Size of concrete mixer and water to add?

I'm a newbie to pouring larger quantities of concrete/mortar/cement. I have a batch I'll need to mix of 3 100 lb bags white portland cement and 5 100 lb bags of tiny pebbles. What is the smallest size mixer I can rent that will do this batch all at once? The local rental has 6 cu ft and 9 cu ft tow-behind mixers. How much water should I add initially? I am afraid of accidentally adding too much water or if I'm too cautious and add it too slowly maybe it would take away a bit from my troweling working time. How long to leave it spinning round the mixer?
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Considering you need sand also I doubt you will get it in one mix.
Without looking it I can't recall the recipe but something like 400-600 pounds of sand (or more) is needed for that much Portland.
BTW Portland is 94 lb bags.
Got up and got the book off the shelf.
A 5 cubic foot batch is: 105 pounds of cement 51 pounds of water 231 pounds of damp sand 315 pounds of stone
Water weights about 8 pounds per gallon. Start with less and temper it with more if you need to.
So as you can see, you may want to revise your recipe a little.
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I should clarify: the tiny pebbles appear to be the sand. The grains of these "pebbles" are no more than 1mm to 2mm wide. So it's like a very coarse sand. I didn't come up with the recipe; I picked up all the bags from the warehouse according to their formula for a certain appearance. Unless somehow they made a major goof up and forgot to give me some fine sand. But I'm pretty sure that what they call "mini pebbles" is actually the coarse grain sand.
With this clarified, how large of a mixer to rent?
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scorpionleather wrote:

Since you have what seems to be a specialized mixture, you might want to ask the company from whom you obtained the "recipe".
Jon
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No sand?, save money and have it delivered mixed.
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wrote:

That is way less than one yard of concrete. Unless you know someone that will bring out some leftover mix, you will pay for several yards to get any ammount delivered. I think around here it used to be a charge of around 3 to 4 yards or they added around the cost of an extra yard for delivery of a small load.
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Probably too late for the OP- but there are lots of places who specialize in short loads these days. Less than a yard will cost about what 2-3yards ought to cost--- but may still be less than portland/aggregate/sand and renting a humongous mixer.
And it is infinitely easier and more reliable. Once it is poured, all you need to do is take care of the mix- not clean up the rental equipment and return it.
Jim
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scorpionleather wrote:

I have no idea how long your mixture will take to cure, but it's sure gonna solidify long before you can trowel a 900+ pounds of cement.
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If I can figure out how many cubic feet these materials will produce, maybe I can mix only half of it or less and end up with extra bags. They wanted to sell me more than enough for the project in their "1 batch" quantity ("you'll end up with extra") but didn't tell me how much extra. So if there is some way I can convert these bags of cement + sand pebble + water to cubic feet any tips on this calculation would be much appreciated.
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scorpionleather wrote:

We can do some back-of-the-envelope calculations. Cement (according to Google references) is about 5,300 lbs/cu yd, or 200 lbs/cu ft. You've got 900 pounds of stuff, plus water, call it 1000 pounds.
That's five cubic feet of cement.
Still, I'd check the set-up time for the concoction (maybe it's on the bag). I don't think I could slather five cubic feet of cement before it became unworkable.
Maybe you could start with a small batch and see how it goes?
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Thanks Bub, I did a different calculation and it matches your results at 5 cubic feet.
The company that made up the formula told me it would take 1.5 to 2 hours before it hardens and gets unworkable with the trowel. Does that sound about right for white portland cement plus coarse sand?
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scorpionleather wrote:

Dunno. I'm barely an apprentice amateur mason trainee. You could simulate your proposed activity thusly:
Kitten in the left hand, skinning knife in the right... No wait...
A. Start stop watch. B. Pick up invisible brick with left hand, get load of invisible cement on trowel in right hand. C. Butter invisible brick, place invisible brick in poisition, fiddle with it D. Admire work, polish results E. Repeat steps B - D ten times. F. Stop stopwatch. G. Divide results by ten. This is time to lay one brick.
Figure out how many bricks are required and multiply by time for one brick. Add some amount, say 20%, for overhead (getting more bricks, having beer, etc.).
Is the result less than 1.5 hours?
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Thats just ONE wheelbarrow of concrete..somehow, I dont think your weight calcs are right.
I've hauled delivered concrete in my large 6CF contractors wheelbarrow from the curb to the front of the house/driveway (didnt want the LARGE concrete truck driving on my driveway) and dumped into prepared forms..not full but probably near 5CF per load...at least 4.. Must have made 12 runs..over 2 yards. Not an impossible task
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