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Old 07-06-2009, 02:25 PM   #1
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can I use 7/8" gravel as aggregate?


I'm adding a support pier under cottage to prop up a beam mid point (advice from others here on an older / previous post re: bouncy floor).

I'm using a 28" Bigfoot footing form with approx 2' of 10" sona tube at the top - down to about 4'. All together I think I need about 5 cubic ft of concrete to fill it (3.5 for the bigfoot, according to the label, and estimating another 1.5 for the tube). Portland cement bag label says it covers approx 15 sq feet for a 4" slab which is approx 5 cu ft.

I have a pile of clean gravel (7/8" max sand/gravel mix) from a local quarry that I had dropped off a year ago. I'm fairly remote and getting a truck out was big $$$. So I'm thinking of just using some of this gravel to mix in the concrete as the aggregate. I'm hoping that as long as I use the right water / cement ratio and drop a couple pieces of vertical rebar in the tube I should end up with a decent result.

I don't want to go crazy analyzing the choice of aggregate or go insane worrying about 5 ft of concrete - but I also don't want the pier falling apart. Any thoughts on this? Has anyone seen a small concrete job go bad because they didn't cherry pick the sand and stone? Also how long should this thing cure before I put any load on it (assuming it's curing underground - am I correct in assuming this will be a while longer?) This will end up being one of 9 piers supporting a 20x20 cabin.

Thanks

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Old 07-06-2009, 06:40 PM   #2
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can I use 7/8" gravel as aggregate?


What your doing sounds fine, considering the Bigfoot is probably overkill for this application. It sounds like the material is what we would call a road gravel in my area. The only possible problem with it is that the material isn't washed at the pit, & generaly has some silt & dirt left in it, which in turn can weaken the concrete. It seems that your mixing about a 5 bag or 3000 psi mix, which is generally adequate for this, but you may want to add an extra half a bag of Portland to help make up for the "dirty" aggregate.

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Old 07-07-2009, 10:29 AM   #3
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can I use 7/8" gravel as aggregate?


thanks jomama45 - maybe that's what this material is generally used for here too (roadwork) - it packs like concrete wherever I spread it around - makes a great level parking / working area around the camp.

now that you mention the dirt and silt issue, I wonder if the fairly heavy rains over the last year would have done anything to wash it out (we've had some wild downpours too) - maybe I could scoop from the top of the pile for the "cleaner" stuff...?

In any case, I'll be sure to top up the cement in the mix - thanks for the tip.

bigfoot - overkill - I kinda thought about that a bit - the worst part of working where I am is the digging (really hard going) and setting that big form in is definitely more work - but if I'm going to go at it I think I might sleep a bit better for the add'l $20 and an extra hour or 2 with the pick and shovel - also makes the beer taste better when the job is done
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