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  • Help with concrete volume?

    Using a concrete calculator and FB's book I'm going to pour a 86" x 73" x 5.5" slab or .74 of a yard. I'm trying to do it the most cost efficient (cheapest LOL) way possible. I don't understand a couple things with concrete. I know I need "Portland cement." What I think my options are, either:

    A: Buy bags of Portland cement, sand, and gravel and mix it myself.

    B: Buy bags of premixed concrete, and mix them myself.

    C: Hire a concrete company to deliver mixed concrete.


    My 2 big questions are :

    Do options A & B provide roughly the same product just different cost and ease?

    AND

    I'm told option A will be the cheapest. I have access to a mixer from a friend. But, I can't figure out how much of each item I will need! For .74 yards how many bags of cement, how much sand, and how much gravel? Can someone give me some ideas? I think I've been reading WAY TOO MUCH! lol

    Thanks! -John

  • #2
    And as usual, when I step back and think... I think I find an answer. So if I go option A, The mixture I found online said "1 part cement, 2 parts sand, 3 parts gravel." I will need

    5- 94lb bags of cement. = 470lbs
    940 lbs of sand
    1410 lbs of gravel

    Am I right? lol

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    • #3
      I'd be interested in folks' ideas as well in terms of option A vs B, what the cost tradeoffs were and was that worth the hassle? Are there benefits of option A other than price to consider?

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      • #4
        A is the cheapest, provided you have a method of hauling bulk aggregate. A and B are about the same amount of physical labor, and C is stupid expensive.

        You are approximately correct on your totals, but when mixing mortar or concrete, it is normal to use volume instead of weight. For example a cubic foot of wet sand weighs a lot more than a cubic foot of dry . sand. So the formula is 1 part portland (by volume), 2 parts sand, 3 parts gravel.

        Using the portland as a baseline since it is a given (one 94# bag=1 cubic foot) 5 cuft portland, 10 cuft sand, 15 cuft portland = 30cuft of concrete in a straight of formula. It will actually be closer to 20 cuft or .714 cubic yard (the reduction in volume is based on the sand and cement filling the voids in the gravel).

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        • #5
          i went with option B. my foundation slab took about 70 bags (its large L shape) which is about 28 mixer loads. it took long enough and strenuous without measuring parts. I will take the premixed shortcut any day , from my calculations savings are nearly negligible.
          Anton.

          My 36" - https://community.fornobravo.com/for...t-bg-build-log

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          • #6
            Option D - depending on where you are - and access to your site - some of the rental center places have a "pre mixed trailer cart" concrete service that is probably somewhere in the middle on cost, but will save you a bunch of time.

            Example only, for my location- http://www.handyandyrentatool.com/ucartconcrete.html
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            • #7
              Originally posted by deejayoh View Post
              Option D - depending on where you are - and access to your site - some of the rental center places have a "pre mixed trailer cart" concrete service that is probably somewhere in the middle on cost, but will save you a bunch of time.

              Example only, for my location- http://www.handyandyrentatool.com/ucartconcrete.html
              That's pretty cool. Tho backing that into my backyard would be tricky since I wouldn't be able to see the container.

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              • #8
                Also remember, if making volume calculations that if mixing the concrete yourself (option A) 3:2:1 (6 parts) will only give you 3 parts of concrete because the sand,cement and water fill all the spaces between the aggregate, so theoretically the volume of aggregate used equals the volume of concrete produced. In practice you'll get around 15% more, especially if the aggregate contains some finer material that has already filled in some of the spaces. This is why weighing the dry materials is a more accurate method although often impractical.
                Last edited by david s; 04-19-2017, 05:27 PM.
                Kindled with zeal and fired with passion.

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                • #9
                  Originally posted by Tscarborough View Post
                  You are approximately correct on your totals, but when mixing mortar or concrete, it is normal to use volume instead of weight. For example a cubic foot of wet sand weighs a lot more than a cubic foot of dry . sand. So the formula is 1 part portland (by volume), 2 parts sand, 3 parts gravel.
                  Thank you for this! I didn't even think about the measurements in volume. How can I tell that 1 ton of sand and 1 ton of gravel will be enough for 5 cubic feet of cement? Is there a calculator or am I already overkill?

                  Originally posted by cnegrelli View Post
                  I'd be interested in folks' ideas as well in terms of option A vs B, what the cost tradeoffs were and was that worth the hassle? Are there benefits of option A other than price to consider?
                  Here is what I found: Option A was overwhelmingly (to me) the cheapest. I will have to buy 5 bags of portland cement @ $11.77 a piece. In order to buy sand I have to buy at least 1 ton. I can pick up however much less than that I need but 1 ton is what I'm picking up! 1 ton of sand is $14.50, and the gravel was the same rule for a minimum. 1 ton of gravel is $28.99. So all in, the total will be less than $120.

                  Option B was closer to $160. The bags may already be mixed, but I figure I am going to have to mix something anyway. Might as well save that $40-50 to spend somewhere else and do a little more work.

                  Option C: I was really happy when the concrete person said it was $120 per yard! I thought my problems were solved! Then she said, "plus a $125 delivery charge. Also a $100 fee for buying less than 5 yards, and a $100 per hour stay fee (because I have a septic tank too close to the back of my house. I'd have to wheel barrow it over) plus taxes and mix fees." She didn't even tell me what the mix fees were...

                  Originally posted by deejayoh View Post
                  Option D - depending on where you are - and access to your site - some of the rental center places have a "pre mixed trailer cart" concrete service that is probably somewhere in the middle on cost, but will save you a bunch of time.
                  What a great idea, and OF COURSE, there is nothing like that near me!

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                  • #10
                    I have a price list from a local nursery that says sand, gravel, and rock run about 2500 to 2700 lb per yard (copy attached). A google search had one site quoting 2600 to 3000 lb per yard. Don't know if any of that helps. I also bought a cheap HF mixer and used it for my cores, hearth, and dry mixing my mortar. I plan(ed) to sell it when I was done, but still have it in the shed to see if I have other projects for it first. I think I saved enough $$ by not renting a mixer to pay for over half of the one I bought.
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