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Old 12-26-2016, 07:59 AM   #1
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How important is gravel?


I am building a concrete pad. It will be about 10' by 20' and 4" thick. I plan to level the dirt and compact it. Install wire in the concrete. So what is the purpose and do I need a 1" gravel layer?


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Old 12-26-2016, 08:10 AM   #2
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Re: How important is gravel?


No, you don't need gravel. Gravel doesn't compact and you don't need plastic either, but the compaction must be correct.

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Old 12-26-2016, 08:14 AM   #3
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Re: How important is gravel?


Depends on where you are planning to pour the slab, what the soil conditions are like, whether there is water that might be an issue, what is your frost level, how hard the base is compacted.

What is the slab going to hold? A building or shed? A Patio for a BBQ? A garage and vehicle?
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Old 12-26-2016, 08:21 AM   #4
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Re: How important is gravel?


The purpose of the gravel is to help with drainage, but a lot depends on where you live, the soil conditions, frost/freezing, etc.
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Old 12-26-2016, 08:24 AM   #5
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Re: How important is gravel?


Going to be in Birmingham Alabama, pad will store Bee Keeping equipment and be enclosed inside a roof/wall.

Not sure about compacting dirt.


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Old 12-26-2016, 08:27 AM   #6
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Re: How important is gravel?


If I have good compacted dirt could I probably skip the gravel?


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Old 12-26-2016, 09:00 AM   #7
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Re: How important is gravel?


1" layer of gravel isn't going do much. It will just compact into the soil.
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Old 12-26-2016, 09:54 AM   #8
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Re: How important is gravel?


If MH," it'll be under a roof, wall" as you say sir, I would grade the space, lay 2" 3/4"stone, place 4mil poly on area and pour it. Seems to me you're not exactly sure what you will do with it after it's poured !! That being the case, I wouldn't want moisture wicking through the slab I just poured and stored important materials in the shed I just built !!


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Old 12-26-2016, 10:52 AM   #9
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Re: How important is gravel?


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Originally Posted by mhhickma View Post
If I have good compacted dirt could I probably skip the gravel?


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Yes, you can skip the gravel, it needs to go into the mix to make concrete , and compact the dirt a minimum of 2 ft. beyond the pad / building line. Moisture doesn't wick up through concrete. That moisture people see on top of a concrete pad / slab, in the parking lots and their walks is surface condensation.
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Old 12-26-2016, 10:54 AM   #10
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Re: How important is gravel?


Thanks everyone. The are I am putting the slab is very narrow and has a slope. Thus adding 2 extra feet of dirt outside pad is tough. So I plan to build a cinder block wall to hold the dirt in.


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Old 12-26-2016, 11:25 AM   #11
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Re: How important is gravel?


The purpose of the gravel is to replace any disturbed soil. It is much easier to scrap out any disturbed soil and replace it with gravel than it is to compact soil. To compact soil it has to be done in increments. About 2" increments if compacting manually. And the soil moisture content must be correct.
Just simpler, easier and cheaper to back fill with gravel.
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Old 12-26-2016, 11:37 AM   #12
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Re: How important is gravel?


you're right SS, no wicking, was just being dramatic. as for the stone, I think pouring w/o stone is asking for water runoff trouble, especially if walls are added. but that's just me.


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Old 12-26-2016, 05:54 PM   #13
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Re: How important is gravel?


I wouldn't skip the gravel, and generally speaking, we use a minimum of 4" under any concrete flatwork we pour. One of the main reasons is that there is generally more than 4" of topsoil/organic material that needs to be removed, so you generally need to import some kind of filler. You might as well use something that's easy to work with, compacts well, drains much better than soil, and is relatively cheap.

As or wicking, concrete undoubtedly can wick water or several inches, or more. The distance it can wick is often arguable, but the fact that it is not water proof and can wick water is not.........
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Old 12-26-2016, 06:27 PM   #14
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Re: How important is gravel?


If you add walls and a roof, which block the wind that removes the moisture coming from below, be sure to add a vapor barrier plastic that retards the moisture vapor. The gravel layer stops capillarity of moisture, gives a reservoir for liquid moisture and minimizes shrinkage stresses and cracking; http://www.wbdg.org/systems-specific...ms/floor-slabs

The poly should be under and around the thickened footing perimeters, especially between any retaining walls abutting it to stop capillarity; https://buildingscience.com/document...059-slab-happy

Be sure to add some conduit in a slab/wall for future electrical work.

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Old 12-27-2016, 07:01 PM   #15
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Re: How important is gravel?


Totally agree with what jomama45 said.

https://www.concretenetwork.com/vapo.../moisture.html

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