Preppng for large concrete slab in a wet area


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Old 07-06-16, 10:36 AM
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Preppng for large concrete slab in a wet area

Hi everyone,

I am planning for the installation of a 1200 square foot concrete patio. In a normal situation we would excavate 10" to account for 6" compacted road crush and a 4" pad. My concern is the very high water table in my area. With all of the rain that we have received lately I am hitting water about 30" down. Living in a northern climate I am concerned with heaving and cracking.

I have spoken with a number of local "experts" and have been presented with a few options. I'm really hoping for some advice and guidance.

1.) The first option presented was the standard approach using 6" road crush under a 4" concrete pad. Their position was that they over spec their pours and are confident this approach is all that is required.

2.) The second option presented was 6" of washed rock, 6 mil poly then the 4" pad. This approach does not use road crush. The logic there is that any water under the pad will wash away via the rock, either to the edges of the pad or back to the weeping tile around the house.

3.) The third option presented was 6" of washed rock, 4" of high density foam insulation then the 4" pad. The logic here is again, any water under the pad will wash away via the rock and any shifting of the pad will be absorbed via the insulation.

4.) The fourth option presented was 6" of road crush under the 4" pad as well as pilings every 10'. Using screw pilings with a post cap that is slightly larger than the piling post. The logic here is the cap is embedded into the concrete. If the pad heaves it will slide up and down the piling post. Also, the 10' spacing is the spacing used for structural stability such as pilings under a footing. The logic here is if the slab settles the pressure placed on the slab by the immovable piling will not crack the concrete as the pilings are close enough to support the weight of the slab.

Knowing very little about concrete you can imagine how confused I am. I'm really looking for some guidance as I have no idea how to proceed.
 
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Old 07-07-16, 11:04 AM
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Personally, I would excavate 12" and compact the sub-grade with a vibratory tamp. I would slope the sub-grade to allow any water under neath to flow to best drainage area. Next I would place a geotextile over the sub-grade. At the drainage end, I would place #1 drainage stone along this edge to help drain away water. I would place (2) 4" lifts of #1 crusher run stone and thoroughly compact each layer. The geotextile leaves a thin layer for water to follow to the drainage stone. It also bridges the area underneath the compacted stone. Before you pour your concrete, I would figure on putting expansion joints every 10' in a grid pattern. Once you pour your concrete, I would saw cut each 10'X10' grid into 5'X5' grids. But that is just what I would do being from WNY, I know about frost and cold weather!
 
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Old 07-07-16, 01:07 PM
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Well, happy to meet you. Sounds like you have similar seasonal temperature swings. Couple of questions if you don't mind.

At the drainage end, I would place #1 drainage stone along this edge to help drain away water.
Would you place the drainage rock under the pad or would this be more like a French drain beyond the concrete? How much drainage rock, how thick and wide should the trough of drainage rock be? Should the drainage rock run to anything like a weeping tile?

I would place (2) 4" lifts of #1 crusher run stone and thoroughly compact each layer.
Is that 4" before or after compacting?

Once you pour your concrete, I would saw cut each 10'X10' grid into 5'X5' grids. But that is just what I would do being from WNY
That's a lot of expansion joints but I get it. I also heard that most cracks start at a corner. Should I plan for expansion joints coming out and away from any corners?
 
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Old 07-07-16, 01:40 PM
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Sorry, one more question. Would there be any benefit to a second layer of geotextile between the 4" layers of crush?
 
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Old 07-08-16, 07:55 AM
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I'll try and answer the question as best as possible.

1. I would make a french drain along the lowest edge you want to drain. You said you had a drain tile next to the foundation. You did not say if the pad would be next to the foundation. If it is then place the lowest edge for drainage so the water does not run toward the foundation along the surface of the concrete. It would flow toward the foundation and freeze in the winter. If you have drainage away from the foundation, make sure the surface of the pad runs toward the low point of your yard.

2. Place the lifts in 4" uncompacted layers. Compact and place the layers on top of each other. You will need to place a final layer of gravel to bring the base to grade. Once you finished compact the last lift. Make the base pad ~ 6" larger than the concrete pad. This will keep the corners from breaking.

