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Old 05-01-2012, 05:23 PM   #1
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Gazebo slab questions


I'm preparing to build a gazebo in back yard, actually more like an outdoor kitchen so we have a nice place to grill regardless of weather. The structure will sit on a 10' x 14' slab (about 4" thick with a perimeter footer about 8-10" wide x 8" deep. I've got two questions:

Regarding anchor bolts: I'll be using galvanized 1/2" j-bolts to anchor the pressure-treated sills. How deep into the concrete should the bolt extend? Local supplier has j-bolts from 6" to 16" (or more, I guess), so what size should I buy?

I've read some articles about using fiber reinforcement in concrete and pouring without reinforcing mesh. Does the fiber actually work? This is a relatively small slab, and will have on a relatively light load on it any time, so do I really need the re-mesh?

I guess I should have mentioned that here in southeast Louisiana we aren't too concerned with frost line, but wind can present some concerns. And where I live there I'm not concerned about codes, just want a good result.

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Old 05-01-2012, 06:15 PM   #2
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Gazebo slab questions


I know Louisiana well, spent a year down there cleaning up after Katrina and Rita.

But about your questions. The steel reinforcing mesh does nothing in a 4 inch slab, however there are others on this forum who disagree with me, so you may want to do a search. The reason I say it does nothing is that you need two inches clear from the face to resist corrosion, which puts the mesh at the center of the slab. This is the neutral axis, so the mesh contributes ZERO to the strength of the concrete. Plus it is almost impossible to get the mesh to the center, so it lacks proper clearance, and starts to rust, which damages the slab. My opinion is to leave out reinforcing on slabs less than 6 inches thick.

As to the fiber, it reduces shrinkage cracking a little, but you get better results by careful placement of the concrete, correct curing procedure, proper mix, and use of control joints. The fibers don't hurt, if it makes you feel better, go for it.

The biggest problem in Louisiana is expansive clay soil. If you are on it, and most of LA is, the soil shrinks and swells with changes in moisture content, which leads to heave and settlement. This will crack your beautiful slab tut suite (or however you spell it in Cajun country). Very hard to prevent this without a deep foundation, so all the prep work you do for the slab can be undone in a couple of years of moisture content change. If you have the energy, time and money, you can dig down four feet or so, backfill with crushed stone or coarse sand, and cast the slab over that. This will minimize problems from expansive soils. Or you can get your soil tested to see how prone it is to shrink/swell, maybe you get lucky. Meanwhile, enjoy your crawfish and ettufe (or however you spell it).

Caio.

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Old 05-01-2012, 06:36 PM   #3
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Gazebo slab questions


I see you made no mention of anything to do with steel reinforsed concrete for the slab. Fiberglass will work fine.
You do need to add some sort of footings for the post, not that much weight but it's to protect againt up lift.
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Old 05-02-2012, 06:35 AM   #4
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Thanks for the replies so far. Still would like some advice regarding anchor bolt depth.
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Old 05-02-2012, 10:36 PM   #5
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Gazebo slab questions


I would shoot for 5-6" minimum anchor bolt embedment, which would likely require an 8" AB. When you put then in, "shake" them straight down & then turn them about 90 degrees when at desired height to aid them in resisting pull-out.


As for the re-enforcement, I'd personally opt for steel, not because I regurgitate what a book that's decades old tell me to use, but due to actual everyday experience...............
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Old 05-03-2012, 06:13 PM   #6
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Originally Posted by jomama45 View Post
...As for the re-enforcement, I'd personally opt for steel...
I think I'll go with a rebar grid. One of the guys I've hired to do the finishing suggested I use plastic cable ties rather than wire to tie the rebar together -- never heard of that, but is it an acceptable option?
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Old 05-04-2012, 11:43 AM   #7
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I think I'll go with a rebar grid. One of the guys I've hired to do the finishing suggested I use plastic cable ties rather than wire to tie the rebar together -- never heard of that, but is it an acceptable option?
The ties are simply meant to hold the bar together until they are encased in concrete, so theoretically, the cable ties will work. Be sure to cut-off the excess "tail" so it doesn't protrude out of the concrete surface when finishing, as well as throw them out and don't let them lay on the subgrade, as they have a tendancy to float to the surface.

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