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I'm wondering if it's okay to pour a 7 x 9 x 4" slab for a shed in batches, in that I'm not able to handle all of the concrete in one day. If so, is there any attack plan that would work the best? Thank you:)

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4 Answers 4

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A 7x9 slab is not very big...unless you are buying bags of concrete mix, loading them in your vehicle, unloading them to the site of the work, mixing them with water from a hose, pouring out of a wheelbarrow, and repeating this until the pour is done. Then it is REALLY big.

Consider this: In many areas there are "mix and pour" concrete trucks that come to your jobsite and mix the concrete right in the truck. and pour it in your forms or in your wheelbarrow. You only pay for the concrete you use.

Less backbreaking work. No spoils or leftovers that you have to find a home for. No trips to the home store. You are done in hours rather than days.

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    A 7' x 9' slab 4 inches thick will require 35 80lb bags (2800lbs total). I think you can get the big box stores to deliver it, otherwise you will probably have to make a few trips. I would rent one of those stand mixers, they can mix 3 bags at a time. I think planning to pour this in 2 pours over 2 days would certainly be doable.
    – Glen Yates
    May 19, 2023 at 14:53
  • Only put 2 bags in a 3 bag mixer. It's less strain on the $120 BS machine made in China, and you. Rent a vehicle, buy the mixer (even if you only use it once, it's worth it) and have them load an entire skid of 60 pound bags of concrete; excess to be returned: still in the truck. 80s are heavy.... Sixty, 60lbs bags = 1cu ft. (56 bags on a pallet)
    – Mazura
    May 19, 2023 at 19:10
  • Doing 2 instead of 3 mixes twice as fast, and completely w/o clumps every time because you can pan the drum down further so that it actually dumps over on itself as it falls off the blades w/o sloshing out.
    – Mazura
    May 19, 2023 at 19:20
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Yes, you can do that. You have to provide something like rebar between the sections to tie then together, and maybe some expansion joint material. Heck, they pour road beds like that all the time. And even the giant Hoover Dam was poured in sections (blocks).

One disadvantage of pouring in sections is that it's more expensive to get small batches of concrete delivered. If you're going to mix your own, that's less of a concern.

I'll leave the details (which I would probably get wrong) to those with more experience in this matter.

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    Not an expansion joint. The slab is too small for that. Just a control joint tooled into the surface.
    – isherwood
    May 19, 2023 at 13:55
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    That's what I meant by leaving the details to the experts!
    – SteveSh
    May 19, 2023 at 14:40
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Addendum to SteveSh's answer...

You want to form the sections separately by running a board across at the point where you'll stop the section. The form board at that point can be shorter in height than the perimeter form so that rebar and extra concrete can pass underneath. It's just needed so you can tool the edge to form the first half of a control joint. This can be done on the fly when you determine how far the first batch goes. Just make sure it's square.

You could accomplish the same thing by holding a straightedge over the forms and running your edger along that, but screwing a board between forms is easy enough and provides more stability.

At the next session you'd remove that intermediate form and continue the pour, once again tooling the control joint, but on the new side.

This leaves you with a standard sidewalk control joint, making your work appear continuous.

enter image description here

Source

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Practical answer

You are dealing with 3/4 yard of concrete.

If you make your slab 7' x 9' x 5" then it will be a yard of concrete. Check local delivery prices and have a heart-to-heart with yourself as to whether the concrete work needs to be days of backbreaking labor or about 2 hours.


The answer to your exact question

How many batches do you plan to do it in?

How much time will elapse in between batches?

Pouring in multiple batches can result in uneven appearance due to moisture content. Definitely place a length of rebar for the next batch to latch onto.

Consider using a concrete bonding agent when pouring the subsequent batch:

enter image description here

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    Bonding agents aren't for vertical joints. They're for horizontal layer situations.
    – isherwood
    May 19, 2023 at 13:49

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