You need a couple of things before you call your concrete supplier and schedule concrete.
1) Your job name/number and location
2) The scheduled pour date and time
3) Mix design number/ bag mix and admixtures
4) Cubic Yards Needed
5) Spacing on trucks
So how do you figure concrete- In the US it's by cubic yards, you times the width by the height by the depth then divide by 27. So a footing that is 16' long x16' wide x 4' deep then divide by 27
16x16x4= so there is 1024 cubic feet and there is 27 cubic feet in one yard so 1024/27=37.92 yards
So would you order exactly 37 yards? If you order the exact you will come up short so rather than getting into a lot of detail about deflection, spillage, weight etc., you have what we call a waste factor. Again we can get into a lot of detail here about uneven sub-grade, composite decking deflection, pump loss, gravel sub grade etc... but if you figure a 5% waste factor on most applications you will be safe. So if you need 37.92 yards times that by 5% gives you 1.85 yards, so 1.85+ 37.92=39.77 yards so you would want to order 40 yards.
When picking your date and time just be aware of how much your pouring- if your trying to pour 500 yards in footing and the job is on a busy road than you would want to pour at off traffic hours for better service. Or what type of mix your pouring if you have a hot mix that has some quick setting admixtures or like a corrosion inhibitor (DCI,CNI) then you don't want to try pouring in the afternoon of a hot sunny day. So, know and understand what’s in your concrete and the environment around and on the job when scheduling concrete pour.
Finally spacing on the trucks, some factors determining the spacing on the trucks would be
first: how many yards an hour can be placed- if you have a pump and are trying to do a large pour-what’s that pump good for, if your using wheelbarrows how many do you have, how long does it take to get from the load section to the pour are and back.
Second: type of pour- there’s a big difference between how many yards an hour you can pour a 4" slab on grade and an 8" slab on grade- take a 30' x 30' area:
* a 4" it takes 30 yards to pour out- so three concrete trucks.
* a 8" it takes 44 yards so 4-1/2 concrete trucks.
The area is the same, so you can dump 4.5 trucks on an 8" slab quicker than you can a 4" slab.
so your spacing might be 7 minutes for an 8" slab and 12 minutes for a 4" slab.
If you’re a new foreman the best advice if it's available for you would be to talk to your finish crew, carpenter crew, concrete supplier and pump operator to come up with a good pour out plan.
And if you don't already have a pre-pour checklist click here and use these free pour cards, not only do they help you think through your pour planning it helps you develop a proper pour process.