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I'm making a 9' x 8' x 3" slab as part of a repair to my driveway. The old driveway concrete has been removed from this space. The new concrete will abut the original driveway on both sides. I'm planning on dividing it into two 4.5' x 8' sections and pouring them separately.

The two concerns I have are:

  1. The largest mixer at my rental place can mix 10 80lb bags and each section will take about 18 bags. Should I further divide the section to do 9 bags at a pour or is it OK to pour two batches one right after/on top of the other.

  2. Do I wait until the first section is cured or partly cured before pouring the adjacent section? There will be an expansion joint spacer between them.

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  • You want to minimize time. 1. i would consider doing more batches as 10 bags can take a while to mix; continuous is ok, as long as it's not a trickle. Use 3+ people. At least pre-open the bags and have water ready to go asap. Don't use rapid set, consider additives that prolong curing, do it on a windless brisk cloudy morning, yada yada. 2. if they don't overlap, it's not terribly important, if they do touch, hurry.
    – dandavis
    Nov 17, 2020 at 20:40
  • @dandavis Thanks - Is there a specific way to mix continuously? On (2) I was thinking of waiting until the first section partially cured so I could walk on when doing the second section.
    – George White
    Nov 17, 2020 at 20:55
  • Have you considered ready-mix delivered? Even though you need less than 1 yd. and around here you pay for 3 yds. minimum, you may still save money over the bags of concrete, the mixer rental, the crew you'll need, and the quality of the finished job.
    – jwh20
    Nov 17, 2020 at 21:38
  • There is, but I wouldn't go for it; it's difficult, unforgiving and failure is highly undesirable. Basically mix a partial batch, then keep topping it off with mix, agg, and water. The hard part is that you need to top off with exactly the right ratio because there's no time to adjust. You might be better off with two smaller mixers and several rapid-fire batches. Obviously, with 10 mixers and 20 helpers, you can do near-continuous. That's expensive and impractical, but illustrates the ideal to strive for.
    – dandavis
    Nov 17, 2020 at 21:39
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    I think you may be being a bit overly cautious. Concrete is fairly forgiving. This question diy.stackexchange.com/questions/70010/… seems to indicate you have about two hours for the concrete to remain bonded to itself. So it's not like the stuff all has to be done in 20 minutes. Mixing the concrete is unlikely to be the biggest issue, make sure you have enough help and tools to spread it.
    – user30371
    Nov 18, 2020 at 2:31

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