3. I know this is a lot of expansion joints but from the pads I have placed and seen placed in the cold weather, it will save you from problems later. You may not want to install saw cuts but it is up to you.

4. As for the corners just make sure you make the base pad larger than the concrete pad and make sure you compact the base thoroughly at the corners.
 
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Old 07-08-16, 08:20 AM
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Thank you. It is tiring having to learn the in's and out's of every project I take on but it gives me comfort knowing I can talk somewhat intelligently with the companies I have bidding for the job.

Just a quick clarification on the French drain; would you have the drain installed under the pad or off of the edge under the sod just as it meets the pad? Would you leverage drainage tile in the French drain then feed that away from the pad? I do have access from the pad location to the back of the property that feeds to a drainage creek servicing the sub-division.

Sorry, for clarification the pad will be against the house foundation. All quotes so far have indicated they will be tying pad into the foundation with rebar and all quotes to date have indicated a 2% grade away from the house.

Once you finished compact the last lift. Make the base pad ~ 6" larger than the concrete pad. This will keep the corners from breaking
So, if my pad will be 4" my base should be 10" or the base should simply be larger than the pad? Is there any benefit in having the base at the edges deeper than the rest of the area?
 
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Old 07-08-16, 06:00 PM
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Standard practice with most DOTs I worked with was to use a 12" min. thick base of large rock (3" or greater), as well-drained as possible. The materials experts swore by such a system, and their explanations as to why it worked escape me at the moment. Draining to daylight would be best, using (smooth) perforated pipe.
 
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Old 07-09-16, 07:38 AM
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Standard practice with most DOTs I worked with was to use a 12" min. thick base of large rock (3" or greater), as well-drained as possible. The materials experts swore by such a system, and their explanations as to why it worked escape me at the moment. Draining to daylight would be best, using (smooth) perforated pipe.
I will admit, this is the most uncertain piece of the entire puzzle the more that I research. The sub grade material is apparently a widely debated topic with no clear winner. I have heard that the best material to use is standard road crush with the lowest level of fines as possible, somewhere in the 5% range. They believe this offers the best compromise for drainage as well as compaction. I have heard others suggest a larger jagged rock is best to allow for the best drainage possible and as long as the cuts are not smooth it should compact okay. If round rock is used it will be like compacting marbles.

I guess the only thing that is certain is my confusion.
 
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Old 07-09-16, 02:54 PM
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I agree, trying to compact marbles is futile; I should have been more specific in specifying at least 50% fractured faces on the rock. A layer of filter fabric on top of it will prevent the voids from clogging up with fines from the crushed base above the rock.
 
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Old 07-10-16, 08:38 AM
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Makes sense, thanks for clarifying. I have to say I'm a little discouraged having been through 4 potential contractors at this point. Each one of them had their own very strong opinions on how it should be managed. Shoot, the fellow I had out last night came in twice as high as the previous high bid; he quoted a 1200 square foot pad at $40k.
 
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Old 07-12-16, 01:58 AM
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Wow. That's more than $33 a S.F. Used to be you could build an entire house for that price. Must not need work, as he obviously quoted high.

Why not do it yourself? The concrete will only cost about 18 x $200, or $3600, with the 200 bucks a yard being a wild guess on my part--could actually be lower in your part of the country. Throw in another $1400 for gravel and rock base, mesh, tool rental and forms, and you're still under $5000. All you need are a few (strong) buddies to help with the placement and finishing.

Hint--don't break out the beer until everything is done!
 
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Old 07-12-16, 06:38 AM
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I wish I was comfortable enough to do the work myself. There's very little I shy away from but that large a pad is more than I could manage. The wife and I are now re-thinking things. Not knowing if the sump pump fix I have coming will resolve my water issue I'm nervous spending that kind of money. We're now thinking a ground level deck with varying elevations might be the way to go. Pressure treated framing and composite decking and it will last for 20 years. It would also allow me to lift boards easily if the water problem continues. Not to mention I would be more than comfortable doing that work myself.

This does present a whole slew of new questions however as draining under the would be deck is still just as critical.
 
 

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