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Old 07-10-2010, 09:45 AM   #1
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Gravel Or Sand


What is the difference between useing gravel or sand as a bed over compacted soil in order to lay a concrete slab? I hear of both.

And, I have always heard that wet sand is 100% compacted. But, in other projects, when watering down a layer of sand and walking on it, your foot sinks about and inch into it. 100% compaction???

Paul

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Old 07-10-2010, 09:55 AM   #2
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Gravel Or Sand


For a patio, if the earth is undisturbed and firm, and at the correct grade, go straight over it with your vapor barrier and concrete. If you need to level, compact or bring up the grade, use 3/4 minus rock, compact that, then the VB and concrete.

My ignorant youth allows me to posit that sand is generally for dinosaurs. (And never put it between the VB and the concrete, a rumored crazy practice specific to the lower West Coast).

Have fun,

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Old 07-10-2010, 10:45 AM   #3
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Gravel Or Sand


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Originally Posted by tttmex View Post
What is the difference between useing gravel or sand as a bed over compacted soil in order to lay a concrete slab? I hear of both.

And, I have always heard that wet sand is 100% compacted. But, in other projects, when watering down a layer of sand and walking on it, your foot sinks about and inch into it. 100% compaction???

Paul
Paul, when you walk over sand, your foot is displacing the sand. But, if you lay down, say a piece of a foot square of plywood and stepped on it, it wouldn't displace it.
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Old 07-10-2010, 10:49 AM   #4
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For a patio, if the earth is undisturbed and firm, and at the correct grade, go straight over it with your vapor barrier and concrete. If you need to level, compact or bring up the grade, use 3/4 minus rock, compact that, then the VB and concrete.

My ignorant youth allows me to posit that sand is generally for dinosaurs. (And never put it between the VB and the concrete, a rumored crazy practice specific to the lower West Coast).

Have fun,
Rory,

What would be the purpose of the vapor barrier under a patio, (unless you were going to convert to conditioned space later)?
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Old 07-10-2010, 10:57 AM   #5
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Gravel Or Sand


I was tought to use sand - one reason was to keep rocks or other from puncturing the vapor barrier.
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Old 07-10-2010, 02:05 PM   #6
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Gravel Or Sand


That's a good point re no need for a vb under an exterior slab. I wonder if the slab is indoors or out?
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Old 07-10-2010, 02:08 PM   #7
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Gravel Or Sand


I would recommend compacted gravel vs. sand anyday. The sand get's tore up too easily from working in the slab.

And I'm curious as to the benefit's of a vapor barrier under exterior flatwork as well?????
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Old 07-10-2010, 03:39 PM   #8
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11 year old report: ftp://imgs.ebuild.com/woc/C99C023.pdf

Be safe, Gary
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Old 07-10-2010, 03:43 PM   #9
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Gravel Or Sand


The benefit of a real vapor barrier (10 mil) is if the area was ever enclosed (addition, 3 season room, shed, etc.) it is already there.

It also prevents capillary moisture from getting into the slab.

I would use a compacted gravel base with a thin sand cushion and then a tough VB.

If you ever have a moisture problem with a slab, you might wish you spent a few pennies for a good VB instead of $$s to tear out, removal and replacement.

Cement finishers can be lazy and do whatever is necessary to make their jobs easier and faster - the same applies to wire and reinforcement even though thye do not make money on the extra materials. - Unless you are shopper nad they are better at selling minimal services. A good buyer is also a good seller.

Dick

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Old 07-11-2010, 03:24 PM   #10
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Gravel Or Sand


How do you compact gravel? Does the tamping method push the gravel into the already compacted subdirt??
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Old 07-11-2010, 04:29 PM   #11
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The gravel (base material) has the right amout of the properly sized particles and enough (not to much fines) to help it compact and bridge over poorer soils. The worse the subgrade or the higher the load, the thicker the base should be. Compacted base should be smooth and have to loose particles coming of if you walk on it.

Base material is common everywhere and is used under concrete and asphalt surfaces, but different place have some local terms that are used.

You say you have compacted "soil" but if it is not good "soil" it could be meaningless.

Dick
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Old 07-11-2010, 04:59 PM   #12
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I hope no one turns there patio into an addition, screen room, etc. without thickening the perimeter edges for the wall and roof added weight.... then a v.b. would be required, if heated space. For a screen room conversion, you would need footings under the slab for the roof posts carrying the roof structure. Wouldn't the v.b. under a patio slab keep the water from draining, if the slab was poured on the leveled sub-grade for a patio? And possibly freeze easier if the slab is covered with snow that melts into the porous concrete slab during the day- pooling there to freeze at night? I would think you wouldn't want a v.b., unless knowing it would be a heated conversion by you, soon. Then I would suggest insulating the perimeter with 2" x 2' deep rigid foam board to counter heat lose. That seems a lot of extra time, energy and money--- adding thickened (12" x 12") edges, perimeter insulation, a heavy gauge v.b., post footings, then raised perimeter 6" wall (to install siding per code), which the B.D may not accept later as the patio probably wasn't inspected when poured. (A big shock when you go for the addition's permit).

I agree with the fines and gravel, though.

Be safe, Gary

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