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Old 04-19-2014, 09:18 PM   #1
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Concrete porch slab


Just demo'd my exisiting 20x6 concrete porch slab that was on a raised masonry block foundation and I'm getting ready to pour a new one on top of existing foundation (which was already back filled within 2 inches of top of block).

My questions are...

1) In order to get the slab to overhang the foundation slightly (1" - 1 1/2" I was planning), how do I build the forms correctly to achieve this? I have some ideas in my head, but not sure if there on par or not.

2) What PSI mix should I order from ready mix co? Slab will be at least 6" thick.

3) Would wire mesh be sufficient, or should I lay in 3/8" or 1/2" rebar?


Thanks for the help!

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Old 04-19-2014, 10:01 PM   #2
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In a case like this you normally just attach a 2x4 to the perimeter of the foundation and then form up from that. That would leave you with a 1.5" overhang. I would use either 3500 or 4000 psi concrete with air entrainment. My personal feeling is that mesh is basically useless because it always ends up on the bottom of the slab instead of the middle where it belongs. You could use #3 or #4 rebar on 3" saddles to get really nice reinforcement or you could probably just get away with adding fiber to the mix and not worry about any steel reinforcement.

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Old 04-19-2014, 10:22 PM   #3
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I would make sure your fill is well tamped/compacted....even with a 4" lip,
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Old 04-20-2014, 07:20 AM   #4
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tnx w/this exception - use #4 rebar on chairs adds flexural strength,,, fiber generally helps fiber makers w/boat pymts imo but nothing else,,,
finish adding compacted fill - put down some vapor barrier - joint as / if necessary

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Old 04-20-2014, 06:30 PM   #5
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If I go with #4 rebar, how far apart should I space it?

Also, should I lay it in like a checker board pattern?
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Old 04-20-2014, 07:25 PM   #6
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Besides correct compaction mentioned by MTN REMODEL LLC there are a couple of things I would do and omit 2 items completely.

1) Treat soil for termites if they are prevalent in your area.

2) Omit the plastic.

3) Omit any rebar or mesh. Not needed on this project at all being the size and thickness it is. It's a porch not the interstate.

4) Place a tapered key ( 1/4, 3/8 or 1/2 inch will suffice ) on top of the form to create a drip edge under the overhang.
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Old 04-20-2014, 07:53 PM   #7
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No way would I consider a first time DIY doing there own slab!
This is a one shot to get it done right deal.
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Old 04-20-2014, 08:03 PM   #8
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Thanks again guys! Regarding the tapered key, can you be any more specific regarding what you mean? Can't exactly picture it in my head...

The base fill has been there 50+ years already (old slab sat on it) so I'm pretty sure it's as solid as it's gonna get...

Joe, I agree 100% with you... I've done footing, driveway and sidewalk forms before, but never on a raised foundation yet.... My stepfather will be doing the finish as he has done concrete before numerous times, I am responsible for the prep though...
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Old 04-20-2014, 08:31 PM   #9
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Quote:
Originally Posted by stevekem View Post
Thanks again guys! Regarding the tapered key, can you be any more specific regarding what you mean? Can't exactly picture it in my head...

The base fill has been there 50+ years already (old slab sat on it) so I'm pretty sure it's as solid as it's gonna get...

Joe, I agree 100% with you... I've done footing, driveway and sidewalk forms before, but never on a raised foundation yet.... My stepfather will be doing the finish as he has done concrete before numerous times, I am responsible for the prep though...
The key / \ is nailed to the top of the horizontal form lumber that creates the overhang. This forms a groove in the bottom of the overhang that prevents any water from traveling back to the foundation wall. The only reason I mention a tapered key is so form removal is easier than would be with a square key. Your stepfather may have a different or even better way.
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Old 04-20-2014, 08:34 PM   #10
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I understand now... thanks for the help!

Quote:
Originally Posted by Fairview View Post
The key / \ is nailed to the top of the horizontal form lumber that creates the overhang. This forms a groove in the bottom of the overhang that prevents any water from traveling back to the foundation wall. The only reason I mention a tapered key is so form removal is easier than would be with a square key. Your stepfather may have a different or even better way.
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Old 04-25-2014, 10:44 PM   #11
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I'm going to be pouring the slab here tomorrow morning, but wanted to ask how long I should wait before pulling the forms off? Till concrete is firm to the touch?

Slab will be about 9" thick and it's supposed to be the 60's with overcast and light rain (will be tented).

Since the 3 sides will be exposed, I'm just not 100% sure if anything else needs done to them before concrete is fully set to clean them up to be nice and smooth. We will be tapping the forms throughout the day as well.
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Old 04-25-2014, 10:50 PM   #12
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You can strip the faces of the stoop to finish them in the same manner as the top (typically broom finish here), but it takes some testing to remove them at the right time. You should not, however, remove the boards supporting the cantilever over the foundation for a day or so.

I would highly recommend oiling the forms, especially if you're going to strip & "face" them. Ordinary cooking oil works well.........
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Old 04-26-2014, 08:36 AM   #13
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so does diesel fuel omg, did i say that ?
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