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  1. #1
    Elite Member AlanB's Avatar
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    Jan 2004
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    2,532
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    Clarksville, TN, USA
    Tractor
    NH 1925

    Default Gravel / rock under concrete.

    OK, I am about to pour my concrete driveway / workpad.

    18' X 30' X 6"

    current base is red clay, but will be digging it out and putting compacted rock / stone down about 8" thick.

    I also know that few people in this area do anything for a base on a drive.

    Now I have heard several explanations as to why the rock was so important, drainage, stability, etc. etc.

    Can someone offer some insight or point me to a website of the why's and wherefores of a crushed rock base under the concrete?

    Thanks.

  2. #2
    Veteran Member
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    Jul 2004
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    1,632
    Location
    Bancroft, Ontario
    Tractor
    JD4300

    Default Re: Gravel / rock under concrete.

    It prevents the base from becoming waterlogged and heaving when it freezes

  3. #3
    Silver Member
    Join Date
    May 2006
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    140
    Location
    Chicago suburb
    Tractor
    Kubota L3400 gear

    Default Re: Gravel / rock under concrete.

    Well,
    my neighbor only used a sand and he thinks he has the best base ever./he compacted it heavy.

    Joe

  4. #4
    Veteran Member Steve_Miller's Avatar
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    Mar 2004
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    1,246
    Location
    Nova Scotia, Canada
    Tractor
    2006 Kioti CK30HST

    Default Re: Gravel / rock under concrete.

    I'm not sure of my state abbreviations, if it is Tennessee then you don't have to worry about the ground freezing, so, if the slab is on higher ground and runoff is not a problem then I would say that the red clay base should be fine. If it is in a hollow then yes, you probably want some rock for stability when the ground may get saturated with water. If it is at all possible try and get the water to drain away from the slab. The main thing when pouring concrete for a slab or footing or any base is to have the water run away from the area. Water is the worse enemy mother nature can throw at you, well maybe a hurricane or a tornado could be worse.


    Steve

  5. #5
    Elite Member AlanB's Avatar
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    Jan 2004
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    2,532
    Location
    Clarksville, TN, USA
    Tractor
    NH 1925

    Default Re: Gravel / rock under concrete.

    Yes, I am in Tennessee.

    My ground is very wet (normally) and we are having an exceptionally dry summer this year, so right now it is dry.

    This driveway is too low. But shy of pumping the water uphill, there is not much way to get much drainage.

    Because of my concerns over drainage, fall, flooding etc. I am electing to go with pavers. At least that way, I can do it over if needed.

    I am laying in about 6" of 2"/3" rock (already done) then about 2" of Crusher run (small rock with fines) then about 1" of gravel sand for a bed to lay the pavers in.

    I am compacting it all in small lifts, maybe about 1" or so with my too small plate compactor (3500 lb force) I am hoping / expecting that by doing it often in small lifts it will be sufficient.

    I think the pavers will also give me some wiggle room if needed with a particular bad place.

    I am laying in a french drain beside the area, to help get the water flowing away, but realisticly, there will be water in this area.

  6. #6
    Silver Member
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    Sep 2005
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    180
    Location
    Near Reno, Nevada
    Tractor
    NH TC35A

    Default Re: Gravel / rock under concrete.

    The main purpose of an aggregate base under a concrete pavement is to keep the subgrade from pumping or erroding, leading to a loss of support under the joints and corners.

    Pumping happens when you have a fine grained subgrade (including clay) that is very wet. Moving wheel loads can cause that material to liquify and move around under the slab, often shooting up through the joints. This leaves voids under the joints and corners, which then causes them to break. Movement of subgrade material also leads to faulting (vertical mis-alignment of joints).

    Frost heave is a complicated and misunderstood phenomenon. It is caused by moisture moving up from underneath a structure (not surface moisture), and can't really be cured by drainage. Frost heave only happens when air temperatures are well below freezing - near-surface water will be frozen, so can't migrate under the structure at that time and cause problems. It is the unfrozen water from deep in the soil (which can't easily be drained away) that gets sucked up and causes frost heaves.

  7. #7
    Silver Member
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    Jul 2006
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    186
    Location
    alabama
    Tractor
    kubota 3130

    Default Re: Gravel / rock under concrete.

    Around here all that is used under concrete is compacted chert of course we dont really have any building codes either.As a matter of fact perk tests have been omitted to cut county costs.

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