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Old 09-11-2011, 08:03 AM   #1
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River rock for concrete base???


We are getting estimates to replace a landscaped area that is currently filled with the round river rock, with a concrete patio slab. The last contractor wanted to reuse the river rock for a base. I'm not a concrete pro, but I don't think that reusing the river rock sounds like a good idea. I thought you wanted a base that would compact nicely - and I don't see how these somewhat rounded rocks, will be able to be compacted.

The contractor has done almost every drive and patio on the street and his work looks good... but I'm skeptical of this proposal. Live in Mid Indiana...

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Old 09-11-2011, 09:11 AM   #2
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River rock for concrete base???


Which river did the rock come from?

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Old 09-11-2011, 09:20 AM   #3
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River rock for concrete base???


Rounded river rock will not really compact as well as better aggregates because it is "too clean" It might be cheaper to use, but is not as good as compactable and there are not enough angular particles. Obviously, an excess of fines in the base is not good.

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Old 09-11-2011, 11:39 AM   #4
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River rock for concrete base???


There is no blanket answer here. What may be a common practice in one area will not work in another. I did a search of the Plasticity Index of Indiana soils and was surprised to find out that you mostly have a clayey soil with PI levels into the high 80's. That means that the soils in your state are expansive just like they are in my area in Texas. What this means to you is that the soil can change its physical size due to water. You do not want a compacted fill under your foundation. What you need is a fill that has a PI of 17 or less. We use what is refered to as #2 base here. It's mostly chip rock and allows water to pass thru it quickly and water does not change the base physical size. River rock is going to have a very low PI but it depends on the size of the rock. If we're talking about 1" or less river rock, there's probably nothing better in your area. Anything larger and you start to lose the support properties.
I'm attaching 2 links for you to read. While you may not fully understand the material, you should be able to glean enough from it so you have an idea what you're facing. Hope this is helpful information.
http://docs.lib.purdue.edu/cgi/viewc...x%20indiana%22

http://www.foundation-repair-guide.c...sive-soil.html
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Old 09-11-2011, 02:08 PM   #5
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River rock for concrete base???


If you were building a foundation to support large loads, such as an industrial building, it would be important to have a well compacted base. You are building a patio, which has very low loads, so the compaction of the base is not as important. What is important is that the base drain well, and that there be minimal settlement or heave of the base. River rock is fine, since it drains well, and the rock itself will not settle once it is minimally compacted. If you are concerned about strength, you can ask the contractor to fill the voids in the river rock with coarse sand, and then compact the sand/rock mix. Very strong.

I would be much more concerned with what is under the river rock. You did not state how thick the river rock is, nor what lies beneath. If you have silty, poor draining soil under say 6 inches of river rock, you may have a frost heave issue. If you have plastic, expansive clay, you may have a soil heave issue. You need to excavate a couple of small test pits through the river rock down to frost level (say four feet) to classify the soil, then you can determine if the soil is suitable for a patio, or if you need to excavate unsuitable soil and replace it with structural fill.
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