: Volume of concrete from one bag of Portland cement
« Reply #12 on: October 24, 2008, 09:23:53 AM »
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.

Offline MountainDon

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Re: Volume of concrete from one bag of Portland cement
« Reply #13 on: October 24, 2008, 09:40:21 AM »
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.
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Offline Squirl

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Re: Volume of concrete from one bag of Portland cement
« Reply #14 on: October 24, 2008, 09:48:22 AM »
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. 

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.

Offline Redoverfarm

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Re: Volume of concrete from one bag of Portland cement
« Reply #15 on: October 24, 2008, 11:51:09 AM »
Too complicated for the mountains. Just shovel 3 gravel, 2 sand and one cement into the mixer several times until you have made enough to fill the hole so to speak.  Math was my poorer subject.  And when you get done you can say "I did it myself and saved 10 cents".  ;D.

With all jokes aside most concrete plants have set minimums on the # of yards. So if you have a small job you either pay or do it yourself.

Offline n74tg

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Re: Volume of concrete from one bag of Portland cement
« Reply #16 on: October 24, 2008, 04:22:16 PM »
Okay, $31 combined cost for pea gravel and sand and $25 for the Portland.  I have about 1/4 yard of sand and pea gravel (each) leftover, the girlfriend has already layed claim to that. 

So $56 combined cost for 3/4 yard. 
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