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Question #Bags of Concrete for Foundation

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  • Question #Bags of Concrete for Foundation

    I was looking at the Materials list and it indicates that you need 46 80lb bags of concrete.

    What is the recommended minimum thickness be...a 3" thicks slab from my calculations would be 26 80lb bags...

    does anyone know of a way to calculate the number of bags required I am not sure my calcs are correct.

  • #2
    Bags of cement

    Really easy answer here. Buy more than you need and take back what you don't use. If you don't, you WILL run short on Sunday evening when the stores are all closed and you've got three square feet to go. Trust me, I've done it, far too many times. You'd think I'd learn.

    "Made are tools, and born are hands"--William Blake, 1757-1827


    • #3
      You need more than you think

      I don't know what kind of concrete you are using, but I used Sakrete and the bag contains a chart of how much you will need for slabs of various sizes. I poured a 5" slab; 30 square feet and it took me 30 sacks. It was a real back breaker.

      With the volume you are using, why not call up a truck? That's what I would have done.
      There is nothing quite so satisfying as drinking a cold beer, while tending a hot fire, in an oven that you built yourself, and making the best pizza that your friends have ever had.


      • #4
        Get more. Especially since you are doing the foundation, if you have extra left you will use it when you do the hearth.

        4 weeks ago I poured my foundation, I had calcuated for the size and shape I was doing, it would take 27 60# bags premix. So I got 30 bags just to be safe. Late saturday afternoon, I rushed to get 10 more bags and used 7 of them. So I was a full 10 bags short.

        When I poured the hearth last weekend, (62inches by 70 inches by 7.5 inches) I calculated it would take 20 #60 bags of premix. 2 bags #80 portland cement and 2 bags (4 cu ft each) of perlite. It actually took 23 bags of premix, 3 bags of #94 portland cement, and almost 3 bags (4 cu ft each) of perlite. Although this time I was prepared with plenty of extra material before I started so it realy wasn't an issue.

        I had calculated it in cubic inches, then divided it by 1728 to get cubic feet and then round up. Since the bags I was getting at Menards were .5 cu ft bags, it was pretty easy to convert cu ft to bags.

        I understand why I was so far off on the foundation calculations (first time doing it, un-even ground so difficult to get precise volume measurments, etc) but for the hearth I really thought I would be closer. I guess that is why I shouldn't quite my day job to build pizza ovens.

        BTW, unless you have plenty of helpers, moving, mixing, pouring, and finishing 40+ bags of concrete will really take it out of you. Plan on some downtime after you get done.


        • #5
          Re: recycled rebar

          Yo' Ya'll.. Being that we're trying to build our oven out of as much junk as we can muster, we're considering going with cast iron window bars as rebar for our foundation. They're solid and tough, and I can't see why it wouldn't work. We dug out our footings a foot and a half deeper around the perimeter of the slab where the stand walls will go and we're using galvanised pipes in those sections. It's not that rebar is expensive, we just think if we use recycled materials, all the save the world chicks will think we're cool. But, we're willing to be slapped back to reality if anyone can tell us why this won't work. Cheers.


          • #6

            There is an Italian potato dish called Verde Trovato (found greens) -- basically whatever you can find in the refrigerator.

            You're building a Forno Trovato; the ultimate chick magnet.
            Pizza Ovens
            Outdoor Fireplaces


            • #7

              Gentlemen of the barrow,

              I've mixed enough concrete by hand to resurface Interstate 5 between LA and San Diego. Advice? Unless you're in super shape, don't do it at all. I'm a part-time professional mason in very good shape, and I'll never hand mix a foundation again--ever, because it's too expensive. Caught your attention, haven't I?

              First off, everyone tries to peg the amount of materials we'll need to a T; never works, no matter how much experience you think you have. Materials lists are just that, lists of materials, but are they complete?

              There's one part of the equation that's hardly ever listed: time. Once you've excavated, built your form, installed the rebar, how much is your time worth to calculate the materials (feckless, cause you'll be short), order the materials, stack the materials, mix the materials, shovel the materials, run for more materials, finish the set concrete, then dispose of the empty bags? Think an hourly that's reasonably on the high side, say 5 bucks, okay, 10 if you're metric. You'll spend a grand of your time quick.

              Another chronically unlisted factor: what is the afterburn of hand work, how long will it last. If you don't pump iron, it'll take weeks for you to recover from two days of hand mixing if you're between 30 and 40. What's that worth? Let's say another grand, for argument's sake, not including the splints and the A535. I'm 59 and work with mortar and concrete several days every week; you do the math.

              You MUST guard against burning out, particularly at such an early stage, when it's just grunt work. It won't do you any good at all to charge out of the chute, because by mid-project you'll be so worn down you'll drop tools and quit until your batteries recharge. My father had a favourite adage about physical work: "Slow down to hurry up." Think deeply about that one, and William Blake's "Rest before labour."

              Okay, here comes the tricky part. Renting an electric cement mixer, not a KitchenAid, I trust, will cost about sixty bucks a day, times two days, equals $120. Subtract that from the two grand in time and agony you've already got in the bank. Voila, you're up $1880. Now, try this one. Call the cement truck dudes. Give them the size and approx depth of your form; those trucks are big and they won't run out in the middle of things. Let's say the trucked in cement will cost five times what the dry stuff would. I'll choose a figure: $1150 versus $250, but I exaggerate, a failing of mine . Even so, you've just spent $900 more than you would if you mixed by hand, with a shovel. Wait a minute. You've got 2 grand in the bank, right? You're still $1100 in the black. More than enough for a couple of boxes of beer, and you haven't listed the water you'll need to wet up those bags and bags of dry mix.

              Take it easy on yourselves, guys; you've still got to spread that wet, heavy concrete by hand, screed level and finish trowel those slabs. That's a fine enough workout. After that, you've still got bricks to lay, parts to assemble, forms to build, wood to stack, dough to make. You'll still get enough exercise by the time you start baking that the wife will say with a twinkle in her eye, "Wow, honey, you haven't had biceps like that since we dated in high school. Here, let me, I'll punch another hole in that belt for you."

              Later portions of oven building take strength, plus finesse, plus patience. I'd recommend you save your energy for those tasks and figuring out what to do with that spare $1100. Donations to the prematurely aged Jim Wills fund gratefully accepted.

              "Made are tools, and born are hands"--William Blake, 1757-1827